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Antarctic Bottom Water has been warming in recent decades throughout most of the oceans and freshening in regions close to its Indian and Pacific sector sources. We assess warming rates on isobars in the eastern Pacific sector of the Southern Ocean using CTD data collected from shipboard surveys from the early 1990s through the late 2010s together with CTD data collected from Deep Argo floats deployed in the region in January 2023. We show cooling and freshening in the temperature‐salinity relation for water colder than ∼0.4°C. We further find a recent acceleration in the regional bottom water warming rate vertically averaged for pressures exceeding 3,700 dbar, with the 2017/18 to 2023/24 trend of 7.5 (±0.9) m°C yr−1 nearly triple the 1992/95 to 2023/24 trend of 2.8 (±0.2) m°C yr−1. The 0.2°C isotherm descent rate for these same time periods nearly quadruples from 7.8 to 28 m yr−1. Plain Language Summary: Cold winds blowing over polynyas (areas of ice‐free water) on the Antarctic continental shelf create sea ice, forming very cold and somewhat salty, hence very dense, waters. These dense shelf waters descend the continental slope to the abyss, mixing with adjacent waters to form Antarctic Bottom Water (AABW). AABW spreads northward from there, filling much of the global abyssal ocean as it mixes with warmer, lighter waters above. AABW has been warming on pressure surfaces, freshening and cooling on density surfaces, and reducing in volume (contracting). These changes are likely a result of melting Antarctic ice sheets, which freshen the shelf waters, making them less dense, hence less able to sink to the bottom. We compare profiles of ocean temperature and salinity in the eastern Pacific sector of the Southern Ocean collected in 2023 and 2024 by robotic freely drifting profilers to data collected from ships from the early 1990s to the late 2010s. We find all of the above listed changes, but also acceleration of the warming, with the rate from 2017/18 to 2023/24 being nearly triple the rate from 1992/95 to 2023/24. The contraction rate has nearly quadrupled. This acceleration has been predicted by high‐resolution climate model simulations. Key Points: Antarctic Bottom Water (AABW) changes in the east Pacific sector of the Southern Ocean are assessed using Deep Argo and ship‐based CTD profilesBottom water warming rates from 2017/18 to 2023/24 nearly triple compared to 1992/95 to 2023/24 rates, contraction rates nearly quadrupleAABW cooling and freshening on isopycnals is also observed in the region, relative to older Circumpolar Deep Water [ABSTRACT FROM AUTHOR]

The Antarctic Slope Current is guided by the topographic gradient of the Antarctic continental slope and creates a dynamical barrier between the continental shelf and the open ocean. The current's vertical structure varies around the continent affecting cross‐slope water mass exchange with consequences for Antarctic mass loss, ventilation of the deep ocean, and carbon uptake. The Antarctic Slope Current is surface‐intensified in many regions but bottom‐intensified in regions of dense overflows. This study investigates the role of dense overflows in modifying the dynamics of the bottom‐intensified flow using a 0.1° global ocean‐sea ice model. The occurrence of bottom‐intensification is tightly linked with dense overflows and bottom speeds correlate with dense overflows on interannual time scales. A lack of vertical connectivity between the bottom and surface flow, however, suggests that the along‐slope bottom water flows are coincidentally co‐located with the Antarctic Slope Current, rather than dynamically a part of the current. Plain Language Summary: The Antarctic Slope Current is a narrow ocean current that travels around Antarctica following the continental slope. It separates the shallow and cold continental shelf from much warmer waters in the open ocean. Intrusions of the relatively warm water across the continental slope impacts melting of Antarctic ice shelves and global sea level rise. Understanding what controls the strength and variability of the Antarctic Slope Current is therefore important. The Antarctic Slope Current usually has the largest velocities near the surface, but there are regions where there is also a strong bottom flow. We use a coupled ocean‐sea ice model and show that the bottom flow is controlled by the export of very dense shelf water that flows down the continental slope in a few locations around the continent. However, the bottom and surface flow does not vary together on interannual time scales which tells us that the two components of the Antarctic Slope Current are largely independent. The result is important as the formation of dense shelf water is expected to reduce in the future which will impact the deep flow, but not the surface component. Key Points: Simulations reveal a close spatial relationship between the bottom‐intensified Antarctic Slope Current and Dense Shelf Water exportInterannual variability of Dense Shelf Water is reflected in the bottom speed of the bottom‐intensified Antarctic Slope CurrentThe surface component varies independently from the bottom‐intensified flow implying two distinct, co‐located currents [ABSTRACT FROM AUTHOR]

Recent progress in understanding Beaufort Gyre (BG) dynamics reveals an important role of ice‐ocean stress in stabilizing BG freshwater content (FWC) over seasonal to interannual timescales. But how the BG's stratification and FWC respond to surface forcing over decadal timescales has not been fully explored. Using a global ocean‐sea ice model, we partition the BG into upper, middle (halocline), and lower (thermocline) layers and perform a volume budget analysis over 1948–2017. We find that the BG's asymmetric geometry (with steep and tight isohalines over continental slopes relative to the deep basin) is key in determining the mean volume transport balance. We further find that a net Ekman suction during 1983–1995 causes the upper and middle layers to deflate isopycnally, while an enhanced Ekman pumping during 1996–2017 causes these layers to inflate both isopycnally and diapycnally, the latter via anomalous flux from the upper to the middle layer. Plain Language Summary: The Beaufort Gyre (BG) has increased its liquid freshwater content (FWC) by 40% in the past two decades. If released and transported downstream to the subpolar North Atlantic Ocean, the excess water might affect the ocean circulation via suppression of deep‐water formation. However, which layer is responsible for BG freshwater accumulations and releases over decadal timescales and the corresponding physical processes remain unclear, hampering our attempts to make future predictions. Here we use an ocean‐sea ice model to explore such changes in its three characteristic layers (upper mixed‐layer water, Pacific Water in the middle layer, and Atlantic Water in the lower layer). We find that the asymmetry of the BG, which has been simplified as a symmetric bowl shape in most previous studies, is important in determining the BG's layered mean state. Over decadal timescales, changes in BG volume are controlled by annual‐mean Ekman pumping/suction resulting from combined wind and ice‐ocean stresses. This study emphasizes the role of asymmetric geometry in determining the BG mean volume balance. It also explores the role of mean flow across the gyre's lateral boundaries in regulating BG's volume and FWC over decadal timescales. Key Points: The Beaufort Gyre's asymmetric geometry is the key to explain a net mean lateral outflow in the upper layer, despite Ekman convergenceIn the mean, the Gyre is fed by a northeast inflow into its middle layer, with outflow to its southwest from both upper and middle layersDeflation/inflation processes are asymmetric in response to anomalous Ekman suction/pumping on decadal timescales [ABSTRACT FROM AUTHOR]

The global ocean mass has significantly changed during recent decades, which may induce changes in the abyssal ocean circulation. Due to the lack of in situ observations, the response of the abyssal South China Sea (SCS) circulation to ocean mass changes remains unclear. The ocean state estimate, Estimating the Circulation and Climate of the Ocean (ECCO), provides the long‐term ocean mass changes in terms of ocean bottom pressure (OBP). Here we use the ECCO OBP data to quantify decadal changes in the abyssal SCS circulation and reveal a weakening trend. The OBP gradient trend manifests in forms of an anomalous anticyclonic circulation that is intensified by topographic effects along the continental slopes to reduce the existing abyssal circulation. The weakening abyssal circulation corresponds to the weakening Luzon Strait overflow that yields a decreasing vorticity budget in the abyssal SCS. Plain Language Summary: Caused by global warming, ice melt and long‐term atmospheric forcing changes, ocean mass has been increased and redistributed over the past decades. The changes in ocean mass distribution may lead to changes in abyssal ocean circulation. The changes in the abyssal South China Sea (SCS) circulation remains unclear due to the lack of in situ observations. This study uses the Estimating the Circulation and Climate of the Ocean (ECCO) ocean state estimate to quantify decadal changes in the abyssal SCS circulation. Our analysis of the ECCO ocean bottom pressure data reveals a weakening trend in the abyssal SCS circulation that is consistent with the recently ascertained weakening Luzon Strait overflow. Key Points: Trends in the ECCO ocean bottom pressure (OBP) gradients indicate a decadal weakening abyssal South China Sea (SCS) circulationThe weakening circulation coincides with the weakening Luzon Strait overflow that yields a decreasing vorticity budget in the abyssal SCSThe anomalous anticyclonic circulation is dominated by OBP gradient trends and intensified by topographic effects along continental slopes [ABSTRACT FROM AUTHOR]

A significant fraction of the Eulerian‐mean downwelling feeding the lower limb of the Atlantic Meridional Overturning Circulation (AMOC) occurs along the subpolar North Atlantic continental slopes and is maintained by along‐boundary densification and large‐scale geostrophic balance. We here use Argo and shipboard hydrography data to map the 2002–2015 long‐term mean density field along the boundary via a dedicated optimal interpolation tool. The overall downstream densification implies an Eulerian‐mean downwelling of 2.12 ± 0.43 Sv at 1100 m depth between Denmark Strait and Flemish Cap. A clear regional pattern emerges with downwelling in the Irminger Sea and western Labrador Sea and upwelling along Greenland western continental slope. Comparisons with independent cross‐basin estimates confirm that vertical overturning transport across the marginal seas of the subpolar North Atlantic mainly occurs along the continental slopes, and suggest the usefulness of hydrographic data in providing quantitative information about the sinking branch of the AMOC. Plain Language Summary: The Atlantic Meridional Overturning Circulation (AMOC), a critical component of the Earth's climate system due to its role in redistributing heat and freshwater between low and high latitudes, is anticipated to decline over the next century. The downwelling of surface waters in the subpolar North Atlantic that feeds the lower limb of AMOC is a vital yet vulnerable process. As revealed by previous theoretical and modeling work, the overall downstream densification along the boundary results in a significant boundary downwelling. Here, the density along the western boundary between Denmark Strait and Flemish Cap is reconstructed to provide a first observation‐based description of the regional and seasonal distribution of this boundary‐focused downwelling in the subpolar North Atlantic. This study not only provides valuable insights into how to improve existing ocean circulation theories of overturning but also contributes to a solid benchmark for evaluating how climate models simulate the sinking branch of the AMOC. Key Points: The long‐term mean full‐depth density field of the subpolar North Atlantic's boundary is reconstructed from hydrography dataThe along‐boundary densification results in a 2.12 ± 0.43 Sv Eulerian‐mean downwelling between Denmark Strait and Flemish CapA first observation‐based regional and seasonal distribution of near‐boundary Eulerian‐mean downwelling is provided [ABSTRACT FROM AUTHOR]

The Gulf of Maine's lunar semidiurnal (M2) ocean tide exhibits spatially coherent amplitude changes of ∼1–3 cm on interannual time scales, though no causative mechanism has been identified. Here we show, using a specially designed numerical modeling framework, that stratification changes account for 32%–48% (Pearson coefficient 0.58–0.69) of the observed M2 variability at tide gauges from 1994 to 2019. Masking experiments and energy diagnoses reveal that the modeled variability is primarily driven by fluctuations in barotropic‐to‐baroclinic energy conversion on the continental slope south of the gulf's mouth, with a 1‐cm amplitude increase at Boston corresponding to a ∼7% (0.30 GW) drop in the area‐integrated conversion rate. Evidence is given for the same process to have caused the decade‐long M2 amplitude decrease in the Gulf of Maine beginning in 1980/81. The study has implications for nuisance flooding predictions and space geodetic analyses seeking highest accuracies. Plain Language Summary: The height of the twice‐daily tide at Boston is about 135 cm, but researchers have long noted that this value fluctuates by about 1–3 cm from year to year. Here we show that the annual tidal height changes—seen in fact throughout the Gulf of Maine—are closely linked to how seawater density is distributed three‐dimensionally in the region. In particular, as tidal currents enter the gulf over steep underwater topography, the vertical distribution of density determines how much of the incoming wave energy is scattered back as internal tides into the deeper Northwest Atlantic. In years where this conversion of wave energy drops by 7% from its nominal value of 4 Gigawatt, the surface tide at Boston typically increases by 1 cm. Climate‐induced changes in ocean temperature and density may strengthen or weaken the conversion effect and thus slightly alter the role of tides in coastal flood events. Key Points: We propagate the M2 tide through realistic, annually varying density structures (1993–2019) in a regional Gulf of Maine modelStratification changes explain 32%–48% of the observed, cm‐level M2 amplitude variability at coastal tide gauges from 1994 to 2019Modeled M2 changes mainly reflect fluctuations in the barotropic‐baroclinic energy conversion rate on the New England continental slope [ABSTRACT FROM AUTHOR]

Primary productivity in the Arctic Ocean is experiencing dramatic changes linked to the receding sea ice cover. The vertical transport of nutrients from deeper water layers is the limiting factor for primary production. Here, we compare coincident profiles of turbulence and nutrients from the Siberian Seas in 2007, 2008, and 2018. In all years, the water column structure in the upstream region of the Arctic Boundary Current promotes upward nutrient transport, in contrast to the regions further downstream, and there are first indications for an eastward progression of these conditions. In summer 2018, strongly enhanced vertical nitrate flux and primary production above the continental slope were observed, likely related to a remote storm. The estimated contribution of these elevated fluxes above the slope to the Pan‐Arctic vertical nitrate supply is comparable with the basin‐wide transport, and is predicted to increase with declining sea ice cover in the future. Plain Language Summary: Microscopic algae, growing in the sunlit surface layer of the ocean, provide food for other species and form the basis of the ecosystem. In the Arctic Ocean, their growth is limited by the availability of nutrients. The main source of these nutrients are waters entering from the Atlantic and Pacific Oceans. These nutrient‐rich waters reside far below the sunlit zone, and vertical mixing is required to bring them upwards to support algal growth. With rapidly declining summer sea ice and changes in the ocean layering, these mixing processes might substantially change. Changes are considered most likely in the region of the steep slopes in the Siberian Seas. To investigate this, we analyze nutrient and mixing measurements in this region from 2007, 2008, and 2018. In 2018, we observed strong mixing, which is connected to ice free conditions and a process that has only recently been described. This strong mixing only happens at the narrow, steep slope region, but might supply the same amount of nutrients to the surface zone as the weak mixing over the much larger area of the deep basins. Key Points: Nutrient fluxes are higher in the western compared with the eastern Laptev SeaEpisodic mixing above the slope might significantly contribute to Pan‐Arctic vertical nitrate supplyIn the central Laptev Sea, a thinning halocline and shoaling Atlantic Water indicate an emerging regime shift in line with Atlantification [ABSTRACT FROM AUTHOR]

Tropical marginal seas host important sedimentary archives that may be exploited to reveal past changes in continental erosion, chemical weathering, and ocean dynamics. However, these records can be challenging to interpret due to the complex interactions between climate and particulate transport across ocean margins. For the southern South China Sea over the last 90 Kyr, we observe a contrasting temporal relationship between the deposition of clay minerals and magnetic minerals, which were associated with two different hydrodynamic modes. Fine‐grained clay minerals can be carried in suspension by ocean currents, leading to a rapid response to regional climate‐driven inputs. In contrast, changes in magnetic mineralogy were linked to glacial‐interglacial sea level variability, from which we infer a control by bedload transport and resuspension. Overall, this study indicates that the transfer pathways and mechanisms imparted by varying hydrodynamic conditions exert a substantial influence on the distribution of terrigenous material in continental margin sediments. Plain Language Summary: Sediments that accumulate in the ocean along continental margins contain the minerals that were eroded and transported by rivers on the nearby land masses. Past changes in the composition of such sediments can be used to reconstruct the history of physical erosion and chemical weathering on the continents, but can also be affected by ocean dynamics over orbital and millennial timescales. Here, clay mineralogy, sediment grain size, and rock magnetic signatures are measured in a core from the continental slope of the southern South China Sea, spanning the last 90 thousand years. Differences between the clay mineral and magnetic records, which are linked to finer and coarser sediment fractions respectively, indicate that specific mechanisms and timescales influenced sediment transport to the core site. This depositional framework must be considered when interpreting the timing of changes in weathering and erosion proxies hosted in sediment records from marginal seas. Specifically, fine‐grained clay minerals may reveal a rapid response to continental weathering changes, whereas the magnetic records are influenced by the transport of denser coarser‐grained minerals under the influence of sea level change. In addition, this framework could potentially serve as an indicator of past sea level change on wide continental shelves. Key Points: In the southern South China Sea, clay mineralogy responds rapidly to climate‐driven changes in inputs on orbital timescalesMagnetic content within the coarser and denser fraction is influenced by sea level change, through bedload and resuspension transportThis depositional framework may serve as a guideline for evaluating sea level change on wide continental shelves over orbital timescales [ABSTRACT FROM AUTHOR]

The North Atlantic deep water (NADW), according to the classic ocean circulation theory, moves southward as a deep western boundary current (DWBC) even though it may veer into interior and then rejoin DWBC when encountering regional circulation features, such as eddy‐driven recirculation. In potential vorticity dynamics, the eastern side of the Mid‐Atlantic Ridge (MAR) may provide a similar topographic support as the continental slope off the western boundary for a southward transport of NADW. In this article, we quantify the mean meridional NADW transports on both sides of the MAR using a data‐assimilated product and find that the flow in the eastern basin contributes about 38±14% $38\pm 14\%$ of the net southward transport of NADW from 50° to 35°N. Our study points to the importance of observing NADW transport variations on the eastern side of the MAR in order to monitor the transport strength of Atlantic Meridional Overturning Circulation. Plain Language Summary: Classic theory suggested that the Deep Western Boundary Current (DWBC) carries the cold and fresh North Atlantic deep water (NADW) from the high latitude to the south. However, float and tracer observations, albeit sparse, have revealed a possible southward pathway in the eastern North Atlantic, that is, east of the Mid‐Atlantic Ridge (MAR). Using an ocean reanalysis product, the Estimating the Circulation and Climate of the Ocean (ECCOV4r4), we show that the steady‐state NADW meridional transport in the basin east of the MAR accounts for about 38±14% $38\pm 14\%$ of the meridional transport in the whole basin between 35° and 50°N. It is found that the NADW circulation pathways are strongly influenced by ocean bottom topography. For example, the eastern flank of the MAR acts as a "western boundary" in the eastern basin against which NADW flows southward. The topographic constraint from the MAR weakens south of 35°N due to changes in the MAR height and smaller planetary vorticity f $f$. The current study has improved our understanding of NADW transport structure at mid‐latitude by quantifying the equatorward transport east of the MAR. Key Points: The eastern Atlantic basin supports nearly 38±14% $38\pm 14\%$ of the southward North Atlantic deep water (NADW) transport in mid‐latitudesThe meridional transport of NADW in the eastern basin is due to the potential vorticity constraint imposed by the Mid‐Atlantic Ridge [ABSTRACT FROM AUTHOR]

Pacific Summer Water (PSW) plays a critical role in the ecosystem of the western Arctic Ocean, impacting sea‐ice melt and providing freshwater to the basin. Most of the water exits the Chukchi Sea shelf through Barrow Canyon, but the manner in which this occurs and the ultimate fate of the water remain uncertain. Using an extensive collection of historical hydrographic and velocity data, we demonstrate how the PSW outflow depends on different wind conditions, dictating whether the warm water progresses eastward or westward away from the canyon. The current carrying the water westward along the continental slope splits into different branches, influenced by the strength and extent of the Beaufort Gyre, while the eastward penetration of PSW along the shelfbreak is limited. Our results provide the first broad‐scale view of how PSW is transferred from the shelf to the basin, highlighting the role of winds, boundary currents, and eddy exchange. Plain Language Summary: Warm Pacific water that flows northward through Bering Strait significantly influences the melting of sea‐ice, the distribution of freshwater, and the ecosystem of the Arctic Ocean. North of Bering Strait, the water progresses poleward across the shallow Chukchi Sea shelf, with most of it draining from the shelf through Barrow Canyon. Presently, the manner in which the flow exits the canyon, as well as its ultimate fate in the basin, remains unclear. We analyzed historical data sets of temperature, salinity, and velocity, and found that the local wind alters the manner in which the water exits Barrow Canyon to the east and west, as well as the corresponding distribution of Pacific Summer Water. The current carrying the water westward along the continental slope subsequently splits into different branches farther downstream. This is modulated by the strength and extent of the Beaufort Gyre, a large recirculating flow in the basin. In contrast, the signature of the warm water in the current flowing to the east is limited. Here, we provide the first observational broad‐scale view of the pathways and evolution of warm Pacific‐origin water from the shelf to the basin, and clarify the effects of wind, boundary currents, and small‐scale turbulent motions. Key Points: Outflow of Pacific Summer Water (PSW) through Barrow Canyon is dictated predominantly by the local windThe westward flow carrying the water away from the canyon bifurcates downstream, modulated by the strength and extent of the Beaufort GyreThe eastward flow emanating from the canyon progresses far downstream, but the signature of the PSW is limited [ABSTRACT FROM AUTHOR]

A bolus is a vortex formed by an internal wave propagating upslope with the shoaling wave. The amount of sediment and biota transported by boluses travelling up the world's continental slopes and onto the continental shelves has not been established but could be vital for maintaining nutrient‐rich coastal ecosystems. Previous laboratory experiments and simulations of shoaling boluses have considered a two‐layer density stratification or a linear density profile. We present experiments on bolus formation and propagation in a stratification with a density profile ρ(z) represented by a tanh‐profile, which describes many ocean pycnoclines. Our laboratory experiments examine pycnoclines with thicknesses varying from nearly zero (a two‐layer system) to large thicknesses where the density varies almost linearly with depth. The bolus volume, displacement upslope, and available potential energy are measured as a function of pycnocline thickness. Maximum upslope displacement is found to be twice that observed for a two‐layer stratification. Plain Language Summary: Interfacial waves can be observed along the surface of the ocean, and as they approach the coastline the waves steepen, break, and ultimately surge up the beach. These waves are analogous to internal waves, which occur within the ocean at the horizontal interface of more and less dense water. These internal waves can be an order of magnitude taller than the waves seen at the beach, and the surge (called a bolus) can move kilometers along the ocean bottom. Previous efforts to model these boluses have approximated the ocean's density as two layers of different density water. However, the ocean has a more gradual transition between the layers, which can be over 100 meters thick. The impact of this transition layer, called a pycnocline, on bolus transport is poorly understood. Here, we perform laboratory experiments to demonstrate that the presence of a thick pycnocline can double the distance the bolus travels upslope and increase the amount of energy for mixing. This demonstrates the need for a better model of bolus propagation to understand how these breaking internal waves move material along the bottom of the ocean. Key Points: Boluses propagate further for pycnoclines of finite thickness than for a discontinuous stratification with two layers of uniform densityThe experiments reveal the existence of a pycnocline thickness that results in a maximum bolus propagation displacement upslopeThin pycnoclines yield large bolus volumes with high available potential energy despite short bolus propagation displacement [ABSTRACT FROM AUTHOR]

Climatic changes have decreased the stability of the Gulf Stream (GS), increasing the frequency at which its meanders interact with the Mid-Atlantic Bight (MAB) continental shelf and slope region. These intrusions are thought to suppress biological productivity by transporting low-nutrient water to the otherwise productive shelf edge region. Here we present evidence of widespread, anomalously intense subsurface diatom hotspots in the MAB slope sea that likely resulted from a GS intrusion in July 2019. The hotspots (at ~50 m) were associated with water mass properties characteristic of GS water (~100 m); it is probable that the hotspots resulted from the upwelling of GS water during its transport into the slope sea, likely by a GS meander directly intruding onto the continental slope east of where the hotspots were observed. Further work is required to unravel how increasingly frequent direct GS intrusions could influence MAB marine ecosystems. Plain Language Summary As the climate has warmed, the changing large-scale circulation of the northwest Atlantic has resulted in increasing western boundary current instability. As a consequence, onshore intrusions of Gulf Stream (GS) water into the Northeast U.S. continental shelf have become increasingly frequent. The impacts of this shift on marine ecosystems have yet to be resolved. While these intrusions of low-nutrient GS water have been thought to potentially diminish biological productivity, we present evidence of an unexpectedly productive subsurface diatom bloom resulting from the direct intrusion of a GS meander toward the continental shelf. These results suggest that changing large-scale circulation has consequences for regional productivity that are not detectable by satellites by virtue of their occurrence well below the surface. [ABSTRACT FROM AUTHOR]

The geodynamic response of continental margins to erosion may reveal current and past changes in the location of the continental slope. We model the three‐dimensional isostatic response to erosion along the retreating Beringian Margin to test the hypothesis that the margin and its canyons were dominantly eroded during the Pleistocene. Our results predict 900 m of flexural uplift, though evidence for this magnitude of uplift is not observed in Pleistocene and younger deposits, suggesting a substantial amount of margin erosion occurred pre‐Pleistocene. Uplift of this magnitude may be represented by a Late Miocene unconformity across the Bering Shelf and upwarped subsurface units at the base of the Beringian Margin. The distribution of uplift from varying canyon incision and margin retreat scenarios has implications for the extent of geodynamic response during erosional periods. Plain Language Summary: Reconstructing the balance between erosion and deposition along continental margins necessitates constraining multiple variables such as sedimentation rates, tectonic subsidence, isostatic compensation, and compaction. However, reconstructing this balance along erosion‐dominated margins is hindered by the inherent removal (from eroding areas) and access challenges (deep‐water, where eroded materials deposit) to geologic records of erosion. We use geodynamics to link erosion and the resulting rebound of the crust to constrain the magnitude and timing of continental margin retreat. We model the isostatic response to a range of submarine erosion scenarios along the Beringian Margin, which are hypothesized to have formed primarily during the Pleistocene. Our results show that the magnitude of uplift predicted by the mass of sediment removed along the Beringian is too substantial to have entirely occurred since the Pleistocene, suggesting the canyons are older than previously thought. Geophysical subsurface data may instead favor isostatic upwarping that began as early as the Miocene suggesting that margin retreat and canyon incision along the deeply incised Beringian Margin has been ongoing for several millions of years. Key Points: Flexural isostatic modeling of the Beringian margin canyons suggests their erosion predates the PleistoceneSlope retreat has a greater impact than canyon enlargement on geodynamic responses to continental margin erosion, even for very large canyonsCumulative erosion of multiple incising canyons predicts a flexural response beyond the flexural wavelength of an individual canyon [ABSTRACT FROM AUTHOR]

The stability of the West Antarctic Ice Sheet (WAIS) depends on ocean heat transport toward its base and remains a source of uncertainty in sea level rise prediction. The Antarctic Slope Current (ASC), a major boundary current of the ocean's global circulation, serves as a dynamic gateway for heat transport toward Antarctica. Here, we use observations collected from the Bellingshausen Sea to propose a mechanistic explanation for the initiation of the westward‐flowing ASC. Waters modified throughout the Bellingshausen Sea by ocean‐sea‐ice and ocean‐ice‐shelf interactions are exported to the continental slope in a narrow, topographically steered western boundary current. This focused outflow produces a localized front at the shelf break that supports the emerging ASC. This mechanism emphasizes the importance of buoyancy forcing, integrated over the continental shelf, as opposed to local wind forcing, in the generation mechanism and suggests the potential for remote control of melt rates of WAIS' largest ice shelves. Plain Language Summary: The Antarctic Slope Current (ASC) is an ocean current that separates warmer, offshore water from colder waters over the continental shelf. The ASC helps to control the movement of warm water on to the continental shelf and ultimately under floating ice shelves. Warm ocean water has contributed to the melting and thinning of West Antarctic ice shelves in recent decades. This study describes where and how the ASC first forms in West Antarctica. Previous studies have suggested that the ASC responds primarily to atmospheric winds blowing over the current itself, whereas here it is argued that processes occurring over a much larger region of the west Antarctic continental shelf may determine the strength and structure of the ASC. These results contribute to our understanding of the long‐term, for example, decades or longer, evolution of the West Antarctic Ice Sheet. Key Points: The initiation of a westward Antarctic Slope Current (ASC) occurs in the western Bellingshausen SeaThe ASC arises from the export of waters, modified by ocean‐ice interactions, to the shelf break, rather than from local wind forcingThis mechanism suggests the potential for remote control of shelf properties as well as West Antarctic ice shelf melt rates [ABSTRACT FROM AUTHOR]

The Gulf Stream is the only pathway in the subtropical North Atlantic by which warm water flows poleward. This transport of warm water and return of cold water at depth is called the Atlantic Meridional Overturning Circulation (AMOC). The dynamic method is applied to hydrocasts collected since the 1930s to estimate upper‐ocean transport (0–1,000 m) between the U.S. Continental Slope and Bermuda and separately to Africa with focus on the longest directly observable timescale. Calculating transport between the Slope and Bermuda eliminates the Gulf Stream's northern and southern recirculation gyres, while calculations between the Slope and Africa remove all other recirculating geostrophic flow. The net Slope‐Bermuda upper‐ocean transport is estimated to be 41.1 ± 0.4 Sv, decreasing by 2.0 ± 0.8 Sv between 1930 and 2020. The AMOC contribution is 18.4 ± 0.6 Sv, decreasing by 0.4 ± 0.6 Sv between 1930 and 2020. Plain Language Summary: The Gulf Stream is the only pathway by which the ocean transports warm water to high latitudes in the North Atlantic and Nordic Seas where it cools, sinks and flows back south at intermediate depths from the subpolar North Atlantic and at greater depths from the Nordic Seas. This circulation pattern, the Atlantic Meridional Overturning Circulation (AMOC), plays an important role in Earth's climate. But the Gulf Stream is also the wester boundary current to the horizontal circulation driven by the trade winds and westerlies at low and mid‐latitudes. The Gulf Stream also drives large recirculations to its south and north. In this study we use hydrographic data taken since the 1930s to calculate the poleward flow in the Gulf Stream, while canceling out the recirculation and wind‐driven transports to estimate the strength of the north‐flowing portion of the AMOC. There is evidence for a 2.0 Sv Gulf Stream slow‐down between 1930 and 2020. Whether and to what extent this reflects a slowdown of the AMOC or wind‐driven circulation cannot be established with certainty. Our estimate of a 0.4 Sv AMOC decrease is reported with low confidence. Key Points: Direct measurements along the Oleander line (∼37°N) show that the depth of the Atlantic Meridional Overturning Circulation (AMOC) maximum is close to 1,000 mNet northward transport between North America and Bermuda equals 41.1 ± 0.4 Sv of which the AMOC accounts for about 18.4 ± 0.6 SvNet northward transport NW of Bermuda slowed by 2 ± 0.8 Sv between 1930 and 2020 with 0.4 ± 0.6 Sv attributed to an AMOC decline [ABSTRACT FROM AUTHOR]

The 2022 volcanic eruption in Tonga caused an unusually large tsunami around the Pacific. It travels with a faster apparent velocity and has larger amplitudes at long distances than what would be expected from a conventional tsunami from the volcanic source. This tsunami was generated by the moving atmospheric Lamb wave and traveled at the speed of the Lamb wave (0.31 km/s). Japanese data showed the amplitude of this first tsunami becomes small when approaching the coast, due to the weaker air‐sea coupling at the shallow depth. This wave split when passing the continental slope, and traveled at the speed of the ocean gravity wave. Therefore, the tsunami observed at the coast is delayed by thousands of seconds from the passage of the Lamb wave. Tsunamis generated by this atmospheric mechanism have not been previously observed by modern digital recording systems and should be considered in the tsunami warning systems. Plain Language Summary: A tsunami is usually generated by the sudden changes of the water heights, and caused by offshore earthquakes, coastal landslides, and submarine volcanic eruptions. The generated vertical displacement of the water propagates as a very long wave, and the tsunami waves become compressed with increased heights as they approach the coast. The 2022 volcanic eruption in Tonga caused an unusually large tsunami, which cannot be explained by conventional sources. The speed and amplitude are very different from theoretical values: the speed is about 0.31 km/s, whereas the average tsunami speed in the Pacific is 0.2 km/s. These data suggest that the tsunami from the Tonga eruption was excited by a pulse of atmospheric pressure as it traveled from the volcano. This source of the tsunami in the atmosphere needs to be considered for the tsunami warning system in the future. Key Points: Lamb wave produced by the volcanic eruption excites the sea surface as it travels across the Pacific and generates a tsunami pulseThe effect of the air‐sea coupling becomes weaker as the Lamb wave approaches the coastThe tsunami wave splits when passing the continental slope of Japan causing more complex waves [ABSTRACT FROM AUTHOR]

Circumpolar Deep Water (CDW) transport across the Antarctic continental slope regulates the delivery of heat to the shelf and its availability to melt floating ice shelves. The cross‐slope density field, calculated from profiles collected by conductivity‐temperature‐depth‐tagged marine mammals on the East Antarctic slope (0–160°E, above 1,000‐ to 3,000‐m isobaths), indicates eddy‐driven overturning: onshore transport of CDW and offshore transport of shallower Antarctic Surface Water. Enhanced eddy activity, determined by a spice standard deviation threshold in the CDW layer, is present over about a third of the East Antarctic slope analyzed. Significantly stronger CDW transport in regions of elevated spice variability produces subsurface temperature anomalies of 0.2–0.25 °C relative to the East Antarctic average. Estimating eddy diffusivity from the hydrography yields about 0.8 m2/s of warm CDW transport to the shelf break in high‐variability regions. Variability of eddy‐induced CDW transport influences the reservoir of heat available for transport across the shelf break. Plain Language Summary: The transport of warm water from the open ocean to the Antarctic continental shelf has global significance, as it regulates the melt of ice shelves and dense water formation. We quantify the strength of eddy‐driven transport of relatively warm Circumpolar Deep Water (CDW) across the continental slope of East Antarctica from the observed cross‐slope density structure. We find that, in general, there is an eddy‐driven overturning: deeper CDW is transported onshore while shallower Antarctic Surface Water is transported offshore. Upon closer inspection, we find that regions with elevated temperature and salinity variability on density surfaces are associated with significantly stronger onshore transport of CDW by eddies. These hot spots of eddy transport produce warm and salty anomalies at the shelf break, impacting the reservoir of heat available to the shelf. Key Points: Cross‐slope eddy transport is inferred from observed density‐layer thickness gradients in East Antarctica between 0°E and 160°EEnhanced onshore transport (≈1.5 × East Antarctic mean) of warm, salty Circumpolar Deep Water occurs in regions of high isopycnal spice variabilityEddy‐induced overturning carries ≈0.8 m2/s of CDW poleward to produce warm, salty anomalies at the shelf break in eddying regions [ABSTRACT FROM AUTHOR]

Simulating Arctic Ocean mesoscale eddies in ocean circulation models presents a great challenge because of their small size. This study employs an unstructured‐mesh ocean‐sea ice model to conduct a decadal‐scale global simulation with a 1‐km Arctic. It provides a basinwide overview of Arctic eddy energetics. Increasing model resolution from 4 to 1 km increases Arctic eddy kinetic energy (EKE) and total kinetic energy (TKE) by about 40% and 15%, respectively. EKE is the highest along main currents over topography slopes, where strong conversion from available potential energy to EKE takes place. It is high in halocline with a maximum typically centered in the depth range of 70–110 m, and in the Atlantic Water layer of the Eurasian Basin as well. The seasonal variability of EKE along the continental slopes of southern Canada and eastern Eurasian basins is similar, stronger in fall and weaker in spring. Plain Language Summary: Ocean mesoscale eddies play crucial roles in the ocean, climate, and ecosystem. While this is presumably also true for Arctic eddies, their dynamics and impacts are far less understood than for lower latitudes: It is a great challenge to resolve the very small Arctic eddies in numerical simulations. Just now, owing to the development of new‐generation models, it becomes possible to resolve Arctic eddies in realistic global ocean configurations. The study presents the results from a first‐ever decadal‐scale global simulation with the Arctic Ocean at 1‐km resolution. An overview of the eddy kinetic energy and its generation is provided for the Arctic deep basin, with a focus on both spatial and seasonal variability. The current results fill some knowledge gaps in Arctic eddy energetics, and also help to identify questions that we will be able to answer with such frontier simulations in the future. Key Points: A decadal‐scale global simulation with the whole Arctic Ocean at 1‐km resolution is carried out using FESOM2A basinwide overview of Arctic eddy energetics is provided with a focus on spatial patterns and seasonal changes of eddy kinetic energyContinental slopes in southern Canada and eastern Eurasian basins are the most energetic regions featuring similar seasonality [ABSTRACT FROM AUTHOR]

Fluid seepage sometimes impacts occurrences of submarine landslides, but the role of landslide deposits in fluid seepage is often neglected. Bathymetric data show that more than 1,000 seabed mounds are scattered over an elongated area at the shelf edge of the northwestern South China Sea. Vertical seismic pipes in the subsurface are spatially correlated with both surface mounds and buried landslide deposits. Detailed analyses within the three‐dimensional (3D) seismic survey show that at least 221 of 231 (∼96%) of the mounds have spatial correlation with the top edges of multiple stages of buried landslide scarps that are stepping seawards from deep to shallow, indicating a causal relationship between buried landslide deposits and fluid seepage. Fluids emitted via differential compaction and hydrofracturing in geological setting characterized by frequent tectonism, flowed upwards along shear surfaces and scarps of landslide deposits and pipes, and eventually provided hard substrates for mound building. Plain Language Summary: Submarine landslides are serious geohazards and the most common sedimentary units on the continental slope. Submarine landslides can be induced by sediment input and processes of becoming rocks, tectonism events such as earthquakes, seawater movement such as tsunamis. Among these factors, fluid seepage‐associated structures such as fluid conduits frequently coexist with landslide deposits; hence, fluid seepage can contribute significantly to occurrences of landslides. However, the impact of landslide deposits on fluid flow is of less concern. Using geophysical data in the northwestern South China Sea, a multitude of seabed mounds are observed in an elongated region along the shelf edge. Most mounds are underlain by pipe‐shaped fluid conduits that link downwards to buried landslide deposits. Mounds have a strong vertical correspondence with the top edges of buried landslide scarps. We conclude that different degrees of compaction between landslide deposits and surrounding sediment and generation of fractures driven by high hydraulic pressure, induced the release of overpressure fluids below bases of landslide deposits, which served as effective fluid conduits and perhaps as part of walls of overpressure fluid layers. The building and release of overpressure fluids were also impacted by ample sediment input and frequent tectonic movements like faulting. Key Points: We uncover more than 1,000 seabed cold‐water coral mounds in the NW South China SeaBuried landslide deposits induced fluid seepage with shear surfaces and scarps as conduits, providing hard substrates for mounds buildingTectonic faulting assisted buried landslide deposits to release overpressure fluids [ABSTRACT FROM AUTHOR]

Intermediate nepheloid layers (INLs) form important pathways for the cross‐slope transport and vertical export of particulate matter, including carbon. While intermediate maxima in particle settling fluxes have been reported in the Eurasian Basin of the Arctic Ocean, direct observations of turbid INLs above the continental slope are still lacking. In this study, we provide the first direct evidence of an INL, coinciding with enhanced mid‐water turbulent dissipation rates, over the Laptev Sea continental slope in summer 2018. Current velocity data show a period of enhanced downslope flow with depressed isopcynals, suggesting that the enhanced turbulent dissipation is probably the consequence of the presence of an unsteady lee wave. Similar events occur mostly during ice free periods, suggesting an increasing frequency of episodic cross‐slope particle transport in the future. The discovery of the INL and the episodic generation mechanism provide new insights into particle transport dynamics in this rapidly changing environment. Plain Language Summary: In the Arctic Ocean deep basins, only a tiny fraction of the algae that grows in the surface layer sinks down to the sea floor. Most of the particles reaching the sea floor originate from the shallower regions closer to the coast. These particles have already settled on the sea floor once, and originate from rivers or algae that grew, died and sank down in shallow regions. Later, these particles are lifted off the ground again by strong turbulent motions and transported toward deeper regions in the middle of the water column. These lift‐off and transport events happen only occasionally, and have not been directly observed in the Eurasian part of the Arctic Ocean yet. Also, we present a new mechanism for the creation of turbulence, which is necessary to lift particles off the sea floor. This mechanism happens mostly during the summer season, when less sea ice is present. Based on this seasonality, it is likely that sediment transport events will become more frequent in the future, when the Arctic sea ice is further declining. Key Points: First direct observation of an intermediate nepheloid layer in the Eurasian part of the Arctic OceanCoinciding strong midwater turbulence is likely caused by a down‐slope current displacing isopycnalsSimilar downslope flow events exhibit a strong seasonality toward the ice free season [ABSTRACT FROM AUTHOR]

Inflow of warm modified Circumpolar Deep Water (CDW) onto the Antarctic continental shelf and into ice shelf cavities is a key driver of Antarctic ice shelf mass loss. While recent research has advanced understanding of CDW heat transport onto the continental shelf, the fate of CDW on the shelf is less understood. Here, we use Lagrangian particle tracking in an ocean‐sea ice model without ice shelf cavities to map the residence time of CDW on the Antarctic continental shelf. Mean residence times vary from <1 month in the East Antarctic to >1 year in the West Antarctic. In regions of dense water formation, transformation of CDW on the shelf limits access of CDW to ice shelves, despite strong onshore CDW heat transport. Elsewhere transformation of CDW on the shelf is weak, implying that temperature on the shelf is limited by heat transport onto the shelf or the offshore heat reservoir. Plain Language Summary: Antarctic ice shelf melt is accelerating and is linked to the inflow of warm ocean waters that melt ice shelves from below. Multiple processes regulate the amount of warm water that reaches the Antarctic ice shelves, and the relative importance of these processes in different regions is not well known. Here, we conduct a virtual particle tracking experiment in a state‐of‐the‐art ocean model to trace the pathways and timescales of warm water on the Antarctic continental shelf. We find that there is a large regional variation in how long warm waters spend on the continental shelf from less than a month to well over a year. In the regions along the continental slope where very dense water formed near the coast flows off the shelf to the abyssal ocean, this is balanced by a strong inflow of warm water onto the continental shelf, which is rapidly converted to denser water, limiting the heat from reaching the ice shelves. Elsewhere warm water can remain on the continental shelf unmodified for a long time, so the heat available for ice shelf melt is instead limited by the strength of the flow of warm water onto the continental shelf. Key Points: Particle tracking in an ocean model shows Circumpolar Deep Water (CDW) residence times on the Antarctic shelf range from a month to several yearsWater mass transformation limits CDW access to ice shelves in Dense Shelf Water formation regions, despite strong onshore CDW transportIn warm shelf regions, long residence times of CDW are associated with weak water mass transformation [ABSTRACT FROM AUTHOR]

Submarine groundwater discharge could impact the transport of critical solutes to the ocean. However, its driver(s), significance over geological time scales, and geographical coverage are poorly understood. We characterize a submarine groundwater seep from the continental slope off northern Norway where substantial amount of meteoric water was detected. We reconstruct the seepage history from textural relationships and U‐Th geochronology of authigenic minerals. We demonstrate how glacial‐interglacial dynamics have promoted submarine groundwater circulation more than 100 km offshore and result in high fluxes of critical solutes to the ocean. Such cryosphere‐hydrosphere coupling is likely common in the circum‐Arctic implying that future decay of glaciers and permafrost in a warming Arctic is expected to attenuate such a coupled process and thus decreases the export of critical solutes. Plain Language Summary: Occurrence of meteoric groundwater (freshwater originated from precipitation such as rain and snow) in the global ocean is an unexpected but seemingly common phenomenon. Here, we report evidence for meteoric groundwater flow at ~800‐m water depth from the coast of northern Norway. Dating of the chemically formed carbonate rocks on the seafloor, an oxidation product of methane, reveals that the groundwater flow was strongest when large ice sheets occupied the nearby shelf. Our results confirm the temporal and geographical scales of meteoric groundwater flow in the Arctic region and highlight its impact on carbon cycling and ocean chemistry. Key Points: We document the connection between glacial dynamics and the discharge of meteoric water from the eastern Norwegian SeaA large quantity of critical solutes discharge to the ocean as a result of water circulationOffshore presence/discharge of freshwater is a common phenomenon in the circum‐Arctic Ocean that have profound impact on ocean chemistry [ABSTRACT FROM AUTHOR]

The meridional coherence, connectivity, and regional inhomogeneity in long‐term sea surface temperature (SST) variability over the Northwest Atlantic continental shelf and slope from 1982–2018 are investigated using observational data sets. A meridionally concurrent large SST warming trend is identified as the dominant signal over the length of the continental shelf and slope between Cape Hatteras in North Carolina and Cape Chidley, Newfoundland and Labrador, Canada. The linear trends are 0.37 ± 0.06 and 0.39 ± 0.06 °C/decade for the shelf and slope regions, respectively. These meridionally averaged SST time series over the shelf and slope are consistent with each other and across multiple longer observational data sets with records dating back to 1900. The coherence between the long‐term meridionally averaged time series over the shelf and slope and basin‐wide averaged SST in the North Atlantic implies approximately two thirds of the warming trend during 1982–2018 may be attributed to natural climate variability and the rest to externally forced change including anthropogenic warming. Plain Language Summary: This study investigates long‐term changes in the sea surface temperature (SST) since 1982 over the Northwest Atlantic continental shelf and slope. In particular, we focus on the changes consistently found from Cape Hatteras, North Carolina, United States, to Cape Chidley, Newfoundland and Labrador, Canada. The SST warming rates concurrently found over this large latitudinal range are 0.37 ± 0.06 and 0.39 ± 0.06 °C/decade for the shelf and slope regions, respectively. Analysis indicates that approximately two thirds of the warming trend during 1982–2018 may be attributed to natural climate variability and the rest to externally forced change including anthropogenic warming. Our findings are further confirmed by comparing with multiple other observational data sets. Key Points: Meridionally coherent surface warming is observed over the Northwest Atlantic continental shelf and slopeThe linear trends in SST exhibit distinct spatial maxima in the slope region off Georges Bank and Cape HatterasLong‐term variations in the meridional means of SST are highly correlated with basin‐averaged values [ABSTRACT FROM AUTHOR]

Bottom drag is believed to be one of the key mechanisms that remove kinetic energy from the ocean's general circulation. However, large uncertainty still remains in global estimates of bottom drag dissipation. One significant source of uncertainty comes from the velocity structures near the bottom where the combination of sloping topography and stratification can reduce the mean flow magnitude, and thus the bottom drag dissipation. Using high‐resolution numerical simulations, we demonstrate that previous estimates of bottom drag dissipation are biased high because they neglect velocity shear in the bottom boundary layer. The estimated bottom drag dissipation associated with geostrophic flows over the continental slopes is at least 56% smaller compared with prior estimates made using total velocities outside the near‐bottom layer. The diagnostics suggest the necessity of resolving the bottom boundary layer structures in coarse‐resolution ocean models and observations in order to close the global kinetic energy budget. Plain Language Summary: When an oceanic flow is close to the seafloor, the bottom drag converts its kinetic energy (KE) to heat through viscous friction, and this dissipation of KE has been shown to be very sensitive to the magnitude of the flow. Despite previous estimates indicating the bottom drag being a significant mechanism for removing KE from the ocean's general circulation, large uncertainty still remains. Using high‐resolution numerical simulations of the Atlantic Ocean, we demonstrate that accounting for the velocity reduction through the oceanic bottom boundary layer reduces KE loss from the balanced flow (in which the pressure gradient force and Coriolis force balance) by at least 56% over the continental slopes. This velocity reduction is due to the presence of sloping topography and ocean stratification near the bottom, which should be resolved in future observational and modeling efforts toward a more complete picture of the ocean's energy budget. Key Points: Thermal wind shear in the bottom boundary layer reduces the magnitude of the near‐bottom flow and bottom stressAccounting for boundary layer shear reduces kinetic energy loss from the geostrophically balanced flow by more than 50%Observations and models that do not resolve the bottom boundary layer will overestimate the energy dissipation due to bottom drag [ABSTRACT FROM AUTHOR]

The genesis of submarine channels is often controlled by gravity flows, but channels can also be formed by oceanographic processes. Using multibeam bathymetry and two‐dimensional seismic data from the western South China Sea, this study reveals how pockmarks can ultimately form channels under the effect of bottom currents and gravity‐driven sedimentary processes. We demonstrate that alongslope and across‐slope channels were initiated by pockmark trains on the seafloor. Discrete pockmarks were elongated due to the erosion of gravity‐driven sedimentary processes and bottom currents, and later coalesced to form immature channels with irregular thalwegs. These gradually evolved into mature channels with continuous overbanks and smooth thalwegs. Submarine channel evolution was significantly influenced by seafloor topography since the Late Miocene. The evolutionary model documented here is a key to understanding how channels are formed in deep‐water environments. Plain Language Summary: Submarine channels are prominent erosional features on continental slopes and basin floors. They are usually formed by ocean currents flowing alongslope and sediment avalanches flowing downslope. Here, we investigate a system of channels on the western South China Sea using geophysical methods. The channels are formed in a region with widespread seafloor depressions (pockmarks) formed by the seepage of fluid into the water column. Gravity‐driven sedimentary processes and ocean currents reshaped these pockmarks, which were ultimately merged together to form immature and irregular channels. Under continued erosion, the immature channels eventually developed mature channels with continuous overbanks and smooth channel floors. This study reveals that ocean currents and gravity processes can form channels with different orientations by eroding pre‐existing pockmarks. Key Points: A complex system of channels in the western South China Sea was formed by the erosion of seafloor pockmarksThe evolution from pockmarks to channel comprises three stages: pockmark train, immature, and mature channelThis on‐going system of channels was initiated in the Late Miocene and significantly influenced by seafloor topography [ABSTRACT FROM AUTHOR]

Abstract: Mooring data collected on the continental slope of the South China Sea show that along‐slope deep sea bottom currents are generated when large spring internal tides (internal waves with tidal frequency) are observed, with the maximum velocity amplitude exceeding 0.15 m/s. The observations are consistent with predictions that near‐bottom breaking of internal waves can result in generation of along‐slope flows when these waves obliquely approach the slope. A linear internal tide model in one horizontal dimension with realistic topography and stratification is used to show that the breaking of internal tides is likely due to near‐critical reflection on the slope. Combining the mooring observations and the model simulation, an along‐slope near‐bottom transport of ~0.5 Sv is estimated. Along‐slope bottom flows caused by breaking internal waves potentially provide a significant way to deform continental slopes and affect deep water exchange between the marginal sea and open ocean. [ABSTRACT FROM AUTHOR]

Bottom currents play a major role in deep‐sea sedimentation, but their significance in the burial of organic carbon is poorly quantified at a global scale. Here we show that Holocene fluxes of organic carbon into the contourite drifts are high, with a global average of 0.09 g cm−2 Kyr−1. At individual drift sites, fluxes are commonly 1–2 orders of magnitude greater than rates in surrounding areas and in global depth‐similar zones. These high fluxes of organic carbon into the contourite drifts are due to high rates of sedimentation. Over the past 50 million years, sedimentation rates at the studied contourite drift sites have overall increased, coincident with decreasing atmospheric CO2 and a cooling global climate. Our work suggests that a ramp‐up of the bottom‐current carbon pump has accelerated removal of CO2 from the atmosphere and oceanic water, thus contributing to the overall global cooling after the Eocene Thermal Maximum. Plain Language Summary: Bottom currents play a major role in deep‐sea sedimentation, but their significance in the burial of organic carbon is poorly quantified at a global scale. Here we examine data from modern contourite drifts (large‐scale, alongslope‐trending bottom‐current deposits) across the globe and show modern fluxes of organic carbon into the drifts are high, with a global average of 0.09 g cm−2 Kyr−1. At individual drift sites, fluxes are commonly 1 to 2 orders of magnitude greater than rates in surrounding areas and in global depth‐similar zones. These high fluxes of organic carbon into the drifts are due to high rates of sedimentation in these deepwater environments, which are driven primarily by vigorous bottom currents—in other words, by a bottom‐current pump that is highly efficient at burying organic carbon. Our work suggests that a ramp‐up of the bottom‐current carbon pump, attributable to progressive intensification of global ocean circulation over the past 50 million years, has accelerated removal of CO2 from the atmosphere and oceanic water, thus contributing to the global cooling after the Eocene Thermal Maximum. Sedimentary records of past organic carbon fluxes in contourite drifts over geologic time could well prove useful in informing predictions of future climate. Key Points: Modern fluxes of organic carbon into the contourite drifts are high, with a global average of 0.09 g cm−2 Kyr−1The fluxes into the drifts are commonly 1–2 orders of magnitude greater than rates in surrounding areas and in global depth‐similar zonesOver the past 50 million years, the bottom‐current pump has accelerated removal of carbon from the oceanic water [ABSTRACT FROM AUTHOR]

The East China Sea Kuroshio (ECS‐Kuroshio) and the Ryukyu Current are the major poleward heat carriers in the North Pacific. Anomalous changes of ECS‐Kuroshio and Ryukyu Current could exert substantial influence on the climate in mid‐latitude regions. However, owing to limited observations and coarse resolution of climate models, how they might change under anthropogenic warming remains unknown. Here, we find an accelerating ECS‐Kuroshio (1.5 Sv) and a decelerating (−2.2 Sv) Ryukyu Current using in‐situ observation during 1958–2022, equivalent to 7% strengthening and 20% weakening in the 65 years. The trend is also simulated by four high‐resolution climate models, with multi‐model ensemble‐mean acceleration (deceleration) of the ECS‐Kuroshio (Ryukyu Current) of 1.2 ± 0.6 Sv (−6.2 ± 2.5 Sv) over 1950–2050. The weakening subtropical wind field reduces their summed transport o. Enhanced stratification, which induces uplift of current system and weaker topography‐flow interaction, leads to the intensifying ECS‐Kuroshio and disappearing Ryukyu Current. Plain Language Summary: The East China Sea Kuroshio (ECS‐Kuroshio) and the Ryukyu Current are the major components of western boundary current (WBC) system in the North Pacific. They transport large amount of heat poleward, maintaining the mid‐latitude region warm and habitable. In this study, we find that in both observation and high‐resolution numerical models, the ECS‐Kuroshio is intensifying and the Ryukyu Current is rapidly declining. Particularly, in climate models, the Ryukyu Current is predicted to experience a 45% weakening during 1950–2050. Weakening wind field in the North Pacific tends to reduce the total transport of WBC system. The rapid weakening of the Ryukyu Current is credit to the enhanced stratification under global warming, which induces uplift of the Kuroshio east of Taiwan. Hence, less water will be blocked by bottom topography (Ilan Ridge) and bifurcates into the Ryukyu Current. Our finding highlights the needs for comprehensive studies on the local and climate effects of the evolution of the WBC system in the North Pacific. Key Points: The East China Sea Kuroshio is intensifying while the Ryukyu Current is rapidly declining in a warming climateThe change is mainly caused by enhanced stratification which induces uplift of current system and weaker topography‐flow interaction [ABSTRACT FROM AUTHOR]

Enhanced sedimentation at glacial margins can produce submarine megaslides (>10,000 km3). We report a single megaslide in the Surveyor Fan, Gulf of Alaska. Minimum extant size is ∼16,124 km2 in area and ∼9,080 km3 in volume. Slope failure occurred ∼1.2 Ma at the onset of the mid‐Pleistocene transition (MPT). With accretion along the Aleutian‐Alaska Trench, the original volume is conservatively ∼16,280 km3, with only a 140 km run‐out due to its blocky, high shear strength nature. We suggest the megaslide was triggered by a major sediment influx at the onset of the MPT, when glacial‐interglacial cycles shifted from 41 to 100 Kyr. The absence of repeat megaslides may reflect a changing balance between seismic strengthening and sediment flux, where later sedimentation driven by cross‐shelf ice streams results in thin, fluidized, non‐cohesive slides. Continued accretion of the Surveyor Fan and megaslide also reduces critical wedge taper, further inhibiting major failure. Plain Language Summary: Sediment flux at glacial margins can produce submarine slides >10,000 km3 in size (megaslides). We report a single megaslide in the Surveyor Fan, Gulf of Alaska. Minimum extant size is ∼16,124 km2 in area and ∼9,080 km3 in volume. Failure occurred ∼1.2 Ma at the onset of the mid‐Pleistocene transition (MPT). Due to accretion along the Aleutian‐Alaska Trench, the original volume is conservatively ∼16,280 km3, but with only a 140 km run‐out due to its blocky, high shear strength nature. We suggest the megaslide was caused by the initial major sediment flux at the onset of the MPT, and that afterward only fluidized, thin, and non‐cohesive slides occurred. This is due to changing balance between seismic strengthening and sediment flux, and accretion of the Surveyor Fan and megaslide which reduces the critical wedge taper inhibiting major failure. Key Points: World's Fifth largest mapped megaslide documented beneath Surveyor Fan Gulf of AlaskaTiming of slope failure linked to onset of mid‐Pleistocene glacial intensificationAbsence of later failures due to changing balance of sediment flux/seismic strengthening and negative feedbacks from critical wedge processes [ABSTRACT FROM AUTHOR]

We conducted two‐dimensional numerical simulations to investigate the mechanisms underlying the strong spatiotemporal correlation observed between submarine landslides and gas hydrate dissociation due to glacial sea‐level drops. Our results suggest that potential plastic deformation or slip could occur at localized and small scales in the shallow‐water portion of the gas hydrate stability zone (GHSZ). This shallow‐water portion of the GHSZ typically lies within the area enclosed by three points: the BGHSZ–seafloor intersection, the seafloor at ∼600 m below sea level (mbsl), and the base of the GHSZ (BGHSZ) at ∼1,050 mbsl in low‐latitude regions. The deep BGHSZ (>1,050 mbsl) could not slip; therefore, the entire BGHSZ was not a complete slip surface. Glacial hydrate dissociation alone is unlikely to cause large‐scale submarine landslides. Observed deep‐water (much greater than 600 mbsl) turbidites containing geochemical evidence of glacial hydrate dissociation potentially formed from erosion or detachment in the GHSZ pinch‐out zone. Plain Language Summary: Many submarine landslides spatiotemporally correlate with gas hydrate dissociation. However, direct mechanical evidence supporting whether the overpressure and deformation due to glacial sea‐level drop‐induced hydrate dissociation are adequate for triggering submarine landslides is lacking. Here, we present two‐dimensional thermal‐hydraulic‐chemical and geomechanical models of a gas‐hydrate system in response to glacial sea‐level drops and conduct sensitivity analyses of the model behavior under a wide range of key conditions from a global perspective. Our simulations suggest that glacial hydrate dissociation might induce plastic deformation or slip at localized and small scales only possibly within the shallow‐water portion of the hydrate stability zone. The deep part (>1,050 m below sea level) of the bottom boundary of the hydrate stability zone could not slip; therefore, the entire bottom boundary of the hydrate stability zone was not a complete slip surface. We demonstrate that glacial hydrate dissociation alone is unlikely to trigger large‐scale submarine landslides. Our work highlights the vicinity of the upper limit of the hydrate stability zone (where the base of the hydrate stability zone intersects the seafloor) as an important area for investigating overpressure and focused fluid flow, localized plastic deformation or slip, and downslope sediment transport related to glacial hydrate dissociation. Key Points: Glacial hydrate dissociation might cause potential plastic deformation or slip at localized and small scales in shallow parts of the GHSZThe large deformation surface at the BGHSZ boundary of the potential plastic deformation zone was not a complete slip surfaceGlacial sea‐level drop‐induced gas hydrate dissociation alone is unlikely to have caused large‐scale submarine landslides [ABSTRACT FROM AUTHOR]

Internal tides (ITs) play a critical role in ocean mixing, and have strong signatures in ocean observations. Here, global IT sea surface height (SSH) in nadir altimetry is compared with an ocean forecast model that assimilates de‐tided SSH from nadir altimetry. The forecast model removes IT SSH variance from nadir altimetry at skill levels comparable to those achieved with empirical analysis of nadir altimetry. Accurate removal of IT SSH is needed to fully reveal lower‐frequency mesoscale eddies and currents in altimeter data. Analysis windows of order 30–120 days, made possible by the frequent (hourly) outputs of the forecast model, remove more IT SSH variance than longer windows. Forecast models offer a promising new approach for global internal tide mapping and altimetry correction. Because they provide information on the full water column, forecast models can also help to improve understanding of the underlying dynamics of ITs. Plain Language Summary: Tidal flow over topographic features on the seafloor generates vertical displacements along the interfaces of ocean layers that have different densities. These vertical displacements at tidal frequencies are known as internal tides. Internal tide displacements are largest well below the sea surface, but also display a sea surface height (SSH) signature that is large enough to be measured by satellite altimeters. Removing internal tide signals from satellite altimeter SSH allows for a more accurate accounting of non‐tidal features, including slowly evolving ocean currents and eddies, that are also measured by altimeters. Here, we show that supercomputer ocean forecast simulations of the global internal tide field are able to remove internal tide SSH from satellite altimeter measurements with a skill level that is comparable to the skill of internal tide SSH removal based upon analysis of the satellite altimeter data itself. Thus, forecast models offer a complementary method for this important task. In addition, forecast models provide information on the entire ocean water column, not just the sea surface. Finally, the hourly outputs of forecast models allow for a greater variety of tidal analysis record lengths than can be achieved with altimeter outputs, which report sea surface height fields much less frequently. Key Points: Global ocean forecast models can accurately simulate both long‐term (phase‐locked) internal tides and their short‐term modulationsOcean forecast models offer a useful complement to empirical models for mapping internal tides and correcting altimetry for internal tidesIn regions of strong internal tides, optimal variance reduction in nadir altimetry is attained through short‐term tidal analyses (∼60 days) [ABSTRACT FROM AUTHOR]

Climate variability over the Tropical and North Pacific Ocean (TPO and NPO, respectively) modulates marginal sea variability. The South China Sea (SCS), the largest marginal sea in the western NPO, is an outstanding example of a region that responds quickly to climate change. However, there is considerable uncertainty regarding the response of the deep SCS to large‐scale climate variability. Multivariate empirical orthogonal function analysis revealed three prominent modes of interconnected temperature anomaly fluctuations within the TPO and NPO. These coherent modes highlight the interactive dynamics among climate variations and reveal their modulation mechanisms for previously less explored potential temperature variabilities in the deep SCS. On the atmospheric bridge, external forces modify the upper‐layer Luzon Strait Transport (LST) by adjusting the Ekman transport and Kuroshio intrusion. For the oceanic pathway, climate variations disturb the deep‐layer LST by adjusting the barotropic flows in the upper layer. Plain Language Summary: This study utilizes multivariate empirical orthogonal function analysis to examine long‐term hydrographic observations and identify coherent variations in surface temperature anomalies in the Tropical and North Pacific Ocean. The leading mode displays in‐phase combinations of the East Pacific El Niño and warm Pacific Decadal Oscillation. The second mode displays a Central Pacific El Niño and North Pacific Meridional Mode‐like mode. The third mode resembles a Central Pacific El Niño and Victoria Mode‐like mode. The objective is to comprehend the synergies between the climatic variabilities in the Tropical and North Pacific Ocean, as well as the processes by which the predominant climate variabilities regulate the undocumented potential temperature variabilities in the deep South China Sea. Extrinsic forcings in the Tropical and North Pacific Ocean regulate the interannual–decadal temperature variabilities by modulating the Luzon Strait Transport. On the atmospheric bridge, these extrinsic forcings affect the upper‐layer Luzon Strait Transport by altering the local Ekman transport and Kuroshio intrusion associated with the bifurcation of the North Equatorial Current. On the oceanic pathway, climate variabilities disturb the deep‐layer Luzon Strait Transport by changing the ocean bottom pressure between the two sides of the Luzon Strait. Key Points: Coherent climate variabilities in the Tropical and North Pacific Ocean are obtained through MVEOF analysisPotential temperature observations from the deep South China Sea indicate interannual–decadal variabilityClimate variabilities affect deep temperature variability through the changing atmospheric bridge and oceanic pathway [ABSTRACT FROM AUTHOR]

Most marine gas hydrate systems follow a vertical pattern with hydrate overlying free gas. Here we document the discovery of a gas to hydrate system in a horizontal sand layer in the Qiongdongnan Basin of the South China Sea. Eight wells were drilled by the Guangzhou Marine Geological Survey in 2021–2022 to investigate the occurrence and mechanisms responsible for the formation of the system. We describe a free gas‐bearing sand reservoir at the center of the system sustained by advecting hot fluids and gas; away from the advecting zone, the cooler, surrounding sand reservoir is filled with hydrate. Observations at this site show that advective heat has a large control on hydrate formation in sands and may be a key mechanism which allows gas migration within the hydrate stability zone and the formation of high‐saturation hydrate in sand layers. Plain Language Summary: Natural gas hydrate, an ice‐like substance composed of water and gas, is commonly found in sediments under the ocean. Most marine hydrate systems follow a vertical pattern where hydrate‐bearing sediments overlie free gas‐bearing sediments; in addition, most hydrate is hosted in marine muds at low concentration. Here we document a horizontal system that transitions from high concentrations of free gas to high concentrations of hydrate in a horizontal sand layer in the northern South China Sea. Two recent drilling expeditions are conducted to explore this unique system. Seismic and logging data suggests that multiple processes including focused fluid flow, capillary sealing and heat transfer control the formation of the system. Key Points: We discover a gas hydrate system where gas transitions to hydrate in a flat‐lying sand layer in the northern South China SeaCapillary sealing occurs at the sand‐clay interface, preventing upward fluid advection and causes fluids to migrate laterallyAdvecting warm fluids and gas are the primary control on the hydrate and free gas system in this sand layer [ABSTRACT FROM AUTHOR]

Sparse offshore coverage of seismic networks has hindered detailed studies of submarine earthquakes and their associated seismic hazard. We present results of our analysis of a diver's recording of acoustic signals from an ML = 5.6 earthquake in the Persian Gulf. We model the signals as a set of several shallow water T phases the frequency and group velocity of which are determined by bathymetry. We show that the audio track from this recording can provide reliable estimates of earthquake location and seismic moment. We also show that the reported shaking in the southern Persian Gulf, >170 km from the source of this small earthquake could result from T waves traveling through the entire width of the basin. Our results point to rudimentary and affordable underwater microphones similar to those used in the divers' cameras as tools to build efficient, low‐cost networks for the study of offshore events and early warning. Plain Language Summary: The lack of offshore seismic stations or the low population of such networks has caused our knowledge of near‐shore earthquake sources to be incomplete. To this end, and as a possible tool to study the source of submarine earthquakes, we have analyzed the video recording from a group of divers who have captured the moments during an earthquake in the Persian Gulf region. This earthquake which occurred on the northern, Iranian side of the Persian Gulf was strongly felt along its southern shorelines, more than 170 km from the epicenter. The analysis of this video, which surfaced on social media, points to a set of trapped acoustic signals in the water, or T‐waves, in close proximity of the source. We show that such recordings can be reliably used to obtain important information regarding the earthquake source, including its location and magnitude. Besides, we make the case that the reported shaking in Dubai was likely due to the arrival of such high‐pitch T‐ waves from the earthquake. Finally, we propose that cheap microphones existing in standard entertainment cameras can be used to build efficient underwater networks for monitoring small to moderate offshore events. Key Points: We use a diver's camera recording to study an offshore, moderate earthquake in the Persian GulfA propagation model and simulations of shallow water T‐waves are presentedLow‐cost, entertainment‐quality, subsea hydrophones run by citizen scientists can augment monitoring of offshore events [ABSTRACT FROM AUTHOR]

Submarine methane seepage can potentially be promoted by the dissociation of the marine hydrates surrounding the continental margins due to oceanic warming since the Last Glacial Maximum. This seepage could be archived by authigenic carbonates at seeping sites, but the time lag caused by heat transmission through the sediment column leads to an inconsistency between the ages of the carbonate and the period of bottom water warming. Here we present the records of the authigenic carbonate crust from drilling site D1 in the Mid‐Okinawa Trough. Uranium–thorium dating results show that the carbonate crust mainly grew downwards during 14–6 ka. Gas hydrates hosted in the relatively thin stability zone dissociated in a rapid response to bottom water warming and intensified the methane seepage. Our study better supports the hypothesis that a considerable amount of methane can be released from marine hydrates due to global climatic changes. Plain Language Summary: Methane hydrates are ice‐like crystalline compounds and often found in deep‐ocean marine sediments. Ocean warming in the past could have destabilized methane hydrates and led to a rapid discharge of free methane gas. Geological information on this methane escape could be preserved by carbonate rocks. To date, the hypothesis of the control of ocean warming on hydrate melting since the Last Glacial Maximum (19,000–26,000 years ago) has not been fully tested. To examine this hypothesis, we used the rock samples of seep‐carbonate retrieved by seafloor drilling in the Mid‐Okinawa Trough. Uranium–thorium dating results show that the seep carbonates formed between 6,000 and 14,000 years ago. It took less time for heat to diffuse downwards from warming seawater to trigger hydrate melting at this location than in most of other oceans at similar water depths. The smaller time lag makes the carbonate rock age close to the period of ocean warming. Our results better support that global ocean warming could potentially release methane from gas hydrates during glacial–interglacial transitions. The consequent methane transport into the oceans and likely also the atmosphere might impact ocean acidification and climatic warming. Key Points: Presentation of the chronology of methane seepage by dating authigenic carbonates collected from drilling core D1 in the Okinawa TroughOcean warming–induced hydrate dissociation led to the downward growth of authigenic carbonate for 14–6 ka after the Last Glacial MaximumThis carbonate record better supports the link of hydrate dissociation with ocean warming due to a smaller thermal lag effect [ABSTRACT FROM AUTHOR]

Mooring data collected at a flat‐topped seamount suggest the generation of pure inertial waves (PIWs; waves with a dominant frequency equal to the local inertial frequency f) by low‐frequency flows over large‐scale topography. Energetic PIWs were observed within a narrow depth range (∼100 m) near the seafloor at the edge of the summit. These waves could be associated with low‐frequency flows. A two‐dimensional nonhydrostatic model was used to show that the observed PIWs are most likely internal wave beams generated by low‐frequency flows over the seamount. Two types of PIWs were identified via observation and model. The first is a PIW that can only travel horizontally. The other propagates upward, with super‐inertial intrinsic frequency that is Doppler‐shifted by the flows to f. Nonlinear triadic interactions among waves with the frequencies [0, f, f] may transfer energy from mean flows to PIWs, promoting energy decay of geostrophic flows over large‐scale topography. Plain Language Summary: Near‐inertial waves (NIWs) are intrinsic motions of the ocean caused by Earth's rotation. Understanding their generation and evolution is key to parameterizing NIWs and the associated mixing in global ocean circulation models. This study presents novel observations suggesting that energetic pure inertial waves (PIWs; i.e., NIWs with a dominant frequency appearing at the local inertial frequency) are generated by low‐frequency flows over large‐scale seamount topography in the abyssal ocean. The nonlinear forcing of low‐frequency flows due to their acceleration near the seamount summit is suggested to be essential for generating PIWs. These findings provide further insight into NIWs in the abyssal ocean, which is important for understanding the distribution of NIWs and the energy decay of geostrophic flows over topography. Key Points: Energetic pure inertial waves (PIWs) were observed within a narrow depth range near the seafloor at a large abyssal seamountNear‐bottom‐enhanced PIWs stem from nonlinear forcing of low‐frequency flows over the seamountLow‐frequency flows over the seamount generate two types of PIWs: nonvertically propagating and upward‐propagating [ABSTRACT FROM AUTHOR]

Abstract Recent progress in understanding Beaufort Gyre (BG) dynamics reveals an important role of ice‐ocean stress in stabilizing BG freshwater content (FWC) over seasonal to interannual timescales. But how the BG's stratification and FWC respond to surface forcing over decadal timescales has not been fully explored. Using a global ocean‐sea ice model, we partition the BG into upper, middle (halocline), and lower (thermocline) layers and perform a volume budget analysis over 1948–2017. We find that the BG's asymmetric geometry (with steep and tight isohalines over continental slopes relative to the deep basin) is key in determining the mean volume transport balance. We further find that a net Ekman suction during 1983–1995 causes the upper and middle layers to deflate isopycnally, while an enhanced Ekman pumping during 1996–2017 causes these layers to inflate both isopycnally and diapycnally, the latter via anomalous flux from the upper to the middle layer.

The International Ocean Discovery Program cored Sites U1517 (Tuaheni landslide) and U1519 (Tuaheni Basin) on the Hikurangi slope, North Island, New Zealand. We employed X‐ray diffraction to document clay mineral assemblages within the muddy sediments, with the aim of testing their potential influences on slope stability. Detrital smectite dominates the clays, with average proportions of 52 wt% at Site U1517 and 53 wt% at Site U1519. Bulk sediment from Site U1517 contains up to ∼29 wt% smectite (average = 21 wt%), high enough to reduce the angle of internal friction (on average) to ∼6°. Stratigraphic sections at both sites are homogeneous; compositional excursions are not evident along inferred slip surfaces or weak layers. Smectite decreases in the "downstream" direction of the East Cape Current, and that spatial trend correlates with lower densities of slide scars. The uniformity of compositional preconditioning, however, points to other factors as determinants of slip nucleation. Plain Language Summary: Expandable clay minerals (smectite) are notorious for weakening sediments and contributing to conditions that promote landslides. The Hikurangi margin offshore North Island, New Zealand, is noted for its abundance of submarine landslides. To assess their possible causes, we analyzed clay mineral assemblages from two sites on the upper trench slope that were sampled by the International Ocean Discovery Program. Cores from Sites U1517 and U1519 display unusual degrees of compositional homogeny, with smectite as the dominant clay mineral. We did not find any compositional anomalies at inferred slip surfaces or along weak layers of the Tuaheni landslide. Instead, the entire stratigraphic succession appears to be relatively weak. With inherited preconditioning, dynamic loading during large earthquakes is probably enough to trigger landslides, especially on steeper slopes. Key Points: Hikurangi trench‐slope sediments, including muds within the Tuaheni landslide complex, contain high concentrations of detrital smectiteSmectite is abundant enough to reduce the bulk sediment's coefficient of friction, with little stratigraphic variabilityThe homogeneous mud composition results from strong currents and does not change significantly along inferred slip surfaces or weak layers [ABSTRACT FROM AUTHOR]

Aged organic carbon (OC) is widely observed in surficial sediments deposited on continental margins, resulting from several factors including old OC input and sedimentological processes. While the supply of old OC to marine sediments has been examined and quantified in a range of continent margin settings, there have been few attempts to constrain the extent to which hydrodynamic processes, specifically lateral transport, contribute to the old age of sedimentary OC. Here, we propose a mathematical index, bAgedOC, to differentiate the causes of sedimentary aged OC, and apply this approach to two typical sediment transport pathways that follow the supply and dispersal of terrestrial OC in the marine environment, and further extrapolate it to submarine canyon, Arctic and deep‐sea environments. The index, representing a modification of a widely used quantitative method, provides a novel approach to distinguish the contribution of lateral transport to the pre‐depositional aging of OC in the oceanic region. Plain Language Summary: Organic carbon (OC) occurring in marine surface continental margin sediments varies in radiocarbon ages, ranging from modern to ancient. While the supply of old terrestrial OC represents a major source of aged OC in marine sediments, sediment redistribution processes in the ocean can also induce old OC ages, although the latter currently remains poorly constrained. We introduce a mathematical method to visualize and distinguish between the causes of aged sedimentary OC that can be applied in regional settings. By applying this method to a suite of surficial sediments from two along‐shelf transects that follow the transport and dispersal of terrestrial OC, we demonstrate the importance of hydrodynamic processes on the sedimentary OC ages. Then we further apply this method to other continental margin settings. Delineating between causes leading to old ages of OC in marine sediments bears upon our understanding of the oceanic carbon cycle. Key Points: A mathematical index is described to differentiate the causes of sedimentary aged OCLateral transport exerts a strong influence on the age spectrum of OC in marine sedimentsLateral transport and terrestrial inputs both contribute to old ages of OC in marine sediments [ABSTRACT FROM AUTHOR]

The formation and spreading of dense deepwater in the polar regions play a key role in one of the most important climate systems, namely ocean meridional overturning circulation, and the deepwater formation is projected to decrease under the global warming. However, the impact of the reduced deepwater formation on the climate system has not been explored in detail. Here, we performed a series of numerical experiments with a climate model where the downward water mass transport through the bottom boundary layer is artificially reduced to quantitatively evaluate its impacts on the transient ocean and climate responses. It is demonstrated that changes in deepwater formation have non‐negligible impacts on not only ocean heat content but also the Earth's radiation budget at the top of the atmosphere: reduction in deepwater formation in high‐latitude oceans causes warming of bottom water, cooling of the ocean surface, and a subsequent decrease in outgoing longwave radiation. Plain Language Summary: The sinking and spreading of cold, dense water into the ocean deep layers at high latitudes plays a crucial role in large‐scale ocean circulation, closely linked to the climate system. In this study, we use a climate model to investigate the impact of the reduced dense deepwater formation on the heat budget of the climate system, aiming at a comprehensive understanding of deepwater formation in a warming climate transition. The results show that changes in the dense water formation have a non‐negligible effect on the heat budget of the atmosphere as well as on the heat content of the ocean. A decrease in deepwater formation in the high‐latitude oceans leads to warming of the bottom waters, cooling the ocean surface, and a concomitant decrease in outgoing longwave radiation. Key Points: We evaluate the impacts of reduced deepwater formation in a climate model by adjusting the parameters of the bottom boundary layerWeakening the downward water mass transport through the bottom boundary layer leads to increased net heat transport to the deep oceanChanges in deepwater formation have non‐negligible impacts on ocean heat content and the atmospheric heat balance [ABSTRACT FROM AUTHOR]

Within the fore‐arc of the Cascadia Subduction Zone, there are significant along‐strike differences in the orientation of splay faults, sediment consolidation, and fault roughness. Here, we use dynamic rupture simulations of megathrust earthquakes on different realizations of a fault system that incorporate fore‐arc properties representative of offshore Oregon and Washington to estimate how splay faults may behave in future megathrust earthquakes in Cascadia. While splay faults were activated in all of our simulations, splay orientation is a primary control on slip amplitude. Seaward vergent faults accommodate significant amounts of slip resulting in large seafloor uplift and significantly larger tsunami amplitudes. For example, our median tsunami heights including splay faults are about a factor of two larger than those that did not include splay fault deformation. We suggest that there is an urgent need to revisit existing approaches to tsunami hazard assessment in Cascadia to include the influence of splay faults. Plain Language Summary: The Cascadia subduction zone has hosted many great earthquakes in the past. Due to the potential of generating great earthquakes, this zone poses a huge earthquake and tsunami hazard. The main fault (or Cascadia megathrust) responsible for generating great earthquakes is about ∼1,000‐km long and is inferred to have variation in the geometrical and mechanical properties. One such difference is the difference between properties of the "forearc" region. The "forearc" is the shallow subsurface region on the landward side adjacent to the Cascadia megathrust. The orientation of the secondary faults (i.e., splays) is different in the forearc for the offshore Washington and Oregon region. Similarly, the consolidation of sediments within the forearc is also different for the Washington and Oregon region. In this study, we performed earthquake and tsunami simulation to understand how the difference in these forearc properties affects the tsunami hazard of the region. We found that the tsunami hazard in the Washington region is higher as compared to the Oregon region due to higher magnitude slip on the splay and megathrust. We find that this difference is mainly due to the difference in the orientation of splay faults. Key Points: Dynamic earthquake rupture simulations of the Cascadia Subduction Zone (CSZ) offshore Washington and Oregon region to investigate updip rupture behaviorLarger tsunami amplitudes observed for the Oregon region suggest a large tsunami hazard in central and southern CascadiaDifferent orientations of the splays within the CSZ forearc result in differences in the amount of slip and tsunami heights [ABSTRACT FROM AUTHOR]

Tropical storms and hurricanes frequently pass over the Straits of Florida, energizing the near‐inertial wave (NIW) field in the strait. Two ship‐based surveys, which were launched shortly after storms, observed velocity shear bands over the western slope of the strait—clear signatures of NIWs. Also, the hydrographic measurements demonstrate the formation of a slantwise critical layer (where isopycnals are parallel with bathymetry) over the western slope, which is known to be a trapping zone for NIWs. A realistic simulation of the Straits confirms the emergence of NIWs under a tropical storm, which is accompanied by inertially modulated bottom‐enhanced mixing over the western slope. The mechanism driving the mixing is that the storm‐generated NIWs radiate downward from the core of the Florida Current, reflect off the eastern slope, and enter the slantwise critical layer over the western slope; upon entering, wave trapping, and amplification lead to the enhanced mixing. Plain Language Summary: Strong winds blowing over the ocean inject a significant amount of energy into the ocean. Some of this energy goes into near‐inertial waves, internal waves radiating from the surface that transmit energy into the ocean interior. Near‐inertial waves (NIWs) are thought to play a role in mixing deep waters and sustaining the global overturning circulation. However, they have not been considered as a key driver of mixing near the seafloor. In this study, we present observations of storm‐generated NIWs over the seafloor of the Straits of Florida by two research cruises launched shortly after tropical storms. Supported by theories, our numerical simulations show supportive evidence that the seafloor mixing in the Straits of Florida can be enhanced by these storm‐generated NIWs. The benthic deep‐water coral ecosystems and the properties of intermediate water masses are potentially influenced by this mixing enhancement. Key Points: A slantwise critical layer for near‐inertial waves is observed over the western slope of the Straits of FloridaSignatures of near‐inertial waves are observed in the slantwise critical layer after tropical storms pass over the Straits of FloridaUpon entering the slantwise critical layer, the waves are trapped and hence amplified, leading to mixing enhancement over the western slope [ABSTRACT FROM AUTHOR]

The possible link between the occurrence of submarine mass failure and climate‐driven factors is highly disputed. This is due largely to the lack of comprehensive records of mass failures in the geologic record for which ages, triggers, and preconditioning factors can be reliably constrained. Such controls would allow accurate testing of cause‐and‐effect relationships. We present a record that comprises 490 earthquake‐triggered mass failure deposits from the Dead Sea depocenter over the past 220 kyr that permits a robust statistical evaluation and correlation with potential preconditioning factors. Our data set reveals that (a) at the orbital‐ and millennial‐scale, variable sedimentation rates are not a preconditioning factor for these mass failure deposits; (b) at the centennial‐to decadal‐scale, earthquake‐triggered mass failures can occur at any lake‐level state; (c) at the orbital‐ and millennial‐scale, the mass failures are more frequent when lake‐levels were high and punctuated by large‐amplitude fluctuations. Plain Language Summary: Some researchers propose that lowering sea‐level leads to mass failures, while, others suggest that raising sea‐level induces mass failures. In contrast, other researchers conclude that no clear correlation exists between mass failures and sea‐level change as the ages of failure events are random. This dispute is due largely to the lack of comprehensive records of mass failures in the geologic record for which ages, triggers, and preconditioning factors can be reliably constrained, thus preventing the testing of cause‐ and ‐effect relationships. We present a record of mass failures from the Dead Sea center over the last 220 kyr. The high‐resolution dating, combined with well‐constrained trigger and preconditioning factors, makes this a unique archive for testing the different hypotheses. Our analysis indicates that mass failures can occur during seismic shaking at any lake‐level state at the centennial‐to decadal‐scale, but are more frequent during lake‐level high‐stands with large‐amplitude fluctuations at orbital‐ and millennial‐scales. Furthermore, we find that sedimentation rate is not a preconditioning factor for mass failures under seismic shaking at both the orbital‐ and millennial‐scales. Key Points: At the orbital‐ and millennial‐scale, variable sedimentation rates are not a preconditioning factor for earthquake‐triggered mass failuresAt the centennial‐to decadal‐scale, earthquake‐triggered mass failures are not statistically correlated with lake‐level stateAt the orbital‐ and millennial‐scale, the mass failures are more frequent during lake‐level high‐stands with large‐amplitude fluctuations [ABSTRACT FROM AUTHOR]

Closely spaced CTD stations showed elevated oxygen within Monterey Submarine Canyon. Anomalously high (2–5 μmol kg−1) dissolved oxygen was found between 600–1,100 m in the O2 minimum, co‐located with a turbulence hotspot caused by convergence of upcanyon, semidiurnal internal tidal energy flux. Furthermore, the oxygen anomaly extended >10 km downcanyon at the same depth and isopycnals of a previously identified intrusion predicted from buoyancy conservation. We show that dissolved oxygen and fine suspended particles act as independent tracers to (a) validate previous microstructure observations of intense turbulence extending >400 m above the bed (mab) at the canyon hotspot, and (b) track boundary‐interior exchange driven by mixing in the form of isopyncal‐spreading of anomalies away from a near‐boundary source. This study demonstrates the use of oxygen, commonly measured with shipboard profiling, as a tool for tracking mixing and lateral dispersal. Plain Language Summary: Continental margins and complex topography at canyons play a critical role in mixing the main ocean thermocline and the vertical transport of heat and carbon. Monterey Submarine Canyon has a persistent hotspot of elevated internal wave energy dissipation between 1,200 and 700 m depth. Using previous data and new analyses we show that this mixing hotspot is also an elevated oxygen hotspot in the middle of the deep oxygen minimum zone (a basin‐wide feature), and that this elevated oxygen anomaly, along with suspended particles and mixed fluid, are expelled from the near‐boundary hotspot and flow offshore ∼10 km within an intrusion. Results improve understanding of how continental‐margin boundary layers remain stratified in the face of persistent energetic turbulence, and suggest potential use of oxygen in future studies of mixing and exchange between continental margins and ocean interior. Key Points: Elevated oxygen in deep oxygen minimum zone is caused by mixing in Monterey Canyon internal wave dissipation hotspotAnomalous oxygen and particles spread offshore in an intrusion, providing evidence for exchange driving by mixing and restratificationDissolved oxygen, often observable via CTD, may be useful for other turbulence and buoyancy‐flux studies when conditions are appropriate [ABSTRACT FROM AUTHOR]

The efficiency of sediment routing from land to the ocean depends on the position of submarine canyon heads with regard to terrestrial sediment sources. We aim to identify the main controls on whether a submarine canyon head remains connected to terrestrial sediment input during Holocene sea‐level rise. Globally, we identified 798 canyon heads that are currently located at the 120m‐depth contour (the Last Glacial Maximum shoreline) and 183 canyon heads that are connected to the shore (within a distance of 6 km) during the present‐day highstand. Regional hotspots of shore‐connected canyons are the Mediterranean active margin and the Pacific coast of Central and South America. We used 34 terrestrial and marine predictor variables to predict shore‐connected canyon occurrence using Bayesian regression. Our analysis shows that steep and narrow shelves facilitate canyon‐head connectivity to the shore. Moreover, shore‐connected canyons occur preferentially along active margins characterized by resistant bedrock and high river‐water discharge. Plain Language Summary: Since the last glaciation about 20,000 years ago, sea level has risen by about 120 m. As a consequence, most coastlines have migrated landward, inundating large shelf areas. Some of these areas are now dissected by submarine canyons. However, with only 4% of the world's submarine canyons reaching today's coastline, these canyons remain the exception. Here, we aim to identify the environmental factors and processes that control whether rates of headward canyon incision can keep pace with landward migration of the coastline during the Holocene. We determine 34 variables that potentially predict whether a canyon remains connected to the coastline. We find that shore‐connected canyons preferentially occur along continental margins with narrow and steep shelves, such as the Mediterranean active margin and the Pacific coast of Central and South America. Moreover, our analysis supports the occurrence of such canyons offshore river basins, that are characterized by resistant bedrock and high water discharge. Such rivers deliver coarse‐grained sediment to submarine canyons, which can erode the canyon head and floor. To this end, our analysis offers new insights into the formation and maintenance of submarine canyons that are required to efficiently transport sediments, pollutants, and organic carbon from rivers to the ocean floor. Key Points: Presently, 183 submarine canyons are connected to the shoreline (within 6 km) along the world's major continentsNarrow shelves and high shelf gradients precondition the maintenance of canyon‐head connectivity to the shoreCanyon heads preferentially remain connected to the shore offshore river catchments with resistant bedrock and high water discharge [ABSTRACT FROM AUTHOR]

Internal wave energy in the Arctic Ocean is often an order of magnitude lower than the midlatitudes. By inhibiting energy input and causing damping, the presence of sea ice is believed to be responsible for low internal wave energy. While a few current studies have shown slightly elevated internal wave energy compared to historical measurements, it has not matched the catastrophic decline in sea ice extent over the same period. We report internal wave energy and mixing estimates that show little difference in the presence of sea ice. To examine possible causes other than sea ice, we adopt the model framework developed in Gill (1984) to explore the importance of previously unexamined factors responsible for the low internal wave energy in the Arctic Ocean. Model results show that low β and shallow mixed layers can result in significant reductions in horizontal kinetic energy in the pycnocline compared to midlatitudes. Plain Language Summary: Kinetic energy below the surface of the ocean is primarily input through the winds and the tides. The wind blows on the ocean, creating currents in a mixed layer at the surface. These currents often make a type of wave called an internal wave that moves downward and distributes that energy to greater depths. The Arctic Ocean is known to be especially quiet and still compared to the midlatitude open ocean. This is often attributed to the presence of a perennial sea ice cover. We use observations from the Canada Basin in summer and fall to show that this energy at depth is not dependent on the presence of ice and run a simple model to show that other factors are important in distributing the kinetic energy from the wind at depth. Key Points: In the Canada Basin, internal wave energy and mixing from shear measurements are similar despite the presence or absence of sea iceModel results show that low values of Beta and shallow mixed layers result in a significant decrease in near‐inertial internal wave energy [ABSTRACT FROM AUTHOR]

Bottom trawling grounds have been expanding to deeper areas of the oceans since the mid‐XXth century, and mitigating strategies aimed to protect fish stocks, such as temporal trawling closures, have recently been implemented. Here we investigated the biogeochemical properties of sediment from a deep‐sea trawling ground in Palamós Canyon (NW Mediterranean) to assess the effects of a 2‐months trawling closure in the recovery of sedimentary organic matter. In comparison to untrawled areas, the continuous erosion and sediment mixing in trawling grounds led to coarser reworked sediments impoverished in organic carbon (∼30% loss) and promoted the degradation of labile compounds (52–70% loss). These impacts persisted after the temporal trawling closure, highlighting that this management strategy is insufficient to restore the seafloor. Considering the continuous expansion of bottom trawling grounds, this activity could have significant and irreversible biogeochemical impacts on ocean margins at a global scale, hampering their carbon burial capacity. Plain Language Summary: Bottom trawling grounds have been expanding to deeper areas of the oceans since the mid‐XXth century, causing substantial effects to fish populations. In order to reduce this impact, new management strategies have been introduced, such as establishing temporal trawling closures to allow fish populations to recover. Bottom trawling not only poses a threat to fish populations but also to the seafloor, since the continuous contact of trawling gear on the bottom erodes large volumes of sediment and reduces the amount and quality of organic matter. In this study, we assessed the effect of a 2‐months trawling closure in deep‐sea bottom trawling grounds in Palamós Canyon (NW Mediterranean) by collecting sediment samples in trawled and untrawled areas during different seasons. Our results show that sediment samples in the trawled site had substantially lower organic matter contents than in the untrawled site, and the remaining organic matter had poor nutritional quality. These impacts persisted after the temporal trawling closure, highlighting that this management strategy is insufficient to restore the seafloor. Considering the continuous expansion of bottom trawling grounds, this activity could have significant and irreversible impacts in organic matter available for benthic species in ocean margins at a global scale. Key Points: Bottom trawling disturbance on the deep seafloor depletes organic matter through erosion and degradation, hampering its preservationA 2‐month seasonal trawling closure is insufficient to restore the seafloor and it is estimated that it would take decades to recoverBottom trawling causes organic carbon losses from the seafloor comparable to those generated on land by tillage of agricultural fields [ABSTRACT FROM AUTHOR]