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Accumulation of radioactive radon gas in indoor air poses a serious risk to human health by increasing the lifetime risk of lung cancer and is classified by IARC as a category one carcinogen. Radon exposure risks are a function of geologic, geographic, building design, and human behavioural variables, and can change over time. Using time series and deep machine learning modelling, we analyzed long-term radon test outcomes as a function of building metrics from 25,489 Canadian and 38,596 Swedish residential properties constructed between 1945 to 2020. While Canadian and Swedish properties built between 1970 and 1980 are comparable (96–103 Bq/m3), innate radon risks subsequently diverge, rising in Canada and falling in Sweden such that 21st Century Canadian houses show 467% greater average radon (131 Bq/m3) relative to Swedish equivalents (28 Bq/m3). These trends are consistent across housing types and regions within each country. The introduction of energy efficiency measures within Canadian and Swedish building codes coincided with opposing radon level trajectories in each nation. Deep machine learning modelling predicts that, without intervention, average Canadian residential radon levels will increase to 176 Bq/m3 by 2050, emphasizing the importance and urgency of future building code intervention to achieve systemic radon reduction in Canada. [ABSTRACT FROM AUTHOR]

Anthropogenic induced climate perturbations are seen in changes in oceanic circulation patterns, and Arctic water masses defined by salinity are vulnerable to change. Biogeography of marine microbial eukaryotes is expected to be impacted by changes in local environmental conditions and advective processes, but tracking the extent of plankton distribution requires understanding routes for both active and passive tracers. To identify such tracers, we focused on samples collected in the western (Canada Basin) and eastern (Nares Strait); extremes of the Canadian High Arctic that are connected by an east flowing current north of Canada. Sequencing of the V4 region of 18S rRNA revealed that Acantharia, a taxonomically and functionally diverse group of large planktonic protists, were particularly common. Arctic acantharians in our study were dominated by two clades belonging to cyst-forming groups. The distribution of one clade suggested successful advective transport from the Pacific sourced water in the Beaufort Gyre to southern Nares Strait, with cells transported along the northern shelf of the Canadian Arctic. A second clade appeared to be a resident taxon of the Canada Basin whose distribution correlated to local environmental conditions, and detection in deeper samples would be consistent with swarmer formation enabling reestablishment the following year. [ABSTRACT FROM AUTHOR]

The Canada Basin is the largest basin in the Arctic Ocean. Its unique physical features have the highest concentration of nutrients being found in the subsurface layer, referred to as the subsurface nutrient maximum layer (SNM). Under climate change in the Arctic, the SNM is an essential material base for primary productivity. However, long-term trends of nutrient variations and dominant factors related to nutrient levels in the SNM are still unclear. In this study, we analyzed the SNM variations and main influencing factors of the Canada Basin based on the Global Ocean Data Analysis Project Version 2 between 1990 and 2015 and the Chinese Arctic Research Expedition between 2010 and 2016. We found that the nutrient concentrations in the SNM were relatively stable for decades [average concentrations of nitrate, phosphate, and silicate were (13.6 ± 2.4) µmol/L, (1.8 ± 0.2) µmol/L, and (31.5 ± 5.7) µmol/L, respectively]. Nutrient reservoirs were dominated by physical processes. Inflow and outflow water of the SNM contributed about 60.4% and −50.2% to the nutrient stocks, respectively, while particle deposition and remineralization in the Canada Basin contributed approximately one-third to the nutrient stocks. Nitrogen fixation and denitrification in the Canada Basin had no substantial impact on nutrient stocks. The overall stabilization of the SNM over the past few decades implied that the SNM would not substantially affect short term primary productivity. Understanding the long-term trends and dominant factors of reservoirs in the SNM will provide useful insights into the changing Canada Basin ecosystem. [ABSTRACT FROM AUTHOR]

The Arctic is one of the regions under the most dramatic climate change. Global warming has led to elevated freshwater inflow into the western Arctic Ocean and significantly altered nutrient structure and biogeochemical cycling. In this work, inorganic dissolved nutrients in the Chukchi Plateau (CP) and adjacent regions were investigated to further understand their characteristics and influencing factors. Results showed that relatively high nutrient concentrations occurred in the water masses with salinity >32 psu, especially there was a nutrient-rich layer influenced by Winter Pacific Water in the water column (15.10±1.96, 2.23±0.26, and 23.46±6.64 µmol/L for DIN, PO4 3- and Si(OH)4, respectively). Contrarily, lower nutrient concentrations occurred in the mixing water of the upper layer (1.76±1.04, 1.15±0.16 and 3.76±2.29 µmol/L for DIN, PO4 3- and Si(OH)4, respectively) with a low DIN/P ratio (1.44±0.59), suggesting DIN has become the potentially biological limiting factor. Furthermore, the freshening and deepening of the upper layer driven by the Beaufort Gyre has hindered the nutrient transport from underlying layer into the upper layer. A maximum chla was observed at 36-75 m water depth, and the phytoplankton biomass decreased from the western to the eastern CP, accompanied by a decreased contribution of micro-sized chla but an increased contribution of small-sized chla (74.0±0.1%). The phytoplankton resource use efficiency for DIN was estimated as (3.2±4.6)×10-3, which was primarily influenced by the physicochemical parameters of water and also largely regulated by the size structure of phytoplankton. [ABSTRACT FROM AUTHOR]

The time series of near‐bottom temperatures collected from September 2018 until August 2020 from an array of three current‐ and pressure‐recording inverted echo sounders showed quasi‐seasonal and quasi‐monthly (∼28 days) variations at a depth of ∼1,300 m near the Chukchi slope in the western Arctic Ocean. They revealed an increase of ∼0.1°C during the winter‐spring period compared with the summer‐fall period. These variations were observed in the data‐assimilated Hybrid Coordinate Ocean Model (HYCOM) outputs near the observation site (correlation coefficient >0.7). They confirmed that variations in near‐bottom temperature are related to changes in the intensity of the Atlantic Water (AW) boundary current, concurrent with the deepening of the lower AW layer by approximately 50 m. The difference in sea surface height (SSH) between the Canada Basin and the Chukchi Shelf increased because of the negative wind stress curl (WSC) and retarded the AW boundary current according to the geostrophic effect. When the near‐bottom temperature increased during the winter‐spring period, the SSH in the Chukchi Shelf was lower than that in the summer‐fall period because of the less negative WSC. Quasi‐monthly variations were related to SSH on the Chukchi Shelf owing to the negative WSC. HYCOM outputs from 1994 to 2015 showed that the AW boundary current weakened more recently than in the past due to the increased melting of sea ice. The results imply that a longer sea‐ice‐free season in the Arctic amplifies changes in the AW boundary current and deep ocean temperature owing to increased atmospheric forcing. Plain Language Summary: Atlantic Water (AW) entering the Arctic Ocean from the Nordic Seas is considered a heat reservoir because it is warmer than the Arctic Ocean. The AW flows along the boundaries around the basin margins, known as the AW boundary current. The AW boundary current gradually deepened after entering and sat at approximately 200–800 m near the Chukchi slope. The AW boundary current exhibited quasi‐seasonal and quasi‐monthly (approximately 28 days) variations, confirmed using near‐bottom temperature data and data‐assimilated Hybrid Coordinate Ocean Model outputs. The variations are due to the geostrophic current caused by the pressure gradient due to the difference in the sea surface height (SSH) between the Chukchi Shelf and Canada Basin, which are located in the southwest and northeast of the Chukchi slope, respectively. Due to the difference in the SSH around the Chukchi slope, the geostrophic current is in the opposite direction to the AW boundary current and can suppress it. Sea surface height is affected by wind stress curl, and sea ice dampens atmospheric forcing. These findings imply that a longer sea‐ice‐free season in the Arctic can amplify changes in the AW boundary current. Key Points: The near‐bottom temperature observed at a depth of ∼1,300 m near the Chukchi slope showed quasi‐seasonal and quasi‐monthly variationsThe variations are related to the wind‐induced change in intensity of the Atlantic Water (AW) boundary current along the Chukchi slopeHYCOM outputs showed a more common occurrence of weaker AW boundary currents when sea ice melted compared to that in the past [ABSTRACT FROM AUTHOR]

Analysis of dissolved oxygen (O2) in the Arctic's surface ocean provides insights into gas transfer between the atmosphere‐ice‐ocean system, water mass dynamics, and biogeochemical processes. In the Arctic Ocean's Canada Basin mixed layer, higher O2 concentrations are generally observed under sea ice compared to open water regions. Annual cycles of O2 and O2 saturation, increasing from summer through spring and then sharply declining to late summer, are tightly linked to sea ice cover. The primary fluxes that influence seasonal variability of O2 are modeled and compared to Ice‐Tethered Profiler O2 observations to understand the relative role of each flux in the annual cycle. Findings suggest that sea ice melt/growth dominates seasonal variations in mixed layer O2, with minor contributions from vertical entrainment and atmospheric exchange. While the influence of biological activity on O2 variability cannot be directly assessed, indirect evidence suggests relatively minor contributions, although with significant uncertainty. Past studies show that O2 molecules are expelled from sea ice during brine rejection; sea ice cover can then inhibit air‐sea gas exchange resulting in winter mixed layers that are super‐saturated. Decreasing mixed layer O2 concentrations and saturation levels are observed during winter months between 2007 and 2019 in the Canada Basin. Only a minor portion of the decreasing trend in wintertime O2 can be attributed to decreased solubility. This suggests the O2 decline may be linked to more efficient air‐sea exchange associated with increased open water areas in the winter sea ice pack that are not necessarily detectable via satellite observations. Plain Language Summary: Dissolved oxygen (O2) is a valuable ocean property that allows us to better understand the exchange of gases between the different ocean layers, sea ice, and atmosphere, and the physical and biological processes that control its variability. Understanding how and why O2 concentrations in the Arctic Ocean mixed layer vary spatially and seasonally is crucial for interpreting its evolution over timescales of years to decades that are influenced by global warming. We use physical and thermodynamical relationships to model the main factors that influence O2 concentrations in the mixed layer of the Arctic Ocean's Canada Basin, which we compare to observations made by Ice‐Tethered Profilers. Model results indicate that seasonal variations in O2 concentrations are dominated by the effects of sea ice growth and melt. Other processes that modulate mixed layer O2, including air‐sea exchange and ocean mixing, have a lesser influence. Between 2007 and 2019, mixed‐layer O2 has decreased in winter months, which we attribute to more openings in the sea‐ice pack during wintertime in the Canada Basin. Key Points: Spatial and seasonal distributions of O2 concentrations in the Canada Basin mixed layer are linked to the seasonal evolution of sea iceModeled fluxes suggest brine rejection and meltwater dilution during sea ice melt/formation dominate seasonal variability of mixed layer O2Decreases in mixed‐layer O2 during winter (over 2007–2019) suggest outgassing, likely driven by changes in the wintertime sea ice pack [ABSTRACT FROM AUTHOR]

Diffusive‐convection (DC) staircases, characterized by interleaving well‐mixed layers and high‐gradient interfaces of thermohaline properties, are commonly observed in the Canada Basin, Arctic. Previous studies suggested a quasi‐steady state of the thermohaline structures of these staircases. In this work, we employ mooring and Ice‐Tethered Profiler (ITP) data to show emergent merger events of staircases mediated by subsurface eddies with warm cores located at 300–500 m depths in 2005, 2007 and 2009. Two types of merger events, the B‐merger (resulting from strengthening of stronger interfaces at the expense of weaker ones) and the H‐merger (produced by vertical drift and collision of staircase interfaces), are documented during the passage of subsurface eddies. The merging staircases are located at 320–450 m depths, with layers of up to 70 m thick and high‐gradient interfaces thinner than 5 m. Mooring measurements reveal the emergence of staircase mergers associated with a background Richardson number dropping from a typical magnitude of O(100) $\mathcal{O}(100)$–O(10) $\mathcal{O}(10)$ to O(0.1) $\mathcal{O}(0.1)$, indicating the persistence of DC staircases in the presence of strong shears. In parallel, ITP data suggest the formation of B‐mergers (H‐mergers) to be favored within the eddy core (flank). The vertical heat fluxes associated with the B‐merger (H‐merger) staircases are estimated to be ∼3 W/m2 (∼1.5 W/m2), which substantially exceed the typical staircase‐driven heat fluxes of O(0.1) $\mathcal{O}(0.1)$ W/m2. These observations imply the significance of subsurface eddies in shaping the halocline structures and thus the vertical heat fluxes in the Arctic Ocean, particularly in a warming climate. Plain Language Summary: In the Arctic Ocean, warm waters derived from the Atlantic Ocean lie beneath surface cold waters, and store sufficient heat capable of melting the Arctic ice cover. Residing between the surface cold and subsurface warm waters are stacked layers, commonly referred to as staircases. These staircases regulate the rate at which heat from underneath may reach the surface and melt the sea ice. This study employs field measurements in the Arctic Ocean to show that staircases can be substantially re‐structured to enhance the upward heat transfer if turbulent spinning bodies of waters named eddies are present at 300–500 m depths. Further calculations show that the existence of up to three such eddies may elevate the overall heat transfer of the western Arctic by up to ∼10%. In‐depth analyses on these processes are thus warranted, particularly in a warming climate that can increasingly "heat up" the subsurface Arctic Ocean. Key Points: Diffusive‐convection staircases are observed to persist and merge under strong vertical shearsBoth B‐mergers and H‐mergers are observed in individual subsurface eddiesVertical heat fluxes across merged staircases in subsurface eddies grow by one order of magnitude [ABSTRACT FROM AUTHOR]

Anthropogenic radionuclides 129I and 236U are used to investigate pathways of the Atlantic Water flow in the Canada Basin, estimate transport timescales, and investigate mixing dynamics within the Atlantic Water layer and the overlying Pacific Water. Transit Time Distribution (TTD) model mean ages indicate water takes 25–35 years to reach the Canada Basin from the entrance of the Arctic, with limited lateral and vertical mixing along the core of the Arctic Ocean Boundary Current. Mode ages obtained from the model yield shorter transport times of 20–32 years. These age estimates agree with previous studies using these radionuclides and ventilation tracers in this region, indicating a steady‐state flow of Atlantic Water for the last 15 years. The distribution of the isotopes in the Atlantic layer indicates two pathways Atlantic Water may take into the basin, supported by the distribution of ages in the TTD model. End‐member mixing models indicate that the Pacific Winter water acquires a 20%–40% Atlantic Water signal of the radionuclides, upwelled over short periods, most likely along the shelf and Barrow Canyon region. Plain Language Summary: Man‐made nuclear products are used to trace the path of Atlantic water from the Atlantic Ocean to the Canada Basin in the Arctic, determine its transit time, and how it interacts with itself and Pacific Water that enters through the Bering Strait. The model suggests water takes 25–35 years to flow from the entrance of the Arctic to the Canada Basin, while a second estimate of age suggests slightly shorter transit times. The results of this study show that Atlantic Water flow has not changed significantly in the last 15 years. The distribution of the isotopes and ages also indicates two possible pathways water may take to the basin. An additional finding is that Pacific Winter Water carries a tracer concentration of 20%–40% of Atlantic Water, probably from Atlantic Water upwelling along the shelf, and in the Barrow Canyon region. Key Points: 129I and 236U trace Atlantic sourced water in the Canada BasinUpwelling of tracer labeled Atlantic Water into the Pacific Winter Water distinguishes the layer from Pacific Summer WaterTransit time distributions yield ages of 25–35 years for the core Atlantic Water [ABSTRACT FROM AUTHOR]

Arctic rivers carry not only large amounts of freshwater but also biogeochemical materials into the ocean and play important roles in Arctic ocean acidification (OA). This study quantitatively evaluated the effects of riverine biogeochemical fluxes (R‐BGC; carbon and nutrients) on the Arctic marine carbonate system and OA using multi‐decadal experiments (1979–2018) with a pan‐Arctic sea ice–ocean model. Improved initial and lateral boundary conditions of carbonate properties, observation‐based riverine biogeochemical data, and land model‐based interannually varying riverine freshwater discharge were adopted to enable more realistic experiments. The model simulated negative trends in aragonite saturation state (Ω) and pH in most regions of Arctic Ocean regardless of R‐BGC. The increased riverine freshwater promoted more OA through the higher dilution effect. Compared to the experiment with riverine discharge of only freshwater, the inclusion of R‐BGC caused positive anomalies in Ω and pH (by ∼0.14 and ∼0.03 in central basins, and by ∼0.15 and ∼0.06 in shelf seas, respectively). In the central basins, these anomalies were caused mostly by carbon (total alkalinity and dissolved inorganic carbon) of the R‐BGC. In the shelf seas, nutrient (nitrate and silicate) fluxes also contributed ∼14% and ∼32% of the anomalies owing to the enhanced primary production and a corresponding reduction in seawater pCO2. R‐BGC mitigated OA (ΔΩ = −1.53 × 10−3 year−1 and ΔpH = −0.56 × 10−3 year−1) in regions where riverine freshwater was accumulated (i.e., the Canada Basin, Chukchi Cap, Eurasian Basin, and East Siberian Sea). This study stressed the importance of including R‐BGC for OA model projection. Plain Language Summary: Ocean acidification (OA) is caused primarily by atmospheric CO2 absorption at the sea surface. In the Arctic Ocean, riverine biogeochemical inputs (R‐BGC; carbon and nutrients carried by riverine water), in addition to freshwater discharge, affect OA. This study quantified the effects of R‐BGC on OA using a couple of model simulations for 1979–2018. Advancing from previous studies, this study conducted more realistic experiments by using improved initial and lateral boundary conditions of carbonate properties, observation‐based riverine biogeochemical data, and the land model‐based interannually varying riverine freshwater discharge data. R‐BGC impacts on OA were found mainly in the surface waters of the Arctic, especially in coastal regions close to the river mouths. During 1989–2018, negative trends in carbonate saturation state and pH (both indicating OA) were simulated regardless of R‐BGC in most regions of Arctic Ocean. Compared to a simulation with riverine input of only freshwater, R‐BGC mitigated the simulated OA in the regions with increased riverine freshwater content. This study stressed the importance of R‐BGC for model‐based OA estimation and future projections. It will improve the understanding of OA under climate change and aid in informing marine ecosystem‐based management. Key Points: Simulations with only riverine freshwater overestimate Arctic ocean acidification and negative trends in aragonite saturation state and pHAddition of riverine biogeochemical fluxes, especially total alkalinity and dissolved inorganic carbon, can reduce this overestimationRiverine nutrient fluxes can also reduce ∼14% and ∼32% of the overestimation in aragonite saturation state and pH in shelf seas [ABSTRACT FROM AUTHOR]

To quantitatively assess seasonal variations in the partial pressure of carbon dioxide (pCO2) in the Pacific sector of the Arctic Ocean (Canada Basin and Chukchi Sea) in 2021, water temperature, salinity, chlorophyll‐a, pCO2, dissolved inorganic carbon (DIC), total alkalinity (TA), and nutrients were measured. During summer 2021, surface water pCO2 in our study area (315 ± 41 μatm) was undersaturated with respect to the atmosphere (404 ± 4 μatm), and the ocean was a sink for atmospheric CO2 (−10.5 ± 11.0 mmol C m−2 day−1). Using DIC, TA, and nutrients in the temperature minimum layer, we estimated the under‐ice pCO2 in the water during the previous winter and calculated changes in pCO2 (δpCO2) due to temperature changes, freshwater inflow, biological activity, and other factors (gas exchange and advection) from winter to summer. In the Chukchi Sea, biological activity and temperature changes had significant impacts on pCO2, whereas in the Canada Basin, the influx of freshwater caused a significant decrease in pCO2. Our results suggested that different types of freshwaters had different effects on pCO2, with sea ice meltwater having a greater effect on reducing pCO2 than river water or snowmelt water. We therefore emphasize the importance of freshwater type and proportion, as well as freshwater supply, for prediction of future pCO2 changes. Plain Language Summary: Assessment of the factors that cause ocean CO2 to vary is important in the Arctic Ocean, which acts as an atmospheric CO2 sink that will change significantly because of global warming. Seawater and sea ice samples were collected in the Pacific sector of the Arctic Ocean to investigate the carbonate chemistry. The partial pressure of carbon dioxide (pCO2) was lower on the sea surface than in the atmosphere during summer, and the ocean absorbed atmospheric CO2. The decreases of pCO2 from winter to summer were driven primarily by biological activity in the Chukchi Sea (partially offset by temperature increases) and by freshwater additions in the Canada Basin. Sea ice meltwater, snow meltwater, and river water had different effects on pCO2, even though all of them diluted the carbonate concentrations of surface water, because of differences in the alkalinity and carbonate chemistry among these freshwater sources. Key Points: The Pacific sector of the Arctic Ocean absorbed atmospheric CO2 in summerIn the Chukchi Sea, biological activity and temperature changed pCO2, while in the Canada Basin, the freshwater caused a decrease in pCO2Sea ice meltwater had a greater effect on reducing pCO2 than river water or snow meltwater [ABSTRACT FROM AUTHOR]

The fluorescence characteristics of dissolved organic matter (DOM) were measured to determine the distribution and drivers of DOM in the dark Canada Basin waters. We studied the relationship between fluorescent DOM (FDOM) and apparent oxygen utilization (AOU) in the whole water column as well as in the main water masses (Pacific Winter Water, Atlantic halocline (AH), Atlantic water—Fram Strait and Barents Sea branches, Deep Temperature Minimum, and Canada Basin deep water). The relative water mass fractions of different water masses were estimated using a five end‐members mixing model. The distribution of water mass fractions was found to be in good agreement with the distribution based on the traditional temperature‐salinity diagram. The fractions of AH waters were linked with AOU, highlighting the significant role played by mineralization in shaping AH humic‐like levels. The slope of the relationship between in situ FDOM—AOU was greater than those reported in the other oceans worldwide. This suggests that the AH is a hot spot for microbial mineralization of humic‐like DOM. On the other hand, the relationships with the humic‐like intensities differed among the four humic‐like components in the >125 m waters, indicating distinct environmental dynamics and biogeochemical roles for each humic‐like component. Plain Language Summary: Fluorescent marine dissolved organic matter in the dark ocean is primarily produced by the microbial decomposition of organic material. In this study, we examine the relationship between fluorescent dissolved organic matter (FDOM) and remineralization‐related variables such as apparent oxygen utilization in the main six water masses in the dark Canada Basin waters. Our study reveals a strong influence of in situ microbial processes on the humic‐like FDOM levels measured by the in situ sensor in the Atlantic halocline waters. We identified four more humic‐like and protein‐like FDOM components in samples collected by the CTD‐rosette that behaved differently. To better understand their sensitivity to in situ mineralization in the dark ocean, future studies should concentrate on in situ monitoring of the blue‐shifted humic‐like and protein‐like FDOM. Key Points: The high in situ fluorescent dissolved organic matter (FDOM) levels were linked to the high Pacific winter water mass fractionsThe slopes of the apparent oxygen utilization–FDOM correlations varied for the four humic‐like componentsThe Atlantic halocline serves as a hot spot for microbial formation of humic‐like FDOM [ABSTRACT FROM AUTHOR]

The internal wave climate in the deep Arctic Ocean, away from the shelves, is quiet because the ice cover shields the ocean from wind energy input, and tidal amplitudes are small. Hence, mixing due to internal wave breaking is small. The shrinking Arctic sea ice cover, however, exposes more open ocean areas to energy transfer by wind. Consequently, more energetic near‐inertial internal waves (NIWs) may carry energy to the bottom, potentially enhancing deep mixing. In the deep Canadian Basin, weakly stratified layers with local buoyancy frequencies smaller than the wave frequency may prevent NIW propagation to the seafloor. We estimate the distribution of these near‐inertial turning depths from temperature and salinity data of the years 2005–2014. Near‐inertial turning depths are ubiquitous in the deep Canadian Basin at ∼2,750 m depth, between 100 and 1,200 m above the bottom. A deep homogeneous layer below 3,300 m is characterized by small squared buoyancy frequencies N2 ∼ 0 with locally unstable layers (N2 < 0). The turning depths reflect NIWs and hence limit their contribution to deep mixing, but the waves create an evanescent perturbation with exponentially decreasing amplitude that can interact with the bathymetry, especially above slopes and ridges where the height of the turning depths above the seafloor is small. After reflection, the main part of the wave energy is trapped between turning depths and the surface, so that a potential increase of wave energy input mainly affects mixing of mid‐depth water masses like the Atlantic Water. Plain Language Summary: Over the last couple of years, Arctic Ocean summer sea ice has decreased. The larger ice‐free regions imply more open areas for wind to act on the ocean surface. As a result, energy from the atmosphere goes into the ocean, triggering more energetic waves in the ocean interior. These internal waves carry energy over longer distances and mix the waters where they break. To better understand this process, we studied temperature and salinity data from 2005 to 2014. Our findings show widespread areas in the deep Canadian Basin, at around 2,750 m depth, where the further propagation of the waves is constrained by weak stratification. These depths are called near‐inertial turning depths, and they shorten the path of near‐inertial waves and isolate the bottom from waves and mixing. In addition, we found weakly stratified and unstable layers below 3,300 m. Our results shed light on the link of surface‐generate waves to the interior mixing rates in the Arctic Ocean. Key Points: In the deep Canadian Basin, there are near‐inertial turning depths between 100 and 1,200 m above the seafloorBelow the turning depths, the deep layer is quasi‐homogeneous and locally unstableNear‐inertial turning depths inhibit internal gravity wave propagation and hence reduce near bottom mixing [ABSTRACT FROM AUTHOR]

The marine environment of the Arctic Ocean has changed rapidly in recent decades. We used reanalysis data and observational data to explore the variations in the upper ocean heat content (OHC) of the Canadian Basin (CB) and the variations in the temperature profiles of the Southern Canadian Basin (SCB). Both the reanalysis data and observational data show increasing trends for the OHC of the CB from 1993 to 2023. Compared to the World Ocean Atlas data (WOA 18/23), the reanalysis data (ORAS5 or GLORYS12V1) significantly underestimated the values of the upper OHC of the Canadian Basin. To explain the OHC differences, the Ice-Tethered Profiler (ITP) observational data were used to analyze the variations in the vertical temperature profiles. We found that the reanalysis data remarkably underestimated the maximum temperatures of the subsurface Pacific warm water and its increasing trend. Based on the short-term prediction results from the Bi-LSTM neural network, we forecasted that the upper OHC will continue to increase in the SCB, mainly due to the warming of the intermediate Atlantic warm water. The research results provide a valuable reference for assessing and improving climate-coupled models. [ABSTRACT FROM AUTHOR]

The Upper Jurassic-Lower Cretaceous sedimentary rocks in the Eastern Barents Sea Basin are up to 2 km thick and represent one of the least studied Arctic intervals. Here, for the first time, we present a detailed analysis of 43,000 km of 2D seismic profiles, as well as well-log and core data from 24 offshore wells with the aim to create a comprehensive sequence stratigraphic framework that can be integrated with the rest of the basin. Results show that (1) seven third-order sequences and five types of clinoforms can be identified based on integrated seismic and well data. The age of each sequence was established based on published biostratigraphic investigations along with new dinocyst interpretations included in this study; (2) the deep marine basin was gradually filled with sediments coming from north, east and south as a response to HALIP, Canada Basin opening and Cimmerian uplift of Novaya Zemlya, and was preserved only in the southwestern part of the Barents Sea Basin at the end of Early Cretaceous and (3) both Eastern Barents Sea and West Siberia Basin share similarities in sedimentary environments and tectonic setting, though the spatial distribution of clastic reservoirs in Upper Jurassic and Lower Cretaceous mega-sequence heavily depends on the source areas that require more provenance focused research. The results presented here can be used in further regional exploration in the area and to better understand the geodynamic evolution of the Greater Barents Sea Basin. [ABSTRACT FROM AUTHOR]

Vertical profiles of temperature microstructure at 95 stations were obtained over the Beaufort shelf and shelfbreak in the southern Canada Basin during a November 2018 research cruise. Two methods for estimating the dissipation rates of temperature variance and turbulent kinetic energy were compared using this data set. Both methods require fitting a theoretical spectrum to observed temperature gradient spectra, but differ in their assumptions. The two methods agree for calculations of the dissipation rate of temperature variance, but not for that of turbulent kinetic energy. After applying a rigorous data rejection framework, estimates of turbulent diffusivity and heat flux are made across different depth ranges. The turbulent diffusivity of temperature is typically enhanced by about one order of magnitude in profiles on the shelf compared to near the shelfbreak, and similarly near the shelfbreak compared to profiles with bottom depth >1,000 m. Depth bin means are shown to vary depending on the averaging method (geometric means tend to be smaller than arithmetic means and maximum likelihood estimates). The statistical distributions of heat flux within the surface, cold halocline, and Atlantic water layer change with depth. Heat fluxes are typically <1 Wm−2, but are greater than 50 Wm−2 in ∼8% of the overall data. These largest fluxes are located almost exclusively within the surface layer, where temperature gradients can be large. Plain Language Summary: In the Arctic Ocean, the mixing of water masses due to turbulence has important impacts on heat transport, influencing sea ice formation and loss. In this study, we quantify mixing using vertical profiles of temperature measured at high spatial resolution that were obtained during a November 2018 research cruise near the shelf and shelfbreak of the Canada Basin. We compare two methods for performing this estimation, and evaluate scenarios when either method might fail. Turbulent mixing rates are found to be higher over the shelf compared to the shelfbreak, and higher over the shelfbreak than the deep ocean, possibly due to interactions between currents and bottom topography. We also quantify rates of heat transport through three distinct water masses (the surface layer, a cold subsurface layer, and a warm water mass originating from the Atlantic Ocean). These findings are valuable for constraining Arctic Ocean heat budgets, as well as for establishing best practices when estimating turbulent mixing from high resolution temperature profiles. Key Points: Average turbulent temperature diffusivity is elevated by 1–2 orders of magnitude on the shelf compared to over the deep slopeA similar magnitude (O $\mathcal{O}$(1 Wm−2)) of heat is fluxed into the cold halocline from the Atlantic Water below as from the overlying surface layerHeat fluxes as high as 50 Wm−2 are occasionally observed in the surface layer [ABSTRACT FROM AUTHOR]

The strong CO2 sink in Arctic Ocean plays a significant role in the global carbon budget. As a high-latitude oceanic ecosystem, the features of sea surface pCO2 and air-sea CO2 flux are significantly influenced by sea ice melt; however, our understanding of pCO2 evolution during sea ice melt remains limited. In this study, we investigate the dynamics of pCO2 during the progression of sea ice melt in the western Arctic Ocean based on data from two cruises conducted in 2010 and 2012. Our findings reveal substantial spatiotemporal variability in surface pCO2 on the Chukchi Sea shelf and Canada Basin, with a boundary along the shelf breaks at depths of 250-500 m isobaths. On the Chukchi Sea shelf, strong biological consumption dominates pCO2 variability. Moreover, in Canada Basin, the pCO2 dynamics are modulated by various processes. During the active sea ice melt stage before sea ice concentration decreases to 15%, biological production through photosynthetic processes and dilution of icemeltwater lead to a reduction in DIC concentration and subsequent decline in pCO2. Further, these effects are counteracted by the air-sea CO2 exchange at the sea surface which tends to increase seawater DIC and subsequently elevate surface pCO2. Compared to the pCO2 reduction resulting from biological production and dilution effects, the contribution of air-sea CO2 exchange is significantly lower. The combined effects of these factors have a significant impact on reducing pCO2 during this stage. Conversely, during the post sea ice melt stage, an increase in pCO2 resulting from high temperatures and air-sea CO2 exchange outweighs its decrease caused by biological production. Their combined effects result in a prevailing increase in sea surface pCO2. We argue that enhanced air-sea CO2 uptake under high wind speeds also contributes to the high sea surface pCO2 observed in 2012, during both active sea ice melt stage and post sea ice melt stage. The present study reports, for the first time, the carbonate dynamics and pCO2 controlling processes during the active sea ice melt stage. These findings have implications for accurate estimation of air-sea CO2 fluxes and improved modeling simulations within the Arctic Ocean. [ABSTRACT FROM AUTHOR]

The aim of this study is to better understand diffusive convection (DC) and its role in the upper ocean dynamic environment and sea ice melting in the Canada Basin. Based on a moored dataset with 6737 profiles collected from August 2003 to August 2011 in the upper layer of the Canada Basin, DC between the warm and salty Atlantic Water (AW) and the colder and less salty Lower Halocline Water (LHW) were investigated. The moorings were designated at four stations: A, B, C, and D, located at the southwestern, southeastern, northeastern, and northwestern parts of the basin, respectively. During the observation period, the temperature, salinity, and depth of the AW and LHW exhibited unique temporal variations. The temperature and salinity of the AW varied among stations, with a decreasing trend from northwest to southeast, consistent with the propagation path of the AW in the Canada Basin. The temperature and salinity of the LHW were similar at all stations. The AW and LHW cores were located between depths of 320–500 m and 160–300 m, respectively, and both gradually deepened over time. Distinct DC staircase structures were observed between the AW and LHW, more pronounced at stations C and D than at stations A and B, which is speculated to be related to eddies at stations A and B during the observation period. The vertical heat fluxes through the DC staircase layer at stations C and D (FHc_C and FHc_D) were estimated using an empirical formula. FHc_C ranged from 0.05 to 0.94 W/m2, and FHc_D ranged from 0.05 to 0.6 W/m2, with the maximum probability value for both at approximately 0.2 W/m2. The effective diffusivities at these two stations (KT_C and KT_D) are similar, ranging from 2 × 10−6 to 3 × 10−5 m2/s, with the highest probability occurring at 6 × 10−6 m2/s. Both the probability density function of the heat flux and the effective diffusivity skewed towards larger values and obey a lognormal distribution, indicating turbulence intermittency of the DC staircase in the Canada Basin. These finding offers new insights into the heat transport and turbulence in the DC staircase, and then bring a deeper understanding of sea ice melting in the Canada Basin. [ABSTRACT FROM AUTHOR]

Obesity and type 2 diabetes mellitus (T2DM) is an alarming problem globally and a growing epidemic. Metabolic surgery has been shown to be successful in treating both obesity and T2DM, usually after other treatments have failed. This study aims to compare Roux-Y gastric bypass and sleeve gastrectomy in determining early diabetic outcomes in obese Malaysian patients with T2DM following surgery. A total of 172 obese patients with T2DM who were assigned to either laparoscopic Roux-en-Y gastric bypass (LRYGB) or laparoscopic sleeve gastrectomy (LSG) were analysed up to a year post-procedure. The patients' T2DM severity were stratified using the Individualized Metabolic Surgery (IMS) score into mild, moderate and severe. Remission rates of diabetes were compared between surgical techniques and within diabetic severity categories. T2DM remission for patients who underwent either surgical technique for mild, moderate or severe disease was 92.9%, 56.2% and 14.7% respectively. Both surgical techniques improved T2DM control for patients in the study. Comparing baseline with results 1 year postoperatively, median HbA1c reduced from 7.40% (IQR 2.60) to 5.80% (IQR 0.80) (p < 0.001), mean total antidiabetic medications use reduced from 1.48 (SD 0.99) to 0.60 (SD 0.86) [p < 0.001], insulin usage reduced from 27.9 to 10.5% (p < 0.001), and T2DM control improved from 27.9 to 82% (p < 0.001). The patients had a median excess BMI loss of 69.4% (IQR 34%) and 53.2% (IQR 36.0%) for RYGB and SG respectively (p = 0.016). At one year following surgery, there is no difference between LRYGB and LSG in terms of diabetic remission. LSG is not inferior to LRYGB in terms of early diabetic outcomes. Milder T2DM shows a better response. LSG is a simpler procedure with a lower risk profile and should be considered as an early treatment option for obese patients with T2DM. [ABSTRACT FROM AUTHOR]

Against the background of wind-forcing change along with Arctic sea ice retreat, the mesoscale processes undergoing distinct variation in the Beaufort Gyre (BG) region are increasingly important to oceanic transport and energy cascades, and these changes subsequently put oceanic stratification into a new state. Here, the varying number and strength of eddies in the central Canada Basin (CB) and Chukchi–Beaufort continental slope are obtained based on mooring observations (2003–2018), altimetry measurements (1993–2019), and reanalysis data (1980–2020). In this paper, the variability in the BG halocline, representing the adjustment of stratification in the upper layer, is shown in order to analyse how variability occurs under changing mesoscale processes. We find that over almost the last 2 decades the halocline depth has deepened by ∼ 40 m in the south of the central gyre, while that in the north has deepened by ∼ 70 m according to multiple datasets. Surrounding the central gyre, the asymmetry of the halocline, with much steeper and deeper isopycnals over the southern continental slope, reduced after 2014. In the meantime, eddy activities in the upper layer from the southern margin of the BG to the abyssal plain have been enhanced. Moreover, the convergence of the eddy lateral flux has increased as the halocline structures on either side, which is at least 120 km from the central gyre, have reached a nearly identical and stable regime. It has been clarified that long-term dynamic eddy modulation through eddy fluxes, facilitating the freshwater redistribution, affects the meridional asymmetry of the BG halocline. Our results provide a better understanding of the eddy modulation processes and their influence on the halocline structure. [ABSTRACT FROM AUTHOR]

The subsurface layer of the Canada Basin contains large reservoirs of dissolved inorganic nitrogen and carbon (DIN and DIC, respectively). Under rapid change of Arctic climate system, these reservoirs may not only serve as potential material bases for primary production, but also for carbon source and sink. However, the long‐term trends in reservoirs and the interaction between reservoirs and the upper ocean are not fully understood. This study used two data sets to evaluate the long‐term trends in the DIN and DIC reservoirs in the subsurface layer of the Canada Basin during 1990–2015. The results indicate an increase of ∼35% in the subsurface DIN reservoir over 25 years, whereas the DIN concentration remained fairly constant. The discrepancy between the DIN reservoir and DIN concentration was primarily owing to the expansion of the subsurface layer, which increased the DIN stock but maintained a relatively stable DIN concentration. Additionally, the ΔDIC/ΔDIN ratio in the subsurface layer increased by ∼33% over 25 years, primarily due to denitrification on the pathway of Pacific inflow, such as Chukchi shelf. In the Canada Basin, the enhanced mixing between the subsurface and surface layers may promote primary production and render the subsurface layer a carbon source. Consequently, when studying the biogeochemical cycles in the changing Arctic Ocean, the long‐term interactions between the subsurface and surface layers in the Canada Basin should be further considered. Plain Language Summary: Nutrients in the subsurface layer of the Canada Basin are potential material bases for primary production. Rapid climate change and enhanced mixing between the subsurface and surface layers impact the biogeochemical cycles in the Canada Basin. Two data sets were collected to determine the dissolved inorganic nitrogen and carbon (DIN and DIC, respectively) concentrations, DIN reservoir, and ΔDIC/ΔDIN ratio in the subsurface layer from 1990 to 2015. The results revealed that the subsurface DIN concentration remained constant, whereas the DIN and DIC reservoirs and ΔDIC/ΔDIN ratio dramatically increased. The increased Pacific inflow and expansion of subsurface layer may have led to an increase in the subsurface DIN and DIC stocks in the Canada Basin. Simultaneously, denitrification on the Chukchi shelf may have slowed the increase in the DIN reservoir, leading to an increase in the ΔDIC/ΔDIN ratio. Increased regional events such as upwellings, eddies, and storms can enhance the mixing of the upper layers (subsurface and surface layers) and thus may promote primary production. However, the increased ΔDIC/ΔDIN ratio implies that the mixing of the upper layers also supplies DIC upwardly, which might negatively impact the air‐sea CO2 flux in the Canada Basin and reduce carbon uptake from the atmosphere. Key Points: The subsurface dissolved inorganic nitrogen (DIN) reservoir in the Canada Basin increased by ∼35% during 1990–2015The subsurface DIN concentration in the Canada Basin remained fairly constant during 1990–2015The ratio of subsurface delta dissolved inorganic carbon (DIC) to delta DIN (ΔDIC/ΔDIN) increased by ∼33% during 1990–2015 [ABSTRACT FROM AUTHOR]

Particulate Mn, given its high adsorptive capacity and oxidation potential, has profound impacts on the cycling of various trace elements and organic matter in the ocean. Moreover, particulate Mn acts as a sink (via oxidation and adsorption) or as a source (via remineralization and photoreduction) term of bioactive dissolved Mn(II). In the Canadian Arctic Ocean, particulate Mn distributions in the water column revealed the presence of distinctively high particulate Mn concentrations and an overwhelming dominance of the non‐lithogenic component to the bulk particulate Mn pool. This phenomenon is of particular importance in halocline waters in the Canada Basin, the Canadian Arctic Archipelago and Baffin Bay, and near‐bottom samples in Baffin Bay. Enhanced microbially‐mediated Mn oxidation in the water column is suggested as the main mechanism driving the non‐lithogenic dominance. Indeed, the microbial community composition data associated with high non‐lithogenic particulate Mn (i.e., Mn oxides) display a high relative abundance of taxa (e.g., f.Pirellulaceae, o.Phycisphaerales, f.Cryomorphaceae, g. Moritella) that have been identified in Mn oxide enriched environments. Furthermore, numerous taxa identified in the Canada Basin halocline water, where non‐lithogenic particulate Mn peaked, are phylogenetically related to known (cultured) Mn‐oxidizing bacteria (MnOB; e.g., Rhodobacteraceae, Oceanospirillaceae, Rhizobiaceae, and other Alphaproteobacteria). Putative MnOB appears to proliferate in certain water masses having a unique set of environmental conditions: low light intensity—alleviating photoinhibition—and high dissolved Mn concentrations, the main drivers known to influence MnOB dynamic, and hence, Mn oxidation. [ABSTRACT FROM AUTHOR]

The Arctic Ocean halocline separates the cold surface mixed layer from the underlying warm Atlantic Water (AW), and thus provides a precondition for sea ice formation. Here, we introduce a new method in which the halocline base depth is determined from vertical stability and compare it to two existing methods. We also propose a novel method for detecting the cold halostad, a layer characterized by a small vertical salinity gradient, which is formed by the Pacific Winter Water in the Canada Basin or by meltwater off the eastern coast of Greenland and off Svalbard. Our main motivation for determining the halocline base depth depending on vertical stability was that vertical stability is closely related to vertical mixing and heat exchange. Vertical stability is a crucial parameter for determining whether the halocline can prevent vertical heat exchange and protect sea ice from warm AW. When applied to measurements from ice-tethered profilers, ships, and moorings, the new method for estimating the halocline base depth provides robust results with few artifacts. Analyzing a case in which water previously homogenized by winter convection was capped by fresh water at the surface suggests that the new method captured the beginning of new halocline formation in the Eurasian Basin. Comparatively large differences between the methods for detecting the halocline base depth were found in warm AW inflow regions for which climate models predict halocline thinning and increased net surface energy fluxes from the ocean to the atmosphere. [ABSTRACT FROM AUTHOR]

A tectonic scheme of the Amerasia Basin was created for the first time on a 1 : 2 500 000 scale on the basis of integrated analysis of geological and geophysical data. The structural framework of the scheme was developed according to the mapping of the basement surface using reflection seismic sections; the surface was detailed on the basis of potential fields. The results of three-dimensional gravity modeling according to calculation of the Moho depths and consolidated earth crust thickness were used as the basic indicators of the origin of the crust. Tectonic elements of different ranks and kinematics were identified on the seismic sections and traced according to the potential fields. The created scheme is the most detailed out of those published previously. It demonstrates the features of the tectonic structure of individual geostructures and their nature and allows us to draw conclusions about the evolution of the Amerasia Basin. [ABSTRACT FROM AUTHOR]

The interannual variability in fluorescent dissolved organic matter (FDOM) was assessed in the Pacific‐ and Atlantic‐derived halocline, and Atlantic waters (AWs) of the Canada Basin during late summer/early fall expeditions in 2007–2017. Fluorescence spectroscopy excitation‐emission matrices coupled with parallel factor analysis were used to validate a seven‐component model. During the 11 years, humic‐like intensities increased by 0.0002–0.0006 r.u. (Raman unit) (p < 0.05) with greater annual rates of change in shallower waters (i.e., Pacific summer water) than in deeper waters (i.e., AWs). No significant temporal trends were observed for protein‐like intensities in any water layer (p > 0.05) due to the labile nature of these components. The increases in humic‐like fluorescence intensities were likely the results of changes in Bering Strait inflow and its modification over the Chukchi Shelf and/or the accumulation of freshwater under anticyclonic wind forcing in the Beaufort gyre. This 11‐year late summer/early fall survey shows the first evidence of changes in FDOM composition in the halocline of the Arctic Ocean basin. Plain Language Summary: The highly stratified Arctic Ocean receives a disproportionate amount of freshwater and plays a large role in the regulation of the global climate. Over the last decade, the effects of sea ice decline, hydrological cycle intensification, and vertical stratification have strengthened. Dissolved organic matter (DOM) is a heterogeneous mixture of non‐living organic compounds and is central to the marine carbon cycle. This study, which is part of the Joint Ocean Ice Study/Arctic Observing Network—Beaufort Gyre Observing System program, aims to assess the level and composition of fluorescent dissolved organic matter (FDOM) in water samples collected in the dark Canada Basin waters of the Arctic Ocean in late summer/early autumn between 2007 and 2017. The results of the present study indicate that microbial activity constitutes a significant source of humic‐like FDOM in the Atlantic—derived halocline waters. During the 11 years, the humic‐like component levels increased in both the Pacific and Atlantic‐origin halocline waters, which were likely the results of changes in Bering Strait inflow, heat transfer over the Chukchi Sea shelves, and/or the accumulation of FDOM‐rich freshwater under clockwise wind forcing in the Beaufort gyre. Key Points: Statistically significant temporal increases were observed in humic‐like fluorescence intensities in the halocline watersIncreases in halocline humic‐like reflect changes in Pacific‐origin flow, organic input, and, Chukchi shelf and advection processesEvidence of in situ microbial production of humic‐like fluorescence was found in the Atlantic‐origin halocline [ABSTRACT FROM AUTHOR]

The Arctic Ocean is undergoing dramatic changes in response to increasing atmospheric concentrations of greenhouse gases. The 2016–2017 Canada Basin Acoustic Propagation Experiment was conducted to assess the effects of the changes in the sea ice and ocean structure in the Beaufort Gyre on low-frequency underwater acoustic propagation and ambient sound. An ocean acoustic tomography array with a radius of 150 km that consisted of six acoustic transceivers and a long vertical receiving array measured the impulse responses of the ocean at a variety of ranges every four hours using broadband signals centered at about 250 Hz. The peak-to-peak low-frequency travel-time variability of the early, resolved ray arrivals that turn deep in the ocean was only a few tens of milliseconds, roughly an order of magnitude smaller than observed in previous tomographic experiments at similar ranges, reflecting the small spatial scale and relative sparseness of mesoscale eddies in the Canada Basin. The high-frequency travel-time fluctuations were approximately 2 ms root-mean-square, roughly comparable to the expected measurement uncertainty, reflecting the low internal-wave energy level. The travel-time spectra show increasing energy at lower frequencies and enhanced semidiurnal variability, presumably due to some combination of the semidiurnal tides and inertial variability. [ABSTRACT FROM AUTHOR]

The hydroacoustic environment of a rapidly warming Arctic Ocean will be impacted by interconnected changes in the physical environment and increased human activity. Previous acoustic calculations will need to be updated to reflect current and future conditions. Earth System Models are important tools for making projections of changes in a wide range of physical processes under future climates. We present a comparison of Arctic acoustic travel times based on output from the Department of Energy's Energy Exascale Earth System Model with measured travel times from the 2016–2017 Canada Basin Acoustic Propagation Experiment and with travel times predicted by empirical temperature and salinity observations. This comparison allows us to test the impact of changes in Arctic sound speed profiles on acoustic travel times and connects Arctic hydroacoustics with the changing Arctic environment as described by a climate model. Plain Language Summary: As the Arctic Ocean changes due to a warming climate and increased human activity, its acoustic environment will also change. Earth System Models that allow interactions between the ocean, sea ice, atmosphere, and other parts of the global water cycle are important tools for investigating conditions under future climates. Measuring acoustic travel time between two points can help constrain the physical properties that control propagation of sound in the ocean. We use the Department of Energy's Energy Exascale Earth System Model to estimate temperature and salinity at different depths in the Arctic Ocean and use these profiles to calculate the time taken for sound waves to travel between a source and a receiver. We compare these estimates to arrival times directly measured in the 2016–2017 Canada Basin Acoustic Propagation Experiment, and to travel times computed for temperature and salinity data measured by instruments in the Arctic. Through this comparison, we test the impact of changes in the Arctic environment on acoustic travel times and demonstrate how a climate model can describe acoustic properties in a changing Arctic Ocean. Key Points: We compare Arctic Ocean acoustic travel times based on an Earth System Model, oceanographic observations, and an acoustic experimentSound speeds in the upper 400 m of the western Arctic Ocean are rapidly changing due to warming and freshening of water layersThe Energy Exascale Earth System Model captures the layered structure of the Canada Basin but not its relative sound speed variations [ABSTRACT FROM AUTHOR]

The Arctic Ocean's Canada Basin (CB) has seen significant changes in ocean properties in the past two decades. A prominent change has been a warming of the Pacific Summer Water (PSW) layer in the central CB. The corresponding change in dissolved oxygen (O2) is analyzed here to provide additional insight into PSW physics and biology, pathways, and evolution. O2 observations are analyzed between 2003 and 2021 from the Joint Ocean Ice Study/Beaufort Gyre Observing System (JOIS/BGOS) field program, which samples CB hydrographic and biogeochemical properties. In the central CB, warming of the PSW layer over 2003–2021 has been accompanied by O2 decreases over this time in the layer. Nutrients and other biogeochemical properties are analyzed to quantify the combined influences of both physical changes and biological changes on the evolution of O2 concentrations in the CB PSW. In the upper portion of the PSW, O2 decreases can be entirely accounted for by surface warming (and corresponding decrease in O2 solubility) of its source waters in the Chukchi Sea region. In the deeper portion of the PSW layer, the observed O2 changes are larger, and are accounted for by a combination of the decreased solubility effect due to warming, and increased organic matter breakdown in warmer waters. Decreasing O2 in a warming Arctic Ocean is consonant with O2 trends in the warming global oceans, and highlights the need for continued observations and analyses. Plain Language Summary: Water properties in the Arctic Ocean's Canada Basin have changed significantly over the past two decades. The most prominent change has been a warming of a water layer sourced from the Pacific Ocean. It is shown here that this warming has been accompanied by a decrease in dissolved oxygen in the same water layer. The warming and oxygen decline are analyzed in context to gain insight into how and why the Arctic Ocean is changing. Ship‐based data from annual icebreaker expeditions, which include measurements of temperature, salinity, dissolved oxygen, and nutrients, are analyzed. The analysis suggests that physical and biological processes have different influences in the different parts of the water layer. In recent decades, widespread dissolved oxygen decreases have also been observed in the global oceans because of ocean warming. This study documents similar changes underway in the Arctic Ocean. Key Points: Recent Pacific Summer Water (PSW) warming in the Canada Basin is accompanied by dissolved oxygen decreases in the same water massDecreased oxygen solubility due to warming of source waters can account for dissolved oxygen decreases in the upper PSWLower PSW dissolved oxygen decreases are attributed to both physical (∼44%) and biological (∼56%) effects of the warming [ABSTRACT FROM AUTHOR]

Under the background of wind forcing change along with Arctic sea ice retreat, the mesoscale processes undergoing distinct variation in Beaufort Gyre (BG) region are more and more significant to oceanic transport and energic cascade, and then these changes put oceanic stratification into a new state. Here the varying eddies and eddy kinetic energy (EKE) in the central Canada Basin (CB) and Chukchi–Beaufort continental slope are obtained based on mooring observations (2003–2018), altimetry measurements (1993–2019) and reanalysis data (1980–2021). In this paper, the variability of halocline in BG representing adjustment of stratification in the upper layer is shown so as to analyze how it occurs under significantly changing mesoscale processes. We find that the halocline depth has deepened by ~40 m while that in the north has deepened by ~70 m in the in the last nearly two decades by multiple data sets. The halocline depth lifting to the north initially was shifted to a final nearly symmetric structure. Eddy strength and Eddy induced low salinity water transportations have been continuously increasing toward the central basin at the mean time the halocline depth and strength among the southern and northern parts in the basin have reached a nearly identical and stable regime. It is clearly clarified that the long-term dynamical eddy modulation through eddy fluxes facilitating the freshwater redistribution inhibited the meridional asymmetry of halocline of the BG. Further research into high-resolution observations and data simulations can helps us to better understand the eddy modulation processes and its influence on large-scale circulation. [ABSTRACT FROM AUTHOR]

Under the background of wind forcing change along with Arctic sea ice retreat, the mesoscale processes undergoing distinct variation in Beaufort Gyre (BG) region are more and more significant to oceanic transport and energic cascade, and then these changes put oceanic stratification into a new state. Here the varying eddies and eddy kinetic energy (EKE) in the central Canada Basin (CB) and Chukchi-Beaufort continental slope are obtained based on mooring observations (2003-2018), altimetry measurements (1993-2019) and reanalysis data (1980-2021). In this paper, the variability of halocline in BG representing adjustment of stratification in the upper layer is shown so as to analyze how it occurs under significantly changing mesoscale processes. We find that the halocline depth has deepened by ~40 m while that in the north has deepened by ~70 m in the in the last nearly two decades by multiple data sets. The halocline depth lifting to the north initially was shifted to a final nearly symmetric structure. Eddy strength and Eddy induced low salinity water transportations have been continuously increasing toward the central basin at the mean time the halocline depth and strength among the southern and northern parts in the basin have reached a nearly identical and stable regime. It is clearly clarified that the long-term dynamical eddy modulation through eddy fluxes facilitating the freshwater redistribution inhibited the meridional asymmetry of halocline of the BG. Further research into high-resolution observations and data simulations can helps us to better understand the eddy modulation processes and its influence on large-scale circulation. [ABSTRACT FROM AUTHOR]

The Chukchi Slope Current is a westward-flowing current along the Chukchi slope, which carries Pacific-origin water from the Chukchi shelf into the Canada Basin and helps set the regional hydrographic structure and ecosystem. Using a set of experiments with an idealized primitive equation numerical model, we investigate the energetics of the slope current during the ice-covered period. Numerical calculations show that the growth of surface eddies is suppressed by the ice friction, while perturbations at mid-depths can grow into eddies, consistent with linear instability analysis. However, because the ice stress is spatially variable, it is able to drive Ekman pumping to decrease the available potential energy (APE) and kinetic energy of both the mean flow and mesoscale eddies over a vertical scale of 100 m, well outside the frictional Ekman layer. The rate at which the APE changes is determined by the vertical density flux, which is negative as the ice-induced Ekman pumping advects lighter (denser) water upward (downward). A scaling analysis shows that Ekman pumping will dominate the release of APE for large-scale flows, but the effect of baroclinic instability is also important when the horizontal scale of the mean flow is the baroclinic deformation radius and the eddy velocity is comparable to the mean flow velocity. Our numerical results highlight the importance of ice friction in the energetics of the slope current and eddies, and this may be relevant to other ice-covered regions. [ABSTRACT FROM AUTHOR]

The aim of this study is to better understand diffusive convection (DC) and its role in the upper ocean dynamic environment and sea ice melting in the Canada Basin. Based on a moored dataset with 6737 profiles collected from August 2003 to August 2011 in the upper layer of the Canada Basin, DC between the warm and salty Atlantic Water (AW) and the colder and less salty Lower Halocline Water (LHW) were investigated. The moorings were designated at four stations: A, B, C, and D, located at the southwestern, southeastern, northeastern, and northwestern parts of the basin, respectively. During the observation period, the temperature, salinity, and depth of the AW and LHW exhibited unique temporal variations. The temperature and salinity of the AW varied among stations, with a decreasing trend from northwest to southeast, consistent with the propagation path of the AW in the Canada Basin. The temperature and salinity of the LHW were similar at all stations. The AW and LHW cores were located between depths of 320–500 m and 160–300 m, respectively, and both gradually deepened over time. Distinct DC staircase structures were observed between the AW and LHW, more pronounced at stations C and D than at stations A and B, which is speculated to be related to eddies at stations A and B during the observation period. The vertical heat fluxes through the DC staircase layer at stations C and D (FHc_C and FHc_D) were estimated using an empirical formula. FHc_C ranged from 0.05 to 0.94 W/m2, and FHc_D ranged from 0.05 to 0.6 W/m2, with the maximum probability value for both at approximately 0.2 W/m2. The effective diffusivities at these two stations (KT_C and KT_D) are similar, ranging from 2 × 10−6 to 3 × 10−5 m2/s, with the highest probability occurring at 6 × 10−6 m2/s. Both the probability density function of the heat flux and the effective diffusivity skewed towards larger values and obey a lognormal distribution, indicating turbulence intermittency of the DC staircase in the Canada Basin. These finding offers new insights into the heat transport and turbulence in the DC staircase, and then bring a deeper understanding of sea ice melting in the Canada Basin.

The Arctic Ocean cold halocline layer (CHL) separates the cold surface mixed layer (SML) from the underlying warm Atlantic water, and thus provides a precondition for sea ice formation. Here, we introduce a new method in which the CHL base depth is diagnosed from vertical stability and compare it to two existing methods. Vertical stability directly affects vertical mixing and heat exchange. When applied to measurements from ice-tethered profilers, ships, and moorings, the new method for estimating the CHL base depth provides robust results with few artifacts. Comparatively large differences between our new method and two existing methods for detecting the CHL base depth were found in regions which are most prone to a CHL retreat in a warming climate. CHL base depth exhibits a seasonal cycle with a maximum depth in winter and also spring, when the SML depth is also at its maximum, but the amplitude of the CHL base depth's seasonal cycle is lower than for the SML for all three methods as expected. We also propose a novel method for detecting the cold halostad layer and study the seasonal cycle employing conservative assumptions to avoid a misclassification (including a lower bound of 50 m for the thickness). Detection of a cold halostad layer was largely confined to the Canada Basin and to the regions off the eastern coast of Greenland and also Svalbard. [ABSTRACT FROM AUTHOR]

Background: The Arctic Ocean receives massive freshwater input and a correspondingly large amount of humic-rich organic matter of terrestrial origin. Global warming, permafrost melt, and a changing hydrological cycle will contribute to an intensification of terrestrial organic matter release to the Arctic Ocean. Although considered recalcitrant to degradation due to complex aromatic structures, humic substances can serve as substrate for microbial growth in terrestrial environments. However, the capacity of marine microbiomes to process aromatic-rich humic substances, and how this processing may contribute to carbon and nutrient cycling in a changing Arctic Ocean, is relatively unexplored. Here, we used a combination of metagenomics and metatranscriptomics to assess the prevalence and diversity of metabolic pathways and bacterial taxa involved in aromatic compound degradation in the salinity-stratified summer waters of the Canada Basin in the western Arctic Ocean. Results: Community-scale meta-omics profiling revealed that 22 complete pathways for processing aromatic compounds were present and expressed in the Canada Basin, including those for aromatic ring fission and upstream funneling pathways to access diverse aromatic compounds of terrestrial origin. A phylogenetically diverse set of functional marker genes and transcripts were associated with fluorescent dissolved organic matter, a component of which is of terrestrial origin. Pathways were common throughout global ocean microbiomes but were more abundant in the Canada Basin. Genome-resolved analyses identified 12 clades of Alphaproteobacteria, including Rhodospirillales, as central contributors to aromatic compound processing. These genomes were mostly restricted in their biogeographical distribution to the Arctic Ocean and were enriched in aromatic compound processing genes compared to their closest relatives from other oceans. Conclusion: Overall, the detection of a phylogenetically diverse set of genes and transcripts implicated in aromatic compound processing supports the view that Arctic Ocean microbiomes have the capacity to metabolize humic substances of terrestrial origin. In addition, the demonstration that bacterial genomes replete with aromatic compound degradation genes exhibit a limited distribution outside of the Arctic Ocean suggests that processing humic substances is an adaptive trait of the Arctic Ocean microbiome. Future increases in terrestrial organic matter input to the Arctic Ocean may increase the prominence of aromatic compound processing bacteria and their contribution to Arctic carbon and nutrient cycles. 2somghBCVjjHAhscSmFASg Video Abstract [ABSTRACT FROM AUTHOR]

The Chukchi Slope Current is a westward-flowing current along the Chukchi slope, which carries Pacific-origin water from the Chukchi shelf into the Canada Basin and helps set the regional hydrographic structure and ecosystem. Using a set of experiments with an idealized primitive equation numerical model, we investigate the energetics of the slope current during the ice-covered period. Numerical calculations show that the growth of surface eddies is suppressed by the ice friction, while perturbations at mid-depths can grow into eddies, consistent with linear instability analysis. However, because the ice stress is spatially variable, it is able to drive Ekman pumping to decrease the available potential energy (APE) and kinetic energy of both the mean flow and mesoscale eddies over a vertical scale of 100 m, well outside the frictional Ekman layer. The rate at which the APE changes is determined by the vertical buoyancy flux, which is negative as the ice-induced Ekman pumping advects lighter (denser) water upward (downward). A scaling analysis shows that Ekman pumping will dominate the release of APE for large scale flows, but the effect of baroclinic instability is also important when the horizontal scale of the mean flow is the baroclinic deformation radius and the eddy velocity is comparable to the mean flow velocity. Our numerical results highlight the importance of ice friction in the energetics of the slope current and eddies, and this may be relevant to other ice-covered regions. [ABSTRACT FROM AUTHOR]

Thirty-four individual thermohaline sheets are identified at depths of 170–400 m in the Canada Basin of the Arctic Ocean by using the hydrographical data measured with the Ice-Tethered Profilers (ITPs) between August 2005 and October 2009. Each sheet is well determined because the salinity within itself remains very stable and the associated salinity anomaly is markedly smaller than the salinity jump between neighboring sheets. These thermohaline sheets are nested between the Lower Halocline Water (LHW) and Atlantic Water (AW) with lateral coherence of hundreds of kilometers and thickness varying from several to dozens of meters. The physical properties, including temperature, heat flux, and vertical turbulent diffusivity, in the sheet are found to be averagely associated with the AW propagation. Spatially, the thermohaline sheet is in a bowl-shaped distribution, which is deepest in the basin center and gradually becomes shallower toward the periphery. The interaction between the LHW and AW could be demonstrated through the property variances in the sheets. The temperature variances in the upper and lower sheets are correlated with the LHW and AW, respectively, transited at the 15th sheet, whereas the depth variance in the sheet is strongly correlated with the LHW. It is proposed that the sheet spatial distribution is mainly dominated by the Ekman convergence with the Beaufort Gyre, slightly modulated with the AW intrusion. Significance Statement: The diffusive convection staircases, composed of consecutive steps containing thick mixed layers and relatively thin interfaces, are prominent between the Lower Halocline Water (LHW) and the Atlantic Water (AW) throughout the Canada Basin. This sheet-like structure is in a bowl shape with lateral coherence over hundreds of kilometers. It is proposed that the distribution of the thermohaline sheet is mainly dominated by the Ekman convergence with Beaufort Gyre, as well as the AW intrusion. The present method of thermohaline-sheet identification would have more implications beyond this work. Since the thermohaline sheet remains mostly stable and coherent on a very large spatial–temporal scale, it might play a similar role as the water mass analysis in numerous applications, e.g., climate change. [ABSTRACT FROM AUTHOR]

In the Canada Basin of the Arctic Ocean, warm and salty Atlantic‐origin Water (AW) lies in the intermediate layer (250–800 m) below a colder and fresher surface layer. It results in a depth range where vertical thermohaline gradients are propitious to double‐diffusion. Indeed, thermohaline staircases are commonly observed and associated with double‐diffusive processes. Using observations from the Beaufort Gyre Exploration Project large database and Ice‐Tethered Profilers, we document the presence of density staircases in the 300–700 m depth range with a striking strong spatial and temporal coherence. However, since 2007, a progressive smoothing of these staircases has occurred, beginning from the western half of the basin. Quantifying this evolution, we find that a general pattern is a clear evolution over time from numerous thick steps (≃40 m) with sharp interfaces to fewer and thinner steps (≃30 m) with smoother interfaces. After 2014, marked density staircases have almost disappeared in most of the Canada Basin. The vanishing of staircases occurs over a few years and coincides with modifications of the large scale circulation and thermohaline large scale horizontal gradients. As the small scale thermohaline structures are thought to play an important role for the vertical and horizontal exchanges of heat within the Canada Basin, the disappearance of the steps may impact the heat distribution at depth, with potential consequences for the evolution of the sea ice cover. Plain Language Summary: In the western side of the Arctic Ocean, Atlantic‐origin water lies around 400 m deep, and is warmer and saltier than the surface layer, resulting in a peculiar stratification compared to the other oceans. Moreover, the interior of this ocean is thought to be quieter than the open ocean. As a result of this unique setting, specific mechanisms based on the difference in temperature and salinity capacity to diffuse can play a role in the formation of step‐like structures often observed in vertical profiles. Using observations of temperature and salinity in the western Arctic, we show that the steps are numerous, thick (≃40 m) and coherent over long time and space in 2005–2010. Yet, in recent years, the steps have started to gradually weaken and even to disappear in the western part of the Canada Basin. As the steps are thought to play an important role for the vertical and horizontal exchanges of heat within the Canada Basin, their disappearance may impact the heat distribution at depth, with potential consequences for the evolution of the sea ice cover. Key Points: Density staircases are detected over the past two decades in in situ observations in the Beaufort GyreDensity staircases have been gradually smoothed, with a transition that begins in the western part of the Canada BasinSmoothing of staircases occurs simultaneously with changes in the large scale dynamics [ABSTRACT FROM AUTHOR]

This study traces dissolved organic matter (DOM) in different water masses of the Arctic Ocean and its effect on the distributions of trace elements (TEs; Fe, Cu, Mn, Ni, Zn, Cd) using fluorescent properties of DOM and the terrigenous biomarker lignin. The Nansen, Amundsen, and Makarov Basins were characterized by the influence of Atlantic water and the fluvial discharge of the Siberian Rivers with high concentrations of terrigenous DOM (tDOM). The Canada Basin and the Chukchi Sea were characterized by Pacific water, modified through contact with productive shelf sediments with elevated levels of marine DOM. Within the surface layer of the Beaufort Gyre, meteoric water (river water and precipitation) was characterized by low concentrations of lignin and tDOM fluorescence proxies as DOM is removed during freezing. High‐resolution in situ fluorescence profiles revealed that DOM distribution closely followed isopycnals, indicating the strong influence of sea‐ice formation and melt, which was also reflected in strong correlations between DOM fluorescence and brine contributions. The relationship of DOM and hydrography to TEs showed that terrigenous and marine DOM were likely carriers of dissolved Fe, Ni, Cu from the Eurasian shelves into the central Arctic Ocean. Chukchi shelf sediments were important sources of dCd, dZn, and dNi, as well as marine ligands that bind and carry these TEs offshore within the upper halocline in the Canada Basin. Our data suggest that tDOM components represent stronger ligands relative to marine DOM components, potentially facilitating the long‐range transport of TE to the North Atlantic. Plain Language Summary: The Arctic Ocean receives a disproportionate large amount of global river discharge and has limited but well‐constrained exchanges with other oceans. This makes the Arctic Ocean unique in terms of dissolved organic matter (DOM) and trace element (TE) sources and distribution. We used data collected during two expeditions spanning the entire Arctic Ocean to characterize the unique distribution of DOM and to study its potential as a water mass tracer and its role in the transport of TE. While the Atlantic‐dominated Nansen Basin was characterized by low levels of the DOM and TE, the central Arctic was dominated by the Transpolar Drift, a current that connects the Eurasian shelves to the Fram Strait and transports DOM from the Siberian Rivers toward the North Atlantic. In contrast, the Chukchi shelf‐Canada Basin region was characterized by the dominance of Pacific water that is enriched by marine DOM from the shallow and productive Chukchi shelf. The distribution of DOM from these different sources was affected by freezing and thawing processes and, therefore, can be used to study water mass transformations and pathways in the Arctic Ocean. Key Points: Dissolved Organic Matter (DOM) distribution in the Arctic Ocean is largely controlled by sea ice formation and melt processesDOM distribution in the Arctic Ocean reveals its potential as a tracer for halocline formation and freshwater source assignmentsTerrigenous and marine DOM are carriers of trace elements from shelves to the open Arctic Ocean, but terrigenous DOM represent stronger ligands [ABSTRACT FROM AUTHOR]

To evaluate the performance of underwater acoustic communication systems, it is typically assumed that the noise at the receiver is uncorrelated spatially and temporally. This assumption underestimates the impact of acoustic ocean ambient noise on the performance of communication systems. In this article, the impact of ocean ambient noise on a coherent acoustic communication system is analyzed. The communication performance is assessed in narrowband conditions at a center frequency of ${\text{2.048}\;\text{kHz}}$ using the noise measurements from two different experiments—DalComm1 and the shallow water Canada Basin acoustic propagation experiment (SW CANAPE). DalComm1 focuses on characterizing the acoustic channel over ranges of 1–10 km on the Nova Scotian littoral, while the spatio-temporal variability of noise propagation in shallow and deep water environments was characterized during the CANAPE experiments. The ambient noise coherence and directionality in both environments were also measured. Two distinct noise modeling methodologies are presented to represent realistic synthetic ambient noise with defined directionality. Further, the synthetic noise is validated against measured ambient noise. The impact of ambient noise characteristics on an optimum space-time filter is characterized. A frame structure with an optimum training duration is also defined for the adaptive filter. It is observed that the bit-error rate of the space-time filter depends on optimizing the training and payload duration in the received signal. [ABSTRACT FROM AUTHOR]

Recent strengthening and westward movement of the Beaufort Gyre (BG), along with the rapid retreat of Arctic sea ice creates a new hotspot for investigating ecosystem responses to environmental changes over the Chukchi Borderland. As an important basis for primary production, nutrient variation and its controls in this hotspot region require to be preliminarily clarified. Using a hindcast simulation of a coupled ocean‐sea ice‐biogeochemical model (NAPA‐BGC) covering 1998–2015, this study suggests a regime shift in annual nitrate concentration in the upper southern Chukchi Borderland with the lower concentration since 2009. The relative contributions of biological and physical processes to annual nitrate variation are quantified via budget estimates. The results show a less temporally variable contribution of biological processes to nitrate variation, while the role of advection shifting from a nitrate source to a nitrate sink since 2009. This suggests physical processes mainly determine the nitrate transition in the upper southern Chukchi Borderland. Driven by the westward shift of the BG, the westward and southward expansion of the upper oligotrophic Canada Basin water narrows the pathway of nitrate‐rich shelf water flowing over the Chukchi Borderland. Meanwhile, the enhanced anticyclonic velocity at the marginal BG leads to an increased westward nitrate flux across the Chukchi Borderland and decreased nitrate supply to the north. Consequently, higher nitrate concentration exists in a narrow region to the south of 75°N, whereas the remaining areas have lower nitrate concentration, contributing to the overall reduction in annual averaged nitrate concentration since 2009 in the southern Chukchi Borderland. Plain Language Summary: With the intensified and westward movement of the Beaufort Gyre (BG) along with the rapid retreat of sea ice in the western Arctic Ocean, the Chukchi Borderland has become a frontier of research on the physical oceanography, biogeochemical cycles, and marine ecosystems. The westward shift of the BG has induced a regime shift in the nitrate concentration state from high to low in the upper southern Chukchi Borderland since 2009. Nitrate budget results show that biological processes contributed to nitrate variation with narrow temporal variability. Advection has shifted from a nitrate source to a nitrate sink since 2009, thus determining the nitrate transition in the upper southern Chukchi Borderland. Under the variation of the BG, upper layer oligotrophic water in the Canada Basin narrows the pathway range of nitrate‐rich shelf water flowing to the Chukchi Borderland. Changes in the anticyclonic circulation velocity structure around 2009 led to an increased westward nitrate flux through the Chukchi Borderland and decreased nitrate supply to the north, resulting in an overall decrease in annual averaged nitrate concentration. As the material basis for phytoplankton growth, the distribution pattern of nitrate will further affect the primary production and biological carbon pump. Key Points: The westward movement of the Beaufort Gyre drives the enhanced westward component of ocean currents in the southern Chukchi BorderlandWith the changed current, westward nitrate flux in the southern Chukchi Borderland increases, while northward flux decreasesNitrate concentration in the upper southern Chukchi Borderland has been shifting from a high level to a low level since 2009 [ABSTRACT FROM AUTHOR]

Measurements of the tracers, 129I, CFC‐11, and SF6 on water samples collected in the Arctic Ocean in 2015 have been used to calculate mean ages, Γ and mixing, Δ parameters using transit time distributions (TTDs) to constrain water circulation and mixing time scales. Values of Γ and Δ determined separately using the two tracer pairs, SF6‐CFC‐11, and 129I‐CFC‐11 are in good agreement for gas solubilities estimated for saturation levels of 0.90, but agreement decreases for other gas saturation levels. Both Γ and Δ increase rapidly with increasing depth below the base of the intermediate water layer (ca. 1,000 m), but maintaining a value of Δ/Γ ≅ 1 supporting the use of this proportionality in applications of TTDs to deep ocean transport of substances such as anthropogenic carbon. Isolines of Γ = 20 years deepening to depths below 1,000 m over the flank of the Mendeleyev Ridge near the North Pole outline the bathymetrically steered, return flow of recently ventilated Atlantic Water toward Fram Strait. Basin interior waters are significantly older with the Γ = 25 years mean age isoline shallowing upward to depths above 500 m in the Makarov, Canada, and Eurasian Basins. Values of Δ remain relatively constant in the 6–10 years range in upper intermediate water across all three basins indicating that flow is principally advective and that the mixing specified by Δ likely occurs upstream of the central basins in regions proximal to the outflow from the Santa Anna Trough. Plain Language Summary: Measurements of the tracers, 129I, CFC‐11, and SF6 on water samples from the Arctic Ocean have been used to calculate mean ages, Γ, and mixing, Δ, parameters using transit time distributions (TTDs) in order to constrain water circulation and mixing time scales. Values of Γ increase with increasing water depth in Atlantic intermediate water and with distance across the Arctic Ocean from the Nordic Seas input region to the Canada Basin. The mixing parameter, Δ, is a metric that relates to the cumulative amount of tracer mixing undergone by a volume element of water. In contrast to Γ, measured values of Δ are relatively constant in arctic intermediate waters indicating strong advective transport throughout the central Arctic Ocean. Values of Γ and Δ determined separately using the tracer pairs, SF6‐CFC‐11, and 129I‐CFC‐11 are in good agreement for gas solubilities at saturation levels of 0.90, but agreement decreases for other gas saturation levels. Both Γ and Δ increase rapidly with increasing depth below the base of the intermediate water layer (ca. 1,000 m), but maintaining a value of Δ/Γ ≅ 1 supporting the use of this proportionality in applications of TTDs to deep ocean transport of anthropogenic carbon. Key Points: Measurements of 129I, CFC‐11, SF6 in Arctic Ocean used to calculate mean ages, Γ and mixing, Δ to constrain water circulation time scalesΓ sections conform to historical ideas of tracer ages; Δ nearly constant in upper intermediate water indicating strong advective flowHigh Δ values in upper halocline are congruent with nutrient maximum indicating that both are governed by winter shelf mixing processes [ABSTRACT FROM AUTHOR]

Interaction between mesoscale eddies and near‐inertial internal waves can contribute to enhanced turbulence mixing but quantitative knowledge from in situ observations is still lacking. This study reveals how eddy/near‐inertial wave interactions (ENIs) can affect the variability of turbulent mixing in the ice‐covered Canada Basin of the Arctic Ocean. We use data from five Ice‐Tethered Profiler with Velocity (ITP‐V) systems that autonomously obtained vertical profiles of horizontal velocity as well as temperature and salinity, which enabled quantification of ENI‐caused turbulent mixing using a fine‐scale parameterization. From the ITP‐V observations in 2013–2015, 67 anticyclones were detected, of which 90% had a deep core at 150–250 m depth. The remaining eddies had a shallow core, typically embedded in the Pacific Summer Water (PSW). Just over one third of the eddies showed evidence of ENI with enhanced near‐inertial internal wave amplitude (NIW) near the eddy cores. For these ENI cases, the parameterized turbulence dissipation rate was O (10−10–10−8 W kg−1), the larger estimates being several orders of magnitude greater than the background level. For the deep eddies, the ENI process can largely be accounted for by the classical theory, NIWs are trapped inside the negative relative vorticity core of anticyclones. For one shallow eddy, the NIW signal was greatest below the core. We postulate that a vertically elongated system of NIWs cannot be constrained vertically within such small‐cored eddies. It is also interpreted that the wave enhancement below the core was supported by the isopycnal slope near the PSW through its geostrophic shear. Plain Language Summary: Two ocean phenomena, eddies and internal waves, are known to interact and contribute to hotspots of strong mixing in the upper ocean. We analyze extensive observations of the interaction of eddies and internal waves in the ice‐covered Arctic Canada Basin using data from several autonomous instruments: the Ice‐Tethered Profiler with Velocity (ITP‐V). The ITP‐V repeatedly measures vertical profiles of ocean temperature, salinity, and currents at 1 m vertical resolution. During 2013–2015, ITP‐Vs encountered 67 individual eddies with core depths ranging between 40 and 250 m. Of these, 90% were found at depths of 150–250 m, while the rest were shallow features. Approximately one third of the observed eddies exhibited internal waves with enhanced amplitudes near the eddy core depths that are inferred to support intensified mixing. For the deep eddies, the amplified waves were mostly observed within the eddies, consistent with previous studies. For one shallow eddy, enhanced wave signals were found below something that has not been previously reported. For this case, we deduced that the sloping stratification associated with the eddy azimuthal flow modified the propagation of waves and resulting in the enhanced mixing on the underside of the eddy. Key Points: High‐resolution autonomous observations of hydrography and currents were conducted in pack ice regions of the Canada Basin, Arctic OceanInteraction between anticyclonic mesoscale eddies and near‐inertial internal waves enhanced turbulent mixing roughly by factor of twoA shallow eddy showed significant wave enhancement below its core resulting from combination of negative vorticity and slanted isopycnals [ABSTRACT FROM AUTHOR]

Atlantic Water (AW) is the largest reservoir of heat in the Arctic Ocean, isolated from the surface and sea ice by a strong halocline. In recent years, AW shoaling and warming are thought to have had an increased influence on sea ice in the Eurasian Basin. In this study, we analyze 59,000 profiles from across the Arctic from the 1970s to 2018 to obtain an observationally based pan‐Arctic picture of the AW layer, and to quantify temporal and spatial changes. The potential temperature maximum of the AW (the AW core) is found to be an easily detectable, and generally effective metric for assessments of AW properties, although temporal trends in AW core properties do not always reflect those of the entire AW layer. The AW core cools and freshens along the AW advection pathway as the AW loses heat and salt through vertical mixing at its upper bound, as well as via likely interaction with cascading shelf flows. In contrast to the Eurasian Basin, where the AW warms (by approximately 0.7°C between 2002 and 2018) in a pulse‐like fashion and has an increased influence on upper ocean heat content, AW in the Canadian Basin cools (by approximately 0.1°C between 2008 and 2018) and becomes more isolated from the surface due to the intensification of the Beaufort Gyre. These opposing AW trends in the Eurasian and Canadian Basins of the Arctic over the last 40 years suggest that AW in these two regions may evolve differently over the coming decades. Plain Language Summary: A few hundred meters beneath the surface of the Arctic Ocean lies a warm, salty layer of Atlantic origin, called Atlantic Water (AW), which is isolated from sea ice and the ocean surface by a vertical salinity gradient that acts as a barrier between the AW and the surface. In recent years, weakening of this barrier and warming of AW in the eastern Arctic have contributed to unprecedented sea‐ice loss. This study analyses 59,000 vertical temperature and salinity profiles from the Arctic Ocean from the 1970s to 2018 to obtain a broad picture of the AW and its variability. The AW temperature maximum is found to be an easily observable, generally effective way to assess how much heat is stored in the AW layer. Over the period studied, the AW in the eastern Arctic warmed and had an increasing influence on the amount of heat in the surface layer, whereas AW heat became increasingly isolated from the surface in the west due to changes in local winds. The emergence of a characteristically different eastern and western Arctic Ocean in the future could have important consequences, both in terms of Arctic sea‐ice loss and global ocean circulation. Key Points: Atlantic Water (AW) is evolving in opposing ways in the eastern and western sectorsData suggest that AW cools during transit via vertical mixing at its upper bound and through interaction with cool dense shelf watersAW core temperature is generally effective in assessing AW heat content but does not always capture temporal trends [ABSTRACT FROM AUTHOR]

The Arctic Ocean plays an important role in global climate and global warming through freshwater and heat exchange with subarctic waters. A better understanding of circulation time scales in the Arctic Ocean is essential to predict changes in climate and biogeochemical cycling in the Arctic Ocean. 129I and 137Cs, which have been discharged from the nuclear fuel-reprocessing facilities, have been employed to determine the time scale of the circulation in the Arctic Ocean. However, its temporal change has not been understood well. In 2017, 2019, and 2020, we measured 129I and 137Cs in the Canada Basin in the Arctic Ocean. Using our new and historical data, we discuss temporal changes in the circulation in the basin between 1993 and 2020. The tracer ages derived from the 129I/137Cs ratio indicate that the transport of the Atlantic water into the Canada Basin was accelerated in 2020. This is consistent with results of recent studies that indicated the intensified inflow of the Atlantic water into the eastern Arctic Ocean in the late 2010s, which is termed "atlantification". Our results confirmed the "atlantification" in the Canada Basin by temporal changes in the transient tracers for the first time. [ABSTRACT FROM AUTHOR]

A large volume of freshwater is incorporated in the relatively fresh (salinity ~32–33) Pacific Ocean waters that are transported north through the Bering Strait relative to deep Atlantic salinity in the Arctic Ocean (salinity ~34.8). These freshened waters help maintain the halocline that separates cold Arctic surface waters from warmer Arctic Ocean waters at depth. The stable oxygen isotope composition of the Bering Sea contribution to the upper Arctic Ocean halocline was established as early as the late 1980's as having a δ18OV-SMOW value of approximately -1.1‰. More recent data indicates a shift to an isotopic composition that is more depleted in 18O (mean δ18O value ~-1.5‰). This shift is supported by a data synthesis of >1400 water samples (salinity from 32.5 to 33.5) from the northern Bering and Chukchi seas, from the years 1987–2020, which show significant year-to-year, seasonal and regional variability. This change in the oxygen isotope composition of water in the upper halocline is consistent with observations of added freshwater in the Canada Basin, and mooring-based estimates of increased freshwater inflows through Bering Strait. Here, we use this isotopic time-series as an independent means of estimating freshwater flux changes through the Bering Strait. We employed a simple end-member mixing model that requires that the volume of freshwater (including runoff and other meteoric water, but not sea ice melt) flowing through Bering Strait has increased by ~40% over the past two decades to account for a change in the isotopic composition of the 33.1 salinity water from a δ18O value of approximately -1.1‰ to a mean of -1.5‰. This freshwater flux change is comparable with independent published measurements made from mooring arrays in the Bering Strait (freshwater fluxes rising from 2000–2500 km3 in 2001 to 3000–3500 km3 in 2011). [ABSTRACT FROM AUTHOR]

Biogeochemical cycles in the Arctic Ocean are sensitive to the transport of materials from continental shelves into central basins by sea ice. However, it is difficult to assess the net effect of this supply mechanism due to the spatial heterogeneity of sea ice content. Manganese (Mn) is a micronutrient and tracer which integrates source fluctuations in space and time while retaining seasonal variability. The Arctic Ocean surface Mn maximum is attributed to freshwater, but studies struggle to distinguish sea ice and river contributions. Informed by observations from 2009 IPY and 2015 Canadian GEOTRACES cruises, we developed a three‐dimensional dissolved Mn model within a 1/12° coupled ocean‐ice model centered on the Canada Basin and the Canadian Arctic Archipelago (CAA). Simulations from 2002 to 2019 indicate that annually, 87%–93% of Mn contributed to the Canada Basin upper ocean is released by sea ice, while rivers, although locally significant, contribute only 2.2%–8.5%. Downstream, sea ice provides 34% of Mn transported from Parry Channel into Baffin Bay. While rivers are often considered the main source of Mn, our findings suggest that in the Canada Basin they are less important than sea ice. However, within the shelf‐dominated CAA, both rivers and sediment resuspension are important. Climate‐induced disruption of the transpolar drift may reduce the Canada Basin Mn maximum and supply downstream. Other micronutrients found in sediments, such as Fe, may be similarly affected. These results highlight the vulnerability of the biogeochemical supply mechanisms in the Arctic Ocean and the subpolar seas to climatic changes. Plain Language Summary: Autumn storms on the Siberian side of the Arctic Ocean churn up sediment that freezes into sea ice. The prevailing ocean currents and winds push this sea ice across the Arctic Ocean toward the Canada Basin, where it melts and releases the sediment into the ocean. Sediment contains manganese and other nutrient elements that help support plankton and life. Using our manganese ocean model, 87%–93% of Mn in the Canada Basin comes from "dirty" sea ice from 2002 to 2019, while rivers supply 2.2%–8.5%. As a result of climate change, less dirty sea ice may make it across the Arctic Ocean, which could reduce this supply system of manganese and other similar micronutrients. This change also has potential impacts downstream: Water from the Canada Basin travels through the shallow Canadian Arctic Archipelago into Baffin Bay and eventually into the North Atlantic. We found that about 34% of Mn transported along this route comes from "dirty" sea ice. In the Canadian Arctic Archipelago, other sources contribute as well: tides churn up sediments from the ocean floor and many rivers flow into the channels. Our study highlights ways in which climate change may impact the nutrient supply systems in the Arctic Ocean. Key Points: We present an ocean model of Manganese (Mn) in the Canadian Arctic that captures observed spatial variationNon‐local sediments transported by sea ice are a key source of micronutrients such as Mn to the Canada BasinRivers are most important for Mn in coastal regions of the Canadian Arctic [ABSTRACT FROM AUTHOR]

The Arctic seasonal halocline impacts the exchange of heat, energy, and nutrients between the surface and the deeper ocean, and it is changing in response to Arctic sea ice melt over the past several decades. Here, we assess seasonal halocline formation in 1975 and 2006-12 by comparing daily,May-September, salinity profiles collected in the Canada Basin under sea ice. We evaluate differences between the two time periods using a one-dimensional (1D) bulk model to quantify differences in freshwater input and vertical mixing. The 1D metrics indicate that two separate factors contribute similarly to stronger stratification in 2006-12 relative to 1975: 1) larger surface freshwater input and 2) less vertical mixing of that freshwater. The larger freshwater input is mainly important in August-September, consistent with a longer melt season in recent years. The reduced vertical mixing is mainly important from June until mid-August, when similar levels of freshwater input in 1975 and 2006-12 are mixed over a different depth range, resulting in different stratification. These results imply that decadal changes to ice-ocean dynamics, in addition to freshwater input, significantly contribute to the stronger seasonal stratification in 2006-12 relative to 1975. These findings highlight the need for near-surface process studies to elucidate the impact of lateral processes and ice-ocean momentum exchange on vertical mixing. Moreover, the results may provide insight for improving the representation of decadal changes to Arctic upper-ocean stratification in climate models that do not capture decadal changes to vertical mixing. [ABSTRACT FROM AUTHOR]

In order to better understand the sources, sinks, and hydrodynamic influences on particulate matter distribution and variability in Arctic basins, we combined transmissometer data from two 2015 fall expeditions: one from Bering Strait (USCGC Healy) and the other from Barents Sea (R/V Polarstern) meeting at the North Pole. These first trans-Arctic sections of beam attenuation were overlain by salinity, temperature, and chlorophyll-a fluorescence (Chl-a) contours. Chl-a sections were overlain by nitrate contours. Particle composition analyzed from filtered samples throughout the full water column aided significantly in discerning sources, distribution, and dynamics of particles. Dense Pacific water moving swiftly through Bering Strait erodes and carries sediment-laden waters onto the Chukchi Shelf and into the Canada Basin. This nutrient-rich Pacific water sinks below a low-salinity, nutrient-poor polar mixed layer, forming a thick lens of high salinity water known as Pacific halocline water (salinity 31-33). Ice and the nutrient-poor mixed layer inhibit photosynthesis in surface waters of Canada and Makarov Basins, but subsurface chlorophyll-a maxima are observed if nutrients and light are available. Surface-water biomass is greater in the Barents Sea than in the Beaufort Gyre because nutrient-rich Atlantic water entering Barents Sea is not isolated from surface waters by ice or strong stratification. Surface water cools, creating high-density water that cascades into ~400 m basins in Barents Sea and into deep Nansen Basin, eroding sediment that creates patches of nepheloid layers in the shallow basins. Intense nepheloid layers in the deep basins are rare, suggesting weak bottom currents. [ABSTRACT FROM AUTHOR]

The sea-ice floe size distribution (FSD) characterizes the sea-ice response to atmospheric and oceanic forcing and is important for understanding and modeling the evolving ice pack in a warming Arctic. FSDs are evaluated from 78 floe-segmented high-resolution (1 m) optical satellite images capturing a range of settings and sea-ice states during spring through fall from 1999 to 2014 in the Canada Basin. For any given image, the structure of the FSD is found to be sensitive to a classification threshold value (i.e., to specify an image pixel as being either water or ice) used in image segmentation, and an approach to account for this sensitivity is presented. The FSDs are found to exhibit a single power-law regime between floe areas 50 m 2 and 5 km 2 , characterized by exponents (slopes in log-log space) in the range - 2.03 to - 1.65. A distinct linear relationship between slopes and sea-ice concentrations is found, with steeper slopes (i.e., a larger proportion of smaller to larger floes) corresponding to lower sea-ice concentrations. Further, a seasonal variation in slopes is found for fixed sites in the Canada Basin that undergo a seasonal cycle in sea-ice concentration, while sites with extensive sea-ice cover year-round do not exhibit any seasonal change in FSD properties. Our results suggest that sea-ice concentration should be considered in any characterization of a time-varying FSD (for use in sea-ice models, for example). [ABSTRACT FROM AUTHOR]