Search

The exchange of heat and freshwater between glaciers and the ocean is dictated by the circulation in glacial fjords and ice shelf cavities. Therefore, our ability to estimate and predict these fluxes depends on our understanding of the circulation mechanisms inside these polar estuaries. Here, we use an exceptionally long observational data set (8+ years) to develop and validate a high‐resolution realistic numerical model of Milne Fiord (Nunavut, Canada), a glacial fjord with an ice shelf at its mouth. Model results show the circulation inside Milne Fiord from 2011 to 2019 is highly three‐dimensional and unsteady. Three distinct circulation modes are identified (eddy, front, barotropic). The shifts between circulation modes are driven by density variations in the offshore coastal current, which restrict vertical stretching, allowing (or not) the coastal current to develop sufficient relative vorticity to enter the fjord. The unsteadiness of the system results in an overturning estuarine circulation with mean velocities ∼50 times smaller than the instantaneous field. Moreover, analysis of the model outputs suggest that at least 2 years of simulation are needed to yield reliable average heat flux estimates in this environment. This work highlights the value of combining long term (>1 year) observations with numerical modeling. This allowed us to uncover the spatial and temporal variability of the system, which impacts how heat and freshwater fluxes can be reliably estimated. Plain Language Summary: Fjords and bays where glaciers end their course in the ocean are common in the Arctic and in Antarctic. The circulation and the interactions between the ocean and glaciers in these polar estuaries has implications for ice loss and sea level rise. It is therefore important to understand the ocean dynamics in glacial fjords (a fjord with a marine terminating glacier) and ice shelf cavities (a water column overlain by the floating extension of a glacier or ice shelf) in order to quantify the impact of the ocean forcing on glaciers. In this study, we use a numerical model to reproduce the ocean conditions in Milne Fiord, a glacial fjord with an ice shelf on the north coast of Ellesmere Island, Nunavut, Canada. The model is validated with in situ observations. The results show that although the velocities are small in Milne Fiord (around 2 cm/s), the circulation is very dynamic. In fact, the circulation pattern is continually changing in response to offshore density variations (no steady‐state). This is problematic for estimating the heat exchange between the ocean and the glacier, because it means limited observations cannot be readily extrapolated in time and space. Key Points: A high‐resolution numerical model of a glacial fjord with an ice shelf (Milne Fiord) was run for 8.3 years and validated with observationsMilne Fiord circulation is highly variable across space and time and switches between three different circulation modes (no steady‐state)The variability of the circulation highlights the need for long term observational programs combined with (validated) numerical models [ABSTRACT FROM AUTHOR]

The Northeast Water Polynya is a significant annually recurring summertime Arctic polynya, located off the coast of Northeast Greenland. It is important for marine wildlife and affects local atmospheric and oceanic processes. In this study, over 40 years of observational and reanalysis products (ERA5 and ORAS5) are analyzed to characterize the polynya's climatology and ascertain forcing mechanisms. The Northeast Water Polynya has high spatiotemporal variability; its location, size and structure vary interannually, and the period for which it is open is changing. We show this variability is largely driven by atmospheric forcing. The polynya extent is determined by the direction of the near‐surface flow regime, and the relative locations of high and low sea‐level pressure centers over the region. The surface conditions also impact the oceanic water column, which has a strong seasonal cycle in potential temperature and salinity, the amplitude of which decreases with depth. The ocean reanalyses also show a significant warming trend at all depths and a freshening near the surface consistent with greater ice melt, but salinification at lower depths (∼200 m). As the Arctic region changes due to anthropogenic forcing, the sea‐ice edge is migrating northwards and the Northeast Water Polynya is generally opening earlier and closing later in the year. This could have significant implications for both the atmosphere and ocean in this complex and rapidly changing environment. Plain Language Summary: Polynya's are areas of open water surrounded by sea ice. These features are important for marine wildlife and can impact how the atmosphere and ocean interact. The Northeast Water Polynya is one example from the Arctic. Each summer, it appears off the coast of Northeast Greenland. In this research we use data from the last 40 years to study this polynya. We find that its location, size and structure varies from year to year. The period for which it is open each year also changes. We conclude that these differences are due to changes in the wind direction near the surface. Winds that flow from south to north over the region are linked to a larger polynya. Winds that flow from north to south are linked to a smaller polynya. With climate change, the Northeast Water Polynya is opening earlier and closing later in the year. In the future, this polynya may no longer form as it does now, which will impact the local environment and wildlife. Key Points: A climatological study of the Northeast Water Polynya reveals spatial, interannual and decadal variabilityThe summertime polynya extent closely correlates to near‐surface winds, largely dictated by the orientation of the pressure gradient across Fram StraitBetween 1980 and 2022, the duration of the Northeast Water Polynya, and associated oceanic warming, increased [ABSTRACT FROM AUTHOR]

Space observations (1979–2020) have shown that, over the past 40 years, years with a decrease in the area of summer ice and their thickness prevailed. Over 10 years, negative trends of anomalies in the area and thickness of the ice are –13 and –15%, respectively. A rapid reduction in the area of old ice (>4-year-old) is also noted, because in 1985 it was estimated at 2.7 million km2, while in March 2010 it was 0.34 million km2. The paper analyses paleo sea ice extent during the Holocene (the last 12 000 years) based on empirical IP25 biomarkers (a sea ice proxy with 25 carbon atoms synthesized by the specific Arctic sea ice diatoms Hasleaspp, which have been proven to be a suitable proxy for paleo-sea ice reconstructions) obtained from deep-sea cores from the North Atlantic. The data showed that, during the warm periods of the Early and Middle Holocene, the area of summer sea ice was reduced to a minimum. This confirms the conclusion made earlier in (Kinnard et al., 2011) that the current trend of reducing the area and thickness of ice is unprecedented over the past 1500 years. There is no complete analogue of the climate in the past corresponding to the current level of the CO2 concentration in the atmosphere. The period with CO2 concentrations in the atmosphere similar to the current level was the warm part of the Middle Pliocene between 3 and 4 million years ago with level of the CO2 concentration 450–500 ppm against approximately 420 ppm at present. Paleo-climate reconstructions for this period estimate the global temperature to be 3.0–3.5 ± 0.5°C higher than at the end of the 19th century. Summer air temperatures in the high latitudes of the Northern Hemisphere exceeded the current ones by 8–10°C, and the sea ice in the Arctic shelf seas was completely absent in the summer. Empirical data and model simulations have shown that presently the main driver of the reduction of the Arctic sea ice area is the increase in concentration of CO2 in the atmosphere. At the present time, old sea ice tends to be replaced by seasonal ice, demonstrating a natural shift from the predominance of permanent ice to an ice-free Arctic. In the case of a continuous increase in CO2 concentration in the atmosphere despite emission control measures, one of the scenarios that happened in the past may occur again. [ABSTRACT FROM AUTHOR]

On 25 February 2022 Antarctic sea ice extent dropped to a satellite‐era record low level of 1.92 × 106 km2, 0.92 × 106 km2 below the long‐term mean. The area of sea ice was also at a record low level of 1.24 × 106 km2. Although no individual sector was at a record low, at the minimum there were negative sea ice anomalies in all sectors of the Southern Ocean, with the largest in the Ross (contributing 46%) and Weddell Seas (26%). The Amundsen Sea Low had a record low depth in October/November 2021, with a series of very deep depressions giving strong offshore winds. These accelerated ice loss during the melt season, creating a 1.00 × 106 km2 coastal polynya in the Ross Sea. In the northern Weddell Sea, westerly winds of record strength led to ice export from the region. Plain Language Summary: Sea ice is a critical part of the Antarctic climate system that is an important breeding habitat for seals and other marine biota, as well as playing an important part in the global ocean circulation. Unlike the Arctic, where sea ice extent has decreased over recent decades, sea ice around the Antarctic actually increased in extent for a large part of the record starting in the late 1970s. However, on 25 February 2022 the extent dropped to a record low level of 1.924 million square kilometers, 1 million square kilometers below the long‐term average. The origins of the sea ice loss can be traced back to extremely deep storms in the Ross Sea in October and November 2021, and the very strong southerly winds that the storms had on their western flank. These moved sea ice away from the Antarctic coast, exposing the ocean and allowing solar heating to warm the ocean and give further sea ice melt. An additional factor was the very strong westerly winds north of the Weddell Sea that moved sea ice out of this basin and toward the east. Key Points: Antarctic sea ice extent dropped to a record low level of 1.92 × 106 km2 on 25 February 2022There were negative sea ice anomalies in all sectors of the Southern Ocean, with the largest in the Ross and Weddell SeasDeep storms in October/November 2021 led to low sea ice concentration and a large coastal polynya that accelerated sea ice loss [ABSTRACT FROM AUTHOR]

The Arctic Ocean has experienced orbital and millennial-scale climate oscillations over the last 500 kilo-annum (ka) involving massive changes in global sea level and components of the Arctic cryosphere, including sea-ice cover, land-based ice sheets and ice shelves. Although these climate events are only partially understood, micropaleontological studies utilizing ostracodes and benthic foraminifera have demonstrated that major changes in faunas have occurred at different timescales that signify ecosystem regime changes linked to sea-ice cover, surface productivity, bottom temperature and other factors. In addition to faunal changes characterizing glacial-interglacial cycles, Arctic sediments contain several unusual faunal events that cannot be explained by orbital-scale sea level and cryospheric changes. One indicator of such events involves the ostracode Rabilimismirabilis (Brady, 1868), a shallow-water species that inhabits continental shelves in themodern Arctic. We conducted studies of the stratigraphic distribution of R. mirabilis in cores from the Northwind, Mendeleev, Lomonosov, and Alpha Ridges; the Siberian and North American (Beaufort Sea) continental margins; and the Lincoln Sea off North Greenland and in the northern Greenland Sherard Osborn Fjord. Evidence from these records suggests that this species occurs as a fossil in deeper water sediment cores on the upper parts of submarine ridges (mainly 700-900 meters water depth, mwd), in significant numbers (from 1%to 50% of total ostracodes) duringMarine Isotope Stages (MIS) 5a (125-109 ka), MIS 4 (71-57 ka), and MIS 3 (57-29 ka). Furthermore, it occurs in cores from various depths on the Siberian margin, the Beaufort and Lincoln Seas duringMIS 1 (the Holocene, ~11-0 ka). These occurrences involve well-preserved, stratigraphically consistent adult and juvenile populations, which are autochthonous in nature and not caused by downslope transport or ice rafting. Based on their age and associated paleoceanographic conditions in the Arctic, we interpret these R. mirabilis events as signifying basin-ward migration during abrupt changes in growth and decay of massive ice shelves and may be useful as biostratigraphic markers. [ABSTRACT FROM AUTHOR]

We mapped the terminus position for every marine‐terminating glacier in the Northern Hemisphere for 2000, 2010, and 2020, including the Greenland Ice Sheet, to provide the first complete measure of their variability. In total, these 1,704 glaciers lost an average of 389.7 ± 1.6 km2 a−1 (total 7,527 ± 31 km2) from 2000 to 2020 with 123 glaciers becoming no longer marine‐terminating over this period. Overall, 85.3% of glaciers retreated, 2.5% advanced, and the remaining 12.3% did not change outside of uncertainty limits. Outlet glaciers of the Greenland Ice Sheet are responsible for 61.9% of total area loss, although their rate of retreat was 34% less in 2010–2020 than 2000–2010. Glaciers with the largest area loss terminate in ice shelves or ice tongues, are surge‐type, have an unstable basal geometry, or have an unusually wide calving margin. Plain Language Summary: North of the equator, 1,704 glaciers touched the ocean in 2000. Here, we present the first analysis to document the frontal position of every one of these glaciers in 2000, 2010, and 2020. We found that 85.3% retreated and are now reduced in area. Only 2.5% of glaciers advanced or increased in area. The remaining 12.3% did not change within uncertainty limits. Total area losses were 389.7 ± 1.6 km2 per year (total 7,527 ± 31 km2) over the 20‐year period. Glaciers flowing from the Greenland Ice Sheet accounted for over 60% of total area losses. We found wide variations in the response of glaciers to similar changes in air and ocean temperature and sea ice concentrations, showing that environmental conditions alone cannot explain why some glaciers retreated more than others. Instead, unique glacier characteristics are the most important factor in controlling the variability of terminus retreat. Glaciers with floating ice at their front (ice shelves or ice tongues), those that undergo periodic changes in their flow velocity (surges), those which have a weak connection to their beds, and glaciers that are unusually wide, experienced the largest area loss from 2000 to 2020. Key Points: There were 1704 marine‐terminating glaciers in 2000 in the Northern Hemisphere, of which 85.3% retreated and 2.5% advanced from 2000 to 2020Tidewater glaciers lost a total area of 7,527 ± 31 km2 from 2000 to 2020, with the Greenland Ice Sheet responsible for 61.9% of total lossesVariations in retreat are best explained by glacier characteristics: ice shelves/tongues, surging, basal geometry, and calving width [ABSTRACT FROM AUTHOR]

Circum-Arctic glacial ice is melting in an unprecedented mode, and release of currently trapped geological methane may act as a positive feedback on ice-sheet retreat during global warming. Evidence for methane release during the penultimate (Eemian, ca. 125 ka) interglacial, a period with less glacial sea ice and higher temperatures than today, is currently absent. Here, we argue that based on foraminiferal isotope studies on drill holes from offshore Svalbard, Norway, methane leakage occurred upon the abrupt Eurasian ice-sheet wastage during terminations of the last (Weichselian) and penultimate (Saalian) glaciations. Progressive increase of methane emissions seems to be first recorded by depleted benthic foraminiferal d13C. This is quickly followed by the precipitation of methane-derived authigenic carbonate as overgrowth inside and outside foraminiferal shells, characterized by heavy d18O and depleted d13C of both benthic and planktonic foraminifera. The similarities between the events observed over both terminations advocate for a common driver for the episodic release of geological methane stocks. Our favored model is recurrent leakage of shallow gas reservoirs below the gas hydrate stability zone along the margin of western Svalbard that can be reactivated upon initial instability of the grounded, marine-based ice sheets. Analogous to this model, with the current acceleration of the Greenland ice melt, instabilities of existing methane reservoirs below and nearby the ice sheet are likely. [ABSTRACT FROM AUTHOR]

The formation of platelet ice is well known to occur under Antarctic sea ice, where subice platelet layers form from supercooled ice shelf water. In the Arctic, however, platelet ice formation has not been extensively observed, and its formation and morphology currently remain enigmatic. Here, we present the first comprehensive, long‐term in situ observations of a decimeter thick subice platelet layer under free‐drifting pack ice of the Central Arctic in winter. Observations carried out with a remotely operated underwater vehicle (ROV) during the midwinter leg of the MOSAiC drift expedition provide clear evidence of the growth of platelet ice layers from supercooled water present in the ocean mixed layer. This platelet formation takes place under all ice types present during the surveys. Oceanographic data from autonomous observing platforms lead us to the conclusion that platelet ice formation is a widespread but yet overlooked feature of Arctic winter sea ice growth. Plain Language Summary: Platelet ice is a particular type of ice that consists of decimeter sized thin ice plates that grow and collect on the underside of sea ice. It is most often related to Antarctic ice shelves and forms from supercooled water with a temperature below the local freezing point. Here we present the first comprehensive observation of platelet ice formation in freely drifting pack ice in the Arctic in winter during the international drift expedition MOSAiC. We investigate its occurrence under the ice with a remotely controlled underice diving robot. Measurements of water temperature from automatic measurement devices distributed around the central MOSAiC ice floe show that supercooled water and thus platelet ice occur widely in the winter Arctic. This way of ice formation in the Arctic has been overlooked during the last century, as direct observations under winter sea ice were not available and contrary to typical Antarctic observations; manifestation of platelet ice in Arctic ice core stratigraphy has been more challenging to identify. Key Points: Here we present extensive observations of platelet ice formation under Arctic winter sea iceThe subice platelet layer appears to form locally due to seed crystals in ocean surface supercooling [ABSTRACT FROM AUTHOR]

This study provides the first documentation of polar bear (Ursus maritimus) maternity denning in snowdrifts around icebergs frozen into the fast ice or grounded on the seafloor. Based on six den observations in north and northeast Greenland during spring surveys in 2018 and 2019 (109 flight hours), together with observations of 20 adult females with 35 cubs of the year (COYs) in adjacent sea ice, we hypothesize that the use of snowdrifts around icebergs for maternity denning is an established behavior in the region and not a random event. Factors influencing maternity denning in snowdrifts around icebergs may include limited suitable drifts on the nearby terrestrial polar desert due to low precipitation, the presence of suitable wind-blown snow banks regardless of the direction of autumn storm winds, cold and stable habitat throughout the winter denning period, and access to ringed seal (Pusa hispida) pupping habitat in the nearby Northeast Water polynya. This type of maternity denning habitat is only available in glaciated regions of the Arctic where marine-terminating glaciers deposit mélange large enough to become grounded offshore and remain in place for months or years. This habitat may become less stable or disappear with long-term climate warming. [ABSTRACT FROM AUTHOR]

Based on sediment cores and geophysical data collected from Petermann Fjord and northern Nares Strait, NW Greenland, an Arctic ice shelf sediment facies is presented that distinguishes sub and pro ice shelf environments. Sediment cores were collected from sites beneath the present day Petermann Ice Tongue (PIT) and in deglacial sediments of northern Nares Strait with a focus on understanding the glacial and oceanographic history over the last 11,000 cal yr BP. The modern sub ice shelf sediment facies in Petermann Fjord is laminated and devoid of coarse clasts (IRD) due to strong basal melting that releases debris (debris filtering) from the basal ice at the grounding zone driven by buoyant subglacial meltwater and entrained Atlantic Water. Laminated sediments in the deep basin proximal to the gounding zone comprise layers of fine mud formed by suspension settling from turbid meltwater plumes (plumites) interrupted by normally graded very fine sand to medium silt layers with sharp basal contacts and rip-up clasts of mud, interpreted as turbidites. An inner fjord sill limits distribution of sediment gravity flows from the grounding zone to the deep inner fjord basin, such that sites on the inner sill and beyond the ice tongue largely only comprise plumites. Bioturbation and foraminiferal abundances increase with distance from the grounding zone. The benthic foraminiferal species, Elphidium clavatum is absent beneath the ice tongue, but dominant in the turbid meltwater influenced environment beyond the ice tongue. The very sparse IRD in sediments beneath the PIT and in the fjord beyond the PIT derives mainly from englacial debris in the ice tongue, side valley glaciers, rock falls from the steep fjord walls and sea ice. We use the modern ice shelf sediment facies characteristics to infer the past presence of ice shelves in northern Nares Strait using analyses of sediment cores from several cruises (OD1507, HLY03, 2001LSSL, RYDER19). On bathymetric highs, bioturbated mud with dispersed IRD overlies a 10–15 m thick, distinctly laminated silt and clay unit with rare coarse clasts and sparse foraminifera which forms a sediment drape of nearly uniform thickness. We interpret these laminated sediments to represent glaciomarine deposition by meltwater plumes emanating from ice streams that terminated in floating ice shelves. IRD layers, shifts in sediment composition (qXRD, MS and XRF) and faunal assemblage changes in the laminated unit document periods of ice-shelf instability sometimes, but not always, coupled with grounding zone retreat. Our deglacial reconstruction, including ice shelves, begins ∼10.7 cal ka BP, with confluent ice streams grounded in Hall Basin fronted by the Robeson Channel ice shelf. Ice shelf breakup and grounding zone retreat to relatively stable grounding zones at Kennedy Channel and the mouth of Petermann Fjord was accomplished by 9.4 cal ka BP when the Hall Basin ice shelf was established. This ice shelf broke up and reformed once prior to the final break up at 8.5 to 8.4 cal ka BP marking ice stream collapse, separation of Greenland and Innuitian ice sheets, and the opening of Nares Strait for Arctic-Atlantic throughflow. The Petermann ice shelf remained in Hall Basin until the Petermann Glacier retreated from the fjord mouth ∼7.1 cal ka BP. The resilience of these northern ice streams to strong early Holocene insolation and subsurface Atlantic Water advection is attributed to their northern aspect, buttressing by narrow passages, and high ice flux from the Greenland Ice Sheet (GIS). [Display omitted] • Arctic ice shelf sediment facies from Petermann Fjord is fine-grained, laminated, and lacks IRD. • Basal debris melts-out near the grounding zone while turbid meltwater plumes deposit fines widely. • Petermann & Nares Strait ice streams terminated in floating ice shelves from 10.7 to 7.1 cal ka BP. • IRD layers in Nares Strait show times of ice-shelf loss—sometimes with grounding zone retreat. • Final Nares ice-shelf break up at 8.5 to 8.4 cal ka BP lead to opening of Nares Strait to throughflow. [ABSTRACT FROM AUTHOR]

Abstract: The discovery in 2011 of a massive phytoplankton bloom underneath first-year sea ice in the western Arctic has prompted an investigation of the spatial and temporal distribution of under-ice phytoplankton blooms. Here, we explore the satellite record from years 1998 to 2012 for evidence of under-ice blooms on the Chukchi Sea shelf. Phytoplankton blooms were categorized as under-ice blooms, probable under-ice blooms, or marginal ice zone blooms, depending on bloom timing in relation to the timing of ice retreat. Annual bloom type maps reveal that under-ice phytoplankton blooms were present in every year of the satellite record. Averaged over all years, the combination of under-ice blooms and probable under-ice blooms covered a portion of the observable study area that was 2.5-fold higher than that of marginal ice zone blooms (71.5% and 28.5%, respectively). This finding strongly contradicts the traditional view that phytoplankton in seasonally ice-covered waters bloom only after ice retreat and instead indicates that blooms are initiated whenever light and nutrient availability is sufficient for photosynthesis, a condition often reached early in the season underneath first-year sea ice on nutrient-rich continental shelves. Spatial patterns in bloom type were distinguished relative to the date of ice retreat, with probable under-ice blooms dominating the nutrient-rich western Chukchi Sea and at higher latitudes where ice retreats later, while marginal ice zone blooms were most common in the southern and eastern Chukchi Sea where ice retreats earlier. Our results suggest that under-ice phytoplankton blooms are widespread in the Chukchi Sea and had been prevalent there for more than a decade prior to their discovery in 2011. [Copyright &y& Elsevier]

Abstract: In 2011, a massive phytoplankton bloom was observed in the Chukchi Sea under first-year sea ice (FYI), an environment in which primary productivity (PP) has historically been low. In this paper, we use a 1-D biological model of the Chukchi shelf ecosystem, in conjunction with in situ chemical and physiological data, to better understand the conditions that facilitated the development of such an unprecedented bloom. In addition, to assess the effects of changing Arctic environmental conditions on net PP (NPP), we perform model runs with varying sea ice and snow thickness, timing of melt, melt ponds, and biological parameters. Results from model runs with conditions similar to 2011 indicate that first-year ice (FYI) with at least 10% melt pond coverage transmits sufficient light to support the growth of shade-adapted Arctic phytoplankton. Increasing pond fraction by 20% enhanced peak under-ice NPP by 26% and produced rates more comparable to those measured during the 2011 bloom, but there was no effect of further increasing pond fraction. One of the important consequences of large under-ice blooms is that they consume a substantial fraction of surface nutrients such that NPP is greatly diminished in the marginal ice zone (MIZ) following ice retreat, where NPP has historically been the highest. In contrast, in model runs with <10% ponds, no under-ice bloom formed, and although peak MIZ NPP increased by 18–30%, this did not result in higher total annual NPP. This suggests that under-ice blooms contribute importantly to total annual NPP. Indeed, in all runs exhibiting under-ice blooms, total annual NPP was higher than in runs with the majority of NPP based in open water. Consistent with this, in model runs where ice melted one month earlier, peak under-ice NPP decreased 30%, and annual NPP was lower as well. The only exception was the case with no sea ice in the region: a weak bloom in early May was followed by low but sustained NPP throughout the entire growth season (almost all of which occurred in deep, subsurface layers), resulting in higher total annual NPP than in cases with sea ice present. Our results also show that both ultraviolet radiation and zooplankton grazers reduce peak open water NPP but have little impact on under-ice NPP, which has important implications for the relative proportion of NPP concentrated in pelagic vs. benthic food webs. Finally, the shift in the relative amount of NPP occurring in under-ice vs. open-water environments may affect total ecosystem productivity. [Copyright &y& Elsevier]

The article focuses on the intensification of water exchange between the Shelf and the Arctic Basin in conditions of ice depletion. It mentions the importance of melting of sea ice in the Arctic Ocean on the national interest of Russia and on the development of mineral deposits on the arctic shelves. It discusses the special role played by shelf convection and cascading among the existing water exchange mechanisms in the Arctic Ocean. It says that a mechanism of dense water formation in the freezing seas was suggested as substitute to the polynya mechanism. Moreover, the so-called thermohaline catastrophe must not have such consequences for the planetary climate.