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Declining sea ice is expected to change the Arctic's physical and biological systems in ways that are difficult to predict. This study used stable isotope compositions (δ13C and δ15N) of archaeological, historic, and modern Pacific walrus (Odobenus rosmarus divergens) bone collagen to investigate the impacts of changing sea ice conditions on walrus diet during the last ~4000 yr. An index of past sea ice conditions was generated using dinocyst-based reconstructions from three locations in the northeastern Chukchi Sea. Archaeological walrus samples were assigned to intervals of high and low sea ice, and δ13C and δ15N were compared across ice states. Mean δ13C and δ15N values were similar for archaeological walruses from intervals of high and low sea ice; however, variability among walruses was greater during low-ice intervals, possibly indicating decreased availability of preferred prey. Overall, sea ice conditions were not a primary driver of changes in walrus diet. The diet of modern walruses was not consistent with archaeological low sea ice intervals. Rather, the low average trophic position of modern walruses (primarily driven by males), with little variability among individuals, suggests that trophic changes to this Arctic ecosystem are still underway or are unprecedented in the last ~4000 yr. [ABSTRACT FROM AUTHOR]

The impact of changes in volume, heat and freshwater fluxes through Arctic gateways on sea ice, circulation and fresh water and heat contents of the Arctic and North Atlantic Oceans is not fully understood. To explore the role played by each gateway, we use a regional sea-ice ocean general circulation model with a fixed atmospheric forcing. We run sensitivity simulations with combinations of Bering Strait (BS) and Canadian Arctic Archipelago (CAA) open and closed inspired by paleogeography of the Arctic. We show that fluxes through BS influence the Arctic, Atlantic and Nordic Seas while the impact of the CAA is more dominant in the Nordic Seas. In the experiments with BS closed, there is a change in the surface circulation of the Arctic with a weakening of the Beaufort Gyre by about thirty percent. As a consequence, the Siberian river discharge is spread offshore to the west, rather than being directly advected away by the Transpolar Drift. This results in a decrease of salinity in the upper 50 m across much of the central Arctic and East Siberian and Chukchi Seas. We also find an increase in stratification between the surface and subsurface layers after closure of BS. Moreover, closure of the BS results in an upward shift of the relatively warm waters lying between 50 and 120 m, as well as a reorganization of heat storage and transport. Consequently, more heat is kept in the upper layers of the Arctic Ocean, thus increasing the heat content in the upper 50 m and leading to a thinner sea ice cover. The CAA closing has a large impact on sea ice, temperature and salinity in the subarctic North Atlantic with opposite responses in the Greenland-Iceland-Norwegian Seas and Baffin Bay. It is also found that CAA being open or closed strongly controls the sea ice export through the Fram Strait. In all our experiments, the changes in temperature and salinity of the Barents and Kara Seas, and in fluxes through Barents Sea Opening are relatively small, suggesting that they are likely controlled by the atmospheric processes. Our results demonstrate the need to take into consideration the fluxes through the Arctic gateways when addressing the ocean and climate changes during deglaciations as well as for predictions of future climate. [ABSTRACT FROM AUTHOR]

The modeling of paleoclimate, using physically based tools, is increasingly seen as a strong out-of-sample test of the models that are used for the projection of future climate changes. New to the Coupled Model Intercomparison Project (CMIP6) is the Tier 1 Last Interglacial experiment for 127 000 years ago (lig127k), designed to address the climate responses to stronger orbital forcing than the midHolocene experiment, using the same state-of-the-art models as for the future and following a common experimental protocol. Here we present a first analysis of a multi-model ensemble of 17 climate models, all of which have completed the CMIP6 DECK (Diagnostic, Evaluation and Characterization of Klima) experiments. The equilibrium climate sensitivity (ECS) of these models varies from 1.8 to 5.6 ∘ C. The seasonal character of the insolation anomalies results in strong summer warming over the Northern Hemisphere continents in the lig127k ensemble as compared to the CMIP6 piControl and much-reduced minimum sea ice in the Arctic. The multi-model results indicate enhanced summer monsoonal precipitation in the Northern Hemisphere and reductions in the Southern Hemisphere. These responses are greater in the lig127k than the CMIP6 midHolocene simulations as expected from the larger insolation anomalies at 127 than 6 ka. New synthesis for surface temperature and precipitation, targeted for 127 ka, have been developed for comparison to the multi-model ensemble. The lig127k model ensemble and data reconstructions are in good agreement for summer temperature anomalies over Canada, Scandinavia, and the North Atlantic and for precipitation over the Northern Hemisphere continents. The model–data comparisons and mismatches point to further study of the sensitivity of the simulations to uncertainties in the boundary conditions and of the uncertainties and sparse coverage in current proxy reconstructions. The CMIP6–Paleoclimate Modeling Intercomparison Project (PMIP4) lig127k simulations, in combination with the proxy record, improve our confidence in future projections of monsoons, surface temperature, and Arctic sea ice, thus providing a key target for model evaluation and optimization. [ABSTRACT FROM AUTHOR]

Continuous and multi-decadal records of faunal abundance and diversity helping to identify the impacts of ongoing global warming on aquatic ecosystems are rare in the coastal Arctic. Here, we used a 50-year-long microfaunal record from a sediment core collected in the Herschel Basin (YC18-HB-GC01; 18 m water depth) to document some aspects of the environmental responses of the southern Canadian coastal Beaufort Sea to climate change. The microfaunal indicators include benthic foraminiferal assemblages, ostracods and tintinnids. The carbonate shells of two foraminiferal species were also analyzed for their stable isotope signatures (δ13C and δ18O). We compiled environmental parameters from 1970 to 2019 for the coastal region, including sea ice data (break-up date, freeze-up date, open season length and mean summer concentration), the wind regime (mean speed, direction of strong winds and the number of storms), hydrological data (freshet date, freshet discharge and mean summer discharge of the Firth and the Mackenzie rivers), and air temperature. Large-scale atmospheric patterns were also taken into consideration. Time-constrained hierarchal clustering analysis of foraminiferal assemblages and environmental parameters revealed a near-synchronous shift around the late 1990s. The microfaunal shift corresponds to an increased abundance of taxa tolerant to variable salinity, turbulent bottom water conditions, and turbid waters towards the present. The same time interval is marked by stronger easterly winds, more frequent storms, reduced sea-ice cover, and a pervasive anticyclonic circulation in the Arctic Ocean (positive Arctic Ocean Oscillation; AOO+). Deeper vertical mixing in the water column in response to intensified winds was fostered by increased open surface waters in summer leading to turbulence, increased particle loading and less saline bottom waters at the study site. Stronger easterly winds probably also resulted in enhanced resuspension events and coastal erosion in addition to a westward spreading of the Mackenzie River plume, altogether contributing to high particulate-matter transport. Increase food availability since ∼2000 was probably linked to enhanced degradation of terrestrial organic carbon, which also implies higher oxygen consumption. The sensitivity of microfaunal communities to environmental variations allowed capturing consequences of climate change on a marine Arctic shelf ecosystem over the last 50 years. [Display omitted] • We identified local effects of recent climate change on Arctic nearshore habitats. • Ecosystem changes occurred on the Canadian Beaufort Sea shelf in the late 1990s. • Microfaunal and environmental changes are linked to enhanced easterly winds. • Variable salinity and enhanced vertical mixing suggest high turbulence after ∼2000. • Taxa tolerating turbid waters/poor food quality relate to enhanced terrestrial input. [ABSTRACT FROM AUTHOR]

Arctic coastal zones serve as a sensitive filter for terrigenous matter input onto the shelves via river discharge and coastal erosion. This material is further distributed across the Arctic by ocean currents and sea ice. The coastal regions are particularly vulnerable to changes related to recent climate change. We compiled a pan-Arctic review that looks into the changing Holocene sources, transport processes and sinks of terrigenous sediment in the Arctic Ocean. Existing palaeoceanographic studies demonstrate how climate warming and the disappearance of ice sheets during the early Holocene initiated eustatic sea-level rise that greatly modified the physiography of the Arctic Ocean. Sedimentation rates over the shelves and slopes were much greater during periods of rapid sea-level rise in the early and middle Holocene, as a result of the relative distance to the terrestrial sediment sources. However, estimates of suspended sediment delivery through major Arctic rivers do not indicate enhanced delivery during this time, which suggests enhanced rates of coastal erosion. The increased supply of terrigenous material to the outer shelves and deep Arctic Ocean in the early and middle Holocene might serve as analogous to forecast changes in the future Arctic. [ABSTRACT FROM AUTHOR]

Arctic sea ice extent is now more than two million square kilometres less than it was in the late twentieth century, with important consequences for the climate, the ocean and traditional lifestyles in the Arctic. Although observations show a more or less continuous decline for the past four or five decades, there are few long-term records with which to assess natural sea ice variability. Until now, the question of whether or not current trends are potentially anomalous has therefore remained unanswerable. Here we use a network of high-resolution terrestrial proxies from the circum-Arctic region to reconstruct past extents of summer sea ice, and show that-although extensive uncertainties remain, especially before the sixteenth century-both the duration and magnitude of the current decline in sea ice seem to be unprecedented for the past 1,450 years. Enhanced advection of warm Atlantic water to the Arctic seems to be the main factor driving the decline of sea ice extent on multidecadal timescales, and may result from nonlinear feedbacks between sea ice and the Atlantic meridional overturning circulation. These results reinforce the assertion that sea ice is an active component of Arctic climate variability and that the recent decrease in summer Arctic sea ice is consistent with anthropogenically forced warming. [ABSTRACT FROM AUTHOR]

Dinoflagellate cyst (dinocyst) assemblages were analyzed in 43 surface sediment samples from the Barents Sea. They can be divided into five major assemblage types, the distribution of which can clearly be linked to the overlying water masses. Notably, a very clear distinction between sites influenced by Atlantic water and Arctic water, respectively, is seen in the change in dominance from O. centrocarpum s.l. to I. minutum and is strongly related to sea-surface temperature. More subtle hydrographical features are also recorded in the assemblages, as can be seen from the spatially coherent distribution of the Atlantic assemblages, which allows discrimination between the Norwegian Coastal Current zone, the Norwegian Atlantic Current zone and two regions of modified Norwegian Atlantic Current waters. Here sea-surface temperature does not seem to be the primary parameter controlling the distribution of the assemblages. Multivariate analyses suggest that the assemblages are associated with the stratification and productivity annual cycles. Cysts of P. dalei seem to favour stratified environments, while O. centrocarpum s.l. might be more adapted to unstable conditions. Cysts of P. dalei are also associated with early spring stratification and productivity, whereas other species such as S. ramosus are related to late spring/summer productivity and stratification. [ABSTRACT FROM AUTHOR]

Cores from site HLY0501-05 on the Alaskan margin in the eastern Chukchi Sea were analyzed for their geochemical (organic carbon, δ13Corg, Corg/N, and CaCO3) and palynological (dinocyst, pollen, and spores) content to document oceanographic changes during the Holocene. The chronology of the cores was established from 210Pb dating of near-surface sediments and 14C dating of bivalve shells. The sediments span the last 9000 years, possibly more, but with a gap between the base of the trigger core and top of the piston core. Sedimentation rates are very high (~156 cm/ka), allowing analyses with a decadal to centennial resolution. The data suggest a shift from a dominantly terrigenous to marine input from the early to late Holocene. Dinocyst assemblages are characterized by relatively high concentrations (600–7200 cysts/cm3) and high species diversity, allowing the use of the modern analogue technique for the reconstruction of sea-ice cover, summer temperature, and salinity. Results indicate a decrease in sea-ice cover and a corresponding, albeit much smaller, increase in summer sea-surface temperature over the past 9000 years. Superimposed on these long-term trends are millennial-scale fluctuations characterized by periods of low sea-ice and high sea-surface temperature and salinity that appear quasi-cyclic with a frequency of about one every 2500–3000 years. The results of this study clearly show that sea-ice cover in the western Arctic Ocean has varied throughout the Holocene. More importantly, there have been times when sea-ice cover was less extensive than at the end of the 20th century. Des carottes provenant du site HLY0501-05 sur la marge de l’Alaska dans la partie est de la mer des Tchouktches ont été analysées pour leur contenu géochimique (carbone organique, δ13Corg, Corg/N, CaCO3) et palynologique (dinokystes, pollen, spores) afin de documenter les changements océanographiques survenus au cours de l’Holocène. La chronologie des carottes a été déterminée à partir d’une datation 210Pb des sédiments près de la surface et d’une datation 14C de coquilles de bivalves. Les sédiments couvrent les derniers 9000 ans, possiblement plus, mais avec une lacune entre la base de la carotte prélevée par carottier à clapet et le haut de la carotte prélevée par carottier à piston. Les taux de sédimentation sont très élevés (~156 cm/ka), permettant des analyses avec une résolution variant de décennale à centennale. Les données suggèrent un approvisionnement qui a passé de surtout terrigène à marin entre l’Holocène précoce et tardif. Les assemblages de sont caractérisés par des concentrations relativement hautes de dinokystes (600–7200 par cm3) et une diversité élevée permettant l’utilisation d’une technique analogique moderne pour la reconstruction de la couverture des glaces de mer, des températures estivales et de la salinité. Les résultats indiquent une diminution de la couverture des glaces de mer et une augmentation, quoique bien moindre, de la température estivale de la surface de la mer au cours des 9000 dernières années. À ces tendances à long terme se superposent des fluctuations à l’échelle millénaire, caractérisées par des périodes de mince glace de mer et de températures élevées de la surface de la mer et une forte salinité qui semblent quasi cycliques, avec une période de récurrence d’une fois tous les 2500 à 3000 ans. Les résultats de cette étude démontrent clairement que la couverture des glaces de mer dans l’océan Arctique occidental a varié tout au long de l’Holocène. De manière encore plus significative, il y a eu des périodes où la couverture des glaces de mer était moins étendue qu’à la fin du 20e siècle. [ABSTRACT FROM AUTHOR]

Surface sediment samples (n = 10), collected between Spitzbergen and Greenland, and two cores raised east (C04) and west (C16) from Fram Strait were analyzed for their chemical and isotopic (Pb) compositions to trace the source of sediments and water masses exchanging between the Arctic and the North Atlantic oceans. In surface sediments, variable major and trace element concentrations suggest variations in both the mineralogy (carbonate and quartz dilution of other silicate minerals) and source regions of detrital supplies, based on Th/Zr and, to a lesser extent, on Th/U ratios. Each core site shows specific but nearly constant Th/Zr ratios, indicating homogeneous source supplies. At both core sites, Pb concentrations and isotopic compositions display similar patterns: homogeneous low Pb and radiogenic crustal signals below 5–10 cm, contrasting with high Pb and less radiogenic anthropogenic inputs at core-tops. However, the differing pre-anthropogenic Pb isotopic ratios in C04 and C16 confirm the involvement of distinct source supplies east and west of Fram Strait. We suggest that this isotopic specificity is mainly owing to inputs of material carried from northwestern Europe by the North Atlantic water mass and from the Laptev Sea by the Transpolar Drift, respectively. Some material from the Greenland margin and possibly from the North Atlantic Ocean may reach this zone as well. Sediments from the western Arctic are not significantly transported into the Fram Strait area, suggesting that the Canadian and the Eurasian basins remained decoupled, at least during the time span of the cored sediments (~2000 years). Des échantillons de sédiments de surface (n = 10), recueillis entre Spitzbergen et le Groenland, et deux carottes prélevées à l’est (C04) et à l’ouest (C16) du détroit de Fram, ont été analysés pour connaître leur composition chimique et isotopique (Pb) dans le but afin de retracer la source des sédiments et des masses d’eau s’échangeant entre les océans Arctique et Atlantique Nord. Dans les sédiments de surface, des concentrations variables d’éléments majeurs et traces suggèrent à la fois des variations de la minéralogie (dilution d’autres minéraux silicatés par les carbonates et le quartz) et des régions sources des apports détritiques, basées sur les rapports Th/Zr et, à un degré moindre sur les rapports Th/U. Chaque site de carottage présente des rapports Th/Zr spécifiques mais presque constants, indiquant une source d’apports homogène. Aux deux sites de carottage, les concentrations en Pb et les compositions isotopiques montrent des patrons semblables : un faible signal homogène Pb et un signal radiogénique de la croûte à des profondeurs de plus de 5–10 cm, ce qui contraste avec les données anthropiques, moins radiogéniques et à Pb élevé au sommet de la carotte. Toutefois, les rapports isotopiques pré-anthropiques différents du Pb dans les carottes C04 et C16 confirment l’implication de deux sources distinctes d’apports à l’est et à l’ouest du détroit de Fram. Cette spécificité isotopique découlerait principalement d’apports de matériel provenant respectivement du nord-ouest de l’Europe par la masse d’eau de l’Atlantique Nord et de la mer de Laptev par la dérive transpolaire. Du matériel de la bordure du Groenland et possiblement de l’océan Atlantique Nord aurait pu aussi atteindre cette zone. Des sédiments de l’Ouest de l’Arctique ne sont pas transportés de manière significative dans le détroit de Fram, suggérant que les bassins canadiens et eurasiens soient demeurés découplés, au moins durant la durée de temps représentée par la carotte de sédiments (~2000 ans). [ABSTRACT FROM AUTHOR]

Dinocyst assemblages and the physical properties of two sediment cores collected in the easternmost part of the main axis of the Northwest Passage, Canadian Arctic Ocean (cores 2004-804-009 BC and 2004-804-009 PC, 74°11.2′N, 81°11.7′W) were used to reconstruct changes in sea-surface conditions and to characterize changes in the depositional environment. Core 2004-804-009 PC spans the last 12 180 calibrated (cal) years BP, with sedimentation rates ranging from 45 to 122 cm/ka. Quantitative estimates of sea-surface parameters reveal relatively large hydrographic variability at millennial time scale. Before 11 000 cal years BP, our records suggest terrigenous inputs related to the last deglaciation. Between 11 000 and 9600 cal years BP, harsh conditions prevailed with August sea-surface temperatures <2 °C and the dominance of heterotrophic taxa. This episode was followed by a gradual increase in the relative abundance of phototrophic taxa and the establishment of milder condition with sea-surface temperature (SST) reaching ~2 °C ~8300 cal years BP, possibly related to increased exchange between the Arctic Ocean and the North Atlantic Ocean. From 6000 cal years BP to the late Holocene, climate variability could be the results of changes in the synoptic-scale atmospheric pattern such as the Arctic oscillation. Les assemblages de dinokystes et les propriétés physiques de deux carottes de sédiments (carottes 2004-804-009 BC et 2004-804-009 PC, 74°11,2′ N, 81°11,7′ O) prélevées dans la partie la plus à l’est de l’axe principal du passage du Nord-Ouest, dans l’Arctique canadien, ont été utilisés pour reconstruire les changements dans les conditions de la surface de la mer et caractériser les changements dans l’environnement de déposition. La carotte 2004-804-009 PC couvre les dernières 12 180 années calendaires avant le présent (années cal. BP) avec des taux de sédimentation qui varient entre 45 et 122 cm/ka. Des estimations quantitatives des paramètres de la surface de la mer révèlent une variabilité hydrographique relativement grande sur une échelle millénaire. Avant 11 000 années cal. BP, nos données suggèrent des intrants terrigènes liés à dernière déglaciation. Entre 11 000 et 9600 années cal. BP, des conditions difficiles prévalaient avec des températures de la surface de la mer en août inférieures à 2  °C et une dominance de taxons hétérotrophes. Cet épisode a été suivi d’une augmentation graduelle de l’abondance relative de taxons phototrophes et l’établissement de conditions plus douces avec des températures de la surface de la mer qui atteignaient environ 2  °C vers 8300 années cal. BP, ce qui est possiblement relié à un échange accru entre l’Arctique et l’océan Atlantique Nord. De 6000 années cal. BP à l’Holocène tardif, la variabilité du climat pourrait découler de changements dans les patrons atmosphériques à l’échelle synoptique, tels que l’oscillation arctique. [ABSTRACT FROM AUTHOR]

Abstract: Proxy evidence suggests that a mean atmospheric state, reminiscent of the positive phase of the North Atlantic/Arctic Oscillation (NAO), persisted throughout the early Holocene and resulted in a dipole pattern in sea-ice concentration between the north-eastern and north-western North Atlantic. A dynamic thermodynamic coupled sea-ice–ocean model is used to simulate the sea-ice concentration and thickness in the Arctic during the early Holocene. It is forced with winds, ocean currents and surface air temperatures (SAT) from recent years with a positive phase of the NAO in conjunction with altered long and shortwave radiation and surface air/ocean temperatures. The simulation reproduces an east/west dipole in sea-ice cover of the Arctic and is compatible with reconstructed sea-ice conditions in the Chukchi Sea (inferred from dinoflagellate cyst analysis of ocean sediment cores). Sensitivity studies were performed to investigate the individual effects of radiation, ocean forcing, SAT and winds on sea-ice cover. Results show that in the East Siberian Sea, SAT is the dominant forcing for changes in sea-ice thickness, whereas winds and SAT are the dominant factors in sea-ice concentration anomalies. In the Barents Sea, sea-ice anomalies are influenced by air and sea surface temperatures, and ocean currents. [ABSTRACT FROM AUTHOR]