For at least the past 50 million years, the Arctic region has had a major role in regulating global climate regimes and their variations through time. In this Review, we discuss the role of the Arctic oceanic basin and its complex bathymetry in controlling ocean circulation and marine cryosphere development. The spatial distribution and depth of various seafloor features, such as ocean gateways, submarine plateaus and continental shelves, influence the pathways of ocean currents, both today and in the past. The Arctic Ocean was an enclosed basin until the Early Eocene (56–48 million years ago), when the Eurasian Basin started to form and a shallow sea connected the Arctic to the Tethys Ocean. The connections with the North Atlantic and the global ocean through shallow and deep gateways prompted the transition from a global greenhouse to icehouse climate. However, the Arctic Ocean remains underexplored, as less than one-quarter of its seafloor is mapped in detail. Future integrated geoscience research, modern bathymetric mapping technology and active international programmes are needed to close these data gaps.

The northern sector of the Greenland Ice Sheet contains some of the ice sheet's last remaining glaciers with floating ice tongues. One of these glaciers is Ryder Glacier, which has been relatively stable in recent decades, in contrast to the neighbouring Petermann and C.H. Ostenfeld glaciers. Understanding Ryder Glacier's future behaviour is important as ice-tongue loss could lead to acceleration and increased ice discharge. Meanwhile, it is unclear whether Greenland-wide modelling attempts are able to accurately resolve the influence of fjord and bedrock topography and small-scale variations in ice dynamics for a glacier like Ryder. To fill these gaps, here we conduct targeted high-resolution modelling of Ryder Glacier until the year 2300. We find that mass loss is dominated by discharge under a low-emissions scenario all the way to 2300, leading to a sea level contribution of between 0.8 and 2 mm depending on the amount of ocean warming. Discharge also plays a key role under a high-emissions scenario up until 2100, after which a strongly negative surface mass balance becomes the dominant driver of mass loss. This negative surface mass balance leads to a much higher sea level rise contribution by 2300 of between 44 and 52 mm, with little sensitivity to the range of ocean warming scenarios used in this study.

The Greenland Ice Sheet's negative mass balance is driven by a sensitivity to a warming atmosphere and ocean. The fidelity of ice-sheet models in accounting for ice–ocean interaction is inherently uncertain and often constrained against recent fluctuations in the ice-sheet margin from the previous decades. The geological record can be used to contextualise ice-sheet mass loss and understand the drivers of changes at the marine margin across climatic shifts and previous extended warm periods, aiding our understanding of future ice-sheet behaviour. Here, we use the Ice-sheet and Sea-level System Model (ISSM) to explore the Holocene evolution of Ryder Glacier draining into Sherard Osborn Fjord, North Greenland. Our modelling results are constrained with terrestrial reconstructions of the paleo-ice-sheet margin and an extensive marine sediment record from Sherard Osborn Fjord that details ice dynamics over the past 12.5 ka years. By employing a consistent mesh resolution of <1 km at the ice–ocean boundary, we assess the importance of atmospheric and oceanic changes to Ryder Glacier's Holocene behaviour. Our simulations show that the initial retreat of the ice margin after the Younger Dryas cold period was driven by a warming climate and the resulting fluctuations in surface mass balance. Changing atmospheric conditions remain the first-order control in the timing of ice retreat during the Holocene. We find ice–ocean interactions become increasingly fundamental to Ryder's retreat in the mid-Holocene, with higher-than-contemporary melt rates required to force grounding line retreat and capture the collapse of the ice tongue during the Holocene Thermal Maximum. Regrowth of the tongue during the neoglacial cooling of the late Holocene is necessary to advance the terrestrial and marine margins of the glacier. Our results stress the importance of accurately resolving the ice–ocean interface in modelling efforts over centennial and millennial timescales, in particular the role of floating ice tongues and submarine melt, and provide vital analogies for the future evolution of Ryder in a warming climate.

Arctic sea ice affects Earth’s albedo, marine productivity and organic matter (OM) transport. Lipid biomarkers have been used to trace OM transport in the Arctic Ocean, but uncertainties remain regarding their spatio-temporal variations and sources over the last glacial cycle. Our study addresses these gaps by analyzing glycerol dialkyl glycerol tetraethers (GDGTs), n-alkanes, and total organic carbon (TOC) in nine central Arctic sediment cores spanning the Marine Isotope Stages (MISs) 3–1. Elevated IIIa/IIa values of branched GDGTs (brGDGTs) in the central Arctic throughout the studied interval suggest a marine origin, contrasting to the #rings_tetra ratios which indicate a terrigenous brGDGT source. We propose that the IIIa/IIa ratio may be a more sensitive indicator of in situ brGDGT production in the central Arctic marine sediments. TOC and biomarker concentrations in the Central Lomonosov Ridge (CLR) cores were higher compared to those from the Lomonosov Ridge off Greenland (LRG) and Morris Jesup Rise (MJR) cores. Low productivity in the central Arctic, along with similarity in the spatial patterns of marine-derived brGDGTs and isoprenoid GDGTs, as well as terrestrial long-chain n-alkanes, suggests that these biomarkers are primarily transported to the central Arctic from the Siberian shelves. This spatial pattern persisted throughout MISs 3–1, suggesting continued sea ice drift during glacial periods, albeit with weakened intensities. Meanwhile, the spatiotemporal variations of the Branched Isoprenoid Tetraether (BIT) index in the region plausibly reflect the relative changes in the crenarchaeol and brGDGT production on the shelf and/or selective degradation of crenarchaeol during its transport.

We investigated the relative contributions of various factors that influence seasonal changes in sea surface partial pressure of CO₂ (pCO₂, calculated from the measured pH and total alkalinity) in four regions of northwestern Greenland: Nares Strait, Lincoln Sea, Sherard Osborn and Petermann fjords. Using the temperature minimum layer as a proxy for winter conditions, we examined pCO₂ dynamics from the onset of sea-ice melt to summer. Our findings revealed significant spatial variability in pCO₂, driven by differences in temperature, freshwater inputs, and biological activity. In particular, in Sherard Osborn Fjord substantial freshwater inputs and strong stratification were found to enhance pCO₂ accumulation, while in Petermann Fjord biological CO₂ uptake was the main driver. This study, conducted in summer 2019, underscores the critical role of northwest Greenland’s coastal waters as a summer CO₂ sink. It highlights the complex interplay of physical and biogeochemical processes in modulating pCO₂, suggesting significant regional differences in CO₂ dynamics between two neighboring fjords.

Our limited knowledge of the marine mammal fauna in northernmost Greenland and Canada, specifically north of 80°N, relies largely on opportunistic observations collected during expeditions with different objectives. The narwhal (Monodon monoceros) migrates long distances in response to ice formation and decay and is notoriously skittish, avoiding areas with ice breakers. Scattered observations from the past 20 years, assessed together with historical observations after 1881, suggest that there is a population of narwhals that uses Hall Basin and its adjacent fjord systems—for example, Nares Strait—as a summer ground. Dating the tusks and bones that have been found shows that narwhals were present in this area as far back as nearly 7000 years ago. The wintering locations of these narwhals remain unknown, highlighting the need to investigate whether they are vulnerable to hunting activities in north-west Greenland. By gaining a better understanding of the narwhals’ winter behaviour and potential hunting risks, we can develop more informed conservation and management strategies for this population.

This study aims to investigate and disentangle the impact of land use and climate variability on the Baltic Sea coastal zone from a millennial perspective. To assess the environmental status of the coastal zone we make use of siliceous microfossils (mainly diatoms), stable nitrogen and carbon isotopes, organic carbon accumulation rates, and lithological changes analyzed in a sediment core collected in Gamlebyviken, Swedish east coast, dated to cover the last 3000 years. Changes in land use and vegetation cover are modelled using pollen stratigraphical data to obtain the percentage coverage of coniferous woodland (Pinus and Picea), deciduous woodland, wetland (Cyperaceae), grassland (including Juniperus) and cropland (cereals) while changes in climatic conditions are assessed through well-documented climatic periods that have occurred in the Baltic Sea region.

The reconstructed regional vegetation cover shows that already 3000 years ago, humans used the landscape for both animal husbandry (grasslands) and farming (cropland), but the impact on the Baltic coastal waters was minor. The diatom accumulation rates were quite high (∼3100–2600 cal yr BP) containing taxa indicative of high nutrient conditions/upwelling, and stable carbon isotopes show that the carbon was produced in the basin but did not result in elevated organic carbon accumulation rates. A gradual change to less marine conditions in Gamlebyviken from about 2500 to 1400 cal yr BP can be attributed to the ongoing land uplift which resulted in a more enclosed embayment with only a narrow inlet area today.

The Medieval Climate Anomaly (1000–700 cal yr BP/950–1250 CE) is a time where extensive eutrophication is registered in the open Baltic Sea, but afforestation is recorded between 1000 and 500 cal yr BP and attributed to the expansion of spruce favored by land-use reorganization with a transition from a one-course rotation system to the three-course rotation system fully established in southern Sweden in the 13th century, and only minor environmental change is recorded in the coastal zone.

The Little Ice Age is documented in our data between 400 and 250 cal yr BP/1550–1700 CE as a decrease in regional cropland (cereals) cover, possibly indicating years of poor crop harvest, and changes in the Baltic coastal zone are evidenced as low carbon and diatom accumulation rates, increase in benthic diatom taxa (low turbidity), and high abundance in diatom taxa associated with sea ice indicating a cold climate.

The most significant changes occurred from about 100 cal yr BP/1850 CE up to present, with a maximum regional cover of grassland and cropland (ca. 35%) at the expense of deciduous woodland, and major changes indicative of a highly eutrophic environment recorded in the coastal zone. Organic carbon accumulation rates peaked in 1968 CE at approximately 134 g C m² yr⁻¹ before subsequently declining to present-day values of 53 g C m² yr⁻¹, mirroring a similar trend observed in diatom accumulation rates. The high organic carbon accumulation rate shows that deep unvegetated accumulation bottoms in the coastal Baltic Sea serve as carbon sinks and are worth exploring for their potential in mitigating climate change.

Variation partitioning shows that 26% of the variance in the diatom assemblages is associated with land use changes. The variables grassland, cropland, and stable nitrogen isotopes are accordingly strong predictors of environmental change in the Baltic coastal zone as reflected by the diatom assemblages.

This study investigates six areas in a historically heavily trawled region of the southern Baltic Sea. Using acoustic geophysical mapping data and sediment cores from three field campaigns (2019, 2020, 2023), we evaluate and quantify the cumulative physical impacts from bottom trawling and the influence of seabed geology on mapped trawl tracks. The results are compared with fishing intensity data over three periods; 2012–2016, 2017–2019 and after the fishery closed. A correlation between fishing intensity and density of mapped trawl tracks exists in the soft sediments of the northern part of the area, while this link is weak in the less trawled southern part, where the seabed is characterized by more consolidated glacial clays and the high density of mapped trawl tracks reflects the preservation of tracks >8 years old. Four years after the closure of the fishery there were no signs of trawl-track degradation in any of the areas. In summary, mapped track densities alone are not a suitable measure of trawling intensity, considering the influence of seabed geology and the persistence of trawl tracks over time. Sediment deformation, observed by CT-scanning, indicates extensive remoulding and coarsening of the upper 20–40 cm of sediments in the trawled areas.

Knowledge about seafloor depth, or bathymetry, is crucial for various marine activities, including scientific research, offshore industry, safety of navigation, and ocean exploration. Mapping the central Arctic Ocean is challenging due to the presence of perennial sea ice, which limits data collection to icebreakers, submarines, and drifting ice stations. The International Bathymetric Chart of the Arctic Ocean (IBCAO) was initiated in 1997 with the goal of updating the Arctic Ocean bathymetric portrayal. The project team has since released four versions, each improving resolution and accuracy. Here, we present IBCAO Version 5.0, which offers a resolution four times as high as Version 4.0, with 100 × 100 m grid cells compared to 200 × 200 m. Over 25% of the Arctic Ocean is now mapped with individual depth soundings, based on a criterion that considers water depth. Version 5.0 also represents significant advancements in data compilation and computing techniques. Despite these improvements, challenges such as sea-ice cover and political dynamics still hinder comprehensive mapping.

The extent and seasonality of Arctic sea ice during the Last Interglacial (129,000 to 115,000 years before present) is poorly known. Sediment-based reconstructions have suggested extensive ice cover in summer, while climate model outputs indicate year-round conditions in the Arctic Ocean ranging from ice free to fully ice covered. Here we use microfossil records from across the central Arctic Ocean to show that sea-ice extent was substantially reduced and summers were probably ice free. The evidence comes from high abundances of the subpolar planktic foraminifera Turborotalita quinqueloba in five newly analysed cores. The northern occurrence of this species is incompatible with perennial sea ice, which would be associated with a thick, low-salinity surface water. Instead, T. quinqueloba's ecological preference implies largely ice-free surface waters with seasonally elevated levels of primary productivity. In the modern ocean, this species thrives in the Fram Strait-Barents Sea 'Arctic-Atlantic gateway' region, implying that the necessary Atlantic Ocean-sourced water masses shoaled towards the surface during the Last Interglacial. This process reflects the ongoing Atlantification of the Arctic Ocean, currently restricted to the Eurasian Basin. Our results establish the Last Interglacial as a prime analogue for studying a seasonally ice-free Arctic Ocean, expected to occur this century. The warm Last Interglacial led to a seasonally ice-free Arctic Ocean and a transformation to Atlantic conditions, according to planktic foraminifera records from central Arctic Ocean sediment cores.