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.

We report the results of amino acid racemization (AAR) analyses of aspartic acid (Asp) and glutamic acid (Glu) in the planktic Neogloboquadrina pachyderma, and the benthic Cibicidoides wuellerstorfi, foraminifera species collected from sediment cores from the Arctic Ocean. The cores were retrieved at various deep-sea sites of the Arctic, which cover a large geographical area from the Greenland and Iceland seas (GIS) to the Alpha and Lomonosov ridges in the central Arctic Ocean. Age models for the investigated sediments were developed by multiple dating and correlation techniques, including oxygen isotope stratigraphy, magnetostratigraphy, biostratigraphy, lithostratigraphy, and cyclostratigraphy. The extent of racemization (D/L values) was determined on 95 samples (1028 subsamples) and shows a progressive increase downcore for both foraminifera species. Differences in the rates of racemization between the species were established by analysing specimens of both species from the same stratigraphic levels (n=21). Aspartic acid (Asp) and glutamic acid (Glu) racemize on average 16 ± 2 % and 23 ± 3 % faster, respectively, in C. wuellerstorfi than in N. pachyderma. The D/L values increase with sample age in nearly all cases, with a trend that follows a simple power function. Scatter around least-squares regression fits are larger for samples from the central Arctic Ocean than for those from the Nordic Seas. Calibrating the rate of racemization in C. wuellerstorfi using independently dated samples from the Greenland and Iceland seas for the past 400 ka enables estimation of sample ages from the central Arctic Ocean, where bottom water temperatures are presently relatively similar. The resulting ages are older than expected when considering the existing age models for the central Arctic Ocean cores. These results confirm that the differences are not due to taxonomic effects on AAR and further warrant a critical evaluation of existing Arctic Ocean age models. A better understanding of temperature histories at the investigated sites, and other environmental factors that may influence racemization rates in central Arctic Ocean sediments, is also needed.

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.

Continental margin sediments contain large reservoirs of methane stored as gas hydrate. Ocean warming will partly destabilize these reservoirs which may lead to the release of substantial, yet unconstrained, amounts of methane. Anaerobic oxidation of methane is the dominant biogeochemical process to reduce methane flux, estimated to consume 90% of the methane produced in marine sediments today. This process is however neglected in the current projections of seafloor methane release from gas hydrate dissociation. Here, we introduce a fully coupled oxidation module to a hydraulic-thermodynamic-geomechanical hydrate model. Our results show that for seafloor warming rates > 1 degrees C century(-1), the efficiency of anaerobic oxidation of methane in low permeability sediments is poor, reducing the seafloor methane emissions by <5%. The results imply an extremely low mitigating effect of anaerobic oxidation of methane on climate warming-induced seafloor methane emissions. Microbial anaerobic oxidation of methane may not substantially mitigate projected warming-induced emissions of methane from marine hydrate-bearing sediments, according to a coupled hydraulic-thermodynamic-geomechanical hydrate model.

The 3.3 million km2 marine ecosystem around the North Pole, defined as the Central Arctic Ocean (CAO), is a blind spot on the map of the world’s fish stocks. The CAO essentially comprises the permanently ice-covered deep basins and ridges outside the continental shelves, and is only accessible by ice-breakers. Traditional trawling for assessing fish stocks is impossible under the thick pack ice, and coherent hydroacoustic surveys are unachievable due to ice-breaking noise. Consequently, nothing is known about the existence of any pelagic fish stocks in the CAO, although juveniles of Boreogadus saida richly occur at the surface associated with the sea ice and ice-associated Arctogadus glacialis has been reported as well. We here present a first indication of a possible mesopelagic fish stock in the CAO. We had the opportunity to analyse a geophysical hydroacoustic data set with 13 time windows of usable acoustic data over a transect from 84.4 °N in the Nansen Basin, across the North Pole (90.0 °N), to 82.4 °N in the Canada Basin. We discovered a deep scattering layer (DSL), suggesting the presence of zooplankton and fish, at 300–600 m of depth in the Atlantic water layer of the CAO. Maximum possible fish abundance and biomass was very low; values of ca. 2,000 individuals km−2 and ca. 50 kg km−2 were calculated for the DSL in the North-Pole area according to a model assuming that all acoustic backscatter represents 15-cm long B. saida and/or A. glacialis. The true abundance and biomass of fish is even lower than this, but cannot be quantified from this dataset due to possible backscatter originating from pneumatophores of physonect siphonophores that are known to occur in the area. Further studies on the DSL of the CAO should include sampling and identification of the backscattering organisms. From our study we can conclude that if the central Arctic DSL contains fish, their biomass is currently too low for any sustainable fishery.