Marine geological and geophysical data from Alpha Ridge in the Arctic Ocean are sparse because of thick perennial sea-ice cover, which prevents access by most surface vessels. Rare seismic data in this area, acquired largely from drifting ice-camps, had shown the hemipelagic drape that covers most of the ridge is highly disrupted within a large (> 90 000 km²) south central region. Here, evidence of pronounced seafloor erosion and debris flows infilling seafloor lows was previously interpreted to be the result of a possible bolide impact. In recent years, several icebreaker expeditions have successfully acquired multibeam bathymetry and sub-bottom profiler data in the western segment of this region. Analysis of these data reveals a complex seafloor morphology characterized by ridges and troughs, angular blocks and escarpments as well as seismic facies characterized by hyperbolic seafloor reflections, and convoluted to incoherent and transparent sub-bottom reflectivity. These features are interpreted as evidence of sediment mass movement with varying degrees of lateral transport deformation. At least two episodes of failure are interpreted based on the presence of both buried and surficial mass-transport features. As multiple events are interpreted, seismicity is the most plausible trigger mechanism rather than bolide impact.

The Marvin Spur is a 450-km-long east–west trending escarpment along the northernmost periphery of the Alpha Ridge, starting about 500 km from the coasts of Ellesmere Island and Greenland off the Arctic Ocean margin of North America and running subparallel to the Amerasian margin of the continental Lomonosov Ridge. This region was investigated as part of the Canada-Sweden Polar Expedition in 2016, from which two seismic profiles are presented. The first is a 165-km-long line along the crest of the Marvin Spur. The second is a 221-km-long line extending southwestward from the spur to the northern flank of the Alpha Ridge within the Cretaceous High Arctic Large Igneous Province (HALIP). Multichannel seismic reflection data were acquired along both lines using a 100-m-long streamer, and the airgun shots were also recorded using 16 sonobuoys and 5 stations on the sea ice to calculate a velocity model for the crust from forward modelling of seismic travel times. The Marvin Spur profile reveals up to 1100 m of sedimentary rocks on top of a 1-km-thick series of basalts (4.5–5.1 km s⁻¹). Upper and lower crust have velocities of 5.8–5.9 km s⁻¹ and 6.2–6.3 km s⁻¹, respectively, with the upper crust being 1–2 km thick compared to around 13 km for the lower crust. A wide-angle double seismic reflection manifests the top and base of a 6-km-thick lower crustal layer that we interpret as magmatic underplating beneath the continental crust of the Marvin Spur. We correlate a high-amplitude magnetic anomaly on Marvin Spur with a comparable anomaly on Lomonosov Ridge by invoking 110 km of dextral strike-slip motion. Assuming that HALIP-related magmatic deposits generate these anomalies, the strike-slip motion pre-dates the main phase of magmatism (latest Cretaceous, 78 Ma). On the northern Alpha Ridge, sediments are around 1-km-thick and cover a 700 to 1700-m-thick series of basalts with velocities of 4.4 to 4.8 km s⁻¹. Below is a 3-km-thick layer with intermediate velocities of 5.6 km s⁻¹ and a lower crust with a velocity of 6.8 km s⁻¹. Moho depth is not resolved seismically, but gravity modelling indicates a total thickness of 13 or 18 km for the igneous crust except for the Fedotov Seamount where Moho deepens by about 5 km. Construction of the seamount occurred in multiple magmatic phases, including flow eruptions during deposition of the Cenozoic sedimentary succession post-dating the main HALIP magmatism.

The continental Lomonosov Ridge spans across the Arctic Ocean and was the subject of a geophysical study in 2016 with two seismic reflection lines crossing the ridge in proximity to the North Pole, one of which continues across the continent–ocean transition zone into the Amundsen Basin. One seismic station and 15 sonobuoys were deployed along these two lines to record seismic wide-angle reflections and refractions for development of a crustal-scale velocity model. Its viability is checked using gravity data from the experiment which are also used to interpolate crustal structure in areas with poor seismic constraints. On the line extending into the Amundsen Basin, continental crust composed of two layers with velocities of 6.0 and 6.5 km s^–1 is encountered beneath the Lomonosov Ridge where the Moho depth is 21 km based on gravity modelling. The crust is overlain by a 1-km-thick layer with velocities of 4.7 km s^–1 coinciding with a zone of positive magnetic anomalies of up to 180 nT. This layer is interpreted to include extrusive volcanic rocks related to the Cretaceous High Arctic Large Igneous Province (HALIP). Within the Amundsen Basin, three distinct crustal domains can be distinguished. Closest to the ridge is a 40-km-wide zone with a crustal thickness around 5 km interpreted as thinned continental crust. Five distinctive faulted basement blocks display high-amplitude reflections along their crests with velocities of 4.6 km s^–1, representing the continuation of the magmatic rocks further upslope. Brozena et al. (2003) interpreted magnetic Chron C25 to be located in this zone but our data are not consistent with this being a seafloor spreading anomaly. In the adjacent crustal domain, heading basinward, the basement flattens and two layers with velocities of 5.2 and 6.8 km s^–1 can be distinguished, where the upper and lower layer have a thickness of 1.5 and 2.0 km, respectively. The upper layer is interpreted as exhumed and highly serpentinized mantle while the lower layer may be less serpentinized mantle with some gabbroic intrusions. This may explain the high-amplitude reflections within the overlying sediments that are interpreted as sill intrusions. Continuing basinward, the last crustal domain represents 4-to 5-km-thick oceanic crust with a variable basement relief and velocities of 4.8 and 6.5 km s–1 at the top of oceanic layers 2 and 3, respectively. It is within this zone that the first true seafloor spreading anomaly Chron C24 is observed, which argues for a similar breakup age in the Eurasia Basin as in the Northeast Atlantic. On the other profile crossing the Lomonosov Ridge, a 60-km-wide intrusion into the lower crust is observed where velocities are increased to 6.9 km s^–1. Gravity modelling supports the interpretation of magmatic underplating beneath the intrusion. Seismic data in this region show that the crust is overlain by a 2-km-thick series of high-amplitude reflections with a velocity of 4.8 km s^–1 in a 30-km-wide zone where strong magnetic anomalies (>800 nT) are observed, suggesting a composition of basalt flows. This part of the Lomonosov Ridge appears therefore to have a HALIP-related magmatic overprint at all crustal levels.

This paper presents a ship-mounted multi-lens camera system for sea-ice monitoring and algorithms to automatically evaluate the sea-ice concentration and to indicate the floe-sizes in a radius of 100 meter around the vessel. During the SWEDARCTIC Arctic Ocean 2016 expedition, 11 camera lenses recorded the sea-ice conditions around the Swedish icebreaker Oden. As an example of the possible use of this image system, the images of six lenses are combined into one 360° panoramic image. To distinguish between water and sea-ice in the images, and thus to evaluate the sea-ice concentration around the vessel, a direct thresholding, the k-means, and a novel adaptive thresholding method are applied. Moreover, an edge detector gives the number of pixels that either form the boundary between sea-ice and water or are part of a visible ice fracture. The ratio between these edge pixels and the total number of pixels containing sea-ice gives an indication of the floe size distribution (FSD) in the image.

Bathymetry (seafloor depth), is a critical parameter providing the geospatial context for a multitude of marine scientific studies. Since 1997, the International Bathymetric Chart of the Arctic Ocean (IBCAO) has been the authoritative source of bathymetry for the Arctic Ocean. IBCAO has merged its efforts with the Nippon Foundation-GEBCO-Seabed 2030 Project, with the goal of mapping all of the oceans by 2030. Here we present the latest version (IBCAO Ver. 4.0), with more than twice the resolution (200 x 200m versus 500 x 500m) and with individual depth soundings constraining three times more area of the Arctic Ocean (similar to 19.8% versus 6.7%), than the previous IBCAO Ver. 3.0 released in 2012. Modern multibeam bathymetry comprises similar to 14.3% in Ver. 4.0 compared to similar to 5.4% in Ver. 3.0. Thus, the new IBCAO Ver. 4.0 has substantially more seafloor morphological information that offers new insights into a range of submarine features and processes; for example, the improved portrayal of Greenland fjords better serves predictive modelling of the fate of the Greenland Ice Sheet. Machine-accessible metadata file describing the reported data: 10.6084/m9.figshare.12369314

The central Arctic Ocean remains largely unexplored when it comes to the presence and cycling of mercury and its methylated forms including mono- and dimethylmercury (MMeHg and DMeHg, respectively). In this study, we quantified total Hg (Hg_T) and methylated Hg species in seawater, ice cores, snow, brine, and water from melt ponds collected during the SWEDARCTIC 2016 expedition to the Amerasian and Eurasian side of the Lomonosov Ridge. In the water column, concentrations of Hg_T, MMeHg and DMeHg ranged from 0.089 to 1.5 pM, <25 to 520 fM and from <1.6 to 160 fM, respectively. Hg_T was enriched in surface waters while MMeHg and DMeHg were low at the surface (i.e. in the polar mixed layer) and enriched at a water depth of around 200–400 m. A 1:2 ratio of DMeHg to MMeHg was observed in the water column suggesting a lower ratio in the central parts of the Arctic Ocean than what has previously been reported from other parts of the Arctic Ocean. At the ice stations, average HgT ranged from 0.97 ± 1.2 pM in the ice cores to 27 ± 17 pM in melt pond waters and average MeHg_T (total MeHg) from 28 ± 15 fM in brine to 130 ± 18 fM in melt pond water. The Hg_T observed in melt ponds and brine was an order of magnitude greater than HgT observed in surface waters and Hg_T in the upper part of the ice-cores was ~4–8 times higher HgT in comparison to lower layers. Our study suggests that ice may act as a source of Hg_T to surface waters but not to be a likely source of the methylated Hg forms. Unlike elemental Hg, DMeHg did not enrich in surface waters covered by ice. Concentrations of DMeHg observed in the ice cores and other samples collected from the ice stations were low, suggesting ice to not act as a source of DMeHg to the atmosphere nor to surface waters.

Within the past decade, an alarm was raised about microplastics in the remote and seemingly pristine Arctic Ocean. To gain further insight about the issue, microplastic abundance, distribution and composition in sea ice cores (n = 25) and waters underlying ice floes (n = 22) were assessed in the Arctic Central Basin (ACB). Potential microplastics were visually isolated and subsequently analysed using Fourier Transform Infrared (FT-IR) Spectroscopy. Microplastic abundance in surface waters underlying ice floes (0–18 particles m⁻³) were orders of magnitude lower than microplastic concentrations in sea ice cores (2–17 particles L⁻¹). No consistent pattern was apparent in the vertical distribution of microplastics within sea ice cores. Backward drift trajectories estimated that cores possibly originated from the Siberian shelves, western Arctic and central Arctic. Knowledge about microplastics in environmental compartments of the Arctic Ocean is important in assessing the potential threats posed by microplastics to polar organisms.

Polar oceans, though remote in location, are not immune to the accumulation of plastic debris. The present study, investigated for the first time, the abundance, distribution and composition of microplastics in sub-surface waters of the Arctic Central Basin. Microplastic sampling was carried out using the bow water system of icebreaker Oden (single depth: 8.5 m) and CTD rosette sampler (multiple depths: 8-4369 m). Potential microplastics were isolated and analysed using Fourier Transform Infrared Spectroscopy (FT-IR). Bow water sampling revealed that the median microplastic abundance in near surface waters of the Polar Mixed Layer (PML) was 0.7 particles m^-3. Regarding the vertical distribution of microplastics in the ACB, microplastic abundance (particles m^-3) in the different water masses was as follows: Polar Mixed Layer (0-375) > Deep and bottom waters (0-104) > Atlantic water (0-95) > Halocline i.e. Atlantic or Pacific (0-83).

Deep sea sediments have emerged as a potential sink for microplastics in the marine environment. The discovery of microplastics in various environmental compartments of the Arctic Central Basin (ACB) suggested that these contaminants were potentially being transported to the deep-sea realm of this oceanic basin. For the first time, the present study conducted a preliminary assessment to determine whether microplastics were present in surficial sediments from the ACB. Gravity and piston corers were used to retrieve sediments from depths of 855–4353 m at 11 sites in the ACB during the Arctic Ocean 2016 (AO16) expedition. Surficial sediments from the various cores were subjected to density flotation with sodium tungstate dihydrate solution (Na2WO4·2H2O, density 1.4 g cm−3). Potential microplastics were isolated and analysed by Fourier Transform Infrared (FT-IR) spectroscopy. Of the surficial samples, 7 of the 11 samples contained synthetic polymers which included polyester (n = 3), polystyrene (n = 2), polyacrylonitrile (n = 1), polypropylene (n = 1), polyvinyl chloride (n = 1) and polyamide (n = 1). Fibres (n = 5) and fragments (n = 4) were recorded in the samples. In order to avoid mis-interpretation, these findings must be taken in the context that (i) sampling equipment did not guarantee retrieval of undisturbed surficial sediments, (ii) low sample volumes were analysed (~10 g per site), (iii) replicate sediment samples per site was not possible, (iv) no air contamination checks were included during sampling and, (v) particles <100 µm were automatically excluded from analysis. While the present study provides preliminary indication that microplastics may be accumulating in the deep-sea realm of the ACB, further work is necessary to assess microplastic abundance, distribution and composition in surficial sediments of the ACB.

The radiosounding network in the Arctic, despite being sparse, is a crucial part of the atmospheric observing system for weather prediction and reanalysis. The spatial coverage of the network was evaluated using a numerical weather prediction model, comparing radiosonde observations from Arctic land stations and expeditions in the central Arctic Ocean with operational analyses and background fields (12-hr forecasts) from European Centre for Medium-Range Weather Forecasts for January 2016 to September 2018. The results show that the impact of radiosonde observations on analyses has large geographical variation. In data-sparse areas, such as the central Arctic Ocean, high-quality radiosonde observations substantially improve the analyses, while satellite observations are not able to compensate for the large spatial gap in the radiosounding network. In areas where the network is reasonably dense, the quality of background field is more related to how radiosonde observations are utilized in the assimilation and to the quality of those observations.

Turborotalita quinqueloba is a species of planktic foraminifera commonly found in the sub-polar North Atlantic along the pathway of Atlantic waters in the Nordic seas and sometimes even in the Arctic Ocean, although its occurrence there remains poorly understood. Existing data show that T. quinqueloba is scarce in Holocene sediments from the central Arctic but abundance levels increase in sediments from the last interglacial period [Marine isotope stage (MIS) 5, 71-120 ka] in cores off the northern coast of Greenland and the southern Mendeleev Ridge. Turborotalita also occurs in earlier Pleistocene interglacials in these regions, with a unique and widespread occurrence of the less known Turborotalita egelida morphotype, proposed as a biostratigraphic marker for MIS 11 (474-374 ka). Here we present results from six new sediment cores, extending from the central to western Lomonosov Ridge, that show a consistent Pleistocene stratigraphy over 575 km. Preliminary semi-quantitative assessments of planktic foraminifer abundance and assemblage composition in two of these records (LOMROG12-7PC and AO16-5PC) reveal two distinct stratigraphic horizons containing Turborotalita in MIS 5. Earlier occurrences in Pleistocene interglacials are recognized, but contain significantly fewer specimens and do not appear to be stratigraphically coeval in the studied sequences. In all instances, the Turborotalita specimens resemble the typical T. quinqueloba morphotype but are smaller (63-125 mu m), smooth-walled and lack the final thickened calcite layer common to adults of the species. These results extend the geographical range for T. quinqueloba in MIS 5 sediments of the Arctic Ocean and provide compelling evidence for recurrent invasions during Pleistocene interglacials.

Constraining the thermal evolution of the Arctic Ocean is hampered by notably sparse heat flow measurements and a complex tectonic history. Previous results from the Lomonosov Ridge in the vicinity of the North Pole, and the adjacent central Amundsen Basin reveal varied values, including those higher than expected considering plate cooling or simple uniform stretching models. Furthermore, in the vicinity of the North Pole an anomalously slow velocity perturbation exists in upper mantle seismic tomography models. However, whether these observations are related to a thermal anomaly in the mantle remains unknown. We present new heat flow results gathered from 17 sediment cores acquired during the "Arctic Ocean 2016" and "SWERUS-C3"€ expeditions on the Swedish icebreaker Oden. Three sites located on oceanic lithosphere in the Amundsen Basin between 7°W-71E° reveal surface thermal conductivity of 1.07-1.26 W/mK and heat flow in the order of 71-95 mW/m2, in line-with or slightly higher (1-21 mW/m2) than expected from oceanic heat flow curves. These results contrast with published results from further east in the Amundsen Basin, which indicated surface heat flow values up to 2 times higher than predicted from oceanic crustal cooling models. Heat flow of 49-61 mW/m2 was recovered from the Amerasia Basin. Sites from the submerged continental fragments of the Lomonosov Ridge and Marvin Spur recovered heat flow in the order of 53-76 and 51-69 mW/m2 respectively. When considering the additional potential surface heat flux from radiogenic heat production in the crust, these variable measurements are broadly in line with predictions from uniform extension models for continental crust. A seismically imaged upper mantle velocity anomaly in the central Arctic Ocean may arise from a combination of compositional and thermal variations but requires additional investigation. Disentangling surface heat flow contributions from crustal, lithospheric and mantle processes, including variable along-ridge rifting rates and timing, density and phase changes, conductive and advective dynamics, and regional tectonics, requires further analysis.

Abstract Double-diffusive convection may occur if both temperature and salinity increase with depth, as in the Arctic Ocean. The process is identifiable by a staircase structure, with mixed layers separated by high-gradient interfaces in temperature and salinity. These staircases, which persist if turbulence levels are weak, are widely present in the Arctic Ocean and responsible for transporting heat toward the overlying sea ice. Acoustic observations (reflection coefficients) from a broadband echo sounder are analyzed here to track the detailed evolution of interfaces in the Arctic's double-diffusive staircase. We infer interface thicknesses from reflection coefficient profiles and find that thicknesses appear to be related to water column displacements. Further, we relate reflection coefficients to interface stratification and interpret stratification changes in the context of turbulence acting to thicken interfaces. The high-resolution capabilities of the echo sounder allow for insights into how double-diffusive heat fluxes and inferred mixing levels may vary in space/time.

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.

Although there is enough heat contained in inflowing warm Atlantic Ocean water to melt all Arctic sea ice within a few years, a cold halocline limits upward heat transport from the Atlantic water. The amount of heat that penetrates the halocline to reach the sea ice is not well known, but vertical heat transport through the halocline layer can significantly increase in the presence of double diffusive convection. Such convection can occur when salinity and temperature gradients share the same sign, often resulting in the formation of thermohaline staircases. Staircase structures in the Arctic Ocean have been previously identified and the associated double diffusive convection has been suggested to influence the Arctic Ocean in general and the fate of the Arctic sea ice cover in particular. A central challenge to understanding the role of double diffusive convection in vertical heat transport is one of observation. Here, we use broadband echo sounders to characterize Arctic thermohaline staircases at their full vertical and horizontal resolution over large spatial areas (100 s of kms). In doing so, we offer new insight into the mechanism of thermohaline staircase evolution and scale, and hence fluxes, with implications for understanding ocean mixing processes and ocean-sea ice interactions.

Two different biostratigraphic approaches are used to identify Marine Isotope Stage 11 (MIS 11) in Arctic Ocean sediments. On the Lomonosov Ridge, globally calibrated nannofossil bioevents constrain the age of sediments back to MIS 13 (Core LOMROG12-3PC). In the Amerasian Basin the unique occurrence of the planktonic foraminifer Turborotalita egelida is increasingly used as a marker for MIS 11. However, the T. egelida horizon has only been dated using cyclostratigraphy. Here we bridge these approaches through investigation of a new core (AO16-8GC) from the Alpha Ridge, Amerasian Basin. AO16-8GC is easily correlated to LOMROG12-3PC and contains the T. egelida horizon, allowing the first comparison between the biostratigraphy of both regions. Based on the nannofossil biochronology of LOMROG12-3PC, the most convincing lithologic correlation between the Alpha and Lomonosov Ridge cores places the T. egelida horizon between MIS 15 and MIS 17. This potentially older age for the T. egelida biohorizon emphasizes the need for continued caution in interpreting paleoceanographic records predating MIS 6, until further work can reconcile the nanno- and microfossil biostratigraphies that are emerging for middle Pleistocene sediments of the central Arctic Ocean.