During the MOSAiC expedition in the Central Arctic Ocean (CAO, 2019-2020), POM was sampled weekly to fortnightly from surface waters and the Chlorophyll a maximum layer (Chl a max) via CTD casts and from bottom sea ice of the floe via ice coring (first- and second-year ice, two layers nearest to the water-ice interface). The POM was filtered onboard (GF/F filters) and deep frozen for the subsequent analysis of a suite of lipid biomarkers, including fatty acids (FA), FA-compound-specific stable isotopes (FA-CSIA), sterols, and highly-branched isoprenoids (HBI). These biomarkers can provide valuable information about the nutritional value, the taxonomic composition (e.g. diatoms vs flagellates), and the origin of the POM that represents the basis of the Central Arctic food web. This dataset comprises the results from the FA analysis only, those from other biomarkers will be submitted in due cause. The separation of the various lipid biomarkers was carried out at the University of Plymouth. After addition of internal standards for each of the 3 components, the filters were saponified with KOH. Thereafter, non-saponifiable lipids (HBI and sterols) were extracted with hexane and purified by open column chromatography (SiO2). Fatty acids were obtained by adding concentrated HCl to the saponified solution and re-extracted with hexane. Further steps of the FA analysis were carried out at the Alfred-Wegener-Institute (AWI) in Bremerhaven. Here samples were converted into fatty acid methyl esters (FAME) and analysed using an Agilent 6890N gas chromatograph. The Clarity chromatography software system (DataApex, Czech Republic) was used for chromatogram data evaluation. FAME were quantified via the internal standard, Tricosanoic acid methyl ester (23:0) (Supelco, Germany) to provide the total amount of FA (TFA) per filter. These TFA quantities per filter can be normalised to the volume of filtered seawater or melted ice core water. Additionally, we provide the mass percentage composition of the TFA, considering 48 individual FA. The FA are presented in shorthand notation, i.e., A:B(n-x), where: A indicates the number of carbon atoms in the straight fatty acid chain, B represents the number of double bonds present, n represents the terminal methyl group and x denotes the position of the first double bond from the terminal end. The biochemical nomenclature of the fatty acids is provided. The dataset is linked to a manuscript that compares pattern seen in sea ice- and water column POM in the CAO with previously published data from Arctic shelf regions. This manuscript focusses mainly on two key long-chain omega-3 FA (eicosapentaenoic acid and docosahexaenoic acid) that are considered essential for the nutrition of higher trophic levels, including humans, and their production to decline with global temperature rise. Contributions by KS were funded by the UK's Natural Environment Research Council MOSAiC Thematic project SYM-PEL: "Quantifying the contribution of sympagic versus pelagic diatoms to Arctic food webs and biogeochemical fluxes: application of source-specific highly branched isoprenoid biomarkers" (NE/S002502/1).

A ground-based radar survey consisting of 35 discrete quad-polarization measurement sites over three field seasons was undertaken on Rutford Ice Stream, West Antarctica. Sites A01 to A10 were collected on 20 January 2017, along a profile orientated perpendicular to the ice flow direction. The 10 sites are located between the central ice flowline and the ice-stream margin along a profile of length 8.5 km with the inter-site spacing decreasing toward the ice-stream margin. Sites B01 to B10 were collected on 05 December 2019, along a profile orientated parallel to the central flowline. The sites were surveyed with the first site 4 km upstream of site A01 and the inter-site distance spacing fixed at 4 km. Sites C01 to C11 were collected on 14 December 2018, and located between sites A01 and A02 at 200 m spacing. Sites D01-D04, collected on 25 January 2019, are downstream of A01 and form a diamond shape with 800 m spacing. At each site, polarimetric radar-sounding measurements were made using an autonomous phase-sensitive radio-echo sounder (ApRES), a frequency-modulated continuous-wave radar. The ApRES has a centre frequency of 300 MHz and a bandwidth of 200 MHz, which results in a range resolution of approximately 40 cm in ice. ApRES radar data were collected as part of the BEAMISH Project (NERC AFI award numbers NE/G014159/1 and NE/G013187/1).

This dataset contains the object ID, morphotaxonomic name, and approximate size (in mm) of over 70,000 organisms identified from select Ocean Floor Observation and Bathymetry System (OFOBS) images from the cruise PS118 to the Weddell Sea from April - May 2019. Images were collected for assessing epibenthic megafauna communities and establishing temporal baselines for future studies. This work has been supported by Natural Environment Research Council (NERC) Independent Research Fellowship NE/S014756/1, awarded to EM. TK is funded by a Cambridge International and Newnham College Scholarship, administered by Cambridge Trust. RW and HG are funded by UK Research and Innovation (UKRI) Future Leaders Fellowship MR/ W01002X/1 "The past, present and future of unique cold-water benthic (seafloor) ecosystems in the Southern Ocean" awarded to RW. HG is also funded by BIOPOLE, funded by the NERC, under National Capability Science Multi-Centre award scheme (NC-SM2).