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This data has been collected and processed as part of the MOSAiC (Multidisciplinary Drifting Observatory for the Study of Arctic Climate) expedition. MOSAiC is a collaborative initiative led by the Alfred Wegener Institute and has received substantial funding from the German Federal Ministry of Education and Research, as well as the US National Science Foundation, Department of Energy, NOAA, and NASA. Numerous other international agencies and institutions have also made significant contributions. The primary objective of this program was to conduct a comprehensive investigation of the evolving Arctic over the course of a year. The expedition took place from October 2019 to October 2020 and was conducted aboard the Research Vessel Ice Breaker (RVIB) Polarstern, involving participants from 20 nations. As part of this submission, we are presenting five distinct datasets. Two of these datasets are related to seawater, two pertain to meltwater, and one pertains to sea ice. The "in-situ" datasets provide information on dissolved methane concentrations and isotope ratios, while the "in-vitro" datasets offer insights into potential methane oxidation rate constants. In the case of sea ice, only "in-vitro" data was collected, as discrete measurements were obtained from another research group. These datasets are the result of the project titled "Collaborative Research: Quantifying microbial controls on the annual cycle of methane and oxygen within the ultraoligotrophic Central Arctic during MOSAiC." The aim of this study was to assess the marine methane metabolism during a one-year period in the Central Arctic Ocean. The results have provided insights into the biogeography of methane hotspots, both in terms of production and oxidation. 

First-year sea-ice thickness, draft, salinity, temperature, and density were measured during near-weekly surveys at the main first-year ice coring site (MCS-FYI) during the MOSAiC expedition (legs 1 to 4). The ice cores were extracted either with a 9-cm (Mark II) or 7.25-cm (Mark III) internal diameter ice corers (Kovacs Enterprise, US). This data set includes data from 23 coring site visits and were performed from 28 October 2019 to 29 July 2020 at coring locations within 130 m to each other in the MOSAiC Central Observatory. During each coring event, ice temperature was measured in situ from a separate temperature core, using Testo 720 thermometers in drill holes with a length of half-core-diameter at 5-cm vertical resolution. Ice bulk practical salinity was measured from melted core sections at 5-cm resolution using a YSI 30 conductivity meter. Ice density was measured using the hydrostatic weighing method (Pustogvar and Kulyakhtin, 2016) from a density core in the freezer laboratory onboard Polarstern at the temperature of –15°C. Relative volumes of brine and gas were estimated from ice salinity, temperature and density using Cox and Weeks (1983) for cold ice and Leppäranta and Manninen (1988) for ice warmer than –2°C.The data contains the event label (1), time (2), and global coordinates (3,4) of each coring measurement and sample IDs (13, 15). Each salinity core has its manually measured ice thickness (5), ice draft (6), core length (7), and mean snow height (22). Each core section has the total length of its top (8) and bottom (9) measured in situ, as well estimated depth of section top (10), bottom (11), and middle (12). The depth estimates assume that the total length of all core sections is equal to the measured ice thickness. Each core section has the value of its practical salinity (14), isotopic values (16, 17, 18) (Meyer et al., 2000), as well as sea ice temperature (19) and ice density (20) interpolated to the depth of salinity measurements. The global coordinates of coring sites were measured directly. When it was not possible, coordinates of the nearby temperature buoy 2019T66 were used. Ice mass balance buoy 2019T66 installation is described in doi:10.1594/PANGAEA.938134. Brine volume (21) fraction estimates are presented only for fraction values from 0 to 30%. Each core section also has comments (23) describing if the sample is from a false bottom, from rafted ice or has any other special characteristics.Macronutrients from the salinity core, and more isotope data will be published in a subsequent version of this data set.