First-year sea-ice salinity, temperature, density, nutrient, oxygen and hydrogen isotope composition from the main coring site (MCS-FYI) during MOSAiC legs 1 to 4 in 2019/2020, version 2

First-year sea-ice thickness, draft, salinity, temperature, nutrient concentrations ([NO3]- [NO2]; [NH4]; [PO4]-; [Si(OH)4]), stable water isotope composition and ice 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 that were performed from 28 October 2019 to 29 July 2020 at coring locations within 130 m of 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, isotope, and nutrient samples were subsampled from the same melted core segment (5-cm resolution). Ice bulk practical salinity was measured using a YSI 30 conductivity meter. Nutrient analyses were conducted shipboard or samples were frozen and analyzed at a shore-based lab. In either case, analyses were conducted colorimetrically using a Seal Analytical AA3 continuous flow auto analyzer (AACE Software, Version 7.09). Measurement of nutrients followed best practices adopted from GO-SHIP recommendations (Hydes et al., 2010; Becker et al., 2020). Ice density was measured using the hydrostatic weighing method in air and kerosene (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), date/time (2), and global coordinates (3,4) of each coring activity. Each salinity/isotope/nutrient core has its manually measured ice thickness (5), ice draft (6), core length (7), and mean snow height (19). Each core section has the total length of its top (8) and bottom (9) measured in situ, as well as the 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 (13), in situ temperature (14), ice density at the laboratory (15) and in situ (16) temperatures, brine volume fraction (17), gas volume fraction (18), nitrate + nitrite (plus error and quality flag; 20, 21, 22), nitrite (23, 24, 25), ammonium (26, 27, 28), phosphate (29, 30, 31), silicate (32, 33, 34), stable water isotopic sample ID and values (35, 36, 37, 38) (Meyer et al., 2000). Temperature, density, and gas volume fraction values were interpolated to the depth of salinity/isotope/nutrient 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. Sea-ice density at in situ temperatures (16) and brine volume (17) fraction estimates are presented only for brine volume fraction values from 0 to 30%. Ice temperatures were assumed to be not higher than -0.1°C. Estimates of sea-ice density at in situ temperatures and gas volume fraction are made using measurements of density and salinity of a separate density core and are not shown if any of these measurements were not performed. Measurements of snow height include the thickness of the surface scattering layer. Each core section also has comments (36) describing if the sample is from a false bottom, from rafted ice, or has any other special characteristics.