Divine, Dmitry V; Salganik, Evgenii; Clemens-Sewall, David; Cimoli, Emiliano; Eggers, Sarah Lena; Takahashi, Keigo; Nicolaus, Marcel: First- and second-year sea-ice density from the coring sites during the CONTRASTS expedition in July-August 2025 [dataset]. PANGAEA, https://doi.pangaea.de/10.1594/PANGAEA.993687 (dataset in review)

Sea-ice salinity, temperature, density, thickness, and draft were measured during surveys at coring sites of the CONTRASTS expedition, with four sampling events conducted at each of three different stations. Ice cores were extracted using a 9-cm (Mark II) internal-diameter ice corer (Kovacs Enterprise, USA). This dataset includes data from 11 coring site visits conducted between 10 July and 26 August 2025, with 14 density, 13 salinity, and 14 temperature cores. The sampling was conducted in the Arctic Ocean between 82.2–85.0°N and 17.9°E to 34.0°W. The locations of the main coring sites, overlaid on optical images for each ice floe and visit, are provided in Linck Rosenhaim et al. (2026, doi:10.1594/PANGAEA.992627). During each coring event, ice temperature was measured in situ from a separate temperature core using Testo 720 thermometers inserted into drill holes with a length equal to half of the core diameter, at 5-cm vertical resolution. Bulk practical salinity was measured from melted 5-cm core segments from the salinity and density cores. Salinity cores were sectioned right after core extraction. Bulk practical salinity was measured using a WTW Cond 3110 conductivity meter. Ice density was determined using the hydrostatic weighing method in air and kerosene (Pustogvar and Kulyakhtin, 2016, doi:10.1016/j.coldregions.2016.09.001) from a separate density core processed in the freezer laboratory onboard R/V Polarstern at temperatures between –17°C and –24°C. Relative volumes of brine and gas were estimated from ice salinity, temperature, and density using Cox and Weeks (1983, doi:10.3189/S0022143000008364) and Leppäranta and Manninen (1988, hdl.handle.net/1834/23905) for ice colder and warmer than –2°C, respectively. This dataset contains the event label (1), date/time (2), geographic coordinates (3–4), core number (8), station number (9), station visit (10), ice age (11), the presence of a melt pond above the ice core (12), and snow or surface scattering layer height (23) for each coring event. For each density core, the following data are provided: manually measured ice thickness (5), ice draft (6), and core length (7). For each core section, the top (13) and bottom (14) depths measured in situ are included. Each core section also includes ice practical salinity (15), laboratory air temperature (16), in situ ice temperature (17), ice density at laboratory (18) and in situ (19) temperatures, brine volume fraction (20) at in situ temperature, and gas volume fraction at laboratory (21) and in situ (22) temperatures. Temperature values were interpolated to the depths of the density measurements. Geographic coordinates of the coring sites were measured directly. Brine volume fraction (20) is reported only for values between 0% and 40%. Ice temperatures were constrained to a maximum of –0.1 °C. Each core section also includes comments (24) describing specific characteristics of the sample.

Acknowledgements: The data collection is part of the expedition of the Research Vessel Polarstern (Knust, 2017, doi:10.17815/jlsrf-3-163) during the expedition CONTRASTS (PS149, grant: AWI_PS149_00). DD's participation in the CONTRASTs cruise was supported by internal funding from the Norwegian Polar Institute. ES was supported through the European Union's Horizon 2020 research and innovation programme under grant agreement No. 101003472 - Arctic PASSION. DCS was supported by the National Science Foundation's Arctic System Science program, award OPP-2138787. EC and KT were supported by the Japan Society for the Promotion of Science under grant agreement No. 24KJ0006 - Roles of algal aggregates on the biological pump in sea ice-covered oceans.