Stable water isotopes (oxygen δ18O and hydrogen δD) of sea ice were measured on ice cores taken by the Biogeochemistry (BGC) team during MOSAIC as a basis for interpreting BGC parameters such as enclosed gases and biogechemical active ingredients (Damm et al., in prep.). Sea ice cores were collected at different BGC stations located on the main ice floe (Central Observatory, CO) and L1 and L2 sites in the Distributed Network of the MOSAiC expedition (Angelopoulos et al., 2021; Mellat et al., 2024) as well as at the Main Coring Sites (MCS) in a multiple team effort (Nicolaus et al., 2022). Level sea ice was categorized into types based on the age: sea ice which grew during the same winter referred to as First-Year-Ice (FYI); sea ice which had survived one or more summer melting periods referred to as Second-Year-Ice (SYI). Sea ice cores were collected using a Kovacs Mark II 9 cm diameter corer. The snow on top of the sea ice was brushed off the top of the cores to minimize the snow affecting the ice surface. Onboard RV Polarstern, the cores were cut in 10 cm sections using a handsaw at 4° C (leg 1) or an electric saw at -20° C (legs 2 and 3). During summer (leg 4) sectioning of cores was partly done in the field, while another part of the cores was transported frozen at -20°C and sectioned within 3 month onshore. Each section was transferred into a gas-tight TedlarTM bag. The closed bags were carefully degassed with a vacuum pump (NKF Neuberger, type N035). Melting occurred within 12 to 15 hours at about 15°C. After shaking the melted ice within the TedlarTM bags, discrete sampling started by first rising the melt water carefully through a Tygon tube connected with the opened valves of the gas-tight bags and then into prepared sample vials. Oxygen and hydrogen isotope analyses were carried out at the ISOLAB Facility at AWI Potsdam (https://hdl.handle.net/10013/sensor.ddc92f54-4c63-492d-81c7-696260694001) with mass spectrometers (DELTA-S Finnigan MAT, USA): https://hdl.handle.net/10013/sensor.af148dea-fe65-4c87-9744-50dc4c81f7c9 https://hdl.handle.net/10013/sensor.62e86761-9fae-4f12-9c10-9b245028ea4c employing the equilibration method (details in Meyer et al., 2000). δ18O and δD values are given in per mil (‰) vs. Vienna standard mean ocean water (V-SMOW) as the standard. The second order parameter d excess was computed according to: d-excess = δD-8 δ18O (Dansgaard, 1964).

In its current form there might be single cores from which isotope data was already published e.g. in Mellat et al, 2024.The dataset in its current form contains all data shown and discussed in Bauch et al, in prep.