Post-Archean Australian Shale normalized 61 heavy-to-light rare earth element ratios (HREE/LREE). The first-year ice, recovered on April 8, 2020, is represented by tracer profiles derived from nine individual ice cores, with depth horizons merged to generate the presented data. In contrast, surface seawater data were collected throughout the ice growth and drift period but are plotted in the first-year ice profiles at ice core depths corresponding to the seawater sampling dates. These depths were calculated using an age model for the sea ice (‘Methods’). The sea ice composition is depicted by black lines, representing average values across varying depth segments, with grey fields indicating 2 standard deviations from measurements. Surface seawater composition, sampled at depths of 2 to 5 m, is represented by color-coded diamonds indicating the sampling date. For the upper 40 cm, where seawater data are unavailable, black diamonds denote low-salinity surface waters in August/September 2020 near the North Pole. Dark grey fields in ( c and e ) for the latter and x-axis error bars for the other seawater data represent 2 standard deviations from measurements for the ε Nd and HREE/LREE values. Y-axis error bars on seawater data indicate ice age uncertainty. In panel ( c ), a color-coded line represents the ice core depth interval corresponding to ice drift when surface seawater salinities were below 32 (December 12, 2019, to February 10, 2020). Depth ranges for the four distinct sea ice intervals i)–iv) are indicated in the δ 18 O profile in ( b ). Differences in absolute values between sea ice and seawater composition arise from brine rejection for salinity, [Nd] and HREE/LREE ratios, and from fractionation during sea ice formation for δ 18 O.
