Rafting of Growing Antarctic Sea Ice Enhances In‐Ice Biogeochemical Activity in Winter.

The study of Antarctic first‐year sea‐ice biogeochemistry has been limited by samples mostly being collected in pack ice during summer, with few winter data available. Measurements from the Antarctic marginal ice zone (AMIZ) have proven even more difficult to obtain. The AMIZ is a broad, circumpolar feature of the Southern Ocean found at different latitudes during the year where light and nutrients are high enough to sustain primary production and influence ecosystem functioning. We present the first biogeochemical data set from growing ice collected in the Atlantic AMIZ during winter 2019, including measurements from young pancake ice (YI) and consolidated first‐year ice (FYI). Measurements of sea‐ice temperature, salinity, crystal structure, δ18O, chlorophyll, and nutrient concentrations were used to investigate the winter sea‐ice habitat and decipher the conditions under which the ice formed and grew. Model simulations support the hypothesis that nutrient accumulation in advancing sea ice cannot be explained by passive seawater entrainment and thermodynamics alone. Our data confirm that winter sea ice is biogeochemically active and accumulates remineralized nutrients. We further propose that mechanical thickening enhances the reservoir of nutrients during the ice growth season. The biogeochemical transition from YI to FYI does not appear to be a linear progression of thickness with habitat space reduction as sea ice consolidates. Instead, FYI bulk biogeochemistry results from multiple cycles of rafting of YI, which conserves the biogeochemical properties of YI in the FYI, ultimately increasing the overall nutrient and chlorophyll content. Plain Language Summary: The Antarctic marginal ice zone (AMIZ) is a region of intense air‐sea interactions where sea ice begins to form during winter. It is generally assumed that little to no biological activity occurs in winter sea ice, with the chemical and biological composition of the ice instead set by seawater that is incorporated during ice formation. We sampled young ice (YI) and first‐year ice (FYI; collectively, growing sea ice) in the Atlantic AMIZ in winter 2019, the first such collection from this region. Our measurements of sea‐ice crystal structure and oxygen isotopes suggest that the FYI formed from repeated breaking and piling up of younger ice. This idea is supported by model experiments showing that the FYI was the result of more than just temperature‐driven thickening of YI. Additionally, our measurements and modeling of nutrients and chlorophyll in the ice strongly suggest that the biological community was active during winter. Our findings challenge the assumption that winter sea ice is biologically inactive and suggest that the biological and chemical signatures of YI are conserved as growing ice is broken up and reformed. The results of our study will help to improve and validate future modeling efforts. Key Points: We present the first biogeochemical data for young and growing sea ice in the Atlantic Antarctic marginal ice zoneWinter sea ice is biogeochemically active and acts as a reservoir of concentrated nutrients during the ice growth seasonThe biogeochemical signature of individual floes is conserved during rafting, leading to enhanced nutrients and chlorophyll‐a in first‐year sea ice [ABSTRACT FROM AUTHOR]