Biogeochemical controls on the concentrations and stable isotope compositions of sea-ice particulate organic matters in the Southern Ocean

The primary production and zooplankton life cycles in the Southern Ocean synchronize with seasonal sea ice melting, during which particulate organic carbon (POC) and nitrogen (PN), including ice algae, are supplied from sea ice into the water column. To understand carbon flow through food webs in productive marginal ice zones, it is necessary to determine the quantity and quality of organic carbon and nitrogen in sea ice. However, knowledge regarding brash sea ice is scarce, even though it is the predominant form at the Antarctic ice edge. In this study, 102 sea-ice samples were collected from the Indian sector (20–160° E) during summer to clarify variations in isotopic baselines and the potential contribution of sea ice-derived carbon and nitrogen to the water column. The δ ¹³ C and δ ¹⁵ N values of all sea-ice particulate matter samples were -24.6±3.2 ‰ and +1.5±2.2 ‰ (mean ± standard deviation), respectively, showing greater variability compared to those in seawater (-28.8±1.0 ‰ for δ ¹³ C and -0.5±1.4 ‰ for δ ¹⁵ N). We identified correlations of δ ¹³ C and δ ¹⁵ N with organic matter and nutrient concentrations and their ratios (e.g., silicate : nitrate), suggesting that nutrient consumption and organic matter decomposition influenced those isotopic characteristics in sea ice. Using freshwater flux from sea ice melt and the measured sea-ice POC concentrations, the POC supply was estimated to be 4.1±0.3 Tg C yr ⁻¹ , equivalent to 14.7±2.8 % of primary production in the marginal ice zone of the studied region. This result suggests that sea ice melt provides significant portion of organic matters potentially supporting Antarctic ecosystems in the seasonal sea ice zone, which could be traced by difference of sea-ice and seawater stable isotope compositions.