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 this 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 δ¹³C and δ¹⁵N values of sea-ice particulate matter were −24.5±3.2‰ and +1.6±2.2‰ (mean±standard deviation), respectively, showing greater variability compared to those in seawater (−28.8±1.0‰ for δ¹³C and −0.5±1.4‰ for δ¹⁵N). We identified correlations of δ¹³C and δ¹⁵N with organic matter and nutrient concentrations and their ratios (e.g., silicate:nitrate), indicating 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, this supply was estimated to be 4.6±0.3 Tg C year^−1, equivalent to >14% of primary production in the marginal ice zone of the studied region. These results suggest that the carbon supply from sea ice is a crucial factor supporting ecosystems in the seasonal sea ice zone.