Stable isotope composition of dissolved inorganic carbon and particulate organic carbon in sea ice from the Ross Sea, Antarctica

[1] We examined controls on the carbon isotopic composition of sea ice brines and organic matter during cruises to the Ross Sea, Antarctica in November/December 1998 and November/December 2006. Brine samples were analyzed for salinity, nutrients, total dissolved inorganic carbon (SCO2), and the C/C ratio of SCO2 (d 13CSCO2). Particulate organic matter from sea ice cores was analyzed for percent particulate organic carbon (POC), percent total particulate nitrogen (TPN), and stable carbon isotopic composition (dCPOC).SCO2 in sea ice brines ranged from 1368 to 7149 mmol kg , equivalent to 1483 to 2519 mmol kg when normalized to 34.5 psu salinity (sSCO2), the average salinity of Ross Sea surface waters. Sea ice primary producers removed up to 34% of the available SCO2, an amount much higher than the maximum removal observed in sea ice free water. Carbonate precipitation and CO2 degassing may reduce sSCO2 by a similar amount (e.g., 30%) in the most hypersaline sea ice environments, although brine volumes are low in very cold ice that supports these brines. Brine d13CSCO2 ranged from −2.6 to +8.0‰ while dCPOC ranged from −30.5 to −9.2‰. Isotopic enrichment of the SCO2 pool via net community production accounts for some but not all carbon isotopic enrichment of sea ice POC. Comparisons of sSCO2, d 13CSCO2, and d CPOC within sea ice suggest that p (the net photosynthetic fractionation factor) for sea ice algae is ∼8‰ smaller than the p observed for phytoplankton in open water regions of the Ross Sea. These results have implications for modeling of carbon uptake and transformation in the ice‐covered ocean and for reconstruction of past sea ice extent based on stable isotopic composition of organic matter in sediment cores.