Sedimentation of C-13-rich organic matter from Antarctic sea-ice algae: A potential indicator of past sea-ice extent

Organic matter collected in sediment traps at a nearshore marine site in Pr ydz Bay, East Antarctica, displayed a pronounced seasonal variation in carb on isotopes (expressed as delta(13)C(OM) ). Winter values were fairly const ant near -20 parts per thousand before increasing to above -15 parts per th ousand in response to inputs from sea-ice algae in early spring. Particulat e organic matter obtained directly from sea-ice cores exhibited very high d elta(13)C(OM) values, typically -15 parts per thousand but as much as -8 pa rts per thousand, much above the values of organic matter suspended in the water column below the sea ice. The sea-ice algae contributed similar to 20 % of the annual flux of organic matter, and the delta(13)C(OM) measurements from a sediment core at the site suggest that this value is representative of the recent past. All of these carbon isotopic compositions are consider ably above the extremely delta(13)C-depleted compositions (generally -25 pa rts per thousand to 30 parts per thousand) that have been found in many oth er Antarctic and Southern Ocean studies. The elevated delta(13)C(OM) values derive from considerable depletion of dissolved carbon dioxide and nutrien ts within the sea ice and to a lesser extent within stratified surface wate rs induced by melting sea ice. Thus, elevated delta(13)C(OM) values in Sout hern Ocean sediment cores may indicate the presence of sea-ice-hosted algae , rather than temperature or surface CO2 pressure (pCO(2)) variations.