CLIMATICALLY LINKED CARBON-ISOTOPE VARIATION DURING THE PAST 430,000 YEARS IN SOUTHERN-OCEAN SEDIMENTS

We use the isotopic composition of carbon from organic matter enclosed within diatom frustules as a proxy for paleoproductivity and paleo-di ssolved carbon dioxide concentrations in ocean surface waters. A South ern Ocean record from south of the Antarctic Polar Front and spanning 430,000 years of carbon isotopic variation in diatomaceous organic mat ter is presented for the first time. The most refractory diatomaceous organic matter fraction was extracted and analyzed to avoid problems a ssociated with diagenesis. The results clearly indicate cyclic changes in organic carbon isotopic ratios, with C-13 depleted values associat ed with all of the last five glacial periods, reflecting changes in su rface water properties and primary productivity. Changes in dissolved carbon dioxide concentrations are the most probable cause of these cyc les, but the possible effect of seawater pH changes cannot be excluded .