LOW ORGANIC-CARBON ACCUMULATION RATES IN BLACK-SEA SEDIMENTS

THE Black Sea, the world's largest anoxic marine basin, is frequently used as a modern analogue for the formation of organic-rich sediments and carbonaceous rocks 1-3, on the widely held assumption that anoxic conditions promote the preferential preservation of organic matter in sediments. Data for testing this hypothesis have so far been equivocal 4-7, but here we use radiocarbon ages obtained using accelerator mass spectrometry for the organic fraction of recent Black Sea sediments to estimate the organic carbon accumulation rates. These range from 0.69 to 2.09 g C m-2 yr-1 and are significantly lower than earlier estimates based on varve counting 6. Depending on the value taken for the rate of primary production in the Black Sea 4,8, between 0.7 and 2.1% of the organic carbon is preserved in the bottom sediments. When compared with carbon accumulation rates in equivalent oxygenated environments 9, these results indicate that the modern Black Sea is not a site of anomalously high organic carbon accumulation. This suggests that anoxic conditions in the water column may not be a prerequisite for the preservation of organic matter in marine sediments, and that models of the origin of carbonaceous facies in the geological record may therefore need to be modified.