Significance of high C/N ratios in organic-carbon-rich Neogene sediments under the Benguela Current upwelling system

Corganic/Ntotal values change from 5 in middle Miocene sediments deposited under the Benguela Current upwelling system to 15 in latest Miocene and Pliocene sediments. Although the change appears to record a larger proportion of land-derived material in younger sediments, bulk δ13Corg values indicate a predominance of marine origin for organic matter throughout the sequence. Marine paleoproductivity, as represented by mass accumulation rates of CaCO3 and organic carbon, increased over this time interval. We postulate that the larger Corganic/Ntotal values record a shift towards greater recycling rates of nitrogen-rich, relative to carbon-rich, organic matter components as the flux of total organic matter to the seafloor increased. Furthermore, we propose that high Corganic/Ntotal values may record enhanced paleoproductivity in other sedimentary sequences dominated by marine organic matter.