Upward increase of kerogen δ13C in the Peru Margin Upper Oligocene: possible implications for the Cenozoic evolution of atmospheric CO2

Abstract Carbon isotopic composition of predominantly marine kerogen in latest Oligocene mudstones of the Peru Margin ODP 682A Hole shows an about 3.5‰ increase with decreasing age. Py-GC and elemental (C/N ratio) analysis of the kerogen plus sulphur isotopic study together with earlier knowledge on geological setting and organic geochemistry results in a better understanding of depositionary environment and allows to separation of the influence of concentration of water dissolved carbon dioxide ( c e ) on kerogen δ 13 C from that of other factors (bacterial degradation, sea surface temperature, DIC δ 13 C, productivity, and admixture of land plant OM). Based on this analysis, the major part of the kerogen shift is considered as a result of the latest Oligocene decrease of marine photosynthetic carbon isotopic fractionation in the Peru Margin photic zone, which in turn possibly reflects a simultaneous drop in atmospheric CO 2 level. Uncertainties in the evaluation of the factors affecting the marine photosynthetic carbon isotopic fractionation and the extent of ocean–atmosphere disequilibrium do not permit calculation of the decrease of the atmospheric CO 2 .