Evidence from nitrogen isotope ratios for enhanced productivity during formation of eastern Mediterranean sapropels

Abstract

THE formation of horizons of organic-rich sediment (sapropels) in the eastern Mediterranean during the Holocene and Upper Pleistocene1 has been ascribed2 to enhanced preservation of organic carbon in bottom waters rendered anoxic by the restriction of deep-water renewal, either by lowered sea level during glacial maximum conditions3 or by the presence of low-salinity surface waters derived from increased runoff2,4. A second possibility is that the sapropels formed as a consequence of higher settling fluxes of organic matter caused by increased primary production connected with the increased runoff5,6. Here we report that in a sediment core from close to the mouth of the Nile, the sapropels have significantly lighter nitrogen isotope ratios (15N/<14N) than the intercalated marl oozes. We adduce evidence that these large differences cannot be caused either by variable mixtures of marine and terrestrial organic matter having different isotopic compositions, or by differences in the extent and type of post-depositional alteration. The differences are consistent, however, with a higher utilization of dissolved nitrate during sapropel formation, implying that the sapropels were ultimately formed as a consequence of high plankton production, which led to high fluxes of organic matter to the sea floor.