Patterns of organic carbon and nitrogen isotopic compositions of latest Pliocene sapropels from six locations across the Mediterranean Sea

Abstract

Inter-layered in the organic-poor sediments that comprise most of the sedimentary record of the Mediterranean Sea are organic-rich sapropels that represent periods of wetter climate, increased primary productivity, and suboxic to anoxic conditions at depth. In order to explore processes important in the spatial and temporal variability of sapropel-forming conditions, we selected sedimentary sequences containing two latest Pliocene sapropels at ODP Sites 974 and 975 in the western basin, the Vrica Plio-Pleistocene type-section, and ODP Sites 964, 967, and 969 in the eastern basin. Our main tools are nitrogen and organic carbon isotopes, which are diagnostic of changes in nutrient origins and availability and in primary productivity. Extremely low δ 15N values (to − 5.1‰) indicate high rates of nitrogen fixation during sapropel formation stimulated by nearly complete consumption of nitrate in the water column due to exceptionally high rates of denitrification. Interruptions in sapropels represent brief intervals of low productivity and diminished nitrogen fixation. The geographical distribution of the interruptions and the presence of spatial patterns in TOC contents, C org/N tot ratios, and δ 15N values suggest a central role of the eastern basin in initiating sapropel formation. The eastern Mediterranean evidently experienced higher rates of primary productivity during sapropel deposition and probably more intense dysoxic conditions in the water column. Organic carbon isotopes present an intriguing geographic trend across the eastern Mediterranean that is probably related to non-biological processes including variations in water temperature, salinity, and amounts of local fluvial runoff.