Paleoceanographic implications of nitrogen and organic carbon isotopic excursions in mid-Pleistocene sapropels from the Tyrrhenian and Levantine Basins, Mediterranean Sea

Comparison of the total nitrogen and organic carbon isotopic compositions of sediment sequences from the Tyrrhenian and Levantine Basins that contain three same-age sapropels reveals evidence of both similarities and differences in mid-Pleistocene sapropel-forming conditions in the western and eastern Mediterranean Sea. Total δ15N values systematically decrease at both locations from 5‰ in background sediments to − 1‰ in the sapropels. The lower δ15N values signify that nitrogen fixation was important to the increased marine productivity implied by the higher organic carbon concentrations, which is consistent with widespread salinity stratification of surface waters when sapropels accumulated. Organic δ13C values systematically increase from − 25‰ in the background sediments to − 22‰ in the sapropels at the Tyrrhenian site, which confirms that increased marine productivity accompanied sapropel deposition. In contrast, δ13C values decrease in the sapropel layers in the Levantine Basin even though organic carbon concentrations are higher in these sapropels. The different types of δ13C excursions associated with increased organic matter production at the two locations indicates a difference in the dissolved carbon pool from a combination of greater fluvial delivery of continental carbon and stronger recycling of organic matter in the Levantine Sea than in the Tyrrhenian Sea. This difference implies that the shifts to wetter climate during times of sapropel deposition were stronger in the eastern than in the western Mediterranean.