RECONCILING SAPROPEL DEPOSITION WITH OLIGOTROPHY IN THE EASTERN MEDITERRANEAN SEA

Quaternary organic-rich sediment layers (sapropels) in the eastern Mediterranean Sea, an otherwise oligotrophic basin with well-oxygenated bottom water, have engendered intensive study during the last half-century. Yet no consensus has emerged on the paleoenvironmental setting that enabled their deposition. In all likelihood, a combination of increased export production and improved preservation of sedimentary organic matter was responsible. An enigmatic recent observation concerns the very low nitrogen isotopic ratios of chlorophyll in late Quaternary sapropels, which are similar to those in living phytoplankton from the eastern Mediterranean Sea today. The nitrogen isotopic values of phytoplankton implied by these chlorophyll values are near zero per mil, that of atmospheric dinitrogen, and are best explained by invoking high rates of nitrogen fixation. Although the biological sources of such high nitrogen fixation rates have not been determined, nitrogen budget calculations for the Mediterranean Sea (Bethoux, J.P. and Copin-Montegut, G., 1986: Limnology and Oceanography, v. 31, p. 1353-1358) and nutrient re-analyses from the Atlantic Ocean (Gruber, N., and Sarmiento, J.L., 1997: Global Biogeochemical Cycles, v. 11, p. 235-266) support the nitrogen isotopic results. The occurrence of widespread nitrogen fixation during sapropel deposition is puzzling, as it implies the existence of stratified, oligotrophic conditions necessary for diazotrophs to thrive. An elaboration of these questions and a possible resolution will be presented.