THE BALTIC SEA AS AN ANALOGUE FOR THE FORMATION OF EASTERN MEDITERRANEAN SAPROPELS

The 1947 - 1948 Swedish Deep Sea Expedition with M/S Albatross revealed a remarkable Late Quaternary development of the eastern Mediterranean Sea (Kullenberg, 1952). Several organic rich layers (sapropels S1 - S11) with black sulphides indicated long periods of stagnation when the preservation of organic matter increased and hydrogen sulphide was accumulated in the water column. In the literature usually two hypothesis are favoured to explain these stagnant periods. 1. High primary production, which during degradation increase oxygen utilisation and, hence, the preservation of organic matter settling to the ocean bottom. 2. A highly stratified water column with bottom water stagnation without oxygen renewal, which would increase organic carbon preservation. The most plausible explanation is a combination of these two hypotheses, which is the present situation in the Baltic Sea, where organic rich layers are accumulated together with black iron sulphides as minute laminae in a sapropelic type of sediments. The Baltic Sea experienced such a stagnation period at around the time of the latest sapropel, S1, and is presently suffering from intermittent stagnation of the bottom water and can thus be used as an analogue to explain the sapropel sequences of the eastern Mediterranean subbasins. The development mirrors the general sedimentological and geochemical sequence of a semi-enclosed sea affected by water stratification due to salinity gradients. The interpretation of the geochemical sapropel sequence as a result of a true sedimentation sequence or diagenesis has been argued upon by several scientists (e.g. Murat and Got, 1987, De Lange et al., 1989, Pruysers et al., 1993 and Passier et al., 1996). This paper presents trace element and stable sulfur isotope data as paleo-geochemical proxies. They form the base for a discussion on redox chemistry and possible diagenesis during formation of sapropelic sediments.