Organic carbon variations of the eastern Mediterranean Holocene sapropel: a key for understanding formation processes

Abstract

This work reports on the organic carbon content of the S1 Holocene sapropel, the youngest and most extensively researched of the Eastern Mediterranean sapropels. The peak organic carbon percentage of the S1 sapropel varies between 1.2% and 3.4% at varying locations. These variations could not be correlated with sapropel thickness, or with spatial location. In contrast, there was a close linear correlation with water depth, the deeper samples containing the highest values of organic carbon. Within the S1 layer, the highest values were found in the top of the sapropel. Even if primary productivity increased during the S1 sapropelic period, the main factors responsible for sapropel deposition were changes in the degradation and preservation of organic matter in the water column and at the sediment–water interface. Water column stratification created an oxygenated surficial layer, 0–350/400 m thick, and a stagnant deep water layer. The majority of the organic material escaping the surficial layer was remineralised either at the sediment–water interface or just below it. Organic matter preserved in sapropelic sediments can be assumed to have remained for a significantly longer time (∼1000 years) within the benthic boundary layer than in the water column. Organic flux decreased with depth through the stagnant deep water layer. When stagnation was established, water exchanges either did not exist or were limited. Vertical oxidant transport depended on eddy diffusion, while oxidant fluxes at the sediment–water interface decreased with depth. The organic matter mineralisation rate also decreased and the burial efficiency increased. The linear relationship between sapropel organic content and depth was reflected in a linear decrease in oxidant flux. Regardless of location, the same organic carbon percentage was preserved at the same depth. In contrast, in the same area but at different depths, organic carbon content ranges between 1.2 and 3.4%. This observation suggests that primary productivity was of the same order of magnitude basinwide. The organic carbon concentration of sediment should be used with caution as a quantitative record of palaeoproductivity.