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

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 be tween 1.2% and 3.4% at varying locations. These variations could not be cor related with sapropel thickness, or with spatial location. In contrast, the re was a close linear correlation with water depth, the deeper samples cont aining the highest values of organic carbon. Within the S1 layer, the highe st values were found in the top of the sapropel. Even if primary productivi ty 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. Wa ter column stratification created an oxygenated surficial layer, 0-350/400 m thick, and a stagnant deep water layer. The majority of the organic mater ial escaping the surficial layer was remineralised either at the sediment-w ater interface or just below it. Organic matter preserved in sapropelic sed iments can be assumed to have remained for a significantly longer time (sim ilar to 1000 years) within the benthic boundary layer than in the water col umn. Organic Aux 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 or ganic matter mineralisation rate also decreased and the burial efficiency i ncreased. The linear relationship between sapropel organic content and dept h was reflected in a linear decrease in oxidant flux. Regardless of locatio n, the same organic carbon percentage was preserved at the same depth. In c ontrast, in the same area but at different depths, organic carbon content r anges between 1.2 and 3.4%. This observation suggests that primary producti vity was of the same order of magnitude basinwide. The organic carbon conce ntration of sediment should be used with caution as a quantitative record o f palaeoproductivity. (C) 2000 Elsevier Science B.V. All rights reserved.