ACTIVE POSTDEPOSITIONAL OXIDATION OF THE MOST RECENT SAPROPEL (S1) INSEDIMENTS OF THE EASTERN MEDITERRANEAN-SEA

Over a wide area of the eastern Mediterranean basin, two Mn-rich layer s have been observed above the most recent sapropel (S1), one immediat ely above the sapropel top and one a few centimetres closer to the sed iment surface. Different mechanisms have been proposed to explain the occurrence of these two Mn peaks: either both Peaks have a diagenetic origin in which case the upper Mn peak is actively forming, or the low er peak is actively forming and the upper peak has a different formati on mechanism. High-resolution porewater, including a gel sampler used for the first time in marine sediments, and solid phase data are now u sed to demonstrate that the oxidation front is located at the level of the lower Mn peak which, therefore, is presently being formed. A bari um-organic carbon relationship is used to calculate the initial organi c carbon profile of the S1 sapropel. The palaeoproductivity profiles g enerated by this method demonstrate that the original sapropel unit wa s bounded by the upper Mn peak. This implies that the interval between the two Mn peaks, where a low organic carbon content is now observed, was originally part of the sapropel. The initially deposited organic carbon has been oxidised by a progressive downwards-moving oxidation f ront. The penetration depth of this oxidation front, i.e., the distanc e between the two Mn peaks, is mainly determined by the organic carbon content, the sediment accumulation rate, and the bioturbation depth. The upper Mn peak appears to have formed as a result of either Mn prec ipitation upon oxygenation of previously anoxic eastern Mediterranean deep water, or preservation of a surficial Mn peak at the end of the h igh productivity episode. In either case the upper Mn peak marks the e nd df sapropel formation as indicated by the Ba profiles. This means t hat formation of the S1 sapropel ceased more recently than is indicate d by radiocarbon dating of the visible top of S1.