Abstract
Over a wide area of the eastern Mediterranean basin, two Mn-rich layers have been observed above the most recent sapropel (S1), one immediately above the sapropel top and one a few centimetres closer to the sediment 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 lower peak is actively forming and the upper peak has a different formation mechanism. High-resolution porewater, including a gel sampler used for the first time in marine sediments, and solid phase data are now used to demonstrate that the oxidation front is located at the level of the lower Mn peak which, therefore, is presently being formed. A barium-organic carbon relationship is used to calculate the initial organic carbon profile of the S1 sapropel. The palaeoproductivity profiles generated by this method demonstrate that the original sapropel unit was 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 front. The penetration depth of this oxidation front, i.e., the distance 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 precipitation upon oxygenation of previously anoxic eastern Mediterranean deep water, or preservation of a surficial Mn peak at the end of the high productivity episode. In either case the upper Mn peak marks the end of sapropel formation as indicated by the Ba profiles. This means that formation of the S1 sapropel ceased more recently than is indicated by radiocarbon dating of the visible top of S1.
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