Review of recent advances in the interpretation of eastern Mediterranean sapropel S1 from geochemical evidence |
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| Institution: | 1. Southampton Oceanography Centre, Empress Dock, Southampton SO14 3ZH, UK;2. Department of Geochemistry, Institute of Earth Sciences, University of Utrecht, P.O. Box 80.021, 3508 TA Utrecht, Netherlands |
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| Abstract: | The sediments of the eastern Mediterranean basin contain Corg-enriched layers (sapropels) interbedded with the Corg-poor sediments which form by far the greater part of the record. While it is generally appreciated that different surface ocean productivity and bottom water conditions are necessary for the formation and preservation of these two sediment types, less attention has been paid to diagenetic effects which are an expected consequence of transitions between dramatically different bottom water oxygenation levels. A geochemical interpretation has emerged of post-depositional oxidation of the most recent sapropel (S1), initially based on the relationship of the Mn, Fe, Corg and S concentration/depth profiles observed around S1, and the characteristic shapes of these elemental profiles known from other situations. This indicates that post-depositional oxidation has removed approximately half of the visual evidence of the sapropel (～6 cm from a total of ～12 cm in the deep basin). The oxidation interpretation from redox-sensitive element redistribution profiles has subsequently been consolidated with evidence from pore water (O2, NO?3, Mn2+ and Fe2+) studies, from characteristic solid phase Ba profiles which yield palaeoproductivity records, and from oxidation-sensitive indicator trace elements (I and Se). So far, these geochemical observations have been concentrated in the deeper central parts of the basin, where sediment accumulation rates are lower than on the basin margins, and radiocarbon dating indicates that S1 formation occurred between 5.3 and 9.0 ky (uncorrected conventional radiocarbon time). It remains to be demonstrated whether or not these times are applicable to the entire E. Mediterranean basin. The implications of these findings to guide sampling in future work on the S1 productivity episode and on older sapropels for palaeoenvironmental investigations are discussed. |
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