Review of recent advances in the interpretation of eastern Mediterranean sapropel S1 from geochemical evidence

摘要

The sediments of the eastern Mediterranean basincontain Corg -poor sediments which form by far the greater part of therecord. while it is generally appreciated that different surface oceanproductivity and bottom water conditions are necessary for theformation and preservation of these two sediment types, less attentionhas been paid to diagenetic effects which are an expectedconsequence of transitions between dramatically different bottomwater oxygenation levels. A geochemical interpretation has emergedof post-depositional oxidation of the most recent sapropel (S1),initially based on the relationship of the Mn, Fe, Corg and Sconcentration/depth profiles observed around S1, and thecharacteristic shapes of these elemental profiles known from othersituations. This indicates that post-depositional oxidation hasremoved appoximately half of the visual evidence of the sapropel (~6cm from a total of ~12 cm in the deep basin). The oxidationinterpretation from redox-sensitive element redistribution profileshas subsequently been consolidated with evidence from pore water(O2, , Mn2+ and Fe2+)studies, from characteristic solid phase Ba profiles which yieldpalaeoproductivity records, and from oxidation-sensitive indicatortrace elements (I and Se). So far, these geochemical observations havebeen concentrated in the deeper central parts of the basin, wheresediment accumulation rates are lower than on the basin margins, andradiocarbon dating indicates that S1 formation occurred between 5.3and 9.0 ky (uncorrected conventional radiocarbon time). It remains tobe demonstrated whether or not these times are applicable to theentire E. Mediterranean basin. The implications of these findings toguide sampling in future work on the S1 productivity episode and onolder sapropels for palaeoenvironmental investigations arediscussed.?1999 Elsevier Science B.V. All rights reserved.