Abstract
The original geochemical signal in marine Corg-rich or formerly Corg-rich sediments is usually highly overprinted by a sequence of diagenetic processes depending on the amount of metabolizable organic carbon and the environmental conditions determining the redox potential. In general, biogenic barium can be used as a tracer for the original
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interval of Corg-rich sediment accumulation even when the latter has been largely removed during subsequent early diagenesis. The investigation of a range of different deep-sea sedimentary environments has shown that the detrital Ba/Al ratio is the crucial factor in the normative approach. Diagenetic processes not only result in the oxidation of organic matter but also in the remobilization of mineral phases as Mn and Fe oxides. The interrelation of a redox-horizon due to non-steady state diagenetic conditions between the last glacial and the Holocene has been reconstructed by the combination of high resolution geochemical and rock-magnetic analyses. The movement of the redox horizon led to the formation of conspicuous solid phase double peaks for Mn and Fe but also to definite anomalies in the vicinity of the redox horizon referring to dissolution of magnetic mineral phases owing to suboxic diagenesis. Such a Mn double peak feature is also known form eastern Mediterranean sapropel S1 sediments, where the double peaks had developed due to post-depositional oxygen penetration and Corg burn-down. However, the magnitudes of the upper Mn peaks are much higher (>3 wt%) within sediments from the eastern Mediterranean intermediate water depth interval (1000 to 2000 m). The change in redox conditions within the water column is responsible for this Mn-enrichment. The intermediate water depth interval preferentially gained Mn2+ that encounters oxygen and precipitates as MnOx where anoxic and oxic waters meet, whereas the Mn2+ got diagenetically lost from the deeper sediments into the anoxic bottom waters. However, the eastern Mediterranean manganese profile shapes can be diagenetically altered in areas that experienced substantial tephra or turbidite depositions such as those initiated by the Santorini explosive eruption in ~1630 B.C. This perturbation can be identified from the increased Sr/Ca and some element/Al ratio profiles. The Sr/Ca ratio, however, is not only increased in the Santorini eruption related turbidites above sapropel S1 but also in the sapropel sediments itself. This enhanced Sr/Ca ratio is associated to the enhanced Sr-rich aragonite content in the sapropel that reaches levels of up to 40 wt.% at the Sirte continental slope (northern Africa). The observed high aragonite contents could be explained by a near-costal source delivering Sr-rich aragonite skeletons or fragments thereof or by secondary diagenetic formation processes that are known to produce Sr-rich aragonite. The relationship of decreasing aragonite values with distance to the African coast and water depth has been emphasized. This relationship together with the SEM observation for the Sirte transect S1 sediments as well as the lack of variation in porosity, all point to a detrital/biogenic near-coastal source for the Sr-rich aragonite needles rather than a diagenetic one.
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