Publication Date:
2023-08-10
Description:
Sequences of sapropels intercalated with hemipelagic mud are a prominent feature of most eastern Mediterranean sediments. The most recent sapropel (S 1 ), recovered in a box core from the Medina Rise in the Ionian Sea, was sampled at ultra-high resolution to evaluate the paleoceanographic conditions during its formation, to characterise its organic matter and to determine post-depositional effects of diagenesis on the geochemistry. The paleoceanographic reconstruction was based on oxygen and carbon isotope compositions of planktonic foraminifera and planktonic foraminiferal census counts to estimate paleo sea surface temperatures. Combined, these results indicate a depleted surface water salinity of about 4 psu and, thus, also surface water density, which at least weakened vertical circulation and thus, bottom water ventilation during sapropel formation. However, sapropel formation may not have been caused by depleted bottom water oxygen levels alone. Significantly increased rates of primary production during times of sapropel deposition are inferred from enhanced barium accumulation rates. The immobility and stability of barium as barite in most Mediterranean sediments makes it a valuable proxy for paleoproductivity in this case. Lipid analyses were conducted for characterisation of sapropel organic matter and estimation of possible carbon sources. Results of the extractable lipids clearly indicate a predominantly marine origin, with dinoflagellates, coccolithophorid algae, other microalgae and eubacteria as main contributing organisms. Comparison with overlying oxidised samples reveals no enrichment of terrestrial organic matter. The ratio of unsaturated long-chain-ketones is strongly affected by sapropel oxidation, so that the derived paleotemperature estimates should used with caution.
Elemental concentrations were measured to recognise effects of the post-depositional oxidation front prograding into the formerly anoxic sediment. As a consequence, the present thickness of the organic-rich layer is only a fifth of its original extent. The maximum dissolution effect of diagenetic reactions cannot explain the observed depletions in carbonate content in the sapropel. Decreased carbonate production during sapropel formation is therefore concluded. Diagenetic relocation of many redox-sensitive elements has occurred. Most elements expected to be associated with organic matter or sulphide-rich sediments display maximum concentrations above or below the present sapropel layer. Similarities are observed in the geochemical behaviour of Ni, Cu, Zn, and Co; and of Mo, V, and Sb. Se exhibits a large, sharp concentration peak and is inferred to be a useful marker for the present boundary of oxic to post-oxic conditions. A model of the oxygen-flux into the sapropel interval closely resembles the observed enrichments of oxidised elements. The sapropel is thus still being oxidised.
Type:
Thesis
,
NonPeerReviewed
Format:
text
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