High-resolution geochemical studies on the most recently-accumulated sapropel S1 in the eastern Mediterranean

Abstract

The sediments of the eastern Mediterranean Sea contain Corg-rich layers, termed sapropels, interbedded with Corg-poor sediments which form by far the greater part of the sedimentary record. This research presents a high-resolution geochemical investigation on slowly- and rapidly-accumulated SI units.Mn, Fe, I and Se profiles in slowly-accumulated SI units (5-10 cm/kyr) have shown that these sapropels are affected by post-depositional oxidation following their formation which produces a much thinner sapropel than was originally deposited. In the slowly-accumulated examples investigated, up to 11.5cm of the original sapropel has been lost through oxidation in as little as 6 kyr. Downward progression of the redox front causes the remobilisation and re-arrangement of redox-sensitive elements (As, Fe, I, Mn, Mo, Se,U and V) around the oxic/post-oxic boundary. Concentration-depth profiles of the redox-sensitive elements in SI have shown that these elements are immobilised in oxic and post-oxic conditions or both. It is noted that I and Se form well-defined peaks at the top of SI, and it is suggested that these two elements may be reliable markers for defining the location of an active oxidation front in the absence of pore water measurements. During this research, it was discovered that Hg behaves similarly to I and Se at the oxic/postoxic boundary, forming a well-defined peak in post-oxic conditions. In turbidite and sapropel examples, Hgis always closely associated with Se and it is suggested that Hg and Se become immobilised through the formation of the selenide mineral tiemannite (HgSe). The presence and persistence of Hg peaks in both recent and ancient sediments (up to 4 Ma) implies that Hg may be a useful diagnostic tool for defining the locations of both active and relict oxidation fronts. Investigation of two rapidly-accumulated Sis from the Adriatic (MD 90-917) and Aegean Seas(LC21) has shown that these cores have suffered negligible post-depositional oxidation due to rapid accumulation rates. The validity of the Ba/Al ratio as a more reliable and persistent productivity index has been confirmed in both LC21 and MD 90-917 where Ba/Al is directly related to Corg content over the entirevisual SI units. In both cores SI appears as a doublet. This doublet is centred on 7.5 kyr BP and geochemical and micropalaeontological evidence indicates that this "saddle" is best interpreted as an episode of improved reventilation and increased deep-water O2 concentrations. A number of redox-sensitive elements are enriched (Cr, Mo, Ni, S, Se, U, V and Zn) in LC21 and MD 90-917, and exhibit double peaks in their concentration depth profiles. The principal routes by which SI develops high authigenie levels of redox-sensitive elementsare: (i) pre-concentration in the water column by biological uptake followed by deposition at the sediment water interface and/or (ii) diffusion from seawater through pore waters into sediments followed by reductionand immobilisation under more reducing conditions. Under more reducing conditions, redox-sensitive elements are immobilised through associations with Corg, pyrite, sulphides or form insoluble solid phases(e.g. U is precipitated and immobilised as UO2(S)). Palaeoproductivity estimates based on Ba/Al weight ratios indicate that productivity was up to 5 times higher during SI formation compared with the present. It is suggested that high productivity wasinitiated by the formation of a deep chlorophyll maximum formed in response to an increased input of freshwater into the eastern Mediterranean. During SI formation, the flux of Corg greatly exceeded the supply of dissolved O2 to the bottom waters and so it is inferred that deposition of SI occurred under anoxic conditions. Interpretations of geochemical parameters such as S/C criteria, I/Corg and V/(V+Ni) ratios and framboidal pyrite sizes all indicate that bottom waters were anoxic-non sulphidic rather than anoxic-sulphidic during SI formation. The geochemical interpretation is in agreement with the benthic foraminiferal data, which show that the two S1 layers have an abundance of low-oxygen tolerant species. It is unclear whether anoxic conditions are induced by increased productivity alone or through a combination of increased productivity and reduction in deep-water formation. Evidence presented indicates that productivity and preservation are both necessary prerequisites for SI deposition. SI initiation and cessation was investigated by AMS 14C dating based upon the Ba/Al ratio criterion. AMS 14C ages for slowly-accumulated sapropels indicate that SI formation started at 10 kyr BP and ended by 5.3 kyr BP. For the rapidly-accumulated cores, the onset and termination of SI is more variable although SI was underway by 9.5kyr BP and complete by 6.0 kyr B.P. The offset in ages is attributed to bioturbation mixing artefacts affecting the Ba/Al signal in the slower-accumulated cores. Since rapidly-accumulated coresare not significantly affected by bioturbation, the best estimate for the duration of SI is 9.5-6.0 kyr B.P.

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