Effects of postglacial seawater intrusion on sediment geochemical characteristics in the Romanian sector of the Black Sea

Pore water and sediment geochemistry in the western Black Sea were investigated on long Calypso piston core samples. Using this type of coring device facilitates the recovery of the thick sediment record necessary to analyze transport-reaction processes in response to the postglacial sea-level rise and intrusion of Mediterranean salt water 9 ka ago, and thus, to better characterize key biogeochemical processes and process changes in response to the shift from lacustrine to marine bottom water composition. Complementary data indicate that organic matter degradation occurs in the upper 15 m of the sediment column. However, sulfate reduction coupled with Anaerobic Methane Oxidation (AOM) is the dominant electron-accepting process and characterized by a shallow Sulfate Methane Transition Zone (SMTZ). Net silica dissolution, total alkalinity (TA) maxima and carbonate peaks are found at shallow depths. Pore water profiles clearly show the uptake of K+, Mg2+ and Na + by, and release of Ca2+ and Sr2+ from the heterogeneous lacustrine sediments, which is likely controlled by chemical reactions of silicate minerals and changes in clay mineral composition. Iron (Fe2+) and manganese (Mn2+) maxima largely coincide with Ca2+ peaks and suggest a close link between Fe2+, Mn2+ and Ca2+ release. We hypothesize that the Fe2+ maxima below the SMTZ result from deep Fe3+ reduction linked to organic matter degradation, either driven by DOC escaping from the shallow sulfate reduction zone or slow degradation of recalcitrant POC. The chemical analysis of dissolved and solid iron species indicates that iron is essentially associated with clay minerals, which suggests that microbial iron reduction is influenced by clay mineral composition and bioavailability of clay mineral-bound Fe(III). Overall, our study suggests that postglacial seawater intrusion plays a major role in shaping redox zonation and geochemical profiles in the lacustrine sediments of the Late Quaternary.


Black sea, Clay minerals, Danube delta, Gas seeps, Iron reduction, Methane oxidation, Sulfate reduction

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Ruffine Livio, Deusner Christian, Haeckel Matthias, Kossel Elke, Toucanne Samuel, Chéron Sandrine, Boissier Audrey, Schmidt Mark, Donval Jean-Pierre, Scholz Florian, Guyader Vivien, Ker Stephan, Riboulot Vincent (2021). Effects of postglacial seawater intrusion on sediment geochemical characteristics in the Romanian sector of the Black Sea. Marine And Petroleum Geology. 123. 104746 (14p.). https://doi.org/10.1016/j.marpetgeo.2020.104746, https://archimer.ifremer.fr/doc/00653/76558/

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