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

Abstract

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.Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site. Highlights► Geochemical analyses highlight multiple diagenesis processes occurring in the sediment. ► Intense methane seepages and organic matter degradation contribute to the sulfate reduction. ► Chemical of dissolved and mineral iron species indicate that iron is associated with clay minerals. ► In response to seawater intrusion, ion exchange, dissolution and reverse weathering reactions change the composition of clay constituting the sediment.

Characterizing the variability of natural gas hydrate composition from a selected site of the Western Black Sea, off Romania

“…1) a light-grey zone free of sediment, with white patches (A, Fig. 4, delineated by a white which is also a GH-rich zone where the brown color indicates the presence of iron-bearing minerals at an elevated concentration in the Black Sea (Egger et al, 2016), as also shown in a complementary study (Ruffine et al, 2021), 3) a dark-grey area with small white patches, which is a less rich GH-zone, at the bottom right-hand side (Fig. 4, C, delineated by a green dotted line) where solid hydrates (or ice) are intermixed with sediments.…”

“…However, previous studies have shown that the limnic sediment is characterized by a lower salinity (Ruffine et al, 2021;Soulet et al, 2010). Therefore, the seepage of brine is unlikely in the study area.…”

“…J o u r n a l P r e -p r o o f X-ray diffraction applied to the sediment in another study (Ruffine et al, 2021) reveals that clays and mica represent between 53 and 67 wt.% of the matrix, with illite being by far the dominant species. The remaining minerals are mainly silicates (feldspar, plagioclase and quartz) and carbonates.…”

“…It was the unique core containing hydrates from a series of >20 cores, and it was mainly composed of Unit 3 lacustrine sediment (Ross and Degens, 1974). The youngest sediments deposited during the Holocene, representing the lacustrine-marine transition (Unit II) and the marine stage (Unit I), were not clearly identifiable; possibly due to a loss of the uppermost part of the core as explained by Ruffine et al (2021). Thus, hydrates were located within the lacustrine sediments.…”

“…While H 2 S is involved in the precipitation of several iron-bearing minerals (especially pyrite, FeS 2 ) (Canfield et al, 1992;Jørgensen et al, 2004;Ruffine et al, 2021) show that H 2 S concentration of the pore water decreases near-zero values within the first meter of sediment and pyrite is present in very small amounts. Thus, H 2 S is largely consumed above the GHOZ and a small amount would remain to be enclathrated as hydrate, as the distance between AOM, OSR, and the GHOZ is short and less than 1 m. Besides, deeply sourced hydrogen sulfide can originate from thermogenic gas reservoirs (Machel, 2001;Machel et al, 1995;Ruffine et al, 2018) or brines that are enriched in this compound (Commeau et al, 1987).…”