Isotope biogeochemistry of the carbon-iron-sulfur cycle in a temperate coastal peatland after flooding by brackish seawater

Abstract

Soils from a coastal peatland (Drammendorf,  southern Baltic Sea) were investigated for the biogeochemical impact of flooding with brackish seawater. The peatland was rewetted in late 2019 through the partial removal of a dyke and brackish water with high sulfate concentration from a lagoon (Kubitzer Bodden) allowed to intrude into the peatland. Soil cores were retrieved about 2 and 3 years after the initial rewetting event. Pore waters were extracted from soil cores using rhizons and samples were analyzed besides physical parameters for major ions, nutrients, water stable isotopes, dissolved inorganic carbon (DIC) concentrations, and stable isotopes in C and S species. Solid phase samples were analyzed for contents of CNS and acid-extractable metal and nutrient species, and the stable isotope composition of acid-volatile sulfide (AVS), chromium-reducible sulfide (CRS, pyrite).Results from the post-event campaigns are compared with pre-flooding conditions. Poree water generally showed a trend towards freshening with depth as remains from the pre-flooding conditions. . Different sites are furthermore characterized by different amounts of diagenetically released dissolved inorganic carbon (DIC). A mixing evaluation of ẟ13C-DIC signatures together with major ion concentrations reveals potential DIC sources, like organic matter/methane oxidation, carbonate dissolution and mixing with seawater-derived DIC. DIC from the dissolution of minor soil carbonates may lead to a relative enrichment of 13C in DIC. Brackish water intrusion and cation exchange are reflected by the downward gradients in pore water compositions, with Na and Mg decreasing and Ca increasing with depth. Soil organic carbon is dominating with inorganic carbon being a minor fraction in most parts. Dissolved pore water sulfate and high total sulfur in the top soils with decrease downward trends further reflect the importance of post-event enhanced sulfur cycling leading to characteristic sulfur isotope signatures. AVS is depleted in the top soils with highest contents at about 5 cmbsf. Pyrite sulfur dominates and show different enrichment zones with the sediment columns. Contents in CRS contents covary with TOC, indicating that the benthic diagenetic system is controlled by organic matter availability, as in normal marine sediments.  The study demonstrates the role of electron acceptor availability for benthic carbon cycling, and the kinetics of biogeochemical interactions upon oxidation of reduced carbon, mineral authigenesis/dissolution, and ion exchange processes. The results of implications for coastal processes in the humid climate zone during times of increasing seawater level rise.