Biogeochemical controls on carbonate dynamics driven by methane and freshwater inputs in shallow sediments of a brackish continental shelf sea

Abstract. Continental shelf seas are essential to the carbon cycle, supporting nearly one-third of global marine primary production and significantly contributing to the burial of organic and inorganic carbon. Processes such as organoclastic sulfate reduction (OSR), methane production or anaerobic oxidation of methane (AOM), occurring in coastal sediments, have a significant impact on the carbon cycling in the marine environment. That impact may be modified by an increased organic matter (OM) supply or freshwater input. In the present interdisciplinary study, we investigated carbonate dynamics in three benthic environments of a brackish continental shelf sea: (1) anoxic sediments dominated by OSR; (2) nearshore sediments with high terrigenous OM input and active methanogenesis; and (3) sediments influenced by groundwater infiltration and pore-water freshening, coupled with methanogenesis. We analyzed pore-water chemistry (DIC, total alkalinity, major ions, nutrients), sediment geochemistry (CH₄, total organic carbon, total nitrogen, total sulfur), and mineralogy, including authigenic carbonates and iron sulfides. We used stable isotopes of DIC (δ¹³C-DIC), methane (δ¹³C-CH₄, δ²H-CH₄) and organic matter (δ¹³C, δ¹⁵N) to trace carbon transformations. We also determined rates of OSR and AOM experimentally. Based on these data, we have proposed conceptual models of carbon cycling in the three sedimentary environments. At the methane-free sediment, organic matter degradation was primarily governed by OSR, producing 53.6 mmol m^-2 d^-1 of DIC within the top 100 cm, accompanied by limited carbonate burial and dominant pyrite accumulation. In contrast, methane-bearing sediments displayed markedly enhanced carbonate dynamics. The role of methane-related processes became increasingly important in sediments influenced by freshwater input, where pore water freshening reduces sulfate availability, shallows the sulfate-methane transition and stimulates exceptionally high AOM rates reaching up to 1077 μmol dm^-3 d^-1, resulting in a DIC production rate of 331 mmol dm^-3 d^-1, more than fivefold higher than OSR. This environment facilitates significant authigenic dolomite burial (ranging from 467 to 994 μmol m^-2 d^-1), illustrating efficient inorganic carbon sequestration within these sediments. Our study provides the first quantitative estimate of authigenic dolomite burial rates in Baltic Sea sediments.