Although the majority of coastal sediments consist of sandy material, in some areas marine ingression caused the submergence of terrestrial carbon‐rich peat soils. This affects the coastal carbon balance, as peat represents a potential carbon source. We performed a column experiment to better understand the coupled flow and biogeochemical processes governing carbon transformations in submerged peat under coastal fresh groundwater (GW) discharge and brackish water intrusion. The columns contained naturally layered sediments with and without peat (organic carbon content in peat 39 ± 14 wt%), alternately supplied with oxygen‐rich brackish water from above and oxygen‐poor, low‐saline GW from below. The low‐saline GW discharge through the peat significantly increased the release and ascent of dissolved organic carbon (DOC) from the peat (δ¹³C_DOC − 26.9‰ to − 27.7‰), which was accompanied by the production of dissolved inorganic carbon (DIC) and emission of carbon dioxide (CO₂), implying DOC mineralization. Oxygen respiration, sulfate () reduction, and methane (CH₄) formation were differently pronounced in the sediments and were accompanied with higher microbial abundances in peat compared to sand with ‐reducing bacteria clearly dominating methanogens. With decreasing salinity and concentrations, CH₄ emission rates increased from 16.5 to 77.3 μmol m⁻² d⁻¹ during a 14‐day, low‐saline GW discharge phase. In contrast, oxygenated brackish water intrusion resulted in lower DOC and DIC pore water concentrations and significantly lower CH₄ and CO₂ emissions. Our study illustrates the strong dependence of carbon cycling in shallow coastal areas with submerged peat deposits on the flow and mixing dynamics within the subterranean estuary.

中文翻译：

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海底地下水排放控制下的沿海泥炭沉积物的碳释放和转化：一项柱实验

尽管大多数沿海沉积物由沙质组成，但在某些地区，海洋入侵导致陆地碳富集的泥炭土壤被淹没。这会影响沿海碳平衡，因为泥炭代表了潜在的碳源。我们进行了一项柱实验，以更好地理解在沿海淡水（GW）排放和微咸水入侵下，控制淹没泥炭中碳转化的耦合流动和生物地球化学过程。色谱柱包含有或没有泥炭的自然分层沉积物（泥炭中的有机碳含量为39±14 wt％），交替提供上方的富氧微咸水和下方的贫氧低盐GW。低盐GW通过泥炭排放，显着增加了泥炭中溶解有机碳（DOC）的释放和上升（δ¹³ C _DOC -26.9‰至-27.7‰），伴随着溶解性无机碳（DIC）的产生和二氧化碳（CO ₂）的排放，这意味着DOC矿化。与沙子相比，泥沙中的氧呼吸，硫酸盐（）还原和甲烷（CH ₄）形成明显不同，并且与泥沙相比，泥炭具有更高的微生物丰度，而减少细菌的细菌显然占优势。随盐度和浓度，CH ₄排放率从16.5提高到77.3  μ摩尔米^-2 d ^-1在14天低盐GW排放阶段。相比之下，含氧微咸水侵入导致DOC和DIC孔隙水浓度降低，并且CH ₄和CO ₂排放量显着降低。我们的研究表明，浅水沿海地区泥炭沉积对碳循环的强烈依赖性对地下河口内的水流和混合动力的影响。