Pockmarks are formed as a result of gas (methane) or/and groundwater outflow from the sea bottom. Methane, the second most important (after CO₂) greenhouse gas, has a significant impact on biogeochemical processes in the bottom sediments by affecting the cycling of some elements, e.g. C, Fe, and S. Active pockmarks may also lead to changes in water column conditions by causing nutrients release from sediments. In the present study, we have focused on the impact of biogeochemical processes in pockmarks (methanogenesis, anaerobic methane oxidation, and groundwater seepage) on the transformation of iron (Fe) and the mineral composition of the sediment. In pore water, concentrations of hydrogen sulfide, phosphate, ammonia, sulfate, chloride, dissolved inorganic carbon, iron, and methane were analyzed. In the sediment, Fe speciation was performed using sequential extraction. The mineral composition was determined using powder X-Ray diffraction and scanning electron microscopy. The results from two pockmarks (with active gas seepage and groundwater infiltration) and two reference stations in the southern Baltic Sea show that geochemical conditions in pockmark sediments are significantly different from those in the typical muddy sea bottom. Pore water in pockmarks is characterized by lower sulfate and higher dissolved carbon concentrations as compared to areas of the seafloor where such structures are absent. This is due to the outflow of groundwater, which was confirmed by lower chloride concentration. In addition, sulfate is used to oxidize methane diffusing from deeper layers. Sediments in pockmarks are enriched in Fe(II) carbonates and depleted in Fe(III) (oxy)hydroxides, resulting from the anaerobic oxidation of methane with Fe(III) (Fe-AOM). Ferrous iron produced in large quantities during Fe-AOM is precipitated with carbonates.

麻点是由于气体（甲烷）或/和地下水从海底流出而形成的。甲烷是第二重要的温室气体（仅次于 CO ₂），通过影响某些元素（例如 C、Fe 和 S）的循环，对底部沉积物中的生物地球化学过程产生重大影响。活性麻点也可能导致水的变化通过引起沉积物中的营养物释放来改变柱条件。在本研究中，我们重点研究麻点中生物地球化学过程（产甲烷、厌氧甲烷氧化和地下水渗漏）对铁 (Fe) 转化和沉积物矿物成分的影响。分析了孔隙水中硫化氢、磷酸盐、氨、硫酸盐、氯化物、溶解的无机碳、铁和甲烷的浓度。在沉积物中，通过连续萃取进行铁形态形成。使用粉末 X 射线衍射和扫描电子显微镜测定矿物成分。波罗的海南部两个麻点（有活跃的气体渗漏和地下水入渗）和两个参考站的结果表明，麻点沉积物的地球化学条件与典型泥质海底的地球化学条件存在显着差异。与不存在此类结构的海底区域相比，麻点中的孔隙水具有较低的硫酸盐浓度和较高的溶解碳浓度。这是由于地下水外流，氯化物浓度降低证实了这一点。此外，硫酸盐还用于氧化从深层扩散的甲烷。麻点中的沉积物富含 Fe(II) 碳酸盐，而缺乏 Fe(III)（羟基）氢氧化物，这是由甲烷与 Fe(III) (Fe-AOM) 进行厌氧氧化而产生的。Fe-AOM 过程中大量产生的二价铁与碳酸盐一起沉淀。