Reduction–oxidation (redox) reaction conditions, which are of great importance for the soil chemistry of coastal marshes, can be temporally dynamic. We present a transect of cores from northwest Florida wherein radical postdepositional changes in the redox regime has created atypical geochemical profiles at the bottom of the sedimentary column. The stratigraphy is consistent along the transect, consisting of, from the bottom upward, carbonate bedrock, a gray clay, an organic mud section, a dense clay layer, and an upper organic mud unit representing the current saltwater marsh. However, the geochemical signature of the lower organic mud unit suggests pervasive redox reactions, although the interval has been identified as representing a freshwater marsh, an unlikely environment for such conditions. Analyses indicate that this discrepancy results from postdepositional diagenesis driven by millennial-scale environmental parameters. Rising sea level that led to the deposition of the capping clay layer, created anaerobic conditions in the freshwater swamp interval, and isolated it hydrologically from the rest of the sediment column. The subsequent infiltration of marine water into this organic material led to sulfate reduction, the buildup of H₂S and FeS, and anoxic conditions. Continued sulfidation eventually resulted in euxinic conditions, as evidenced by elevated levels of Fe, S, and especially Mo, the diagnostic marker of euxinia. Because this chemical transformation occurred long after the original deposition the geochemical signature does not reflect soil chemistry at the time of deposition and cannot be used to infer syn-depositional environmental conditions, emphasizing the importance of recognizing diagenetic processes in paleoenvironmental studies.

中文翻译：

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淡水沼泽如何产生盐沼的化学特征？

还原-氧化（氧化还原）反应条件对沿海沼泽地的土壤化学非常重要，可以随时间变化。我们提出了一个来自佛罗里达州西北部的岩心样板，其中氧化还原制度中的激进沉积后变化已在沉积柱底部形成了非典型地球化学剖面。地层沿断面一致，由下往上依次为碳酸盐岩基岩，灰色黏土，有机泥段，致密黏土层和代表当前咸水沼泽的上部有机泥浆单元。然而，下部有机泥单元的地球化学特征暗示了普遍的氧化还原反应，尽管该区间被确定为代表淡水沼泽，在这种情况下这种环境不太可能发生。分析表明，这种差异是由千禧年规模的环境参数驱动的沉积后成岩作用引起的。不断上升的海平面导致覆盖的粘土层沉积，在淡水沼泽地带创造了厌氧条件，并在水文上将其与沉积物柱的其余部分隔离开来。随后海水渗透到该有机物质中导致硫酸盐还原，H的积累₂ S和FeS，以及缺氧条件。持续的硫化最终导致了富营养状态，这是由富营养的诊断标记物Fe，S，尤其是Mo含量升高所证明的。因为这种化学转化发生在原始沉积之后很长时间，所以地球化学特征不能反映沉积时的土壤化学，也不能用于推断同沉积环境条件，从而强调了在古环境研究中认识成岩过程的重要性。