Characterization of Fe-S minerals influenced by buried ancient woods in the intertidal zone, East China Sea

An ancient wood layer dated at about 5600 cal. a BP by AMS¹⁴C dating was discovered in the intertidal zone, East China Sea. Samples affected by ancient woods, including fresh coast bedrock, weathering bedrock, seepage water from coast, seepage water from ancient wood layer, intertidal seawater, fresh water, beach mud, ancient wood barks and ancient peat, were collected for geochemical analysis. The beach mud and the bacteriogenic iron oxides (BIOS) in coastal seepage water were analyzed by mineralogical and high-resolution transmission electron microscopy (HRTEM)-selected area electron diffraction (SAED) analysis. Inorganic sulfur compositions and δ ³⁴S of the ancient peat and the beach mud were determined. The results showed that Fe, Mn, S (SO ₄ ^2- ) were enriched in the intertidal area at different levels, very likely caused by fermentation of ancient woods. The presence of abundant ironoxidizing bacteria (FeOB) and sulfate-reducing bacteria (SRB) in this intertidal zone was confirmed by HRTEM-SAED observation, and these bacteria were involved in Fe-S cycle to induce extracellular biomineralization. The negative δ ³⁴S_V-CDT (-2.9‰) likely indicated the biogenic origin of iron-sulfide minerals in the beach mud at high sulfate reduction rate (SRR). These findings are helpful for understanding the biogeochemical Fe-S cycle and biomineralization process at high organic matter deposition rate and high SRR in the intertidal zone, estuary, or near shoreline.