Characteristics and possible formation process of ferromanganese beachrock in an intertidal zone of East China Sea influenced by buried ancient woods

The ferromanganese (Fe[sbnd]Mn) deposits could be found in many shallow water and deep-ocean places. Lots of studies have been performed to explore the occurrence, formation and economic value of Fe[sbnd]Mn nodules or crusts on the deep ocean floor. However, the occurrence and formation of Fe[sbnd]Mn beachrocks at the coastal zones has rarely been reported. In the present study, we collected two types of dendritic Fe[sbnd]Mn beachrock samples from an intertidal zone of Zhoushan Archipelago, East China Sea, where was significantly amended by an ancient wood layer formed during 5900–5600 cal. yr B.P. Then the microscale analysis including electron microprobe analyzer (EMPA), X-ray powder diffraction (XRD), and scanning electron microscopy-energy dispersive X-ray spectrometer (SEM-EDS) were performed to study the microscale characteristics on these Fe[sbnd]Mn beachrocks. Combined with our previous studies, a possible formation process was proposed: (1) Buried ancient woods produced enough organic acids to accelerate the weathering of nearby bedrock via chemical or biological reactions; (2) The seepage water could be regarded as a bridge to connect the ancients woods and the intertidal zones, and large amounts of dissolved Fe²⁺ and Mn²⁺ can be transported to the sandy beach and intertidal zones; (3) Precipitation of insoluble Fe[sbnd]Mn oxides occurred with the involvement of microbial processes and direct inorganic chemical sorption under mixing of seepage water and seawater; (4) After that, these Fe[sbnd]Mn oxides would experience nucleation, crystallization growth, and then diagenetic process to form the Fe[sbnd]Mn beachrocks. The present study spotted a light on the microscale features, formation process and biogeochemical cycling of nearshore/coastal diagenetic Fe-Mn deposits.