Trace Metal Geochemistry and Diagenetic Processes in Estuarine Sediments of the Imo River Delta

Trace metal behaviour in estuarine sediments is controlled by complex diagenetic processes including redox cycling, sulfide precipitation, and organic matter decomposition that determine both metal burial and potential remobilisation into overlying waters. This study examined the trace metal geochemistry and diagenetic processes in sediment cores from the Imo River Delta, South East Nigeria. Sediment cores (up to 60 centimetres depth) were collected from three estuarine stations representing different depositional environments. Sequential extraction analysis was applied to quantify metal (iron, manganese, zinc, copper, lead, arsenic, and cadmium) partitioning across five operationally defined fractions. Porewater chemistry including pH, dissolved oxygen, sulfate, and dissolved iron and manganese was measured at 2-centimetre depth intervals. Stable sulfur isotopes were used to trace diagenetic sulfide formation. Scanning electron microscopy with energy dispersive X-ray spectroscopy was applied to identify metal-bearing mineral phases. Iron and manganese showed classic redox cycling profiles with enrichment in porewater at the sulfate-methane transition zone. Lead and copper were predominantly associated with organic matter and sulfide fractions, suggesting low remobilisation risk under stable anoxic conditions. Arsenic was anomalously enriched in the authigenic fraction and showed evidence of desorption from iron oxyhydroxides during iron reduction. Stable sulfur isotope fractionation patterns confirmed active microbial sulfate reduction at all stations. The study provides a detailed biogeochemical characterisation of trace metal cycling in a tropical delta system and identifies arsenic remobilisation as a potential groundwater contamination pathway. Keywords: trace metals, sediment geochemistry, diagenesis, Imo River Delta, arsenic.