Enrichment effect and environmental control of clay reactive iron in the Changjiang River estuary and East China Sea

Chemical speciation analysis of iron (Fe) is a crucial method for understanding sediment provenance, environmental evolution, and the biogeochemical cycling of iron in various environments. However, there are limitations in studying iron speciation, especially in sediments in different grain sizes, which hinders the comprehensive understanding of the iron cycle. In this study, we focused on the surface sediments from the Changjiang River estuary to East China Sea shelf. We employed a six-step extraction method to obtain the concentrations of total Fe (Fe_T), highly reactive Fe (Fe_HR), poorly reactive Fe (Fe_PR), and unreactive Fe (Fe_U) in both bulk sediment samples and their clay fractions. Results show an order of Fe_HR＞Fe_PR＞Fe_U in abundance. Fe_T and Fe_HR contents in the bulk sample were closely related to the mean grain size and the concentrations of clay, TOC, and Al, indicating that clay minerals rich in organic matter are prone to enrich Fe_HR. The Fe_HR/Fe_T ratio in the clay fraction increased by 10% and the Fe_PR/Fe_T ratio in the clay fraction decreased by 10% compared to the bulk sample, indicating an enrichment effect of Fe_HR on clay minerals. The dynamic estuarine environment controlled the distribution of Fe speciation in sediments, with higher Fe_T and Fe_HR contents observed in the bulk sediment samples from the turbidity maximum zone of the Changjiang River estuary, which significantly influenced by grain size. The clay fraction could effectively eliminate the influence of grain size, with Fe_T and Fe_HR being heavily retained in the low-salinity region at the forefront of the turbidity maximum zone, while in the medium to high-salinity offshore areas, the sources of Fe remained relatively stable, being mainly the mixture of Fe_HR-rich sediment from the Changjiang River and Fe_HR-poor sediment from the shelf. This study revealed that the migration of clay fractions from the watershed to the estuary and shelf might dominate the distribution and cycling of highly reactivity Fe at the land-sea interface, and provided important insights into the sources and sinks of particulate Fe in the ocean, geochemical cycling, and their environmental effects.