Source-sink process of reactive iron in shelf sediments controlled by Holocene depositional environments and redox states: Insight from the Central Yellow Sea Mud

Iron in shelf sediments is crucial in the marine iron cycle, significantly impacting the global C-S-Fe biogeochemical cycles. However, its fate in shelf sediments remains unclear. This study focuses on sediment core YSCW-1 from the Central Yellow Sea Mud, utilizing sediment grain size, redox-sensitive elements U and Mo, and iron speciation to explore the control mechanisms of redox states and sedimentary environments on highly reactive iron since the Holocene. Sediments in core YSCW-1 can be classified into U1, U2, U3, and U4 units based on lithology, grain size, and geochemical characteristics. The results indicate that iron in the Central Yellow Sea Mud is relatively enriched in the mud-depocenter, occurring in fine-grained terrestrial sediments derived from continental chemical weathering, but its fate varies among different units. In U1 (9.3–9.0 ka), iron was mainly sourced from the Yellow River, deposited under oxic–suboxic conditions with pyrite iron as the dominant reactive iron speciation. In U2 (9.0–7.3 ka), an estuarine environment with low salinity and oxidizing conditions contributed to the increase of carbonate iron. In U3 (7.3–3.1 ka), the establishment of modern circulation in the Yellow Sea led to the dominance of Yangtze River-derived sediments, and the stable, low-energy environment promoted the prevalence of pyrite iron. U4 was deposited under oxic–suboxic conditions, with the iron dissimilatory reduction being the dominant process of organic matter mineralization, resulting in a decline in pyrite iron and an increase in magnetic iron. Our findings suggest that sedimentary environments and redox states jointly control the process of iron speciation on the continental shelf.