Iron geochemistry in surface sediments of the subtropical northern South China Sea and a comparison with the temperate East China Sea

The influence of differential chemical weathering intensity on the contents and spatial distribution of highly reactive iron (Fe_HR) in the marginal sea sediments is not well documented. Here, we used sequential extractions to quantify various Fe pools in offshore surface sediments of the northern South China Sea (nSCS), which receives multiple fluvial sediments with distinct chemical weathering intensity, and then to unravel whether and if so, to what extent, the contents and spatial distributions of Fe_HR are linked to chemical index of alternation (CIA) of the sediments. Our results show that, despite the inputs of Fe_HR-rich, highly chemically weathered fluvial sediments, Fe_HR contents in offshore sediments of the nSCS are almost independent of the CIA values, and Fe_HR was not enriched relative to the global average of Fe_HR in oxic sediments, probably due to effective trapping of Fe_HR in the estuary and dilution by moderately chemically weathered Taiwan-sourced sediments. Our results indicate that the signature of Fe_HR enrichment previously observed in the offshore sediments, which are discharged from a highly chemically weathered mountainous island (Hainan Island) of the nSCS, becomes lost beyond the local zone around the island. Relative to the marginal sediments of the nSCS (water depth <1000 m), the higher Fe_HR contents in the deep-water sediments (water depth larger than 1000 m) can be attributed to higher smectite contents, lower inorganic carbon contents, and low dilution by Taiwan-sourced fluvial material in the deep-water area. A comparison of the subtropical nSCS and the temperate East China Sea indicates that chemical weathering intensity is not the overarching factor governing Fe_HR/Fe_T ratios in the two climatically different regions, though chemical weathering intensity along with depositional/diagenetic regimes is an important factor controlling the sizes and transformation of some specific Fe_HR pools.