Sulfate reduction and formation of iron sulfide minerals in nearshore sediments from Qi'ao Island, Pearl River Estuary, Southern China

Sulfate reduction usually leads to the formation of iron sulfide minerals in marine sediments and strongly influences the global coupled carbon-iron-sulfur biogeochemical cycles in marine sediments. Here, we report sulfate reduction and pyrite formation from the coastal sediments near Qi'ao Island, Pearl River Estuary using a combination of sedimentary pore water and solid sediment geochemical analyses. Strong enrichments in reactive iron (276.8-358.2 μmol/g) were observed, especially at the sites located near the river outlets. High contents of reactive iron and low values of the degree of iron pyritization (DOP) indicate that the limiting effect of reactive iron on pyrite formation can be excluded at these sites. In the upper sedimentary layer, conservative mixing of river water and marine water reinforces the fact that sulfate reduction is not appreciable. The limited sulfate supply and low sulfate reduction rates lead to not enough dissolved sulfide available for formation of iron sulfide minerals. However, below this dilution-mixing layer, pore water geochemical profiles indicate that a fraction of sulfate consumption is mediated by upward-diffusing methane. An additional sulfide is produced from anaerobic oxidation of methane (AOM) and later reacts with reactive iron, resulting in enhancement of pyrite formation in the sediments. By contrast, it appears that sulfate concentration is a primary factor controlling for pyrite formation in these estuarine environments. Our study highlights the need in future work for an integrated analysis of the hydrologically driven change in the bottom seawater sulfate concentrations to better understand the regulation factors of global sulfur reservoirs in coastal sediments of estuarine environments, as sulfur burial is an important factor in determining the past atmospheric oxygen level.