Fe-P-S geochemical characteristics of sediments in the Shenhu area of northern South China Sea and their implications for methane leakage

The Shenhu area is a key area in China for gas hydrate exploration and trial production. Research results suggest that such elements as Fe-P-S, are easily affected by the anaerobic oxidation of the methane (AOM) to form the minerals of pyrite and viviante. To study the methane seep intensity and hydrate accumulation potential of the area through Fe-P-S and other elements and their geochemical characteristics has reference significance for further understanding the gas hydrate accumulation mechanisms. In this paper, the core sediments from the Site 2A of the Shenhu area in the north of South China Sea are selected as the research target. Upon the basis of previous studies, the relationship among Fe, P, S and their implications for methane leakage are studied with the analysis data of major and trace elements, iron-bound phosphorus, authigenic apatite phosphorus, carbonate iron, magnetite iron, reducing iron, chromium-reduced sulfur (CRS), sulfur isotopes, and total organic carbon (TOC). All the data of TOC, P/Ti, Al/Ti, Ba/Ti, high active iron (Fe_HR) and other productivity indicators from the Site 2A station indicate a medium level of primary productivity. The primary productivity below 600 cmbsf (centimeter below sea floor) in water depth increases slightly deepwards. CRS content increases below 600 cmbsf and its sulfur isotope is obviously positive, which indicates the existence of AOM. The Mg/Ca and Sr/Ca are indicators of high magnesium calcite and aragonite in sediments, and Sr/Ti and Ba/Ti contents related to authigenic carbonate. They all leave peaks at 600 cmbsf, and the water around this depth, the contents of Iron-bound phosphorus and Authigenic apatite phosphorus are also high. Therefore, we speculate that the depth of 600 cmbsf is most probably the upper boundary of Sulfate Methane Transition Zone (SMTZ). The research further suggests that the content of iron-bound phosphorus increases significantly in the SMTZ, CRS and δ³⁴S in pyrite and may indicate the content of pyrite, so that methane seep can be recognized by different forms of Fe, P and S elements.