Sulfur speciation in marine sediments impacted by gas emissions in the northern part of the South China Sea

Abstract

Three cores of marine sediments from the Shenhu area in the northern part of the South China Sea were analyzed by XANES analysis for sulfur speciation. The area has been investigated for the presence of hydrocarbons and potential gas hydrate formations. Cored samples of site 4B showed a specific profile of sulfur speciation with sharp and frequent variations in relative contents of sulfate and sulfide, which differed greatly from the profiles obtained for the sediments taken at sites 5B and 6A. The upper part of core 4B (of 0–95 cm) was soft and rich in pore water, containing mainly coarse silt sand. The lower part of the core (i.e., depth > 95 cm) was relatively dryer and darker in color, and dominated by silts and clay resembling sediments from mud volcanoes. The sulfur speciation results revealed that sulfate makes up almost 100 percent of all sulfur species in the upper part of the core 4B, which indicates strong oxidizing conditions, whereas the lower part of the same core has high relative contents of sulfide, sometimes close to 100% S2−. In the lower part of the core, the relative content of sulfide and sulfate changes rapidly and frequently, indicating rapid changes of oxidizing and reducing conditions. On the other hand, the vertical profiles of sulfur species for the cores from sites 5B and 6A are relatively consistent with lower sulfide contents indicative of stable and weaker reducing conditions. We hypothesize that the frequent and sharp variations in the ratios of sulfide to sulfate at site 4B may indicate some intermittent eruption of methane with clay from petroleum reservoirs underneath the sea floor over a relatively short period of time.