Geochemical sedimentary evidence from core 973-2 for methane activity near the Jiulong Methane Reef in the northern South China Sea

The Jiulong Methane Reef, located on the northern slope of the South China Sea, is characterized by several features, such as bottom-simulating reflections and authigenic carbonates, which are indicative of methane seeps that are currently occurring as well as those that occurred in the past. However, to date, the effect that these methane seeps have on the sedimentary environment is not completely clear. To provide further insights into the biogeochemical processes involved in methane seeps, a 6.73 m piston core (973-2) was retrieved from this area in 2011 to perform an in-depth analysis. The chronology of the core has been established by AMS(14)C dating, and sedimentary events since the last glacial period have been recorded. The results indicate abnormally low abundance and diversity, as well as high infaunal percentages of benthic foraminiferal assemblages in the lower part of the core from 673 to 350 centimeters below the sea floor (cmbsf), indicating impacts from methane activities. Major elemental barium, bromine, and titanium, which serve as proxies of the paleoproductivity, organic contents, and terrestrial supply, respectively, are found to make little contribution in CaCO3 content. However, regardless of the factors mentioned above, the CaCO3 content is still higher in the lower part of core, indicating the presence of authigenic carbonates. In addition, the total organic carbon/total sulfur ratios of less than 400 cmbsf are found to be beyond the normal range, which is 2.8 +/- 0.8, reflecting the effects of methane oxidation. In one of the sections, the trace metals and molybdenum profiles show enrichments that are closely related to those of the methane seeps. The results indicate that this area experienced methane seep events in the last glacial period.