Anaerobic Oxidation Of Methane In Sediments From The Pearl River Estusry To South China Sea

The process of anaerobic oxidation of methane (AOM) can effectively remove methane from marine sediments before it reaches the sediment-water interface. Therefore, AOM plays a significant role in the global budgets of methane and the emission of methane into the atmosphere, where it acts as a strong green-house gas. Furthermore, methane is oxidized with concurrent sulfate reduction, producing inorganic carbon and volatile sulfur, and in this way AOM affects the biogeochemical cycle of carbon and sulfur in marine sediment.In the present study, AOM was investigated in the coastal sediments from the Pearl River estuary (Qi'ao Island and Guishan Island) to South China sea. By examining the profile distribution of pore-water geochemical data, including the methane and sulfate concentration, pH value andΣCO₂ concentration, and carbon isotope composition of CH₄ and SCO₂, it was found that the process of AOM was indeed occurred in the sediments. Based on the PROFILE modeling calculation, the portion of total sulfate reduction via AOM is 9.0%, 8.4%, 45.5% and 92%, respectively, and the percentage ofΣCO₂ added to the pore-water is 4.7%, 72.4%, 29.45 and 85.2% correspondingly for four surveyed sites.The depths of sulfate-methane interface (SMI) in sediments from four surveyed sites were not the same, which became deeper from the Pearl River estuary to South China sea. This is due to the fact the SMI depths are controlled by the amount of sedimentary organic matter supplied. The greater amount of the sedimentary organic matter is favorable for facilitation of the process of organic matter mineralization and the rapid depletion of pore-water sulfate, and on the other hand, causes the increase of the methane flux into the reaction zone between methane and sulfate, which results in a portion of sulfate supported by AOM. Accordingly, the SMI was shifted toward the sediment surface.Additionally, the distribution of sulfate reducing rates and their changes with the temperature in sedimentary profile from the Pearl River Estuary were determined by using the radioactive ³⁵SO₄^2- technique. Results show that the rates of sulfate reduction were controlled by the amount of organic matter and temperature, and the optimistic temperature for sulfate reduction was between 27℃to 39℃. However, the maximum of sulfate reducing rates was in 36℃which is almost the same to in situ temperature in the Pearl River Estuary, and it was revealed that there exists mesophilic sulfate reducing bacteria in the sediments and these microorganisms have adaptability for their subsistence environment.