Methane in the South China Sea and the Western Philippine Sea

Approximately 700 water samples from the South China Sea (SCS) and 300 water samples from the western Philippine Sea (wPS) were collected during eight cruises from August 2003 to July 2007 to determine methane (CH4) distributions from the surface to a depth of 4250 m. The surface CH4 concentrations exceeded atmospheric equilibrium, both in the SCS and the wPS, and the concentrations were 4.5 +/- 3.6 and 3.0 +/- 1.2 nmol L-1, respectively. The sea-to-air fluxes were calculated, and the SCS and the wPS were found to emit CH4 to the atmosphere at 8.6 +/- 6.4 mu mol m(-2) d(-1) and 4.9 +/- 4.9 mu mol m(-2) d(-1), respectively. In the SCS, CH4 emissions were higher over the continental shelf (11.0 +/- 7.4 mu mol m(-2) d(-1)) than over the deep ocean (6.1 +/- 6.0 mu mol m(-2) d(-1)), owing to greater biological productivity and closer coupling with the sediments on the continental shelf. The SCS emitted 30.1x10(6) mol d(-1) CH4 to the atmosphere and exported 1.82x10(6) mol d(-1) CH4 to the wPS. The concentrations of both CH4 and chlorophyll a were high in the 150 m surface layer of the wPS, but were not significantly correlated with each other. CH4 concentrations generally declined with increasing depth below the euphotic zone but remained constant below 1,000 m, both in the SCS and the wPS. Some high CH4 concentrations were observed at mid-depths and bottom waters in the SCS, and were most likely caused by the release of CH4 from gas hydrates or gas seepage.