Distribution, source and flux of methane in the western Pearl River Estuary and northern South China Sea

Abstract

The distribution of methane in the western Pearl River Estuary (PRE) and northern South China Sea (SCS) was investigated during an oceanic cruise in September, 2006. Results show that the methane concentration in surface water ranged from 6.9 to 173.7 nmol L − 1 at the 27 stations sampled in the western PRE. The methane saturation ratio ranged from 329 to 7896%, and the average sea-to-air methane flux was about 63.5 ± 32.2 μmol m − 2 d − 1 . The methane concentration in surface water was high in the western margin of the upper estuary, near marshes and shoals. The methane concentration decreased rapidly from the estuarine margin to the open coastal ocean, resulting from salinity (sulfate) increase in the overlying water, increasing water depth, diffusion across the air–sea interface and dilution of seawater during estuarine mixing. The major methane source in the PRE is thought to be methanogenic degradation in organic-enriched sediments. In the northern SCS, the concentration of methane varied from 2.4 to 5.9 nmol L − 1 in surface water at the 55 testing stations. The methane saturation ratio was between 134 and 297 %, and the sea-to-air methane flux was estimated to be 15.6 ± 8.0 μmol m − 2 d − 1 . Two stations located at E7 (15° 59.405″; 17° 59.165″) and E6 (115° 0.065″; 17° 0.032″) showed abnormal methane concentrations of 9.5 and 13.8 nmol L − 1 in surface water, respectively, and were probably affected by pollution from allochthonous oil. A subsurface methane maximum (4–5 nmol L − 1 ) occurred in water that was 150–200 m deep within this area. This maximum may be due to accumulation of plankton productive particulates and limited sea–air exchange in the sea surface layer. Abnormally high methane values of 4.3 and 8.7 nmol L − 1 were measured in the bottom waters of stations B8 and E4, respectively. These can be interpreted as being due to methane seeps from seafloor sediments.