Distributions and fluxes of methane in the East China Sea and the Yellow Sea in spring

Abstract

[1] Distributions and fluxes of methane were determined during two surveys in March-May 2001 in the Yellow Sea and the East China Sea. Methane concentrations in the surface and bottom waters range from 2.52 to 5.48 and 2.81 to 8.17 nM, respectively. The distributions of methane are influenced obviously by the Yangtze River effluent and Kuroshio water. CH 4 input via the Yangtze River is estimated to be 3.17 mol/s, of which a considerable part may be lost by air-sea exchange during estuarine mixing. Net CH 4 flux exported from the shelf to the Kuroshio is about 1.84 mol/s. Methane enrichments in bottom waters occur widely, which reveals sediment sources of CH 4 . However, the CH 4 input from the sediments of the studied region in spring is lower than other shelf regions due to low organic carbon in the sediments and high O 2 contents in the water column. The sea-to-air methane fluxes are estimated to be 1.36 ± 1.45 and 2.30 ± 2.36 mmol m À2 d À1 using Liss and Merlivat [1986] and Wanninkhof [1992] relationships, respectively, and the estimated spring emission rate of methane ranges from 9.32 Â 10 À3 to 15.7 Â 10 À3 Tg CH 4 yr À1 . However, these estimations suffer from the neglect of seasonal variability and should be taken as a low limit. Therefore more measurement campaigns should be carried out to enhance our understanding of this particular oceanic region.

Abnormally High CH4 Concentrations in Seawater at Mid-Depths on the Continental Slopes of the Northern South China Sea

“…Some high values do, nevertheless, exist at mid-depths (around 1000 m). These signals may be related to the sediments on the upper continental slope, which are richer in organic carbon (Tsurushima et al 1996;Zhang et al 2004). Thus, the unusually high concentrations of CH 4 can most likely be attributed to the anoxic generation of CH 4 and the release of CH 4 from gas hydrates.…”

Marine hypoxia/anoxia as a source of CH<sub>4</sub> and N<sub>2</sub>O

“…Further offshore over the mid-shelf in ECS, where the water column is more stratified, CH 4 content of near-bottom samples can be 50-100% higher than at the surface. In the deeper water, data from interior of the Kuroshio show CH 4 concentrations declining with depth down to 1000 m (Zhang et al, 2004). Over the ECS shelf, the upwelled Kuroshio subsurface waters have relatively low O 2 (125-156 µM) with lower CH 4 concentrations than in near-bottom waters over the inner shelf.…”

“…9a) shows a concentration gradient from the Changjiang Estuary to the Kuroshio Surface Waters with considerable seasonal and inter-annual variations (Zhang et al, 2008a). Elevated CH 4 concentrations occur in the water column of inner and mid-shelf but outside the high-turbidity plumes from the Changjiang, corresponding to the phytoplankton blooms caused by a decrease in turbidity that seems to limit photosynthesis closer to the coast (Zhang et al, 2004). Further offshore over the mid-shelf in ECS, where the water column is more stratified, CH 4 content of near-bottom samples can be 50-100% higher than at the surface.…”

Enhanced methane emissions from oil and gas exploration areas to the atmosphere – The central Bohai Sea

“…All the water samples were analyzed in the land laboratory within 60 days of collection. [CH 4 ] was extracted using a gas-stripping method (Swinnerton and Linnenbom, 1967;Zhang et al, 2004). Briefly, [CH 4 ] was purged from the seawater, and water vapors and carbon dioxide were removed.…”

“…Briefly, [CH 4 ] was purged from the seawater, and water vapors and carbon dioxide were removed. Then CH 4 was concentrated in a cold U-shaped stainless steel trap packed with 80/100 mesh Porapak Q; the Porapak Q trap was heated and the released CH 4 was injected into a Shimadzu GC-14B analyzer and detected by an FID (Zhang et al, 2004). The detection limit is about 0.1 nmol L À1 , with overall uncertainty of <3% of the contents (Zhang et al, 2004).…”