Studies On Biogeochemistry Of Dissolved Methane And Nitrous Oxide In The Coastal Waters Of China

CH4 and N2O are important atmospheric trace gases, which play significant roles in the global warming and atmospheric chemistry. Global oceans are net natural sources of atmospheric CH4 and N2O. Although coastal regions such as continental shelves, estuaries and bays only occupy a small part of the world ocean, they appear to be responsible for a large part of the oceanic CH4 and N2O emission. Therefore studies on the biogeochemistry of dissolved CH4 and N2O in coastal waters will be helpful to estimate the contribution of oceanic emissions to the atmospheric CH4 and N2O on a global scale, and to predict the influence of oceanic emissions to the global climate.In the present dissertation, distributions, sources, sinks and fluxes of CH4 and N2O in the coastal waters of China, i.e. the Yellow Sea (YS) and the East China Sea (ECS), the Yangtze River Estuaries and the Jiaozhou Bay, are studied detailedly andsystematically for the first time. The main research work is as follows:1. The gas-stripping chromatographic methods have been developed in our lab for the determination of the dissolved CH4 and N2O in the seawaters based on the references. The detection limits are 0.06 and 0.08 nmol/L, and the precisions are <3% and <5% for CH4 and N2O, respectively. The head-space method has been developed for the determination of the dissolved N2O in the seawaters. The detection limit is 1.04 nmol/L and the precision is <5%. Sample storage experiments have been done to test reliability of the storage procedure used in this study, and the results show that the decrease of CH4 concentration during a period of 50 days is less than 3% and the effects of storage appear negligible.2. Distributions and fluxes of methane are determined during surveys on the YS and ECS in March-May 2001 and on east-northern part of ECS in September 2003. The results show that the average methane concentrations in the surface and bottom waters in the YS in spring are 3.43 + 0.23 and 3.70 0.63 nmol/L,respectively, while those in the ECS are 3.24?.59 and 4.281.26 nmol/L, respectively. The average methane concentrations in the surface and bottom waters in the ECS in summer are 12.814.0 and 22.4+21.2 nmol/L. In general, methane concentrations in the waters of the ECS show obvious seasonal variations, with those in summer higher than in spring by a factor of 4-5. enrichment in the near-bottom waters is found at most stations of the YS and ECS in both summer and spring, which underlines that there exists important sources of methane in deep waters or bottom sediments. The distributions of methane in the YS and ECS are influenced by the methane-rich Yangtze River effluent and methane-poor Kuroshio water. Hence the horizontal distributions of methane in the surface and bottom waters in spring illustrate a decrease of methane concentration onshore to offshore and a high methane plume occurs off the Yangtze River Estuary, which extending northeastward along the axis to the Cheju Island. In the eastern side of the ECS shelf, low methane concentrations less than 2.5 nmol/L is found in the surface waters influenced by Kuroshio. The vertical distributions of dissolved methane at 4 sections in the ECS in summer are studied and the results show that the distribution is influenced by Yangtze River diluted water, Kuroshio, Taiwan Warm Current, Tsurushima Warm current and methane release from sediments, hence the vertical distribution of methane along different sections are quite variable. Based on the average volume transport between the ECS shelf and Kuroshio, and an estimate of the methane concentrations associated with the inflow and outflow water, the net CH4 loss through interactions between the ECS shelf water and Kuroshio is estimated to be about 1.84 and 3.72 mol/s in spring and summer, respectively, which implies that the marginal seas are a significant net source of methane to the ocean interior. The surface water of the YS and ECS is supersaturated in spring and summer, which indicates that the ECS and YS are net sources of atmospheric methane. Lis...