| Cold seeps are special areas on the seafloor,characterized by methane-rich fluid seepage and a sequence of biogeochemical reactions.The main carbon-related biogeochemical reactions taking place in this area,may include organoclastic reduction,methanogenesis,anaerobic oxidation of methane?AOM?,and authigenic carbonate precipitation.To our knowledge,the whole reactions mentioned above may play a key role in carbon sequestration,nutrient elements cycling,climate change,and evolution of life.In this dissertation,porefluid data of 112 cores in Dongsha,Shenhu,Qiongdongnan and beikang were used.Combining geochemical analysis and numerical modeling,my purposes are to unveil the spatial rate distributions of aforementioned biogeochemical reactions and rates of carbon turnover in the four cold seep areas of South China Sea.Additionally,an attempt was made to infer the methane seepage pattern in Haiyang IV area,South China Sea,by considering the sedimentary lithofacies and results of numerical modeling.Based on inverse distance weighted spatial interpolation,biogeochemical rate spatial distributions were obtained.Spatial distribution of organic degradation in Shenhu is heavily influenced by topography featured by valleys and ridges.Contrarily,the topography do not show much influence on rate distributions of AOM and authigenic carbonate precipitation,but contour lines still show northwest-southeast trending.Owing to high input of terrestrial organic carbon and kuroshio branched current in eastern Dongsha,rates of organic degradation are higher than western Dongsha.Since the impact of the active continental margin,more methane sourced from different pathways can contribute to higher rates of AOM in eastern Dongsha.Also,the rate distribution of authigenic carbonate precipitation shows the same pattern as AOM in Dongsha.Owing to the difference of origin and migration pathway of organic matter,rates of organic carbon degradation in southern Qiongdongnan are higher than in the north.Gas chimneys,faults,and mud diapirs in northern Qiongdongnan provide adequate methane for the AOM in the subseafloor sediments,rates of AOM and authigenic carbonate precipitation,thus,show higher values in the north than in the south.To summarize all the computation results,the average rate of organic degradation in Dongsha is biggest,followed by Beikang,Qiongdongnan and Shenhu.This relationship between areas may be controlled by origins of organic matter,sedimentation rates,and clay mineral assemblages.Among the factors,the clay mineral assemblage which have different preservation capabilities of organic matter may serve as the main cause for such relationship.Similarily,effluxes of dissolved inorganic carbon and rates of AOM have the same comparative relationship as organic degradation,which may be attributed to differences of extra methane supplies by hydrate dissociation or bubble dissolution.Rates of authigenic carbonate precipitation in four areas fall into the same range,but the rate in Qiongdongnan is slightly higher the other three areas as the precipitation constant adopted in Qiongdongnan is slightly higher.Computation results reveal that the efflux of dissolved methane in Qiongdongnan is biggest,followed by Dongsha,Beikang and Shenhu.In comparison with major cold seep areas worldwide,carbon effluxes and rates of carbon-related biogeochemical reactions in the four study areas have comparable values with cold seeps on the passive continental margins.According to sediment lithofacies analyses,multiply layers of turbidite have been distinguished.In combination with carbonate contents and seismic images,sediments in Haiyang IV area may be inferred to be mass transport deposits.Generally speaking,the phenomenon that ion concentrations of porewater in the upper several meters have the same values as in seawater is ascribled to non-local transport,such as bubble irrigation.By analyzing multiple ion concentration profiles,it's postulated that transient methane seepage may also generate the same feature of profiles.Subsequently,nonsteady reaction-transport modeling was run and it's found that concentration-depth profiles can be perfectly reproduced under this assumption.Results of the modeling also show that the onset of seepage event are about 160 BP and 200 BP in the two cores.Advection velocitys in the two cores are 5 cm yr-1 and 3cm yr-1 respectively,confined by the modeling.On the basis of the distribution of coldseep-based faunal community and near-seafloor hydrate around the cold seep,i speculate that methane-rich fluids may migrate to the surrounding sediments,which is confirmed to be mass transport deposits,along the fissure or coarse-grained layer followed by the clogging by the hydrate in the seepage center. |
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