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Sulfur Cycle In Sediments Of The Chinese Marginal Seas During Early Diagenesis

Posted on:2006-12-22Degree:DoctorType:Dissertation
Country:ChinaCandidate:X Q PuFull Text:PDF
GTID:1100360182993848Subject:Marine geology
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Sulfur is one of the major components in seawater. Sulfur cycle in marine sedimentsduring diagenesis resulted in the geological evolution of the redox condition on thesurface of the Earth and that of the atmospheric composition. It has direct influence andcontrol on the marine carbon cycle and the biogeochemical behaviors or cycles of largenumber of trace elements, particularly heavy metals that may play an important role in themarine ecosystem. In addition, as marine sulfur cycle is found to be intertangled with themarine carbon cycle, study on the marine sulfur cycle will certainly shed light on thegas-hydrate, a potential new energy resource for the 21th century, that are usually burieddeep in the continental shelf or slope sediments. Consequently, the sulfur cycle has longbeen an important subject of study, and recently, it started to attract the attention of theChinese marine scientists.In this dissertation, the sediment samples taken from the Yellow Sea continental shelf,the Licun estuary of the Jiaozhou Bay, and the South China Sea continental slope, wereanalysed for grain size, contents of acid volatile sulfide (AVS), contents of pyrite, reactiveFe, organic carbon, pore water sulfate concentration, pyrite and its sulfur isotope. Basedon our data and the data in the East China Sea from previous work by others, theformation of sulfide and its controlling factors in the process of sulfur cycle in theChinese marinal seas were discussed. Based on the analysis of the reactive metals and thepyritized metals in the sediments of the Licun estuary, the transformation of AVS to pyriteand trace metals distribution in AVS and pyrite were discussed. The redox condition,which is crucial for the distribution of sulfides in the sediments, was discussed throughanalysis of the reactive metal profiles in the sediments of the Yellow Sea. Based on theenvironmentally sensitive group of grain size data, the characters of sedimentaryenvironments were discussed. Combining AVS, sulfur isotope of pyrite, organic carbon,inorganic carbon and N/C ratio, the abnormality of the sulfur cycle caused by upwardmethane flux and the indication of gas hydrate formation at depth were discussed.We determined the gain sizes of sediments collected from the Yellow Sea and theSouth China Sea, applying three different pretreatment schemes. Our data indicated thatsediment grain size, measured after organic matter and carbonate removal, reflected wellthe characteristics of terrigenic fraction of the sediment. At site NH-1 of the South ChinaSea, three size fractions (i.e.,1.3~2μm,10~15μm, and 28~50μm) of sediment wereidentified to reflect sedimentary environments. The fine-grained fractions (1.3~2μm and10~15μm) consisted mainly of particles that settle from the upper water column, whilefraction 28~50μm were composed of materials that were transported by bottom watercurrents.AVS content in the sediments of the Yellow Sea and the South China Sea wasdetermined. We found that AVS content decreased in the following order: Licun estuary inthe Jiaozhou Bay > Jiaozhou Bay > the East China Sea continental slope > the Yellow Seacontinental shelf > the East China Sea continental shelf > the South China Sea slope.Based upon our results of sulfide, reactive metals, and pyritization of sediment corescollected from the Yellow Sea and South China Sea, and published results of sedimentsfrom the East China Sea, we concluded that sediment organic carbon content was themajor factor that controlled the formation of sulfide in sediments of the Chinese MarginalSeas. Reactive iron would become the controlling factor only in environments with veryhigh organic carbon supply, such as the Licun estuary. The quantity and quality of organicmatter, sedimentation rate, redox condition, bottom water temperature, and burial time allcould influence the formation, quantity and transform of sulfides in the sediments.In sediment core NH-1, retrieved from the South China Sea, a pyrite contentmaximum was found at the depth of 141.5cm. Extensive sulfate reduction took place atdepths between 120-141.5cm. Combining previous results obtained from cores collectedto the west of our core, we thought that the depth of 141.5cm might represent theboundary of Late Pleistocene glacial and Holocene post-glacial period. The high AVScontents appeared near the bottom of the sediment core, which was abnormal as AVS inmost marine sediments had mid-depth maximum. Both δ34S of pyrite and AVS contentshowed abnormal value at the 247.5~380.5cm interval of the sediment core NH-1,suggesting intensive methane upward flux and possibly the formation of gas-hydrate atgreater depth at this location.Both active Fe and Mn profiles showed maximums at the surface layer to subsurfacelayer. The depth of Fe maximum was usually deeper than that of Mn, which was inaccordance with the fact that Fe reduction began at deeper depth than Mn's reduction.Reactive Mn, therefore, had a higher mobility in the sediment than reactive Fe. ReactiveMn enrichment in oxidizing environment and its absent in reducing environment couldserve as a sensitive indicator of bottom water redox conditions.
Keywords/Search Tags:Sulfur Cycle, Yellow Sea, Jiaozhou Bay, South China Sea, Grain size, early diagenesis, acid volatile sulfide(AVS), pyrite, sulfur isotope, active metals, pyritizatio metals, organic carbon, gas hydrate
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