The Storage And Evolution Of Actived Silica In Sediments From The Continental Slope Of Northern South China Sea And Their Indication For The Hydrate

With the successful exploitation of gas hydrates in the South China Sea,hydrates have become one of the most important renewable clean energy sources in China,which is worthy of further study.A large number of research data have shown that there is gas hydrates storage in the continental slope of northern South China Sea,and many geochemical data indicate that the sea level drop in the study area during the colder period of geologic history resulted in several large-scale decomposition of natural gas hydrates and the shallow SMTZ nowadays.As an important element in the biogeochemical cycle,silicon is one of the most important elements of marine primary producers such as diatoms.In the early diagenesis of sediments,silicon is closely related to important elements such as carbon and sulfur,among which the weathering of silicates and the methanogenesis of organic matter complement each other,and the alkalinity required for the reverse weathering diagenesis of active silica is mainly provided by SO42-and other electron acceptors oxidizing organic matter.Therefore,the study on the storage and evolution of active silica in sediments in the continental slope of northern South China Sea is not only helpful to enrich the gaps in the study of the evolution of active silica in the South China Sea gas hydrate occurrence area but also provide a new idea for identifying the depth of SMTZ and methane emission records,thus enriching the research data marine gas hydrate in China.This article selects Core 973-3,973-4 and 973-5 from Southwest Taiwan Basin on the continental slope of northern South China Sea for researching.Main experiments include sediment BSi extraction,two-step extraction of active silica,Tessier continuous extraction of silica and elemental analysis.Combining our group's previous results of geological and geochemical studies in northern South China Sea and similar researches,the following consequences are acquired:(1)Comparison of BSi content with sediment particle size and TOC content in Core 973-3 and 973-4,the BSi content in sediments is mainly affected by terrestrial dilution and turbidite deposition.The influence of dilution of terrestrial dilution in Core 973-5 is tiny than that in two other cores.BSi content of some layers in Core 973-4 and 973-5 is affected by methane emission during extremely cold period.(2)The average ratio of TOC/BSi in Core 973-4 is 1.17,which is much lower than 7.7 in siliceous organism,which indicates that the dissolution rate of biological silicon is much less than decomposition rate of organic matter from biological death to sediment.The vertical variation of TOC/BSi ratio indicates that in early diagenesis,the consumption rate of BSi is not lower than the decomposition rate of organic matter.The TOC/BSi ratio in Core 973-4 recorded three sudden cooling events after the last glaciation.(3)IMOF-Si and OSF-Si are the dominant forms of Tessier method.The variation of IMOF-Si content is closely related to the composition of highly reactive iron.It is considered that the IMOF-Si extracted by Tessier method should be "highly reactive iron(manganese)form silica",and it is an important source of active silica for reverse weathering except BSi in sediments.The content of OSF-Si is synergistic with the content of BSi,so it is considered to be the form of biological silicon and residual organic matter.(4)BSi is the main part of active silica in sediments.Due to the thicker turbidite deposit layer,the content of Si-HCl in Core 973-3 is higher,and there is no significant positive correlation between the Si-Alk content and the BSi content.Reverse weathering of active silica in Core 973-3 is obviously stronger than that in the other two cores.(5)According to the indication of reverse weathering,the SMI in Core 973-3 is at about 950cm,similarly,910cm in Core 973-4 and 750cm in Core 973-5,which is close to the results obtained by other methods.Shallow SMI indicates the presence of gas hydrate reservoirs.(6)The higher CIA*value and weaker reserve weathering indicate that the shallow sediments at Core 973-4 have higher methanogenic capacity,which is also due to its higher organic matter content.(7)The evolution of active silica in hydrate area have two obvious characteristics:?Methane leakage during extremely cold periods bring about that BSi content could not record high productivity but be obviously lower than warm periods.?Reverse weathering of active silica can only react in a narrow zone.