The Coupling Of Carbon And Sulfur In Sediments In The Early Diagenesis Of Methane Hydrate Potential Area Of Northern South China Sea

As an ideal alternative energy in the21st century, gas hydrate has become a research focus in the global science field. The existing of gas hydrate is mainly under the influence of temperature, pressure and composition. Not only the variations of sea level, deposition rate and the sea water temperature at the bottom, but also sundry geological hazards, such as underwater landslides and earthquake can lead to the decomposition and release of gas hydrates. Therefore, by studying in coupling of carbon and sulfate in the gas hydrate potential zone in northern South China Sea, the accumulation process and the decomposition and release mechanism of gas hydrates under a gusty condition. As a potential zone of gas hydrate, the northern South China Sea also experienced the methane decomposition and release in geological history. Methane will be released and go upwards, which, subsequently, join in the anaerobic oxidation of methane-sulfate-reduction. Anaerobic methane oxidation-sulfate reduction (AOM-SR) is one of the most important reactions in the early diagenetic process of sediments, which is affected by the pore water in sediments and the carbon sources, such as organic matter and the lifting methane.Moreover, it claims a guiding significance in the gas hydrates exploitation in our country. This research is on the basis of the National Major Fundamental Research and Development Project "Basic study on gas hydrate accumulation laws and exploitation in the South China Sea"(No.2009CB219501), the National Nature Science Fundation Project "Research on the sulfate-methane system prosumer mechanism and its interface (SMI) characteristics of the sediments in the northern South China Sea"(project number:40976035), and "Research tics on biomarkers and molecular biology of methanogenesis and anaerobic oxidation of methane in gas hydrate system"(project number:41276046).We chose the core973-4from the Taixinan area as the object of this research, for instance, the determination of geochemical characteristics, the observation of authigenic pyrite via microscope, the recovering of sedimentary environment, etc. Combining with the relevant research materials and data (e.g., the regional geological features, other potential areas of methane hydrate in the northern South China Sea and, etc.), the following consequences are an outline of this study:(1) The concentration of organic carbon in the core973-4from the Taixinan area amounts to0.39-1.26%, and the correspondingly data of TS stands at0.2-0.7%. At the depth of898cm, AVS surges to9315μmol/g. Besides, the concentration of834S in pyrite fluctuates between17.9%o and-44.4%o. By the apparent geochemical characteristics of973-4column, a comprehensive judgment comes into being:in973-4column, there are two sulfate reduction reactions-the anoxygenous oxidation of organic matter at the depth of360-440cm and AOM at the depth of580-900cm. The zone of the anoxygenous oxidation of organic matter could be divided into the upper portion locating at580-700cm and the lower portion locating at700-900cm. The boundary of these two parts above indicated mainly by the mutation of sulfate gradient in pore water and the diverseness and second growth of pyrite crystal(2) In973-4column, the depth of SMI is900cm. According to the carbon isotope and dating, the shallow SMI of973-4column may claim the existing of gas hydrate in Taixinan area.(3) Through analyzing the variations, with depth, of sulfate concentration in multiple sections in the northern South China Sea, the conclusions is the shifty sulfate concentration, which goes with its depth, in the northern South China Sea could be separated into three following parts:the surface oxidation zone of organic sulfur, the zone of the anoxygenous oxidation of organic matter, the central transitional zone and the zone of AOM. The surface oxidation zone of organic sulfur existing or not hinges on, frequently, three cases:the concentrations and types of organic matter in the sediments, regional geological structure and the influence of ocean current. Generally, the three cases above present all together. Correspondingly, the central transitional zone existing or not hinges on the anoxygenous oxidation of organic matter in the upper portion and the methane flux leaking upward in the lower portion. (4) Generally, the depth of SMI has a close relationship with the existence of gas hydrates reserves in the lower portion. Multiple reasons contribute to the depth of SMI in gas hydrate potential area in the northern South China Sea. In this article, a filtering methodology for interferential data is put forward. By this methodology, the several depths of SMI in different sections have been recalculated in the northern South China Sea.(5) The formation mechanisms of pyrite in973-4column can be partitioned into the type Ⅰ filling and type Ⅱ filling. The former type means filling among bioclastics and sediment particles, and metasomatically filling in bioclastics. At macro level, framboids pyrite. At macro level, pyrite has three categories, that is to say, framboids pyrite, tubular pyrite and massive pyrite. In terms of distribution in the aggregate forms of pyrite, tubular pyrite is a common existence in the upper portion, massive pyrite usually presents in the middle portion, and framboids pyrite shows in deeper portion relatively. Filling pyrite distributes all over every layer. Pyrite formation depends on bioclastics type and the collection period gap between sediment particle sizes. The aggregate forms of pyrite hinge on the types of bioclastics and the scales of the gap between sediments during formation.(6) In973-4column, pyrite conveys multiple single crystals, for instance, cut corner cube, cube, octahedron, pentagonal dodecahedron and an aggregation consisting of cube and pentagonal dodecahedron. Thereinto, octahedron, acting as the main form of pyrite existences, spreads widely in every layer. By contrast, other crystals appear in the sediments at700cm, which depth represents a boundary between the upper and lower portions of SMIZ. Various crystals indicate a shifty microenvironment, which attributes to various restrictive conditions of AOM. A possible inducement may be that the decomposition of methane hydrate, in the methane hydrate zone, is unstable and occasional, which due to a substantial fluctuation of methane flux. Consequently, the SO42-and CH4in SMIZ as different restrictive conditions of AOM show an on-going fluctuation too, which, eventually, effects on the microenvironment and path of pyrite formation.(7) The early diagenetic process of sediments in this study area, in the first place, the more active materials will reduce sulfate out of the oxidation of organic matter. With the consumption of active organic matter and the increasing depth, the process above is restrained, the leaving organic matter with less activity will be used by methanogens in the methane-producing zone. Then the methane stemming from last procedure will come into the methane hydrate under the appropriate condition. Due to the changing occurrence conditions of methane hydrate, methane will be released and go upwards, which, subsequently, join in the anaerobic oxidation of methane-sulfate-reduction and then completes interaction between carbon and sulfur in early diagenetic process.