Geobiological Processes Of Methane Seeps In The Marinoan Cap Carbonate In Southeast Of The Three Gorges

Cap carbonate, in the end of the Marinoan ice age, may be the only know occurrence of a Precambrian seep carbonate. Tepee-like structures, sheet crack, extremely negative carbon isotope signatures, and barite fans in post-glacial cap carbonate, which have been interpreted as products of methane oxidation at the time of deposition. The presence of δ13C values as low as-48%o provide unequivocal evidence for methane influence. However, some scientists believe that negative δ13C signals were not recorded until after the deposition of the cap dolostone, which denies exceptional carbon isotope evidence for the methane release event during Marinoan deglaciation.Authigentic carbonates often occur with cold seep and have become as one of the key part in research into cold seep. Cold seep carbonates often be divided into two types, one is Chemoherm carbonates, the other is seep carbonate. Chemoherm carbonates signify sustained focused seepage of methane-rich fluids exiting into the free bottom water. Seepage-associated carbonates are different from cheoherms in forming carbonates slabs, concretions, crusts and tubes. The chimneys and tubes are often completely of partially filled by more generation of aragonite of HMC. Carbonate formation at methane seeps results from the anaerobic oxidation of methane (AOM) which is performed by a consortium of methanotrophic archaea and sulphate reducing bacteria.This paper based on field survey of Marinoan cap carbonate in Southeast of the Three Gorges area, and detailed sedimentological, petrographic and isotope data, in order to discuss the extremely negative carbon isotope formation mechanism. Almost all samples in this investigation show negative δ13C values, most of them between-1and-5%o (VPDB), which accord with the rest of the world. Compared with δ13C values,δ18O values have a large range. In Youxi section, almost all of the δ18O values of cap carbonate are greater than-10‰(VPDB). In Xiangjiawan and Xijiaoao section, about half of the samples are greater than-10‰. By contrast, in Gucheng section, almost all samples are less than-10%o(VPDB). In summary, The W/R ratio of fluids is typically high with respect to O and low with respect to C, so carbonate δ18O is driven to lower values through higher-temperature equilibration while δ13C undergoes little change. Therefore, using co-variation between δ18O and δ13C to evaluate the degree of diagenesis is better than relying solely on δ18O values. Here we report negative δ13C values as low as-32%o(VPDB) in the middle and upper part of cap carbonate from two additional sections (Xiangjiawan and Xijiaao section). These negative δ13C values all obtained from bulk sample which contain well-preserved stromatolite beds. The macrostructure contain low domes with irregular crinkly millimeter-scale lamination and upward fibrous laminae. The dominate microfarics are clotted dolomicrite, mm-scale laminae of dark organic-rich layers alternate with microspar layers, fibro-radiate cement and dispersed aggregates of hematite/limonite after pyrite. Isotopic compositions of micro-drilled samples show that dark organic-rich layers and upward fibrous cement laminae exhibits extremely negative δ13C values. By contrast, matrix dolomite presents relatively negative δ13C values ranging from-1.33‰to-7.25%o(VPDB).Previous studies show that highly negative δ13C values mainly occur in isopachous cements and blocky calcite that fill stromatactis-like cavities, sheet cracks, and fractures. These findings imply that methane-related activity did not occur until after the deposition of the cap carbonate. In this thesis, a systematic study of macroscopic, microscopic petrographic and carbon isotope was carried out. The results display that highly negative δ13C values in the middle and upper part of cap carbonate consist with the emergence of microbial mat. We found dispersed aggregates of iron mineralization in microbial mat which may be hematite/limonite after pyrite. Besides, the parallel lamination is sometimes deformed and disrupted. These entire phenomena enable us imagine that the methane-bearing fluids migrated upwards to the base of the sulfate reduction (SR) zone, where methane was oxidized (CH4+SO42-â†'HCO3-+HS-+H2O) and the AOM zone developed. The bicarbonate produced in this reaction was significantly depleted in δ13C, which has been recorded in the cap carbonate.