Calcium isotope fractionation and its controlling factors over authigenic carbonates in the cold seeps of the northern South China Sea.

In this study, we analyzed stable calcium isotope results of authigenic carbonates from two cold seep areas of the Dongsha area and the Baiyun Sag in the northern South China Sea. The stable isotopes of carbon and oxygen as well as the mineral composition of authigenic carbonates were used to investigate control calcium isotope fractionation. The δCa ratios of the southwestern Dongsha area samples ranged from 1.21‰ to 1.52‰ and the ratio of the Baiyun Sag sample was 1.55‰ of the SRM915a isotope standard. X-ray diffraction analysis showed that the carbonate samples consisted of dolomite, calcite and aragonite, with small amounts of high-Mg calcite and siderite. The δC values of the carbonates of the southwestern Dongsha area varied between −49.21‰ and −16.86‰ of the Vienna PeeDee Belemnite (VPDB) standard and the δO values ranged from 2.25‰ to 3.72‰ VPDB. The δC value of the Baiyun Sag sample was 2.36‰ VPDB and the δO value was 0.44‰ VPDB. The δC values of the carbonates of the southwestern Dongsha area revealed there is methane seeping into this area, with a variable contribution of methane-derived carbon. The sampled carbonates covered a range of δC values suggesting a dominant methane carbon source for the light samples and mixtures of δC values for the heavier samples, with possibly an organic or seawater carbon source. The δO values indicated that there is enrichment in O, which is related to the larger oxygen isotope fractionation in dolomite compared to calcite. The results of the Baiyun Sag sample exhibited normal seawater carbon and oxygen isotopic values, indicating that this sample is not related to methane seepage but instead to precipitation from seawater. The relatively high δCa values indicated either precipitation at comparatively high rates in pore-water regimes with high alkalinity, or precipitation from an evolved heavy fluid with high degrees of Ca consumption (Raleigh type fractionation). The dolomite samples from the Dongsha area revealed a clear correlation between the carbon and calcium isotope composition, indicating a link between the amount and/or rate of carbonate precipitation and methane contribution to the bicarbonate source. The results of the three stable isotope systems, mineralogy and petrography, show that mineral composition, the geochemical environment of authigenic carbonates and carbon source can control the calcium isotope fractionation. [ABSTRACT FROM AUTHOR]