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 delta Ca-44/40 ratios of the southwestern Dongsha area samples ranged from 1.21aEuro degrees to 1.52aEuro degrees and the ratio of the Baiyun Sag sample was 1.55aEuro degrees 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 delta C-13 values of the carbonates of the southwestern Dongsha area varied between -49.21aEuro degrees and -16.86aEuro degrees of the Vienna PeeDee Belemnite (VPDB) standard and the delta O-18 values ranged from 2.25aEuro degrees to 3.72aEuro degrees VPDB. The delta C-13 value of the Baiyun Sag sample was 2.36aEuro degrees VPDB and the delta O-18 value was 0.44aEuro degrees VPDB. The delta C-13 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 delta C-13 values suggesting a dominant methane carbon source for the light samples and mixtures of delta C-13 values for the heavier samples, with possibly an organic or seawater carbon source. The delta O-18 values indicated that there is enrichment in O-18, 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 delta Ca-44/40 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.