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Clumped isotope geochemistry of island carbonates in the South China Sea: Implications for early diagenesis and dolomitization
Marine Geology ( IF 2.9 ) Pub Date : 2021-05-15 , DOI: 10.1016/j.margeo.2021.106513
Yangrui Guo , Wenfeng Deng , Xi Liu , Kai Kong , Wen Yan , Gangjian Wei

Studying the diagenesis of island carbonates can be hampered by the difficulty of accurately constraining the mineral formation temperature. The carbonate clumped isotope composition (measured as the Δ47 value) is based on the temperature-dependent binding of 13C to 18O, and offers a promising means of calculating the formation temperature. In this study, we report the Δ47 values of island carbonates from a ~1 km-long drill core from the Meiji Reef in the South China Sea, which are integrated with petrography, carbon (δ13C) and oxygen isotope (δ18O) compositions, and trace element contents to constrain the diagenetic environment. We find that most of the digenetic alteration of the initial carbonates likely occurred above a water depth of ~200 m below sea level at temperatures between ~15 and 28 °C, and that the fluids involved during the formation of the diagenetic calcites and dolomites have similar δ18O values to modern seawater. Considering the estimated secular variation in seawater δ18O, the dolomite likely formed from an evaporative fluid source via a brine reflux mechanism and with a high convection rate during the late Miocene. Although isotopic compositions of the reef carbonates were overprinted by early diagenetic alteration, the Δ47 values can reflect the temperature of their depositional environment if the diagenesis occurred at depths similar to the depositional depth. These results can help refine the existing dolomitization model of tropical islands and increase the utility of Δ47 values of diagenetic reef carbonates in shallow burial settings as valuable records of the shallow water and climate conditions.

更新日期:2021-05-18
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