Clumped isotope geochemistry of island carbonates in the South China Sea: Implications for early diagenesis and dolomitization

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 Delta(47) value) is based on the temperature-dependent binding of C-13 to O-18, and offers a promising means of calculating the formation temperature. In this study, we report the Delta(47) values of island carbonates from a similar to 1 km-long drill core from the Meiji Reef in the South China Sea, which are integrated with petrography, carbon (delta C-13) and oxygen isotope (delta O-18) 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 similar to 200 m below sea level at temperatures between similar to 15 and 28 degrees C, and that the fluids involved during the formation of the diagenetic calcites and dolomites have similar delta O-18 values to modern seawater. Considering the estimated secular variation in seawater delta O-18, 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 Delta(47) 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 Delta(47) values of diagenetic reef carbonates in shallow burial settings as valuable records of the shallow water and climate conditions.

Automated summary

This study focuses on the Delta(47) values of island carbonates from the Meiji Reef in the South China Sea to constrain the diagenetic environment, finding that most diagenetic alteration likely occurred at water depths around 200 meters below sea level at temperatures between 15 and 28 degrees Celsius. The fluids involved in the formation of diagenetic calcites and dolomites have similar delta O-18 values to modern seawater.