Pristine or altered, what can early diagenesis tell us in shallow-water carbonates?

Abstract

Early marine and meteoric diagenesis is commonly proposed to alter the primary geochemical signals of shallow-marine carbonates, hindering their use as a reliable archive for ancient seawater chemistry. However, there are still uncertainties on the extent and controlling factors of early diagenetic alteration for shallow-marine carbonates. In this study, we compile and compare radiogenic Sr, Li, and U isotope data of carbonate island reefs and platforms from the South China Sea and the Bahamas to better constrain the early diagenetic effects on geochemical signals in carbonates. Multiple diagenetic zones are identified in carbonate drillcores from the South China Sea, and temporally correlated with similar zones in the Bahamas. The radiogenic Sr, Li and U isotope records from shallow-marine carbonates in these two geographically distant regions are affected by multiple factors, including compositions of diagenetic fluids, local depositional environments, fluid advection/diffusion rates, and carbonate recrystallization rates. These factors ultimately determined the diagenetic extents and regimes (fluid-buffered or sediment-buffered) of shallow-marine carbonates, which vary across different isotopic systems. A full understanding of early diagenetic history and its effects on different isotopic proxies will validate the use of shallow-marine carbonate as an archive for ancient seawater information, and contribute to tracking the temporal changes in paleo-climate and -environment.