Late Cenozoic reef carbonates, built layer by layer by shallow-water coral communities, record paleoenvironmental conditions from surface to intermediate depths. A striking feature is the occurrence of several-hundred-meter-thick dolomite units formed between 11 Ma and 1 Ma. Because ambient seawater at that time was generally unfavorable for dolomitization, this thick reef dolomite points to anomalous seawater chemistry, making it a unique archive for investigating long-term deviations in marine environmental conditions.
This study focuses on reef carbonates from the southern South China Sea. By quantifying the relationships between dolomitization degree and the contents and sulfur isotopic compositions of carbonate-associated sulfate, iodine contents, and molybdenum isotope ratios, we demonstrate that sulfate-related geochemical processes play a critical role in driving dolomitization. Consequently, reef dolomitization appears to require chemical conditions that depart substantially from normal seawater, and the dolomitization degree can serve as a sensitive indicator of such departures. The resulting age vs. dolomitization degree curve thus provides a continuous record of seawater condition evolution in the South China Sea since 11 Ma.
Our work not only advances the understanding of the “dolomite problem” but also transforms a mineralogical parameter into a potential paleoceanographic tool, offering independent, quantitative evidence for studying marine environmental changes since the late Miocene.

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