Magnesium isotopic compositions and origin of Neogene dolomites in Xisha Islands, South China Sea

A common consideration of dolomite-based records for paleoseawater geochemistry reconstruction is the process of dolomite precipitation, which is closely related to the source of relevant elements. "Island dolomite" has been proposed as having great potential for recording paleoseawater geochemical characteristics, however, dolomitization should be strictly constrained in advance. As a major element in dolomites, magnesium (Mg) and its isotopes are critically involved in dolomitization, which may serve as a tool for providing direct insights into this process. Here, we report the Mg isotopic compositions of reef dolomites from the well Xike-1 reef core in Xisha Islands, South China Sea. The geochemical and mineralogical characteristics suggest that these reef dolomites experienced limited diagenetic alteration, and retained primary geochemical characteristics after dolomitization was complete. These reef dolomites reached chemical equilibrium with contemporaneous seawater during dolomitization, which were buffered by coeval seawater. Our results suggest that the development of carbonate platform and the formation of reef dolomites in Xisha Islands were related to tectonic subsidence. These reef dolomites were deposited during periods of decreasing tectonic subsidence rate. Then, dolomitization model for these reef dolomites was established, which provides an effective basis for reconstructing the chemical characteristics of paleoseawater. The occurrence of near-invariant seawater delta Mg-26 value during the past similar to 23 Myr was subsequently verified via cross-checks provided by different seawater delta Mg-26 archives in this and previous studies. Our results highlight the great potential of seawater-buffered dolomites with limited postdepositional diagenetic alteration for constructing paleoseawater geochemistry though the geological history.