Toward a better understanding of the controlling factors on the Mg isotope compositions of shallow-water carbonates: Insights from core samples in the South China Sea

The Mg isotope composition (δ²⁶Mg) of shallow-water carbonates provides valuable insights into the marine Mg cycle, past climate, and diagenesis. However, the effects of the mixing zone and meteoric diagenesis on the δ²⁶Mg of carbonates remain poorly understood, and the variations of δ²⁶Mg during diagenesis differ globally under diverse environmental settings. In this study, we investigate the controlling factors affecting δ²⁶Mg of shallow-water carbonates during diagenesis across different mineralogies (aragonite, calcite, and dolomite) and diagenetic realms (meteoric, marine, and mixing zone diagenesis, as well as dolomitization) using bulk carbonate samples from Well NK-1, Meiji Atoll, South China Sea, by integrating mineralogical, geochemical analyses with numerical modeling. Our results reveal that δ²⁶Mg of carbonate is primarily controlled by both mineralogy and diagenetic realms, with varying degrees of diagenetic alteration. Dolomites with a fluid-buffered origin and well-preserved high-Mg calcite exhibit δ²⁶Mg offsets of ∼ −2 ‰ and ∼ −2.4 ‰ from coeval seawater, respectively, making them reliable archives for seawater δ²⁶Mg reconstruction. In contrast, the δ²⁶Mg of aragonite-dominated samples is easily influenced by mixed calcite due to the low Mg content in aragonite. Moreover, calcite displays a large δ²⁶Mg variation from −5.1 ‰ to −3.1 ‰, with the constrained Mg isotope fractionation during meteoric, mixing zone, and marine diagenesis overlapping between −4.5 ‰ and − 4 ‰. This suggests that aragonite and low-Mg calcite are not ideal archives of seawater δ²⁶Mg, whereas the least altered limestone sample may represent a lower limit for coeval seawater δ²⁶Mg with an offset of ∼ −2.4 ‰. By comparing our geochemical data with other modern shallow-water carbonates, we propose that variations of limestone δ²⁶Mg from global sites are primarily controlled by both diagenetic realms and the degree of alteration. This finding underscores the importance of using more severely altered samples to assess the fidelity of carbonates in recording seawater chemistry. Our study enhances the understanding of the behavior of Mg isotopes in carbonates during diagenesis, evaluates the reliability of carbonate minerals as seawater δ²⁶Mg archives, and clarifies key controls on carbonate δ²⁶Mg across global settings.