Abstract: The magnesium isotope composition (δ²⁶Mg) of shallow-water carbonates provides critical constraints on the marine Mg cycle, paleoclimate, and diagenesis. However, the impact of meteoric and mixing-zone diagenesis on carbonate δ ²⁶Mg remains poorly constrained, and diagenetic effects exhibit significant regional variability. Here, we investigate how mineralogy (aragonite, calcite, and dolomite) and diagenetic environments (meteoric, marine, mixing-zone, and dolomitization) influence bulk carbonate δ ²⁶Mg. We integrate mineralogical and geochemical analyses with numerical modeling using samples from Well NK-1 (Meiji Atoll, South China Sea). Results indicate that carbonate δ ²⁶Mg is co-controlled by mineral composition and the diagenetic environment. Fluid-buffered dolomites and well-preserved high-Mg calcites exhibit fractionation offsets of ~−2.0‰ and ~−2.4‰ relative to coeval seawater, respectively, making them reliable archives for seawater δ²⁶Mg reconstruction. In contrast, aragonite-dominated samples are highly susceptible to alteration by secondary calcite due to the low Mg content of aragonite. Furthermore, calcite δ ²⁶Mg varies widely (−5.1‰ to −3.1‰), corresponding to isotope fractionation factors between −4.5‰ and −4.0‰ during meteoric, mixing-zone, and marine diagenesis. This suggests that aragonite and low-Mg calcite are poor seawater archives; however, the least-altered limestone samples may constrain the lower limit of coeval seawater δ²⁶Mg (offset ~−2.4‰). Global comparisons indicate that limestone δ²⁶Mg variability is primarily controlled by the diagenetic setting and the degree of alteration. This study underscores the importance of evaluating diagenetic alteration to assess the fidelity of seawater chemistry records, refines our understanding of Mg isotope behavior during diagenesis, and clarifies the key controls on global carbonate δ ²⁶Mg.

Key words: Meiji Atoll, Well NK-1, Diagenesis, Recrystallization, Calcite, Meteoric diagenesis