Mg and Ca isotopes of phreatic groundwater on a carbonate island: implications for meteoric diagenesis of carbonates

Meteoric diagenesis can significantly overprint primary geochemical signatures of marine carbonates. Constraining the associated geochemical differentiation is therefore essential for distinguishing primary signals from alteration, but remains difficult based on altered-carbonate geochemistry alone. We investigated major ion concentrations and Ca – Mg isotopic compositions ( δ 44/40 Ca and δ 26 Mg) of groundwater from Yongshu Island (southern South China Sea), a site marked by extensive freshwater–seawater mixing within its subsurface aquifers. By using Cl − as a conservative seawater tracer in a binary mixing model, the freshwater δ 26 Mg value of −2.1 ‰ was obtained, whereas the freshwater δ 44/40 Ca value (∼0 ‰) was assumed to equal that of seawater, as it showed no correlation with Ca 2+ concentration. The inferred δ 26 Mg and δ 44/40 Ca values of freshwater are markedly higher than those expected from carbonate dissolution. Together with box modeling of meteoric diagenetic water–rock reactions, these elevated isotopic values are inferred to arise from fluid isotopic evolution under sediment-buffered conditions within the vadose zone. The newly obtained δ 26 Mg and δ 44/40 Ca compositions of freshwater were used to model meteoric phreatic diagenesis of carbonate rocks. The modeled results can well reproduce the coupled Ca – Mg isotopic evolution observed in young meteoric water-altered carbonates from coral islands in the South China Sea and the Great Barrier Reef, Australia. These results are interpreted to reflect predominantly fluid-buffered meteoric diagenesis within the meteoric phreatic zone. Differentiating the sediment-buffered vadose zone from the fluid-buffered phreatic zone allows for a coherent interpretation of the Ca–Mg isotopic compositions in both meteoric limestones and reef-island freshwater. Elucidating Mg and Ca isotopic behavior in the vadose and phreatic zones during meteoric diagenesis will provide a robust framework for interpreting paleoenvironmental signals preserved in freshwater-altered carbonates.