The effects of mineralogy and early diagenesis on the Cenozoic carbonate Ca and Mg isotopic records from the South China Sea

Abstract

The δ^44/40Ca and δ²⁶Mg values of marine carbonate are widely used to reconstruct paleo-seawater chemistry. However, post-depositional diagenesis often alters the geochemical signatures, leading to biased interpretations of the seawater archives. Here, we present multi-proxy (C-O-Mg-Ca) isotopic data from late Cenozoic carbonates of the NK-1 core in the South China Sea to evaluate the extent of meteoric and marine diagenetic effects on Ca and Mg isotope compositions. The core-top unconsolidated carbonate sediments (0–20 m), composed of aragonite and high-Mg calcite, yield an average δ^44/40Ca value of 0.84 ‰ (NIST SRM-915a) and δ²⁶Mg value of −3.07 ‰ (DSM-3). Within the meteoric zones (20–120, 443–503, 539–670 m), δ^44/40Ca and δ²⁶Mg values in limestone range from 0.57 ‰ to 1.46 ‰ and − 5.02 ‰ to −2.98 ‰, respectively, showing positive correlations with δ¹³C and δ¹⁸O values. The numerical early diagenetic modelling suggests that δ^44/40Ca and δ²⁶Mg values of limestone decrease under freshwater diagenesis, mainly controlled by the mixing ratios between freshwater and seawater. In contrast, limestone within the marine diagenetic zone (670–700 m) exhibits uniformly high δ^44/40Ca (∼1.48 ‰) and δ²⁶Mg (∼ − 4.08 ‰) values, coupled with positive δ¹³C and δ¹⁸O values, indicating seawater-buffered diagenetic conditions. For dolomite, the high δ^44/40Ca (1.39 ± 0.15 ‰) and consistent δ²⁶Mg (−2.85 ± 0.12 ‰) values suggest the dolomitization occurred under fluid-buffered conditions. The reconstructed late Cenozoic seawater calcium and magnesium isotopic compositions by the NK-1 core dolomite are consistent with the globally published data from other coeval dolomites and various proxies (e.g., carbonate ooze, foraminifera, barite). This highlights the potential of penecontemporaneous dolomite as a reliable recorder of paleo-seawater Ca and Mg isotopic compositions.