A dolomite-based record of seawater calcium isotope composition over the Neogene

The calcium isotope composition of seawater (delta Ca-44/40(sw)) records important information on the evolution of the calcium and carbon cycles over geologic time. Over the past two decades, many attempts have been made to reconstruct the Neogene delta Ca-44/40(sw) variation using various calcium isotope records of seawater or seawater-like pore fluids. Nonetheless, large discrepancies still exist among these Neogene delta Ca-44/40(sw) records. Here we report delta Ca-44/40 values in a suite of Neogene island dolomites (20.8-3.4 Ma, n = 18) from the South China Sea. Combining multiple geochemical proxies (delta Ca-44/40, delta C-13, and delta O-18, this study; Sr/Ca, & sum;REY, delta Li-7, delta Mg-26, Sr-87/Sr-86, and Delta(47), previous studies) and evidence from petrography, fluid inclusions, and magnetostratigraphy, we suggest that the dolomitization for these dolomites was buffered by seawater-like fluids in the shallow marine burial domain or even marine diagenetic domain (<180 m) over a considerable period of time (<2 Myr). Then, we apply the equilibrium calcium isotope fractionation factor between dolomite and seawater of -0.52 +/- 0.20 parts per thousand (2SD, n = 13) estimated from the literature to these fluid-buffered dolomites and reconstruct a new Neogene delta Ca-44/40(sw) record. After using the Locally Weighted Regression (LOWESS) curve fitting method, our dolomite-based delta Ca-44/40(sw) record is consistent with the reconciled seawater record from other calcium isotope archives, including shark teeth, corals, foraminifera, brachiopods, barites, phosphates, and bulk pelagic carbonates. Our results support the use of fabric-retentive early marine diagenetic dolomites to trace the evolution of delta Ca-44/40(sw) through Earth's history.