Boron isotopic compositions of middle Miocene to recent shallow-water carbonates from the South China Sea: Assessing diagenetic effects and implications for paleoclimate changes

The partial pressure of atmospheric CO₂ (pCO₂) significantly influences global climate change and biological evolution through geological history. Boron isotopic composition (δ¹¹B) in carbonates has been used to reconstruct the paleo-pH of seawater, providing insight into atmospheric pCO₂ levels. However, the fidelity of δ¹¹B records in marine carbonates due to diagenesis remains uncertain. Here, to understand how diagenetic processes influence B isotopic records in marine carbonates, we examined B concentrations and δ¹¹B values of modern corals (from Hainan Island), unconsolidated shallow pushcores (Jiuzhang A and Jiuzhang B from Nansha Islands, South China Sea), and a long drillcore (XK-1 from Xisha Islands, South China Sea) covering the Late Miocene to Holocene periods. Our coral samples show a uniform δ¹¹B range (20.16 ± 0.85%), consistent with previously published values for modern corals in other open oceans. The δ¹¹B values of the unlithified carbonate sediments from Jiuzhang A and B pushcores vary within a narrow range (18.70 ± 0.84%), yielding pH and CO₂ concentrations consistent with the range between modern and pre-industrial values. Since we did not observe any statistically significant covariations between traditionally established diagenetic proxies (such as δ¹³C, δ¹⁸O, Mn/Sr, and Al/Ca) and δ¹¹B values for non-dolomitized samples in the XK-1 drillcore, we suggest that δ¹¹B values of bulk limestones are not significantly affected by typical diagenesis (meteoric, mixed-diagenetic, and marine diagenetic processes), likely due to limited post-depositional recrystallization of our study carbonates. In contrast, dolomitization significantly decreases δ¹¹B values of bulk carbonates, rendering dolomites unsuitable as archives for reconstructing seawater δ¹¹B_borate values. Our study supports that marine limestone with limited recrystallization or dolomitization have the potential to record ambient seawater pH values with high resolution. The established secular seawater pH variations based on XK-1 δ¹¹B records provide information about ocean acidification over the past 5.1 million years.