What boron systems in coral Porites lobata tell us?--Implications for Seawater pH Reconstruction

Abstract

How habitat, long-term ocean warming and acidification affect coral calcifying fluid carbonate chemistry remains poorly understood, limiting our ability to assess coral susceptibility and reconstruct past ocean conditions. Boron isotopes (d¹¹B) and B/Ca ratios in Porites corals have been used to infer the pH of coral's calcifying fluid (pH_CF). While the extent to which these proxies capture seawater pH (pH_SW) and carbonate chemistry remains uncertain, the boron systems in Porites corals appear sensitive to environmental pH variations and continues to be a major focus of coral geochemistry research. Here, we present seasonally resolved d¹¹B and B/Ca records from Porites lobata collected from Lanyu, Taiwan, spanning two intervals: 1975-1986, and 2006-2021. We compare these results with published records from the same species to assess how environmental and physiological factors influence boron system in Porites lobata.Our results show a narrower and lower range of dissolve inorganic carbon (DIC) concentrations in the calcifying fluid of P. lobata, suggesting distinct external DIC supply or utilization compared to other Porites species. We also observed significant declines in pH_CF in both the Taiwan and Maldives records, with an accelerated decrease after 2010. Assuming equilibrium between the calcifying fluid and ambient seawater in terms of total boron concentration and isotopic composition, the estimated pH declines over the past 50 years align well with model projections. However, we detect an offset between records from northern and southern Lanyu, which cannot be explained by temperature-dependent growth effect alone, indicating additional influences on coral calcification. Expanding pH_CF datasets across geographic regions will be essential for constraining the response of Porites corals to ocean acidification and improving proxy-based reconstructions of past ocean chemistry.