Ocean acidification in the tropical Indian Ocean over the past 37 years: Insights from δ11B and B/Ca records in a Maldives coral

Boron isotopes ($\delta$¹¹B) in coral skeletons of Porites have been widely applied to reconstruct past seawater pH (pH_SW) on decadal to centennial timescales. However, due to biological regulation within corals, an additional transfer function is required to estimate ambient seawater chemistry during the skeleton growth under the calcification site fluid pH. Temperature may also interfere with coral calcification fluid pH (pH_CF) due to changes in kinetics of coral aragonite precipitation, or buffering capacity in coral calcification fluid. To decipher how coral Porites adjusts pH_CF in response to pH_SW from complex environmental controls, long-term records from sites with least fluctuations in environmental conditions other than pH_SW are essential. Here we present a 37-year record of coral $\delta$¹¹B and B/Ca ratios derived from a coral core collected from southern Maldives, the tropical Indian Ocean. Our results show no clear seasonality in the coral $\delta$¹¹B and B/Ca ratios between monsoons, but a long-term decline in coral pH_CF is evident across the entire record. When applying different existing transfer functions, we also observe discrepancies among the calculated pH_CF values, model results and short-term instrumental data. Calculated calcification fluid dissolved inorganic carbon concentration ([DIC]_CF) values are relatively low compared to literature, suggesting that coral calcification fluid carbonate chemistry may be under different levels of control, even within the same coral taxa. Thus, coral records from a wider geographic range are required to better quantify coral response to ocean acidification, and our results can serve as a baseline for future comparisons.