Deep-sea coral δ13C: A tool to reconstruct the difference between seawater pH and δ11B-derived calcifying fluid pH

The boron isotopic composition (δ11B) of coral skeleton is a proxy for seawater pH. However, δ11B-based pH estimates must account for the pH difference between seawater and the coral calcifying fluid, ΔpH. We report that skeletal δ11B and ΔpH are related to the skeletal carbon isotopic composition (δ13C) in four genera of deep-sea corals collected across a natural pH range of 7.89-8.09, with ΔpH related to δ13C by ΔpH=0.029×δ13C+0.929, r2=0.717. Seawater pH can be reconstructed by determining ΔpH from δ13C and subtracting it from the δ11B-derived calcifying fluid pH. The uncertainty for reconstructions is ±0.12 pH units (2 standard deviations) if estimated from regression prediction intervals or between ±0.04 and ±0.06 pH units if estimated from confidence intervals. Our new approach quantifies and corrects for vital effects, offering improved accuracy relative to an existing δ11B versus seawater pH calibration with deep-sea scleractinian corals.