Quantifying the pH ‘vital effect’ in the temperate zooxanthellate coral Cladocora caespitosa: Validation of the boron seawater pH proxy

Boron isotopic and elemental systematics are used to define the vital effects for the temperate shallow water Mediterranean coral Cladocora caespitosa. The corals are from a range of seawater pH conditions (pHT ~ 7.6 to ~ 8.1) and environmental settings: (1) naturally living colonies harvested from normal pH waters offshore Levanto, (2) colonies transplanted nearby a subsea volcanic vent system, and (3) corals cultured in aquaria exposed to high (700 μatm) and near present day (400 μatm) pCO2 levels. B/Ca compositions measured using laser ablation inductively coupled mass spectrometry (LA-ICPMS) show that boron uptake by C. caespitosa cultured at different pCO2 levels is independent of ambient seawater pH being mainly controlled by temperature-dependent calcification. In contrast, the boron isotope compositions (δ11Bcarb) of the full suite of corals determined by positive thermal ionisation mass spectrometry (PTIMS) shows a clear trend of decreasing δ11Bcarb (from 26.7 to 22.2‰) with decreasing seawater pH, reflecting the strong pH dependence of the boron isotope system. The δ11Bcarb compositions together with measurements of ambient seawater parameters enable calibration of the boron pH proxy for C. caespitosa, by using a new approach that defines the relationship between ambient seawater pH (pHsw) and the internally controlled pH at the site of calcification (pHbiol). C. caespitosa exhibits a linear relationship between pHsw and the shift in pH due to physiological processes (ΔpH = pHbiol − pHsw) giving the regression ΔpHClad = 4.80 − 0.52 × pHsw for this species. We further apply this method (“ΔpH–pHsw”) to calibrate tropical species of Porites, Acropora, and Stylophora reported in the literature. The temperate and tropical species calibrations are all linearly correlated (r2 > 0.9) and the biological fractionation component (ΔpH) between species varies within ~ 0.2 pH units. Our “ΔpH–pHsw” approach provides a robust and accurate tool to reconstruct palaeoseawater pHsw for both temperate and tropical corals, further validating the boron fractionation factor (αB3–B4 = 1.0272) determined experimentally by Klochko et al. (2006) and the boron isotope pH proxy, both of which have been the foci of considerable debate.

Research Highlights►Boron isotope ratios in Cladocora caespitosa decrease with decreasing seawater pH. ►B/Ca elemental ratios reflect ambient seawater temperatures not pH. ►Ambient seawater pH and biologically controlled pH are linearly correlated. ►The biological vital effects on δ11B during coral calcification is quantified. ►A new robust approach to reconstruct seawater pH is presented.