Evaluation of boron isotope ratio as a pH proxy in the deep sea coral Desmophyllum dianthus: Evidence of physiological pH adjustment

The boron isotope ratio (δ ¹¹B) of foraminifers and tropical corals has been proposed to record seawater pH. To test the veracity and practicality of this potential paleo-pH proxy in deep sea corals, samples of skeletal material from twelve archived modern Desmophyllum dianthus (D. dianthus) corals from a depth range of 274-1470m in the Atlantic, Pacific, and Southern Oceans, ambient pH range 7.57-8.05, were analyzed for δ ¹¹B. The δ ¹¹B values for these corals, spanning a range from 23.56 to 27.88, are found to be related to seawater borate δ ¹¹B by the linear regression: δ ¹¹B _coral=(0.76±0.28) δ ¹¹B _borate+(14.67±4.19) (1 standard error (SE)). The D. dianthus δ ¹¹B values are greater than those measured in tropical corals, and suggest substantial physiological modification of pH in the calcifying space by a value that is an inverse function of seawater pH. This mechanism partially compensates for the range of ocean pH and aragonite saturation at which this species grows, enhancing aragonite precipitation and suggesting an adaptation mechanism to low pH environments in intermediate and deep waters. Consistent with the findings of Trotter et al. (2011) for tropical surface corals, the data suggest an inverse correlation between the magnitude of a biologically driven pH offset recorded in the coral skeleton, and the seawater pH, described by the equation: ΔpH=pH recorded by coral-seawater pH=-(0.75±0.12) pH _w+(6.88±0.93) (1 SE). Error analysis based on 95% confidence interval(CI) and the standard deviation of the regression residuals suggests that the uncertainty of seawater pH reconstructed from δ ¹¹B _coral is ±0.07 to 0.12 pH units. This study demonstrates the applicability of δ ¹¹B in D. dianthus to record ambient seawater pH and holds promise for reconstructing oceanic pH distribution and history using fossil corals.