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

Patrick Martin,Joseph A Stewart,Helen K White,J Murray Roberts,Sebastian Hennige,Nathalie F Goodkin,Elisabeth L Sikes,Gavin L Foster

doi:10.1002/2015gl066494

Abstract

AbstractThe 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.

Highlights

Past changes in atmospheric carbon dioxide concentrations are closely linked to changes in the vast pool of dissolved inorganic carbon in the deep ocean [Broecker, 1982; Burke and Robinson, 2012; Martínez-Botí et al, 2015; Sigman and Boyle, 2000; Yu et al, 2010]
Regional differences in δ13C of dissolved inorganic carbon (DIC) between our sampling locations were
Positive relationship between δ18O and δ13C (Figure 2a), consistent with previous coral data, and indicative of strong vital effects likely related to coral pH upregulation [Adkins et al, 2003; McConnaughey, 1989; Spiro et al, 2000]

Summary

Introduction

Past changes in atmospheric carbon dioxide concentrations are closely linked to changes in the vast pool of dissolved inorganic carbon in the deep ocean [Broecker, 1982; Burke and Robinson, 2012; Martínez-Botí et al, 2015; Sigman and Boyle, 2000; Yu et al, 2010]. The boron isotope ratio, δ11B, of marine biocarbonates is an important seawater pH proxy [Hemming, 2009; Pelejero and Calvo, 2007], which has been explored in a range of marine calcifiers, including shallow-water [Hemming and Hanson, 1992; Hönisch et al, 2004; Liu et al, 2009] and deep-sea corals [Anagnostou et al, 2012; Farmer et al, 2015; McCulloch et al, 2012b]. While sediment cores allow paleoceanographic reconstructions that cover extended time periods, their temporal resolution is coarse because sediment accumulation rates in the deep sea are typically only 1–3 cm kyrÀ1. Expanding paleoceanographic studies of δ11B to deep-sea coral samples will enable reconstructions of past CO2 fluxes into and out of the deep ocean carbon pool with much higher temporal resolution

Methods

Results

Conclusion