Understanding cold bias: Variable response of skeletal Sr/Ca to seawater pCO2 in acclimated massive Porites corals.

Catherine Cole,Christopher Hintz,Nicola Allison,Kenneth Hintz,Adrian Finch

doi:10.1038/srep26888

Catherine Cole, Christopher Hintz + Show 3 more

Open Access

https://doi.org/10.1038/srep26888

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Abstract

Coral skeletal Sr/Ca is a palaeothermometer commonly used to produce high resolution seasonal sea surface temperature (SST) records and to investigate the amplitude and frequency of ENSO and interdecadal climate events. The proxy relationship is typically calibrated by matching seasonal SST and skeletal Sr/Ca maxima and minima in modern corals. Applying these calibrations to fossil corals assumes that the temperature sensitivity of skeletal Sr/Ca is conserved, despite substantial changes in seawater carbonate chemistry between the modern and glacial ocean. We present Sr/Ca analyses of 3 genotypes of massive Porites spp. corals (the genus most commonly used for palaeoclimate reconstruction), cultured under seawater pCO2 reflecting modern, future (year 2100) and last glacial maximum (LGM) conditions. Skeletal Sr/Ca is indistinguishable between duplicate colonies of the same genotype cultured under the same conditions, but varies significantly in response to seawater pCO2 in two genotypes of Porites lutea, whilst Porites murrayensis is unaffected. Within P. lutea, the response is not systematic: skeletal Sr/Ca increases significantly (by 2–4%) at high seawater pCO2 relative to modern in both genotypes, and also increases significantly (by 4%) at low seawater pCO2 in one genotype. This magnitude of variation equates to errors in reconstructed SST of up to −5 °C.

Highlights

The influence of pH and dissolved inorganic carbon (DIC) on coral skeletal Sr/Ca is poorly constrained
We test the direct impact of variations in seawater pCO2 on skeletal Sr/Ca in massive Porites spp. corals, the coral genus most commonly used for palaeoclimate reconstruction
Final seawater pCO2 target levels ranged from the last glacial maximum (LGM; ~180 μatm), through present day (~400 μatm) to levels projected by the year 2100 (~750 μatm)

Summary

Introduction

The influence of pH and DIC on coral skeletal Sr/Ca is poorly constrained. Culture studies investigating the effect of reduced seawater pH across a range of ocean acidification scenarios yield mixed results: seawater pH and Sr/Ca were negatively correlated in Acropora digitifera[26] and newly settled recruits of Favia fragum[27] but were unrelated in Montipora capitata[13]. Seawater temperatures do not vary significantly between the treatments (Table 1) and we observe excellent agreement (within 0.3%) in the KDSr/Ca of duplicate sub colonies of the same coral genotype within each treatment (Fig. 2), indicating that minor differences in coral positioning and lighting in each tank do not affect skeletal Sr incorporation.

Results

Conclusion