The effect of skeletal mass accumulation in Porites on coral Sr/Ca and δ¹⁸O paleothermometry

Abstract

[1] Paleotemperature estimates based on coral Sr/Ca have not been widely accepted because the reconstructed glacial-Holocene shift in tropical sea-surface temperature ($4-6°C) is larger than that indicated by foraminiferal Mg/Ca ($2-4°C). We show that corals over-estimate changes in sea-surface temperature (SST) because their records are attenuated during skeletogenesis within the living tissue layer. To quantify this process, we microprofiled skeletal mass accumulation within the tissue layer of Porites from Australasian coral reefs and laboratory culturing experiments. The results show that the sensitivity of the Sr/Ca and d 18 O thermometers in Porites will be suppressed, variable, and dependent on the relationship between skeletal growth rate and mass accumulation within the tissue layer. Our findings help explain why d 18 O-SST sensitivities for Porites range from À0.08‰/°C to À0.22‰/°C and are always less than the value of À0.23‰/°C established for biogenic aragonite. Based on this observation, we recalibrated the coral Sr/Ca thermometer to determine a revised sensitivity of À0.084 mmol/mol/°C. After rescaling, most of the published Sr/Ca-SST estimates for the Indo-Pacific region for the last $14,000 years (À7°C to +2°C relative to modern) fall within the 95% confidence envelope of the foraminiferal Mg/Ca-SST records. We conclude that two types of calibration scales are required for coral paleothermometry; an attenuated Porites-specific thermometer sensitivity for studies of seasonal to interannual change in SST and, importantly, the rescaled À0.084 mmol/mol/°C Sr/Ca sensitivity for studies of 20th-century trends and millennial-scale changes in mean SST. The calibration-scaling concept will apply to the development of transfer functions for all geochemical tracers in corals.

Evaluation of annual resolution coral geochemical records as climate proxies in the Great Barrier Reef of Australia

“…It is worth noting that this calibration is almost identical to the modified Sr/Ca-SST equation used to model annual mean SST by Gagan et al (2012). Both the slope (-0.0824) and intercept (11.209) of our equation are very close to those of their equation (-0.084 and 11.278, respectively, Fig.…”

“…However, the detailed procedures best used for this calibration require further investigation. The agreement of our Sr/Ca-SST equation with that of Gagan et al (2012) again indicates that our calibration is reliable and that it can be applied to annual mean SST reconstruction on the GBR.…”

“…7). The equation of Gagan et al (2012) corrects for the attenuation effect inherent within the geochemical records and is believed to provide accurate annual mean SST records (Gagan et al 2012). It is reasonable to assume that data obtained from corals analyzed using an annual sampling resolution incorporates all of the significant seasonal factors, and any seasonality inherent within the climatic and environmental signals has been averaged out.…”

“…Some studies have evaluated the climatic significance of annual coral records averaged using monthly or sub-annual values from each year (Lough 2004;Smith et al 2006;Pfeiffer et al 2009). However, such approaches may suffer from biases caused by the attenuation of the coral geochemical records, which leads to an overestimate of the changes in sea surface temperature (SST) over decadal to millennial timescales (Gagan et al 2012).…”

Pronounced interannual variability in tropical South Pacific temperatures during Heinrich Stadial 1

“…S10). Our estimates of a reduction in mean SST by 3.5 °C relative to today, using our seasonal Tahiti coral Sr/Ca-SST relationship or by only 2.4 °C, using a possibly more suitable relationship for mean Indo-Pacific SST changes 32 , however, have relatively large uncertainties. As commonly observed at various locations 27,28,33 , between-colony offsets in mean Sr/Ca are evident among the modern Tahiti corals (Supplementary Methods and Supplementary Fig.…”