Interpreting sea surface temperature from strontium/calcium ratios in Montastrea corals: Link with growth rate and implications for proxy reconstructions

[1] We analyzed strontium/calcium ratios (Sr/Ca) in four colonies of the Atlantic coral genus Montastrea with growth rates ranging from 2.3 to 12.6 mm a(-1). Derived Sr/Ca-sea surface temperature (SST) calibrations exhibit significant differences among the four colonies that cannot be explained by variations in SST or seawater Sr/Ca. For a single coral Sr/Ca ratio of 8.8 mmol mol(-1), the four calibrations predict SSTs ranging from 24.0 degrees to 30.9 degrees C. We find that differences in the Sr/Ca-SST relationships are correlated systematically with the average annual extension rate (ext) of each colony such that Sr/Ca ( mmol mol(-1)) = 11.82 (+/- 0.13) - 0.058 (+/- 0.004) x ext ( mm a(-1)) - 0.092 (+/- 0.005) x SST (degrees C). This observation is consistent with previous reports of a link between coral Sr/Ca and growth rate. Verification of our growth-dependent Sr/Ca - SST calibration using a coral excluded from the calibration reconstructs the mean and seasonal amplitude of the actual recorded SST to within 0.3 degrees C. Applying a traditional, nongrowth-dependent Sr/Ca - SST calibration derived from a modern Montastrea to the Sr/Ca ratios of a conspecific coral that grew during the early Little Ice Age (LIA) ( 400 years B. P.) suggests that Caribbean SSTs were > 5 degrees C cooler than today. Conversely, application of our growth-dependent Sr/Ca - SST calibration to Sr/Ca ratios derived from the LIA coral indicates that SSTs during the 5-year period analyzed were within error (+/- 1.4 degrees C) of modern values.