Improved calibration of the Sr/Ca-temperature relationship in the sclerosponge Ceratoporella nicholsoni: Re-evaluating Sr/Ca derived records of post-industrial era warming

Although the Sr/Ca ratio in the skeletons of sclerosponges appears to be robustly correlated with water temperature, the measurement of Sr/Ca ratios from several specimens by different workers yields unexpectedly large (up to 5 °C) increases in temperature over the past 150 years. This is considerably greater than spatially and temporally averaged observational temperature increases. One possible culprit for the larger than anticipated temperature changes in these samples arises from the calibration between temperature and Sr/Ca ratios of the skeleton that was derived from specimens grown in a relatively narrow range of high temperatures (26 to 30 °C). In order to improve the calibration, we acquired a number of samples of sclerosponges collected at lower average water temperatures (different localities and deeper water depths), and combined measurements of the Sr/Ca ratios from bulk samples representing the last ~15 years of growth with the previous calibration dataset. Analyses of these samples have produced a revised relationship between temperature and Sr/Ca ratios in the skeletons of sclerosponges. Application of this new calibration equation to published sclerosponge data reduces the magnitude of the increase in temperature reconstructed from Sr/Ca ratios to values in better agreement with spatially and temporally gridded observational ocean temperature data sets. However, the acceleration of warming indicated by nearly all Caribbean sclerosponge records over the last several decades remains unchanged by this revised calibration.