Sr/Ca-temperature calibration for the coral Montastraea cavernosa

Abstract

The Sr/Ca ratio in coral skeletons is an important proxy for the reconstruction of sea surface temperatures (SST) of the environment and helps our knowledge about SST variability in the tropics. In the western tropical South Atlantic where the species Montastraea cavernosa is one of the most important reef-builders, only a few species have been evaluated as coral-based archives for environmental changes. Here we evaluate the Sr/Ca ratio recorded in the thecal wall and other skeletal structures of the species M. cavernosa and its implications for paleothermometry investigations. Sampling along the thecal wall generated a record of Sr/Ca ratios with well-defined annual cycles, but also with noisy intervals. A linear regression of the most recent 8 years of well-defined Sr/Ca ratios and SST results in the calibration equation Sr/Ca (mmol/mol) = −0.040 (±0.005) * SST (°C) + 10.15 (±0.15). This is the first calibration equation for this species. The skeletal elements of M. cavernosa were characterized by heterogeneous Sr/Ca ratios, with higher values for the columellae and septum, which could potentially generate a cold bias in reconstructed SST of up to 7.5 °C. The variability of Sr/Ca ratios in these structures overwhelms the annual seasonality of Sr/Ca ratios in the thecal wall, and the difference in Sr/Ca ratios observed from distinct skeletal structures may be related to biological or vital effects influencing the uptake of Sr into different coral structures. Furthermore, improving sampling techniques for M. cavernosa should be considered in future studies for the successful recovery of robust geochemical signals, eventually expanding the number of species used as natural archives in the western tropical South Atlantic.