High-resolution Sr/Ca records in sclerosponges calibrated to temperature in situ

Abstract

Ratios of strontium to calcium have been analyzed by laser- ablation inductively coupled plasma-mass spectrometry (LA-ICP- MS) in a skeletal section of the sclerosponge Ceratoporella nichol- soni. The growth period, representative of 3 yr, was stained in the skeleton with a fluorochrome (calcein). Temperatures were record- ed at 2 h intervals within the shallow, cryptic reef enclosure that the sclerosponge inhabited on the northern coast of Jamaica, al- lowing the formulation of a direct empirical relationship between Sr/Ca and temperature. To verify this calibration, Sr/Ca ratios of two sclerosponges of the same species from depths of 67 m and 136 m in Exuma Sound, Bahamas, were analyzed by LA-ICP-MS and compared to the temperatures from these depths over a decade prior to collection. The result is an independently verified, high- resolution empirical calibration for the temperature sensitivity of Sr/Ca ratios in the aragonite skeletons of sclerosponges from Ja- maica and the Bahamas. The calibration is a first for C. nicholsoni and indicates that sclerosponges are more sensitive temperature recorders than zooxanthellate corals. It represents an important step in establishing skeletal geochemistry of sclerosponges as a proxy of temperature in the upper 250 m of the ocean.