Application of Sr/Ca Ratios in Sclerosponges as Temperature Proxy

We investigated Sr/Ca ratios of sclerosponge skeletons collected from the Caribbean and the Pacific Ocean. Sclerosponges build massive calcareous skeletons in isotopic and chemical equilibrium with seawater. Because of their slow growth rate of 0.1-0.4 mm/y, they cover time intervals of decades and centuries. Sclerosponges do not depend on photosynthesis and live as deep as several hundreds of meters below sea level. Thus data derived from sclerosponges can greatly extend proxy records from reef corals. Three species of sclerosponges have been investigated: Ceratoporella nicholsoni from the Caribbean and Vaceletia sp. and V. crypta from the southwestern Pacific ocean (Coral Sea, Fiji). The specimens were sampled from different water depth between 10 and 350 m with temperatures ranging from 16 to 28° C. The modern temperature differences were used to calibrate the Sr/Ca thermometer. In all species we find a negative correlation of Sr/Ca with temperature. The different species show different Sr/Ca ratios at similar water temperatures, however they point to similar temperature sensitivity of Sr/Ca in all skeletons of about -0.1 mmol/mol/° C. We apply the Sr/Ca temperature proxy to establish a temperature reconstruction for Jamaica at 20 m below sea level with a roughly monthly temporal resolution in a 50 year time frame in the mid 17th century. This high resolution Sr/Ca record reflects annual temperature variations of up to 1.5° C. Comparison of the mean Sr/Ca ratio of this interval with the mean of the second half of the 20th century yield a temperature increase of about 2° C.