How brain corals record climate: an integration of skeletal structure, growth and chemistry of Diploria labyrinthiformis from Bermuda

Abstract

The aragonite skeleton of massive reef-building corals contains a record of the oceanic environment in which they grow. However, reading of the record requires understanding of how it is archived, a process complicated by the elaborate skeletal construction and seasonal growth patterns that characterize many species. In this study, we assess the utility of the massive brain coral Diploria labyrinthiformis as an archive of sea surface temperature (SST) variability in the western North Atlantic. In situ staining of live colonies combined with microscale analysis of skeletal chemistry indi- cate that D. labyrinthiformis grows throughout the year on Bermuda and records the full annual cycle of SST variability. However, skeleton accreted during the summer is overlain (thickened) by skeleton accreted during the subsequent fall and winter. As a result, conventional coarse sampling for δ 18 O enables seasonal δ 18 O cycles to be resolved but these do not capture the full amplitude of the annual SST cycle. Our data show that the shallow gradient of the δ 18 O-SST regression equation derived for D. labyrinthiformis (-0.113‰ oC -1 ) relative to the expected -0.22‰ oC -1 for marine skeletons results from dampening of the summertime peak in δ 18 O. In contrast, skeleton accreted during the winter is not thickened and wintertime δ 18 O captures the interannual wintertime SST variability at this site. Using SIMS ion microprobe to analyse strontium to calcium ratios (Sr/Ca), we avoided the thickening deposits and were able to resolve the full amplitude of the annual Sr/Ca cycle. The Sr/Ca-SST relationship obtained for D. labyrinthiformis (-0.0843 mmol/mol oC -1 ) corresponds to that derived from fast-growing tropical reef corals. X-ray intensity ratios, used as a proxy for skeletal density, reveal the expected seasonal changes associated with growth banding as well as variability on inter- annual and decadal timescales. These variations are well correlated with wintertime SST variability in the subtropical gyre and may be a valuable proxy thermometer for the North Atlantic.