Replicating and calibrating Haitian microatoll coral Sr/Ca variations to build confidence in sea surface temperature reconstructions

Paleoclimatologists reconstruct sea surface temperature (SST) from corals by sampling boulder-shaped massive corals along the vertical growth direction or the assumed maximum growth axis to avoid kinetic effects related to variations in growth rate. Microatolls are a morphological variation of long-lived massive coral colonies that grow laterally just beneath the ocean surface. Cross-sections from five Siderastrea siderea microatolls, 35 km west of Port-au-Prince Haiti (18.479070°N, 72.668659°W), were recovered following the 2010 earthquake. Two colonies (10BELOC4 and 10LEO1), ~5.5 km apart, were sampled along paths oriented at 25° and 91° from the vertical growth direction; respectively, to determine differences in path orientation and between colonies. We find mean linear extension rates are similar (4.2 ±1.2 mm/year-1, 1σ for 10LEO1; and 4.3 ±1.3 mm/year-1, 1σ for 10BELOC4) with no significant correlation between intercolony linear extension rates (r=0.52, p=0.18, n=8) or between linear extension rates and SST (r=-0.20, p=0.31, n=29 for 10LEO1). We find similar intercolony seasonal variability in coral Sr/Ca (1.35 ±0.04 mmol/mol, 1σ for 10LEO1; and 1.41 ±0.05 mmol/mol, 1σ for 10BELOC4) and no significant difference (p<0.001) in mean coral Sr/Ca (8.90 ±0.05 mmol/mol, 1σ for 10LEO1; and 8.83 ±0.06 mmol/mol, 1σ for 10BELOC4; n=100). Calibration of coral Sr/Ca to OISST for the 1º grid including our study site results in a slope similar to previous studies using boulder-shaped S. siderea colonies (Sr/Ca=7.91-0.04(SST), r2=0.96, n=57). These results suggest that reliable SST reconstructions that span many centuries can be derived from large S. siderea microatolls.