Multi-element sea surface temperature measurement using coral substructures

We conducted EMP and SEM analyses to study the structure, Sr/Ca and Mg/Ca ratios of the Coral Dichocoenia. Both Sr/Ca and Mg/Ca ratios were used to reconstruct the seawater temperature at which the coral grew. The concentrations of Sr and Ca, and Sr/Ca ratio have no significant variations across the alternating narrow-thick bands, but concentrations of Na and Mg, and the Mg/Ca ratio vary significantly across those bands. The uniform distribution of Sr/Ca ratios suggests that this ratio is less sensitive to temperature and biological effects relative to Mg/Ca ratio. Using a Rayleigh-based, multi-element (RBME) coral thermometer, the average temperatures of the thick bands and narrow bands on a Dichocoenia aff. tuberosa longitudinal section were estimated as 25.1 °C with an range between 22.5 °C and 28.3 °C, and 24.9 °C with an range between 21.4 °C and 27.5 °C, respectively. The average temperatures in both thick bands and narrow bands are about 2 °C lower than the current SST level in Costa Rica region. The distinct calculated calcifying fluid Mg/Ca and Sr/Ca ratios of narrow band (Sr/Ca =0.009; Mg/Ca =2.3) and thick band (Sr/Ca =0.010; Mg/Ca =1.2) not only suggests the two types of band are not sharing the same calcifying fluid, but also indicates the two types of bands may utilize distinctive growth mechanisms or may grow in different seasons associated with varying biological effects. The calcifying fluid Mg/Ca and Sr/Ca ratios generated from the thick bands (Sr/Ca=0.010;Mg/Ca=1.2) in coral Dichocoenia afftuberosa (sample 108302) agree with the thick bands in other Dichocoenia corals such as coral Dichocoenia tuberosa (sample 118669), which implies that the thick bands of these corals may share a similar growth mechanism.