Investigation of a Northern Gulf of Mexico brain coral, Diploria strigosa, for Climate Reconstructions

Paleoclimatologists have produced sea-surface temperature (SST) reconstructions from coral geochemistry by sampling fast growing brain corals commonly found in the Atlantic Ocean. Many researchers avoid deep coral reefs, >10-m water depth, because the geochemistry of these corals may not reflect SST due to water stratification and to avoid possible kinetic effects due to reduced coral growth with increasing water depth. Flower Garden Banks is the northernmost coral reef in the Gulf of Mexico with the top of the reef at 18 m from the surface. In 1979, the U.S. Geological Survey recovered a core from a small Diploria strigosa colony in the East Flower Garden Bank (27°54'N, 93°35'W) from a depth of ~21 m. Measurements of the x-radiographs reveal an average linear growth rate of 5.4 mm/year (±1.06, 1σ; n = 38). We analyzed approximately monthly samples (0.3 mm/sample) extracted along the growth axis for Sr/Ca and δ¹⁸O. These geochemical proxies for temperature reveal seasonal cycles; however, the section from 1968-1970 contains a 1.16% shift in the mean Sr/Ca with higher Sr/Ca values present only in the summer samples, resulting in a reduced seasonal cycle. Shifts in coral geochemistry have been noted in other studies with the source of the shifts attributed to diagenesis, sampling artifacts, or growth effects. Our analysis of scanning electron microscope images did not find indications of diagenesis. To test if our sampling rate biased our results, we sampled at a higher resolution (0.2 mm/sample) and found similar results for Sr/Ca. The anomalous section coincides with a 39% reduction in linear extension rates, leaving a growth effect as the remaining cause; however, this shift is unlike the growth effect observed in other species. We conclude that corals from this reef show promise for paleoclimate studies; however, further studies with larger colonies are needed.