Evaluating the effect of coral topography on the climate signal in porites spp.

Understanding how the record of environmental variations in the surface waters of tropical oceans is encoded in the skeletal geochemistry of corals is essential to deciding how to extract a signal from their skeletons. A few previous investigators have studied coral growth patterns and corresponding geochemical signals; however despite this attention, questions remain about the best way to extract a climate record from coral cores with regard to skeletal topography and drill path location. Utilizing a slabbed core of Porites lutea from Espiritu Santo, Vanuatu (~15°06'S, 166°52'E) we extracted coral samples along growth bands to evaluate geochemical variations within skeletal isochrons. Coral samples were analyzed using an inductively coupled plasma-optical emission spectrometer to determine Sr/Ca ratios, a proxy for sea surface temperature (SST.) Ninety-three Sr/Ca determinations were made from the first isochron; the average value was 8.982 ± 0.030 mmol/mol. The variation about the mean is nearly identical to the analytical precision associated with the Sr/Ca determination. These results suggest that drilling a diachronous path along any axis of growth in a coral skeleton should yield similar results. A second isochron extracted from the coral is being processed to replicate these initial results. In a second experiment, utilizing another slabbed core of Porites lutea from Efate, Vanuatu (~17°34'S, 168°14'E) two sets of paths were drilled through sections of coral characterized by pronounced skeletal topography. In each of these pairs, one path was drilled along an axis of minimum growth and another along an axis of maximum growth. Coral samples were analyzed in the same way as described above to determine Sr/Ca ratios and these analytical determinations are being compared to the instrumental record of SST variations in the region. These results will show whether growth topography is an important variable to monitor when extracting a climate signal from Porites lutea.