18O Fractionation in the coralline aragonite of Porites lobata: Implications in oceanic paleothermometry studies

Abstract

This study is fundamentally focused on a simple, but relatively unpondered problem: the coral δ¹⁸O temperature sensitivity (dδ/dT). Previous paleotemperature studies of corals have involved the use of either: 1) the calcite’s δ¹⁸O temperature sensitivity for mollusks (Epstein et al., 1953) or 2) the slope value (Δδ/ΔT) obtained by comparing annual temperature extrema versus annual δ¹⁸O extrema in the annual growth bands of coral skeletons. Because both perspectives involve the use of different values for the thermal sensitivity of the coral skeleton, significant discrepancies may be obtained while reconstructing oceanic paleotemperatures. In order to solve this problem, a field calibration was conducted by comparing the δ¹⁸O composition of coral skeletons of Porites lobata with sea surface temperature (SST). A paleotemperature equation for P. lobata was determined by comparing the sclerochronologicaly determined monthly skeletal δ¹⁸O values with monthly SST values for the Costa Rican coast. Additionally, when δw effects were considered in the coral skeletons, it was found that Porites yields a temperature dependent ¹⁸O isotopic fractionation between 0.222 and 0.235‰/ºC. This study reveals that the thermal sensitivity of the coralline aragonite is slightly larger than that for mollusk’s calcite. Therefore, future oceanic paleotemperature applications that involve corals should avoid the use of the thermal sensitivity of the δ¹⁸O of biogenic calcite, as has been done in the past. The use of the slope value proposed here may also eliminate the common problem of the seasonal ΔTºC not matching the Δ(δc-δw) observed in the coral skeletons.