Magnesium isotope variability in aragonitic corals: a new paleothermometer?

Geochemical variations preserved in the aragonite skeletons of massive corals have tremendous potential to produce highly-resolved paleotemperature reconstructions spanning many centuries. However, this potential has not been fully realized because physiological processes, referred to as “vital effects,” may significantly influence commonly used paleotemperature proxies (e.g. Sr/Ca, Mg/Ca and δ18O) and thus preclude a straightforward interpretation of geochemical data in terms of environmental variability. The magnesium isotope ratio (δ26Mg) of corals may be a promising new tool for reconstructing SST that is potentially less sensitive to “vital effects.” The low mass of Mg leads to isotopic fractionations between seawater and biogenic carbonate that are large enough to be sensitive to subtle SST variations, while its long residence time (~15.5 Ma), homogenous concentration (1270 ppm) and uniform isotopic composition (δ26Mg = -0.81 ± 0.05‰) in the ocean make it less likely to be altered by seawater δ26Mg variability. Here, we present a test of coralline δ26Mg as an SST proxy using a suite of Porites spp. corals from the Great Barrier Reef. Initial results measured by MC-ICP-MS show seasonal variations in δ26Mg that are coherent with SST, δ18O, Sr/Ca, Mg/Ca and U/Ca variability. An empirical calibration to local SST suggests a temperature sensitivity of ~0.0145‰/°C, which is very similar to the that of inorganically-precipitated aragonite. These promising results suggest further development of the coralline δ26Mg thermometer may result in more accurate tropical SST reconstructions and an improved understanding of the biomineralization process.