The influence of temperature and vital effects on magnesium isotope variability in Porites and Astrangia corals

The magnesium isotope composition (δ26Mg) of sub-annually and bulk-sampled Porites corals is presented and suggests that the coral–seawater fractionation factor (∆26Mg) is temperature-dependent with a sensitivity of ~0.03‰°C−1. Although the apparent temperature dependence of ∆26Mg in Porites corals could be used empirically to reconstruct paleoclimate, its temperature sensitivity is about three times that of abiogenic aragonites, which indicates that kinetic and/or biologically-induced processes (collectively known as vital effects) are also important. Comparison with data from abiogenic aragonite precipitation experiments suggests that ∆26Mg vital effects vary seasonally in Porites. The ∆26Mg of skeleton deposited in winter is within the analytical error of abiogenic aragonite, but that of skeletal aragonite grown during summer exhibits significantly smaller fractionations that may be associated with light-enhanced calcification. The observed vital effect is consistent with