Possible controls on Li, Na, and Mg incorporation into aragonite coral skeletons

We present ion probe measurements of Li/Ca, Na/Ca and Mg/Ca ratios of three scleractinian corals (Lophelia pertusa, Desmophyllum cristagalli, Porites lutea). These ratios are systematically enriched in Rapid Accretion Deposits (RADs) compared to surrounding fibres, or Thickening Deposits (TDs), and present huge variations that cannot be ascribed to any environmental parameter change. Moreover, these elemental ratios are positively correlated in the three coral species. We explore different mechanisms to explain these observations: (1) mixing between different carrier phases, (2) influence of specific ion pumps, (3) precipitation rate effects, (4) Rayleigh fractionation, and (5) pH change in the calcifying fluid. Of these possibilities, the most likely proposal seems to be kinetic effects that have influence on the number of defects (i.e., modification of the crystallographic structure), which are linked to the precipitation rate of the skeleton, and the partition coefficients are the cause of the positive correlation between Li, Na, and Mg in the coral skeleton. Temperature has an indirect influence on the skeletal concentration of these elements through its effect on the skeletal growth rate.