Resumen de Trace element and isotopic composition of coral skeletons as proxies for paleoclimate reconstructions and the role of coral physiology in controlling the geochemical composition during the skeletogenesis
The geochemical composition of the aragonite exoskeleton of corals is increasingly recognized as a valuable tool for paleoceanographic studies and the deep-water specimens are considered among the most promising archives for sub-decadal scale resolution of intermediate and bathyal oceanic variability. However, the chemical heterogeneities observed at micron and nanometer size scales suggest that coral physiology imprints a “vital effect” upon different structural regions, which potentially complicates and distorts their interpretations and hence the paleoceanographic reconstructions. A number of geochemical models have been proposed, based on detailed geochemical investigations of stable isotopes (oxygen and carbon) and trace elements in coral skeletons. The large oxygen and carbon isotope variability has been related to the so-called “kinetic” and “carbonate” models, with the former linked to the kinetic fractionation of the two isotopes during calcification (McConnaughey 1989) and the latter considering the internal pH as the key parameter (Adkins et al., 2003). The “carbonate” model has been questioned by recent boron isotopic results that predict the presence of amorphous calcium carbonate (ACC) as a transient precursor phase during the biomineralization process. Trace element heterogeneity has been explained by growth-rate related mechanisms, Rayleigh fractionation and differences in cellular function within the calicoblastic cell layer. The physiological component needs to be quantified or removed in order to obtain reliable paleoclimate reconstructions and new promising approaches have been recently tested. These involve the use of geochemically similar elements (i.e. lithium and magnesium) and the application of non-traditional and radiogenic isotopes, which are independent of biological fractionation.
