High-resolution geochemical records from Mediterranean cold-water corals: proxies for paleoclimate and paleoenvironmental reconstructions and the role of coral physiology

The chemical composition of the skeletal hard parts of the azooxanthellate scleractinian coral Desmophyllum dianthus retrieved from various sites in the Mediterranean Basin was analysed by laser ablation ICP-MS in order to investigate the potential of this species as paleoenvironmental and paleoclimate archive and assess the organism’s physiology control on the chemical signal The coral calcification is a biologically-mediated process which can induce large geochemical offset from the expected equilibrium values and this is particularly evident once the analytical spatial resolution is increased. Our data suggest that the majority of the elements analysed (B/Ca, Mg/Ca, Sr/Ca, U/Ca, P/Ca) show a strong spatial heterogeneity linked to the coral microstructure (centres of calcification vs. fibrous aragonite), with some of the elements varying by more than a factor of four. Given the near constant environment in which azooxanthellate corals live, factors other than temperature, such as the different precipitation rate and/or the different organic content between COCs and FA, might be responsible for the geochemical heterogeneity. The skeletal portion close to the outer septal surface displays low variability of the geochemical signal and so it is considered the best candidate for paleoenvironmental reconstructions. Therefore, the possibility to retrieve reliable paleoenvironmental information from D. dianthus can be achieved by a selective sampling of specific ultra-structures, in order to minimise ‘vital effect’, non-carbonatic contaminations, and reduce noise on paleoclimate and paleoenvironmental signals.