Partitioning of Divalent Elements between Calcium Carbonates and Water: A Solid Solution Model and Implications for Palaeoenvironmental and Palaeoecological Reconstructions from Biogenic Carbonates

Abstract

Divalent element incorporation in biogenic calcium carbonates depends on various factors such as the crystal structure (calcite versus aragonite), water temperature, water composition, biological activity (group and genus of the organism, skeleton growth rate)... Thus they are potential tracers of palaeotemperatures, palaeoenvironments, secular seawater composition variations. For instance, the fractionation of Sr between coral aragonitic skeletons and seawater has been extensively used as a proxy to surface seawater temperature and empirically calibrated on living organismes (Weber, 1973). More recently, it has been shown that other divalent elements such as Mg display an even greater temperature dependence of their fractionation between coral aragonite and seawater (Mitsuguchi et al, 1996). In more complex organisms such as bivalves, seasonal variations are observed but are more difficult to relate to temperature variations because of the strong influence of the organism metabolism on the shell compositions. Finally, the mechanisms of incorporation the trace or minor elements determine the fractionation in several ways. Trace metals may be trapped in organism skeletons as adsorbed species on the calcium carbonate surface, in substitution of calcium on crystallographic sites, or in fluid or organic matrix inclusions in the more or less porous matrix. These different sites also determine the resistance of the original geochemical signature of the biogenic carbonate to alteration through geological times. Thermodynamic modelling of the equilibrium partitioning between calcium carbonates and water can be attempted and compared with empirical calibrations in order assess the factors governing the actual composition of biogenic shells. We test here a simple elastic model of trace element substitution energies which has been successfully applied to the description of trace element partioning between minerals and liquids at high temperature (Blundy and Wood, 1994) to estimate the equilibrium constants for divalent metals between calcite or aragonite and water and compare it with available experimental data and empirical calibrations on various biogenic carbonates in order to decipher the crystal-chemical and biological sensu lato controls.