MINOR ELEMENT DISTRIBUTION FUNCTIONS FOR CARBONATE: IMPLICATIONS FOR SECULAR VARIATION IN SEAWATER ELEMENTAL CHEMISTRY

It has been proposed that oceanic Mg/Ca chemistry has varied during the Phanerozoic and that this variation has controlled primary carbonate mineralogies between aragonite-dominated and calcite-dominated “seas”. Carbonate, being a ubiquitous precipitate both in modern and ancient oceans, has the prospect of being a powerful proxy of paleoseawater chemistry. But before its full potential can be realized, the mechanism of minor element incorporation must be more full understood. In the past this process has been generalized by the distribution coefficient.

However, using one distribution coefficient “to rule them all” presupposes that distribution occurs as a linear process. Empirical data from abiotic (Fuchtbauer and Hardie, 1976; Mucci and Morse, 1983) and biotic calcite (Stanley, 2004; Ries, 2004, 2005) suggest that at least in terms of varying fluid chemistry non-linear distribution functions govern incorporation of Mg into a precipitating calcite.

Plots of distribution functions illustrate several important points concerning the distribution of Mg into marine calcites: (1) For the empirical datasets, whether or not distribution coefficients vary, the lower the fluid Mg/Ca, the lower the solid Mg/Ca. None of the datasets display anticorrelation where solid Mg/Ca would decrease with increasing fluid Mg/Ca. (2) Whether or not distribution coefficients vary, the total range in solid Mg/Ca increases with increasing fluid Mg/Ca. (3) Kinetics cannot be ignored when considering Mg/Ca distribution since it operates even on abiotic precipitates. (4) Distribution coefficients decrease exponentially with increasing Fluid Mg/Ca. (5) The aforementioned four observations are equally valid for biotic phases as for abiotic phases. (6) Organisms that produce a CaCO3 skeleton prefer low- to no-Mg calcite. (7) None of the taxa reported in the literature have the biomineralization pathways necessary to maintain a constant skeletal Mg/Ca in the face of changing fluid Mg/Ca.