An Elemental Fractionation Mechanism Common to Biogenic Calcium Carbonate

Abstract

Biological modulation of element incorporation presents a major hurdle in the interpretation of geochemical data as an environmental proxy, detailed understanding and quantitative evaluation of the mechanism of elemental fractionation both being essential for reliable reconstruction of an environment. Biogenic calcium carbonate has a specific skeletal microstructure, which is strongly controlled by biomineralization. Since primary processes are more likely reflected on a smaller spatial scale, elemental distribution patterns associated with skeletal microstructure should provide unique information on biological elemental fluctuations, which cannot be determined from large-scale analysis. To study elemental fractionation mechanisms, microscale elemental distribution patterns have been studied in coral skeletons and bivalve and foraminiferal shells and the skeletal microstructure, sulfur distribution, and organic features compared. The microanalytical studies revealed two characteristic patterns that were common to all studied biogenic calcium carbonates, even though the specimens examined represented different phyla: (1) significant compositional heterogeneities that could not be explained by changes in the ambient environment and (2) a strong correlation of "metal/Ca" ratios with all or some of sulfur distribution, skeletal microstructure, and organic character. Based on these common features, I propose a mechanism of elemental fractionation, commonly applicable to biogenic calcium carbonates and involving both composition and/or concentration of organics in the calcifying fluid, that facilitates preferential elemental incorporation into biogenic calcium carbonate.