Control of Solution Chemistry and Mineralization Pathway on the Partitioning of Mg and Sr in ACC and Calcite
Abstract
The inability of the classical terrace-ledge-kink (TLK) step growth model to explain the growth of some Mg and Sr-bearing carbonates in biogeochemical settings is a long-standing enigma to fully understanding mineralization. Evidence from structural biology shows that some calcifying organisms overcome the limitations of mineralization by the step growth process by employing a non-classical pathway that begins with the formation and storage of an amorphous calcium carbonate (ACC) precursor that subsequently transforms to calcite. The community does not have a complete picture of the factors that regulate the composition of minerals formed by this process. This study quantifies inorganic controls on the Mg and Sr content of calcites that form from ACC. Experiments were designed to produce ACC with different Mg and Sr compositions by systematically altering initial Me/Ca ratios, carbonate concentrations, and pH. Subsamples of the ACC were analyzed for Mg and/or Sr content and the remainder was allowed to transform within solution. We find the composition of ACC and calcite is controlled by the interplay of three inorganic factors that affect partitioning: 1) initial Me/Ca ratio, 2) carbonate concentration, and 3) pH. In addition, Mg partition coefficients increase with temperature in ACC and calcite, whereas Sr partitioning is independent of temperature. Measured Sr partition coefficient values in calcite that form from ACC are much higher than those reported for biogenic calcites that form via classical growth, which suggests partition values may have the potential to be used as indicators of the mineralization pathway. Further analysis of partition coefficients also suggests the need to revisit how these values are determined and interpreted in studies of biogenic calcites.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.V41C2810B
- Keywords:
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- 1042 GEOCHEMISTRY Mineral and crystal chemistry;
- 0419 BIOGEOSCIENCES Biomineralization;
- 0460 BIOGEOSCIENCES Marine systems