Peptides Enhance Mg Content of Calcite: Toward a Process-Based Understanding of Proxy Models
Abstract
Investigations of modern organisms relating magnesium content of calcified skeletons to temperature often exhibit unexplained deviations from the signature expected for inorganically precipitated calcite. These 'vital effects' are believed to have kinetic and taxonomic origins but the mechanistic basis for measured offsets remains unclear. A complicating factor is that mineralization is isolated from the external environment within an organic-rich matrix whose roles in mineralization are implicated but not well understood. Here we show that a simple hydrophilic peptide, sharing the same acidic character as macromolecules isolated from sites of calcification, increases the magnesium content of calcite up to 3 mol%. Using in situ AFM, we demonstrate that (Asp3Gly)6Asp3 also enhances growth rate and step edge energy of calcite compared to inorganic controls. Kinetic and thermodynamic measurements indicate that biomolecules interact with calcite surfaces to lower the energy barrier to desolvating the more strongly hydrated magnesium ion, thereby increasing the probability of its incorporation relative to calcium. Comparisons to previous studies that correlate Mg content of carbonate minerals with temperature show this peptide-induced Mg- enhancement is equivalent to offsets of several degrees Centigrade. The findings suggest local macromolecule chemistry influences Mg signatures- a plausible origin of vital effects. Further, studies of nonskeletal carbonates have long-asked whether the natural marine humic and protein substances found in sedimentary environments may influence mineralization. These insights provide a physical basis for anecdotal evidence that organic chemistry modulates the mineralization of inorganic carbonates.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFMPP51C1515D
- Keywords:
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- 0419 Biomineralization;
- 0424 Biosignatures and proxies;
- 1042 Mineral and crystal chemistry (3620);
- 3939 Physical thermodynamics;
- 4835 Marine inorganic chemistry (1050)