HR: 1330h
AN: B12C-0793    [PDF]
TI: The Role of $\mathrm{Ca^{2+} / CO_3^{2-}}$ Ratio in Calcite Dissolution and Growth: Implications for Mechanistic Control of Biomineralization
AU: Arvidson, R S
EM: rsa4046@ruf.rice.edu
AF: Rice University, Dept. of Earth Science - MS 126 P.O. Box 1892, Houston, TX 77251-1892 United States
AU: * Davis, K J
EM: kjdavis@rice.edu
AF: Rice University, Dept. of Earth Science - MS 126 P.O. Box 1892, Houston, TX 77251-1892 United States
AU: Luttge, A
EM: aluttge@rice.edu
AF: Rice University, Dept. of Earth Science - MS 126 P.O. Box 1892, Houston, TX 77251-1892 United States
AB: The hypothesis that secular variations in the Mg/Ca ratio of seawater have exerted a fundamental control over the mineralogy and abundance of both skeletal and nonskeletal carbonates has received substantial support from both experimental and field data. In this context, ongoing efforts directed at understanding the mechanistic basis for interaction of Mg and Sr with carbonate mineral surfaces during growth (e.g., Davis et al. 2000) are of obvious importance. However, a growing body of experimental dissolution data records additional site-specific interactions between the surface and dissolved free carbon species and carbonate complexes. We suggest these data may provide additional insight into mechanisms by which organisms maintain skeletal integrity under variable conditions, including the possible development of surface precursors of mixed carbonate phases. For example, recent data have shown that kink dynamics along the fast, obtuse $(+)$ step directions are highly sensitive to the ratio of magnesium to carbonate ion. We have used these observations as the basis for exploration of the relationship between the simple ratio of dissolved calcium to carbonate ion and surface dynamics. In sets of carefully designed experiments, we sought to maintain (1) a constant distance from equilibrium by varying $\mathrm{Ca^{2+} / CO_3^{2-}}$ ratio at constant IAP, (2) constant $\mathrm{Ca^{2+} / CO_3^{2-}}$ at variable IAP, (3) all under conditions of both over- and undersaturation ranging from far to close to equilibrium. Using an integrated approach, observations were made over a wide range of space and time scales using both AFM and VSI (vertical scanning interferometry). These coupled observations provide resolution of the relationship between the overall rate of reaction (total change in surface topography) and detailed observations of characteristic step dynamics developed during both dissolution and growth. Our preliminary results confirm a strong sensitivity of the conventional fast step direction to changes in $\mathrm{Ca^{2+} / CO_3^{2-}}$ ratio; the conventional slow step is relatively insensitive by comparison. These results imply additional complexity in the relationship between step morphology, impurity incorporation, and saturation state at $\mathrm{Ca^{2+} / CO_3^{2-}}$ activity ratios of $\sim \!1$ (Teng et al. 1999), and invite more extensive treatment of differential roles of cation and anion during attachment and detachment. Davis et al. (2000) Science 290, 1134-1137. Teng et al. (1999) Geochimica et Cosmochimica Acta 63, 2507-2512.
DE: 1045 Low-temperature geochemistry
DE: 1050 Marine geochemistry (4835, 4850)
DE: 3947 Surfaces and interfaces
DE: 4835 Inorganic marine chemistry
DE: 5112 Microstructure
SC: Biogeosciences [B]
MN: 2003 Fall Meeting