The goal of the project is to improve the current understanding of the behavior and dynamics of biogenic Mg-calcite minerals in the natural environment. This knowledge is critical to better understand the impacts of ocean acidification on marine Mg-calcite calcifiers and carbonate mineral dissolution, and how these key parameters are represented in models in order to predict the impacts of ocean acidification on marine ecosystems, biogeochemical cycles, oceanic carbon uptake, and ultimately the global carbon cycle. The research is specifically conducted in order to resolve the limitations and discrepancies currently observed in model calculations based on the different Mg-calcite solubility curves. Three hypotheses to be tested in this project include: (1) The average Mg-calcite mineral composition in surface sediments and Mg-calcite calcifiers is in metastable equilibrium with surface seawater based on the biogenic �??minimally prepared�?� solubility curve; (2) The average Mg-calcite composition in calcareous hard-parts of a particular calcifying species decrease as a function of CO32- along a natural gradient in CO2 chemistry across the Bermuda platform; and (3) The average Mg-calcite composition in surface sediments decrease as a function of: a) decreasing surface seawater [CO32- ] across the Bermuda platform; b) decreasing pore water [CO32- ] with increasing sediment depth; and c) decreasing mineral grain size as a result of selective mineral dissolution based on mineral stability. To address these hypotheses a careful characterization of the marine CO2 system in both surface seawater and pore water will be conducted concurrent with detailed characterization of carbonate mineral properties including grain size, grain constituents, and composition along a natural gradient in CO2 chemistry across the Bermuda coral reef platform. The mineralogy of major marine Mg calcite calcifiers will also be analyzed in order to characterize the mineral composition of the sediment source material. It is believed that a careful characterization of Mg-calcite composition in surface sediments as well as in the source material (i.e., living organisms) together with seawater CO2 parameters will provide important information in terms of the reactivity and dynamics of these minerals in the natural environment and in relation to the existing experimental solubility curves.