Major advances have recently been made in quantifying the chemical parameters which control carbonate mineral reaction kinetics in natural waters. One of the primary factors influencing rates of reaction is the amount of surface area per unit mass available for reaction. For relatively smooth, nonporous carbonates, the relationship between reactive and total surface area measured by gas adsorption has proven simple. However, the dominant carbonates in the marine environment are biogenic and exhibit complex microstructures. We have performed experiments to determine dinghy what proportion of the total surface area of these biogenic carbonates is actually available for reaction during dissolution. Three biogenic carbonates having widely differing microstructures ( Halimeda, coral, and echinoid) were chosen for study and comparison with rhombic calcite. The influence of grain size on the relation between total surface area, determined from gas adsorption, and reactive surface area, determined relative to rhombic calcite during dissolution in constant composition solutions, was of primary concern. Unlike rhombic calcites, total surface area for texturally complex biogenic grains varied slightly or not at all with increasing grain size. However, both rhombic calcite and biogenic carbonate dissolution rates were inversely related to grain radius. This implies that much of the total surface area of biogenic carbonates is unavailable for reaction with the aqueous solution. Dissolution rates were a function of both grain size and microstructure. We have defined an empirical roughness factor which quantifies the difference in reactive surface area between rhombic calcite and a given biogenic grain of equivalent size. The roughness factor increased with grain microstructural complexity but was independent of grain size over the grain-size range (51-513 microns) studied. The maximum roughness factor value observed was 7 for Halimeda relative to a value of 1 for rhombic calcite. Our findings indicate that both microstructure and grain size can play important roles in controlling reactivity of biogenic carbonates during diagenesis. The relative significance of grain size versus microstructure for carbonate dissolution rates in natural systems is discussed.

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