The influence of surface state and saturation state on the dissolution kinetics of biogenic aragonite in seawater

Abstract

Laboratory observations of aragonite dissolution in seawater, under temperature and pressure conditions that approximate the natural oceanic environment, indicate that R = k{prime}((CO{sup 2{minus}}{sub 3}){sub s} - (CO{sup 2{minus}}{sub 3})){sup n} is an appropriate expression describing the dependence of dissolution rate on seawater saturation, state, where R is aragonite dissolution rate in percent per day, (CO{sup 2{minus}}{sub 3}){sub s} is the carbonate ion concentration at saturation, and (CO{sup 2{minus}}{sub 3}) is the in-situ carbonate ion concentration. Under conditions in which the influence of surface alteration is minimized, plots of dissolution rate, R, versus ((CO{sup 2{minus}}{sub 3}){sub s} - (CO{sup 2{minus}}{sub 3})) approach linearity. Consequently, in the absence of surface alteration effects, our results suggest that the reaction order, n, in the above expression should be equal to one. Use of single pteropod shells in extended experimental sequences indicates that progressive roughening of the shell surface by dissolution can substantially enhance shell dissolution rates. Surface alteration leads to variable values of the rate constant, k{prime}, in the expression above. For rate measurements obtained at increasing degrees of both undersaturation and shell roughness, the multiplicative factors k{prime} and ((CO{sup 2{minus}}{sub 3}){sub s} - (CO{sup 2{minus}}{sub 3})) give rise to curvature in plotsmore » of rate versus ((CO{sup 2{minus}}{sub 3}){sub s} - (CO{sup 2{minus}}{sub 3})). For models in which variations in k{prime} are not explicitly acknowledged, dissolution rates are generally described successfully with reaction orders (n) greater than one. Our experiments, performed at variable pressure, were modeled using several realistic partial molar volume changes ({delta} V) for aragonite dissolution in seawater.« less