A LABORATORY METHOD FOR MEASURING CARBONATE DISSOLUTION RATES

Seldom do field observed dissolution rates of carbonates match theoretical calculations. The development of this new laboratory method attempts to reconcile this disparity by dissolving samples of carbonates in a uniform environment. Two calcite samples were prepared by cutting a small (~1 cm cube) of calcite and polishing it on one side while covering the other sides with marine epoxy and cyanoacrylate glue to control the surface area of the samples. On an individual basis, each sample was placed in a styrene cage which was then submerged in approximately 0.65 L of deionized water. During the experiment, CO₂ was pumped into the water to ensure the saturation of H₂CO₃. Conductivity was also measured to observe the liberation of ions, namely Ca²⁺, from the sample. The beaker was placed on a stir plate to ensure even mixing of water and was sealed using a plastic beaker cover and parafilm. Each experimental trial was conducted over several days. Small aliquots of the beaker’s water were collected approximately every 12 hours, and later analyzed for Ca²⁺ concentration through atomic emission spectroscopy which was used to calibrate the conductivity - Ca²⁺ relationship, and from this dissolution rates of the samples can be calculated.

During the course of these experiments it became apparent that liquid-based probes and even gel-based probes inherently leak buffer solution, making them unsuitable when conductivity is also being measured over long periods of time. This prohibits the use of pH probes or ion-selective electrodes for either calcium or magnesium which would have removed the need for atomic emission spectroscopy. Preliminary results from several trials show a dissolution rate of about 1000 cm*ka^-1 (±6%) for a calcite sample in a uniform and H₂CO₃-saturated solution; which is not outside the realm of reason given the idealized nature of the experimental design.