A NEW LABORATORY METHOD FOR MEASURING THE DISSOLUTION RATES OF CARBONATE ROCKS

Observed denudation rates in the field do not always match the theoretical rates. In Michigan’s Upper Peninsula this was the case when investigating grike sizes. The field observations suggested a much greater rate of dissolution than the theoretical rates; the development of this research project and methodology attempts to reconcile this.

Three dolostone samples were collected from the mid-Silurian Niagara Escarpment (Engadine Group) in a grike field in Michigan’s Upper Peninsula. To analyze these samples, a new method for measuring dissolution rates of carbonates was developed. A small (~1 cm cube) of rock was cut and polished on one end while all other sides were covered in 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 a liter 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 Ca²⁺ and Mg²⁺ from the host rock. 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 experiment was run for several days at a time. Small aliquots of the beaker’s water were periodically removed and later ran for Ca²⁺ concentration through atomic emission spectroscopy which was used to calibrate the conductivity - Ca²⁺ relationship. Samples were also analyzed in thin section to ascertain any petrographic control on dissolution.

Petrographic analysis determined that all samples were dolowackestones with no major distinguishing features. During the experiments it was found that liquid-based pH probes inherently leak buffer solution, making them unsuitable when conductivity is also being measured over long periods of time. No difference was found in the experiment when using either deionized water or ultra-pure deionized water. Experimental data from samples are still forthcoming; however, based on preliminary data, it does appear that the experimental apparatus does work and that the dolostones are dissolving. This method is a work in progress.