CALCULATING DISSOLUTION RATES OF CARBONATE ROCKS: A NEW LABORATORY METHOD

The dissolution of carbonate rocks such as limestone and dolostone can be seen in karst landscapes all around the planet. The dissolution of these rocks can lead to the lowering of a landscape surface, denudation. The rate at which this denudation occurs naturally is difficult to measure in the field due to the time scales involved. It is also difficult to determine through theoretical calculations due to the heterogeneous nature of carbonate rocks. Therefore, it may be difficult to reconcile field observations and theoretical data. Some attempts have been made in the laboratory setting to overcome and reconcile these differences, and this abstract discusses a new variation of those methods.

Samples of calcite, eogenetic limestone from Eleuthera, The Bahamas, and telogenetic dolostone from Michigan’s Upper Peninsula were analyzed. Samples were individually tested by being submerged in a deionized water solution that was continuously saturated with CO₂ while conductivity and pH levels were recorded at regular intervals to ensure CO₂ saturation. Each sample was prepared for the experiment by being cut into an approximate 1.5cm x 1.5cm x 1.5cm cube and covered in electron-negative epoxy and cyanoacrylate glue. One side was left exposed and was subsequently polished. A stationary cage made of styrene held the sample and was placed in a 1000ml beaker so that the sample was suspended in the center and the exposed side faced the bottom where a magnetic stir bar continuously agitated the solution. The entire setup was covered in Parafilm to prohibit evaporation.

Each sample went through three trials for a period of 3-5 days and between each trial the exposed side was repolished to ensure consistency. Preliminary results indicate that the design of the experiment was effective when comparing the conductivity curves between trials and samples. Conductivity acted as a proxy for Ca²⁺ and Mg²⁺ ions that were liberated through the dissolution. As expected the calcite had the fastest relative rate of dissolution, followed by the eogenetic limestone and the telogenetic dolostone. Future work entails collecting water samples during the experiment and utilizing them to calibrate the conductivity values to Ca²⁺ and Mg²⁺ concentrations in order to calculate the true dissolution rate.