NON-DESTRUCTIVE EXPERIMENTS AND MODELING OF CALCIUM CARBONATE FORMATION IN SOILS

Soil carbonates in paleosols are commonly used in geoscience research to reconstruct both climate of the past and the ancient elevation of Earth’s land surface. A limitation of using soil carbonates to reconstruct past environments is the uncertainty associated with what season (e.g. spring vs. summer) the carbonate originally formed. To improve climate reconstructions, the factors that control precipitation and dissolution of soil carbonates must be further explored. This study aims to demonstrate the interplay of these factors by examining changes of CO₂ and O₂ concentrations. In this study, CO₂ and O₂ concentrations were measured using a Field Metabolic System (Sable Systems) for bottles containing slurries of DI water and soil with a sandy loam texture obtained from Stengl-Lost Pines, a field site near Bastrop, Texas. The respiratory quotient (RQ; ∆CO₂/∆O₂) was calculated for each bottle to determine the respiratory quotient. The measured soil microbial RQ value was 1.0, and further experiments were conducted using the same soil to non-destructively monitor carbonate precipitation and dissolution. CO₂ and O₂ concentrations, volumetric water content, and temperature were measured in four containers of soil to analyze calcite formation and dissolution. Calcite was added to two containers, while the other two were calcite-free controls. Water was slowly added to the containers to imitate rainfall and stimulate microbial respiration. The apparent respiratory quotient (ARQ; 0.76*∆CO2/∆O2, where 0.76 accounts for the faster diffusion of O₂) can be used to determine the timing of carbonate formation and dissolution. Measured ARQ values were slightly lower than 1.0 immediately after watering in the controls, which is consistent with CO₂ dissolving in water, before stabilizing at a value of 0.9 for the following six days. The ARQ values in the calcite treatment tubs decreased more substantially to a value of 0.4, as CO₂ was consumed via calcite dissolution. After approximately 24 hours, the ARQ values for the treatment tubs increased to 1.3, indicating excess CO₂ released during calcite precipitation. This study demonstrates the effectiveness of monitoring carbonate precipitation and dissolution by measuring ARQ values through time. A production-diffusion model will be used to place the experiment results within a predictive and quantitative framework to further the understanding of soil carbonate formation.