GSA Annual Meeting in Indianapolis, Indiana, USA - 2018

Paper No. 4-4
Presentation Time: 9:00 AM

TRACKING SOIL CARBONATE DISSOLUTION AND PRECIPITATION THROUGH HIGH-PRECISION MONITORING OF SOIL PORE SPACE OAND CO2


GALLAGHER, Timothy M., Department of Geological Sciences, the University of Texas at Austin, Austin, TX 78712, CALDWELL, Todd G., Bureau of Economic Geology, Jackson School of Geosciences, University of Texas at Austin, University Station, Box X, Austin, TX 78713 and BREECKER, Daniel O., Department of Geological Sciences, University of Texas at Austin, Austin, TX 78712

Authigenically formed soil carbonates occur in many dryland soils. They can accumulate in soils over thousands of years, gradually driving changes in soil hydrology, chemistry, and nutrient availability. Developing a complete understanding of the seasonal processes that control soil carbonate formation has proven difficult, due in large part to uncertainties in relating short term instrumental records to the geochemistry of carbonates that have been accumulating for millennia.

In an effort to better understand the processes controlling soil carbonate pedogenesis, we designed a series of controlled laboratory and field experiments in which powdered calcite was added to previously carbonate-free soils. In order to non-destructively track soil carbonate dissolution and precipitation, soil moisture, temperature and the concentrations of pore space O2 and CO2 were monitored. Soil CO2 and O2 were measured using a continuous flow analyzer coupled with an automated soil gas sampling system. The concentration of soil CO2 is affected by the dissolution/precipitation of calcium carbonate and changes in soil respiration, both of which are mediated by soil moisture and temperature, while soil O2 levels are primarily controlled by changes in soil respiration. These simultaneous measurements allow us to separate the carbonate dissolution/precipitation CO2 signal from changes driven by respiration.

Our results indicate that after soil wetting, large, rapid calcium carbonate dissolution events are followed by significantly longer periods of gradual calcium carbonate formation. Similar trends in oxygen were observed between treatment and control soils, while treatment exhibited a smaller and lagged peak in CO2. Additionally, large differences in the δ13C values of soil-respired CO2 were observed between treatment and control soils during carbonate dissolution events, with δ13C values 10 ‰ higher in the treatment soils. These results indicate that the conceptual open system with fixed, respiration-controlled CO2 does not adequately describe the soil system during rapid wetting events. Instead, a numerical model in which gas-water equilibration occurs in a closed system with constant sum CO2 constraint at each time step more closely explains the experimental observations.