INVESTIGATING EVAPORATION AND SOIL WATER MOVEMENT BY MEASURING THE STABLE ISOTOPE COMPOSITION OF WATER IN VERTISOLS

In this study we trace water through Vertisols using stable oxygen and hydrogen isotope ratios. Common on floodplains, Vertisols are frequently cultivated and good modern analogs for paleosols typically used to reconstruct paleoclimates. We hypothesize that: 1) evaporation occurs deeper in Vertisols than other soils (typically 20-30 cm) due to drought-induced cracking and 2) soil water at the depth of soil carbonate formation (140 cm in the soil studied) has δ¹⁸O values similar to mean annual precipitation, as typically assumed for paleoVertisol carbonates. From September 2012 to July 2013, we collected samples from the top 140 cm of a Trinity River floodplain Vertisol on Richland Creek Wildlife Management Area in Texas. Water was extracted from soil samples for isotopic analysis via vacuum distillation and gravimetric water contents were measured. Measured soil water δ¹⁸O and δD values ranged from -2‰ to -8‰ and -20‰ to -50‰ respectively. All of the water samples measured thus far plot (δD versus δ¹⁸O) within the field defining a local meteoric water line (LMWL) for nearby Waco, TX. The soil water δ¹⁸O deviations (‰) from the Waco LMWL indicate that samples collected above/below 50 cm tend to lie on the right/left of the LMWL. Evaporation of water from the soil above 50 cm is consistent with this observation but is not required by the data. Isotopic compositions and water contents are more variable above 50 cm than they are below 50 cm, except in July when soil cracks were observed and 140 cm δ¹⁸O values ranged from -5‰ to -8‰. The soil water δ¹⁸O values are lower during the winter and higher during the summer. Observed δ¹⁸O variations might result from evaporation and/or mixing with precipitation and downward flowing water. Below 100 cm, soil water δ¹⁸O values converge to -6.6 ± 0.8 ‰ (1s, n=25), which is significantly lower (p < 0.0001) than the mean δ¹⁸O value of Waco precipitation (-3.7 ± 2.7 ‰, 1s, n=96). These more negative δ¹⁸O values at depth could result from preferential percolation of winter rain/large rain events due to the variable shrink-swell controlled permeability of these soils. If the measured range in isotope compositions does record preferential percolation, then paleoVertisols may record the range in isotopic composition of ancient precipitation.