INVESTIGATING SOIL WATER MOVEMENT AND PEDOGENIC CARBONATES FORMATION BY MEASURING THE STABLE ISOTOPE COMPOSITION OF WATER IN VERTISOLS

In this study we trace water using stable oxygen and hydrogen isotope ratios in clay-rich soils (Vertisols) that are common on floodplains in central Texas and the uplands of the Blackland Prairie. Our objectives are twofold. First, a better understanding of water movement in these soils would help improve water management, irrigation operations and models of contaminant transfer from the surface to groundwater. Second, water movement in these soils influences the formation and oxygen isotope composition of pedogenic carbonate. Floodplain Vertisols are good modern analogs for many of the paleosols typically used to reconstruct past climates. Therefore improved understanding of modern Vertisols helps interpret the oxygen isotope composition of paleosol carbonate nodules. We hypothesize that: 1) evaporation occurs deeper in Vertisols than other soils and 2) that peodgenic carbonate records the oxygen isotope composition of mean annual precipitation. Previous natural abundance isotope tracer work indicates that evaporation is typically limited to the top 20cm in soils (Gazis 2004). Evaporation from clayey soil is expected to be different because clay rich soils crack when dry, exposing deeper parts of the soil to air. On September 17, 2012, we collected soil samples from the top meter of a Trinity River floodplain Vertisol southeast of Dallas, Texas. Soil water was extracted from soil samples by boiling under vacuum for isotopic analysis and gravimetric water content was measured for each soil sample. Our results show that water content generally decreases with depth in the top meter and that soil samples with the lowest water content have the lowest d¹⁸O and dD values (d¹⁸O = -6‰ at 100 cm). These trends in water content and isotopic composition are the opposite of the trends expected for evaporation of water. The d¹⁸O value of soil water at 100 cm is substantially lower than the d¹⁸O value of mean annual precipitation in Austin and similar to the d¹⁸O value of precipitation that falls during large events (d¹⁸O = -7‰ Pape, 2010). This suggests that large rainstorms may preferentially recharge the deeper portions of these slowly permeable (when wet) soils, but more monitoring is required for verification. If true, the d¹⁸O value of pedogenic carbonate in clay rich paleosols may be biased toward the d¹⁸O values of large events.