Stable Isotope Compositions in Modern Vertisols: The Influence of Microtopography and Climate

Claudia I. Mora, Dana L. Miller and Steven G. Driese, Department of Geological Sciences, University of Tennessee, Knoxville, TN 37996

Stable carbon isotope profiles for soil organic matter (SOM) and pedogenic carbonate (PC) in modern Vertisols from the Lake Charles series of east coastal Texas record systematic inflections that can be interpreted as a coherent record of climate change over the past ~35000yrs. A depth-time relationship is inferred from previous Texas soil studies. The bases of the profiles have low δ¹³C values, typical of cool, Late Pleistocene C3-dominated ecosystems. At mid-profile, more

positive δ¹³C values suggest a greater proportion of C4 plants, interpreted to be a response to mid-Holocene warming. At the very top of the profiles, δ¹³C values are more negative, a trend seen in other studies of very young (100-1000yr) uncultivated Texas soils, suggesting a recent increase in C3. Microhigh and microlow soil environments express the same trends, but at higher levels in the soil microhigh profile. Although PC and SOM compositions covary, SOM preserves isotopic trends more coherently than PC. PC does not show the upper-most ecosystem shift, likely due to the time lag for carbonate formation. The preservation of systematic carbon isotope inflections is inconsistent with large-scale "self-mulching" of Vertisols and points to the utility of these soils in paleoclimate studies.