TRACKING QUATERNARY ECOSYSTEM SHIFTS IN A MODERN VERTISOL CLIMOSEQUENCE, TEXAS COASTAL PRAIRIE

Recent studies of pedogenic carbonate and soil organic matter (SOM) in a modern Vertisol climosequence in the Texas Coastal Prairie document a shift from a C3- to a C4- dominated ecosystem over the past 35,000 years. We investigated a Vertisol profile near Victoria, Texas, in the drier portion of the climosequence (90-100cm/yr). Samples were collected in ~3m vertical profiles in the high and low positions of the vertisol microtopography. In the microhigh, d¹³C values of SOM increase monotonically from -24.1‰ PDB at depth to -14.7 at the top. d¹³C values of pedogenic carbonate decrease from -5.6 at depth to -7.23 at 1.5m and then increase to -6.8 at top. In the microlow, d¹³C of carbonate decrease from -5.3 at depth to -7.5 at the top with a 2‰ positive excursion at ~1m; organic matter values are under investigation. The d¹³C of SOM record a mixed (~60% C3) ecosystem in the Late Pleistocene changing to nearly pure C4 by mid-Holocene and suggest a significantly greater C4 floral component in the drier, compared to the wetter, portions of the climosequence. Coexisting carbonate and SOM are not in isotopic equilibrium except deep in the profile. By comparison, isotopic fractionations in wetter portions of the climosequence all approach equilibrium.