Occurance and stable isotope compositions of soil carbonate and organic matter within a climatic transect of modern Vertisols along the coastal prairie of Texas

Stable carbon and oxygen isotopes from pedogenic carbonate and soil organic matter in a modem Vertisol preserve coherent isotopic records that reflect changes in climate and vegetation during pedogenesis. Three sites from Lake Charles series Vertisols, on the Coastal Prairie of Texas showed similar and systematic carbon isotope inflections with depth. These inflections suggest the following climate/ecosystem changes: base of the profiles record cooler conditions, warmer/drier conditions are recorded at mid-profile, and evidence for cooler/wetter conditions again is present at the top of the profiles which agrees with a historical increase in C₃ vegetation seen in Texas and Oklahoma. Although both soil organic matter and pedogenic carbonate have similar depth profiles, coexisting organic matter and carbonate are not contemporaneous. Pedogenic carbonates do not show the uppermost, negative shift in isotopic composition, most likely the result of insufficient time to crystallize carbonate with modern signatures. Stable carbon isotopes of soil organic matter and pedogenic carbonate appear to be sensitive to soil horizonation and microtopography. The coherent record of stable carbon isotopes preserved in these Vertisols indicate that these soils have not experienced significant "self-mulching" or whole sale pedoturbation. Instead, the systematic soil morphology and isotopic profiles suggest that Vertisols may preserve useful paleoclimate records.