We analyzed carbonates and bulk organic matter from paleosols from the upper Miocene Ogallala Formation in the Texas Panhandle, U.S.A. Stable carbon and oxygen isotopes were measured on pedogenic carbonates, rivertine tufa, and reworked carbonate clasts in fluvial channels. Bulk organic matter samples were analyzed along depth gradients in paleosol profiles. Using a Monte Carlo uncertainty analysis, we interpret carbonate δ13C (range = -8.5 to -5.2 ‰ VPDB) and organic matter δ13C values (range = -25.9 to -24.2 ‰ VPDB) to be consistent with C4 biomass low in abundance and variability at the study site, but with large uncertainties that would be overlooked using simple linear mixing model approaches. All carbonate δ18O values showed a very narrow range (-5.4 to -4.0 ‰ VPDB), suggesting a common meteoric water source.