Research Article DOI: 10.2110/jsr.2010.018 PEDOGENIC CARBONATES IN TEXAS: STABLE-ISOTOPE DISTRIBUTIONS AND THEIR IMPLICATIONS FOR RECONSTRUCTING REGION-WIDE PALEOENVIRONMENTS

ABSTRACT: Pedogenic carbonates are ubiquitous in paleosols through geologic history, and have been widely used to speculate on paleoclimatic conditions during their formation. They are widespread and well developed in large areas of Texas. A majority are of late Quaternary age and have formed across a range of climatic conditions on various types of substrates. To evaluate the usefulness of pedogenic carbonates as environmental indicators, this study investigates regional-scale trends in stable-isotope compositions of pedogenic carbonates across approximately 900 km of Texas. The d13C values of 49 pedogenic carbonate profiles (182 analyses) on various host strata (e.g., limestone, igneous rocks), range from 210.6 % to +0.7 % V-PDB. Most d13C values are evenly distributed between 29 % and 21%, and the d18O values range from 29.1 % to +0.2 % V-PDB, mainly between 26 % and 22%. There is no correlation between carbon and oxygen isotopic values (R2 = 0.0002). Pedogenic carbonates in the Texas Gulf Coastal Plains, Southern High Plains, and west Texas exhibit higher d13C values than those in south and central Texas, which corresponds to the spatial change in vegetation between C4-dominated and C3-dominated plants. The marked increase in d13C values of the pedogenic carbonates from the Tertiary strata into the overlying Quaternary strata confirms the invasion of a C4 biomass into the Southern High Plains since the early Pliocene. From east to west, the d18O values of late Quaternary pedogenic carbonates decrease gradually, a trend that mimics the spatial variation of d18O values of modern meteoric water primarily due to the increased distance from the Gulf of Mexico (source of most of th