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Pedogenic Carbonates in Texas: Stable-Isotope Distributions and Their Implications for Reconstructing Region-Wide Paleoenvironments

¹ Department of Earth and Atmospheric Sciences, University of Houston, Houston, Texas 77204-5007, U.S.A.; txcaliche{at}gmail.com
² Department of Earth and Atmospheric Sciences, University of Houston, Houston, Texas 77204-5007, U.S.A.

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 ¹³C values of 49 pedogenic carbonate profiles (182 analyses) on various host strata (e.g., limestone, igneous rocks), range from –10.6 to +0.7 V-PDB. Most ¹³C values are evenly distributed between –9 and –1, and the ¹⁸O values range from –9.1 to +0.2 V-PDB, mainly between –6 and –2. 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 ¹³C 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 ¹³C 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 ¹⁸O values of late Quaternary pedogenic carbonates decrease gradually, a trend that mimics the spatial variation of ¹⁸O values of modern meteoric water primarily due to the increased distance from the Gulf of Mexico (source of most of the meteoric water).

In contrast to the regional trends, the stable-isotope values of some pedogenic carbonates developed on marine limestone and calcareous alluvium in central Texas and south Texas have isotopic signatures inherited from the host strata. In addition, some of the ¹³C values of pedogenic carbonate nodules in the Gulf Coastal Plains have been affected by saline groundwater. Soil carbonates formed in the shallow parts of the profile, such as petrocalcic horizons, may have also been influenced by the atmospheric CO₂. Nevertheless, climatic trends are readily apparent in the isotopic data, thus demonstrating their potential value in region-wide paleoclimatic analyses.