Dating calcic soils under marginal climatic or lithologic conditions using a soil developmental index: An example from the Stockton Plateau, Terrell County, Texas
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A mature pedogenic carbonate horizon (Stage IV) has developed in a fine-grained soil above Pliocene-Miocene alluvial gravels derived from Cretaceous limestone of the Stockton Plateau, and Paleozoic limestone, chert, and novaculite from the Marathon uplift. Other calcic soil horizons have been age-dated in the southwestern U.S. using a soil developmental index. However, this soil is located in an area classified by the Soil Conservation Service as having marginal conditions for pedogenic carbonate accumulation at the present. These marginal conditions are attributed either to climatic conditions that are too moist or to the presence of non-pedogenic carbonate accumulations in soil. We examined the carbonate question by applying soil morphologic, electron microprobe, and stable isotopic analyses to assess the carbonate origin of the soil horizon. The soil morphology indicates that the calcic soil in question has developed an average 109 cm thick K horizon below surface depths of 68-80 cm. In gravelly areas of the soil a 15-20 cm thick laminar Km horizon caps the K horizon. Microprobe analyses on portions of the Km horizon show that CaCO3 is the dominant material present and displaced the clays forming laminar and pisolith features. The carbonate chemistry of the Km horizon is dominated by CaCO3 (98 percent by weight) with minor amounts of MgCO3 (0.7 parts per million). Bulk isotopic analyses were run on the K horizon and local Cretaceous (Segovia) limestone. 18O differences between the K and Km horizons (-3.0 to -1.7) and local limestone (-6.1 to -4.9) indicate the K and Km horizon carbonates are the result of soil processes. Transects across Km coated limestone clasts show an 18O shift of 2-4 across the soil-rock interface. After ruling out atmospheric mixing and isotopic inheritance from limestone as factors influencing isotopic composition, the 13C results (-3.4 to 1.4) are interpreted as reflecting carbonate precipitation in a C4 vegetative environment. These results imply pedogenesis is responsible for the carbonate in the soil. Thus, a calcic soil development index can be applied to obtain an age-date. The index analysis yielded an average amount of secondary carbonate of 92 g/cm2 for the K horizon in the soil resulting in an average soil development age of 18020 Kyr. These results correlate well with other Stage IV K horizons in west Texas and southern New Mexico.
