Charles T. Hallmark

The Buttermilk Creek Complex and the Origins of Clovis at the Debra L. Friedkin Site, Texas

A large artifact assemblage dating to 15,000 years ago lies beneath a Clovis assemblage in central Texas. Compelling archaeological evidence of an occupation older than Clovis (~12.8 to 13.1 thousand years ago) in North America is present at only a few sites, and the stone tool assemblages from these sites are small and varied. The Debra L. Friedkin site, Texas, contains an assemblage of 15,528 artifacts that define the Buttermilk Creek Complex, which stratigraphically underlies a Clovis assemblage and dates between ~13.2 and 15.5 thousand years ago. The Buttermilk Creek Complex confirms the emerging view that people occupied the Americas before Clovis and provides a large artifact assemblage to explore Clovis origins.

Quantifying pedogenic carbonate accumulations using stable carbon isotopes

Abstract Four pedons from a late Quaternary chronosequence developed in calcareous alluvium in central Texas were investigated to assess the ability of the stable C isotope method to partition and quantify pedogenic carbonate accumulations. To quantify pedogenic carbonate accumulations with this method, δ 13 C values of bulk, pedogenic, and parent carbonate must be known. For each pedon, δ 13 C values of bulk carbonate were measured on a horizon by horizon basis. The parent carbonate end-member for all pedons was approximated by averaging the bulk δ 13 C values for all horizons from the weakly developed floodplain soil. The diffusion model of Cerling (1984) and Quade et al. (1989) was used to estimate the pedogenic carbonate end-member. Quantification of pedogenic carbonate accumulations by the isotopic method was compared to quantitative estimates conducted by field morphology, binocular light microscope point counts, and thin-section point counts. Results suggest that the isotopic method is superior to other methods of quantifying pedogenic carbonate accumulations. Whole-soil pedogenic carbonate accumulations as calculated by the isotopic method for the chronosequence were: (1) 1 to 4 vol.% by 2000 years; (2) 4 to 15 vol.% by 5000 years; and (3) 1 to 12 vol.% after 15,000 years. These results reveal net soil carbonate loss through time for central Texas climates. This corroborates similar conclusions drawn for the same chronosequence using mass balance analysis to calculate the flux of carbonate. Net carbonate loss through time also indicates that these soils do not serve as long-term carbonate C sequesters.

Clay minerals of four soils formed in eolian and tephra materials in Iceland

Abstract Clay minerals in a Vitricryand and three Haplocryands derived from eolian and tephra materials in different parts of Iceland and in glacial till samples representative of underlying strata were studied by a combination of methods. The soils are primarily used as rangelands and are highly susceptible to wind erosion. Allophanes, imogolite and poorly-crystalline ferrihydrite are abundant (> 700 g/kg of the clay) throughout the soils, including A horizons. This clay mineralogy would give rise to stable, low-density aggregates that influence the susceptibility of the soils to wind erosion. It also affects chemical properties such as high phosphate retention of the soils, even of the Cryands with relatively low clay contents (160–275 g/kg). Imogolite was found in some but not all horizons of all soils. No particular relationship of imogolite to measured Si/Al molar ratios of the oxalate-oxalic acid soluble fractions of the clays (0.51 to 1.13) was evident. Layer silicate minerals and laminar opaline silicas are either not present or occur in small quantities. Plagioclase, augite and/or their weathering residues were found in the clay and silt separates. Dark-colored glass is predominant in the fine sand throughout the soils and the tills. One till sample had a clay mineral composition similar to the eolian-andic soils, while the other contained smectite and possibly a spherical halloysite-smectite intermediate, but not allophane and imogolite, as major weathering products. The clay mineral composition of these soils and tills was compared with those derived from basic tephras in other regions.