A paleoclimatic record for a southern Great Plains locality (the Aubrey Clovis site in north-central Texas) has been established using stable carbon and oxygen isotopes. Detailed composite stratigraphic sections, constrained by 14 C ages, place the age of these deposits between 14,200 and 1600 yr B.P. Calcium carbonate samples of lacustrine and pedogenic origin were analyzed. Oxygen isotopic compositions of most of these in situ carbonates reflect equilibrium precipitation from local meteoric waters. Oxygen isotope values reflect changes in the composition of meteoric waters tied to changes in the isotopic composition of moisture derived from the Gulf of Mexico. Oxygen isotopic variability at the Aubrey site is coincident with marine isotopic records from the gulf that vary due to changes in Laurentide ice sheet volume and meltwater influx. The stable carbon isotopic record, reflecting changing biomass through time, corroborates humid versus arid interpretations based on sedimentology and rates of alluviation. A middle Holocene arid period was in contrast to moist early and late Holocene climate, affirming interpretations of other workers studying southern Great Plains Holocene climate history.

Rates of fluvial sedimentation exhibit spatial and temporal variability that is important with respect to the study of prehistoric archaeological sites. Sedimentation rates within fluvial basins vary in response to internal dynamics, geomorphic controls, and external factors including climate and tectonics. Fluvial rates of sedimentation may be estimated using sedimentary, pedogenic, biogenic, and radiometric evidence. Holocene rates of sedimentation vary by three orders of magnitude, as shown by radiometric data from numerous localities in North America. Sedimentation rates define rates of matrix accumulation in archaeological sites. These act as controls on site construction and site modification. Artifact densities, spatial patterning, and preservation are all conditioned by rates of matrix accumulation. These dimensions of prehistoric sites are critical to the evaluation of variability within and between archaeological sites and to the study of past settlement/subsistence systems.

Waters and Stafford (Reports, 23 February 2007, p. 1122) provided useful information about the age of some Clovis sites but have not definitively established the temporal span of this cultural complex in the Americas. Only a continuing program of radiometric dating and careful stratigraphic correlations can address the lingering ambiguity about the emergence and spread of Clovis culture.