Paleoclimate Reconstruction Based on the Stable Carbon Isotopic Composition of Bone Collagen from Small Herbivores Recovered in Archaeological Assemblages

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With National Science Foundation support, Drs. Raymond Mauldin, Debajyoti Paul, and Robert Hard will conduct collaborative research focused on developing short-term, local measures of past ecological conditions using patterns of stable carbon isotopes in collagen extracted from small herbivores from archaeological sites. The two year project brings together staff from the Laboratory for Stable Isotope Geochemistry (LSG), graduate and undergraduate students from the Department of Geology at the University of Texas at San Antonio (UTSA), researchers from the Center for Archaeological Research (CAR), and graduate and undergraduate students from the Department of Anthropology at UTSA. Most paleoclimate reconstructions in archaeology operate at large temporal scales (hundreds of years) and unknown, poorly defined, or extremely large spatial scales. Human foragers, however, do not initiate adaptive shifts in response to changes that occur over centuries, but in response to short term events that commonly operate at a seasonal or yearly timeframes, and that cover kilometers rather than continents. While long-term, large scale climate changes will certainly influence which adaptive strategies are ultimately successful, it is at these smaller temporal and spatial scales that adaptive decisions are initiated. This research differs from most other archaeological paleoclimate investigations in that it targets small herbivores, such as jack rabbits and cottontails, which can provide data on climate at scales appropriate for exploring human adaptations. These herbivores consume a variety of plants that use one of two different photosynthetic pathways, termed C4 or C3, to extract carbon dioxide from their environment. Measurements of carbon isotope ratios in collagen extracted from herbivore bones can provide an estimate of the relative contribution of C4 or C3 plants in herbivore diet. As these two different plant pathways are favored by different environmental conditions, with C4 plants more common in warm, arid settings, and C3 plants more common in cooler, moister settings, the relative contribution of C4 or C3 plants in jack rabbit and cottontail bone collagen can provide data to reconstruct aspects of past vegetation and climate. Jack rabbits and cottontails have a number of distinct advantages in the development of appropriate spatial and temporal measures of past climates. They have short life spans, seldom exceeding 3 years in length. Lifetime range sizes are commonly less than 1 square kilometer. As such, the diets of these small herbivores reflect local, short-term patterns of vegetation. Collagen extracted from jack rabbit and cottontail bones, collected from well-dated archaeological contexts, can provide a record of vegetation, and shifts in vegetation, at short temporal and small spatial scales directly relevant to understanding human adaptations. While using archaeological samples obtained from three ecologically distinct areas of Texas, and focused on hunter-gather adaptations that span the last 4,000 years, this research has broad implications for investigations of human adaptation. Small herbivores are present in archaeological and paleontological sites throughout much of the world, and from a variety of temporal contexts. Therefore, the findings of this collaborative research between the LSG (Geology), Anthropology, and CAR at UTSA will be of interest to the broader scientific community, both nationally and globally.