Reconstructing Paleoenvironments of Southwestern New Mexico Archaeological Sites 1. LOPEZ, Adrianne, 1. SUAREZ, Marina B., 2. FREDERICK, Charles and 3. HARD, Robert, 1. Department of Geological Sciences, The University of Texas at San Antonio, 2. Department of Geography and the Environment, The University of Texas at Austin, 3. Department of Archaeology and Anthropology, The University of Texas at San Antonio

Background & Purpose Boot-Heel New Mexico on border of Chihuahua. Artifacts discovered ranging from 4000BCE – 1200 AD. Is this the area of introduction of maize agriculture? Two trenches were dug, soil samples taken throughout. Carbonate nodules and snails collected. Samples of modern vegetation were also collected.

Background & Purpose Looking for variation in d13C to see changes in vegetation Looking for variation in d18O to see changes in moisture availability and temperature 1cm

d13C vs. d18O d13C is influenced by the decay of plant matter and CO2 diffusion in to the soil. C3 plants usually range from -33 to -23 ‰ vs. VPBD C4 plants usually range from -16 to -9 ‰ vs. VPBD d18O of carbonates are influenced by the temperature during carbonate production and the d18O of meteoric water. Also influenced by the amount of rainfall, seasonal changes, and evaporation of water in soils. Shells precipitated will record the d18O of the surrounding water.

Methods Soil samples are crushed and dried out. Samples are decarbonated with 3M HCl Decarbonated samples are dried in an oven and weighed out in tin capsules Organic samples are also dried out, crushed, and weighed out Samples are analyzed with a Thermo Finnigan Delta+ XP Isotope Ratio Mass Spectrometer via Costech EA to obtain d13C data. Data is corrected to the VPDB scale.

Methods Carbonate samples are drilled out with micro-drill Snail samples are crushed Samples are weighed in to vials and flushed with helium Samples are reacted with 100% phosphoric acid at 70C to release CO2 CO2 was transferred to a Thermo Finnigan Delta+ XP Isotope Ratio Mass Spectrometer via a Thermo Gasbench II. Data is corrected to the VPDB scale.

Results from Trench 1 d13C for Trench 1 range from -24.7 to -15.7‰ vs. VPDB.

Results for Trench 2 d13C values for Trench 2 range from -26.2 -16.3‰ vs. VPDB.

Results for Carbonate & Snail d13C avg: -3.4 to -0.3 ‰ vs VPDB Snail fragments were recovered from trench 2. d18O avg = -7.8 vs. VPDB Avg. -3.4 ‰ d18O and -6.09 ‰ for d13C.

Interpretations Trench 1 Trench 2 d13C data from both trenches shows fluctuations between C3 and C4 dominant environments. Lighter d13C values are characteristic of an environment dominated by desert shrub type plants, characteristic of dry conditions. Modern Vegetation averaged -15.8‰ vs. VPDB for grasses, and -25.6‰ vs. VPDB for woody shrubs. C3&C4 C3 *Increasing values indicate a shift towards mixed vegetation and wetter conditions

Interpretations Carbonate Data The d18O of meteoric water for the formation of the soil carbonates was -7.4‰ vs. VSMOW. Modern precipitation values average -8.4 ‰. Ancient d18O of meteoric water is 1 per mil heavier than modern The d13C carb data reflect values typical of mixed C3-C4 vegetation. Snail d18O values were much heavier, perhaps indicating evaporatively enriched ponds.

Conclusions Organic carbon data suggest a fluctuating environment from C3 dominant to mixed C3 and C4. Organic rich and drab color muds in the base of both trenches could reflect pond or shallow lake conditions. The lightest d13C org values may be characteristic of aquatic plants and/or algae. Enriched d18O values for the snail are also in support of enriched pond or shallow lake conditions. Environment would have been capable of supporting maize agriculture because wetter conditions than today likely occurred in the past.

References and Acknowedgments Leng, M., 2006, Isotopes in Palaeoenvironmental Research, Vol. 10. Ch. 1 Isotopes in Water; Ch. 4 Isotopes in Lake Sediments: AA Dordrecht, The Netherlands. Koch, P. L., (1998) Isotopic Reconstruction of Past Continental Environments. Annual Review, Earth Planet, Science, 573-613. Sharp, Z., 2007,Principals of Stable Isotope Geochemistry, 1st ed.: Upper Saddle River , New Jersey, Pearson Education Inc. Skeleton, P., Spicer, R. A., Kelly, S. P., Gilmore, I., 2003, The Cretaceous World,: Trumpington Street, Cambridge, UK. The Open University. Darling, W. G., Bath, A. H., Gibson, J. J., Rosanski, K., 2006, Isotopes in Paleoenvironmental Research; Ch. 1 Isotopes in Water; AA Dordrecht, The Netherlands, Springer. Funding for travel was provided by the Grants for Research Advancement and Transformation program in the Office of the Vice President for Research at UTSA, awarded to Dr. Robert Hard and Dr. Marina Suarez. Adrianne Lopez had funding provided by The Minority Science and Engineering Improvement Program MSEIP (Grant Award no. P120A150110) Assistance in the lab was provided by Ruby Patterson and Chris Ray.