RECONSTRUCTING PALEOENVIRONMENTS OF SOUTHWESTERN NEW MEXICO ARCHAEOLOGICAL SITES

Archaeological localities in southwestern New Mexico were investigated to improve understanding of Holocene paleoenvironments in an area where the introduction of maize agriculture to the American southwest is thought to occur. Two trenches from near archeological sites were correlated using carbon isotopes of sedimentary organic carbon from buried soils. In addition, modern plant material was analyzed for comparison, as well as small soil carbonates and snails recovered from the trenches. Isotopic values analyzed from trench 1 range from -24.7 to -15.7 vs. VPDB, and from trench 2 from -26.2 to -16.3 vs. VPDB. This data suggests shifting mixes of C3 and C4 vegetation likely due to changes in moisture patterns. In this setting (Chihuahuan Desert), shifts towards lower d¹³C values (C3 plants), represent shifts toward drier conditions supporting woody shrubs, whereas C4 grasses represent wetter conditions. Values obtained from modern plant samples average -15.8 for grasses and -25.6 for woody shrubs; consistent with the range of values obtained for buried soils. Organic plant samples have also been analyzed to compare the current isotopic values to previous plant data.

Carbonate isotope data from trench 1 plotted in oxygen vs. carbon isotope space revealed typical meteoric calcite lines that average -7.8‰ vs VPDB for oxygen and ranged from -3.0 to -0.3 ‰ for carbon. Snails recovered from trench 2 were significantly heavier in O-isotope values and lighter in C-isotope values (d¹⁸O = -3.4 and d¹³C = -6.1 ‰). Using a modern mean annual temperature, meteoric water responsible for precipitation of the soil carbonate was -7.4 ‰ vs VSMOW which is slightly more enriched than current precipitation that is -8.4 per mil vs. SMOW. This suggests similar conditions to today (at least during the time represented by soil carbonate development).

While much work is still needed to constrain environmental changes, the data suggest fairly significant fluctuations in vegetation and likely moisture conditions represented by the buried soils. Conditions likely fluctuated between dry conditions similar to today to wetter conditions represented by grasslands.