ISOTOPIC RECORD OF HOLOCENE PALEOCLIMATE FROM PALEOSOLS IN NOCHIXTLÁN VALLEY, OAXACA, MEXICO

Mesoamerica has a long record of human-environment interaction. In the Nochixtlán Valley, Oaxaca, Mexico, six episodes of erosion have been identified between 4,220 BC and the present. Within the Rio Verde drainage highland erosion produced lowland deposition, causing geomorphic and ecological changes (1). The two earliest periods of erosion are likely the result of climatic changes, four subsequent episodes correspond to demographic and land use changes identified in the archeological record (2). Correlation of erosional episodes to changes in demography and land use suggests an anthropogenic origin. Few regional climate records exist for Oaxaca; those that do exist are lowland records. Paleoenvironmental records throughout the Holocene, pre- and post-dating human influence are necessary to build a climate record to evaluate human-environment interaction.

Here we present isotopic data from 100+ paleosols in cutbank exposures of the Rio Verde’s tributaries within the Nochixtlán Valley. More than 40 ¹⁴C dates from soil organic matter in paleosols yield dates 10,500 BC to the present and restrict periods of soil formation to ≥ 500 years. Preliminary temperatures calculated from carbonate clumped isotope thermometry range from 19˚C to 28˚C. Initial results for δ¹³C of pedogenic carbonates indicate soils were dominated by C₄ plants or contained a mix of C₃ and C₄ vegetation. δ¹⁸O soil water data range from ‑9.4‰ to ‑3.9‰ (SMOW), calculated using soil temperatures from mass-47 enrichments of CO₂ (Δ₄₇), which fall within values of meteoric waters in the region today. Soil temperatures calculated from Δ₄₇ values of modern pedogenic carbonates are within the range of regional maximum annual temperatures for the Nochixtlán Valley and correspond to temperatures during months with low precipitation and peak potential evapotranspiration. Modern δ¹⁸O and Δ₄₇ data suggest pedogenic carbonates from buried soils record conditions in the Nochixtlán Valley during the warmest, driest part of the year.

These initial data demonstrate the potential for paleosol isotopic records to provide a context to intensification of agriculture and potential anthropogenic environmental effects in Oaxaca.

1. A. M. Goman, A. A. Joyce, R. G. Mueller, Quaternary Res 63, 250 (2005).

2. A. A. Joyce, R. G. Mueller, World Archaeol 29, 75 (Jun, 1997).