PRELIMINARY PHYTOLITH RESULTS FROM ALLUVIAL ARROYO SEDIMENTS AT THE “CLIFF SITE”, MIXTECA ALTA HIGHLANDS, OAXACA, MEXICO

We report preliminary analyses of phytolith samples extracted from a Holocene stratigraphic section at the “Cliff Site” in a tributary arroyo to the Río Culebra in the semi-arid highlands of the Mixteca Alta in northern Oaxaca, Mexico. Radiocarbon dating on bulk sediment samples and charcoal from the stratigraphic section indicate deposition of sediment between 14,000 cal. yrs BP to the 16th century CE. The small tributary to the Cliff Site drains a watershed of approximately 8 km², which includes a large archaeological site (Inguiteria) that was surrounded by cultivated terraced fields during the late Holocene. As such, agricultural modification to the landscape and presence of cultivars are expected to influence the phytolith composition of the latest Holocene portion of the stratigraphic sequence. Phytolith records are valuable corroborates for other paleoenvironmental proxies that elicit data on past vegetation cover. Biases of differential preservation and production by plant taxa are considered both in the sampling methods and data analysis. We highlight two general trends in the data: 1) a greater proportion of woody species phytoliths over grass phytoliths (C₃ and C₄ species) across strata and 2) higher C₄ species over C₃ species proportions except where grass phytoliths dominate. Calculated aridity indices and proportions of phytoliths indicate regular oscillation in the proportions of warmer-adapted and cooler-adapted plant communities with a preponderance of semi-arid adapted species over much of the Holocene. This phytolith record is a paleoenvironmental proxy augmenting similar records from sediment in more diverse geomorphological settings. As a paleoenvironmental proxy it also contributes to current understanding of human land-use histories in a dynamic environmental setting spanning the Holocene epoch.