PALEOLIMNOLOGICAL EVIDENCE FOR EARLY FORMATIVE OCCUPATION OF THE RÍO VERDE VALLEY, OAXACA, MEXICO

Over the past decade, intensive archaeological and geomorphological studies of the Lower Río Verde, Oaxaca, Mexico, have documented dynamic landscape change that ended during the Terminal Formative. At some point prior to ~2000 years ago the previously meandering Río Verde River changed morphology to a braided stream pattern, possibly as the result of erosion in the highlands. The change in river morphology is thought to have resulted in the expansion of the agriculturally rich flood plain of the lower Río Verde; archaeological data indicates that settlement patterns were affected and the population increased significantly by the Late Formative (2350-2065 cal B.P.).

However, earlier occupation of the region is difficult to determine with traditional archaeological methods because of the significant alluviation. A palaeolimnological study was initiated in order to examine Holocene vegetation and land use changes in the region.

We present multiproxy paleolimnological data from two sites (Loma Reyes ‘D' and Charco Barro) located on the old paleochannel of the Río Verde. Analysis of microscopic and macroscopic charcoal concentrations indicate a complex history of burning, presumably for agriculture, starting in the late Early Formative. Peaks in magnetic susceptibility indicate increased sediment erosion. This combined data indicates land clearance for agriculture approximately one thousand years earlier than previously suspected from the archaeological data. Unfortunately, pollen was poorly preserved in the floodplain sites; it is hoped that pending analysis of the sediments for carbon isotopes and phytoliths will aid in paleoecological reconstruction of the local vegetation. The regional vegetation history is reconstructed from a 5m long sediment core taken from Laguna Pastoría, a nearby estuarine lagoon.