| The modern Malpaso Valley, located in the northerwestern frontier of Mesoamerica, is a desertified landscape characterized by erosion, arroyo incision, and invasion of farmland by arid-adapted thorn-scrub. Modern farmers achieve successful harvests only one year out of five. Yet once, the valley was a locus for large-scale prehispanic settlement that persisted for at least 400 years. By AD 500, the Mesoamerican site of La Quemada became the principal center for a network of over 200 contemporaneous villages. Around AD 900 the valley was abandoned, however, and no major settlements were established in the region until it became a focus for Spanish silver mining in the 16 th century.;Phytolith analysis and geomorphological study of 18 backhoe trenches and alluvial exposures in the Malpaso Valley's floodplain allow me to evaluate the occurrence of climatic change and the effects of human land use over the last 4,000 years. The results demonstrate that the roots of today's desertification are less than 500 years old. A semi-arid climatic regime (similar to that of today) took effect ∼ 425--73 BC but did not contribute to ancient or modern desertification. This trend may have even encouraged sedentism, as the first evidence for agriculture occurs in the alluvial record at this time. No drought episode correlates with the valley's abandonment, indicating that archaeologists' current explanations for cultural collapse must be reevaluated. Although prehispanic peoples modified the landscape for at least 1,400 years, their practices did not negatively impact the environment. Rather, European mining and farming, coupled with forest clearance in the 16th century, initiated a trajectory of degradation still visible today.;Archaeologists have speculated over the timing and causes for the valley's arid state. Current research suggests that frontier farming and sedentism lagged behind that of the Mesoamerican core by almost 2,000 years. Pedro Armillas hypothesized over forty years ago that an increase in regional rainfall to levels suitable for farming triggered the frontier's settlement. This shift later reversed and drought provoked regional abandonment by AD 1000. Other researchers posit that detrimental human impact produced abandonment. |
