Late Quaternary Climatic Geomorphology, Volcanism, and Geoarchaeology of Carrizo Wash, Little Colorado River Headwaters, USA

摘要

Isolating the climatic mechanisms driving Holocene geomorphic change and deciphering the role of landscape change in prehistoric cultural processes both require well-dated and relatively continuous alluvial chronologies. This study presents a centennial-scale, latest Pleistocene and Holocene chronology based on alluvial fan, floodplain, groundwater-discharge, and volcanic deposits for the Carrizo Wash watershed, a Little Colorado River headwater drainage on the southeastern Colorado Plateau. More than 200 radiocarbon dates provide chronometric control. The age of Zuni Salt Lake volcanic eruptions was re-evaluated using radiocarbon and luminescence dating. Two eruptive phases ~13.3 ka and ~11.8 ka suggest closely spaced, monogenetic events. These terminal Pleistocene ages for the eruptions are significantly younger and substantially more precise than previous argon method ages. Sediment exposed in modern arroyos is dominated by middle Holocene (~7.1–4.9 ka) alluvium in valley contexts, whereas piedmont alluvium dates primarily to the late Holocene (~4.3–2.7 ka). Extensive prehistoric channel entrenchment occurred ~4.9 and 0.8 ka. Localized incision occurred ~1.9 and 1.2 ka, and possibly ~7.5 and 2.7 ka. Extended drought typically preceded arroyo cutting, and entrenchment was associated with increased climate variability, major shifts in precipitation amount or seasonality, and reduced flooding. Accelerated valley and piedmont aggradation appears related to increased flooding and runoff associated with reduced vegetation cover during periods of low effective moisture resulting from enhanced North American Monsoon circulation and weak ENSO conditions. Conversely, slow or stalled deposition appears connected to reduced flooding and runoff fostered by denser vegetation during times of increased effective moisture caused by enhanced El Niños and increased winter precipitation. Ground-water discharge deposits at Cienega Amarilla indicate that spring discharge was greatest and water tables most elevated ~2.3–1.6 ka. Spring discharge appears to reflect variations in El Niño frequency and intensity and the resultant variations in winter precipitation. Study results suggest that predicted increased drought and enhanced or delayed monsoons associated with modern climate change could initiate accelerated erosion of upland areas and increased flooding in southern Colorado Plateau headwater tributaries. Archaeological implications include temporal biases associated with surface site distributions and changing viability of floodwater and water-table farming over time.