CONNECTING UPLAND EROSION, TRIBUTARY AND MAINSTEM FLUVIAL ACTIVITY, AND DUST EMISSION ON THE COLORADO PLATEAU’S SOUTHWESTERN EDGE DURING THE QUATERNARY PERIOD

Understanding how landscapes evolve is critical to predicting how land surfaces will respond to anthropogenic changes and requires comparing widespread and well-dated climatic, ecological, and geomorphological records. Here, we present such a record from Stoneman Lake (2050 masl), Arizona, USA, a small collapse feature in basalt on the southwestern edge of the Colorado Plateau. We accompany palynological and microscopic charcoal records from a sediment core with timeseries of catchment erosion and coarse dust deposition (both mass accumulation rates derived from end member modeling analysis of laser granulometric data) to generate a more comprehensive view on the timescales and drivers of sediment transport in the Middle Verde River watershed over the last 250 ka. Glacial-interglacial to millennial-scale climate changes affect effective moisture, temperature, vegetation, and forest fire regimes in the lake basin. The highest erosion rates correspond to early interglacial periods (MIS 5e and the Early Holocene) but remain elevated until late glacial conditions (MIS 2 and 6a) when erosion drastically decreases. Nearby (25 – 40-km-away) sources of the coarse dust are Quaternary-aged fluvial and alluvial deposits upwind and downslope in the Verde River mainstem (~1000 masl). Periods of sediment aggradation here typically occur during glacial periods (MIS 4 and 6) and correspond to increased coarse dust flux into Stoneman Lake. By comparing parallel records of upland critical zone processes and sediment delivery to the mainstem environment, we show the nature of connectivity across a steep topographic gradient in a semi-arid continental setting. Our results suggest that soils in the upland hillslope setting of this watershed develop during cool and wet late glacial climates characterized by low forest fire activity and spruce and fir forests. Upon glacial termination, these soils take <10³ years to erode, responding to decreased effective moisture, vegetation changeover to a montane pine forest, and increased fire activity. Because mainstem aggradation does not appear to occur during this time, sediments from these eroded soils likely accumulate in intervening tributary environments, only to be scoured tens of thousands of years later when glacial conditions return and fluvial discharge increases.