DENUDATION, DEPOSITION, AND DUST: HOW LONG-TERM CLIMATE CHANGE DRIVES SEDIMENT TRANSPORT ON THE MARGIN OF THE COLORADO PLATEAU

The landscapes of the western United States continue to experience change resulting from human activity. Among these changes, enhanced aridity and fire intensity increase tree mortality and can therefore alter ecosystems. While connections between hydroclimate, ecology, and land surface processes are currently playing out in real-time, their long-term effects on the landscape are not as well understood. The unconformity-free sediments of Stoneman Lake, Arizona, USA, taken in core STL14, contain indicators of hydroclimate (sedimentary facies), paleoecology (pollen), fire history (charcoal), and sediment transport (mass accumulation rates) spanning multiple glacial cycles. Set just above the piñon-juniper–Ponderosa pine ecotone on the southwestern edge of the Colorado Plateau, this record, especially in the context of past transitions towards warmer and drier conditions, demonstrates significant landscape and ecosystem changes that present long-term challenges for similar settings across the region. For instance, this upland setting incurred a ~2.5 - 10x increase in sediment transport rates accompanied by dramatic changes in forest composition and fire regime following the last two glacial terminations. The STL record also allows us to distinguish and quantify accumulation of local sediment, far-traveled eolian dusts, and proximally-sourced loess using mineralogy, geochemistry, and grain size end member modeling. The accumulation rate of loess is enhanced during periods of alluvial and fluvial aggradation along the middle Verde River Valley, glacial MISs 2, 4, and 6. Comparing the timing of loess accumulation to that of erosion at Stoneman Lake illuminates the pace of geomorphic coupling between the Valley’s mainstem river and its upland hillslopes. This dataset supports the following model: hillslope sediment mantles develop during cooler and wetter glacials in association with extensive forest canopies. Following transitions toward warmer and dryer interglacials, rapid erosion of hillslope sediments coincides with increasing forest fires, forest migration, and changing precipitation modes. Sediment storage in intervening tributaries lasts until the next glacial episode when increasing discharge erodes and inundates the valley mainstem with this sediment.