CLIMATE AND SEASONALITY EFFECTS ON DUNE MOBILITY, SOUTHERN COLORADO PLATEAU TRIBAL LANDS, SOUTHWESTERN USA

An extensive mantle of active, partly active and relict sand dunes on the southern Colorado Plateau exhibit diverse forms, ages, and states of mobility. Sediment supply responses from multiple sources are strongly dependent upon seasonality and landscape traits, even while a warming climate is associated with new dune formation and mobility. Declining precipitation, a shift from snowfall to rainfall and increased potential evapotranspiration contribute to reduced surface water availability, resulting in increased sediment supply and dune formation and growth downwind of riparian areas. Moreover, precipitation that occurs during winter storms and the late summer monsoon season is out of phase with periods of high wind energy (typically occurring in spring). Therefore, within the current climate regime, the season most prone to dune mobility (early spring) occurs before annual plant germination, leading to inherently high sediment transport rates. Once dunes are brought to a highly active state via the absence of stabilizing vegetation cover, a return to average climatic regimes would be insufficient to reverse dune activity. Falling dunes preserved in canyons perpendicular to sediment transport all suggest that dune activity correlates with aridity, including well-resolved formation of falling dunes during MIS 5c (92.3 ± 7.4 ka), more broadly correlated dunes preserved from early MIS 3 (57- 51 ka), as well as mid-Holocene dune ages. Holocene dune ages fall within distinct periods, and also suggest that dune activity occurred in response to Colorado Plateau climatic fluctuations. Recent dune mobility further suggests that as yet unresolved paleo-dune stabilization lag times that could last for centuries are highly likely after drought-incurred periods of mobility, or that a change to cool, calm or wet conditions is required. Further examination of Holocene dunes may assist in resolving the question of conditions required for dune stabilization (in sand sheets) after dune mobilization during periods of aridity (preserved in falling dunes). Multiple lines of evidence suggest that interpretation of dune activity must account for seasonality, vegetation response and landscape history to unravel whether climate or sediment supply characteristics dominate in promoting dune mobility.