SOUTHWESTERN U.S. GEOMORPHIC ACTIVITY OVER TWO GLACIAL CYCLES (MIS 7-1) FROM GRAIN SIZE ANALYSIS OF EOLIAN SEDIMENTS FROM STONEMAN LAKE, AZ

The study of preserved geomorphic surfaces has permitted the development and refinement of models describing how landscapes respond to climate change. Unfortunately, preservation and age control of such surfaces deteriorates with time, so spatially widespread and temporally constrained evidence of pre-Last Glacial surface processes is sparse. The southwest has experienced a range of climatic conditions through the Quaternary and thus we do not yet understand the range of resultant landscape responses. Core STL14 from Stoneman Lake, Arizona, USA is a continuous sediment record containing numerous paleoenvironmental indicators of the last ~1.3 Ma, a unique archive for the aridlands of southwestern North America. Previous mineral and geochemical analyses of the core indicate significant amounts of atmospheric dust deposition, accumulations that contain signals from upwind. We use laser granulometry to examine the top ~13 meters of eolian record, an interval spanning MIS 7 – 1 (~230 ka – 0). End member (EM) modeling separates particle size distributions into sub-populations whose modes are influenced by the weathering and transport histories of their source material. Stratigraphic variation of EM volumes thus indicates shifts in sediment delivery. A total of five EMs were identified explaining ~99% of the data set’s variance; two are dust. EM-3 (13 µm mode; 28% of total clastics) was likely emitted >100 km away, increases after major climatic transitions (MIS 6/5 and 2/1), and correlates with regional changes occurring on alluvial fans and playas in the Southern Great Basin, Mojave, and Sonoran Deserts. Loessic EM-4 (38 µm mode; 19% of total clastics) is highest during glacials (MIS 6, 4, and 2) and probably originates from aggrading floodplains of the nearby Verde River located only 10’s of km away. Dust mass accumulation rate (the product of dry bulk density, sedimentation rate, and %dust) correlates with %EM-3. This suggests glacial-timescale dust fluxes in southwestern North America are broadly controlled by sediment availability, increasing when regional climate changes trigger widespread geomorphic responses.