ALLUVIAL FAN RESPONSE TO STRENGTHENING OF LATE HOLOCENE ENSO VARIABILITY IN THE SONORAN DESERT, SOUTHWEST ARIZONA

The integration of soil stratigraphy, geomorphic mapping, and radiocarbon dating of alluvial deposits offer insight to the timing and paleoclimatic context of fan sedimentation near Yuma, Arizona. Of the total Quaternary alluvium identified in an area of 3400 km², approximately 10% was deposited during this late Holocene episode of aggradation, and is characterized by regionally continuous alluvial fan and plain cut-and-fill terraces. Fan terrace deposits with weakly developed gravelly soils and fluvial-type sedimentology yielded a date of 3200-2950 cal yr B.P. from charcoalized iron wood (Olneya tesota). Alluvial plain deposits with weakly developed buried sandy soils yielded a date of 2460-2300 cal yr B.P. from a terrestrial snail shell. To consider the role of climatic variability and larger scale circulation patterns as a driver of alluvial fan aggradation, precipitation records were analyzed to form historical analogs to late Holocene fan aggradation and to understand the frequency, magnitude, and source of precipitation events. The historical record indicates numerous above average precipitation correlated to the Southern Oscillation Index (SOI) in the region but lacks any significant alluvial fan response. Monsoonal precipitation accounted for ~35% of the annual total but lacked significant correlation to SOI. Extreme precipitation associated with dissipating tropical cyclones occurs during both El Nino and La Nina conditions. Of the 47 tropical cyclones to have occurred in the southwest U.S. between AD 1933 and 2008, 26 produced measurable precipitation with 23% occurring during strong El Nino, 12% during moderate El Nino, and 58% occurring during neither (-0.5<SOI<0.5). If the atmospheric circulation patterns during the late Holocene were similar to historical climate, it appears that a greater frequency of extreme storms of high intensity, short duration related to dissipating tropical cyclones or mesoscale convective systems would be required as the climatic drivers for the aggradation. The timing of aggradation from 3200 to 2300 cal yr B.P. correlates well with other paleoclimatic proxy records in the southwest U.S. and eastern Pacific region, which indicate an intensification of the El Niño-Southern Oscillation (ENSO) climatic pattern and rapid climate change during this period.