THE OIS-4 GLACIAL – ALLUVIAL DICHOTOMY

Two primary sediment records of landscape response to climate change in southwestern North America are alluvial fan stratigraphy and glacial deposits. OIS-2 alpine glacier advances have exceeded and therefore obliterated the moraine record of OIS-4 advances throughout much of the Sierra Nevada and Rocky Mountains, although at higher latitudes in both hemispheres OIS-4 commonly exceeded OIS-2 ice volumes. Alluvial fans sourced from non-glaciated catchments in the southwest, however, reveal the opposite: it appears that the piedmont surface area comprises fan complexes of OIS-4 age that equals or exceeds the area covered by fan complexes OIS-6 age and exceeds the area of fans of all other ages. There are numerous implications of this dichotomy of the sedimentary record. (1) The recently developing OIS-4 chronology obliges reconsideration of the age of penultimate moraines and extensive piedmont alluvium that are broadly interpreted to correlate with OIS-6. (2) The new chronology also compels reassessment of landscape evolution models, soils development indices, and weathering rates calibrated against the incorrect OIS-6 ages. (3) If the southwest alluvial fan stratigraphy is dominated by an OIS-4 “pulse”, then soils chronosequence dating may provide a cost-effective means of establishing slip rates across expansive tectonically active regions. We demonstrate the significance of the OIS-4 alluvium pulse in the Mojave and Sonoran deserts using a mixture of boulder, surface pebble, and episodic-erosion-corrected depth profile TCN (36Cl and 10Be) ages combined with soils stratigraphy from basins with different climate regimes (currently spanning hyper-arid to semi-arid), elevations, and basin lithologies. Questions needing to be addressed include: What is the glaciological reason that the temperate alpine OIS-2 advances exceeded OIS-4 advances? What duration of time is represented by the OIS-4 alluvial “pulse” (TCN chronology currently lacks the precision needed to address this)? What mechanisms triggered such wholesale evacuation of sediment stored in highland catchments to generate such extensive alluvial basin fill during OIS-4? How can we meaningfully quantify the impact of the OIS-4 landscape response in southwestern North America and elsewhere?