Soil-Landscape Evolution and Paleoenvironmental Significance of Aeolian and Aeolian-Fluival Interaction Records in the Mohawk Valley, Southwestern Arizona

Abstract

The response of dune systems to long term climatic change in the Sonoran Desert remains poorly understood, limiting our ability to forecast future dune field expansion and accurately reconstruct past environments from composite aeolian records. The dune fields of the Mohawk Valley in southwestern Arizona preserve thick soil stratigraphic archives that can be used to track aeolian landscape changes well into the Pleistocene. The interaction of ephemeral drainages with sand dunes has further resulted in the formation of dune dammed water bodies that retain local records of flood frequency, runoff intensity, and aeolian-fluvial interaction dynamics. The association of Indigenous archaeological sites with many of these playas suggest they played an important role in precontact land use of the interior desert. In order to better understand long term aeolian landscape evolution and the paleoenvironmental significance of dune dammed playa records in the Mohawk Valley this dissertation focused on 1) using trace element geochemistry and mineralogical analysis to determine the origin of aeolian sediments in the valley, 2) using luminescence (SAR & TT-OSL, n=55) and radiocarbon (n=12) geochronometers to place age constraints on aeolian accretion/stability cycles, 3) reconstructing the composition of stabilizing dune vegetation from the δ 13C values of pedogenic carbonate in buried dune soils, and 4) characterizing and numerically dating stratigraphic successions in dune dammed water bodies and adjacent dune settings. This research revealed that most dune sands show strong geochemical similarities with local valley floor alluvium ultimately derived from granitic bedrock sources in valley-margin mountain catchments. Luminescence dating constrained Pleistocene dune activity to ~120, 36-28, 18.5-17, and 14.5-12.6 ka (ka=kilo-annum=1,000 yrs). The intervening periods of dune stability are marked by the formation of dust-enriched calcareous soils. Crescentic and linear dune activity in the Holocene was dated to 9.9, 8.2-8.1, 2.6-2.1, 0.6-0.5, 0.21-0.15, and <0.08 ka. The δ 13C values of carbonate nodules from buried Pleistocene dune soils indicate carbonate formation under low soil respiration rates (0.2-1.0 mmol/m2/hr) and dune stabilizing plant communities dominated by C4 grasses (as high as 70% between 35-32 ka). SAR-OSL and 14C ages from dune dammed playa deposits revealed enhanced periods of floodwater capture between ~13.6-12.4, 12.3-8.2, 6.4-5.0, 4.5-2.0, and 0.8-0.5 ka. Less frequent and lower intensity flooding marked the middle Holocene from 8.2-6.9 ka. The results of this study indicate aeolian activity over the last 120,000 years was associated with a diverse range of paleoclimatic conditions, from the warmer/drier than modern climatic regime of the last full interglacial to the wetter/cooler conditions of the latest Pleistocene. This finding suggests sediment supply and climatic factors both contributed to complex aeolian landscape responses in the late Quaternary. Prolonged periods of dune stability and soil formation in the Pleistocene occurred during wetter/cooler than modern climates across late MIS5 (MIS=Marine Isotope Stage) to MIS3 and late MIS3/early MIS2 through the LGM (Last Glacial Maximum). The predominance of C4 grasses on stabilized Pleistocene dune surfaces was likely a product of low atmospheric CO2 levels and/or the continued presence of summer monsoonal precipitation during soil formation. Dune dammed playa catchments provided important records of local runoff frequency and intensity; however, paleoenvironmental interpretations from these deposits must be considered within the context of both paleoclimatic change and aeolian-fluvial interaction processes. Future episodes of enhanced alluvial sediment supply following periods of flooding and/or increased anthropogenic landscape disturbances could result in the partial or complete reactivation of the Mohawk Valley dune fields in the 21st century.