A PEDOGENIC RECORD OF ENVIRONMENTAL CHANGE IN THE UPPER SAN PEDRO RIVER BASIN

Quantifying the timing of soil diagnostic horizon development is key to understanding how Quaternary environmental change controls the distribution of soil-landscape patterns observed today. The objective of this study was to constrain the timing of subsurface soil property development on a fan remnant in southeastern Arizona, testing the hypothesis that observed argillic and calcic horizons formed asynchronously under glacial and interglacial climate conditions, respectively. The study site was located in the Upper San Pedro River basin near Tombstone, AZ on the mid-early Pleistocene alluvial deposit colloquially referred to as the Gleeson Road Conglomerate. We sampled a soil profile exposed by a road cut that contains a well expressed ~1 m thick argillic horizon that has a ~0.5 m thick Stage III calcic horizon embedded within it. The location of the calcic horizon within the argillic suggests the argillic horizon formed prior to any substantial carbonate accumulation. The depth profile was sampled for in situ ¹⁰Be concentrations to determine profile age and erosion rates, U/Th concentrations of bulk carbonates, individual carbonate nodules and cobble coatings, to constrain carbonate ages, and the secondary phyllosilicates were isolated and their oxygen and hydrogen isotope ratios measured to determine the soil-climate conditions under which they formed. The ¹⁰Be data indicated a most probable erosion rate of 0.89 cm ky^-1 (0.53-1.55 cm ky^-1), and most probable age of 140 ka (68-336 ka). Preliminary U/Th analyses of bulk soil carbonates suggest an age range of 20-100 ka, although with large error. Additional U/Th of individual carbonate nodules and cobble rinds, coupled with phyllosilicate isotope data, will provide constraint on the timing and environmental conditions under which the soil diagnostic horizons formed.