6 Surficial geology and neotectonics in the West: Celebrating and advancing the legacy of Kirk Bryan through field-based geomorphology studies (Posters)

This session invites and examines recent field-based geomorphology studies in the West. Contributions regarding soils geomorphology, neotectonics, pediments, fluvial terraces, and landscape development are invited.

Monday, 18 May 2026
8:00 AM - 5:30 PM
Alvarado D/E

Constraining Meteoric 10Be Deposition Rates in Arid Soils from an Alluvial Fan Chronosequence Near Yuma, Arizona, USA

Paper number: 6-2
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

The utility of meteoric 10Be (10Bemet) as tool to quantify rates and process of soil development has global applications in diverse environments. We assume closed-system behavior of 10Bemet in this study to constrain long-term (e.g., 103-105 years) 10Bemet surface deposition rates in southwest AZ and expand the potential use of 10Bemet in the southwest US. We characterized four pedons developed on Pleistocene-Holocene alluvial fan deposits and sampled bulk soil materials to extract 10Bemet inventories on a soil-horizon basis and establish a local calibration site. Additional samples were collected from several locations within adjacent active washes to help establish background inventories of 10Bemet. Soils are Typic Haplocalcids and Gypsic Haplogypsids that exhibit surface V (Av) horizons of variable thickness and structural development formed below desert pavements. Parent materials consist of gravelly volcaniclastic sediments derived from local exposures of Oligo-Miocene volcanic rocks. Pedons express progressive thickening and development of a Bw-Bky-BCky horizon sequence that reflects relative soil age. We used standard acid leaching and column extraction techniques to isolate meteoric Be for analysis of samples via accelerator mass spectrometry. We determine total 10Bemet inventories by converting measured 10Be/9Be ratios to horizon-based inventories and summing those through each soil profile. We use a reference depth of comparison that is common for each pedon as well as the total depth of exposed soil to compare summed inventories against independent soil ages. Independent soil ages for each pedon are 3-5 ka (CRF-01), 36 ka (CRF-02), 115 ka (CRF-04), and 234 ka (CRF-05), based on optically stimulated luminescence and surface exposure depth-profile dating techniques. These ages provide a five-point calibration when the modern alluvium is also considered. When compared to the 10Bemet inventories, we expect a local long-term deposition rate of 1-2 x 106 at/cm2/yr based on deposition rates from available sources but also note the potential for significant variations due to a variety of local factors that will be explored.

Geological Society of America Abstracts with Programs. Vol. 58, No. 4

doi: 10.1130/abs/2026RM-410588

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