Evaluating the reliability of Late Quaternary landform ages: Integrating 10Be cosmogenic surface exposure dating with U-series dating of pedogenic carbonate on alluvial and fluvial deposits, Sonoran desert, California

Abstract:

To assess the reliability of Quaternary age determinations of alluvial and fluvial deposits across the Sonoran Desert (Coachella Valley and Anza Borrego) in southern California, we applied both ¹⁰Be exposure age dating of surface clasts and U-series dating of pedogenic carbonate from subsurface clast-coatings to the same deposits. We consider agreement between dates from the two techniques to indicate reliable age estimates because each technique is subject to distinct assumptions and therefore their systematic uncertainties are largely independent. ¹⁰Be exposure dates should yield maximum ages when no correction is made for inheritance and post-depositional erosion is negligible. U-series dating, in contrast, provides minimum dates because pedogenic carbonate forms after deposition. Our results show that: (1) For deposits ca. 70 ka or younger, ¹⁰Be and U-series dates were generally concordant. We note, however, that in most cases U-series soil dates exceed ¹⁰Be exposure dates that are corrected for inheritance when using ¹⁰Be in modern alluvium. This suggests that ¹⁰Be concentrations of modern alluvium may exceed the ¹⁰Be acquired by late Pleistocene deposits during fluvial transport and hillslope residence (i.e., Pleistocene inherited ¹⁰Be). (2) For deposits older than ~70 ka, U-series dates are significantly younger than the ¹⁰Be dates. This implies that U-series dates in this region may significantly underestimate the depositional age of older alluvium, probably because of delayed onset of deposition, slow accumulation, or poor preservation of secondary carbonate in response to climatic controls. Thus, whenever possible, multiple dating methods should be applied to obtain reliable ages for late Quaternary deposits.