Assessing the reliability of U-series and 10Be dating techniques on alluvial fans in the Anza Borrego Desert, California

We combine 10Be exposure dating with U-series dating of pedogenic carbonate from subsurface clast-coatings to date deposition of mid-Holocene to late Pleistocene alluvial fans at four sites in the Anza Borrego desert of California. Since each dating technique is subject to distinct assumptions, their systematic uncertainties are largely independent. We therefore consider 10Be and U-series dates that are consistent to be highly reliable. If post-depositional erosion has not significantly modified a fan surface, exposure dates should yield maximum ages when no correction is made for inheritance. U-series dating, in contrast, provides minimum dates because pedogenic carbonate forms after fan deposition. Accordingly, agreement between dates from the two techniques (or lack thereof) provides a stringent test of their reliability. Our results show that: (1) concordant 10Be and U-series dates were measured for most fans, thereby providing robust age estimates, (2) U-series dating of pedogenic carbonate can be successfully applied to alluvium as young as mid-Holocene and (3) 10Be inheritance of the studied Holocene fan deposits appears to be significantly lower than implied by 10Be concentrations in active wash deposits.

► Combining 10Be and U-series dating techniques allow for robust ages of alluvial fan deposits.
► U-series dating of pedogenic carbonate can be successfully applied to mid-Holocene alluvium.
► 10Be measured in active washes appear to not represent inheritance of target alluvial fans.