The Transport History of Alluvial Fan Sediment Inferred From Multiple Geochronometers

Abstract

We present a multi-chronometer approach to refine the age of an alluvial fan and to infer sediment transport and deposition history in the Anza Borrego Desert region of Southern California. We measure in situ produced cosmogenic carbon-14 (14C) from boulders on the fan surface and infrared stimulated luminescence (IRSL) ages from single feldspar grains within the alluvium. Our new IRSL age [5.3 ± 0.5 ka (±1σ)] is in excellent agreement with existing uranium-series [U-series; 5.3 ± 0.2 (±2σ)] ages of pedogenic carbonates. The IRSL and U-series ages show that in situ 14C measurements [6.6 ± 1.1 ka (±1σ)] from boulders contain inherited nuclides from prior exposure in the upstream catchment, much like measurements of the longer-lived nuclide, beryllium-10 (10Be). However, in situ 14C ages are closer to the preferred ages inferred from IRSL and U-series and with less scatter than comparative 10Be ages. Our data demonstrate that a multi-geochronometer approach will produce ages of alluvial fan surfaces with the greatest degree of confidence. We then apply the paired 14C and 10Be concentrations to infer the prior exposure and storage duration of the sampled boulders of 3.1 ± 3.2 and 4.6 ± 2.3 Kyr, respectively. A mixture model analysis of the single grain IRSL ages suggests bimodal storage durations prior to remobilization with peaks at ca. 2 and 10 Kyr. We demonstrate that cosmogenic nuclide inheritance and single grain IRSL equivalent dose distributions can provide additional information regarding sediment transport history prior to deposition on the alluvial fan.