How old are the Mojave topographic dunes? The implications of new luminescence dating analyses from the Cady Mountains, Mojave Desert, southwest USA

The aeolian landforms of the Mojave Desert in the SW USA have been studied in detail over the last three decades, particularly in terms of their relationship to the region's topography and Pleistocene climate / sediment supply histories, as well as wider developments, such as the aeolian “sediment state” concept. In this context, the evolution of the Mojave River and its associated palaeolakes is thought to have been a key control on long-term sediment supply to aeolian systems, and luminescence dating chronologies for a range of (resulting) aeolian landforms have been related to these hydrological changes. Here we argue that at least some of these aeolian chronologies need to be re-assessed.

We focus on luminescence chronologies for aeolian landforms within and marginal to the Cady Mountains, a mountain block adjacent to the Mojave River and palaeolake Manix, east of Barstow, California. We demonstrate that quartz in this locale exhibits several malign luminescence properties, and that low temperature K-feldspar infrared stimulated luminescence (IRSL) consistently exhibits high anomalous fading rates. Both contribute to age underestimation. We address these issues via post-infrared IRSL (pIRIR) and post-isothermal post-IR (pIt-IR) analyses of K-feldspars. The resulting ages span the last ~120 ka and imply phases of aeolian activity of a substantially greater antiquity than previously inferred. Notably, at one well-studied site – the Soldier Mountain sand ramp – the new ages suggest a landform dating not to Marine Isotope Stage (MIS) 1 or 2, as previously suggested, but more likely to MIS 5. The Cady Mountain record indicates that the only period of the last glacial cycle lacking evidence for aeolian sedimentation is ~40–9 ka, broadly consistent with expectations of increased regional humidity. These results also suggest that site topographic context may influence the age structure of aeolian deposits. In this instance, sand ramps consistently represent the oldest type of deposit (range ~ 40–120 ka), while early-mid Holocene dune accumulation is associated with sandsheets and valley-fill sands. Based on these findings, we argue that there is a need to critically re-assess the existing regional luminescence age database, and that there is potential to significantly revise our understanding of the region's aeolian system responses, and associated paleoenvironmental interpretations.