SINGLE PEBBLE COSMOGENIC 10BE ANALYSES REVEAL LONG-TERM EPISODIC DISCHARGE OF THE MOJAVE RIVER, SOUTHERN CALIFORNIA, USA

Pluvial lake stages in the Mojave Desert, and the rest of the southwestern USA, approximately coincided with glacial stages. Much of the evidence for this link comes from reconstructed shoreline and sedimentary records of pluvial lakes along the course of the Mojave River, which stretches ~200 km across the Mojave Desert from the San Bernardino Mountains to Silver Lake playa north of Baker, CA. It has been argued that the Mojave River must have had past periods of sustained high discharge in order to support Mojave Desert pluvial lakes. However, although there is an extensive body of research into Mojave Desert pluvial lake records as a proxy for discharge, no attempt has been made to more directly measure the timing of past Mojave River discharge.

We collected ten rounded to well rounded, equidimensional to somewhat elongated, quartzite pebbles from a desert pavement developed on a strath terrace of the Mojave River that is cut into and overlies pluvial Lake Manix deposits ~40 km downstream of Barstow, CA, and measured their individual concentrations of ¹⁰Be. These quartzite pebbles are primarily Precambrian or lower Cambrian in age and have been most recently recycled from late Miocene aged sedimentary units exposed exclusively in the San Bernardino Mountains to the southwest. New luminescence dating places the age of the strath between 25.4 ± 0.71 ka (quartz OSL) and 24.3 ± 1.09 ka (IRSL). This is similar to the 24.5 ka estimate of Lake Manix draining made from lake records. The individual pebbles have ¹⁰Be concentrations between 1.62 ± 0.08 and 17.81 ± 0.96 x10⁵ atoms/g, corresponding to modeled exposure ages between 26.41 ± 2.71 and 310.81 ± 34.40 ka at 1s. Moreover, a normal kernel density analysis of the individual pebble ages shows 3 distinct populations of pebbles at 30.71 ^+6.06/_-4.76 ka, 70.53 ^+8.65/_-7.79 ka, and 129.83 ^+19.48/_-23.37ka.

From these data, we infer that (1) An extended pulse, or set of pulses, of river transport indicates greater water availability in the San Bernardino Mountains, the source of ~90% of Mojave River discharge; (2) based on the difference between the youngest individual pebble age and the timing of Lake Manix draining, pebbles could travel the ~200 km from their source in one pulse, and; (3) the ages of these pulses occur before full glacial conditions as determined from various proxy records.