Modern sediments from a variety of depositional environments in the hyperarid southern Arava region, Israel, were analyzed for their residual equivalent dose (De). Colluvial and fluvial sediments from drainage systems of different sizes and parent materials were collected and alkali feldspars were measured using the single-aliquot additive-dose protocol. All samples showed a large scatter in the Des. For most types of the sediments, the average De is between 2 and 11 Gy, equivalent to residual ages of 1-5 ka. The lowest De is usually in the range of 0.1-4 Gy, confirming that the single-aliquot protocol can identify the best bleached grains. Newly weathered material from active channels gave the largest De's of up to 30 Gy. Reworked fluvial sediments, derived from terraces and fans, gave progressively smaller De's with increasing level of recycling. The degree of bleaching of colluvial sediments strongly depends on the face of the slope. The high De's and the large scatter result from the mode of sediment transport. Brief rain episodes result in flash floods, where large volumes of sediment are transported over short distances and only a small part of the sediment is exposed at any time to the sun. Only repeated cycles of transportation can homogenize the sediment and fully expose all its particles to the sun. Single-grain or small-aliquot protocols can overcome the high and variable residual De values and enable better dating of sediments in hyperarid environments.
Bibliographical noteFunding Information:
This research was supported by the Israel Science Foundation funded by the Israel Academy of Science and Humanities. We would like to thank R. Madmon for field assistance and S. Zeffran for sample preparation. An earlier version of this manuscript benefited from comments by an anonymous reviewer.