Residual ages of modern sediments in an hyperarid region, Israel

Modern sediments from a variety of depositional environments in the hyperarid southern Arava region, Israel, were analyzed for their residual equivalent dose (D_e). 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 D_es. For most types of the sediments, the average D_e is between 2 and 11 Gy, equivalent to residual ages of 1-5 ka. The lowest D_e 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 D_e's of up to 30 Gy. Reworked fluvial sediments, derived from terraces and fans, gave progressively smaller D_e's with increasing level of recycling. The degree of bleaching of colluvial sediments strongly depends on the face of the slope. The high D_e'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 D_e values and enable better dating of sediments in hyperarid environments.