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 % ). Colluvial and #uvial sediments from drainage systems of di!erent 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 % s. For most types of the sediments, the average D % is between 2 and 11 Gy, equivalent to residual ages of 1}5 ka. The lowest D % is usually in the range of 0.1}4 Gy, con"rming that the single-aliquot protocol can identify the best bleached grains. Newly weathered material from active channels gave the largest D % ’s of up to 30 Gy. Reworked #uvial sediments, derived from terraces and fans, gave progressively smaller D % ’s with increasing level of recycling. The degree of bleaching of colluvial sediments strongly depends on the face of the slope. The high D % ’s and the large scatter result from the mode of sediment transport. Brief rain episodes result in #ash #oods, 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 % values and enable better dating of sediments in hyperarid environments. ( 2000 Elsevier Science Ltd. All rights reserved.