Residual ages of modern sediments in an hyperarid region, Israel

Modern sediments from a Variety of depositional environments in the hyperar id southern Arava region, Israel, were analyzed for their residual equivale nt dose (D-e). Colluvial and fluvial sediments from drainage systems of dif ferent sizes and parent materials were collected and alkali feldspars were measured using the single-aliquot additive-dose protocol. All samples showe d a large scatter in the D(e)s. For most types of the sediments, the averag e D-e is between 2 and 11 Gy equivalent to residual ages of 1-5 ka. The low est D-e is usually in the range of 0.1-4 Gy, confirming that the single-ali quot protocol can identify the best bleached grains. Newly weathered materi al from active channels gave the largest D-e's of up to 30 Gy. Reworked flu vial sediments, derived from terraces and fans, gave progressively smaller D-e's with increasing level of recycling. The degree of bleaching of colluv ial 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 e pisodes result in Bash floods, when large volumes of sediment are transport ed 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 o r small-aliquot protocols can overcome the high and variable residual D-e v alues and enable better dating of sediments in hyperarid environments. (C) 2000 Elsevier Science Ltd. All rights reserved.