CLIFF-TALUS CO-EVOLUTION UNDER LONG-TERM NE NEGEV HYPERARIDITY

Talus flatirons are ubiquitous in arid and semi-arid regions. Despite their steepness, talus slopes in hyperarid areas store sediment for long periods (~500 ka) and thus constitute a valuable sedimentary and environmental archive. Talus flatiron sequences are generally associated with mesas composed of horizontal erodible sedimentary rocks capped by resistant bedrock. Talus sequences indicate alternating phases of deposition and erosion leading to talus flatirons ringing mesas and cliffs. The oldest and youngest talus groups are farthest and nearest to the source cliff, respectively. The hypothesis that the systematic spatial distribution of talus generations is related to glacial-interglacial cycles and relatively high rates of cliff retreat during interglacial was first proposed in the Negev; here we test and challenge this proposal. Three groups of talus flatirons from the Negev, Israel, present gypsic-salic soils typical of hyperarid (<80 mm yr^-1) climate. Calcic soil horizons are completely absent, indicating long-term hyperaridity. ¹⁰Be concentration indicate middle Pleistocene deposition and abandonment ages: the abandonment of the older talus group occurred 497 +176/-114 ka. The middle talus group initial deposition was at 551 +80/-142 ka and its abandonment at 270 +17/-28 ka. These ages, combined with talus flatiron positions yield bedrock cliff retreat rates of 6-12 m/Ma and 200 m/Ma for the talus apex, respectively, in opposite directions. These results indicate that (a) talus formation and erosion cycles are unrelated to glacial-interglacial cycles, and (b) significant cliff retreat is not the primary cause for the parallel position of talus flatiron generations relative to cliffs.