Paper No. 3
Presentation Time: 8:45 AM
Pedogenesis In Response to Quaternary-Scale Aridification of the Negev Desert, Israel
Although wetter Quaternary episodes were suggested for the Negev desert, we demonstrate that Reg soils sensitive to changes in precipitation indicate that in contrast with the northern Negev, the southern Negev has been hyperarid throughout the Quaternary. Tectonically stable, and well-preserved Pliocene alluvial surfaces of the Paran watershed (S Negev) allowed continuous development of cumulative/welded surficial soils. In-situ desert pavement and alluvial parent material 10Be and 36Cl concentrations constrained the ages of surfce formations/abandonment. The desert pavement chert clasts are 1.5-1.8 Ma with a maximum possible extremely slow erosion rates of 2-5 cm/Ma. The complex soil profiles developed after ~1.9 Ma, initially with moderate calcic soil horizon at ~1 m depth and later with only the arid to hyperarid gypsic and gypsic-saline horizons. Regionally, the gypsic-saline soil developed since >300 ka (OSL ages) and is associated with massive dust accumulation. This pedogenesis indicates transition from Plio-Pleistocene semi-arid conditions to early Pleistocene arid and extremely arid conditions since at least the middle Pleistocene. At the same time stage V-IV calcic soils formed in the northern Negev. This indicates a steep climatic gradient and that the proposed wetter Quaternary episodes were restricted to the northern Negev, (currently mildly arid, 100-250 mm/yr). This steep gradient in the Negev paleoclimate and the permanency of the southern Negev Quaternary hyperaridity is explained by the combination of the factors controlling regional rainfall: the southward-decreasing depth of the atmospheric boundary layer with distance from the Mediterranean, the altitude of the central Negev Highland (1000m)that probably reached its final altitude during the early Pleistocene, and the location of the southeastern Mediterranean shoreline. Their interaction prevents or allows the passage of rain into the southern Negev. We demonstrate that climatically, the Negev desert is not a simple extension of the Sahara, which the Negev is frequently related to.