A NEW INSIGHT INTO PEDOGENIC PROCESSES IN AN EXTREMELY ARID ENVIRONMENT AND ITS PALEOCLIMATIC IMPLICATIONS - THE NEGEV DESERT, ISRAEL

It is well known that wet phases occurred in the Negev desert during the Pleistocene and the Holocene. The question is whether these phases can be detected in the soils and paleosols. Alluvial fan chronosequences in the extremely arid areas of the Negev desert (< 80 mm rainfall) are characterized by salic and gypsic Reg soils. These chronosequences are of Pleistocene and Holocene age and range between 4 ka and 100 ka. Although the soils that developed on stable surfaces are cumulative and polygenetic, none of them show calcic horizons. The only environment in this extremely arid area in which Quaternary calcic soils were detected is streambeds of ephemeral flash flood channels. In these channels calcic soils were developed under the active alluvial fill. Stratigraphic surveys conducted in the alluvial fill of several such channels in the southern Arava Valley, Dead Sea Rift, indicate the existence of an identical and highly distinctive fluvio pedogenic unit (FPU) 50-100 cm beneath the surface. The FPU has some initial attributes of a soil such as horizonation, calcium carbonate deposition, iron oxides release and accumulation of fine-grained material. The FPU forms at the lower limit of the contemporary scour and fill processes under wet conditions but not under intensive leaching. It is tied to the cumulative influence of persistent differences in water availability to various parts of the channel before and during flood events, which form a moisture regime equivalent to about 350 mm annual rainfall. This moisture regime is conducive to the formation of a calcic soil. The existence of the FPU in active channels and in terraces in this extremely arid area shows that when there is enough moisture for calcic soil to develop, such a soil will form. The lack of calcic soils and the widespread presence of Holocene and Pleistocene salic-gypsic soils emphasize that hyper arid conditions prevailed during the last 100 ka in most of the Negev desert. This conclusion is also supported by the isotopic composition of carbon and oxygen of the calcic nodules in the paleo and recent FPU’s (d ¹³C ranges from –2‰ to +3.5‰ and d ¹⁸O from –8‰ to +6‰). There was no indication for humid climatic conditions in the FPU’s and the Reg soils.