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Stratigraphic studies conducted on the alluvial fill of several ephemeral stream channels in the southern Arava Valley, Dead Sea Rift, indicate the existence of a calcic 'soil' developed 50-100 cm beneath the channel surfaces. This anomalous calcic 'soil' does not fall within the categories of any standard soil or pateosol definition and thus has been termed "Fluvial Pedogenic Unit" (FPU). The FPU occurs at the lower limit of the contemporary scour-and-fill processes in the aerated zone and it defines the maximum scour-and-fill during floods. The FPU is also found in late Pleistocene and Holocene terraces, termed paleo-FPU, indicating that the hydrological regime of past floods was similar to the present one.

The formation of a calcic FPU in this unique environmental setting is associated with the cumulative influence of persistent differences in water availability to various parts of the channel before and during flood events. The carbonate accumulation results from low recurrence of flood events, relatively limited moisture amounts and almost no involvement of vegetation in its formation. The isotopic analyses of calcic crusts in the FPU and in the paleo-FPU indicate that and climatic conditions prevailed during their formation. This indicates that the flood regime under and climatic conditions characterized most of the Negev Desert during the late Pleistocene and the Holocene. However, a clear difference in the delta(18)O and the delta(13)C values was detected between the active channels and the terraces. This difference suggests a change in the rainwater composition, rather than changes of temperature or amounts of rainfall. (c) 2006 Elsevier Ltd and INQUA. All rights reserved.