One of the most significant environmental processes that occurred at the transition from the last glacial phase into the present inter-glacial phase in arid regions was the shift from aggradation to incision in the drainage systems. This is evident by the sharp transition from a fluvial regime depositing fine-grained sediment within the wadis to intensive incision which formed gullies and narrow channels that dissected the late Pleistocene sediments. In order to investigate this transition, we studied three small-scale basins in the arid region of the Negev Highlands, southern Israel. Although the selected basins drain toward different base levels, their geomorphological parameters, particle size distribution of alluvial units and their OSL ages are similar. Sediments from the penultimate glacial cycle are found in patches in the bigger catchments. Fluvial loess was widely deposited since at least 67. ka until after 28. ka, covering valleys and slopes. Between ~ 28 and ~ 24. ka, loess was washed from the slopes into the channels, exposing the underlying colluvium. At ~ 24. ka erosion began with the transport of slope colluvium as gravels into the valleys that eroded the underlying loess sediments. Incision became dominant at ~. 12. ka and is still ongoing and intensifying. Dust and reworked loess continued to be deposited during the main incision stages. It is proposed that the transition from aggradation to incision was controlled by rates of loess supply and removal. Until ~ 24. ka dust choked the drainage system and only after reduction in dust supply was erosion and incision possible. It began first on the slopes and then in the channels. Our results show that an increase in precipitation is not a prerequisite for initiation of incision as is often assumed. Similar processes are described in other arid zones around the world.
Bibliographical noteFunding Information:
The research was funded by the Earth Science Research Administration of the Israel Ministry of Energy and Water Resources in Jerusalem , grant number 29-17-051 . We thank Y. Rephael, Y. Mizrachi and I. Swaed for assistance in fieldwork, Z. Dolgin for help in sample preparation, D. Stuber and O. Yoffe for chemical analyses and M. Ben-Israel and R. Krasilshikov for PSD analysis. We thank two anonymous reviewers for their insightful comments that improved this manuscript.
© 2015 Elsevier B.V.
- Loess sediments
- Negev Highlands
- OSL dating