Controls on aggradation and incision in the NE Negev, Israel, since the middle Pleistocene

Abstract

We investigated the mid-Pleistocene to recent aggradation-incision pattern of two drainage systems (Nahal Peres and Nahal Tahmas) in the hyperarid north eastern Negev desert, southern Israel. Although these drainage systems drain into the tectonically active Dead Sea basin, lake level fluctuations cannot account for the aggradation-incision pattern as bedrock knickpoints disconnect the investigated parts of these drainage systems from base level influence. We applied geomorphic mapping, soil stratigraphy, optically stimulated luminescence (OSL) and cosmogenic (in situ 10Be) exposure dating to reconstruct cycles of aggradation and incision of alluvial terraces and to study their temporal association with regional periods of humidity and aridity and global glacial-interglacial cycles. The spatial and temporal relationships between the alluvial units suggest changes in the drainage system behavior since the middle Pleistocene, and show a pattern in which prolonged periods of sediment aggradation alternated with short periods of rapid and intense degradation through erosion and incision into sediment and bedrock. We obtain ages for several Pleistocene-Holocene periods of incision: ~1.1Ma, ~300ka, ~120ka, ~20ka, ~12ka and ~2ka. Although broadly synchronous, the Nahal Peres and Nahal Tahmas systems exhibit temporal differences in aggradation and incision.Hyperarid conditions have persisted in the region at least since the middle Pleistocene, as evidenced by gypsic-salic soils that ubiquitously cap the investigated alluvial terraces. This observation is consistent with other observations throughout the Negev indicating prolonged aridity. Thus, alternation between sediment aggradation and degradation cannot be correlated in a simple and straightforward way to climatic changes. We explain the temporal differences in aggradation and incision between Nahal Peres and Nahal Tahmas as resulting from the differences in stream gradient, basin hypsometry, and drainage basin area (which influences the amount of water passing through the system). Regardless of humidity source (tropical plumes, Red Sea trough or Mediterranean fronts) and the frequency and intensity of rainfall in the region, our study shows that: 1) unlike other climatic proxies, such as speleothems, the evolution of alluvial terraces, which involves many stages (sediment generation, transport, deposition, and subsequent incision), results in a significant time gap between the actual ages of terraces and the timing of external triggers such as climatic fluctuations, and 2) such time-gaps may vary, even within nearby drainage systems, due to differences in drainage systems specific characteristics, and thus result in asynchronicity (or at least poor synchronicity) of alluvial terrace deposition and incision.