EVIDENCE FOR LARGE PALEOFLOODS DURING LATE OXYGEN ISOTOPE STAGE 3 FROM NAHAL NETAFIM CATCHMENT, NEGEV DESERT, ISRAEL

Sedimentological evidence for 27 large paleofloods were found in a cave in Nahal Netafim, a small catchment, in the hyperarid Southern Negev Desert. The sediments are fine-grained flood slackwater deposits which accumulated at elevations of 2.3-5.3 m above the sandy gravel channel bed. The elevation of these deposits represents the maximal stage of the flood. Eight sedimentary units were dated using the luminesence method, and their ages range from 34.3±1.4 to 27.1±1.0 ka. The Late Pleistocene elevation of the channel bed needed for the computations of peak discharges was determined using Fluvio-Pedogenic Layers (FPL)found near the cave. These red layers represent the base of the active layer in the channel. One FPL was found in the present channel at a depth of 1.1 m, dated to ~9 ka and the second within a low alluvial terrace (stratigraphically older) - 0.3 m above present channel bed. The upper FPL was correlated to FPL’s in the nearby Nahal Yael catchment, dated to 20-30 ka. This FPL represents the Late Pleistocene channel bed of Nahal Netafim. The calculated peak discharges for these paleofloods range from 240 to 600 m3/s. The peak discharge of 10 out of the 27 paleofloods in the cave is higher than 500 m3/s and deviates from the envelope curve of paleofloods for the Negev Desert. This indicates that flood magnitudes during the late Pleistocene were much higher than during the middle-late Holocene. The average frequency of large floods in Nahal Netafim is about 250 years compared to about 1,000 years in other similar catchments during the middle-late Holocene. The hydrological regime of the late Pleistocene is characterized by high frequency of very large floods compared to the middle-late Holocene to present. This is probably related to slightly different climatic conditions with higher rainfall intensities and/or duration, though still arid. Comparing our data to other paleohydrological and paleoclimatological data sources supports this hypothesis.