Large floods during late Oxygen Isotope Stage 3, southern Negev desert, Israel

Abstract

Slackwater deposits were found in a cave in the Nahal Netafim catchment (35 km2), near the head of the Gulf of Aqaba in the southern Negev, Israel. The sedimentological record includes 27 large paleofloods, dated by infrared stimulated luminescence to 33,000-29,000 years ago. The scatter of the ages and their large uncertainties prevented an assessment of the exact duration of the record and the specific timing of each flood. Bayesian analysis was used to adjust the dense dating results so that the time interval between the first and last flood deposit preserved in the cave could be estimated. Minimum peak discharges were reconstructed based on the estimated elevation of the Late Pleistocene channel bed as indicated by fluvio-pedogenic layers found near the cave. The average frequency of these large floods (200-600 m3 s-1) for the period between 33,000 and 29,000 years ago is about 1 flood per 150 years, while for the mid-late Holocene it is only 1 large flood per 1000 years. Eight floods out of the 27 recorded deviate from the envelope curves of mid-late Holocene paleofloods and measured floods in the hyperarid Negev desert, indicating a different hydroloclimatological regime. The anomalous large floods are hypothesized to have resulted from an increase in regional rainfall intensity and/or duration, attributed to increased frequency of the Red Sea Trough low-pressure system that affects the region. Available records indicate that the northern Negev and areas farther north in Israel controlled by Mediterranean pressure systems were wetter 40-20 ka BP. At the same time, the southern Negev, probably in response to the Red Sea Trough system, also experienced short episodes of more and/or larger rainstorms. The timing of these episodes in both the northern and the southern Negev towards the Last Glacial Maximum points to a potential synchronous strengthening of both the Mediterranean and Red Sea systems, currently acting at different seasons. These episodes of increased storminess in the area were brief and were not able to alter the general hyperarid conditions in this area.