Pliocene-Pleistocene climate of the northern margin of Saharan-Arabian Desert recorded in speleothems from the Negev Desert, Israel

The Middle-Late Pliocene climate was 2-3°C warmer than today, but with similar levels of atmospheric CO2. This period reflects climate conditions expected in the near future, and is therefore an important target for current data-modeling studies. This study reconstructs the Pliocene-Quaternary evolution of arid conditions on the northern margin of the Saharan-Arabian desert, using radiometrically (U-Pb) dated periods of speleothem deposition from three caves of the Negev Desert, Israel. Speleothem growth started between ~3.75 and ~3Ma, at the end of the first significant tectonic uplift of the western shoulder of the Dead Sea Rift. Major speleothem deposition, indicating wet conditions, occurred during the Pliocene around ~3.1Ma, with subsequent aridity during the last 3Myr, punctuated by short wet episodes - the Pleistocene Negev Humid Periods (NHP). The oldest dated NHP occurred between ~1.7 and ~1.25Ma, and other short humid episodes continued intermittently later. Speleothem δ18O values (-6.9‰ to -11.2‰) show that the humid episodes were associated with periods of low global ice volume and warm temperatures. After correction for rainfall δ18O changes associated with ice volume and temperature effects, the observed relative constancy of speleothem δ18O values from Pliocene to Late Pleistocene indicates rainfall was from a common source, most probably the eastern Mediterranean Sea. The humid Pliocene conditions could be generated by more southerly position of the Mediterranean coast (allowing the access of Mediterranean precipitation to the Negev), as well as by warmer Atlantic Sea Surface Temperatures, that weakened the Azores High Pressure Cell. Maximum amounts of precipitation were 500-600mm/a during the Pliocene and >300mm/a during Pleistocene NHP. The Pliocene and the earliest Pleistocene NHP are associated with formation of lakes in the Negev. Low 87Sr/86Sr ratios of ~0.7078 in the Pliocene speleothems are indicative of low dust supply, low water residence time in the vadose zone and relatively high weathering rates of the cave host rock. Increase of 87Sr/86Sr ratios to 0.7082-0.7083 in the Pleistocene suggest an increased supply of desert dust, high water residence time in the vadose zone and reduced host rock weathering.