Paleoclimate and location of the border between Mediterranean climate region and the Saharo–Arabian Desert as revealed by speleothems from the northern Negev Desert, Israel

Abstract

Speleothem bearing karstic caves of the northern Negev Desert, southern Israel, provides an ideal site for reconstructing the paleoclimate and paleo-location of the border between Mediterranean climate region and the Saharo–Arabian Desert. Major periods of speleothem deposition (representing humid periods) were determined by high resolution 230Th–U dating and corresponding studies of stable isotope composition were used to identify the source of rainfall during humid periods and the vegetation type. Major humid intervals occurred during glacials at 190–150 ka, 76–25 ka, 23–13 ka and interglacials at 200–190 ka, 137–123 ka and 84–77 ka. The dominant rainfall source was the Eastern Mediterranean Sea, with a possible small contribution from southern tropical sources during the interglacial periods. When the interglacial interval rainfall was of Eastern Mediterranean origin, the minimum annual rainfall was ∼ 300–350 mm; approximately twice than of the present-day. Lower minimum amounts of precipitation could have occurred during glacial periods, due to the cooler temperatures and reduced evaporation. Although during most of the humid periods the vegetation remained steppe with mixed C3 + C4 vegetation, Mediterranean C3 type steppe-forest vegetation invaded southward for short periods, and the climate in the northern Negev became closer to Mediterranean type than at present. The climate was similar to present, or even more arid, during intervals when speleothem deposition did not occur: 150–144 ka, 141–140 ka, 117–96 ka, 92–85 ka, 25–23 ka, and 13 ka–present-day. Precipitation increase occurred in the northern Negev during the interglacial monsoonal intensity maxima at 198 ka, 127 ka, 83 ka and glacial monsoonal maxima at 176 ka, 151 ka, 61 ka and 33 ka. However, during interglacial monsoonal maxima at 105 ka and 11 ka, the northern Negev was arid whereas during glacial monsoonal minima it was usually humid. This implies that there is not always synchroneity between monsoonal activity and humidity in the region. Oxygen isotopic values of the northern Negev speleothems are systematically lower than contemporaneous speleothems of central and northern Israel. This part is attributed to the increased rainout of the heavy isotopes by Rayleigh fractionation processes, possibly due to the farther distance from the Mediterranean coast.