THE RISE AND FALL OF LATE PLEISTOCENE PALEOLAKES IN SOUTHERN JORDAN

Several Late Pleistocene paleolakes have been reported in southern Jordan and used to interpret past climate. They include both large lakes such as Paleolake Hasa, Paleolake Jafr, and those in Mudawwara and Umari, as well as smaller lakes such as Jurf ed Darawish. In addition, fine-grained sediments at Jebel Hamra Feidan in the Jordan Valley have been interpreted as Lake Lisan deposits, even though they occur at elevations well above known highstands for the paleolake. These purported lakes occupied a variety of settings ranging from broad and shallow closed depositional basins (e.g. Jafr, Mudawwara) to localities where basin closure does not exist today, including the deeply incised stream channels at Wadi Hasa. At Wadi Hasa, previous researchers invoked mechanisms such as side-canyon damming or movement along tectonic faults to allow for lake formation and demise. Over the last eight years we have studied paleo-hydrologic deposits throughout southern Jordan, including those in Wadi Hasa, Jebel Hamra Feidan, and Al Jafr Basin. At all locations, excluding the coquina deposits at Mudawwara, Umari, and Azraq, geologic evidence indicates that the depositional environments were wetlands or springs – environments fed by ground-water discharge – and not lakes. This interpretation is based on the sedimentology of the deposits, their spatial distribution, lateral facies assemblatges, the presence of archaeological materials, root voids, and other trace fossils, as well as invertebrate fossil assemblages. The interpretation of these hydrologic deposits as palustrine instead of lacustrine has significant climatic and paleoenvironmental implications. Ground-water discharge deposits form where water tables approach or breach the ground surface. They form during wetter periods in arid lands and can be used to infer past climate conditions. However, wetlands are not as sensitive to evaporation as large, shallow lakes because recharging waters are shielded from the atmosphere during transport through the aquifer. Thus, wetlands do not require the extreme increases in precipitation needed to sustain lake systems. The interpretation of many paleohydrologic deposits in southern Jordan as wetlands has important implications for paleogeography and the search for human occupation in these areas during wetter times.