Climate Inferred from Geology and Archaeology

Early climatic interpretations for the Lisan and later formations (Late Pleistocene and Holocene—Neev and Emery, 1967, figs. 16, 17) were supported and updated by information from additional coreholes. Although most new and old coreholes bottomed at relatively shallow depths, 20 to 30 m, four of them reached greater depths, 74, 80, 161, and 285 m beneath the 1960 floor of the Dead Sea south basin. The sequences consist of alternating layers of marl and rocksalt. Most marls were deposited from dilute brine during high lake levels and contain alternating laminae of chemical deposits of white aragonite, gray gypsum, and fine-grained detritus consisting of yellowish, brown, green, or dark gray carbonates, quartz, and clay. The detrital fraction is coarser and more dominant toward the deltas, especially near Amazyahu escarpment in the south. Rocksalt layers indicate deposition from more concentrated brine when the levels dropped to about -400 m m.s.l. Lower elevations could have been reached when the sea continued to shrink and when the runoff-to-evaporation ratio diminished, bringing the south basin to complete dessication. As neither the geochemical nature (ionic ratios) of the brines nor the physiography of the terminal water body has changed at least since Late Pleistocene or Lisan Lake time (Katz, Kolodny, and Nissenbaum, 1977), it is probable that through the past 60,000 years rocksalt was precipitated only when the water surface was at or below the critical level of -400 m m.s.l. Gamma-ray logs for some of the new coreholes provide more objective and precise depths of marl and rocksalt layers than do actual samples of sediments. Content of radiogenic minerals in the rocksalt is negligible compared with that in the marl; thus, these layers identify changing physical environments and climates as well as correlating stratigraphy. On gamma-ray logs the peaks or highest intensities of positive anomalies indicate that marl layers or wet climatic subphases and their thicknesses on the logs are proportional to their duration. Presence of negative anomalies or very low levels of gamma radiation show both the existence and thickness of rocksalt layers that denote dry climatic phases.