Petrography,strontium,barium and uranium concentrations,and strontium and uranium isotope ratios in speleothems as palaeoclimatic proxies: Soreq Cave,Israel

Abstract

The reconstruction of the palaeoclimate of the eastern Mediterranean region for the last 60 ka BP is based on theδ¹⁸O andδ¹³C variations of speleothems from Soreq Cave, Israel. Climatic conditions during most of the time interval between 60 and 17 ka BP (the period equivalent to the last glacial) were relatively cold and dry, while they were warmer and wetter from 17 ka BP to the present. At ~17 ka BP, there was a major climatic change with a sharp increase in annual rainfall and temperature and a very wet period occurring between 8.5 and 7.0 ka BP. During the colder and drier period, large, detritus-free, preferentially oriented calcite crystals were deposited from slow-moving water. As a result of a sharp change in the hydrological regime at ~17 ka BP, fast-moving water started entrainment of the soil and carrying detrital material into the cave, and the calcite crystals deposited became small and anhedral. Coinciding with the petrographic and isotopic changes, a sharp drop occurred in the concentrations of strontium, barium and uranium, and in the ratios ⁸⁷Sr/⁸⁶Sr and (²³⁴U/²³⁸U)₀, which reached minimum values during the wettest period. This drop reflects enhanced weathering of the soil dolomite host rock. During colder and drier periods, higher trace-element concentrations and higher isotopic ratios reflect an increase in the contribution of salts derived from exo genic sources (sea spray and aeolian dust), and a reduced contribution of weathering from the host dolo mites.