Journal of Conference Abstracts

Petrography, Chemistry and Isotope Geochemistry of Speleothems as Paleoclimate Proxies: Soreq Cave, Israel

Miryam Bar-Matthews (matthews@mail.gsi.gov.il)¹, Avner Ayalon (ayalon@mail.gsi.gov.il)¹ & Aaron Kaufman²

¹ Geological Survey of Israel, 30, Malchei Israel St., Jerusalem 95501, Israel

² Weizman Institute of Science, Rehovot 76100, Israel

The stable isotope composition of accurately dated speleothems of the Soreq cave (Israel) has been shown to be a powerful tool for the determination of the rainfall amounts, average annual temperatures and the contribution of C3 and C4 type vegetation, in the Eastern Mediterranean region during the last 60 Ky. The petrography, trace element concentrations and strontium and uranium isotope ratios correspondingly determine the character of the infiltrating water, the origin of dust particles and the types of weathering reactions. Climatic conditions during most of the time interval between 60 and 17 Ky (the period equivalent to the last glacial) were relatively cold and dry, whereas warmer and wetter conditions prevailed from 17 Ky to the present. At 17 Ky, there was a major climatic change, with a sharp increase in annual rainfall and temperature and the occurrence of a deluge period between 8.5 and 7.0 Ky. During the colder and drier periods, large, detritus-free, preferentially oriented calcite crystals were deposited from slowly infliltrating water. As a result of the sharp change in the hydrological regime at 17 Ky, fast-migrating water started leaching the soil and carried detrital material into the cave, and calcite crystals became small and anhedral. Coinciding with these petrographic and isotopic changes, a sharp drop occurred in the concentrations of strontium, barium, and uranium, and the ⁸⁷Sr/⁸⁶Sr and (²³⁴U/²³⁸U)₀ ratios reached minimum values during the deluge period. The figure shows the Soreq cave speleothem ⁸⁷Sr/⁸⁶Sr values superimposed on a ¹⁸O vs. age plot and shows that the isotopic values of all the speleothems fall into two distinct groups. Group A consists of samples from 60-17 Ky with higher ⁸⁷Sr/⁸⁶Sr ratios, higher strontium concentrations and relatively high ¹⁸O values. Group B are samples from 17 Ky to present-day, with lower ⁸⁷Sr/⁸⁶Sr ratios and strontium concentrations and relatively low ¹⁸O values. The higher trace element concentrations and isotopic ratios during colder and drier periods reflect an increase in the contribution of salts derived from exogenic sources (sea spray and aeolian dust), and a reduced contribution of weathering of the host dolomites. The isotopic ratios of group B samples, which were deposited in a warm, wet interglacial climate, clearly fall closer to that of the host rock.

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