ENVIRONMENTAL CONTROLS OF SPELEOTHEM MINERALOGY IN A KARSTIC DOLOMITIC TERRAIN (SOREQ CAVE, ISRAEL)

Pleistocene speleothems of the Soreq Cave, Beth Shemesh, Isreal, situated within karstic Cretaceous dolomitic rocks, consist of two major mineralogical types: (1) monomineralic, low-magnesium calcite (LMC; 0.5 and 7.5 mol% MgCO3) straw stalactites, stalagmites, helictites, and pool deposits; (2) mineralogically heterogeneous deposits in the form of conical stalactites, flowstones and cave corals produced from thin-flowing solution films or sprays, in which LMC alternates with high-magnesium calcite (HMC; 13 to 33 mol% MgCO3), dolomite (33-50 mol% MgCO3) and LMC laminations. A unique feature of the type (2) is the continuous approximately 0-50 mol% range of MgCO3. SEM and delta-O-18 studies show that all calcites and dolomites grew by direct precipitation from local groundwater solutions. LMC formation represents equilibrium precipitation from dripping-water solution with Mg/Ca < approximately 2, whereas HMC and dolomite precipitation occurred from solution films enriched in Mg as the result of prior closed system (with respect to Mg/Ca) crystallization of LMC. delta-C-13 compositions of LMC, HMC, and dolomites are in the range - 12 to -8 parts per thousand and agree with model calculations. Mineralogical features such as the oscillations between LMC and Mg-enriched composite zones may indicate environmental alternations in the past. Aragonites occur as late infillings or as overgrowths on present day or paleosurfaces. Their isotopic compositions, distinctly different from other carbonates, may indicate evaporation (in the case of aragonite on a present day surface) or factors that enriched C-13 (in paleo deposits). Overgrown parts of weathered speleothems are depleted in O-18 with respect to the original material and may represent periods of climatic change, such as glacial half-arrow-pointing-both-left-and-right interglacial transitions. SEM mineralogical and petrographic studies of of speleothems may thus provide important indications of paleoclimatic change.