Formation of lacustrine dolomite in the late Miocene marginal lakes of the east mediterranean (Northern Israel)

The study focuses on the formation of lacustrine dolomite in late Miocene lakes, located at the East Mediterranean margins (Northern Israel). These lakes deposited the sediments of the Bira (Tortonian) and Gesher (Messinian) formations that comprise sequences of dolostone and limestone. Dolostones are bedded, consist of small-sized (<7 mu m), Ca-rich (52 to 56 mol %) crystals with relatively low ordering degrees, and present evidence for replacement of CaCO3 components. Limestones are comprised of a wackestone to mudstone matrix, freshwater macrofossils and intraclasts (mainly in the Bira Formation). Sodium concentrations and isotope compositions differ between limestones and dolostones: Na = similar to 100 to 150 ppm; similar to 1000 to 2000 ppm; delta O-18 = -3 center dot 8 to -1 center dot 6 parts per thousand; -2 center dot 0 to +4 center dot 3 parts per thousand; delta C-13 = -9 center dot 0 to -3 center dot 4 parts per thousand; -7 center dot 8 to 0 parts per thousand (VPDB), respectively. These results indicate a climate-related sedimentation during the Tortonian and early Messinian. Wet conditions and positive freshwater inflow into the carbonate lake led to calcite precipitation due to intense phytoplankton blooms (limestone formation). Dry conditions and enhanced evaporation led to precipitation of evaporitic CaCO3 in a terminal lake, which caused an increased Mg/Ca ratio in the residual waters and penecontemporaneous dolomitization (dolostone formation). The alternating lithofacies pattern reveals eleven short-term wet-dry climate-cycles during the Tortonian and early Messinian. A shift in the environmental conditions under which dolomite formed is indicated by a temporal decrease in delta O-18 of dolostones and Na content of dolomite crystals. These variations point to decreasing evaporation degrees and/or an increased mixing with meteoric waters towards the late Messinian. A temporal decrease in delta C-13 of dolostones and limestones and appearance of microbial structures in close association with dolomite suggest that microbial activity had an important role in allowing dolomite formation during the Messinian. Microbial mediation was apparently the main process that enabled local growth of dolomite under wet conditions during the latest Messinian.