Temperature and salinity effects on 18O fractionation for rapidly precipitated carbonates:: Laboratory experiments with alkaline lake water

The precipitation of lacustrine carbonates cart occur relatively rapidly over large ranges of salinity, alkalinity and temperature. Few studies have directly addressed oxygen fractionation of stable isotopes during the formation of these carbonates. We present results from experiments that elucidate the factors which regulate the isotopic composition of carbonates precipitated from lake water. Water from Mono Lake was diluted by different amounts with Creek water from the Mono Basin. Carbonates were precipitated in these solutions by adding CaCl2 solution or powder. The experimental results show that rapidly precipitated inorganic calcite, typical of some lacustrine environments, may record equilibrium O-18 fractionation as has been demonstrated previously for slowly inorganically growing, or biologically precipitated carbonates. However, apparent disequilibrium fractionation is observed in carbonates precipitated rapidly from waters of extremely high salinities and low temperatures. This appears of result from the formation of carbonate polymorphs, such as gaylussite. Rapidly precipitated pure calcite exhibits O-18 enrichments with respect to predicted equilibrium values. The temperature correlation of O-18 fractionation appears to hold at relatively high temperatures (>10 degrees C) but the slope of the temperature/fractionation equation is salinity dependent, although the cause is presently unclear.