TEMPERATURE AND SALINITY EFFECTS ON O-18 FRACTIONATION FOR RAPIDLY PRECIPITATED CARBONATES - LABORATORY EXPERIMENTS WITH ALKALINE LAKE WATER - PERSPECTIVE

An important factor involved in determining the isotopic composition o f precipitating lake minerals is their mineralogy. In the Dead Sea (Is rael) the aragonite-calcite ratio of lake carbonate sediments can be c orrelated with delta(13)C and delta(18)O fractionation increasing arag onite concentration conforms to an increase in the heavier carbon and oxygen isotopes. The enrichment of the heavier isotopes is explained b y strong evaporation during the formation of aragonite, when the light er isotopes are preferentially removed as part of Got. Calcite was for med as a result of degradation of gypsum through the activity of bacte ria which preferentially use lighter isotopes from their carbon source .In a hypersaline lake at Salt Flat, Texas (US), layers of isotopicall y-heavy and isotopically-light carbonate sediments alternate in beds s imilar to those of the Dead Sea. As in the Dead Sea, the light layers are composed for the most part of calcite which has resulted from the bacterial decomposition of gypsum. At Salt Flat the isotopically heavy carbonate is dolomite, and not aragonite as in the Dead Sea.