RELATIONSHIP BETWEEN C-13 AND O-18 FRACTIONATION AND CHANGES IN MAJOR ELEMENT COMPOSITION IN A RECENT CALCITE-DEPOSITING SPRING - MODEL OF CHEMICAL VARIATIONS WITH INORGANIC CACO3 PRECIPITATION

A theoretical model is derived in which isotopic fractionations can be calculated as a function of variations in dissolved carbonate species on CO2 degassing and calcite precipitation. This model is tested by application to a calcite-depositing spring system near Westerhof, Germany. In agreement with the model, 13C of the dissolved carbonate species changes systematically along the flow path. The difference in δ values between the upper and lower part of the stream is about 1‰. The 13C content of the precipitated calcite is different from that expected from the theoretical partitioning. The isotopic composition of the solid CaCO3 is similar to that of the dissolved carbonate, though in theory it should be isotopically heavier by about 2.4‰. The 18O composition of dissolved carbonate and H2O is constant along the stream. Calculated calcite-water temperatures differ by about +5°C from the observed temperatures demonstrating isotopic disequilibrium between the water and precipitated solid. This is attributed to kinetic effects during CaCO3 deposition from a highly supersaturated solution, in which precipitation is faster than equilibration with respect to isotopes. Plant populations in the water have virtually no influence on CO2 degassing, calcite saturation and isotopic fractionation. Measurements of PCO, SC and 13C within a diurnal cycle demonstrate that metabolic effects are below the detection limit in a system with a high supply-rate of dissolved carbonate species. The observed variations are due to differences in CO2 degassing and calcite precipitation, caused by continuously changing hydrodynamic conditions and carbonate nucleation rates. © 1979.