Physics
Scientific paper
Aug 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007gecoa..71.3948c&link_type=abstract
Geochimica et Cosmochimica Acta, Volume 71, Issue 16, p. 3948-3957.
Physics
11
Scientific paper
The δ18O of ground water (-13.54 ± 0.05 ‰) and inorganically precipitated Holocene vein calcite (+14.56 ± 0.03 ‰) from Devils Hole cave #2 in southcentral Nevada yield an oxygen isotopic fractionation factor between calcite and water at 33.7 °C of 1.02849 ± 0.00013 (1000 ln αcalcite water = 28.09 ± 0.13). Using the commonly accepted value of ∂(αcalcite water)/∂T of -0.00020 K-1, this corresponds to a 1000 ln αcalcite water value at 25 °C of 29.80, which differs substantially from the current accepted value of 28.3. Use of previously published oxygen isotopic fractionation factors would yield a calcite precipitation temperature in Devils Hole that is 8 °C lower than the measured ground water temperature. Alternatively, previously published fractionation factors would yield a δ18O of water, from which the calcite precipitated, that is too negative by 1.5 ‰ using a temperature of 33.7 °C. Several lines of evidence indicate that the geochemical environment of Devils Hole has been remarkably constant for at least 10 ka. Accordingly, a re-evaluation of calcite water oxygen isotopic fractionation factor may be in order. Assuming the Devils Hole oxygen isotopic value of αcalcite water represents thermodynamic equilibrium, many marine carbonates are precipitated with a δ18O value that is too low, apparently due to a kinetic isotopic fractionation that preferentially enriches 16O in the solid carbonate over 18O, feigning oxygen isotopic equilibrium.
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