Browsing Radiocarbon, Volume 28 (1986) by Subjects
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13C Variation in Limestone on an Aquifer-Wide Scale and Its Effects on Groundwater 14C Dating ModelsIn modeling the initial 14C activity of ground waters, the delta-13C of marine limestone is taken conventionally to vary little about 0 per mil PDB. This variation was found to be 6.28 per mil in samples taken over intervals from 10^-2 to 1^5 meters in the Mooney Falls Member of the Redwall Limestone in northern Arizona. Such a variation will cause appreciable variability in the results of all four initial activity models tested. The variability, due primarily to a numerical instability in the models dependent on this parameter, can introduce significant uncertainty into groundwater "age" calculations.
14C Dating of Calcareous Tufa from Different Environments14C dates of carbonate and organic matter fractions are compared with a series of calcareous tufa samples from selected sites representing different geochemical environments and associated with different bedrocks. Results obtained in this study indicate values of apparent age ranging from 940 +/- 110 yr for calcareous tufas associated with Pleistocene sediments to 2000 +/- 110 yr and even ca 4000 yr for tufas from sites associated with Jurassic or Cretaceous limestone. It was found also that within each of the investigated sites the value of apparent age does not change significantly with the age of the tufa layer.