Use of 39 Ar and 14 C for Groundwater Dating

• Published in: Radiocarbon
• Main Authors: Loosli, Heinz Hugo, Oeschger, Hans
• Format: Article in Journal/Newspaper
• Language: English
• Published: Cambridge University Press (CUP) 1980
• Subjects: General Earth and Planetary Sciences; Archeology; Greenland
• Online Access: http://dx.doi.org/10.1017/s0033822200010250; https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0033822200010250

Cosmic-ray produced atmospheric 39 Ar activity (T 1/2 = 269 yr) has been determined at 0.11 ±0.012 dpm/lt argon. Ice samples from two profiles in Greenland bore holes showed conclusively that 39 Ar dating leads to correct ages. Corrections can be made for possible contamination of the samples with ambient air during field extraction and during laboratory processing by measuring 85 Kr in the same samples. The following isotopes: 14 C, 39 Ar, 85 Kr, 3 H, partly 32 Si, 13 C, and 18 O were investigated in 20 groundwater samples. Unexpectedly large discrepancies between “ 14 C ages” and “ 39 Ar ages” were observed for many of these samples. For example, a horizontal profile of a confined sandstone aquifer in the Franconian Albvorland showed decreasing 39 Ar and 14 C activities from respectively, 100 percent to 17 percent and 80 percent to 0.3 percent of modern activity, corresponding to elapsed time periods of 700 and >20,000 years, respectively. It seems unlikely that gas exchange through the aquiclude is the cause of this discrepancy. It can neither be explained by only assuming that the water represents a mixing of components with different ages. We detected the possibility of underground production of 39 Ar in thermal spring water from Zurzach, Switzerland. Its argon showed higher specific activity than atmospheric argon. Elsewhere, however, samples were found containing no detectable 39 Ar activity: two wells of a confined carstic aquifer in Ingolstadt, Germany, show less than 7 percent atmospheric activity. In our opinion, the assumptions on which the 14 C method in hydrology are based need to be critically re-examined. It is possible that for some aquifers carbonate exchange between solid and liquid phases in the aquifer changes 14 C results to a larger degree than generally assumed.