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Field Degassing as a New Sampling Method for 14C Analyses in Old Groundwater

  • Reika Yokochi (a1), Ryan Bernier (a1), Roland Purtschert (a2), Jake C. Zappala (a3) (a4), Yoseph Yechieli (a5) (a6), Eilon Adar (a6), Wei Jiang (a3) (a7), Zheng-Tian Lu (a3) (a4) (a7), Peter Mueller (a3), Gerard Olack (a1) and Roi Ram (a6)...
Abstract
Abstract

Radiocarbon (14C) activity in groundwater can be used to determine subsurface residence time up to ∼40 kyr, providing crucial information on dynamic properties of groundwater and on paleoclimate. However, commonly applied sampling methods for dissolved inorganic carbon (DIC-14C) are prone to low level of modern atmospheric contamination, resulting in underestimation of groundwater ages that cluster around 30–40 kyr. We extract CO2 gas from groundwater using a device originally developed for studies of noble gas radionuclides. Carbon is collected in the gas phase, eliminating the possibility of fostering microbial activities and aqueous chemical reactions during sample storage. This method collects CO2-14C and radiokrypton (81Kr and 85Kr) samples simultaneously. The presence of any shorter-lived 85Kr is used to evaluate the degree of atmospheric contamination during sampling or mixing of young groundwater. Most groundwater samples showed lower CO2-14C activities than those of DIC-14C, presumably due to the absence of atmospheric contamination. Samples with 81Kr age exceeding 150 kyr have no detectable CO2-14C except where mixing sources of young groundwater is suspected. These field data serve as confirmations for the reliability of the newly presented sample collection and CO2-14C method, and for the outstanding roles of radiokrypton isotopes in characterizing old groundwater.

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Copyright
This is an Open Access article, distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives licence (http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is unaltered and is properly cited. The written permission of Cambridge University Press must be obtained for commercial re-use or in order to create a derivative work.
Corresponding author
*Corresponding author. Email: yokochi@uchicago.edu.
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Radiocarbon
  • ISSN: 0033-8222
  • EISSN: 1945-5755
  • URL: /core/journals/radiocarbon
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