Dating of water with Atom Trap Trace Analysis (ATTA) of 39Ar
In hydrology the age of water, i.e. the time between last equilibrium with atmosphere and the sampling of the water, is of great interest. With ocean circulation, climate reconstruction and groundwater management just a few possible applications are named. By measuring the concentration of a radioactive isotope, such as 14C, the age of a water sample can be determined. The dating range of a radioactive isotope is given by its half-life (see figure 1). For water that is between 50 and 1000 years old, there is a dating gap for which only one radioisotope is available: 39Ar.
Fig. 1: Dating ranges for radioactive isotopes commonly employed for dating water (left) defined by the corresponding half-lives (right)
39Ar is an ideal tracer because it is of cosmogenic origin with negligible anthropogenic contribution and had a constant atmospheric concentration over the last 1000 years. Moreover, as a noble gas it is conservative and not involved in geochemical processes, which e.g. complicates the dating with 14C. However, the routine dating with 39Ar in environmental sciences is hampered by its extremely low atmospheric abundance of 39Ar/Ar ~ 10-15 corresponding to merely ~ 8000 39Ar atoms in a liter of modern water. So far, it has been accessible only by Low-Level Counting (LLC) in the underground laboratory in Bern requiring a sample size of several tons of water and a measuring time of 8-60 days. The latter requirement mainly limits the applicability of 39Ar dating with LLC to groundwaters, where degassing several tons of water from a well is typically feasible.
Fig. 2: Picture of the 39Ar-ATTA setup in Heidelberg.
Atom Trap Trace Analysis (ATTA) is an atom optical technique that has been developed for rare krypton isotopes. It is now available for routine analysis and several environmental studies based on radiometric krypton dating with ATTA have been conducted. The method makes use of the high selectivity of resonant photon scattering involved in laser cooling in order to distinguish the desired rare isotope from the abundant isotopes. Since it is not bound to radioactivity, it allows for a fast analysis while requiring only comparatively small sample sizes independent of the isotope’s half-life. In the course of this project a setup for ATTA of 39Ar (figure 2) has been developed in our collaborating group of Prof. Markus Oberthaler at the Kirchhoff-Institute for physics (www.kip.uni-heidelberg.de/matterwaveoptics/research/atta/).
For 39Ar-analysis of groundwater samples with ATTA, the dissolved argon gas needs to be extracted from the water. Our group has developed an instrument for degassing several tons of water in the field (see figure 3) which is based on membrane contactors that are permeable for the dissolved gases in the water.
Fig. 3: Impression of groundwater sampling with the degassing instruments.
The gas extracted from the water is compressed into 9 l-containers for easy transport to the laboratory. There, the argon is seperated from the other gases with a gas-chromatic setup (figure 4), that has been built in the course of the project. Together with the ATTA group at the Kirchhoff-Institute we could for the first time date groundwater samples with ATTA of 39Ar [Ritterbusch et al., 2014]. After having dated large groundwater samples, the next challenge is 39Ar-dating in small water and ice samples of about 1 kg, which requires the development of facilities for the extraction of argon from small samples of water and ice with high purity and high extraction efficiency.
Fig. 4: Schematic (left) and picture (right) of the setup for the extraction of argon.
- Aeschbach-Hertig, W., 2014. Radiokrypton dating finally takes off. Proc. Natl. Acad. Sci. 111: 6856-6857.
- Ritterbusch, F., S. Ebser, J. Welte, T. Reichel, A. Kersting, R. Purtschert, W. Aeschbach-Hertig, M. K. Oberthaler, 2014. Groundwater dating with Atom Trap Trace Analysis of 39Ar. Geophys. Res. Lett., in press. doi: 10.1002/2014GL061120.
- Welte, J., F. Ritterbusch, I. Steinke, M. Henrich, W. Aeschbach-Hertig, and M. K. Oberthaler, 2010. Towards the realization of atom trap trace analysis for 39Ar. New J. Phys. 12, 065031(14pp)
- Welte, J., I. Steinke, M. Henrich, F. Ritterbusch, M. K. Oberthaler, W. Aeschbach-Hertig, W. H. Schwarz, and M. Trieloff, 2009. Hyperfine spectroscopy of the 1s5-2p9 transition of 39Ar. Rev. Sci. Instrum. 80, 113109, doi:10.1063/1.3257691.
- Reichel, T., 2013. Groundwater Degassing and Separation of Argon from Air for 39Ar Dating with ATTA. PhD thesis, University of Heidelberg, 142 pp. pdf (55.7 MB)
- Kersting, A., 2013. A new method of krypton purication for groundwater dating with atom trap trace analysis. Diploma thesis, University of Heidelberg, 76 pp. pdf (11.8 MB)
- Schwefel, R., 2012. Methoden zur Probenaufbereitung von Eis- und Grundwasserproben zur 39Ar-Datierung mittels "atom trap trace analysis". Diploma thesis, University of Heidelberg, 107 pp. pdf (7.0 MB)
- Kollefrath, A., 2011. Test einer Entgasungsanlage. Bachelor thesis, University of Heidelberg, 65 pp. pdf (5.4 MB)
ATTA-pioneers in Argonne: http://www.phy.anl.gov/mep/atta/research/atta.html
ATTA at Kirchhoff-Institute Heidelberg: www.kip.uni-heidelberg.de/matterwaveoptics/research/atta/
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