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Continental degassing of 4He by surficial discharge of deep groundwater

Nature Geoscience volume 8, pages 35–39 (2015)Cite this article

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Abstract

Radiogenic 4He is produced by the decay of uranium and thorium in the Earth’s mantle and crust. From here, it is degassed to the atmosphere1,2,3,4,5 and eventually escapes to space1,5,6. Assuming that all of the 4He produced is degassed, about 70% of the total 4He degassed from Earth comes from the continental crust2,3,4,5,7. However, the outgoing flux of crustal 4He has not been directly measured at the Earth’s surface2 and the migration pathways are poorly understood2,3,4,7,8. Here we present measurements of helium isotopes and the long-lived cosmogenic radio-isotope 81Kr in the deep, continental-scale Guarani aquifer in Brazil and show that crustal 4He reaches the atmosphere primarily by the surficial discharge of deep groundwater. We estimate that 4He in Guarani groundwater discharge accounts for about 20% of the assumed global flux from continental crust, and that other large aquifers may account for about 33%. Old groundwater ages suggest that 4He in the Guarani aquifer accumulates over half- to one-million-year timescales. We conclude that 4He degassing from the continents is regulated by groundwater discharge, rather than episodic tectonic events, and suggest that the assumed steady state between crustal production and degassing of 4He, and its resulting atmospheric residence time, should be re-examined.

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Figure 1: Sampling locations, 4He concentrations and 81Kr ages for groundwater samples from the Guarani aquifer.
Figure 2: Guarani groundwater ages estimated by 14C,81Kr and 4He.
Figure 3: Observed and modelled 4He concentrations in Guarani groundwater samples.

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Acknowledgements

W.J., Z-T.L., P.M. and the Laboratory for Radiokrypton Dating at Argonne are supported by DOE, Office of Nuclear Physics, under contract DE-AC02-06CH11357. Development of the ATTA-3 instrument was supported in part by NSF EAR-0651161. C. Sambandam, L-F. Han, D. Hillegonds, P. Klaus, S. Terzer and E. Izweski of IAEA assisted in noble gas analysis or with graphic illustrations.

Author information

Authors and Affiliations

  1. Isotope Hydrology Section, International Atomic Energy Agency, A-1400 Vienna, Austria

    Pradeep K. Aggarwal, Takuya Matsumoto & Luis J. Araguas-Araguas

  2. Department of Geological Sciences, University of Delaware, Newark, Delaware 19716, USA

    Neil C. Sturchio

  3. CEA and LEBAC, Universidade Estadual Paulista (UNESP), Avenida 24 A, 1515 13506-900 Rio Claro - SP, Brazil

    Hung K. Chang & Didier Gastmans

  4. Physics Division, Argonne National Laboratory, Argonne, Illinois 60439, USA

    Wei Jiang, Zheng-Tian Lu & Peter Mueller

  5. Department of Physics and Enrico Fermi Institute, The University of Chicago, Chicago, Illinois 60637, USA

    Zheng-Tian Lu

  6. Department of Geophysical Sciences, The University of Chicago, Chicago, Illinois 60637, USA

    Reika Yokochi

  7. Climate and Environmental Physics, Physics Institute, University of Bern, 3012 Bern, Switzerland

    Roland Purtschert

  8. National Science Foundation, Arlington, Virginia 22230, USA

    Thomas Torgersen

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  1. Pradeep K. Aggarwal
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Contributions

P.K.A. initiated the project and conceived of the proposed mechanism for crustal degassing; P.K.A., N.C.S., H.K.C. and D.G. designed and conducted the sampling campaign with assistance from L.J.A-A. and W.J.; R.Y. and R.P. purified krypton, W.J., Z-T.L. and P.M. measured krypton isotopes, T.M. conducted noble gas analysis and model calculations; P.K.A. and T.M. wrote the paper with assistance from L.J.A-A.; H.K.C., D.G., N.C.S. and T.T. contributed to data evaluation and presentation. All authors reviewed and commented on the manuscript.

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Correspondence to Pradeep K. Aggarwal.

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Aggarwal, P., Matsumoto, T., Sturchio, N. et al. Continental degassing of 4He by surficial discharge of deep groundwater. Nature Geosci 8, 35–39 (2015). https://doi.org/10.1038/ngeo2302

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