Letter | Published:

Continental degassing of 4He by surficial discharge of deep groundwater

Nature Geoscience volume 8, pages 3539 (2015) | Download Citation

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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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

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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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.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Pradeep K. Aggarwal.

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DOI

https://doi.org/10.1038/ngeo2302

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