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Continental degassing of 4He by surficial discharge of deep groundwater

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.

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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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The authors declare no competing financial interests.

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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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