Aggarwal, Pradeep K.Matsumoto, TakuyaSturchio, Neil C.Chang, Hung K. [UNESP]Gastmans, Didier [UNESP]Araguas-Araguas, Luis J.Jiang, WeiLu, Zheng-TianMueller, PeterYokochi, ReikaPurtschert, RolandTorgersen, Thomas2015-10-212015-10-212015-01-01Nature Geoscience, v. 8, n. 1, p. 35-39, 2015.1752-0894http://hdl.handle.net/11449/129057Radiogenic He-4 is produced by the decay of uranium and thorium in the Earths mantle and crust. From here, it is degassed to the atmosphere(1-5) and eventually escapes to space(1,5,6). Assuming that all of the He-4 produced is degassed, about 70% of the total He-4 degassed from Earth comes from the continental crust(2,-5,7). However, the outgoing flux of crustal He-4 has not been directly measured at the Earths surface(2) and the migration pathways are poorly understood(2-4,7,8). Here we present measurements of helium isotopes and the long-lived cosmogenic radio-isotope Kr-81 in the deep, continental-scale Guarani aquifer in Brazil and show that crustal He-4 reaches the atmosphere primarily by the surficial discharge of deep groundwater. We estimate that He-4 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 He-4 in the Guarani aquifer accumulates over half- to one-million-year timescales. We conclude that He-4 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 He-4, and its resulting atmospheric residence time, should be re-examined.35-39engArtigo10.1038/NGEO2302WOS:000346825000012Acesso restrito1989662459244838