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 atmosphere and eventually escapes to space. Assuming that all of the 4He produced is degassed, about 70% of the total 4He degassed from Earth comes from the continental crust. However, the outgoing flux of crustal 4He has not been directly measured at the Earth's surface and the migration pathways are poorly understood. Here we present measurements of helium isotopes and the long-lived cosmogenic radio-isotope 81 Kr 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.