Continental degassing of 4He by surficial discharge of deep groundwater

Radiogenic He 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 He degassed from Earth comes from the continental crust. However, the outgoing flux of crustal He 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 Kr in the deep, continental-scale Guarani aquifer in Brazil and show that crustal He reaches the atmosphere primarily by the surficial discharge of deep groundwater. We estimate that He 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 in the Guarani aquifer accumulates over half- to one-millionyear timescales. We conclude that He 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, and its resulting atmospheric residence time, should be re-examined.

Die Ethnisierung des Politischen: Identitätspolitiken in Lateinamerika, Asien und den USA

Ob die Delawaren in den USA, Indios in Mexiko oder die Kiranti-Bewegung in Nepal – seit knapp 20 Jahren fordern ethnische Bewegungen die Nationalstaaten in der außereuropäischen Welt heraus. Sie verlangen die Anerkennung und politische Repräsentation ethnischer Minderheiten und fordern, sie in die Definition der Nation einzuschließen. Die Autorinnen und Autoren des Bandes zeigen, dass diese »Ethnisierung des Politischen« bis ins frühe 19. Jahrhundert zurückreicht und die Identität einiger Nationalstaaten Amerikas und Asiens nachhaltig infrage stellt.