Insights into deep carbon derived from noble gases

Science and society are faced with two challenges that are inextricably linked: fossil-fuel energy dependence and rising levels of atmospheric carbon dioxide. Management of remaining hydrocarbon resources, the search for cleaner fuels and increasing interest in subsurface carbon storage all require a better understanding of the deep terrestrial carbon cycle. The coupling of noble gas and carbon chemistry provides an innovative approach to understanding this deep carbon. Whereas carbon geochemistry and isotopic signatures record the history of inorganic and organic reactions that control carbon mobility, the inert noble gases provide unique tracers of fluid origin, transport and age. Together, they have been used to show that groundwater has a key role as both the sink for geologically sequestered carbon dioxide, and in the transport and emplacement of hydrocarbon gas deposits. Furthermore, these tracers have also been used to show that groundwater and subsurface microbiology jointly influence the formation and alteration of fossil-fuel deposits to an extent not previously recognized. The age and distribution of groundwater in fractures in the Earth's crust exert important controls on the Earth's deepest microbial communities.