Falling through the cracks: The role of fractures in Earth-atmosphere gas exchange

[1] If we are to understand global warming, and in particular global water-cycling, then it is vital to explore the links between atmospheric conditions, earth processes and major global cycles. One arena that has been heretofore ignored is the effect on global dynamics of earth fractures that are open to the atmosphere. Historically, these fractures have been studied merely as participants in aquifer recharge or aquifer contamination during periods of infiltration. In general, they are considered inactive when there is no precipitation. This paper puts forward in-situ continuous field measurements demonstrating that during no-flow periods, fractures breathe via convection on a daily basis, enhancing atmospheric exchange by several orders of magnitude compared to the non-fractured crust. We quantify the timing, persistence and characteristics of this mechanism. The convective exchange mechanism is pervasive, occurring daily with peak flux exchange at night and in winter, the reverse of most other surfac