Breathing Earth, venting cracks

The couplings between Earth’s solid surface and atmosphere are a rich area for study. For example, it is known that soil “respiration” plays a large role in the global water cycle. Researchers have long assumed that diffusion is the dominant mechanism for transferring gases across the interface between air and soil or rock, enhanced somewhat by wind- and pressure-fluctuation-driven transport. But scientists working in Israel’s Negev Desert have uncovered a surprisingly important new mechanism: In regions where Earth’s porous surface has cracks, fractures, or other discontinuities, thermal convection can expel, on a daily basis, up to 200 times more gas than diffusion, depending on the surrounding conditions. The team from Ben Gurion and Oregon State universities installed arrays of sensors in the large—2-cm-wide, 1-m-deep—crack in the foreground of the photograph: Temperature and relative-humidity data were acquired every 10 minutes for more than two years. Every day, the Sun’s warmth propagates slowly down through the rock until, as the evening cools, the air in the crack becomes less dense than the overlying atmosphere, and convection sets in. Venting of warm, moist air from the rock surrounding the crack, along with the entrainment of cool dry atmospheric air, then continues until dawn. Convection takes place for up to 19 hours a day in the winter, 12 during the summer. The process, say the researchers, is natural and pervasive and could have a large impact on Earth-atmosphere gas exchange. Also, because water vapor, carbon dioxide, and other gases are involved, there may be implications for climate change studies. (N. Weisbrod et al., Geophys. Res. Lett. 36 , L02401, 2009, http://dx.doi.org/10.1029/2008GL036096 .)