Methane from the deep

Methane from the deep could be a significant feedback mechanism in climate change. As oil and gas companies well know, many hydrocarbon deposits lie beneath the sea floor. Over geologic time scales, some oil and gas (primarily methane, CH₄) deposits can slowly seep through cracks in the sedimentary rock and reach the ocean floor, where buoyancy takes over. The ensuing natural oil slick on the ocean’s surface then degrades through both bacterial action and evaporation of the volatile chemicals. What’s left is a tar residue that sinks back to the ocean floor. This chain of events inspired a team of Earth scientists to examine sea-floor cores, drilled off the coast of California, for deposits of tarry sand as a proxy for CH₄ release. They found several episodes of enhanced CH₄ emission in Earth’s history, notably from 16 000 to 14 000 years ago and from 11 000 to 10 000 years ago. Those two eras correspond to periods of globally melting glaciers and rising atmospheric temperatures. The researchers also note that hydrates—semisolid CH₄ agglomerates that can exist in the cold, high-pressure sea-floor environment—are common near hydrocarbon seeps. According to team leader Tessa Hill (University of California, Davis), a possible scenario is that the hydrates first become unstable due to a warmer ocean bottom at higher pressure from rising sea levels; the roused hydrates then perturb the sea floor to enhance hydrocarbon seepage and CH₄ emission. Because CH₄ is a potent greenhouse gas, global warming increases and brings about further sea-floor instabilities. The natural process is slow to start, but it gathers steam in a positive feedback loop.(T. M. Hill et al., Proc. Natl. Acad. Sci. USA 103 , 13570, 2006 http://dx.doi.org/10.1073/pnas.0601304103 ).