CARBON buried in the Earth could ultimately determine the fate of our planet’s atmosphere. So concluded a pioneering meeting last week about the Earth’s long-neglected “deep” carbon cycle.
Carbon is locked away down in the Earth’s crust: in magma and old carbonate rocks buried by plate tectonics, in fossil fuels like coal and oil, and in ice lattices beneath the ocean bed. It has long been assumed that this carbon was largely cut off from the surface, and could safely be ignored when analysing the effect of greenhouse gases on climate.
Now it seems there may be much more “deep carbon” ready to spew out than we thought.
This realisation could have profound implications for our climate, argues Robert Hazen of the Carnegie Institution, who organised the meeting at the institution’s Geophysical Laboratory in Washington DC. “We may be on the verge of a transformational moment…a glimpse of new, unexplored scientific territory,” he says.
Perhaps the greatest threat of an unexpected release of carbon from the deep comes from an indirect effect of human-made CO2. Global warming could destabilise some deep carbon reserves, notably in clathrates - ice lattices which are found beneath the ocean floor and continental permafrost, and even under freshwater lakes like Lake Baikal in Siberia (pictured). These ice structures may hold trillions of tonnes of methane.
“We are extremely concerned that clathrates are the largest single source of greenhouse gases that could be added to the atmosphere,” says Hazen. “If you raise temperatures even slightly, they could be released.” According to Ronald Cohen, a geophysicist at the Carnegie Institution, natural warming caused large releases of methane around 55 million years ago.
Though the deep carbon cycle could theoretically absorb human-made emissions, Hazen points out that this would take millions of years. Catastrophic methane emissions could happen over just a few decades.
Natural processes such as volcanism are also known to bring carbon to the surface, but there may be other mechanisms to release buried carbon that have not been considered by mainstream climate science. For example, there is growing evidence that microbes living deep in the crust may be converting carbon into forms that can migrate to the surface - notably methane.
Vladimir Kutcherov of the Royal Institute of Technology in Stockholm, Sweden, speculates that unknown non-biological chemical reactions may also be able to produce methane or hydrocarbons that seep up through the crust. For example, methane or petroleum might be produced when carbonate rocks react with water and iron upon being subducted into the mantle. Kutcherov and colleagues say hydrocarbon deposits from Kidd Creek in Ontario, Canada, have an isotopic signature suggesting they are not organic in origin - though this claim was disputed by others at the meeting.
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