One of the places where Nature was thought to suck up and bury atmospheric carbon may actually be doing just the opposite, according to new research on the volcanics of deep-sea ridges.
Seismic profiles of the rifting Guaymas Basin in the Gulf of California show molten rock from a deep-sea ridge squeezing far and wide as "sills" of magma into the layers of ocean floor sediments. The sediments, which are loaded with organic debris, are baked by the magma and so release vast amounts of greenhouse gases.
The discovery has big implications for calculating the Earth's current carbon budget, as well as for how past climates were thrown in to disarray by "intrusive" volcanic activity beneath the seafloor -- a place that had always previously been considered a carbon sink, not a carbon source.
"It was one of those 'Ahah!' moments," said researcher Daniel Lizarralde of the Woods Hole Oceanographic Institution, describing the discovery of the sills in the seismic data.
What especially surprised the researchers was how far the magma sills were located from the narrow ridge where magma is typically found at most ocean floor spreading centers. In the Guaymas Basin the sills of magma were not just a few kilometers (1.8 miles) away from the spreading center ridge, as expected, but up to 50 kilometers (31 miles) away. That vastly increases the volume of sediments the hot rock can cook and from which methane and other carbon-rich gases can be released.
Lizarralde and his colleagues published their discovery in the Nov. 14 issue of the journal Nature Geoscience.
"Sills derived from intrusive volcanism in sedimentary basins have been linked to huge natural methane fluxes in the past," commented David Goldberg of the Lamont-Doherty Earth Observatory. In a separate article in the same issue of Nature Geoscience, Goldberg cites, for example, the 55-million-year-old sills of the Norwegian margin, which is suspected of having caused global climate change.
Goldberg also explains that what makes these sills such potent sources of carbon gases is the fact they are buried. Thick blankets of sediment allow the magma to bake the buried organic material at very high temperatures and drive off ten times more carbon dioxide than if the magma had just poured out onto the seafloor.
All that gas buoys up through the sediments and into the ocean, heating up the deep waters. Methane that is release can stimulate deep sea biological activity.
Exactly how much carbon these sills and others like them in small ocean basins elsewhere are contributing to the atmospheric carbon is still not known, said Lizarralde. For one thing, this is the first place such wide-ranging sills have been discovered. And then there is the question of how much of the carbon-rich gases are recaptured by deep-sea organisms -- and so never reached the ocean's surface.
"We don't know how much is immediately taken out by seafloor fauna," said Lizarralde. That will take a lot more deep sea exploration to determine.