The ocean’s version of the La Brea Tarpits has been discovered off the Santa Barbara coast, so-called asphalt volcanoes that probably added a lot of methane to the atmosphere when they were active some 35,000 years ago and deposited massive flows of petroleum offshore.
The underwater volcanoes are part of a larger structure of tar deposits in the area, and although the volcanoes themselves are not active, oil has been bubbling steadily out of nearby seeps in the underground rock for thousands of years.Â Just sail some 10 miles offshore and the surface of the ocean has a oily sheen and smells, says Chris Reddy, a scientist at WHOI and co-author of a paper on the asphalt volcanoes that appeared in April’s Nature Geoscience. Some 20,000 liters a day is released; about half the oil that enters the world’s oceans comes from natural seeps like these.
The volcanoes are dormant now, but at one time may have been an important regional flux of methane,Â a potent greenhouse gas. Finding natural sources of methane like these are critical to understanding how methane is released into the atmosphere.
“These sites likely put methane in to the atmosphere when they were active, but the amounts from individual sites were likely small compared to the global total,” said Dave Valentine, a geochemistry professor at UC Santa Barbara and co-author of the paper.
Valentine and his colleagues discovered the volcanoes on a diving expedition in the area. They were curious about some unusual sea floor topography they had noticed, and then sent an autonomous underwater vehicle to snap some photographs.What was revealed were seven domed volcanoes in all, larger in area than a football field, the largest of which was taller than a six story building. How do you miss these so close to a heavily populated coastline? Well, they were deep enough that diving expeditions never reached those peaks.
The volcanoes are thought to be made entirely of asphalt rooted deep below the subsurface to their base. It’s like a massive bumpy parking lot down there. Valentine says they’re not a source of global warming today, although the surrounding seeps are adding a relatively small, ever-present source to the atmosphere.
“These sources contribute to the background methane that exists in the atmosphere, and are rather minor compared to the impact of human activities over the last two centuries, rice agriculture and dairy farms being examples of sources impacted by humans,” he says.
But geochemists are often thinking in terms of scale.
“In a longer term view, changes in emission from the subsurface may have had significant impacts on climate, but it would take many such features as we have found to make a global impact.”