Oil seeping from the seafloor may have contributed to climate change long before the internal combustion engine did. The petroleum deposits are rich in the powerful greenhouse gas methane, which, according to a new study, may have played a major role in two previous episodes of global warming.
Bedrock below the ocean bottom keeps a lid on oil reservoirs, but it's not an impermeable cap. Small cracks allow petroleum and methane to bubble to the surface. Once there, the petroleum oxidizes and turns to tar, which sinks. Meanwhile, the methane drifts into the atmosphere, where it makes up about 15% of the total amount of the gas sent skyward by natural sources such as wetlands and melting tundra. Humans contribute slightly more than all natural sources combined.
But does undersea methane make up a larger piece of the pie during periods of global warming? Paleo-oceanographers Tessa Hill of the University of California (UC), Davis, James Kennett of UC Santa Barbara, and collaborators attacked the question by looking at tar deposits from sediment cores taken off the Santa Barbara coast. They found 3 times more tar mixed into the sand from the last two major warming periods, 11,000 and 15,000 years ago, than was seen on average. This suggests that 3 times more oil was released from seeps during those periods, and 3 times more methane along with it, the team reports online this week in Proceeding of the National Academy of Sciences. The team suggests that global warming may have first melted undersea methane ice, disturbing the sea floor and opening new cracks in the oil reservoirs.
"This is a source of methane that we might have assumed in the past was stable," says Hill. "As it turns out, it's very sensitive to climate change. I would anticipate that it would be sensitive to climate change in the future as well." If methane was released similarly from all the other marine reservoirs worldwide, it would account for nearly half the increase in atmospheric methane during those warming periods, says Hill, who believes this methane may have driven further warming. The researchers acknowledge, however, that global warming would probably affect different petroleum deposits differently, so such a simultaneous release is unlikely; further research will be needed at other oil seeps around the world, they say.
The evidence from Santa Barbara is "beautiful," says Jérôme Chappellaz, an atmospheric scientist at the Laboratoire de Glaciologie et Géophysique de l'Environnement in St Martin d'Hères Cedex, France. But he cautions against extrapolating to all marine oil reservoirs around the world. Evidence from carbon testing of ice core samples points away from marine methane sources and toward wetlands and melting tundra, he says.