
David Valentine and colleagues at the University of California, Santa Barbara, surveyed the sea floor and discovered the mounds, the largest of which rises 20 metres above the seabed, made from tar. Some were still releasing methane. It is the first time that asphalt volcanoes have been identified in the area. Valentine says they formed as sticky hydrocarbons seeped from the seabed around 40,000 years ago.
Methane would also have been released at a rate that greatly exceeds today's output, with devastating consequences for the local ecosystem. The gas would have attracted bacteria that metabolise methane and deplete oxygen. That fits with analysis of sea-floor sediments, which suggests that a dead zone of around 600 square kilometres formed here about 40,000 years ago.
Journal reference: Nature Geoscience, DOI: 10.1038/ngeo848
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