Abstract
Every year, natural petroleum seepage emits 0.2–2 Tg of oil to the ocean1. Significant oil seepage can build large underwater mounds, consisting of tar deposits with morphologies similar to volcanic lava flows, known as asphalt volcanoes2,3. Such events are typically accompanied by large fluxes of the greenhouse gas methane4,5. Marine sediments from the Santa Barbara basin, California, contain a record of elevated methane concentrations6, anoxia and tar deposition7 during the Pleistocene epoch that had been attributed to dissolution of methane hydrates. However, the region is known to have exhibited oil seepage in the past7. Here, we document the discovery of seven extinct asphalt volcanoes off the coast of southern California. The morphology of the deposits and geochemistry of samples taken from the two largest structures supports their classification as asphalt volcanoes, derived from a common source. We estimate that the two structures resulted from seepage of 0.07–0.4 Tg of oil, accompanied by the emission of 0.35–1.8 Tg of methane. Radiocarbon dating of carbonate deposits entrained with the asphalt indicates formation of the volcanoes between 44 and 31 kyr ago. The timing and volume of erupted hydrocarbons from the asphalt structures can explain some or all of the documented methane release and tar accumulation in the Santa Barbara basin during the Pleistocene.
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Acknowledgements
We thank the captains and crew of the R/V Atlantis, the pilots and crew of the DSV Alvin, the crew of the AUV Sentry, as well as the scientific parties of the SEEPS 07 and SEEPS 09 expeditions. G. Paradis and L. Xu assisted with analyses. E. Keller provided information and insight about the site. J. Cook and M. Heintz provided assistance with figures. A. Sylvester, C. Busby and M. Love provided helpful comments.
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D.L.V. and C.M.R. designed the study and conducted the sampling; D.L.V., C.M.R., R.K.N., C.F., E.E.P. and T.M.H. carried out laboratory measurements; O.P., D.R.Y., R.C. and S.C.B. participated in sampling and analysed data; B.A.C., M.S. and C.N.R. estimated volumes of the features; D.L.V. wrote the manuscript.
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Valentine, D., Reddy, C., Farwell, C. et al. Asphalt volcanoes as a potential source of methane to late Pleistocene coastal waters. Nature Geosci 3, 345–348 (2010). https://doi.org/10.1038/ngeo848
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DOI: https://doi.org/10.1038/ngeo848
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