Asphalt volcanoes as a potential source of methane to late Pleistocene coastal waters

Authors

David L. Valentine, University of California, Santa Barbara
Christopher M. Reddy, Woods Hole Oceanographic Institution
Christopher Farwell, University of California, Santa Barbara
Tessa M. Hill, University of California, Davis
Oscar Pizarro, The University of Sydney
Dana R. Yoerger, Woods Hole Oceanographic Institution
Richard Camilli, Woods Hole Oceanographic Institution
Robert K. Nelson, Woods Hole Oceanographic Institution
Emily E. Peacock, Woods Hole Oceanographic Institution
Sarah C. Bagby, University of California, Santa Barbara
Brian A. Clarke, University of California, Santa Barbara
Christopher N. Roman, University of Rhode Island
Morgan Soloway, University of California, Santa Barbara

Document Type

Article

Date of Original Version

5-1-2010

Abstract

Every year, natural petroleum seepage emits 0.2-2 Tg of oil to the ocean. Significant oil seepage can build large underwater mounds, consisting of tar deposits with morphologies similar to volcanic lava flows, known as asphalt volcanoes. Such events are typically accompanied by large fluxes of the greenhouse gas methane. Marine sediments from the Santa Barbara basin, California, contain a record of elevated methane concentrations, anoxia and tar deposition 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 past. 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. © 2010 Macmillan Publishers Limited.

Publication Title, e.g., Journal

Nature Geoscience

Volume

3

Issue

5

Citation/Publisher Attribution

Valentine, David L., Christopher M. Reddy, Christopher Farwell, Tessa M. Hill, Oscar Pizarro, Dana R. Yoerger, Richard Camilli, Robert K. Nelson, Emily E. Peacock, Sarah C. Bagby, Brian A. Clarke, Christopher N. Roman, and Morgan Soloway. "Asphalt volcanoes as a potential source of methane to late Pleistocene coastal waters." Nature Geoscience 3, 5 (2010): 345-348. doi: 10.1038/ngeo848.