Tip from USGS and MBARI Scientists Leads to Discovery of Methane Seep off San Diego, California, by Scripps Graduate Students
A tip from U.S. Geological Survey (USGS) scientists and their colleague at the Monterey Bay Aquarium Research Institute (MBARI) led to the discovery of a deep-sea methane seep by graduate students from Scripps Institution of Oceanography at the University of California, San Diego. Evidence for the seep was discovered during the San Diego Coastal Expedition in July 2012.
Methane seeps are known in several areas off the coast of California, but this is the first such finding off San Diego County. The seep was discovered about 20 miles west of Del Mar, in a fault zone known as the San Diego Trough fault zone. Methane, the most common form of natural gas, exists in sediment beneath the seafloor along many of the world’s continental margins. Faults can provide a pathway for methane to “seep” upward toward the seafloor.
The newly discovered seep occurs at a mound on the seafloor that is 1,036 meters (3,400 feet) beneath the sea surface, has an area the size of a city block, and rises to the height of a two-story building. USGS scientists Jamie Conrad and Holly Ryan and MBARI scientist Charlie Paull surveyed the mound in 2010 as part of an effort to determine a slip rate for the San Diego Trough fault zone. Their findings, to be published December 2012 in the Bulletin of the Seismological Society of America, will affect estimates of earthquake hazards in the region.
Conrad, Ryan, and Paull were familiar with the mound from a seismic-reflection profile (a side view of sub-seafloor sediment layers) collected in 2000 by the late USGS geologist Bill Normark (http://soundwaves.usgs.gov/2008/06/staff.html). The profile reminded Paull of a “mud volcano” in Santa Monica Bay where a core collected during a 2003 USGS expedition recovered methane hydrate (an icelike crystalline solid in which methane gas molecules are trapped; see http://soundwaves.usgs.gov/2006/03/research.html). Because of Paull’s hunch that the mound would be associated with methane, and because it is in a complex area of the San Diego Trough fault zone that they wanted to investigate, Conrad, Ryan, and Paull included the mound in their 2010 survey. Using an autonomous underwater vehicle (AUV) developed by MBARI (http://www.mbari.org/auv/), they mapped the bathymetry of the mound at ultra-high resolution, revealing distinctive patches of rough seafloor that are typically associated with fluid venting.
In late 2011, Conrad got a call from Scripps graduate student Jillian Maloney, a geologist helping two fellow graduate students—biological oceanographers Ben Grupe and Alexis Pasulka—plan a search for offshore methane seeps. Such seeps host unusual biological communities about which much remains to be learned. When Maloney asked Conrad where they might find undersea gas vents, he suggested a couple of spots, including the mound that he and his colleagues had mapped off San Diego.
During the following summer’s San Diego Coastal Expedition, the Seeps Team, now a group of five graduate students, included the mound in their search for methane seeps. “It was farther from shore than we had planned to go,” said Maloney, “but we had some extra time, so we went out to take a look.”
They mapped the mound with the research vessel’s multibeam sonar and subbottom echosounder, both of which send out sound waves that reflect off objects in the water column, the seafloor, and the sedimentary layers beneath the seafloor. The reflected sound signals produce images of the seafloor surface (in the case of the multibeam sonar) and sub-seafloor sedimentary layers (in the case of the sub-bottom echosounder). (Learn more about sonar-mapping methods at http://earthguide.ucsd.edu/earthguide/diagrams/sonar/sonar.html.)
“In the ‘chirp’ data [the sub-bottom data], we saw the most convincing evidence for a seep that we had seen all cruise—a distinctive disruption of the subsurface layers that’s typical of fluid seepage,” said Maloney.
Ordinarily, the next step would be to send the vessel’s remotely operated vehicle (ROV) down to image the suspected seep. But at 1,036 meters (3,400 feet) below the surface, the mound was too deep for the ROV’s tether. Pasulka and Grupe decided to drop a multicore sampler, a frame that supports a ring of eight 1-meter-long corers. When the sampler came back on deck, its contents added to the evidence that the team had discovered a seep.
“As soon as we began slicing a core open, we started smelling hydrogen sulfide, and about 10 cm down into the core we started seeing carbonate nodules,” said Pasulka. “As we continued to sift through the mud and examine the organisms under the microscope, it gradually became clear that we had hit a seep.”
Carbonate nodules and hydrogen sulfide (which has a rotten-egg smell) are both products of anaerobic oxidation of methane by microbes that use methane as their main energy source. The organisms in the cores included siboglinids, threadlike worms that have no digestive system but gain nutrition from symbiotic bacteria living inside them, and vesicomyid clams, which gain nutrition from bacteria living on their gills. The bacteria metabolize the hydrogen sulfide produced by methane oxidation. “These symbiotic bacteria make a lot of energy,” said Pasulka, “with enough left over to be used by their hosts.”
The various microbes that produce energy from chemicals in seep fluids form the base of food webs for these rich ecosystems, making methane seeps biological oases in seafloor areas that otherwise are sparsely populated.
Pasulka and Grupe, who are interested in the biology of seep ecosystems, are excited to have discovered a seep so close to San Diego. The site’s relative proximity to Scripps—about 3 to 4 hours by boat—will allow repeated visits over months and years to study the ecosystem and observe how it changes. The Seeps Team is already planning to revisit the site during the San Diego Coastal Expedition’s second leg in December 2012, when they hope to get video footage and additional samples.
To learn more about the seep discovery and the process that led to it, read the Scripps news release and Jillian Maloney’s blog post. Read additional blogs from the expedition—and watch for posts during the December 2012 leg—at https://sites.google.com/site/sandiegoseaflex/blog. Visit the expedition’s Facebook page at http://www.facebook.com/sdcoastex.
in this issue:
Methane Seep off San Diego, California