During a recent oceanographic expedition off San Diego, graduate student researchers from Scripps Institution of Oceanography at UC San Diego discovered convincing evidence of a deep-sea site where methane is likely seeping out of the seafloor, the first such finding off San Diego County.
Such "methane seeps" are fascinating environments because of their extraordinary chemical features and often bizarre marine life. The area of interest, roughly 20 miles west of Del Mar, is centered on a fault zone known as the San Diego Trough Fault zone. Methane, a clear, highly combustible gas, exists in the earth's crust under the seafloor along many of the world's continental margins. Faults can provide a pathway for methane to "seep" upward toward the seafloor.
The Scripps graduate students made the discovery during the recent San Diego Coastal Expedition (bit.ly/sdcoastex), a multidisciplinary voyage conceived and executed by Scripps graduate students. The cruise was funded by the University of California Ship Funds Program, which supports student research at sea and provides seagoing leadership opportunities.
While conducting surveys in search of methane seeps aboard Scripps' research vessel Melville, the graduate students mapped a distinct mound on the seafloor at 1,036 meters depth (3,400 feet), spanning the size of a city block and rising to the height of a two-story building. The area had been recommended by Jamie Conrad, Holly Ryan (U.S. Geological Survey) and Charles Paull (Monterey Bay Aquarium Research Institute), who surveyed the faults in 2010.
A sediment core sample reveals black marks where sulfide is present, an indication of a methane seep environment.
"Below the mound," described Scripps geosciences graduate student Jillian Maloney, "we observed a disruption in subsurface sediment layers indicative of fluid seepage."
The Scripps researchers then deployed instruments to collect sediment cores, gathering further evidence such as seep-dwelling animals, sulfidic-smelling black mud and carbonate nodules. These samples are currently being analyzed in Scripps laboratories for chemical clues and other telling elements of the environment.
Organisms collected from the site include thread-like tubeworms called siboglinids and several clams. Siboglinids lack a mouth and digestive system and gain nutrition via a symbiotic relationship with bacteria living inside them, while many clams at seeps get some of their food from sulfide-loving bacteria living on their gills.
Siboglinids (left), which lack a mouth and digestive system, live inside tubes (right) in methane or sulfide-rich environments.
While food is scarce in much of the cold, dark ocean depths, it is abundant at seeps due to the bacteria that proliferate around the methane source. Microbes there are eaten by worms, snails, crabs and clams, leading to a rich and productive community that helps sustain the surrounding deep-sea ecosystem.
"These chemosynthetic ecosystems are considered 'hot spots' of life on the seafloor in an otherwise desert-like landscape," said San Diego Coastal Expedition team member Alexis Pasulka, a Scripps biological oceanography graduate student. "New forms of life are continuously being discovered in these environments. Therefore, it is important to study these ecosystems not only to further appreciate the diversity of life in our oceans, but also so that we can better understand how these ecosystems contribute to overall ocean productivity and the carbon cycle."
Methane is a potent greenhouse gas, and researchers don't yet fully understand the magnitude to which seeping methane in the ocean contributes additional carbon to the atmosphere. Moreover, on many continental margins, frozen methane hydrates could represent a future energy source. Along the West Coast, methane seeps are known to exist off Oregon, California (near Eureka, Monterey Bay, Point Conception and Santa Monica), in the Gulf of California and off Costa Rica.
When exposed to acid, carbon dioxide bubbles from these pieces of carbonate rock
(see video). Microbes are known to produce carbonate in methane environments.
"This is a significant and exciting discovery in part because of the possibilities for future research at Scripps," said biological oceanography graduate student Benjamin Grupe, a member of the seep contingent on the San Diego Coastal Expedition. "The existence of a methane seep just a few hours from San Diego should allow Scripps scientists to visit frequently, studying how this dynamic ecosystem changes over days, months and years. Such regular data collection is difficult at most cold seeps, which rarely occur so close to ports or research institutions."
Grupe will lead a follow-up cruise in December that will revisit the newly discovered seep to collect additional samples and learn more about this ecosystem. The team of graduate students hopes to raise funds to employ technologies such as video-driven coring instruments and towed video cameras that will give them an up-close look at the methane seep.
The search for local seeps was one focus area of the multidisciplinary San Diego Coastal Expedition, which included teams of students investigating the oceanography and marine ecosystems off San Diego and led by chief scientist Christina Frieder. In addition to Grupe, Pasulka and Maloney, other members of the seep team included geophysics graduate students Valerie Sahakian and Rachel Marcuson.
R/V Melville, the oldest ship in the U.S. academic fleet, is owned by the U.S. Navy and has been operated by Scripps Oceanography for all of its 41 years.
"The students should be congratulated on their hard work and perseverance leading to this exciting find," said Lisa Levin, a Scripps professor who has studied methane seep ecosystems in most of the world's oceans. "Other scientists have suspected that methane seeps were present in the San Diego region, but these new data and samples provide the first convincing evidence. We know very little about what lives in deep waters-the planet's largest ecosystem-so it is not unexpected to find surprises on the deep-sea floor right in our own backyard. Having a 'local' seep should be a great boon to deep-sea research, education and public outreach at Scripps."