Natural Methane Removal in the Gulf of Mexico
There is a relationship between known hydrocarbon (oil and natural gas) discoveries at great depth in the Gulf of Mexico and hydrocarbon seepage such as Methane at the seafloor. Chemosynthetic communities are associated with these seeps. They are remarkable in that they utilize a carbon source independent of photosynthesis and the sun-dependent photosynthetic food chain that supports all other life on Earth. Chemosynthetic communities occur in isolated areas with thin veneers of sediment only a few meters thick. Calling the results "extremely surprising," researchers from the University of California, Santa Barbara and Texas A&M University report that methane gas concentrations in the Gulf of Mexico have returned to near normal levels only months after a massive release occurred following the Deepwater Horizon oil rig explosion.
Findings from the recent research study, led by oceanographers John Kessler of Texas A&M and David Valentine of UC Santa Barbara, were published today in Science Xpress in advance of their publication in the journal Science. The findings show the removal of more than 200,000 tons of dissolved methane through the action of bacteria blooms that completely consumed the immense gas plumes the team had identified in mid-June 2010. At that time, the team reported finding methane gas in amounts 100,000 times above normal levels. But, about 120 days after the initial spill, they could find only normal concentrations of methane and clear evidence of complete methane respiration.
"What we observed in June was a horizon of deep water laden with methane and other hydrocarbon gases," Valentine said. "When we returned in September and October and tracked these waters, we found the gases were gone. In their place were residual methane-eating bacteria, and a 1 million ton deficit in dissolved oxygen that we attribute to respiration of methane by these bacteria."
Kessler added: "Based on our measurements from earlier in the summer and previous other measurements of methane respiration rates around the world, it appeared that (Deepwater Horizon) methane would be present in the Gulf for years to come. Instead, the methane respiration rates increased to levels higher than have ever been recorded, ultimately consuming it and prohibiting its release to the atmosphere."
While the scientists' research documents the changing conditions of the Gulf waters, it also sheds some light on how the planet functions naturally. Nature is surprisingly resilient and resistant to releases and other sudden changes in the environment.
Kessler noted: "The seafloor stores large quantities of methane, a potent greenhouse gas, which has been suspected to be released naturally, modulating global climate. What the Deepwater Horizon incident has taught us is that releases of methane with similar characteristics will not have the capacity to influence climate."
The research team collected thousands of water samples at 207 locations covering an area of about 36,000 square miles. The researchers based their conclusions on measurements of dissolved methane concentrations, dissolved oxygen concentrations, methane oxidation rates and microbial community structure.
Others have analyzed remote sensing images from space that reveal the presence of natural oil slicks across the north-central Gulf of Mexico especially in water depths greater than 3,000 feet. Estimated seepage rates ranged from 4 barrels/day to 70 barrels/day compared. This evidence considerably increases the area where chemosynthetic communities dependent on hydrocarbon seepage may be expected.
The densest aggregations of chemosynthetic organisms have been found at water depths of around 1500 feet and deeper. The best known of these communities was named Bush Hill by the investigators who first described it. It is a surprisingly large and dense community of chemosynthetic tube worms and mussels at a site of natural petroleum and gas seepage. The seep site is a small knoll that rises about130 feet above the surrounding seafloor in about 1,900 feet water depth.
For further information: http://www.universityofcalifornia.edu/news/article/24754