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Up to 500,000 tons of gaseous hydrocarbons released into the deep ocean during the BP Deepwater Horizon disaster, according to a new study just released.

Such a large gas discharge, which generated concentrations 75,000 times the norm, could result in small-scale zones of “extensive and persistent depletion of oxygen” as microbial processes degrade the gaseous hydrocarbons, according to the University of Georgia-led study co-authored by Florida State University oceanographer Ian MacDonald.

The Macondo Well blowout discharged not only liquid oil, but also hydrocarbon gases, such as methane and pentane, which deposited into the water column. Researchers normally do not quantify gases for oil spills, but they said in this instance, documenting the amount of hydrocarbon gases released by the blowout is critical to understanding the discharge’s true extent, the fate of the released hydrocarbons, and potential impacts on the deep oceanic systems.

The researchers said the 1,480-meter depth of the blowout (nearly 1 mile) is highly significant because deep-sea processes (high pressure, low temperature) entrapped the released gaseous hydrocarbons in a deep (1,000-1,300m) layer of the water column.

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The methane and other gases likely will remain deep in the water column and microbes will consumed them in a process known as oxidation, which en masse can lead to low-oxygen waters, said UGA Professor of Marine Sciences Samantha Joye, the lead researcher on the project.

“We’re not talking about extensive hypoxic areas offshore in the Gulf of Mexico,” Joye said. “But the microbial oxidation of the methane and other alkanes will remove oxygen from the system for quite a while because the time scale for the replenishment of oxygen at that depth is many decades.”

Some of the larger gaseous hydrocarbons documented, such as pentane, have significant health implications for humans and potentially for marine life, said report co-author Ira Leifer of the University of California-Santa Barbara.

The study concludes that separating the gas-induced oxygen depletion from that due to liquid hydrocarbons is difficult, absent further research, because all hydrocarbons contribute to oxygen depletion. Therefore, documenting the total mass of hydrocarbons discharged is critical for understanding the long-term implications for the Gulf’s microbial communities, food chain and overall system.

Joye’s team examined samples from 70 sites around the leaking wellhead during a research cruise aboard the R/V Walton Smith during late May and early June of 2010. They combined their data with estimates of the volume of oil released to arrive at a figure that allows scientists to quantify, for the first time, the gas discharge in terms of equivalent barrels of oil. They calculated a gas discharge that’s the equivalent of either 1.6 to 1.9 or 2.2 to 3.1 million barrels of oil, depending on the method used. Although the estimate reflects the uncertainty still surrounding the discharge, even the lowest magnitude represents a significant increase in the total hydrocarbon discharge.

“These calculations increase the accepted government estimates by about one-third,” MacDonald said.

The ever-shifting small-scale currents in the Gulf likely have dissipated the plumes and the low oxygen zones associated with them, Joye said, making them difficult if not impossible to find at this point in time. Although gliders are a new platform used for research, scientists typically search for subsurface features by dropping instruments from research vessels, a process that’s analogous to looking for a feature on the Earth’s surface by randomly dropping instruments from a height of 1,500 meters (about 5,000 feet) in the atmosphere.

“It’s like searching for a needle in the haystack,” Joye said. “We may never know what happened to all of that gas.”

Joye cautioned against assuming that microbes will rapidly consume the gases released from the well. Undoubtedly, the methane is a feast for them, Joye said, but she also noted the microbes need nutrients, such as nitrogen, copper and iron. These nutrients are in scarce supply in the Gulf’s deep waters, Joye said, and once they go away the microbes will cease to grow — regardless of how much methane is available.

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