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Deepwater Horizon blowout released half a million tonnes of hydrocarbon gases

In the early months of the Deepwater Horizon oil spill, scant attention was paid to the huge volume of gases that gushed into the Gulf of Mexico, along with the estimated 62,500-68,000 barrels of oil per day.

Researchers report this week in Nature Geoscience that some 500,000 tonnes of gases – comprising roughly 40% of the total hydrocarbons – were discharged from the blown-out BP well. These gases could lead to small pockets of low oxygen in the waters surrounding the wellhead for years or even decades to come, the researchers say.

A previous study in Science concluded that the massive amounts of methane gas injected into the deep waters of the Gulf during the spill were devoured by microbes in a matter of months.

But Samantha Joye, a biogeochemist at the University of Georgia in Athens and lead author of the new study, says that her team detected gases at roughly 100 times background levels as late as December 2010, five months after the well was finally capped. Whereas previous teams sampled for gases largely in the southwest direction of the wellhead, the gases her group observed were in other regions, Joye says.

“I don’t think you can conclude based on the data available right now that all the gas is gone from the system,” says Joye. “Unlike the oil, gas doesn’t sediment to the bottom – it’s dissolved in the water and it floats around,” Joye adds. “So it’s not a problem that’s going to go away on a short time frame.”

Microbial consumption of gases in these areas was probably limited by a lack of key nutrients such as nitrogen, copper, and iron, which are in short supply in the deep waters of the Gulf, says Joye. In other words, microbial populations likely swelled to consume the feast of gases, but only until they ran out of the nutrients that they need for growth.

It is not yet clear whether low-oxygen zones will affect deepwater organisms, such as deepwater corals. Although pockets of low oxygen could persist for decades due to the slow turnover of deep waters, the size and magnitude of the problem does not compare to the seasonal “dead zone” in the Mississippi River basin, says Joye.

Posted on behalf of Amanda Mascarelli

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