Methane's hidden impact in Gulf oil spill
Large quantities of methane released by BP's oil blowout aren't fouling beaches like the Gulf oil spill is, but could endanger a key link in the undersea food chain.
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A 10-day research cruise in mid-June took measurements over a distance that ranged from about 1,600 feet from the blowout to eight miles away. The team, led by David Valentine from the University of California at Santa Barbara and John Kessler from Texas A&M University, found that methane concentrations "were low in the surface water and overlying air, very high at depths greater than 3,000 feet, and somewhat elevated in between," Dr. Valentine writes in an email exchange.Skip to next paragraph
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"We are interpreting this data to mean that the vast majority of the methane that escapes the top hat is trapped at depths of around one kilometer, and that only small amounts are likely to escape through the ocean to the atmosphere," he says.
The methane remains a captive of deep water because in temperate and tropical oceans, sea water forms stable layers that don't readily mix upward, he explains.
During a research cruise in late May, Joye and her colleagues gathered measurements in some 90 locations inside a 30 mile by 40 mile "box" of ocean near the drill site. At each location, the team lowered an array of sampling bottles – each triggered to capture a water sample at a different depth.
Analysis of the dissolved gas content in the samples revealed a layer within five miles of the blowout in which the dissolved methane was six times higher than the dissolved oxygen. The 600-foot thick layer of water ranged in depth from 3,300 feet to 4,300 feet below the surface.
The bottom line: Methane-loving microbes would find themselves in the equivalent of an all-you-can-eat-and-then-some diner.
Enough food – the methane – is present that, in principle, microbes could use up all of the oxygen in that "lens" of seawater, dropping oxygen levels to zero – a level more akin to the bottom of the Black Sea than to the Gulf of Mexico's annual oxygen-depleted "dead zone."
That breakdown of methane occurs very slowly. And in practice, Joye cautions, microbes need oxygen, too. So at some point their activity could slow or stop before all of the oxygen in a methane-heavy parcel of water disappeared.
Still, microbial breakdown of the methane could reduce oxygen concentrations to levels untenable for a range of marine creatures. And just as a lack of vertical mixing in the deep water is holding the dissolved methane at depth, that lack of mixing keeps high levels of dissolved oxygen at the surface from replenishing oxygen levels in the deep water.
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