By Sid Perkins
Massive releases of methane from arctic seafloors could create oxygen-poor dead zones, acidify the seas and disrupt ecosystems in broad parts of the northern oceans, new preliminary analyses suggest.
Such a cascade of geochemical and ecological ills could result if global warming triggers a widespread release of methane from deep below the Arctic seas, scientists propose in the June 28 Geophysical Research Letters.
Worldwide, particularly in deeply buried permafrost and in high-latitude ocean sediments where pressures are high and temperatures are below freezing, icy deposits called hydrates hold immense amounts of methane (SN: 6/25/05, p. 410). Studies indicate that seafloor sediments beneath the Kara, Barents and East Siberian seas in the Arctic Ocean, as well as the Sea of Okhotsk and the Barents Sea in the North Pacific, have large reservoirs of the planet-warming greenhouse gas, says study coauthor Scott M. Elliott, a marine biogeochemist at Los Alamos National Laboratory in New Mexico.
Many oceanographic surveys have already discovered plumes of methane rising from the ocean floor, particularly in the Arctic, Elliott notes. The climate warming expected in coming decades will likely extend even into the deep sea, melting or destabilizing hydrates and releasing their trapped methane, he explains. Some scientists estimate that increased temperatures across some swaths of ocean floor between 300 and 600 meters deep — where methane hydrates are now stable but may not be in the future — could eventually release as much as 16,000 metric tons of methane each year.
That methane would be an unexpected bounty for methane-munching marine microbes that consume dissolved oxygen and produce carbon dioxide. As a result, the researchers’ model suggests, the waters down-current of a large methane plume, especially in an ocean basin with poor circulation, could lose as much as 95 percent of their oxygen.
The ocean acidification that resulted from the increased carbon dioxide would rival that seen in surface waters under today’s atmosphere, which is already stifling the growth of phytoplankton, rendering the shells of marine snails thinner (SN: 10/20/07, p. 245) and affecting marine ecosystems worldwide (SN: 7/17/04, p. 35).
“This will be a truly big environmental pollution problem in the next few decades,” Elliott contends. “This problem is not going to go away.”
Besides generating large volumes of acidified water and low-oxygen dead zones, the microbial activity will rob the waters of key nutrients — including nitrate, copper and iron — that otherwise would be used by microorganisms that don’t feed on methane. Many of these nutrients are already sparse, and the resulting shift in populations among the fiercely competitive microorganisms at the base of the ocean’s food chain in many regions could be devastating, the researchers suggest.
“This is an interesting possibility,” says David Valentine, a microbial geochemist at University of California, Santa Barbara. The team “has taken what we know about methane-consuming organisms and placed it in the context of a warming Arctic,” he notes.
Nevertheless, he continues, the largest rates of methane release considered by the researchers “are considerably larger than scientists have seen in the Arctic recently.
“They picked a very large [methane] flux, in my estimation”, he says. “But there’s very little doubt that if methane emissions are as large as this, there will be severe biological and geochemical impacts.”
Future work will refine the new study’s preliminary results, says Elliott. In areas where river deltas inject organic material and dissolved trace elements into the sea, for instance, it’s not clear how all of the intricately related processes will affect water chemistry.
On the whole, though, the cascade of ecological effects envisioned by Elliott and his colleagues are a reasonable scenario, Valentine says. “The same sort of processes are seen in the dead zones in many lakes and oceans today,” he notes.
