Far North Science

For years, they were the mystery bumps dimpling the Beaufort Sea north of Alaska and Canada.

Hundreds of small hills rose from the flat seabed, creating an eerie topography in the still, frigid gloom. Some loom more than 100 feet over the sediment in a place covered by ice most of the year. What could have caused them?

Kim Fulton-Bennett (c) 2007 MBARI

Scientists figured they were just “pingos” — mounds formed by ice-cores and permafrost on the Arctic tundra — that had been inundated when the ocean rose after the last ice age ended.

But new research from the Monterey Bay Aquarium Research Institute suggests these mounds may have another cause, one that could contribute dramatically to climate change by adding the super greenhouse gas methane to the atmosphere.

A team of MBARI scientists found evidence that these mounds grow because methane bubbles upward from buried hydrate layers deep beneath the sea floor.

The oozing gas “squeezes deep sediments up onto the seafloor like toothpaste from a tube,” according to MBARI geologists Charlie Paull and William Ussler and their coauthors. Their research appeared in the January issue of Geophysical Research Letters.

The scientists mapped eight of the hills and took samples. After analyzing the gas, they concluded it came from methane hydrate, a buried frozen mixture of water and methane that remains stable only under high pressures and temperatures far below freezing. It led to a startling explanation.

When the Beaufort Sea flooded at the end of the ice age, the seawater slowly began to “warm” the buried hydrates over thousands of years, eventually triggering large releases of methane gas.

According to Paull, “We don’t know if this gas and sediment was burped up in a single year, or moved slowly like a glacier.” In either case, Paull’s data suggest that pingo-like features are growing in response to warming that started thousands of years ago.

Thus, their growth is not a result of human-induced global warming. However, Paull’s research does show that pingo-like features are still growing and releasing methane today.

Because methane is a potent greenhouse gas, climate scientists would like to know how
much is bubbling up from the seafloor worldwide. Future research on methane hydrates and pingo-like features may help address this question.

As Paull phrased it, “Pingo-like features are one of the places where we see methane coming up through the seafloor. As yet we don’t know how important they are, since we don’t know how much gas is coming up in the Arctic as a whole or in other seafloor areas.”

This study also provides scientists with clues to how buried methane hydrate deposits
might behave in other parts of the world in response to global warming.

According to Paull, “One of the questions we’re trying to answer is ‘What do buried hydrates do when they are suddenly warmed up?’ In this case, we have a field experiment that’s been going on for thousands of years.”