February
6, 2007
METHANE
BUBBLING THROUGH SEAFLOOR CREATES UNDERSEA HILLS
According
to a recent paper
published by MBARI geologists and their colleagues, methane gas
bubbling
through seafloor sediments has created hundreds of low hills on the
floor of
the Arctic Ocean.
These enigmatic features,
which can grow up to 40 meters (130 feet) tall and several hundred
meters
across, have puzzled scientists ever since they were first discovered
in the
1940s.
Writing
in the January issue of Geophysical
Research Letters, MBARI geologists Charlie Paull and William
Ussler and
their coauthors described the results of field work they conducted on
the Beaufort Sea Shelf, offshore of the north coast
of Canada.
In this area of year-round sea ice and permafrost, the team spent over
a month
mapping the seafloor and collecting sediment cores and gas samples from
these
underwater hills, which they call "pingo-like features."
"Pingos,"
small,
dome-shaped, ice-cored hills, are found in many Arctic regions.
"Pingo-like features" are similar in shape and size to pingos on
land, but are found underwater, on the continental shelf in several
parts of
the Arctic.
Previous studies have suggested
that pingo-like features are pingos that formed on land but were
submerged when
sea level rose following the end of the last ice age, over 10,000 years
ago.
Based
on their geologic
fieldwork and subsequent chemical analysis of the gas and sediments
from eight
pingo-like features, Paull and his coauthors propose an alternative
hypothesis:
Pingo-like features form when methane hydrate (a frozen mixture of gas
and
seawater) decomposes beneath the seafloor, releasing gas that squeezes
deep
sediments up onto the seafloor like toothpaste from a tube.
The
geologists based this
hypothesis on a number of observations and measurements. First, sound
waves
bounced through the pingo-like features showed that they were not built
up from
layers, but consist of a jumbled mixture of sediment and small nodules
of
fresh-water (rather than salt-water) ice. Carbon-14 dating of organic
matter in
the sediment at the crests of several hills showed that this sediment
was
deposited before the last ice age, thousands of years before sediments
on the
surrounding seafloor. Finally, many of the pingo-like features were
surrounded
by shallow "moats," where the seafloor within a kilometer of the hill
had apparently subsided.
Even
with evidence that
pingo-like features were made of older, deeper sediment that had been
pushed up
from beneath the seafloor, the geologists still had to figure out what
geologic
process could generate enough pressure to lift seafloor sediments. The
most
obvious source of such pressure was methane gas, which the researchers
observed
bubbling out of the tops of several pingo-like features.
After
chemically analyzing this
gas, the researchers concluded that it originated as methane hydrate,
an
ice-like mixture of water and methane that forms within sediments under
much of
the Arctic seafloor and beneath permafrost areas on land. Methane
hydrate can
only remain solid at low temperatures and high pressures. Such
conditions exist
several hundred meters below the seafloor in this part of the Arctic Ocean.
The
researchers suggested that
such buried hydrates might be decomposing and releasing large amounts
of
methane gas. This seemed possible because the seafloor in this area has
been
gradually warming over the last 10,000 years, after being flooded as
sea levels
rose at the end of the last ice age. Although within a few degrees of
freezing,
the seawater in this region is at least 10 degrees Centigrade (20
degrees
Fahrenheit) warmer than permafrost-filled soil. Thus, when the ice
sheets from
the last ice age melted and the ocean flooded the continental shelves,
it
caused the seafloor sediment to become warmer.
Over thousands of
years, the
scientists believe, this "wave" of warming moved downward through the
sediment. Eventually it reached the frozen methane hydrates, hundreds
of meters
down. Even a slight temperature increase could have caused some of the
buried
methane hydrates to decompose, releasing methane into the surrounding
sediments.
Paull and Ussler's
data suggest
that this newly released methane migrated sideways under the seafloor,
held in
place by an impermeable layer of frozen soil that lies between the
hydrates and
the seafloor. Eventually it collected and moved toward the surface
along faults
or in other areas where the sediments were relatively weak.
Eventually
the extruded sediment
collected to form the low undersea hills visible on bathymetric charts.
At the
same time, areas on either side of the mounds, where much of the gas
and
sediment originated, slowly collapsed, forming the deeper "moats"
observed by the researchers.
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."
##
Contact:
Kim
Fulton-Bennett
Monterey Bay Aquarium
Research Institute
831-775-1835
kfb@mbari.org
This
text
derived from:
http://www.mbari.org/news/news_releases/2007/paull-plfs.html
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