Malin Waage et al,
Geological controls
of giant crater development
on the Arctic seafloor.
2020
Scientific Reports
DOI: 10.1038/s41598-020-65018-9
SUMMARY:
The craters are up to
one kilometer wide
and 35 meters deep.
They are leaking methane.
Exploration of petroleum
resources in the Barents Sea
is a hot topic in Norway.
The area is a part of
a vulnerable Arctic
ecosystem.
But the area's geological
system is poorly understood.
A recent study
in Scientific Reports
looked beneath these
craters in the ocean floor
and revealed the
geological structures.
"It turns out that this area
has a very old fault system—
essentially, cracks in bedrock
that likely formed 250 million
years ago," says Malin Waage,
a postdoc at CAGE, Centre for
Arctic Gas Hydrate, Environment
and Climate, and the first author
of the study.
The deep origin of craters
and mounds was discovered
using cutting edge 3-D
seismic technology that can
penetrate deep into the
ocean floor and help scientists
visualize the structures
in the hard bedrock
underneath.
250-million-year-old cracks
in the seafloor feed greenhouse
gas methane into giant craters
in the Barents Sea.
More than 100 craters
are found in the area.
"Our 3-D survey covered
approximately 20 percent
of the entire crater area.
We believe that it is important
to understand if similar fault
systems exist in the larger
context of the Barents Sea,
because they potentially
could pose a threat
to marine operations."
"There is still very much
that we don't know about
this system.
But we are currently
collecting and
analyzing
new data in the
Barents Sea,
which is
dominated
by similar
crater structures.
This can help us map
in greater detail the
fault systems and
associated
weakness,"
says Waage.
The oil industry
needs to know:
(1)
Will these weak structures
lead to unpredictable and
explosive methane release ?
(2)
Can such a release
be triggered by
drilling for oil. ?
(3)
Will the methane gas
reach the atmosphere
and cause greenhouse
warming ?
DETAILS:
A 2017 study, in PNAS,
mapped several methane
mounds, some 500 meters
wide, in the Barents Sea.
The mounds
were considered
to be signs of
impending methane
expulsions that
created the craters.
Another 2017 study,
published in Science,
described hundreds
of massive, kilometer-
wide craters on the
ocean floor in the
Barents Sea.
Now, more than 600
gas flares were identified
in and around these craters,
releasing the greenhouse
gas steadily into the water.
Malin Waage also said:
"Craters and mounds appear
along different fault structures
in this system.
These structures control
the size, placement and
shape of the craters.
The methane that is leaking
through the seafloor originates
from these deep structures
and is coming up through t
hese cracks."
"Our previous studies
in the area hypothesized
that climate warming
and the retreat of
the ice sheet some
20,000 years ago caused
the gas hydrates beneath
the ice to melt, leading to
abrupt methane release
and creating craters,"
said Waage.
Gas hydrates are a
solid form of methane
that is stable in
the cold temperatures
and high pressure
that an enormous
ice sheet provides.
As the ocean warmed up,
and the pressure of the ice
sheet lifted, the methane ice
in the seafloor melted,
and the craters formed.
"This study, however,
adds several layers
to that picture,
as we now see
that there has been
a structural weakness
beneath these giant
craters for much longer
than the last 20,000 years.
Deep below the seafloor,
the expansion of gas
and release of water
built up a muddy slurry
that eventually erupted
through the fractures
and caused seafloor
collapses and craters
in the hard bedrock.
Think of it as a building:
The roof of a building
can cave in if the ground
structure is weak.
We believe that this
is what happened
in the crater area after
the last glaciation,"
says Waage.