Methane is
stored in permafrost in the form of methane hydrates, also known as klathrates.
Methane stored in Siberian permafrost is mostly of biogenic origin due to the burial of organic matter but sometimes there is some mixing with deeper thermogenic
methane.
Unexplained
craters first appeared on the permafrost-covered Yamala Peninsula in Siberia in
2014. Methane outgassing offshore of the Yamal Peninsula also occurs. I wrote about
this in my 2022 book Natural Gas and Decarbonization from which the next
section is derived, in order to give some context and to show some of the modeling
and mechanisms for methane seeps on the West Yamal shelf in shallow waters close
to the peninsula.
Methane Seeps
There are methane seeps in many places in the world. Methane is trapped
in large quantities in klathrates, or gas hydrates, on the ocean floor and in
the permafrost of the Arctic tundra. Some seeps are long-established and ongoing,
and some are newly formed. Offshore the Yamal peninsula in Siberia the
permafrost is thawing due partially to warming ocean temperatures but mostly
due to geothermal heat flux where heat from within the earth is melting
it. In the Kara Sea the permafrost extends to below the ocean where during the
last glacial maximum sea level was far lower and the land extended out from the
sea. The permafrost thickness on land is up to a half mile but is thinner in
the sea now where both warmer ocean temperatures and especially geothermal heat
flux are thawing it. It is leaking in a small band where the water is shallow,
not deep enough to hold it in with hydrostatic pressure. Gas hydrates on the
ocean floor are held and sealed by the hydrostatic pressure of the water column
but methane in permafrost under the ocean nearer to shore with a thinner water
column is sealed more by the cold temperatures of the permafrost and may leak.
The accelerated climate change temperature increase response of the Arctic can
potentially make the submerged permafrost thaw faster, releasing more methane
in these areas. On land in Yamal peninsula in Siberia are craters that have
sunk as sinkholes that are releasing methane from the permafrost in some
places.[1]
In some places sea level rise that newly covers tundra that contains
klathrates could lead to them being released into the atmosphere, but the quantity
is not expected to be significant. One reason is that much of the methane
released to the ocean is oxidized, often by microbes. One estimate is that only
about 1% of dissolved methane in the ocean makes it to the atmosphere. There is
some concern that human endeavors that dig into marine sediments like pipelines
and undersea cables might disturb them enough to release klathrates but nothing
like this has been observed. Since the areas where oceanic klathrates occur are
vast it is not possible to quantify in great detail but perhaps the satellites
designed for methane detection can be able to detect new sources or increases
in outgassing. The USSR, Canada, the US and more recently Japan and India have
done some research projects to test recovering of methane klathrates through
drilling. However, there are technical issues that make the costs of recovery
uneconomic. Before fracking unleased vast new quantities of natural gas, the
research into klathrates was more poignant perhaps than today.[2]
Recently, in Antarctica, scientists have gotten the opportunity to see a
new undersea methane seep forming which may aid in understanding these
phenomena. It is the first active seep found in Antarctica. A microbial mat on
the sea floor below the frozen ocean revealed the presence of the seep. The
microbes found there were not the same kind as found at seeps in other
locations which suggested to some researchers that there may be a succession
pattern of microbes that inhabit seeps.[3]
New Model Explains Methane Crater-Forming Explosions
on the Yamal Peninsula
Since 2014
methane craters have been found on the Yamal Peninsula and the nearby Gydan Peninsula
in the Siberian Arctic. The big crater found in 204 measures approximately 70
meters (230 feet) across at its widest point. Clearly, something ne is causing
this and that something very likely is associated with warming permafrost
influenced by anthropogenic global warming, which has been has occurred more
strongly in the Arctic region (as predicted by models) in a phenomenon known as
‘Arctic acceleration.’ The new research offers the most detailed modeling yet
and proposal of the mechanism of these explosive methane ejections into the
atmosphere. The authors found that permafrost warming alone would not be enough
to produce these explosions or “blowouts” and that explanations based on
contact with deeper thermogenic natural gas accumulations are not plausible due
to the gas being overwhelmingly of biogenic origin.
The authors
propose that the melting permafrost causes significant amounts of meltwater
below the surface that flows down via a process called osmosis into lenses of
highly saline water kept in a liquid state by pressure, known as cryopegs. The
meltwater increases the pressure within the cryopeg until it has enough pressure
to fracture the frozen soil above and form a crater. The gas trapped just below
it is then released. This is a purely physical process, they say, since they
ruled out any chemical reaction process to account for the blowouts. According
to Phys.org:
“The Yamal Peninsula's thick, clayey permafrost acts
as an osmotic barrier—and warming is changing it. This 180 to 300-meter (590 to
980-foot)-thick layer stays permanently frozen throughout the year. An
"active layer" of topsoil above it thaws and re-freezes seasonally.”
“Interspersed throughout the tundra and sandwiched
within the permafrost lie unusual, one-meter-thick layers of unfrozen,
high-salinity water called cryopegs, kept liquid by a combination of pressure
and salinity. Underneath the cryopegs sits a layer of crystallized
methane-water solids, called methane hydrates, which are kept stable by high
pressure and low temperature.”
The image below seems to show the first two parts of the process. Part 3 is probably the meltwater reaching the cryopeg and part 4 is the pressure blowout/explosion.
Osmotic pressure from the pressure differential between
the two liquids makes the meltwater flow down toward the cryopegs. When the
meltwater reaches the cryopegs it increases the pressure until the ground above
fractures. It is similar to the process of hydraulic fracturing where the
fracture gradient is overcome by increasing hydraulic pressure to cause the
cracking of nearby rock or soil. Essentially, osmosis is pressure pumping into
the cryopeg until it exceeds the fracture gradient of the overlying sediments.
“The increasing pressure creates cracks in the soil that
travel upward from the cryopeg toward the surface. The pressure gradient then
reverses: the cracked soil causes a sudden drop in pressure at depth. That
pressure change damages the methane hydrates below the cryopeg, which causes a
release of methane gas and a physical explosion.”
A 2019 study of
gas hydrates in permafrost compared the shear strength and deformation styles
of unfrozen hydrate-free, frozen hydrate-free, unfrozen hydrate-bearing, and
frozen hydrate-bearing sediments. The presence of hydrates and whether it was
frozen or not was found to strongly affect how these sediments deform under
stress, in this case, high pressure.
“The shear characteristics and deformation behavior of
four types of artificial sediments were investigated at different conditions,
including unfrozen hydrate-free, frozen hydrate-free, unfrozen hydrate-bearing,
and frozen hydrate-bearing sediments. Results show that ice and gas hydrates
distinctively affect the shearing characteristics and deformation behavior of
the specimens, though they are both water-based crystalline solids.”
“Methane hydrate plays a dominant role in the
geomechanical properties of the simulated permafrost sediments.”
The blowouts
release a mix of gas and water. Pockmarked Arctic lake bottoms on the Yamal Peninsula
are thought to be craters formed by similar processes. A past period of Arctic warming
has been hypothesized as a reason for their formation. It has also been noted
that gas sometimes leaks around wellbores drilled into permafrost. This was in
the past thought to be from oil and gas reservoirs in rocks far below leaking
up but no it seems much more likely that this is from gas hydrates trapped
within the permafrost, presumably below these cryopegs. A 2018 paper in Cold
Regions Science and Technology noted that the type of sediment, its
composition, and grain size have an effect on the sensitivity of the hydrates to
temperature changes. The hydrates decompose at lower temperatures in clay
sediments than in sandy sediments.
“Intrapermafrost metastable hydrates can exist in salted
clay sediments if ice coating formation around hydrates during
self-preservation process is possible. But unlike in intrapermafrost sandy
sediments metastable hydrates in salted clay sediments decompose at much lower
temperatures (in the experiment conducted that was −6.75–−6.57 °C), than
in sandy sediments.”
The new research implicating
osmotic pressure as a mechanism for increasing cryopeg pressure also suggests a
similar mechanism for methane releases on the shelf offshore of the Yamala
Peninsula. The authors also suggest what future research should aim to clarify:
“Further work may explore the typical volume of gas that
is released in these explosions, and their potential height into the
atmosphere. Furthermore, it is relevant to assess the number of currently
existing cryopegs. The model may also be verified by checking if the explosions
occur most often during or just after summer.”
The researchers also note that this area has a very specific
set of geological circumstances that re conducive to these blowout craters, not
likely to be widely replicated. Even so, the cryopegs should be mapped and the
volume of potentially releasable methane associated with them should be
determined since it is thought that the total volume would be enough to have a significant
impact on global warming.
References:
Study
offers new explanation for Siberia's permafrost craters. Science X staff.
Phys.org. September 26, 2024. Study
offers new explanation for Siberia's permafrost craters (msn.com)
Osmosis
Drives Explosions and Methane Release in Siberian Permafrost. Ana M. O.
Morgado, Luis A. M. Rocha, Julyan H. E. Cartwright, and Silvana S. S. Cardoso. American
Geophysical Union. Geophysical Research Letters. First published: 26 September
2024. Osmosis
Drives Explosions and Methane Release in Siberian Permafrost - Morgado - 2024 -
Geophysical Research Letters - Wiley Online Library
Experimental
modeling of methane release from intrapermafrost relic gas hydrates when
sediment temperature change. V.S. Yakushev, A.P. Semenov, V.I. Bogoyavlensky, V.I.
Medvedev, and I.V. Bogoyavlensky. Cold
Regions Science and Technology. Volume 149, May 2018, Pages 46-50.
Experimental
modeling of methane release from intrapermafrost relic gas hydrates when
sediment temperature change - ScienceDirect
Gas
Hydrates in Permafrost: Distinctive Effect of Gas Hydrates and Ice on the
Geomechanical Properties of Simulated Hydrate-Bearing Permafrost Sediments. J.
Yang, A. Hassanpouryouzband, B. Tohidi, E. Chuvilin, B. Bukhanov, V. Istomin,
A. Cheremisin. JGR Solid Earth. Volume124, Issue3. March 2019. Pages 2551-2563.
Gas
Hydrates in Permafrost: Distinctive Effect of Gas Hydrates and Ice on the
Geomechanical Properties of Simulated Hydrate‐Bearing Permafrost Sediments -
Yang - 2019 - Journal of Geophysical Research: Solid Earth - Wiley Online
Library
[1] Portnov, Alexey, Mienert, Jurgen, and
Serov, Pavel, October 2014. Modeling the evolution of climate-sensitive Arctic
subsea permafrost in regions of extensive gas expulsion at the West Yamal
shelf. JGR Biogeosciences. Modeling the evolution of
climate‐sensitive Arctic subsea permafrost in regions of extensive gas
expulsion at the West Yamal shelf - Portnov - 2014 - Journal of Geophysical
Research: Biogeosciences - Wiley Online Library
[2]
Barry, Justin P., 2008. Deep Ocean Methane Clathrates: An Important New Source
for Energy? A Thesis in Chemistry Education Presented to the Faculty of the
University of Pennsylvania in partial fulfillment of the requirement of the
Degree of Master of Chemistry Education At University of Pennsylvania 2008. Microsoft
Word - Justin Barry-Thesis-Methane Clathrates-Final Copy.doc (upenn.edu)
[3]
Sakharkar, Ashwini, July 23, 2020. First active Methane Seep discovered in
Antarctica. Tech Explorist. First
active Methane Seep discovered in Antarctica - Tech Explorist
No comments:
Post a Comment