67 / 78
A GLOBAL OUTLOOK ON METHANE GAS HYDRATES
67
Two key questions are:
• What are the methane sources?
• How effective are the methane sinks that consume methane
before it reaches the atmosphere?
Knowing the sources can reveal whether the system has been
steadily releasing methane at these rates in response to long-term
climate change and/or whether the methane release rates are likely
to accelerate as the system responds to short-term warming.
One possible source is methane brought in by the six largest
Eurasian rivers, although Shakhova
et al.
(2010a) suggest most
of the riverine methane is oxidized in the rivers prior to reaching
the ESAS. Given the geologic history of the ESAS, it is more likely
that methane is coming out of the ESAS sediment (Fig. TB-3.2.2).
The sediment drape on the ESAS is organic-rich (Vetrov and
Romankevich 2004; Shakhova
et al.
2010a). The upper layers were
frozen as permafrost until increasing sea levels, starting 7 000
to 15 000 years ago, flooded the region (Shakhova
et al.
2010b)
and raised the ground-surface temperature above freezing. The
permafrost has been thawing ever since as heat and salt from
overlying sea water penetrate deeper into the sediment. Shakhova
et al.
(2010a) summarize four methane sources in this thawing,
organic-rich system:
1. Methane can be produced via microbial breakdown of organic
material in the shallow, modern ESAS sediment, which was
never frozen.
2. As permafrost thaws, the newly unfrozen, older organic material
also becomes available for microbes to consume, producing
methane as a by-product of that consumption.
3. Gas hydrates, thought to exist across a significant portion of the
ESAS (Soloviev 2002; Shakhova
et al.
2010a), may be dissociating
and releasing methane in response to heat transferred down
from the sea floor.
4. Methane may be leaking up through the thinning or thawed
permafrost from a deeper petroleum system.
The present-day methane release from ESAS sediments is
thought to be occurring in response to long-term sediment
warming resulting from seawater flooding the ESAS region
(Shakhova
et al.
2010a), rather than to recent atmospheric
warming trends. However, it is not yet certain which sources
contribute to the observed seawater-methane concentrations.
Methane consumption efficiency, the combined removal of
methane due to dissolution and to microbial processes in the
soil and water column (see Volume 1 Chapter 2), is also not well-
constrained in the ESAS region. Quantifying methane sources
and sinks remains a requirement for establishing the long-term
climatic impact of methane released to the atmosphere.
Figure TB-3.2.2:
Methane plumes in the East Siberian Arctic
Shelf (ESAS). The extremely shallow ESAS environment allows
gas-bubble plumes to reach the water surface, facilitating the
transfer ofmethane fromthe sediment to the atmosphere (Image
courtesy of I. Semiletov, unpublished data from cruise-2011).