FROZEN HEAT | Volume 1

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).

A GLOBAL OUTLOOK ON METHANE GAS HYDRATES 67

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