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Friday, January 6, 2012

Comparison of subsea hydrate stability between paleo events at Barrow, Alaska, and the East Siberian Arctic Shelf now

Gas Hydrate Stability Zone Dynamics and Global Climate Change

V. E. Romanovsky and T. E. Osterkamp, Department of Geology & Geophysics, Geophysical Institute

The results of investigations of ice cores from Greenland and Antarctica, deep-sea sediment cores, as well as other paleoclimate investigations, show that, during the Pleistocene-Holocene time, significant changes in global temperature and greenhouse gas content in the atmosphere occurred. In general, such changes were associated with ice age rhythms. Preliminary analysis of these data shows that the changes in greenhouse gas contents did not always lead the corresponding global temperature changes. Moreover, the global temperature changes preceded the greenhouse gas concentration variations. Certain connections exist between global sea level changes and greenhouse gas concentrations in the atmosphere. Although the data about sea level changes are more reliable only for the last 20-25 thousand years, it can be noted that in most cases the sea level changes lead the greenhouse gas concentration changes.

The key to understanding the relationship between sea level variations and the change in concentration of greenhouse gases may be the gas hydrate stability zone and subsea permafrost dynamics within the arctic shelves. During the ice ages of the Pleistocene, these shelves were emergent and subjected to deep freezing several times. Tremendous amounts of gas, particularly methane, were trapped by the permafrost layer within the shelves. The thermodynamic conditions in this case were proper for thick layers (up to 600-1,000 m) of gas hydrates. The intensity of gas hydrate created within the subsea sediments varies between sites. This depends on the organic substance content in the sediments and on thermodynamic conditions. According to the most common estimations, the total volume of carbon in the form of gas hydrates in the ocean sediments is 10,000 to 11,000 Gt.

Figure 1. (below) 15,000 years ago. The shelf at Barrow was exposed and frozen. The hydrate stability zone was largest.
Fig. 1

Figure 2. (below) 4,000 years ago. Subsea permafrost and hydrate stability zone continued to degrade. The free gas at 65-80 km offshore could have been released to the atmosphere.
Fig. 2
Fig. 3
Figure 3. (above) Present time. Subsea permafrost at Barrow exists 8-10 km offshore. The contribution of greenhouse gases to the atmosphere from hydrate destabilization during the last 3,000 years was much less than 3,000-4,000 years ago, assuming rapid transport of the gases to the seabed.

There are three main areas of gas hydrate occurrences: the region of continuous onshore permafrost, the region of subsea permafrost within the arctic shelves, and the oceanic region of the outer continental margin. The first and the third regions are currently characterized by fairly stable conditions. Due to gas hydrate stability zone dynamics, changes in the concentration of greenhouse gases can be initiated only in the region of continental shelves with gas hydrates.

The extensive Russian arctic shelves play an especially important role because of their large area and usual shallow sea depth. Sea level changes and history of climatic variations during the Late Pleistocene through Holocene determine the subsea permafrost existence and dynamics. During the ice ages, low sea levels exposed much of the vast Siberian shelf to cold air temperatures which allowed permafrost to aggrade in these shelves. Temperature and pressure values associated with these conditions are favorable for the formation of gas hydrates. These shelves have now been submerged and the cold surface boundary condition has been replaced by a warm one. Consequently, the permafrost in these shelves is now degrading and warmer temperatures have made the gas hydrates unstable. When the permafrost disappears, the gas from the decomposed hydrates can pass through the sediments and water to the atmosphere creating a significant positive feedback loop to enhance climatic warming.

Estimates of the potential gas fluxes to the atmosphere and the timing of these gas fluxes require detailed information on the dynamics of thawing subsea permafrost and gas hydrate decomposition. A numerical model was used to investigate the potential stability fields of the gas hydrates associated with subsea permafrost.

To estimate the dynamics of possible methane release to the atmosphere, curves for the volume of destabilized sediments in time were constructed for Cape Thompson and Barrow. These curves showed extremely sharp peaks, which means that most of the gas could have been released into the atmosphere during a short period of time (less than one millennium). The timing of these events was different for Cape Thompson (1 ka ago) and Barrow (3 ka ago).

These calculations can, with proper environmental and paleogeographical conditions, be used to estimate permafrost and gas hydrate equilibrium zone dynamics within the continental shelves of Siberia and North America.

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