Offshore permafrost and gas hydrate stability zone on the shelf of East Siberian Seas

Abstract

Dynamics of the submarine permafrost regime, including distribution, thickness, and temporal evolution, was modeled for the Laptev and East Siberian Sea shelf zones. This work included simulation of the permafrost-related gas hydrate stability zone (GHSZ). Simulations were compared with field observations. Model sensitivity runs were performed using different boundary conditions, including a variety of geological conditions as well as two distinct geothermal heat flows (45 and 70 mW/m 2 ). The heat flows used are typical for the coastal lowlands of the Laptev Sea and East Siberian Sea. Use of two different geological deposits, that is, unconsolidated Cainozoic strata and solid bedrock, resulted in the significantly different magnitudes of permafrost thickness, a result of their different physical and thermal properties. Both parameters, the thickness of the submarine permafrost on the shelf and the related development of the GHSZ, were simulated for the last four glacial-eustatic cycles (400,000 years). The results show that the most recently formed permafrost is continuous to the 60-m isobath; at the greater depths of the outer part of the shelf it changes to discontinuous and ''patchy'' permafrost. However, model results suggest that the entire Arctic shelf is underlain by relic permafrost in a state stable enough for gas hydrates. Permafrost, as well as the GHSZ, is currently storing probable significant greenhouse gas sources, especially methane that has formed by the decomposition of gas hydrates at greater depth. During climate cooling and associated marine regression, permafrost aggradation takes place due to the low temperatures and the direct exposure of the shelf to the atmosphere. Permafrost degradation takes place during climate warming and marine transgression. However, the temperature of transgressing seawater in contact with the former terrestrial permafrost landscape remains below zero, ranging from À0.5 to À1.8°C, meaning permafrost degradation does not immediately occur. The submerged permafrost degrades slowly, undergoing a transformation in form from ice bonded terrestrial permafrost to ice bearing submarine permafrost that does not possess a temperature gradient. Finally the thickness of ice bearing permafrost decreases from its lower boundary due to the geothermal heat flow. The modeling indicated several other features. There exists a time lag between extreme states in climatic forcing and associated extreme states of permafrost thickness. For example, permafrost continued to degrade for up to 10,000 years following a temperature decline had begun after a climate optimum. Another result showed that the dynamic of permafrost thickness and the variation of the GHSZ are similar but not identical. For example, it can be shown that in recent time permafrost degradation has taken place at the outer part of the shelf whereas the GHSZ is stable or even thickening.

Defining the updip extent of the gas hydrate stability zone on continental margins with low geothermal gradients

“…The required time to affect bulge geometries is on the order of thousands of years. For example, regions of arctic permafrost that formed on continental margins exposed to the atmosphere during the last glacial period, <10 ka ago, are still only partially melted on submarine shelves that have flooded during the Holocene [e.g., Romanovskii et al, 2005]. Longer-period fluctuations in seafloor temperature, such as those on the timescale of glacial cycles or longer, will be able to overprint the effect of the HSZ bulge.…”

Preliminary Geospatial Analysis of Arctic Ocean Hydrocarbon Resources

“…The dark blue color in Figure 3.6 represents the highest potential for continuous methane gas hydrate occurrence, the lighter blue shades represent progressively less favorable conditions. Boundaries for the thick (~200 m) hydrate deposit (dark blue) are confined to water depths greater than 30m and less than or equal to 100 m. Romanovskii et al (2005) did not eliminate the 0-30m water depth zone. We have given this zone a lower hydrate occurrence probability to more appropriately categorize estuaries and near shore environments where the thermal regime may not favor gas hydrate stability.…”

“…The approach to mapping potential methane gas hydrate occurrence in the polar region is based on Wood and Jung (2008) as shown in Figure 3.4 and on bathymetry modeling adapted from Romanovskii et al (2005). Both Max and Lowrie (1993) and Wood and Jung (2008) directed their attention to nonpermafrost related factors controlling gas hydrate occurrence in the marine environment.…”

“…3.6 Romanovskii et al (2005) focused on the East Siberia Arctic Shelf (Figure 3.5), which has a flat bathymetry and relatively shallow water depths (not exceeding 100 m) and which can extend over 1000 km from the shoreline. By reconstructing the permafrost evolution over the last 400,000 years, these authors modeled the evolution of the gas hydrate stability zone as well.…”

“…Index map of investigation area (taken from Romanovskii et al, 2005) Building on the research presented by Romanovskii et al (2005), we developed a map of potential methane gas hydrate occurrence for the entire arctic shelf (Figure 3.6). The criteria were selected based on modeling results by Romanovskii et al (2005) that suggested the arctic shelf is underlain by relic permafrost stable enough to support gas hydrate formation. It should be noted that inflow of rivers (shown in Figure 3.6) and active tectonic zones, which would be dominant factors controlling hydrate stability, were not considered because of scale.…”