Paull, Charles K.; Ussler, William, III; Dallimore, Scott R.; Blasco, Steve M.; Lorenson, Thomas D.; Melling, Humfrey; Medioli, Barbara E.; Nixon, F. Mark; McLaughlin, Fiona A.
<1> The Arctic shelf is currently undergoing dramatic thermal changes caused by the continued warming associated with Holocene sea level rise. During this transgression, comparatively warm waters have flooded over cold permafrost areas of the Arctic Shelf. A thermal pulse of more than 10°C is still propagating down into the submerged sediment and may be decomposing gas hydrate as well as permafrost. A search for gas venting on the Arctic seafloor focused on pingo-like-features (PLFs) on the Beaufort Sea Shelf because they may be a direct consequence of gas hydrate decomposition at depth. Vibracores collected from eight PLFs had systematically elevated methane concentrations. ROV observations revealed streams of methane-rich gas bubbles coming from the crests of PLFs. We offer a scenario of how PLFs may be growing offshore as a result of gas pressure associated with gas hydrate decomposition.
EDIT
<3> This paper focuses on evidence of degassing from decomposing Arctic gas hydrate deposits beneath the southern Beaufort Shelf. This area is arguably one of the most gas-hydrate-rich regions in the Arctic
. During the Pleistocene much of the Beaufort Shelf was an emergent, unglaciated coastal plain exposed to very cold mean annual surface temperatures , which aggraded thick permafrost. The geothermal regime is conditioned by this permafrost interval and as a result methane hydrate may be theoretically stable to depths as great as 1500 m . Marine transgression during the Holocene has caused much of the Beaufort Shelf to be inundated by relatively warm marine and estuarine waters. This has imposed a step change from mean annual land surface temperatures perhaps as low as −20°C during glacial times to present bottom water temperatures which are no colder than −1.8°C. The consequence of this warming is to reduce the vertical extent of both permafrost and the stability zone in which gas-hydrate-bearing sediments may occur. Because of the slow process of heat conduction, the transient effects of the transgression are still occurring . Thus, methane gas release, if it is occurring, should be continuing today.
EDIT
Figure 1. (top) A strip of multibeam data, core locations (open circles), and (bottom) a seismic profile (A-A′) associated with the Kaglulik PLF. Note the PLF is surrounded by a collapse moat (M) filled with layered sediments containing normal faults (F) in Figure 1 (bottom). General location of the study area is indicated with the yellow box within the regional map in Figure 1 (top, left inset). A map of the Beaufort Sea Shelf shows locations of vibracoring sites in Figure 1 (top, right inset). Dots indicate PLFs at which multiple cores were taken, crosses indicate background shelf cores, and the red box outlines the area covered with more detailed map of Kaglulik PLF. Enhanced TIF <7.5 MB>
Figure 2. Schematic drawing outlining PLF and moat formation (M) associated with gas hydrate decomposition. (a) Cross-section of the permafrost-bearing Arctic seafloor (SF) (previously <−10°C) after being transgressed by Arctic Ocean water (<−1°C). As the subsurface warms, the top of the gas hydrate stability zone will move downward. Warming results in gas hydrate decomposition in a gradually thickening zone (brown), releasing gaseous methane into the sediments (yellow). Bubble formation associated with this phase change will create overpressured conditions. (b) Shows how material may flow (red arrows) both laterally and vertically in response to overpressure. Displaced sediments rise upward to form the PLF and allow the gas to vent (VG). As the pressure is dissipated through both the transfer of solids and degassing, subsidence in the area immediately surrounding the PLF (black arrows) creates the moat. Enhanced TIF <9.9 MB>
<18> We propose that gas release and bubble formation associated with decomposing gas hydrates at depth causes expansion of the sediment matrix that drives the upward extrusion of sediment to form the PLFs. Decomposition of intra-permafrost methane hydrate can supply substantial quantities of methane gas that generate large localized over-pressures. At the pressure and temperature conditions at the top of the gas hydrate stability field, gas hydrate will decompose into water ice and gas. Because ice has essentially the same density as gas hydrate, any gas released during decomposition will create gas expansion voids and create local over pressures. Substantial overpressures will not be maintained because they will exceed the mechanical strength of shallow sediments. As pressures build within subsurface horizons, gas is forced through weaknesses in the overlying permafrost layers (Figure 2). Extruded material builds up on the seafloor to form the PLF. The observed amount of vertical displacement of the PLFs implies that material moves laterally within the over-pressured horizons to these zones of weakness, then upward to the seafloor. The source of the displaced material and pressure to drive the vertical expansion may extend over a much larger area than the PLF itself. As sediment migration and gas venting proceeds, subsurface volume losses ultimately result in the collapse and formation of moat basins around the sites of sediment expulsion (Figure 2).
<19> Several lines of evidence suggest that these processes may be operative in the formation of Beaufort Sea PLFs. Elevated formation pressures, up to 1.6 times hydrostatic conditions, have been measured in several offshore exploration wells, including the Kopanoar PLF site where sub-permafrost gas hydrate has been documented . Venting of gas at PLF summits has been observed in video footage from ROV dives. High methane concentration and a rapid decrease in sulfate concentration in cores from PLF crests, contrasting with the absence of these features in moat and background sites, suggest a focused methane flux occurs through the PLF from a gas source at depth. The molecular composition and carbon isotope signature indicate that the venting gas is microbial and derived from pre-Holocene carbon sources. Gas with similar chemistry occurs within the permafrost interval above deeper gas hydrate deposits in the Mackenzie Delta .
No link, subscription service.
Printer-friendly format
Email this thread to a friend
Bookmark this thread
(1000+ posts)
Send PM |
Profile |
Ignore
