Reply #73: AGU - Origin Of Pingo-Like Features On The Beaufort Sea Shelf (Re. Methane Hydrates) [View All]

[hatrack]

Origin of pingo-like features on the Beaufort Sea shelf and their possible relationship to decomposing methane gas hydrates

Charles K. Paull and William Ussler III

Monterey Bay Aquarium Research Institute,
Moss Landing, California, USA

Scott R. Dallimore

Natural Resources Canada,
Sidney, British Columbia, Canada

Steve M. Blasco

Natural Resources Canada,
Dartmouth, Nova Scotia, Canada

Thomas D. Lorenson

U.S. Geological Survey,
Menlo Park, California, USA

Humfrey Melling

Fisheries and Oceans Canada,
Sidney, British Columbia, Canada

Barbara E. Medioli and F. Mark Nixon

Natural Resources Canada,
Ottawa, Ontario, Canada

Fiona A. McLaughlin

Fisheries and Oceans Canada,
Sidney, British Columbia, Canada

Abstract

<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


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).

EDIT

<22> Upon warming caused by transgression, dissociation of intra-permafrost gas hydrate would first occur at the top of the methane hydrate stability field at temperatures substantially less than zero degrees Celsius. In the environment where the gas hydrate is dissociating, decomposing gas hydrate, free gas, and freshwater ice co-exist. For liquid water to occur immediately above the gas hydrate stability zone, substantial quantities of salt or other physical-chemical inhibitors are required. The occurrence of freshwater ice in the PLFs argues against the existence of brines in these sediments.

<23> Industry coring has confirmed that at Admirals Finger PLF, high ground ice contents extend to at least 40 m below the surface. With 30% volumetric ice fraction, the freezing of ground water within a gasified sediment fabric can account for approximately 12 m of heave at the sea floor. Because the relief of many PLFs is more than 12 m, additional material movement is needed to satisfy mass balance and the age of the material.

No link, subscription service