4.1. Marine Origin for PLFs
 Eight PLF sites were studied on the Beaufort Shelf, some more than a hundred kilometers distance from each other. The similarity of field observations, geological and geochemical data is striking and indicates that these features are forming in a similar manner.
 Because disseminated organic matter in the PLF sediments is distinctly older than that in the surrounding moat sediments, we conclude that the PLF sediments pre-date the latest marine transgression [Hill et al., 1993]. The presence of pristine marine bivalve shells in growth position and sediment ages that post-date the local transgression indicate that the moat sediments formed in a marine environment since transgression. The sediment age and PLF morphology are consistent with the PLFs being constructional features formed from material extruded from depth. The moats indicate local subsidence of the sea floor has formed small basins surrounding the PLFs that collect recent marine deposition (Figure 1).
4.2. Model for the Formation of PLFs
 A broader question is whether the formation of PLFs is related to thermal equilibration of the modern seafloor with warmer, transgressing Arctic Ocean water. The temperature and pressures within the terrestrial permafrost environment that existed on the Arctic Shelf prior to transgression indicate that gas hydrate was stable at burial depths as shallow as 150 m. Intra-permafrost gas hydrate (gas hydrate within ice-bonded permafrost) has been observed in Mackenzie Delta core samples at 451 m depth and inferred, on the basis of in situ gas concentrations, at depths as shallow as 119 m [Dallimore and Collett, 1995]. Sub-permafrost gas hydrate has also been observed in many offshore exploration wells [Weaver and Stewart, 1982].
 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).
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.
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 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 [Weaver and Stewart, 1982]. 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 [Dallimore et al., 1999; Lorenson et al., 1999].
 The settling of the moats surrounding PLFs is also consistent with the dissociation of a laterally extensive gas hydrate deposit at depth. The zone of dissociating gas hydrate would initially experience an increase in pressure. With time, local stresses may exceed the overburden strength, causing sediment failure and vertical migration of gas and sediment within a conduit beneath the nascent PLF. As gas and sediment move upward, venting and heaving of the sea floor is expected. At the same time, the relief of stress within and export of material from the area surrounding the conduit would promote consolidation and in turn subsidence of the sea floor to form a moat basin (Figure 2).
 The occurrence of vesicular-textured freshwater ice comprising up to 30% of the volume of the PLF sediments is consistent with this model. The void shape and spacing is similar to gas exsolution voids observed in deep-sea cores [Paull and Ussler, 2001]. We suggest this ice texture, which has not been described in the literature, is consistent with an initial gasified sediment texture and subsequent infilling of the gas voids by freshwater ice.
 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.
 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.
 The occurrence of freshwater ice within the shallow sediments sampled within the Beaufort Shelf is unique to the PLFs and not the surrounding sediments. This implies a subsurface source for the freshwater. Decomposition of gas hydrate within the permafrost would leave behind freshwater ice, because bottom water temperature data and thermal models indicate ground temperatures remain below the freezing point for freshwater for hundreds of meters below the seafloor. Thus, freshwater ice may be carried upwards in the extruded sediments. How water migrates into the gas voids and optically clear ice forms in the subsurface remains unclear.
 We propose that methane gas hydrate decomposition is a key factor in PLF formation (Figure 2). This process provides an explanation for the source of the observed venting methane, the uplifted, older freshwater-ice-bearing sediments on the PLF, and the existence of collapse depressions surrounding PLFs.