Book 2.indb - US Climate Change Science Program

Abrupt Climate Changerelease is large enough, the rising column ofseawater in contact with the bubbles may saturatewith methane, or the bubbles can be larger,potentially increasing the escape efficiency tothe atmosphere.4.1.5 Fate of Methane Hydrate in theWater ColumnPure methane hydrate is buoyant in seawater, sofloating hydrate is another source of methanedelivery from the sediment to the atmosphere(Brewer et al., 2002). In sandy sediment, thehydrate tends to fill the existing pore structureof the sediment, potentially entraining sufficientsediment to prevent the hydrate/sedimentmixture from floating, while in fine-grainedsediments, bubbles and hydrate grow by fracturingthe cohesion of the sediment, resulting inirregular blobs of bubbles (Gardiner et al., 2003;Boudreau et al., 2005) or pure hydrate. Breweret al. (2002) and Paull et al. (2003) stirredsurface sediments from Hydrate Ridge usingthe mechanical arm of a submersible remotelyoperated vehicle and found that hydrate didmanage to shed its sediment load enough tofloat. Hydrate pieces of 0.1 m survived a 750-mascent through the water column. Paull et al.(2003) described a scenario for a submarinelandslide in which the hydrates would graduallymake their way free of the turbidity currentcomprised of the sediment and seawater slurry.4.1.6 Fate of Dissolved Methane in theWater ColumnMethane is unstable to bacterial oxidation inoxic seawater. Rehder et al. (1999) inferred amethane oxidation lifetime in the high-latitudeNorth Atlantic of 50 years. Methane oxidationis faster in the deep ocean near a particularmethane source, where its concentration ishigher (turnover time 1.5 years), than it is in thesurface ocean (turnover time of decades) (Valentineet al., 2001). Water-column concentrationand isotopic measurements indicate completewater-column oxidation of the released methaneat Hydrate Ridge (Grant and Whiticar, 2002;Heeschen et al., 2005).An oxidation lifetime of 50 years leaves plentyof time for transport of methane gas to theatmosphere. Typical gas-exchange time scalesfor gas evasion from the surface ocean wouldbe about 3–5 m per day. A surface mixed layer100 m deep would approach equilibrium (degas)in about a month. Even a 1,000-m-thick wintermixed layer would degas about 30% during a3-month winter window. The ventilation timeof subsurface waters depends on the depth andthe fluid trajectories in the water (Luyten etal., 1983), but 50 years is enough time that asignificant fraction of the dissolved methanefrom bubbles might reach the atmosphere beforeit is oxidized.4.2 Geologic Data Relevant to PastHydrate Release4.2.1 The Storegga LandslideOne of the largest exposed submarine landslidesin the ocean is the Storegga Landslide in theNorwegian continental margin (Mienert etal., 2000, 2005; Bryn et al., 2005). The slideexcavated on average the top 250 m of sedimentover a swath hundreds of kilometers wide,stretching halfway from Norway to Greenland(Fig. 5.10). There have been comparable slideson the Norwegian margin every approximately100 kyr, roughly synchronous with the glacialcycles (Solheim et al., 2005). The last one,Storegga proper, occurred about 8,150 yearsago, after deglaciation. It generated a tsunamiin what is now the United Kingdom (D’Hondtet al., 2004; Smith et al., 2004). The Storeggaslide area contains methane hydrate depositsas indicated by a bottom simulating seismicreflector (BSR) (Bunz and Mienert, 2004;Mienert et al., 2005; Zillmer et al., 2005a,b)corresponding to the base of the hydrate stabilityzone (HSZ) at 200–300 m, and pockmarksPure methanehydrate is buoyant inseawater, so floatinghydrate is anothersource of methanedelivery from thesediment to theatmosphere.183