Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science Program Chapter 5Methane releasedfrom sediments inthe ocean may reachthe atmospheredirectly, or it maydissolve in theocean.chunks MacDonald et al. saw had vanishedwhen they returned a year later; presumably ithad detached and floated away.Collett and Kuuskraa (1998) estimate that500 GtC might reside as hydrates in the gulfsediments, but Milkov (2004) estimates only5 GtC. The equilibrium temperature changein the deep ocean to a large, 5,000-GtC fossilfuel release could be 3 °C (Archer et al., 2004).Milkov and Sassen (2003) subjected a twodimensionalmodel of the hydrate deposits inthe Gulf to a 4 °C temperature increase andpredicted that 2 GtC from hydrate would melt.However, there are no observations to suggestthat methane emission rates are currently accelerating,and temperature changes in Gulf ofMexico deep waters in the next 100 years arelikely to be smaller than 3–4 °C. Sassen et al.(2001b) find no molecular fractionation of gasesin near-surface hydrate deposits that would beindicative of partial dissolution, and suggestthat the reservoir may in fact be growing.Other examples of structural deposits includethe summit of Hydrate Ridge, off the coastof Oregon (Torres et al., 2004; Tréhu et al.,2004b), and the Niger Delta (Brooks et al.,2000). The distribution of hydrate at HydrateRidge indicates up-dip flow along sand layers(Weinberger et al., 2005). Gas is forced intosandy layers where it accumulates until the gaspressure forces it to vent to the surface (Tréhuet al., 2004a). Tréhu et al. (2004b) estimatethat 30–40% of pore space is occupied byhydrate, while gas fractions are 2–4%. Methaneemerges to the sea floor with bubble vents andsubsurface flows of 1 m s –1 , and in regions withbacterial mats and vesicomyid clams (Torreset al., 2002). Further examples of structuraldeposits include the Peru Margin (Pecher etal., 2001) and Nankai Trough, Japan (Nouzéet al., 2004).Mud volcanoes are produced by focused-upwardfluid flow into the ocean and are sometimes associatedwith hydrate and petroleum deposits.Mud volcanoes often trap methane in hydratedeposits that encircle the channels of fluid flow(Milkov, 2000; Milkov et al., 2004). The fluidflow channels associated with mud volcanoesare ringed with the seismic images of hydratedeposits, with authigenic carbonates, and withpockmarks (Dimitrov and Woodside, 2003)indicative of anoxic methane oxidation. Milkov(2000) estimates that mud volcanoes contain atmost 0.5 GtC of methane in hydrate, about 100times his estimate of the annual supply.4.1.4 Fate of Methane Released asBubblesMethane released from sediments in the oceanmay reach the atmosphere directly, or it maydissolve in the ocean. Bubbles are not generallya very efficient means of transporting methanethrough the ocean to the atmosphere. Rehder etal. (2002) compared the dissolution kinetics ofmethane and argon and found enhanced lifetimeof methane bubbles below the saturation depthin the ocean, about 500 m, because a hydratefilm on the surface of the methane bubblesinhibited gas exchange. Bubbles dissolve moreslowly from petroleum seeps, where oily filmson the surface of the bubble inhibit gas exchange,also changing the shapes of the bubbles(Leifer and MacDonald, 2003). On a largerscale, however, Leifer et al. (2000) diagnosedthat the rate of bubble dissolution is limitedby turbulent transport of methane-rich waterout of the bubble stream into the open watercolumn. The magnitude of the surface dissolutioninhibition seems small; in the Rehder et al.(2002) study, a 2-cm bubble dissolves within30 m above the stability zone, and only 110 mbelow the stability zone. Acoustic imaging ofthe bubble plume from Hydrate Ridge showedbubbles surviving from 600–700 m water depth,where they were released to just above thestability zone at 400 m (Heeschen et al., 2003).One could imagine hydrate-film dissolutioninhibition as a mechanism to concentrate therelease of methane into the upper water column,but not really as a mechanism to get methanethrough the ocean directly to the atmosphere.Methane can reach the atmosphere if themethane bubbles are released in waters that areonly a few tens of meters deep, as in the caseof melting the ice complex in Siberia (Xu etal., 2001; Shakhova et al., 2005; Washburn etal., 2005), or during periods of lower sea level(Luyendyk et al., 2005). If the rate of methane182