Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science Program Chapter 5migration through the stability zone or a landslide,or the bubbles could remain enmeshed inthe sediment matrix. Hydrate moves down tothe base of the stability zone by the accumulationof overlying sediment at the sea floor, somelting of hydrate at the stability zone takesplace continuously, not just in association withocean warming.When hydrate melts, most of the releasedmethane goes into the gas phase to formbubbles, assuming that the porewaters werealready saturated in dissolved methane. Thefate of the new bubbles could be to remain inplace, to migrate, or to diffuse away and reactchemically (Hinrichs et al., 1999; Wakeham etal., 2003), and it is difficult to predict which willoccur. The potential for gas migration throughthe stability zone is one of the more significantuncertainties in forecasting the ocean hydrateresponse to anthropogenic warming (Harveyand Huang, 1995).In cohesive sediments, bubbles expand by fracturingthe sediment matrix, resulting in elongatedshapes (Boudreau et al., 2005). Bubblestend to rise because they are less dense than thewater they are surrounded by, even at the 200+atmosphere pressures in sediments of the deepsea. If the pressure in the gas phase exceeds thelithostatic pressure in the sediment, fracture andgas escape can occur (Flemings et al., 2003).Modeled and measured (Dickens et al., 1995)porewater pressures in the sediment columnat Blake Ridge approach lithostatic pressures,indicating that new gas bubbles added to thesediment might be able to escape to the overlyingwater by this mechanism.A differential-pressure mechanism begins tooperate when the bubbles occupy more thanabout 10% of the volume of the pore spaces(Hornbach et al., 2004). If a connected bubblespans a large enough depth range, the pressureof the porewater will be higher at the bottom ofthe bubble than it is at the top, because of theweight of the porewater over that depth span.The pressure inside the bubble will be morenearly constant over the depth span, because thecompressed gas is not as dense as the porewateris. This will result in a pressure gradient atthe top and the bottom of the bubble, tendingto push the bubble upward. Hornbach et al.(2004) postulated that this mechanism mightbe responsible for allowing methane to escapefrom the sediment column, and they calculatedthe maximum thickness of an interconnectedbubble zone required, before the bubbles wouldbreak through the overlying sediment column.In their calculations, and in stratigraphic deposits(they refer to them as “basin settings”),the thickness of the bubble column increasesas the stability zone gets thicker. It takes morepressure to break through a thicker stabilityzone, so a taller column of gas is required. Incompressional settings, where the dominantforce is directed sideways by tectonics, ratherthan downward by gravity, the bubble layeris never as thick, reflecting an easier path tomethane escape.Multiple lines of evidence indicate that gas canbe transported through the hydrate stabilityzone without freezing into hydrate. Seismicstudies at Blake Ridge have observed the presenceof bubbles along faults in the sedimentmatrix (Taylor et al., 2000). Faults have beencorrelated with sites of methane gas emissionfrom the sea floor (Aoki et al., 2000; Zühlsdorffet al., 2000; Zühlsdorff and Spiess, 2004). Seismicstudies often show “wipeout zones” wherethe bubble zone beneath the hydrate stabilityzone is missing, and all of the layered structureof the sediment column within the stability zoneis smoothed out. These are interpreted to be areaswhere gas has broken through the structureof the sediment to escape to the ocean (Riedel etal., 2002; Wood et al., 2002; Hill et al., 2004).Bubbles associated with seismic wipeout zonesare observed within the depth range that shouldbe within the hydrate stability zone, assuming180