Brief intervals of profound global environmental change have punctuated Earth's history. During some of these times, including the Permian-Triassic boundary, the 13C/12C ratio of carbonate and organic matter at numerous widespread locations decreased significantly (>2‰). These negative δ13C excursions probably signify rapid and massive inputs of 13C-depleted carbon to the atmosphere and some to all portions of the ocean. Increasingly, methane has been implicated as the source of the 13C-depleted carbon, with most work suggesting a gas hydrate origin. Specifically, some environmental perturbation causing dissociation of gas hydrate in marine sediment to free gas bubbles, which then escape through sediment failure such as slumping (e.g., Katz et al., 1999). Recently, however, Ryskin (2003) has presented an alternative scenario whereby significant amounts of dissolved methane accumulated in stagnant deep waters and drove methane eruptions somewhat analogous to the carbon dioxide expulsions documented at Lake Nyos (e.g., Kling et al., 1987; Zhang, 1996). The purpose of this comment is to point out a basic problem with this model.
Ryskin (2003) suggests that methane solubility will steadily increase with depth in the ocean so that very high concentrations can accumulate in deep water and that free gas can exsolve from this water. Indeed, he states that the mole fraction of dissolved methane can reach ∼4.3 × 10−3 beneath 4 km of water. This view is incorrect because he has neglected the clathrate phase. Contrary to his assertion that “. . . it is immaterial whether some part of this methane flux results from dissociation of methane hydrates. . . ” (p. 741), the ability of methane and water to crystallize as a clathrate at high pressure and low temperature necessarily complicates any scenario that invokes the buildup of methane in the deep ocean. Phase diagrams appropriate for understanding the methane-seawater system (Fig. 1) clearly show that the mole fraction of methane dissolved in water at 4 km depth cannot accumulate beyond ∼1.2 × 10−3, otherwise the solid clathrate structure would precipitate. However, without the confines of sediment, deep waters of the ocean cannot store significant amounts of methane in methane hydrate because it floats. Initially, this may seem like a trivial point that merely drops the theoretical maximum for methane concentrations in the deep ocean, albeit significantly. However, as outlined by Ryskin (2003) and others (Zhang, 1996), gas eruptions occur when gas exsolves from solution, which decreases the density of a water parcel, which then causes the water parcel to rise, which releases additional gas. The formation of methane hydrate at relatively low methane concentrations necessarily precludes the exsolution of dissolved methane to free gas bubbles in deep water. Thus, a methane eruption cannot initiate from great depth. One might suggest that methane-driven eruptions could occur when large masses of floating gas hydrate rise into warm, shallow water and dissociate (Zhang, 2003). The enormous amount of methane required to cause a prominent negative δ13C anomaly would, however, necessarily mean that the floating gas hydrate was somehow dislodged from sediment beneath the seafloor.
I thank Y.X. Zhang for making the plots for this comment.
