Natural Gas Hydrates: Cause and Effect of Global Climate Change?
Presiding: B Buffett, University of Chicago; C Ruppel, National Science Foundation
U33A-01 INVITED 13:30h
Shallow Methane Hydrates: Rates, Mechanisms of Formation and Environmental Significance.
Shallow gas hydrates have been identified at more than 20 locations worldwide, and are commonly associated with observations of bubble discharge at the seafloor. These deposits are host to active chemosynthetic communities and are likely to play a predominant role in energy, climate and carbon cycle issues associated with hydrate processes. Because seafloor gas hydrates are not in equilibrium with seawater, these deposits require a constant supply of methane to replace loss by continuous diffusion to bottom water. We will summarize evidence documenting that at the shallow deposits on Hydrate Ridge (OR) methane must be delivered in the free gas phase and present simple models used to infer formation rates, which are orders of magnitude higher than those for hydrates formed deeper in the sediment column (Torres et al., 2004). At Hydrate Ridge, methane gas is channeled from deep accretionary margin sequences to the gas hydrate stability zone (GHSZ) through a permeable layer that has been mapped seismically (Horizon A). High gas pressure in this horizon can drive gas through the GHSZ to the seafloor (Trehu et al., 2004). We will review current ideas that address mechanisms whereby gas migrates from Horizon A to the seafloor, including inhibition by capillary effects and the development of a high salinity front that can shift the hydrate stability field enough to allow for methane transport as a gas phase.
U33A-02 13:45h
Sensitivity of the Methane Hydrate Stability Zone around Seafloor Seeps to Changes in Sea Level and Bottom Water Warming
Finite element modeling of fluid flux associated with seafloor seeps shows that the vast majority of the fluid (and inferred methane) flux occurs in the immediate vicinity of the seep. This phenomenon is responsible for concentrating methane hydrates in the shallow sediments at seep sites or even creating mounds of methane hydrate on the seafloor as reported on the Cascadia Margin and Gulf of Mexico. The modeling also shows that the heat transport due to the fluid advection is responsible for much more closely spaced isotherms at the seep compared to away from the seep. A bottom water temperature change will therefore affect a smaller volume (up to orders of magnitude smaller) at a seep as compared to a location without significant advection, (the volume is made even smaller by anomalously saline pore water in the Gulf of Mexico). However, the increased concentration of methane hydrate at the seep may more than compensate for the smaller overall volume, making highly localized seep sites as important to volumetric methane release as far greater areas with no active advection. Further, because the methane hydrate at seeps is physically concentrated near the seafloor, not distributed through hundreds of meters of sediment, the time required for dissociation from a thermal pulse is far shorter, and not only is the migration path through the sediment much smaller, but migration pathways are already established. These factors suggest that even though the hydrate at seeps may be a relatively small fraction of the global hydrate reservoir, it may be a very significant, if not dominant fraction of the volatile methane hydrate reservoir - that reservoir susceptible to dissociation from realistic geologic perturbations, and therefore important for climate change.
U33A-03 14:00h
Linear-to-Volcanic Response in a Self-Organizing Driven Fluid Mixture by an Interacting Lattice Gas Computer Simulation
A computer simulation model is used to study the transport, flow, and self-organizing morphology in a multi-component fluid mixture. We consider a mixture of two immiscible components (A, B) driven by a hydrostatic pressure bias from a source (reservoir) at the bottom on a cubic lattice. Mobile particles (A, B - representing heavier gas hydrate/sediment particles and fluid) in equal number are distributed randomly on half of the lattice sites initially where a site cannot be occupied by more than one particle. The empty (pore) sites can act as a component of an effective medium. A set of interactions among these constituents is considered in addition to excluded volume hard-core interaction. The hydrostatic pressure bias (H) is implemented probabilistically to drive particles (A, B) against gravity. We use the Metropolis algorithm to move these particles stochastically. Periodic boundary conditions are used along the transverse directions while the longitudinal ends (top and bottom) are open. Thus, particles can escape from the top or bottom; however, they can enter the lattice only from the source at the bottom in this model. The probability of release of constituents (A, B) depend on their current relative concentrations in the lattice. Particles flow, re-distribute, and their concentrations change as the simulation proceeds. The flux rate of particle flow becomes constant and the morphology become stable in the long time steady-state limit. We observe a variety of self-organized structures, which exhibit dense phase at the bottom and dilute (gaseous) phase on the top with a bi-continuous phase in between. The flux rate (j) shows linear response at low hydrostatic bias but is different for constituents (A, B) with different molecular weight. The response becomes non-linear in high-bias regime; there it diverges (essentially erupts) for higher molecular weight components while it decreases for the lighter component.
U33A-04 INVITED 14:15h
New Perspectives on Methane in Quaternary Climate Change: Modern Methane Seeps, Intermediate Water Temperature History and Thermogenic Seepage
Several lines of evidence from late Quaternary sediment records implicate methane as a forcing mechanism for rapid climate change via releases from the hydrate reservoir. One such line of evidence, negative Δ13C excursions in marine sediments, is interpreted to reflect the transport of methane to the water column and atmosphere. However, it is controversial whether such late Quaternary Δ13C excursions actually represent methane release, or instead local changes in sediment-porewater geochemistry. Clarification of this issue is crucial for the interpretation and understanding of the nature of past rapid climate change. We present the results of several studies in Santa Barbara Basin, California and Hydrate Ridge, Oregon that focus on understanding methane release in the modern environment and geologic record. Modern methane seeps at Hydrate Ridge, Oregon provided an opportunity to investigate the influence of methane on the Δ13C composition of foraminifera from these environments. Three species of benthic foraminifera were picked for live (stained) and fossil specimens to assess the potential role of post-depositional authigenic carbonate Δ13C values. Individual living foraminifera from seep sites recorded Δ13C values to negative 21.2‰, indicating the isotopic influence of methane. No statistical difference existed between isotopic values of live vs. fossil specimens, ruling out a significant role for authigenic carbonate. These investigations demonstrate that living foraminifera are capable of recording high concentrations of methane via their carbon isotopic composition. Previous high-resolution studies of Santa Barbara Basin (ODP Site 893) indicated that Δ13C excursions associated with interstadial warming were brief (5-10 years duration) and synchronous within benthic and planktonic species. Newly collected cores provided material to investigate a continuous deglacial record of intermediate waters, which reside at the critical depth range for the destabilization of methane hydrate. Intermediate water temperatures appear to warm synchronously with surface waters (2-3°C), indicating early warming on the California margin approximately 2 kyr prior to Termination IA. Negative Δ13C values of planktonic foraminifera indicate the influence of an isotopically-depleted carbon source in surface waters at this time. In an exciting and new discovery, the amount of tar preserved in basin sediments was quantitatively analyzed for the past 32 kyr. These records, from two sites in the basin, indicate that tar seepage from thermogenic hydrocarbon reservoirs increased during Termination IA and IB deglacial warming. These records indicate that during time periods of abrupt warming, both the hydrate and thermogenic methane reservoirs may increase outputs of methane to the water column and atmosphere. These and other new records of methane hydrate destabilization indicate that geologic evidence is growing in support of late Quaternary instability of the gas hydrate reservoir and associated episodic transfer of methane into the ocean/atmosphere system.
U33A-05 14:30h
Time-dependent response of the marine clathrate reservoir to climatic and anthropogenic forcing.
Mechanistic models for the distribution of methane clathrate in marine sediments predict a steady-state inventory of 5000 Gton C. These models also predict a strong sensitivity to changes in ocean temperature. Increasing the ocean temperature by 1.5° C is expected to decrease the steady-state inventory by roughly a factor of 2. The time scale for adjustment to a new steady state is not well known. However, we can parameterize the time-dependent behavior using first-order rate constants. Methane accumulation is expected to occur on time scales of several million years, whereas methane release could be comparatively fast. We calculate the rate of methane release by dividing the clathrate reservoir into fast and slow parts. The fast part responds through slumping of continental margins, whereas the slow part responds by diffusion and oxidation of methane below the seafloor. The evolution of the clathrate reservoir through geologic time is sensitive to the choice of rate constants. When the time scale for the fast part is too short, runaway melting is caused by the positive feedback from radiative forcing (assuming CH4 has oxidized to CO2). Thus the existence of the present-day inventory imposes a constraint on the rate of release. The time scale for accumulation must by greater 5 Myr to avoid unrealistic fluctuations in Δ13C during glacial cycles, but shorter than 10 Myr to allow the clathrate reservoir to build up during the geologically recent cooling. The constrained model predicts a methane release of 200 GTon C or less on deglaciations. Future methane releases of 2000--4000 GTon C are expected in response to a 2000 GTon C anthropogenic carbon release. Anthropogenic climate change differs from deglaciations in that it warms the ocean to temperature not seen in millions of years.
U33A-06 14:45h
Ecosystem Modelling for Impact Assessment of Possible Methane Leakage during Methane Hydrate Utilization
Natural methane hydrate has been scientifically studied as a carbon reservoir globally. However, in Japan, the potential for energy resource has been industrially highlighted. There is less domestic oil and natural gas resources in Japan, but many potential deposition areas for methane hydrate in ocean around Japan are the reasons. Less CO2 discharge from methane compared with coal, oil and conventional natural gas when the same calorie value we get is considered as the advantage for energy resource. However, because methane hydrate distributes in shallower sediment layer in ocean floor, accidental leakage of methane may occur while we utilize methane hydrate. Methane itself has 21-times impact on the greenhouse effect, if it reaches the atmosphere. Therefore, it is necessary to estimate the behavior in the environment after the leakage, if we want to use methane hydrate as energy resource. The mass balance after leakage of methane on seafloor and in water column is numerically studied through the analyses of methane emissions from natural cold seepages and hydrothermal activities in this research. The outline structure of mass balance ecosystem model creating is introduced and some preliminary examination results from the test calculation are discussed.