GC14A-01 INVITED
Roles of Clathrate Hydrates in Crustal Heating and Volatile Storage/Release on Earth, Mars, and Beyond
Clathrate hydrates are stable through much of the Solar System. These materials and hydrate-like amorphous associations of water with N2, CO, CH4, CO2, O2 and other molecules could, in fact, constitute the bulk of the non-rock components of some icy satellites, comets, and Kuiper Belt Objects. CO2 clathrate is thermodynamically stable at the Martian South Pole surface and could form a significant fraction of both Martian polar caps and icy permafrost distributed across one-third of the Martian surface. CH4 clathrate is the largest clathrate material in Earth's permafrost and cold seafloor regions, and it may be a major volatile reservoir on Mars, too. CO2 clathrate is less abundant on Earth but it might store most of Mars' CO2 inventory and thus may be one of the critical components in the climate system of that planet, just as CH4 clathrate is for Earth. These ice-like phases not only store biologically, geologically, and climatologically important gases, but they also are natural thermal insulators. Thus, they retard the conductive flow of geothermal heat, and thick accumulations of them can modify geotherms, cause brines to exist where otherwise they would not, and induce low-grade metamorphism of upper crustal rocks underlying the insulating bodies. This mechanism of crustal heating may be especially important in assisting hydrogeologic activity on Mars, gas-rich carbonaceous asteroids, icy satellites, and Kuiper Belt Objects. These worlds, compared to Earth, are comparatively energy starved and frozen but may partly make up for their deficit of joules by having large accumulations of joule-conserving hydrates. Thick, continuous layers of clathrate may seal in gases and produce high gas fugacities in aquifers underlying the clathrates, thus producing gas-rich reservoirs capable of erupting violently. This may have happened repeatedly in Earth history, with global climatic consequences for abrupt climate change. We have hypothesized that such eruptions may have occurred during interglacial epochs and formed super-size maar craters in Bering Land Bridge National Preserve (Alaska). On Mars, clathrates and gas-saturated aquifers apparently played some role in the largest flood- and debris-flow-forming events in that planet's history, with vast consequences for landform development and resurfacing. This heating phenomenon also has possible implications for carbon sequestration as a means of climate change mediation on Earth; besides other concerns about their long-term stability, artificial hydrates produced by carbon dioxide pumping onto the seafloor might heat up and become unstable over time due to normal background radiogenic heat flux.
GC14A-02
Impacts, Salinity, and Climate Change: Global-Scale Triggers for Hydrate Dissociation
The release of greenhouse gases from gas hydrate reservoirs likely leads to increased global temperatures initiating a positive feed-back trend of increased warming and further hydrate dissociation. However, the initial geologic trigger which instigates hydrate dissociation may vary. Impacts will disrupt both the water column and shallow sediments in marine and continental shelf environments or expose and/or heat hydrate deposits in the subsurface within permafrost. The effects of marine or coastal impacts may be seen far from the point of impact as waves or surges disrupt sediments hundreds of meters below the water surface and reduce temperature and chemical stratifications within the water column. These disruptions could perhaps destabilize regional scale (i.e. Gulf of Mexico) gas hydrate deposits. Regional changes in salinity may also result in significant hydrate dissociation as the activity of water decreases due to mixing of higher salinity fluids or evaporation. Salinity- triggered hydrate dissociation is likely to be most effective in relatively isolated basins such as the Mediterranean, Black Sea, or perhaps ancient seas on Mars.
GC14A-03 INVITED
A Spatially-Resolved Model of the Global Ocean Hydrate Reservoir
We have developed the first mechanistic model of the geographic distribution of the oceanic methane hydrate reservoir. In our previously published study (Buffett and Archer, EPSL 227: 185-199, 2004), we accounted for heterogeneity by binning the sea floor according to water depth, temperature, and oxygen concentration. Here we recompose the map of the global sea floor to examine the distribution of hydrate regionally, and to predict the impact of anthropogenic ocean warming on hydrate stability. We pay particular attention to the volume fraction of bubbles upon melting at the base of the stability zone, motivated by the idea that methane escape to the ocean depends on the bubble fraction exceeding a critical interconnected value. Our model finds the Arctic to be special in this regard, producing a greater bubble volume upon melting than the global average, because the cold water column supports hydrate stability at lower pressure than elsewhere. The Arctic is also special because warming can reach these shallow depths more quickly than it will reach the deeper depths in the rest of the world, and because of the polar amplification of warming. In spite of the Arctic hydrate sensitivity, however, experiments with the hydrate representation within the CLIMBER intermediate complexity model of the atmosphere and ocean predict the methane release response of hydrates in the coming centuries to be small.
GC14A-04
Estimates of Methane Production Rates Based on d13C of the Residual DIC in Pore Fluids from the Cascadia Margin
An outstanding question in carbon cycling models, particularly those that involve gas hydrate formation in accretionary margins, is the rate of methane generation in anoxic sediments. This rate is a key parameter in numerical models involving gas hydrate formation, magnitude of the deposits and recharge of the gas hydrate reservoir following a destabilization event. Pore fluid samples recovered from ~100 to 200 mbsf along a transect of sites drilled in the Cascadia convergent margin during ODP Legs 146 and 204 and IODP Leg 311 show a distinct enrichment in the d13C values of the residual dissolved CO2 with progression from a reference site drilled west of the deformation front (Site 888, d13C: -5 ppt) to Site U1329 (d13C: +32 ppt) located ~65 km from the shore, at the eastern limit of gas hydrate occurrence in the Cascadia margin. Assuming that methane production proceeds primarily by carbonate reduction at relatively rapid rates in a closed system (Rayleigh distillation) with a fractionation factor of 1.07, the measured d13C-DIC at Site U1329 corresponds to ~50% removal of the available dissolved carbonate pool that is assumed to be characterized by 10 mM ?CO2, having a d13C of -20 ppt. Although it is hard to know the available DIC pool, 10 mM represents an average value based on alkalinity measurements at these sites. These estimates correspond to an in situ production of 5 mmol/l methane. Sediments from Site U1329 have been dated as late Miocene, ~5.3 Myr old, thus based on the assumption that the methane was generated over this period, in sediments with porosity of 50%, the first order approximation of the calculated rate of methane production is 0.4 mmol/m3yr. Our estimated methane production rate compares well with the published rates for Leg 204, that vary from 10 mmol/m3yr (upper 100 mbsf) to 0.1 mmol/m3yr (deeper than 100 mbsf), based on an alkalinity model. However, estimates using methane production rates for starved methanogens and the numbers of methanogens in the sediments for most of the Leg 204 samples, yielded values of ~0.005 mmol/m3yr, although 25% of these samples yielded estimates that were higher than this value.
GC14A-05
Compound Natural Gas Hydrate: A Natural System for Separation of Hydrate-Forming Gases
Natural processes that separate materials from a mixture may exert a major influence on the development of the atmospheres and surfaces of planets, moons, and other planetary bodies. Natural distillation and gravity separation, amongst others, are well known means of differentiating materials through liquid-gas partitioning. One of the least known attributes of clathrate (gas) hydrates is their potential effect on the evolution of planetary system oceans and atmospheres. Gas hydrates separate gases from mixtures of gases by concentrating preferred hydrate-forming materials (HFM) guests within the water-molecule cage structure of crystalline hydrate. Different HFMs have very different fields of stability. When multiple hydrate formers are present, a preference series based on their selective uptake exists. Compound hydrate, which is formed from two or more species of HFM, extract preferred HFM from a mixture in very different proportions to their relative percentages of the original mixture. These compound hydrates can have different formation and dissociation conditions depending on the evolution of the environment. That is, the phase boundary of the compound hydrate that is required for dissociation lies along a lower pressure – higher temperature course. Compound hydrates respond to variations in temperature, pressure, and HFM composition. On Earth, the primary naturally occurring hydrate of interest to global climate modeling is methane hydrate. Oceanic hydrate on Earth is the largest store of carbon in the biosphere that is immediately reactive to environmental change, and is capable of releasing large amounts of methane into the atmosphere over a short geological time span. Hydrate formation is essentially metastable and is very sensitive to environmental change and to gas flux. Where natural variations in temperature and pressure varies so that hydrate will form and dissociate in some cyclical manner, such as in oceans where sea level is capable of rising and falling, and which warms and cools, and in atmospheres in which temperature swings take place, the compound hydrate system can both sequester and release HFM selectively. When there is strong gas flux hydrate will tend to form; when gas flux falls below a certain level, hydrate may either dissolve or dissociate. On other bodies in the solar system, such as on Titan, ethane, propane, nitrogen, noble gases, and other HFMs may be selectively withdrawn from gas and liquid phases and sequestered within hydrate, or selectively released when climate swings occur, which can cause positive or negative feedback to atmospheric composition and greenhouse intensity. Where carbon-based biosphere conditions exist, the hydrate system will interact with it in the same manner as it does on Earth.
GC14A-06
Oceanic gas hydrate instability and dissociation in response to climate change
Paleooceanographic evidence has been used to postulate that methane from oceanic hydrates may have had a significant role in regulating past global climate, implicating global oceanic deposits of methane gas hydrate as the main culprit for a remarkably rapid sequence of global warming effects that occurred during the late Quaternary period. However, the behavior of contemporary oceanic methane hydrate deposits subjected to rapid temperature changes, like those predicted under future climate change scenarios, is poorly understood, and existing studies focus on deep hydrate deposits under equilibrium conditions. We simulated the dynamic response of several types of oceanic gas hydrate accumulations to temperature changes at the seafloor and assessed the potential for methane release into the ecosystem. The properties of benthic sediments, the saturation, stability, and distribution of the hydrates, the ocean depth, the geothermal gradient, and the effects of biogeochemical activity were considered. The results suggest that while many deep hydrate deposits are indeed stable under the influence of rapid seafloor temperature variations, shallow deposits, such as those found in arctic regions or in the Gulf of Mexico, can undergo rapid dissociation and produce significant carbon fluxes over a period of decades. These results may be used to provide a source term to regional or global climate models to determine the impact of gas hydrate deposits on global climate.