Methane clathrates

Figure 3.49 summarizes the oxygen isotope curve for the last 65 Ma. The most pronounced warming trend is expressed by a 1.5%o decrease in 8 0 and occurred early in the Cenozoic from 59 to 52 Ma, with a peak in Early Eocene. Coinciding with this event is a brief negative carbon isotope excursion, explained as a massive release of methane into the atmosphere (Norris and Rohl 1999). These authors used high resolution analysis of sedimentary cores to show that two thirds of the carbon shift occurred just in a few thousand years, indicating a catastrophic release of carbon from methane clathrates into the ocean and atmosphere. 

Several hundred to several thousand feet beneath the ocean floor in permafrost and continental edge regions lies a potentially vast source of natural gas in excess of 10 cubic meters of gas hydrates, consisting largely of methane clathrate (53-55). Gas... 

Buffet, B. Archer, D. (2004). Global inventory of methane clathrate sensitivity to... 

Shimada, W. Takeya, S. Kamata, Y. Uchida, T. Nagao, J. Ebinuma, T. Narita, H. (2005). Texture Change of Ice on Anomalously Preserved Methane Clathrate Hydrate. J. Phys. Chem. B, 109, 5802-5807. 

Shpakov, V.P. Tse, J.S. Tulk, C.A. Kvamme, B. Belosludov, V.R. (1998). Elastic moduli calculation and instability in structure I methane clathrate hydrate. Chemical Physics Letter, 282, 107-114. 

Figure 7.1 Burning snowballs of methane clathrate hydrate (image courtesy of the US Geological Survey).

Clathrate hydrates are inclusion compounds formed by the enclosure of a small guest molecule within a hydrogen bonded cage of solid-state water. Clathrate hydrates are co-crystals and are thus distinct from ice, which is made of pure water, and hence can have different physical properties to ice such as a different melting point. The classic example of a clathrate hydrate is the burning snowball of methane clathrate hydrate. The combustion of the methane in the clathrate is self-sustaining, Figure 7.1. Many... 

The same phenomenology must be important locally on Earth, too, where thick evaporite deposits of hydrated salts and local thick beds of methane clathrate in permafrost or seafloor sediments should influence the thermal environment of the crust. The predicted control on the crust s thermal state by hydrate deposits should have consequences for the localization of hydrothermal springs around and within evaporite basins, hydrothermal metamorphism... 

As with other compounds, solution effects can elevate the condensation temperatures of clathrate guest species. Sill and Wilkening calculated that in a gas of solar composition the major clathrate, and the first to form, will be ice-methane, and that noble gases can substitute for the methane at temperatures higher than decomposition temperatures for noble gas clathrates. They calculate, for example, that in a total nebular pressure of 2 x 10 atm (high in comparison with most model pressures currently considered of about 10 4 atm ), ice-methane clathrate at 80 K will have dissolved 99% of the available Xe (and substantially smaller amounts of the other noble gases). 

The Sill and Wilkening proposal that clathrates formed in the cold outer parts of the solar system and then transported to the inner solar system (e.g., in comets) might help account for the atmospheres of the terrestrial planets. They contend that infall of 1 ppm of ice-methane clathrate with noble gases dissolved as just described could account for the present inventories of Ar, Kr, and Xe in the terrestrial atmosphere. 

N.J. English, J.M.D. Macelroy, Structural and dynamical properties of methane clathrate hydrates. J. Comput. Chem. 24, 1569-1581 (2003)... 

The origin of the methane clathrates interests me. How are they formed in the sediments Do they come from below, or above If they would come from above, I cannot see how they ever could form there in such high concentrations because they first go through a higher temperature regime. It is something that I do not understand, I am afraid. 

Methane clathrate decomposition has been implicated in the Latest Paleocene Thermal Maximum (—55 Ma ago) by an extraordinary injection of isotopically light carbon into the carbon cycle (Dickens, 2000, 2001a) and in Quaternary interstadials as indicated by observations of isotopically light foraminifera in Santa Barbara Basin sediments (Kennett et al., 2000). Dickens (2001a) compares the functioning of the CH4 hydrate to a bacterially mediated capacitor. 

Other times of unusually extensive tropical paleosols were the latest Paleocene (55 Ma Taylor et ai, 1992), latest Eocene (35 Ma Bestland et ai, 1996 Retallack et al, 2000), and Middle Miocene (16 Ma Schwarz, 1997). These events are notable as short-lived (<0.5 Myr) spikeUke warmings in both stable isotopic records from the ocean (Veizer et al., 2000 Zachos et al, 2001) and stomatal index studies (Retallack, 2001b). The latest Paleocene warm spike is associated with such profound carbon isotopic lightening that it can only reasonably be attributed to the methane from isotopically light methane clathrates from the ocean floor or permafrost (Koch, 1998). Short-term physical forcings are thus also recorded in the paleosol record of paleoclimate. 

Harvey L. D. D. and Huang Z. (1995) Evaluation of the potential impact of methane clathrate destabilisation on future global warming. J. Geophys. Res. 100, 2905—2926. 

An important step in understanding the local structure around a nonpolar solute in water was made by Jorgensen et al. Using Monte Carlo simulations based on an intermolecular potential, which contained Lennard-Jones and Coulomb contributions, they determined the number of water molecules in the first hydration layer (located between the first maximum and the first minimum of the radial distribution function) around a nonpolar solute in water. This number (20.3 for methane, 23 for ethane, etc.) was surprisingly large compared with the coordination numbers in cold water and ice (4.4 and 4, respectively). These results provided evidence that major changes occur in the water structure around a nonpolar solute and that the perturbed structure is similar to that of the water—methane clathrates, ... 

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