Oceans sediments, hydrates

Transport in solution or aqueous suspension is the major mechanism for metal movement from the land to the oceans and ultimately to burial in ocean sediments. In solution, the hydrated metal ion and inorganic and organic complexes can all account for major portions of the total metal load. Relatively pure metal ores exist in many places, and metals from these ores may enter an aquatic system as a result of weathering. For most metals a more common sequence is for a small amount of the ore to dissolve, for the metal ions to adsorb onto other particulate matter suspended in flowing water, and for the metal to be carried as part of the particulate load of a stream in this fashion. The very insoluble oxides of Fe, Si, and A1 (including clays), and particulate organic matter, are the most important solid adsorbents on which metals are "carried."... 

Ocean sediments with hydrates typically contain low amounts of biogenic methane. 

In a recent ocean hydrate formation state-of-the-art summary, Trehu et al. (2006) listed the effects of fluid flow and sediment lithology. Ocean hydrate deposits are distributed on a spectrum between two types in ocean sediments (1) focused high flux (FHF) gas hydrates, and (2) distributed low flux (DLF) gas hydrates. In FHF hydrates the gas comes from a large sediment volume channeled through a high-permeability sand to the point of hydrate formation, and these hydrates are typically in the upper tens of meters of the sediment. In contrast, the DLF hydrates are generated near where the hydrates are formed, and fluid flow is responsible for movement of the gas within the gas hydrate occurrence zone (GHOZ). 

Figure 7.11 Envelopes of methane hydrate stability (a) in Permafrost and (b) in Ocean sediment. (Reproduced from Kvenvolden, K.A., Chem. Geol., 71, 41 (1988). With permission from Elsevier Science Publishers.)...

For hydrates in ocean sediments, the technology for detecting the BSR was determined in 1953 with the development of a precision ocean depth recorder (Hamblin, 1985, p. 11). In this technique a sonic wave penetrates (and is reflected from) the ocean floor, with the time recorded for the return of the reflected wave to the source. Velocity contrasts beneath the ocean floor mark a change in material density, such as would be obtained by hydrate-filled sediments overlying a gas. BSRs related to hydrates are normally taken as indications of velocity contrasts between velocity in hydrated sediments and a gas, marked by a sharp decrease in sonic compressional velocity (Vp) and a sharp increase in shear velocity (Vs) (Ecker et al., 1996). 

The sediment setting will control the target to some degree. Hydrate deposits in sandy ocean sediments (e.g., the Gulf of Mexico) likely... 

Moridis, GJ., Sloan, E.D., Gas Production Potential of Disperse Low-Saturation Hydrate Accumulations in Oceanic Sediments, Energy Conversion and Management Journal, accepted (2006), also as LBNL Report 61446, August 2006. 

In ocean drilling, hydrated sediment cores are often obtained. Because the cores frequently traverse warm waters for periods of about 1 h, hydrated cores dissociate and release gas, to yield higher pressures. When core liners are retrieved on the deck of a drilling vessel, frequently the warm weather can cause additional hydrate dissolution, resulting in further pressure increases. The modeling of this dissociation has been done by Wright et al. (2005) and by Davies et al. (2006). 

The storage of methane as hydrates offers a potentially vast natural gas resource. As to the question of how much hydrate there is right now, there is no definitive answer. However, the worldwide amount of carbon bound in gas hydrates has been estimated to total twice the amount of carbon to be found in all known fossil fuels originally on Earth. Additionally, conventional gas resources appear to be trapped beneath methane hydrate layers in ocean sediments.22... 

Klauda, ).B. and Sandler, S.I. (2005) Global distribution of methane hydrate in ocean sediments. Energ. Fuel, 19, 459. 

Gas Hydrate Gas-rich layer of ice found in ocean sediments. 

The hydrophobic effect can lead in the extrane case to the formation of crystalline hydrates, such as the well-known chlorine hydrate Clj.SHjO, and CH. nHjO, found at great depth in ocean sediments, and recently of interest (Kleinberg and Brewer,... 

There are also large reservoirs of CH4 stored as methane clathrate methane hydrate), a crystalline ice-like structure of water and CH4 that can exist at high pressures and low temperatures, such as may occur in areas of permafrost and beneath certain ocean sediments. Figure 4.51 shows the stability diagram of methane clathrate. Approximately 2 X10 Tg CH4 are estimated to be stored as methane clathrate in the ocean floor (Archer et al., 2009). 

Since methane is almost always a byproduct of organic decay, it is not surprising that vast potential reserves of methane have been found trapped in ocean floor sediments. Methane forms continually by tiny bacteria breaking down the remains of sea life. In the early 197Qs it was discovered that this methane can dissolve under the enormous pressure and cold temperatures found at the ocean bottom. It becomes locked in a cage of water molecules to form a methane hydrate (methane weakly combined chemically with water). This "stored" methane is a resource often extending hundreds of meters down from the sea floor. 

Lorenson, T.D., Collett, T.S., Gas content and composition of gas hydrate from sediments of the southeastern North American continental margin, in Proc. ODP, Sci. Results, (Pauli, C.K., Matsumoto, R., Wallace, P.J., and Dillon, W.P. eds.), College Station, TX (Ocean Drilling Program), 164, 37—46 (2000). 

More importantly, the result of Booth et al. also suggests that only massive hydrate samples can survive the trip from the bottom of the ocean to ship deck. For example, if the massive MAT Guatemala 2 sample (topmost in Figure 7.7) were recovered at constant pressure, the temperature would need to rise more than 16°C before the sample reached the three-phase line, where dissociation would begin. This result is consistent not only with laboratory determinations for dispersed hydrates (Kumar et al., 2004 Pauli et al., 2005 Wright and Dallimore, 2005) but also shows the parallel of recovered core dissociation with radial dissociation due to depressurization in pipelines, modified for sediment content (Davies and Sloan, 2006). 

Figures 7.11a,b are arbitrary examples of the depths of hydrate phase stability in permafrost and in oceans, respectively. In each figure the dashed lines represent the geothermal gradients as a function of depth. The slopes of the dashed lines are discontinuous both at the base of the permafrost and the water-sediment interface, where changes in thermal conductivity cause new thermal gradients. The solid lines were drawn from the methane hydrate P-T phase equilibrium data, with the pressure converted to depth assuming hydrostatic conditions in both the water and sediment. In each diagram the intersections of the solid (phase boundary) and dashed (geothermal gradient) lines provide the lower depth boundary of the hydrate stability fields.

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