HR: 14:50h
AN: OS52C-05 INVITED     [PDF]
TI: Properies of sea Floor Hydrates From Hydrate Ridge/Cascadia Margin
AU: * Bohrmann, G
EM: gbohrmann@uni-bremen.de
AF: Department of Geosciences, University of Bremen, Klagenfurter Str., Bremen, D-28359 Germany
AU: Abegg, F
AF: GEOMAR Research Center for Marine Geosciences, Wischhofstr. 1-3, Kiel, D-24148 Germany
AU: Suess, E
AF: GEOMAR Research Center for Marine Geosciences, Wischhofstr. 1-3, Kiel, D-24148 Germany
AB: Near-surface methane hydrates are well known from southern and northern summits at Hydrate Ridge. Fabric analyses of such samples indicate that at least parts of the hydrate are formed from free methane gas. Free gas migrates upwards through the sediment column and is also indicated by gas bubbles emanating at the seafloor. These bubbles form plumes in the water column. In most cases pure white hydrate occurs here in layers millimeters to several decimeters thick. On a macroscopic scale the fabric varies from highly porous, with pore diameters of up to several cm, to massive with no visible pores. Bulk densities range from 0.35-0.75 g cm-3 and are inversely correlated with the pore volume, which ranged from 10 - 70 vol percent. A total end-member density of pure natural methane hydrate of 0.79 AŸƒ_sA,Añ0.13 g cm-3 is estimated differing considerably from the theoretical value of 0.91 g cm-3. The low bulk density and the porous fabric result from the formation of hydrate from bubbles of methane. Our CT investigations on samples obtained by autoclave coring technology show that at least parts of the pores within pure hydrate layers are filled by free gas in situ, which additionally lowers the material density in its natural environment. Low densities create high positive buoyancy as well as low acoustic velocity of hydrate-rich sediments. Strong buoyancy facilitates the rapid transfer of solid methane hydrates from the seafloor to the atmosphere by hydrate floats, and hence affects the greenhouse gas budget. Furthermore, the low acoustic velocity affects the subsurface distribution as well as the thickness of hydrated strata estimated from seismic velocities and hence the estimate of the amount of hydrate stored in off-shore sedimentary formations.
DE: 1615 Biogeochemical processes (4805)
DE: 1800 HYDROLOGY
DE: 3022 Marine sediments--processes and transport
DE: 5100 PHYSICAL PROPERTIES OF ROCKS
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting