HR: 15:25h
AN: OS42B-08    [PDF]
TI: Exploring Ideas About Why Methane Generation and Hydrate Formation Occur So Abundantly in the Bering Sea Basin
AU: * Scholl, D W
EM: dscholl@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025 United States
AU: Barth, G A
EM: gbarth@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025 United States
AU: Childs, J R
EM: jchilds@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025 United States
AB: INTRODUCTION: Beneath the virtually flat surface of the abyssal floor (3700-4100 m) of the Bering Sea Basin, geophysical data record a regionally persistent BSR at a depth of ~450 m. Above vertical chimneys of ascending gas (~0.5 km in dia, 1-2 km in height) the horizontal attitude of the BSR is deflected (domed) upward at what are called VAMP (Velocity-Amplitude) structures. VAMPs are thought to record the massive deposition of interstitial hydrate that forms a velocity pull-up lens in a sequence of flat-lying turbidite and diatom beds. The areally extensive (~450,000 km2) BSR and abundance (thousands) of gas chimneys and associated VAMPs testify to high, basin-wide flux of methane. Why should this be so? Taking a basin analysis approach, the following combination of depositional, diagenetic, and oceanographic factors are hypothesized to be involved. SEDIMENT COLUMN: The basin floor is underlain by a 3-12-km thick sequence of mostly Eocene and younger mudstone and turbidite deposits, the Miocene, Pliocene, and Quaternary part of which is, or was, richly diatomaceous. During the past 2-3 Myr a ~1-km-thick, water- (porosity = 50-60%) and diatom-rich turbidite section accumulated rapidly over the latest Miocene basin floor. Below the turbidite sequence silica diagenesis has formed porcelaneous shale via the downsection conversion of the opal-A of diatom tests to opal-CT and, at great depths, chalcedonic quartz or chert. HEATFLOW: The average flux of heat across the basin floor is 55-60 mW/m2 (n= 31, range 42-90), which is higher than could be produced by its basement of Cretaceous oceanic crust. The near-sea floor thermal gradient is 50-60 deg C/km. PRODUCTIVITY: The Bering Sea Basin is one of Earth's most productive fishing grounds, a circumstance linked to the regional upwelling here of part of the nutrient-rich global conveyor belt of circulating thermohaline water. Drilling and dredging document that high surface water productivity, in particular for diatoms, has been maintained over the basin since at least the early Miocene. WORKING HYPOTHESIS: We suggest that the rapid deposition of a basin-wide blanket of glacial age turbidite deposits warmed underlying organic- and diatom-rich hemipelagic beds of early Pliocene and older age. Warming accelerated conversion of diatomaceous units to porcelaneous shale, a process that is accompanied by rock volume contraction, fracturing, and the release of heat and water. Subsurface heating stimulated the production of thermogenic methane that ascended toward the seafloor along fracture pathways in the shale. Methane entering the turbidite and diatomaceous section locally nourished the growth of massive hydrate deposits to form an abundance of VAMP structures, each releasing heat of crystallization. This flux of methane to the basin floor is thus linked to the late Cenozoic warming of middle Tertiary beds that enhanced the production of thermocatalytic gases and facilitated their upward advection through fractured siliceous shale to form wide-spread deposits of methane hydrate in porous late Cenozoic beds.
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3022 Marine sediments--processes and transport
DE: 3025 Marine seismics (0935)
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting