HR: 08:00h
AN: OS41C-01 INVITED     [Abstracts]
TI: Uncertainties in Modeling the Global Inventory of Methane Hydrate
AU: * Buffett, B
EM: buffett@geosci.uchicago.edu
AF: University of Chicago, 5734 S. Ellis Avenue, Chicago, IL 60637 United States
AU: Archer, D
EM: d-archer@uchicago.edu
AF: University of Chicago, 5734 S. Ellis Avenue, Chicago, IL 60637 United States
AB: The challenge in estimating the global inventory of methane hydrate lies in extrapolating the results of a few well-studies field sites to the entire ocean. The inherent uncertainty is revealed in recent estimates of the inventory, which vary from 500 Gton C (Milkov, 2004) to 74,000 Gton C (Klauda & Sandler, 2005). We address the problem of extrapolating the hydrate inventory by developing a mechanistic model for the distribution of methane hydrate in marine sediments. The inventory is determined by binning the seafloor area according to water depth, temperature, and O₂ concentration. Organic carbon rain to the sea floor is treated as a simple function of water depth, and carbon burial for each bin is estimated using a sediment diagenesis model. The predicted concentration of organic carbon is fed into a geophysical model for hydrate formation to calculate steady-state profiles of dissolved, frozen and gaseous methane. We estimate the amount of methane in ocean sediments by multiplying the sediment column inventories by the corresponding binned seafloor areas. Our estimate of resulting methane inventory is most sensitive to the efficiency of methane production from organic matter and to the rate of fluid flow within the sediment column. We choose values for these parameters by fitting the model predictions to observations from several field sites. Our best fit yields a global estimate of 3× 10¹⁸ g of carbon (3000 Gton C) in clathrate and 2× 10¹⁸ g (2000 Gton C) in methane bubbles. Unfortunately, the choice of parameters values required to fit the observations is not unique and we have no assurances that any mutually consistent set of parameter values is suitable for the entire ocean. More data from new field sites would help to reduce the uncertainty, but more progress might be made by compiling new types of observations. Of particular interest are the regional properties of hydrate occurrences, such as the area fraction of continental margins with detectable bottom-simulating reflectors (BSRs). Roughly 30% of the area of the continental margin is predicted to have hydrate, and about 20% about of these locations are expected to have detectable BSR due to the presence of bubble volumes in excess of 1% of the pore volume. Regional occurrences of hydrate or BSRs could be slightly higher or lower than the global average, depending on the regional water depth and O₂ concentration. Compilations of the frequency of hydrate and/or BSR occurrences would provide a valuable constraint on the global inventory of methane hydrate.
DE: 0428 Carbon cycling (4806)
DE: 0714 Clathrate
DE: 3004 Gas and hydrate systems
DE: 3022 Marine sediments: processes and transport
SC: Ocean Sciences [OS]
MN: Fall Meeting 2005