Estimation of the global amount of submarine gas hydrates formed via microbial methane formation based on numerical reaction-transport modeling and a novel parameterization of Holocene sedimentation

Abstract

This study provides new estimates for the global offshore methane hydrate inventory formed due to microbial CH 4 production under Quaternary and Holocene boundary conditions. A multi-1D model for particular organic carbon (POC) degradation, gas hydrate formation and dissolution is presented. The novel reaction-transport model contains an open three-phase system of two solid compounds (organic carbon, gas hydrates), three dissolved species (methane, sulfates, inorganic carbon) and one gaseous phase (free methane). The model computes time-resolved concentration profiles for all compounds by accounting for chemical reactions as well as diffusive and advective transport processes. The reaction module builds upon a new kinetic model of POC degradation which considers a down-core decrease in reactivity of organic matter. Various chemical reactions such as organic carbon decay, anaerobic oxidation of methane, methanogenesis, and sulfate reduction are resolved using appropriate kinetic rate laws and constants. Gas hydrates and free gas form if the concentration of dissolved methane exceeds the pressure, temperature, and salinity-dependent solubility limits of hydrates and/or free gas, with a rate given by kinetic parameters. Global input grids have been compiled from a variety of oceanographic, geological and geophysical data sets including a new parameterization of sedimentation rates in terms of water depth.We find prominent gas hydrate provinces offshore Central America where sediments are rich in organic carbon and in the Arctic Ocean where low bottom water temperatures stabilize methane hydrates. The world's total gas hydrate inventory is estimated at 0:82 Â 10 13 m 3 -2:10 Â 10 15 m 3 CH 4 (at STP conditions) or, equivalently, 4.18-995 Gt of methane carbon. The first value refers to present day conditions estimated using the relatively low Holocene sedimentation rates; the second value corresponds to a scenario of higher Quaternary sedimentation rates along continental margins.Our results clearly show that in-situ POC degradation is at present not an efficient hydrate forming process. Significant hydrate deposits in marine settings are more likely to have formed at times of higher sedimentation during the Quaternary or as a consequence of upward fluid transport at continental margins.

The challenges of quantifying the carbon stored in Arctic marine gas hydrate

“…Notation: P 0 seabed pressure; T 0 , seabed temperature; TG, thermal gradient; K th, thermal conductivity; calculated using six different methane hydrate phase boundaries Quinby-Hunt, 1994, 1997;Distribution Coefficient Method or K vsi -Method, Sloan and Koh, 2008;Moridis et al, 2008;Tischenko et al, 2005;Lu and Sultan, 2008) and assuming 3.5 wt% salinity and steady state conditions. B) Holocene sedimentation rate calculated using the water depth vs sediment accumulation relationship from Burwicz et al (2011). C) Mass of carbon stored in GH estimated using Piñero's et al (2013) …”

Modeling the fate of methane hydrates under global warming

“…The reason for this lies in the nonlinear pressure-and temperature-dependent solubility limit of methane in seawater. Example simulations with the full transient reaction transport model upon which the employed transfer function is built [Burwicz et al, 2011] show that the time the system takes to reequilibrate to a new steady state is much longer than the here considered time scales of seafloor warming. It is therefore legitimate to assume that the GHI scales linearly with the reduction in the stability zone thickness in the global warming experiments.…”

“…Figure 2) without clear convergence [cf. Kvenvolden and Lorenson, 2001;Buffett and Archer, 2004;Milkov, 2004;Klauda and Sandler, 2005;Archer et al, 2009;Boswell and Collett, 2011;Burwicz et al, 2011;Wallmann et al, 2012;Piñero et al, 2013]. Estimates based on the global extrapolation of known hydrate deposits [e.g.…”

“…Estimates based on the global extrapolation of known hydrate deposits [e.g. , Milkov, 2004] suffer from the sparse global data coverage, while the uncertainty in model-based estimates [e.g., Klauda and Sandler, 2005;Archer et al, 2009;Burwicz et al, 2011;Wallmann et al, 2012] results from differences in model assumptions and variable quality of global input data sets.…”

“…It is therefore reasonable to shift the main deposition areas from the shelf to the deeper continental slope and rise. Hence, the sedimentation rate was increased over a ∼500 km wide range around the continental margins by moving deposited material from shelf regions (z = 0-200 m) to continental slopes [Burwicz et al, 2011]. At distance greater than 500 km to the coast sedimentation rates are computed via equation (8).…”