Abstract
Abstract. The accumulation of gas hydrates in marine sediments is essentially controlled by the accumulation of particulate organic carbon (POC) which is microbially converted into methane, the thickness of the gas hydrate stability zone (GHSZ) where methane can be trapped, the sedimentation rate (SR) that controls the time that POC and the generated methane stays within the GHSZ, and the delivery of methane from deep-seated sediments by ascending pore fluids and gas into the GHSZ. Recently, Wallmann et al. (2012) presented transfer functions to predict the gas hydrate inventory in diffusion-controlled geological systems based on SR, POC and GHSZ thickness for two different scenarios: normal and full compacting sediments. We apply these functions to global data sets of bathymetry, heat flow, seafloor temperature, POC input and SR, estimating a global mass of carbon stored in marine methane hydrates from 3 to 455 Gt of carbon (GtC) depending on the sedimentation and compaction conditions. The global sediment volume of the GHSZ in continental margins is estimated to be 60–67 × 1015 m3, with a total of 7 × 1015 m3 of pore volume (available for GH accumulation). However, seepage of methane-rich fluids is known to have a pronounced effect on gas hydrate accumulation. Therefore, we carried out a set of systematic model runs with the transport-reaction code in order to derive an extended transfer function explicitly considering upward fluid advection. Using averaged fluid velocities for active margins, which were derived from mass balance considerations, this extended transfer function predicts the enhanced gas hydrate accumulation along the continental margins worldwide. Different scenarios were investigated resulting in a global mass of sub-seafloor gas hydrates of ~ 550 GtC. Overall, our systematic approach allows to clearly and quantitatively distinguish between the effect of biogenic methane generation from POC and fluid advection on the accumulation of gas hydrate, and hence, provides a simple prognostic tool for the estimation of large-scale and global gas hydrate inventories in marine sediments.
Highlights
High sedimentation rates are calculated for the Oregon, the southern Argentinean, and the Indian margins, where gas hydrates have been predicted or drilled (e.g. Trehu et al, 2004; Collett et al, 2008)
Our study shows that the global inventory of marine GH can be estimated by the application of transfer functions that were derived from systematic runs of a numerical transportreaction model
GH formation in marine sediments is determined by the thickness of the GHSZ, the particulate organic carbon (POC), sedimentation rate (SR), and upward fluid advection
Summary
Hydrology and Earth SystemSubmarine more than4g0asrehgyiodnrastews or(lGdHwS)idechaiaevnend cbtheeeensir recovered in presence has been deduced from geophysical, geochemical and geological evidences at more than 100 continental margin sites (e.g. Mazurenko and Soloviev, 2003; Milkov and Sassen, 2002; Lorenson and KvOencveoladenn, 2S0c07ie). nThceey have been recognized as a key factor with respect to past and future climate change (e.g. Hester and Brewer, 2009; Dickens, 2001a; Adam, 2002; Archer, 2007; Lunt et al, 2011; Biastoch et al., 2011) and are considered as a new unconventional resource of natural gas (e.g. Sloan, 2003; Boswell, 2009). Mazurenko and Soloviev, 2003; Milkov and Sassen, 2002; Lorenson and KvOencveoladenn, 2S0c07ie). Hester and Brewer, 2009; Dickens, 2001a; Adam, 2002; Archer, 2007; Lunt et al, 2011; Biastoch et al., 2011) and are considered as a new unconventional resource of natural gas In contrast to the general progress in gas hydrate research made so far, the global abundance of GH in marine sediments still remains poorly constrained. Estimates are generally based on extrapolation of field data Kvenvolden and Claypool, 1988; Dickens, 2001b; Milkov, 2004) and Published by Copernicus Publications on behalf of the European Geosciences Union. E. Pinero et al.: Estimation of the global inventory of methane hydrates
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