Estimation of the global inventory of methane hydrates in marine sediments using transfer functions

[摘要] The accumulation of gas hydrates in marine sediments is essentiallycontrolled by the accumulation of particulate organic carbon (POC) which ismicrobially converted into methane, the thickness of the gas hydratestability zone (GHSZ) where methane can be trapped, the sedimentation rate(SR) that controls the time that POC and the generated methane stays withinthe GHSZ, and the delivery of methane from deep-seated sediments byascending pore fluids and gas into the GHSZ. Recently, Wallmann et al.(2012) presented transfer functions to predict the gas hydrate inventory indiffusion-controlled geological systems based on SR, POC and GHSZ thicknessfor two different scenarios: normal and full compacting sediments. We applythese functions to global data sets of bathymetry, heat flow, seafloortemperature, POC input and SR, estimating a global mass of carbon stored inmarine methane hydrates from 3 to 455 Gt of carbon (GtC) depending on thesedimentation and compaction conditions. The global sediment volume of theGHSZ in continental margins is estimated to be 60–67 × 10¹⁵ m³,with a total of 7 × 10¹⁵ m³ of pore volume (available for GHaccumulation). However, seepage of methane-rich fluids is known to have apronounced effect on gas hydrate accumulation. Therefore, we carried out aset of systematic model runs with the transport-reaction code in order toderive an extended transfer function explicitly considering upward fluidadvection. Using averaged fluid velocities for active margins, which werederived from mass balance considerations, this extended transfer functionpredicts the enhanced gas hydrate accumulation along the continental marginsworldwide. Different scenarios were investigated resulting in a global massof sub-seafloor gas hydrates of ~ 550 GtC. Overall, oursystematic approach allows to clearly and quantitatively distinguish betweenthe effect of biogenic methane generation from POC and fluid advection onthe accumulation of gas hydrate, and hence, provides a simple prognostic toolfor the estimation of large-scale and global gas hydrate inventories inmarine sediments.

[学科分类] 地球化学与岩石