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

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 × 10<sup>15</sup> m<sup>3</sup>, with a total of 7 × 10<sup>15</sup> m<sup>3</sup> 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

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

“…Increasing by an order of magnitude the sedimentation rates shown in Figure 3b and keeping the upward FF constant at 0.01 cm yr -1 , results in an Arctic average hydrate saturation of 0.2%. For a fix FF, increasing the SR tends to reduce the GH inventory (Piñero et al, 2013). Using Holocene sedimentation rates ( Fig.…”

“…from Wallmann et al (2012) for normally and fully compacted sediments, respectively, and M C _3 from Piñero et al (2013) for an upwards advective flow velocity of 0.01 cm yr -1 . In M C _1, M C _2, and M C _3 the first number is the mass carbon calculated using the sedimentation rates from Method #1 and the second using Method #2.…”

“…To consider other possible sources of dissolved methane into the GHSZ, Piñero et al (2013) transfer function (Eq. 1) was also applied.…”

“…1) was also applied. These transfer functions are fitting equations to the numerical results from a reactive transport code that considers the dominant physical and biogeochemical processes and parameters including: sediment compaction, the solubility of methane in pore water, the formation and dissociation of GH and formation and dissolution of free methane gas in pore water, diffusive and advective transport of dissolved constituents, input and degradation of particulate organic carbon (POC) and particulate organic nitrogen (PON) via sulfate reduction and methanogenesis, anaerobic oxidation of methane (AOM), and formation and adsorption of ammonium, dissolved inorganic carbon (DIC) and methane (Piñero et al, 2013). The input parameters for the transfer functions are: (i) thickness GHSZ (H GHSZ , m), (ii) sedimentation rate (SR, cm kyr -1 ), (iii) POC (wt %), and (iv) upward advective fluid flow from mechanisms other than mechanical compaction (FF, cm yr -1 ).…”

“…Here, we illustrate the influence of uncertainties in the parameters controlling the thickness of the GHSZ by considering a rather large perturbation of ±30% in the calculation of the Arctic marine GH inventory from published state-of-the-art transfer functions (Wallmann et al, 2012;Piñero et al, 2013). We also present an analysis where some of these estimates are constrained with GH saturations derived from geophysical data and propose an explicit function that allows the estimation of an equivalent upward fluid flow of methane-rich fluids into the GHSZ required satisfying geophysically-derived saturations.…”