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 (POCar) which is microbially converted into methane, the thickness of the gas hydrate stability zone (GHSZ) where methane can be trapped, and the delivery of methane from deep-seated sediments by ascending pore fluids and gas into the GHSZ. Recently, Marquardt et al. (2010) developed a transfer function to predict the gas hydrate inventory in diffusion-controlled geological systems based on POCar and GHSZ. We present a new parameterization of this function and apply it to global datasets of bathymetry, heat flow, seafloor temperature and organic carbon accumulation estimating a global mass of only 91 Gt of carbon (GtC) stored in marine methane hydrates. 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 and passive margins, which were derived from mass balance considerations, this extended transfer function predicts the formation of gas hydrates along the continental margins worldwide. Different scenarios were investigated resulting in a global mass of sub-seafloor gas hydrates of 400–1100 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

Marine‐controlled source electromagnetic study of methane seeps and gas hydrates at Opouawe Bank, Hikurangi Margin, New Zealand

“…At shallow depth, where the areas of high resistivity and the seismic conduits reach the seafloor, free gas may be prevailing as seafloor doming [Koch et al, 2015] and active seafloor venting suggest. Together with our results from Opouawe Bank, these studies support the concept of gas hydrate sweet spots in areas of high fluid flow and gas venting as suggested by, e.g., Liu and Flemings [2007], Boswell and Collett [2011], and Piñero et al [2013]. Insight from such experiments should be easily transferred and applied to other seep site areas within the gas hydrate stability field.…”

“…At intermediate depths between 50 and 200 m below seafloor, saturations are between 40 and 80% and gas hydrate may be the dominating pore filling constituent. At shallow depths from 10 m to the seafloor, free gas dominates as seismic data and gas plumes suggest.Of particular interest are areas of enhanced fluid flow and methane seepage from deepwater sediments which are often found along compressive margins and have been associated with the formation of locally higher gas hydrate concentrations [e.g., Collett et al, 2009;Piñero et al, 2013]. These seep structures are characterized by the migration of methane-rich fluids and/or free gas along subvertical faults and fissures into the gas hydrate stability zone (GHSZ).…”

Paleo‐methane emissions recorded in foraminifera near the landward limit of the gas hydrate stability zone offshore western Svalbard

“…Methane hydrate, an ice-like compound which forms under high pressure and low temperature conditions from methane-saturated water, is a natural temperature-sensitive methane reservoir [Hester and Brewer, 2009]. Significant quantities of methane are stored as hydrate in marine sediments [e.g., Dickens, 2011;Piñero et al, 2013]. The volume of the gas hydrate stability zone (GHSZ) decreases in response to increasing ocean bottom water temperatures, causing a likely release of gaseous methane [e.g., Dickens, 2011;Reagan and Moridis, 2007].…”

Mechanistic insights into a hydrate contribution to the Paleocene‐Eocene carbon cycle perturbation from coupled thermohydraulic simulations

“…If such catastrophic mechanisms are excluded, and our model runs are broadly representative, then the CIE can be explained by hydrate dissociation only if (i) fractures were present or formed during hydrate dissociation to enhance the permeability and (ii) the minimum hydrate inventory is at least double the approximately 2000 Pg C [e.g., Dickens et al, 1995] required to account for the CIE based on isotopic mass balance considerations. Given a warm Paleocene ocean and therefore a more restricted hydrate stability field than at present, such a large hydrate inventory is difficult to reconcile with model-based estimates of the modern inventory of approximately 550-3000 Pg C [Buffett and Archer, 2004;Kretschmer et al, 2015;Piñero et al, 2013;Yamamoto et al, 2014]. These inventories might be reconciled if the modern inventory is underestimated by models [Beaudoin et al, 2014] and/or if higher seabed temperatures stimulated significantly greater methanogenesis in the late Paleocene than today [Gu et al, 2011].…”