Transient seafloor venting on continental slopes from warming‐induced methane hydrate dissociation

Abstract

Methane held in frozen hydrate cages within marine sediment comprises one of the largest carbon reservoirs on the planet. Recent submarine observations of widespread methane seepage may record hydrate dissociation due to oceanic warming, which consequently may further amplify climate change. Here we simulate the effect of seafloor warming on marine hydrate deposits using a multiphase flow model. We show that hydrate dissociation, gas migration, and subsequent hydrate formation cangenerate temporary methane venting into the ocean through the hydrate stability zone. Methane seeps venting through the hydrate stability zone on the eastern Atlantic margin may record this process due to warming begun thousands of years ago. Our results contrast with the traditional view that venting occurs only updip of the hydrate stability zone.

Overestimating climate warming‐induced methane gas escape from the seafloor by neglecting multiphase flow dynamics

“…The average dissociation rate for the HS approach is 5–10 times faster than the coupled T + H simulations depending on the initial thickness of the MHSZ and the intrinsic permeability (Figure b). Although faster dissociation rates for the HS approach compared to a multiphase flow model were also reported by Darnell and Flemings [], the difference seem to be less pronounced in their study.…”

“…To overcome the limitations of the HS approach, detailed site‐specific and regional assessments of hydrate dissociation have been conducted using more complex multiphase models [e.g., Moridis et al ., ; Reagan and Moridis , , ; Reagan et al ., ; Marín‐Moreno et al ., ; Thatcher et al ., ; Darnell and Flemings , ; Stranne et al ., ]. However, these studies have not quantitatively investigated limitations of the HS approach and how these may influence current global and basin‐scale estimates of CH 4 gas release from the seabed.…”

“…It is widely recognized that the HS approach neglects important dynamic processes that decrease the rate of dissociation and the amount of CH 4 that can escape from the seafloor [Darnell and Flemings, 2015]. These include (1) increasing pore pressures during hydrate dissociation [Xu and Germanovich, 2006], (2) freshening of pore waters during dissociation [Xu and Germanovich, 2006], (3) the endothermic dissociation reaction [Holder et al, 1988], and (4) the mobility of gas within the sediments [Reagan and Moridis, 2008] amount that is ultimately retained in the seafloor [Stranne et al, 2016] (Figure 1).…”

A densification mechanism to model the mechanical effect of methane hydrates in sandy sediments

“…Hyodo et al estimated the hydrate saturation within the sediment based on the stoichiometry of the hydrate formation reaction and assuming that all the methane gas injected converted into hydrate. Several studies have proposed that hydrate and gas can coexist under hydrate stability conditions . In particular, Sahoo et al show experimental evidence in which hydrate formation stops with up to 13% of gas still on the sediment under favorable pressure, temperature, and salinity conditions.…”

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

“…If the overlying ocean warms, hydrate that has accumulated beneath the seabed over a long period can dissociate and methane may be released into the ocean. Present-day venting into the oceans at several locations may be attributed to such a mechanism [Darnell and Flemings, 2015;Phrampus and Hornbach, 2012;Phrampus et al, 2014;Westbrook et al, 2009], although the origin of the methane involved remains controversial [Berndt et al, 2014]. Widespread hydrate dissociation has the potential to lead to a positive feedback in which the released methane and its oxidation product, carbon dioxide, enhance warming [Archer and Buffett, 2005].…”