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PUBLICATIONS Geophysical Research Letters RESEARCH LETTER 10.1002/2016GL070049 Key Points: • Neglecting dynamic processes associated with hydrate dissociation and multiphase flow leads to largely overestimated dissociation rates • A significant portion of the produced gas is retained within the sediments on a centennial time scale • Simplistic models can overestimate future climate warming-induced CH4 gas release by orders of magnitude

Supporting Information: • Supporting Information S1 • Movie S1 • Movie S2 • Movie S3 • Movie S4 Correspondence to: C. Stranne, [email protected]

Citation: Stranne, C., M. O'Regan, and M. Jakobsson (2016), Overestimating climate warming-induced methane gas escape from the seafloor by neglecting multiphase flow dynamics, Geophys. Res. Lett., 43, 8703–8712, doi:10.1002/ 2016GL070049. Received 15 JUN 2016 Accepted 5 AUG 2016 Accepted article online 10 AUG 2016 Published online 30 AUG 2016

Overestimating climate warming-induced methane gas escape from the seafloor by neglecting multiphase flow dynamics C. Stranne1,2,3, M. O'Regan1,2, and M. Jakobsson1,2 1

Department of Geological Sciences, Stockholm University, Stockholm, Sweden, 2Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden, 3Center for Coastal and Ocean Mapping/Joint Hydrographic Center, Durham, New Hampshire, USA

Abstract

Continental margins host large quantities of methane stored partly as hydrates in sediments. Release of methane through hydrate dissociation is implicated as a possible feedback mechanism to climate change. Large-scale estimates of future warming-induced methane release are commonly based on a hydrate stability approach that omits dynamic processes. Here we use the multiphase flow model TOUGH + hydrate (T + H) to quantitatively investigate how dynamic processes affect dissociation rates and methane release. The simulations involve shallow, 20–100 m thick hydrate deposits, forced by a bottom water temperature increase of 0.03°C yr1 over 100 years. We show that on a centennial time scale, the hydrate stability approach can overestimate gas escape quantities by orders of magnitude. Our results indicate a time lag of > 40 years between the onset of warming and gas escape, meaning that recent climate warming may soon be manifested as widespread gas seepages along the world's continental margins.

1. Introduction Continental margins host large quantities of methane (CH4) stored as hydrates [Beaudoin et al., 2014]. Methane hydrates are crystalline compounds of CH4 and water that are stable under elevated pressure and low temperature conditions such as those found in sediments along continental slopes and the abyssal plains. Due to observed and predicted global warming [Intergovernmental Panel on Climate Change, 2013], there is concern that increased ocean temperatures will lead to destabilization of natural marine hydrate deposits with subsequent seafloor CH4 escape, which could impact ocean chemistry, atmospheric greenhouse gas loads, or both [Dickens, 2003; Archer, 2007, 2015; Hester and Brewer, 2009; Elliott et al., 2010, 2011; Isaksen et al., 2011; Lunt et al., 2011; Phrampus and Hornbach, 2012]. Studies of CH4 dynamics in hydrate-bearing sediments have been performed with analytical and numerical models spanning a wide range of complexity and spatial scales [Yousif et al., 1991; Rempel and Buffett, 1997; Moridis et al., 1998; Ji et al., 2001; Xu, 2004; Sun et al., 2005; Liu and Flemings, 2007; Kwon et al., 2010; Moridis, 2014]. To a first order, the vertical extension of the methane hydrate stability zone (MHSZ) is a function of pore pressure (P), temperature (T), and salinity (S) [Tishchenko et al., 2005]. As a consequence, the MHSZ exists across most continental margins as a thin lens that thickens downslope because of increasing pressure and typically decreasing temperature [Dickens, 2001] (Figure 1a). The thickness of the MHSZ will decrease as seafloor temperatures increase in a warming climate. The change in thickness can be calculated for a new steady state seafloor temperature [Giustiniani et al., 2013] or as a function of time assuming conductive heat transfer [e.g., Biastoch et al., 2011; Phrampus and Hornbach, 2012] (Figure 1a). By combining estimates of sediment porosity and initial hydrate saturation, the change in thickness of the MHSZ is often translated into a corresponding amount of dissociated hydrate and seafloor CH4 gas escape. This method is here referred to as the hydrate stability (HS) approach. Due to its simplicity, it is commonly used to estimate CH4 gas escape from ocean warming scenarios on both regional [Biastoch et al., 2011; Giustiniani et al., 2013] and global [Kretschmer et al., 2015] scales.

©2016. American Geophysical Union. All Rights Reserved.

STRANNE ET AL.

It is widely recognized that the HS approach neglects important dynamic processes that decrease the rate of dissociation and the amount of CH4 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] and the

WARMING-INDUCED SEAFLOOR CH4 GAS ESCAPE

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Geophysical Research Letters

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10.1002/2016GL070049

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Figure 1. (a) Shelf slope MHSZ at initial steady state (t0), during the equilibration (tn), and at the new steady state (t∞). (b) Schematic illustration of the complexity of multiphase fluid flow at the dissociation front (within the three-phase coexistence layer); gas permeability increases, pressure increases, and salinity decreases as hydrate dissociates in pore space. Locally, the endothermic dissociation of hydrate removes heat and slows further dissociation. Also shown is residual gas trapped within the sediments.

amount that is ultimately retained in the seafloor [Stranne et al., 2016] (Figure 1). Although accounted for in this study, the dilution effect tends to be small for low hydrate saturations (lower than ~10%) [Xu and Germanovich, 2006], and therefore, we will not discuss its isolated contribution here. 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., 2005b; Reagan and Moridis, 2008, 2009; Reagan et al., 2011; Marín-Moreno et al., 2013; Thatcher et al., 2013; Darnell and Flemings, 2015; Stranne et al., 2016]. However, these studies have not quantitatively investigated limitations of the HS approach and how these may influence current global and basin-scale estimates of CH4 gas release from the seabed. In this study we investigate how a numerical multiphase flow model, TOUGH + hydrate (T + H), compares to the HS approach in terms of dissociation rate and seafloor CH4 gas escape. The T + H model takes into account the negative feedback from gas expansion and freshwater dilution on the dissociation rate and the retarding endothermic dissociation reaction. We investigate the individual contributions from the retarding pressure feedback and the endothermic dissociation reaction and the influences of intrinsic permeability on dissociation (through pore pressure buildup) and on sediment gas retention.

2. TOUGH + Hydrate Numerical Model The TOUGH + hydrate model [Moridis, 2014] is used in this study to address the short-term (