The sensitivity of gas hydrate reservoirs to climate change: Perspectives from a new combined model for permafrost-related and marine settings
Introduction
Gas hydrates are ice-like crystalline compounds of water and gas (in nature predominantly methane), that occupy the pore space of sediments if low temperature and high pressure conditions prevail, and if a sufficiently large volume of gas is available (Davie and Buffett, 2001). Natural gas hydrates have been discovered in marine sediments on continental margins, in the sedimentary infill of deep lake basins, as well as in association with continental and relic sub-shelf permafrost (Collett et al., 2009). Milkov (2004) estimates the global volume of natural gas contained in submarine hydrate reservoirs to be in the range of 1 to 5 × 1015 m3 (equivalent to ± 500–2500 Gton of methane carbon). In addition to this, about 20 Gton of carbon is estimated to be stored in Arctic permafrost-related hydrate deposits (Ruppel, 2015). This vast amount of methane, locked up in hydrate reservoirs at relatively shallow depths in the geosphere (compared to regular natural gas accumulations), has drawn the attention of the research community because it may constitute a potential energy resource (Koh et al., 2012), and because it may play a role in the global carbon cycle and influence the Earth's climate (e.g. Nisbet, 1990, Dickens et al., 1995, Kvenvolden, 2002, Kennett et al., 2003, Archer, 2007, Archer et al., 2009, Ruppel, 2011, Ruppel and Kessler, 2017). Awareness of the latter issue originated in the 1980′s, with a.o. Kvenvolden (1988) realizing that gas hydrate reservoirs may release significant amounts of methane (CH4) and its oxidation product carbon dioxide (CO2), both greenhouse gases, to the atmosphere, when hydrates decompose as a result of global warming. The subsequent increase in atmospheric greenhouse gas levels would contribute to already warming temperatures and thus maintain or even amplify the dissociation of the hydrates. As such, this positive feedback mechanism would mark a close interrelationship between geologically rapid climate change and fluctuations in atmospheric greenhouse gas levels on the one hand, and variations in the size of the global gas hydrate reservoir on the other (Fig. 1). Hydrate reservoirs in high-latitude areas are believed to be most sensitive to this process, because climatic changes are most pronounced here (e.g. IPCC, 2013) and hydrates exist below shallower water depths. Hypotheses relying on this hydrate-climate coupling have been formulated within the framework of Quaternary ‘glacial to interglacial’ transitions (e.g. Nisbet, 1990, Paull et al., 1991, Loehle, 1993, Poort et al., 2005) and higher-frequency ‘stadial to interstadial’ climatic oscillations (Kennett et al., 2003), in the debate of present-day climate change (e.g. Westbrook et al., 2009, Biastoch et al., 2011, Hunter et al., 2013, Marín-Moreno et al., 2013, Ruppel and Kessler, 2017), as well as in the context of the Paleocene-Eocene thermal maximum (PETM) and other past hyperthermal events (Dickens et al., 1995).
However, the role of gas hydrates in the carbon cycle and global climate, although being suggestive, remains controversial. A wide range of studies have contributed to the debate, presenting either field observations, measurements and/or quantitative modelling results in favor of or against a significant role for gas hydrates in controlling Earth's climate. The discussion is complex because gas hydrate systems are governed by the interplay of geological, chemical, physical and biological processes, each working in a certain direction, at a specific rate, and according to a small or large set of imposed parameters (Xu and Ruppel, 1999). Many studies have focused on only one single aspect of the gas hydrate system, hereby ignoring the potential impact of interrelated processes on the presented results. Moreover, many studies focus on one specific study area and one specific hydrate-bearing geological setting, with permafrost-related hydrate settings being strongly under-represented. As a result, a general consensus has not yet been reached.
Obviously, a more integrated approach and general perspective are necessary. A recent study by Ruppel and Kessler (2017) reviews the full climate – hydrate feedback cycle shown in Fig. 1, with the main focus on present-day, anthropogenically forced climate change. The here presented study concentrates more deeply on the first step in this cycle and reviews how gas hydrate reservoirs react when they are subjected to changing climatic conditions. Specifically, the objectives of this work are to: (1) provide a concise overview of key studies and examples, to illustrate the available tools and above-outlined issues when evaluating the sensitivity of gas hydrate reservoirs to climate change; and (2) complement this review with a new, simple one-dimensional hydrate dissociation model, though based on a large and critically chosen set of parameters and processes, in order to thoroughly evaluate where, how fast, in what fashion and to what extent gas hydrates can dissociate when climatic oscillations disturb hydrate stability conditions. In this study, we specifically focus on the case of Quaternary and present-day climate variability. The environmental changes associated with the PETM took place on different timescales and within a different palaeogeographical context. Hence the discussion on the role of hydrate decomposition in the PETM is not addressed here, but is for example reviewed by Dickens (2011). Importantly, the here presented model is applicable to both permafrost and marine environments, which allows to assess and compare four distinct hydrate-bearing settings across a hypothetic high-latitude continental margin (i.e. i) thick onshore permafrost, ii) relic sub-shelf permafrost, iii) upper continental slope and iv) lower continental slope/continental rise; see Fig. 2). By means of a sensitivity analysis we illustrate how variations in certain model parameters affect the model outcome. Changes in temperature and pressure (through sea level variations) representative for the last deglaciation following the Last Glacial Maximum (LGM) and for contemporary climate change are then simulated, in order to determine the climate-sensitivity of gas hydrate reservoirs in these two case studies. The model incorporates a number of important factors that have not always been fully considered in previous studies, such as i) the rates of the involved processes, especially the dissociation rate, but also the (re-)formation rate of hydrates, and ii) the effective occurrence of hydrate in the subsurface, as the sedimentary column in which gas hydrates can theoretically occur is not necessarily entirely filled with hydrates.
Section snippets
Review of studies on the sensitivity of gas hydrates to climate change
Natural gas hydrate reservoirs were discovered and recognized as a major pool of carbon in the shallow geosphere during the 1960′s and 1970′s. Soon, scientists started to realize that changes in the size of the global hydrate inventory could interfere with the Earth's carbon cycle and climate (e.g. Kvenvolden, 1988, MacDonald, 1990, Kvenvolden, 1993). Early studies have primarily focused on the role of melting methane hydrates in Quaternary ‘glacial to interglacial’ transitions. For example,
Modelling gas hydrate reservoir response to climatic variations
In this study, we seek to develop a relatively simple, but robust numerical model, that is applicable in any study area and in both permafrost-related and marine environments. We opted for a one-dimensional model because most hydrates occur in stratigraphically constrained hydrate reservoirs which lack significant lateral variation (Archer et al., 2009). The parameters used in the model are summarized in Table 2. Methane is assumed to be the only gas component in the system, so that all values
Case studies
The model is used to simulate the hydrate stability response to two specific cases of environmental change: (i) the deglaciation following the LGM, which happened on a timescale of several thousands of years, and (ii) present-day anthropogenic climate warming, occurring on a shorter timescale of a few tens to hundreds of years. We specifically focus on hydrate reservoirs across a high-latitude continental margin which are inferred to be the most sensitive to environmental changes (e.g. Archer
Evaluation of the sensitivity of gas hydrates to the last deglaciation and Quaternary climatic oscillations
The results of the simulation of the last deglaciation (CASE LGM) help to shed a light on the hypothesized role of gas hydrates in Quaternary climatic oscillations (see Table 1 for a list of previous studies). Most strikingly, in each of the considered geographic settings, the modelled hydrate dissociation timescale is at least in the order of tens of thousands of years. This does not support a rapid feedback with warming climate, as proposed by Nisbet (1990) and Loehle (1993) for the last
Conclusions
During the past decades, the potential role of gas hydrate systems in global climate has been the motive of extensive, international research. The large volumes of methane contained in gas hydrate reservoirs and the dependence of their stability on pressure and temperature suggest that perturbations in the size of the global gas hydrate inventory could drive variations in atmospheric greenhouse gas levels, and thus impact on the Earth's climate. However, to date there is little consensus on the
Acknowledgements
TM is supported by a doctoral scholarship of the Ghent University Special Research Fund (BOF). The authors wish to thank Dr. Nabil Sultan and one anonymous reviewer for constructive reviews which significantly enhanced the quality of this manuscript. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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