Abstract
Abstract. Atmospheric methane from episodic gas hydrate (GH) destabilization, the "clathrate gun" hypothesis, is proposed to affect past climates, possibly since the Phanerozoic began or earlier. In the terrestrial Beaufort-Mackenzie Basin (BMB), GHs occur commonly below thick ice-bearing permafrost (IBP), but they are rare within it. Two end-member GH models, where gas is either trapped conventionally (Case 1) or where it is trapped dynamically by GH formation (Case 2), were simulated using profile (1-D) models and a 14 Myr ground surface temperature (GST) history based on marine isotopic data, adjusted to the study setting, constrained by deep heat flow, sedimentary succession conductivity, and observed IBP and Type I GH contacts in Mallik wells. Models consider latent heat effects throughout the IBP and GH intervals. Case 1 GHs formed at ~0.9 km depth only ~1 Myr ago by in situ transformation of conventionally trapped natural gas. Case 2 GHs begin to form at ~290–300 m ~6 Myr ago in the absence of lithological migration barriers. During glacial intervals Case 2 GH layers expand both downward and upward as the permafrost grows downward through and intercalated with GHs. The distinctive model results suggest that most BMB GHs resemble Case 1 models, based on the observed distinct and separate occurrences of GHs and IBP and the lack of observed GH intercalations in IBP. Case 2 GHs formed >255 m, below a persistent ice-filled permafrost layer that is as effective a seal to upward methane migration as are Case 1 lithological seals. All models respond to GST variations, but in a delayed and muted manner such that GH layers continue to grow even as the GST begins to increase. The models show that the GH stability zone history is buffered strongly by IBP during the interglacials. Thick IBP and GHs could have persisted since ~1.0 Myr ago and ~4.0 Myr ago for Cases 1 and 2, respectively. Offshore BMB IBP and GHs formed terrestrially during Pleistocene sea level low stands. Where IBP is sufficiently thick, both IBP and GHs persist even where inundated by a Holocene sea level rise and both are also expected to persist into the next glacial even if atmospheric CO2 doubles. We do not address the "clathrate gun" hypothesis directly, but our models show that sub-IBP GHs respond to, rather than cause GST changes, due to both how GST changes propagates with depth and latent heat effects. Models show that many thick GH accumulations are prevented from contributing methane to the atmosphere, because they are almost certainly trapped below either ice-filled IBP or lithological barriers. Where permafrost is sufficiently thick, combinations of geological structure, thermal processes and material properties make sub-IBP GHs unlikely sources for significant atmospheric methane fluxes. Our sub-IBP GH model histories suggest that similar models applied to other GH settings could improve the understanding of GHs and their potential to affect climate.
Highlights
1.1 Proposed gas hydrate impacts on past climate and their future climate change potentialGas Hydrates (GHs) are proposed to have exerted a major control on past climates, both for the glacial-interglacial periods (Nisbet, 2002; Kennett et al, 2003), and for other important geological events extending back to the start of the Phanerozoic or earlier (e.g., Benton and Twitchett, 2003; Kirschvink and Raub, 2004)
We do not address the “clathrate gun” hypothesis directly, but our models show that sub-ice-bearing permafrost (IBP) GHs respond to, rather than cause ground surface temperature (GST) changes, due to both how GST changes propagates with depth and latent heat effects
Most of the thinning occurs in the IBP, which buffers the effect on the GH layer, such that the accompanying GH layer thinning is very small and within the range attributed to previous interglacial variations, in spite of the accelerated surface warming that is anticipated to accompany a doubling of atmospheric CO2
Summary
1.1 Proposed gas hydrate impacts on past climate and their future climate change potential. J. Majorowicz et al.: Inferred gas hydrate and permafrost stability history models linked to climate change destabilization, called commonly the “clathrate gun” hypothesis (Kennett et al, 2003), is the trigger for, or a significant cause of, important global climate changes including glacialinterglacial transitions. Our previous analysis, using a 1-D model (Majorowicz et al, 2008), of Holocene-Pleistocene temperature history implications for IBP and GH layers in onshore BMB during the last 600 ka showed that GHs can be stable through interglacial intervals despite large GST variations due to the buffering effect of overlying IBP and latent heat effects. Models for the formation and persistence of subIBP GHs provide us insights into the geological history of GHs, their probable formation age, and their potential to be a source of atmospheric methane flux that might affect climates past and future. The work will evaluate the climate risks posed by future global and regional temperature change to IBP and GH stability and their potential to serve as a future methane source to the atmosphere
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