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| Titel |
Inferred gas hydrate and permafrost stability history models linked to climate change in the Beaufort-Mackenzie Basin, Arctic Canada |
| VerfasserIn |
J. Majorowicz, J. Safanda, K. Osadetz |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1814-9324
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| Digitales Dokument |
URL |
| Erschienen |
In: Climate of the Past ; 8, no. 2 ; Nr. 8, no. 2 (2012-03-30), S.667-682 |
| Datensatznummer |
250005476
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| Publikation (Nr.) |
 copernicus.org/cp-8-667-2012.pdf |
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| Zusammenfassung |
| 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. |
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