HR: 09:50h
AN: B41D-06    [Abstracts]
TI: Arctic Methane and the 8.2 kyr BP Event: Past Warning of Future Danger?
AU: * Nisbet, E G
EM: e.nisbet@gl.rhul.ac.uk
AF: Dept. Geology Royal Holloway, Univ. London, Egham, Surrey, TW200EX United Kingdom
AB: Methane mixing ratio is closely linked to temperature in the record. This implies strong feedbacks, but which factor drives and which responds remains controversial. Understanding near-past examples may tell us much about the possible future. The 8.2 kyr BP climate event (Spahni et al., 2003) is generally attributed to 8.45 kyr BP drainage of Glacial Lake Agassiz. However, the 8.2 kyr BP Storegga slide (Bondevik et al., 2003) and associated mega-earthquakes (with 5-15m offsets -Arvidsson, 1996) may have also contributed. Ice from the slide and surface freshwater introduced by floods from simultaneous lake ruptures may have temporarily collected in the northern Norwegian and Barents Seas. Earthquake uplift may have reduced Atlantic inflow over the then-upwarped entry to the Barents Sea. Permafrost and hydrate carried to depth by the slide would have introduced freshwater and gas to the deep Norwegian basin, perhaps briefly weakening Atlantic meridional overturning circulation. All these factors may have extended the time of winter ice cover in the Barents, Kara, and Norwegian seas, cooling boreal wetlands and reducing methane emissions. Conversely, hydrate in Storegga debris would have emitted methane for a century, perhaps hastening return of normality. If the 8.2 kyr BP event was simply the product of special events geologically unlikely today, that briefly shut down boreal wetland methane emissions, then there is little concern. But the 8.2 kyr BP event's lesson may lie as much in the rapid recovery as in the cooling:- that on a quasi-modern Earth, methane can increase sharply, by up to 100 ppb in a few years, just as it did in the glacial terminations (Nisbet, 2002). Future boreal warming, if moisture is adequate, could sharply increase methane and VOC emissions from boreal wetland and forest. Disseminated hydrate decay as well as local Arctic methane pockmark blowouts may occur in the mid-late 21st century if the Barents and Kara seafloors warm markedly. Slumps and earthquakes may occur in the high Arctic as ice melts. Though perhaps not large compared to other anthropogenic forcing effects, these short-term inputs to an already warming global climate system could perhaps trigger northward shift of northern summer jet streams, with significant consequences. Arvidsson, R. (1996) Science 274: 744-746 Bondevik. S et al.. (2003) EOS 84, 289, 293 Nisbet, E.G. (2002) Phil. Trans. R. Soc. Lond. A 360, 581-607 Spahni, R., et al. (2003) G.R.L., 30, 25-1-25-4.
DE: 9315 Arctic region
DE: 1610 Atmosphere (0315, 0325)
DE: 1615 Biogeochemical processes (4805)
DE: 0315 Biosphere/atmosphere interactions
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting