HR: 17:45h
AN: B44C-08    [Abstracts]
TI: Assessing the Evidence for Extensive Wildfires at the Cretaceous-Tertiary Boundary
AU: * Belcher, C M
EM: c.belcher@gl.rhul.ac.uk
AF: Dept of Geology, Royal Holloway University of London, Egham, Surrey, TW20 0EX United Kingdom
AU: Collinson, M E
EM: m.collinson@gl.rhul.ac.uk
AF: Dept of Geology, Royal Holloway University of London, Egham, Surrey, TW20 0EX United Kingdom
AU: Finch, P
EM: p.finch@rhul.ac.uk
AF: Centre for Chemical Sciences, Royal Holloway University of London, Egham, Surrey, TW20 0EX United Kingdom
AU: Scott, A C
EM: a.scott@gl.rhul.ac.uk
AF: Dept of Geology, Royal Holloway University of London, Egham, Surrey, TW20 0EX United Kingdom
AB: Models of the Cretaceous-Tertiary impact at Chicxulub have suggested that the thermal radiation released by the impact would have been sufficient to ignite extensive wildfires. Eight non-marine K-T sequences stretching from New Mexico to Saskatchewan have been studied in order to test this hypothesis. A multi-proxy approach has been devised by identifying and using key palaeo wildfire proxies (charcoal, soot and polyaromatic hydrocarbons (PAH's)) in combination to assess the extent of biomass burning as part of the K-T events. Soot and PAH's cannot be used to indicate fire location, as soot and PAH's from one large fire could be spread globally. The morphology of the soot and nature of the PAH's present can be used to determine their source, allowing identification of those created by biomass burning versus those from coal, gas and hydrocarbons in the K-T rocks. In contrast to soot and PAH's charcoal is a product uniquely produced by the combustion of vegetation. Charcoal in non-marine rocks provides an excellent tool to record the distribution of wildfires and therefore assess the extent of any thermal radiation associated with the impact at Chicxulub. Quantitative data from three different measures of charcoal abundance ({\it in situ} in polished blocks of rock and macro- and microscopic charcoal particles released from sieving of demineralised sediment) reveal that the K-T boundary rocks across the Western Interior of North America contain significantly less charcoal than is typical of the Cretaceous background of this area. The Cretaceous sedimentary rocks contain between 4 and 9 times (according to the measure used) more charcoal particles than the K-T sedimentary rocks. Taphonomic factors do not explain this difference. Furthermore non-charred plant remains are also abundant in the K-T rock layers. Re-assessment of the record of soot and PAH's reported in the K-T rocks suggests that the morphology of the soot and the signature of the PAH's is more consistent with them being sourced from the vaporization of hydrocarbon material rather than biomass burning. We conclude that there was no significant wildfire across North America as part of the K-T events. The below background levels of charcoal in the K-T rocks allows the ground temperatures following the K-T impact to be constrained to no more than $545\deg$C at any point and not above $325\deg$C for any significant period. This indicates that the impact at Chicxulub did not generate sufficient thermal power to ignite extensive wildfires.
DE: 1600 GLOBAL CHANGE (New category)
DE: 1630 Impact phenomena
DE: 0400 Biogeosciences
DE: 0614 Biological effects
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting