25th International Meeting on Organic Geochemistry IMOG 2011

P-212
Constraining stable carbon and hydrogen isotope excursions of
fire events from controlled burning experiments and
applications to the Triassic-Jurassic extinction event
Caroline Jaraula 1 , Kliti Grice 1 , Christiane Vitzthum von Eckstadt 1 , David Kelly 2 , Stephen
Clayton 1 , Luis Felipe Opazo 3 , Richard Twitchett 3
1 Curtin University, Bentley, Western Australia, Australia, 2 Curtin University, Margaret River, Western
Australia, Australia, 3 University of Plymouth, Plymouth, United Kingdom (corresponding
author:C.Jaraula@curtin.edu.au)
Controlled burning experiments were conducted
separately on Australian C3 eucalyptus trees (Marri,
Karri, Jarrah), conifer, wild oats &C4 spinifex &
kangaroo grasses to constrain isotopic shifts for
plants due to burning. Unburnt biomass, resulting
ashes & emitted volatile organic compounds (VOCs)
were analysed for changes in biomarker distributions
& their stable C & H isotopic compositions. The
biomass primarily contain odd/over/even
predominance of long chain n-alkanes (n-C27 to n-C33)
& their δ 13 C are consistent to those reported for C3 (-
22 to -42‰) & C4 (-9 to -29‰) plants (e.g. Rieley et
al., 1991). VOCs trapped on a TenaxTA resin were
thermally desorbed for compound-specific isotope
analyses & yield heavier δ 13 C (Table 1; Vitzthum von
Eckstaedt et al., 2010 submitted) than ash or
biomass. In the ash, the predominant n-alkanes are
shorter (n-C20 to n-C23) & useful in tracking fire
history. Experiments are underway to ascertain
Table 1. Compound-specific C isotope of VOCs
compound C3 plants δ 13 C (‰) C4 plants δ 13 C (‰)
Max Min Max Min
Benzene -26.3 -30.4 -14.7 -17.8
Toluene -26.3 -29.4 -14.0 -18.1
Ethylbenzene -26.3 -26.3
m-xylene -25.2 -26.8
Styrene -25.4 -29.0 -13.2 -16.4
Naphthalene -24.6 -26.5 -17.4 -18.3
Bulk (‰) -24.6 -30.4 -13.2 -18.3
whether these alkanes are from burnt wood or leaves.
On the average, δ 13 Cn-alkanes in C3 plant ash are 4‰
enriched compared to the unburnt biomass, whereas
C4 plant ash are depleted by 6.39‰. It is likely that
burning releases 13 C-depleted hydrocarbons occluded
in phytoliths, which are more prominent in C4
grasses, causing the overall depletion of δ 13 Cn-alkanes
in C4 ash consistent with previous studies (e.g. Krull
et al., 2003). These n-alkanes attributed to biomass
burning is investigated across several localities
bearing the Triassic/Jurassic boundary deposits (Tr/J)
as combustion biomarkers & fossil charcoal have
been reported in abundance. H & O indices from rockeval
pyrolysis of Tr/J classify St. Audries Bay, Engl&
as most immature, with the highest TOC typical of
Type II deposits compared with those from Lyme
Regis & Larne, Northern Ireland. Each locality is
characterised according to their change in terrestrial
plant biomarkers (i.e. waxes), marine-derived
biomarkers & HMW PAHs across Tr/J. Compoundspecific
δ 13 C & δD are tracked before, during & after
the extinction event (Grice et al., 2010 this volume;
Williford et al., 2010), compared with marine
carbonates, & their isotopic excursions hypothesized
to be influenced by massive fire events associated
with the extinction event.
References
Grice K, Nabbefeld B, Twitchett R, Summons RE,
Hays L, Williford K, McElwain J, Holman A, Böttcher
M 2011. Exploring mass extinction events
(Triassic/Jurassic & Permian/Triassic): Association
with global warming events. IMOG these
proceedings.
Krull E, Skjemstad JO, Graetz D, Grice K, Dunning W,
Cook G, Parr JF 2003. 13 C-depleted charcoal from
C4 grasses & the role of occluded carbon in
phytoliths. Org Geochem 34, 1337-1352.
Rieley G, Collier RJ, Jones DM, Eglinton G 1991. The
biogeochemistry of Ellsmere Lake, UK-I Source
correlation of leaf wax inputs to the sedimentary
lipid record. Org Geochem 17, 901-912.
Williford KH, Grice K, Holman A, McElwain JC.
Molecular & Stable Isotopic Signatures of Extreme
Heat Stress at the Triassic/Jurassic Boundary
Nature Geoscience Submitted Jan 2011.
Vitzthum von Eckstaedt C, Grice K, Ioppolo-Armanios
M, Chidlow G. δD & δ13C analyses of
atmospheric volatile organic compounds by
thermal desorption gas chromatography
isotope ratio mass spectrometry. Journ of
Chrom A. Submitted Jan 2011.
351