25th International Meeting on Organic Geochemistry IMOG 2011

O-08
An integrated lipid biomarker and chemostratigraphic record of
biospheric evolution through the late ordovician mass extinction
event
Megan Rohrssen, Gordon Love
University of California, Riverside, Riverside, California, United States of America (corresponding
author:mrohr001@ucr.edu)
The Late Ordovician (~450-440 Ma)
witnessed the two-phased mass extinction of an
estimated 84% of all marine animal species, one of
the largest extinctions in the Phanerozoic eon [1].
Unlike other mass extinctions, marine fossil
communities after the Ordovician-Silurian (O-S)
boundary appear very similar to those that preceded
it. Bolide impact and gamma-ray bursts have both
been proposed as extinction triggers, but these
mechanisms lack robust supporting evidence for the
end Ordovician [e.g. 2]. There is strong evidence,
however, for major Southern hemisphere
glaciation/deglaciation episodes, with accompanying
water column temperature and chemistry change, sea
level fluctuations and positive carbon and oxygen
isotope excursions [3-4], putatively coincident with the
recorded faunal extinction.
Anticosti Island, Quebec, Canada, provides
an exceptional opportunity to assemble nearly
continuous chemostratigraphic records of the Late
Ordovician in a shallow, paleotropical mixed
carbonate / siliciclastic setting. In this study we have
used lipid biomarkers from bitumen and kerogens
from sedimentary rocks exposed in outcrop and drill
core to gain insight into the taxonomy of the major
marine primary producers, water column depositional
conditions and overall microbial community structure
during the Hirnantian Isotopes of Carbon Excursion
(HICE) and mass extinction.
Lipid biomarkers from Anticosti outcrops
have low thermal maturities consistent with the burial
history of the island and lack signs of younger
petroleum-derived hydrocarbons (e.g. zero oleanane
from angiosperms, HBIs from diatoms). In addition,
Anticosti biomarkers have low C28/C29 sterane ratios
(