25th International Meeting on Organic Geochemistry IMOG 2011

P-062
A diagenetic origin for the moretane anomaly at the Permian
Triassic Boundary
Katherine French 1 , Nicholas Tosca 1 ,Changqun Cao 3 , Gordon Love 2 , Roger Summons 1
1 Massachusetts Institute of Technology, Cambridge, MA, United States of America, 2 University of California
Riverside, Riverside, CA, United States of America, 3 Nanjing Institute of Geology and Palaeontology,
Nanjing, China (corresponding author:klfrench@mit.edu)
The Permian-Triassic Boundary (PTB;
~252.2 Ma) marks the greatest loss of biodiversity in
Earth‘s history with an estimated 95% of species
becoming extinct (1). Many mechanisms have been
proposed as triggers for this mass extinction event
and these are still a matter of considerable debate.
Despite the persisting ambiguity of how the event was
initiated, it has become increasingly evident that both
the marine and terrestrial ecosystems deteriorated
rapidly (2-4). Geochemical techniques have helped to
elucidate the nature of the PTB transition but have
simultaneously opened new questions that require
explanation.
Anomalous C30 moretane (17β(H), 21α(H)hopane)
abundances at the PTB have been reported.
The moretane/hopane ratio is widely used as a
thermal maturity indicator, but like other geochemical
parameters, organic matter sources and depositional
environment can modulate the measured
moretane/hopane ratios of lipid extracts (5). Previous
workers have eliminated thermal maturity as the
underlying cause of the observed C30 moretane
excursions at Meishan. Alternatively, they have
proposed that the C30 moretane/hopane excursion at
the PTB signals either enhanced terrigenous input to
a marginal marine ecosystem, increased marine
acidification, or lithological effects (6-8). Improved
understanding of the moretane record stands to
further constrain our knowledge of the PTB.
For this purpose, we present the
moretane/hopane record of extended desmethyl
homohopanes and methylated hopanes from the
Meishan section in South China. The samples are
from a drill core that spans the lead-up and recovery
intervals and are expected to have minimum surficial
weathering alteration. The desmethyl homohopane
(C31-C34) and 2α- and 3β-methylated hopanes echo
the same moretane/hopane stratigraphic trend
previously observed for C30 hopanes (6). These new
results require a mechanism that can account for the
parallel excursions in the desmethyl and methylated
hopane records. Our results do not preclude the
possibility that enhanced terrigenous organic input
was characteristic of events at the PTB, but this
mechanism does not account for the observed
moretane anomalies in the Meishan section.
Carbonate measurements and clay analysis implicate
lithology as the key factor for generating the
moretane/hopane excursions at the PTB.
Furthermore, our results indicate that caution must be
used when interpreting moretane/hopane data.
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