25th International Meeting on Organic Geochemistry IMOG 2011

P-173
Oxidation and physical protection of organic matter by mineral
matrix : their influence on the diagenesis of hopanoids
Yann Hautevelle, Apolline Lefort, Pierre Faure, Raymond Michels
UMR7566 G2R, CNRS, Nancy Université, Vandoeuvre-lès-Nancy, France (corresponding
author:yann.hautevelle@g2r.uhp-nancy.fr)
Hopanoids are among the most abundant
organic compounds in the geosphere. Their
distribution is classically studied in organic
geochemistry for assessing the thermal history of
sedimentary series. Indeed, it is now well known that
hopanoids are biosynthesized under a
thermodynamic unstable biological conformation.
Furthermore, thermal immature sedimentary rocks are
typically characterized by the presence of hopenes.
During thermal stress, hopenes are hydrogenized into
hopanes, the 22S isomer appears for each
homohopanes and the biological conformation is
transformed into the more stable geological
conformation.
However, some of our work suggests that the
classical interpretation of hopane distributions, based
on the influence of heating, should be moderated due
to the fact that other stresses, like oxidation, have a
similar incidence on the diagenesis of hopanoids.
Indeed, experiments show that oxidation of the
thermally immature Callovian-Oxfordian claystones
from the east of the Paris basin also induces a
progressive decrease of hopenes, the apparition of
22S isomers as well as the transformation of
biological conformation into the geological.
We also studied these Callovian-Oxfordian
claystones as well as their Oxfordian surrounding
limestones. The claystones are characterized by a
clearly immature distribution of hopanoids
(abundance of hopenes, hopanes in the biological
conformation and the large predominance of the 22R
epimers over the 22S). Surprisingly, a more mature
distribution was found in the limestones (lower
abundance of hopenes, the presence of higher
proportion of hopanes in the geological conformation
and the significant presence of 22S epimers). In this
case, thermicity cannot be invoked to explain this
discrepancy because the Oxfordian limestones lie
above the claystones and their temperature has not
exceeded 50°C since deposition. So, another
explanation must be advanced to account for this
difference. As oxidation seems to have a similar effect
than thermicity on hopanoid distributions, we can
expect that the ―mature‖ distribution characterizing the
limestones can be related to a more intense post-
depositional oxidation. At the opposite, the hopane
distribution indicates that the organic matter of the
claystones was not oxidized although the deposit
conditions were also oxic. This can be explained by a
protective matrix effect of clay minerals.
In the Cretaceous siltites of Marcoule, we show that
hopanoids also underwent oxidation but only in
specific layers associated to bioturbation. In this case,
oxidation seems to be favored by the disturbation of
the embedding sediment which lowers the protective
matrix effect.
More recently, we focused our investigation
on the carbonate Oxfordian-Kimmeridgian platform at
Gudmont-Villiers (Eastern Paris basin). These
limestones were deposited in a lagoon & shallow
marine environments. They present an alternation of
carbonate mudstone and grainstone beds. These
deposits underwent a similar and very low thermicity
after their deposition since they are located above the
Callovo-Oxfordian claystones. In this case, carbonate
mudstone beds also present a typically ―immature‖
distribution of hopanes while the grainstones show a
more ―mature‖ distribution. Once again, thermicity
cannot be at the origin of this discrepancy. However,
mudstones are associated to a settling hydrodynamic
regime and are rather impermeable, while grainstones
are related to more agitated conditions and typically
porous at deposition time. In mudstones organic
matter is much more protected from oxidation than in
grainstones.
This work clearly shows that care must be
taken when interpreting the distribution of hopanoids
in terms of thermicity. Yet, differences in hopanes
distributions as a function of facies may be combined
with other biomarkers to infer diagenetic/
palaeoenvironmental conditions during deposition.
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