25th International Meeting on Organic Geochemistry IMOG 2011

P-454
Tracing of palaeofloristic changes in the Paris basin (France)
during Jurassic: contribution of the retene/cadalene ratio
Yueming Lu, Yann Hautevelle, Raymond Michels
UMR7566 G2R, CNRS, Nancy Université Faculté des Sciences Boulevard des Aiguillettes BP 70239,
Vandoeuvre-lès-Nancy, France (corresponding author:yueming.lu@g2r.uhp-nancy.fr)
Vascular plants synthesize a large diversity
of low molecular weight compounds and more
particularly bioterpenoids. Many biochemical studies
on extant plants have pointed out that certain
bioterpenoids are specific and only occur in precise
taxa while others are generic and are widely
distributed in plant kingdom. Because these
bioterpenoids can be preserved within sediments,
their diagenetic counterparts (also called
geoterpenoids) are useful proxies of ancient
vegetation. Therefore, vascular plant biomarkers can
be used as a chemostratigraphic tool for tracing
palaeovegetation changes on hinterlands. Because
these changes are under climatic control, they can
also be used for tracing climatic evolutions through
geological times.
Among all these plant biomarkers, cadalene
and retene are certainly the most well-known.
Cadalene is a sesquiterpenoid considered as a
generic plant biomarker because it comes from the
diagenesis of cadinenes and cadinols. Indeed, these
biological compounds are synthesized by many plant
taxa. At the opposite, retene is a more specific
diterpenoid since it comes from the diagenesis of
biological abietanoids (abietic acid, ferruginol, etc.)
mainly produced by conifers.
This is why, the ratio retene/cadalene
(Re/Ca) can be used, in a first approach, to trace
paleofloristic changes through geological times (van
Aarssen et al., 2000). Then, a careful study of plant
biomarkers can allow to assess more precise
information on palaeofloras.
Hautevelle et al., (2006) pointed out a
significant increase of the Re/Ca ratio at the end of
the Lower Oxfordian in the eastern part of the Paris
basin. This evolution is also remarkably synchronous
with the progressive installation of the Oxfordian
carbonate platform. A careful study of the
diterpenoids associated to retene shows that their
distribution is very typical of those of fossil Pinaceae.
This increase in the Re/Ca ratio were thus interpreted
as an increase of the proportion of Pinaceae on the
London-Brabant massif at the end of the Lower
Oxfordian which can be linked to an increase of
aridity.
Then, this same approach was extended to
the study of the whole Jurassic of the Paris basin. For
this, we calculated the Re/Ca ratio for nearly 60
samples coming from the A901 core. It is located in
the Northern part of the Paris basin and the studied
sedimentary series are dated from Hettangian to
Middle Oxfordian. Calculated ratios show significant
evolutions through the Jurassic indicating
palaeofloristic and palaeoclimatic changes.
Furthermore, these evolutions seem to be correlated
to the 2 nd order alternations of carbonate platforms
and argillaceous deposits. Unfortunately, it is not
possible to determine more precisely these floristic
and climatic changes as it was the case for the Lower
Oxfordian.
More recently, we studied two another cores
(EST 432 & EST 433) drilled in the eastern part of the
Paris basin and allowing the study of the whole
Jurassic. More than 50 samples were studied in order
to calculate the Re/Ca ratio. The preliminary study
shows that we found the same evolutionary trend as
that obtained in the northern part of the Paris basin.
Furthermore, the evolution of the Re/Ca ratio
through Jurassic in the Paris basin appears to be
quite similar to that published by van Aarssen et al.
(2000) concerning the Jurassic of the Carnarvon
Basin (Australia). Such similarities in places very
remote lead us to believe that these evolutions are
linked to global palaeofloristic and palaeoclimatic
changes.
References:
Hautevelle, Y., R. Michels, F. Malartre, and A.
Trouiller (2006), Vascular plant biomarkers as proxies
for palaeoflora and palaeoclimatic changes at the
Dogger/Malm transition of the Paris Basin (France),
Organic Geochemistry, 37(5), 610-625.
van Aarssen, B. G. K., R. Alexander, and R. I. Kagi
(2000), Higher plant biomarkers reflect
palaeovegetation changes during Jurassic times,
Geochimica et Cosmochimica Acta, 64(8), 1417-
1424.
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