HR: 0800h
AN: PP11B-0573    [Abstracts]
TI: Calcareous Nannofossil Paleofertility Indicators During the Valanginian Carbon Perturbation in an Epicontinental Basin System (Vocontian, SE France)
AU: * duchamp, s
EM: duchamp@geol.u-psud.fr
AF: UMR 8148 IDES, batiment 504, orsay, 91400 France, Metropolitan
AU: gardin, s
EM: gardin@ccr.jussieu.fr
AF: CNRS-UMR 5143 CEPAGE, tour 56-46 4, place Jussieu, Paris, 75252 France, Metropolitan
AU: fiet, n
EM: fiet@geol.u-psud.fr
AF: UMR 8148 IDES, batiment 504, orsay, 91400 France, Metropolitan
AU: bartolini, a
EM: chiara@ccr.jussieu.fr
AF: CNRS-UMR 5143 CEPAGE, tour 56-46 4, place Jussieu, Paris, 75252 France, Metropolitan
AU: blamart, d
EM: dominique.blamart@lsce.cnrs-gif.fr
AF: Laboratoire LSCE UMR CNRS-CEA, batiment 12, Avenue de la Terrasse, Gif sur Yvette, 91198 France, Metropolitan
AU: pagel, m
EM: pagel@geol.u-psud.fr
AF: UMR 8148 IDES, batiment 504, orsay, 91400 France, Metropolitan
AB: A major and global delta 13C positive excursion is recognized in sediments of the Late Valanginian-Hauterivian interval. This perturbation coincides with a widespread eutrophication of marine ecosystems associated with organic carbon rich deposits and a crisis of carbonate producing biota. Subaerial volcanism of the Parana-Etendeka large igneous province (ca. 132 Ma) was presumably responsible for an increase of CO2, triggering greenhouse conditions, increased weathering and elevated nutrient transfer rates to oceans. High resolution carbon isotope stratigraphy coupled with a quantitative calcareous nannofossils study has been done at Angles section, located in an epicontinental basin of the northern margin of the Tethys (Vocontian basin, SE France). In this kind of environment we can unravel the influence of weathering on delta 13C variations and on abundance fluctuations of primary producers (calcareous nannofossils). We test open sea calcareous nannofossils described in the litterature as good paleofertility indicators to elaborate new nutrient index and reconstruct sea surface trophic conditions in this epicontinental basin. Some calcareous nannofossil taxa fluctuations appear very sensitive to delta 13C variations. The beginning of the delta 13C perturbation (minimim delta 13C values in Hirsutus ammonite biozone) is associated with relatively abundant {\it Watzauneria sp.}, generally considered as an oligotrophic taxon. The slow increase of delta 13C values in Hirsutus to Campylotoxus ammonite biozone is paralleled by the progressive decline of {\it Watzauneria sp.} and by instantaneously rise of specimens considered as mesotrophic indicators such as {\it Discorabdus rotatorus.} and {\it Lithraphidites sp.}. The abrupt increase of the delta 13C signal to a peak (Campylotoxus to Verrucosum biozone) is marked by the definitive decline of {\it Watzauneria sp.} whereas nannofossils usually indicating high fertility conditions ({\it Zeughrabdotus fissus.} and {\it Biscutum sp.}) abound significantly. The plateau of maximum delta 13C values seems to correspond to mesotrophic conditions, with some fluctuations towards more oligotrophic conditions, as testifyed by the ponctual slight abundance of {\it Discorabdus rotatorus.} and {\it Lithraphidites sp.} associated with the ponctual high abundance of {\it Watzauneria sp.}. At last, the return to post-perturbation low delta 13C values (Nicklesi to Callidiscus biozone) is characterized by a continuously rise of the oligotrophic indicator. {\it Biscutum sp.} wanes progressively whereas {\it Zeughrabdotus fissus} definitively exctincts. Most of open sea nannofossils paleofertility indicators seem to have coherent pattern with the variations of the delta 13C signal. Thus, they can be used as nutrient index in epicontinental basin systems. This is not the case for {\it Rhagodiscus asper}, generally considered as an oligotrophic taxon in open sea, while in an epicontinental basin system such as the Vocontian basin, it seems to have the same behaviour as mesotrophic open sea indicators (such as {\it Discorabdus rotatorus.} and {\it Lithraphidites sp.}). In this specific environment, calcareous nannofossils show some "local" response to the global Valanginian perturbation.
DE: 1610 Atmosphere (0315, 0325)
DE: 1615 Biogeochemical processes (4805)
DE: 1620 Climate dynamics (3309)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting