HR: 0800h
AN: PP51B-0599    [Abstracts]
TI: The Lower Toarcian Carbon-Isotope Perturbation: Geochemical And Biological Evolution During A Methane Release Event
AU: * Hermoso, M
EM: hermoso@ccr.jussieu.fr
AF: Université Pierre et Marie Curie - JE 2477 Biominéralisations et Paléoenvironnements, 4, place Jussieu - Case postale 116, Paris, 75005 France
AU: Le Callonnec, L
PP51B-0599 AF: Université Pierre et Marie Curie - JE 2477 Biominéralisations et Paléoenvironnements, 4, place Jussieu - Case postale 116, Paris, 75005 France
AU: Minoletti, F
PP51B-0599 AF: Université Pierre et Marie Curie - JE 2477 Biominéralisations et Paléoenvironnements, 4, place Jussieu - Case postale 116, Paris, 75005 France
AU: Renard, M
PP51B-0599 AF: Université Pierre et Marie Curie - JE 2477 Biominéralisations et Paléoenvironnements, 4, place Jussieu - Case postale 116, Paris, 75005 France
AU: Emmanuel, L
PP51B-0599 AF: Université Pierre et Marie Curie - JE 2477 Biominéralisations et Paléoenvironnements, 4, place Jussieu - Case postale 116, Paris, 75005 France
AB: During the lower toarcian anoxic event, the occurrence of a global negative excursion of the δ¹³C is now well documented. Several interpretations have been proposed: recycling of dissolved inorganic carbon, methane hydrates release from sediments and more recently a thermogenic methane release of Gondwana coals (McElwain et al., 2005).
In this study, the evolution of the composition of the sediments is discussed as well as the types of carbonate producers and the geochemical responses (δ¹³C, δ¹⁸O, traces elements in carbonates: Mg, Fe, Mn, Sr) during the carbon isotopic shift. A granulometric separation technique (Minoletti et al., 2001) was applied to define the distribution of each type of calcareous particles (coccoliths, Schizophreaella, macrocrystals, micarbs). The quantification of these particles during the δ¹³C event and the geochemical signature of each group allow to discriminate variations recorded by primary and diagenetic compounds.
In the boreal zone, the "Schistes carton" Fm. of the Paris Basin exhibits a negative excursion with a magnitude of 6‰. This decrease can be decomposed in various stages assuming several input events. A previous warming phase (δ¹⁸O decrease) should evoke a thermal destabilization of methane hydrates. In sediments preceding the carbon-isotope shift, ankeritic carbonates (Fe-Mg rich carbonate rhomboedrons) dominate over other carbonate particles. The origin of Fe-Mg flux in carbonates could be explained by more reducing conditions (due to the oxidation of methane into carbon dioxide).
During the negative carbon-isotope event, an important decrease in the CaCO₃ content is clearly observed and can be explained by an acidification of marine water and a higher dissolution of calcitic particles. Changes in the sea water composition can be deduced from granulometric fractions analyses. Biogenic-rich fractions (mainly coccoliths) effectively record the negative carbon-isotope shift as well as the bulk.
A cyclostratigraphic analysis using magnetic susceptibility variations allows to estimate the duration of the δ¹³C shift interval. The identification and counting of obliquity's cycles lead to an approximate duration of 100 ky (E. Huret, pers. comm., work in progress).
In the tethyan realm (Umbria-Marche Basin and Lusitanian Basin) the sedimentary and geochemical records show significant differences compared with the boreal domain. Moreover, correlations between the two domains are difficult because of discrepanciesproblems in the biostratigraphic frameworks. The magnitude of the δ¹³C shift is smaller (-3‰ tethyan vs. -6‰ boreal) but its extension is wider (respectively 20 vs. 4 meters). These differences (magnitude vs. extension) can be explained by a higher accumulation rate in the Tethys than in the northern Europe.

McElwain J. C., Wade-Murphy J. and Hesselbo S. P. (2005) - Changes in carbon dioxide during an oceanic anoxic event linked to intrusion into Gondwana coals. Nature (435) pp. 479-482.
Minoletti F., Gardin S., Nicot E., Renard M., Spezzaferri S. (2001) - Mise au point d'un protocole expérimental de séparation granulométrique d'assemblages de nannofossiles calcaires: applications paléoécologiques et géochimiques. Bull. Soc. Géol. Fr. (172) pp. 437-446.

DE: 1051 Sedimentary geochemistry
DE: 1630 Impacts of global change (1225)
DE: 4802 Anoxic environments (0404, 1803, 4834, 4902)
DE: 4806 Carbon cycling (0428)
DE: 9611 Jurassic
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005