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| Titel |
Astrochronology of the Valanginian Stage (Early Cretaceous) : implications for the origin of the Weissert Event |
| VerfasserIn |
Mathieu Martinez, Jean-François Deconinck, Pierre Pellenard, Stéphane Reboulet, Laurent Riquier |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250076872
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| Zusammenfassung |
| Due to the scarcity of available radioisotopic ages in the Lower Cretaceous, the
Geologic Time Scale presents uncertainties that impact palaeoceanographic and
palaeoclimatic reconstructions. Particularly, the chronological relationship between
the Mid-Valanginian carbon-isotope excursion (namely the ‘Weissert Event’) and
the activity of the Paraná-Etendeka Large Igneous Province is debated. To better
constrain this relationship, an astrochronology of the Valanginian Stage is proposed
based on high-resolution gamma-ray spectrometry measurements performed on five
biostratigraphically well-constrained sections throughout the Vocontian Basin (SE
France).
The Valanginian sediments of the Vocontian Basin are composed of decimetric
hemipelagic marl-limestone alternations. These lithologic cycles are attributed to orbital
forcing because marls and limestones display significant differences within clay mineralogy,
geochemistry and faunal assemblages and these marl-limestone alternations are correlated
throughout the Western Tethys and the Atlantic Ocean. Among the analyzed sections, Vergol
(GSSP candidate for the Berriasian-Valanginian boundary), La Charce (GSSP candidate for
the Valanginian-Hauterivian boundary) and Angles (Valanginian Hypostratotype) are
standard sections for the Valanginian Stage since all ammonite zones and subzones are
precisely identified and bounded.
Spectral analyses were performed using the multi-taper method and amplitude
spectrograms on the gamma-ray signals. The comparison between sedimentary frequency
ratios derived from the spectral analyses and orbital frequency ratios calculated from
astronomical solutions allows the identification of a pervasive dominance of the precession
and the 405 kyr-eccentricity cycles throughout the Valanginian Stage. A duration
of 5.1 myr is proposed for the Valanginian Stage on the base of the recognition
of the 405 kyr-eccentricity cycles. This duration is in agreement with the orbital
calibration proposed from δ13C measurements in the Maiolica Formation (Central
Italy).
By anchoring this proposed astrochronology with available radioisotopic ages for the
Berriasian-Hauterivian interval, it appears that the Paraná-Etendeka activity started ~2 myr
after the onset of the Weissert Event and therefore can not have induced the carbon-isotope
excursion. Instead, following Westermann et al. (2010), we propose that continental carbon
organic storage accompanied by carbonate-platform drownings are responsible for the first
major carbon-isotope shift of the Cretaceous.
Bibliography :
Westermann, S., Föllmi, K.B., Adatte, T., Matera, V., Schnyder, J., Fleitmann, D.,
Fiet, N., Ploch, I., Duchamp-Alphonse, S., 2010. The Valanginian δ13C excrusion
may no be an expression of a global oceanic anoxic event. EPSL 290, 118-131. |
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