Mathieu Moiroud face au diffractomètre aux rayons X
L’étude “High-resolution clay mineralogy as a proxy for orbital tuning: example of the Hauterivian-Barremian transition in the Betic Cordillera (SE Spain)” par Moiroud, M., Martinez, M., Deconinck, J.-F., Monna, F., Pellenard, P., Riquier, L., Company, M., publiée à Sedimentary Geology, 282, 336-346. [pdf]
Mathieu Moiroud (Mathieu.Moiroud@u-bourgogne.fr), aujourd’hui en thèse au Laboratoire Biogéosciences, vient de publier les résultats de son M2 effectué sous la direction de Pierre Pellenard et de Jean-François Deconinck. Cette nouvelle étude est complémentaire à celle publiée cette année par Mathieu Martinez, (cf Martinez, et al., 2012), également en thèse au laboratoire Biogéosciences.
Il s’agit ici d’analyses spectrales opérées sur la coupe de Rio Argos, qui est candidate au GSSP. Les variables testées sont les rapports kaolinite/chlorite, kaolinite/illite, kaolinite/(I/S) et la concentration en carbonate de calcium. La composition en minéraux argileux s’avère intimement liée à la lithologie, et dérive des variations climatiques induites par les cycles astronomiques. La calibration orbitale permet de proposer des durées pour les zones à T. hugii et P. ohmi.
Résumé: The Río Argos section, located in the Subbetic Domain of the Betic Cordillera (SE Spain), exhibits well-exposed, undisturbed hemipelagic marl-limestone alternations attributed to orbital forcing. This section encompasses the sediments at the Hauterivian-Barremian transition (Early Cretaceous), including the Faraoni Oceanic Anoxic Event. A high-resolution mineralogical study was performed on the total carbonate content and the clay fraction by X-ray diffraction, to test the ability of clay minerals to record astroclimate forcing, assuming very limited influence from burial diagenesis on clay minerals and the dominance of inherited materials. The clay mineral composition is systematically related to the lithology, with enrichment in illite/smectite mixed-layers (I/S) in calcareous beds, and enrichment in kaolinite and illite in marly interbeds. Kaolinite/chlorite, kaolinite/illite, and kaolinite/(I/S) ratios as well as calcium carbonate content were tested as climate variables, using the Multi-Taper Method for spectral analyses. The identification of orbital cycles was performed by comparing frequency ratios of sedimentary cycles to frequency ratios calculated for the Hauterivian from the La2004 astronomical cycles. Orbital calibrations generated from the four mineralogical proxies are consistent, providing durations of ca. 535 kyr for the T. hugii and ca. 645 kyr for the P. ohmi ammonite zones. The link between clay mineral signature and orbital cycles suggests that bed-interbed alternations are produced by the cyclic high-frequency fluctuations of continental runoff, mainly driven by precession.