Developing a chronostratigraphic tool for climatic archives : absolute dating (K/Ar and ⁴⁰Ar/³⁹Ar) and paleomagnetism applied to lavas

• Main Author: Sasco, Romain
• Other Authors: Laboratoire des Sciences du Climat et de l'Environnement Gif-sur-Yvette (LSCE), Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Paris-Saclay-Centre National de la Recherche Scientifique (CNRS), Université Paris Sud - Paris XI, Hervé Guillou
• Format: Doctoral or Postdoctoral Thesis
• Language: French
• Published: HAL CCSD 2015
• Subjects: Earth magnetic field; Paleomagnetism; Paleointensity; K-Ar dating; ⁴⁰Ar/³⁹Ar dating; Age-paleointensity data; Timescale calibration; Ardèche; Canary Islands; Champ magnétique terrestre; Paléomagnétisme; Paléointensité; Datation K-Ar; Datation ⁴⁰Ar/³⁹Ar; Couple âge-paléointensité; Calibration d’échelle de temps; Îles Canaries; [SDU.STU]Sciences of the Universe [physics]/Earth Sciences; ice core
• Online Access: https://tel.archives-ouvertes.fr/tel-01266050; https://tel.archives-ouvertes.fr/tel-01266050/document; https://tel.archives-ouvertes.fr/tel-01266050/file/VA2_SASCO_ROMAIN_28012015.pdf

The understanding of climatic mechanisms and rapid climate changes requires a high-resolution, robust, and precise timescale which allows long-distance and multi-archives correlations.An appropriate tool to construct such a timescale is provided by the Earth magnetic field (EMF). The EMF is independent from climatic variations and its past evolution, global at the surface of the Earth, is recorded by most of the geological/climatic archives. Sedimentary sequences provide continuous records of relative intensities of the EMF on timescales usually based on ice core age models or orbital tuning. Lavas, though discontinuously emitted through time, record the absolute intensity of the EMF during their cooling at the surface of the Earth. Lavas are dated using 2 complementary methods: ⁴⁰Ar/³⁹Ar and K-Ar, both independent from climatic parameters. Lavas have therefore the potential to deliver tie-points (age-paleointensity couples) enabling the time calibration of sedimentary sequences and their transfer onto absolute intensity scale and chronological time scale. This timescale can then be transferred to other climatic archives. The present study focusses on the last 200 ka with lavas sampled from young volcanoes of Ardèche (South Massif Central, France) and recent phases of volcanism in the Canary Islands.Lava flows from Ardèche provided unexploitable paleointensity results and ages with large uncertainties. Therefore, they failed to provide suitable tie-points. However, our geochronological results evidence how crucial the combination of both the K-Ar and 40Ar/39Ar methods is to test the accuracy and geological meaning of the ages. Ardèche lavas have abundant mantellic and crustal xenoliths, potential carriers of excess ⁴⁰Ar*. Our study suggests that the argon excess is located in sites that decrepitate at low temperature (<600°C). Because ⁴⁰Ar/³⁹Ar ages are not affected by excess ⁴⁰Ar*, they provide reliable results. The new age dataset indicates that the volcanic activity of Ardèche can be divided in 3 phases: ...