The Laschamp Excursion in Lava Flows From the Massif Central (France): New Data and Synthesis

The Laschamp excursion, originally identified by N. Bonhommet in the French Massif Central, has been intensively investigated worldwide. Based on its short duration, this excursion has been very often used as a chronostratigraphic tiepoint by different communities. Recently, however, Plenier et al., (2007) have proposed a duration of about 6000 years for this excursion, in sharp contrast with previous estimates based on sedimentary and volcanic studies, as well as on analysis of the 10Be and 36Cl records in ice cores (Laj et al., 2000; 2004; Wagner et al., 2000; Guillou et al., 2004; Svensson et al., 2006; Singer et al., 2009). If correct this new evaluation would make this chronostratigraphic tie point largely less interesting, and would also have an impact of geodynamo mechanisms proposed to explain differences between excursions and reversals of the geomagnetic field (Gubbins, 1999). To further investigate this point, we have conducted a new study in the Massif Central. In addition to the three original lavas exposed at La Louchadière, Laschamp and Olby, recently dated by Guillou et al. (2004) and by Singer et al. (2009), we have intensively investigated several other outcrops characterized by transitional paleomagnetic directions, including different outcrops of the same lava flows at different localities separated by up to a few km, using a coupled paleomagnetic/ K/Ar and 40Ar/39Ar approach. All the obtained 40Ar/39Ar and K/Ar ages are tightly grouped and very consistent with the previous estimates of the Laschamp excursion of Guillou et al (2004) and Singer et al. (2009) around 41 ka. Geomagnetic field intensities (Thellier & Thellier) also show transitional values around 16 μT, however slightly higher than those obtained at the Laschamp and Olby outcrops (Roperch et al., 1988).The paleomagnetic directions, while all transitional, are sometimes significantly different from each other, indicating that the different outcrops have recorded different " snapshots " of the excursion, although the data do not allow to describe the dynamics of the excursion because the ages are undistinguishable from one another. This clearly indicates that the duration of the excursion is shorter or of the same order of magnitude as the accuracy of the radiometric dating (~ 1000-1500 years). These results, therefore, confirm a short duration of the Laschamp excursion. The hypothesis of a 6000 years duration can be confidently rejected. The results will also be discussed in combination with the most recent published results from lavas unambiguously erupted during the same period (from Iceland and New Zealand) and compared with new results from the Mono Lake excursion (Kissel et al, this session). It clearly appears that the two excursions are well separated in time, the Mono Lake occurring about 7 ka after the Laschamp excursion with 3 millennial climatic cycles (Dansgaard-Oeschger cycles 10 to 7) separating them.