On the age of the Laschamp geomagnetic excursion

Department of Geology and Geophysics, University of Wisconsin-Madison, 1215 West Dayton Street, Madison, WI 53706, USA
Earth and Planetary Science Letters (Impact Factor: 4.73). 11/2004; 227(3-4):331-343. DOI: 10.1016/j.epsl.2004.09.018


The Laschamp geomagnetic excursion is a critical tie-point found directly in deep-sea sediment cores and revealed in polar ice as an abrupt change in the rate of cosmogenic nuclide flux. Despite the importance of this excursion to quantifying paleoclimate proxy records archived in sediment and ice, and to providing an independent calibration of the radiocarbon calendar, its timing remains poorly known. Previous K–Ar, 40Ar/39Ar, and U–Th isochron determinations from lava flows at the type locality in the Massif Central, France, vary widely, are imprecise, and suggest a mean age of about 46.2±2.5 ka (±2σ). Results of 6 new unspiked K–Ar and 13 40Ar/39Ar incremental heating experiments on subsamples from three sites on the Laschamp and Olby flows are concordant and give a weighted mean age of 40.4±1.1 ka (2σ uncertainty including analytical sources only) that is 10% younger than the previous estimates. Considering that the 40K→40Ar decay constant is not known to a precision better than ±2.4%, the most probable radioisotopic age for the Laschamp excursion is 40.4±2.0 ka (2σ, analytical plus decay constant uncertainties). This new age for the Laschamp excursion agrees precisely with that deduced from the NAPIS-75 deep-sea sediment paleointensity stack when calibrated against the GISP2 ice core chronology using the O isotopes in ice and the magnetic properties of the marine cores.

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    • "Ar step-heating analyses of multiple aliquants, to achieve 'reasonable' precision levels of w10% (e.g. Singer and Pringle, 1996; Singer et al., 2002; Guillou et al., 2004; Singer et al., 2004; Jicha and Singer, 2006; Matchan and Phillips, 2011). Until now, the ultimate precision of individual measurements has been limited by the available noble gas mass spectrometers, most of which are essentially singlecollector instruments, based on 1980s designs (e.g. "
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    • "The detailed procedure for the age model is described in the supporting information and the ages of specific boundaries are given in Table S1. In addition, Optically Stimulated Luminescence (OSL) dating results (Figure 2a, black circles) and the age of Laschamp event [Guillou et al., 2004] (Figure 2b, grey bar) detected in this section Figure 1. Map for sampling site and comparing sites in earlier studies. "
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