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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    ABSTRACT: The acquisition of high precision geochronological data for young basalts, particularly low potassium examples <500 ka old, is a non-trivial task. This is due to low 40Ar radiogenic argon yields, high atmospheric argon levels and, sometimes, extraneous argon contamination. In this study we present new, high precision argon isotopic data for a basalt flow from the Mount Rouse volcano of the Newer Volcanic Province, southeast Australia. These data were generated using a new generation, multi-collector, ARGUSVI mass spectrometer. We show that the multi-collector system achieves an order of magnitude improvement in analytical precision and a concomitant improvement in our ability to detect minor isotopic disturbances, compared to ‘conventional’ single-collector mass spectrometry. We report a revised eruption age of 284.4 ± 1.8 ka (0.6%, 95% CI) for Mount Rouse, based on laser step-heating experiments of several groundmass aliquants (50-100 mg) from a single sample (180-250 μm size fraction). This result is considerably more precise than our previous estimate of 303 ± 13 ka (4.3%, 2σ), obtained using an older generation VG3600 mass spectrometer and larger (∼200 mg) sample aliquants.
    Quatemary Geochronology 08/2014; 22. DOI:10.1016/j.quageo.2014.02.005 · 2.69 Impact Factor
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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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    ABSTRACT: [1] Cosmogenic 10Be is a promising precipitation index, because its fallout flux in sediments is mainly controlled by wet precipitation after its production in the atmosphere. Here we report on a new study for reconstructing precipitation during the last 130 ka using 10Be measurements from Chinese loess, with multi-variable linear regression to remove the geomagnetic field modulation and dust flux dilution effects from the loess 10Be record. The broad similarity between our result and speleothem δ18O indicates that the new precipitation record is robust. It also records an interesting increase in precipitation that occurred during Marine Isotope Stage 3 (MIS 3), exhibiting a similar rainfall amount with that of MIS 5, suggesting that MIS 3 is a special period with strengthened summer Monsoon intensity. By comparison with a stacked marine isotope record and a summer insolation record, our precipitation data clearly shows a close correspondence with Northern Hemisphere summer (JJA) solar insolation changes on orbital timescales. During MIS 3, our record follows the insolation differential between 30°N and 30°S, suggesting that rising rainfall changes during MIS 3 are a response to the Inter-hemispheric summer insolation differential forcing.
    Journal of Geophysical Research: Solid Earth 01/2014; 119(1). DOI:10.1002/2013JB010296 · 3.44 Impact Factor
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    • "Reversed and transitional directions in these flows, with correspondingly low field intensities, have since been dated by a number of authors (e.g. Guillou et al. 2004; Plenier et al. 2007; Singer et al. 2009). The most recent radioisotopic dating of the Laschamp and Olby flows using "
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    ABSTRACT: Geomagnetic excursions are brief deviations of the geomagnetic field from behaviour expected during `normal secular' variation. The Laschamp excursion at ˜41 ka was one such deviation. Previously published records suggest rapid changes in field direction and a concurrent substantial decrease in field intensity associated with this excursion. Accurate dating of excursions, and determination of their durations from multiple locations, is vital to our understanding of global field behaviour during these deviations. We present here high-resolution palaeomagnetic records of the Laschamp excursion obtained from two Ocean Drilling Program (ODP) Sites, 1061 and 1062 on the Blake-Bahama Outer Ridge (ODP Leg 172). High sedimentation rates (˜30-40 cm kyr-1) at these locations allow determination of transitional field behaviour during the excursion. Palaeomagnetic measurements of discrete samples from four cores reveal a single excursional feature, across an interval of 30 cm, associated with a broader palaeointensity low. We determine the age and duration of the Laschamp excursion using a stratigraphy linked to the δ18O record from the Greenland ice cores. This chronology dates the Laschamp excursion at the Blake Ridge to 41.3 ka. The excursion is characterized by rapid transitions (less than 200 yr) between stable normal polarity and a partially reversed polarity state. The palaeointensity record is in good agreement between the two sites, revealing two prominent minima. The first minimum is associated with the Laschamp excursion at 41 ka and the second corresponds to the Mono Lake excursion at ˜35.5 ka. We determine that the directional excursion during the Laschamp at this location was no longer than ˜400 yr, occurring within a palaeointensity minimum that lasted 2000 yr. The Laschamp excursion at this location is much shorter in duration than the Blake and Iceland Basin excursions.
    Geophysical Journal International 11/2013; 195(3-3):1519-1533. DOI:10.1093/gji/ggt327 · 2.56 Impact Factor
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