On the age of the Laschamp geomagnetic excursion

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Abstract

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.

Introduction

Accurate and precise dating of paleoclimate proxies is of paramount importance in deciphering the time scale of climatic changes recorded in ice cores and sedimentary archives. The temporal calibration of such records is commonly based on layer-counting techniques or time-series modeling of variations in δ18O coupled to orbital-forcing climate theory. The resulting chronology is only relative, however, and requires absolute tie-points to recast the recorded climatic variations into a quantitative time frame.

Direct dating by radiogenic isotopes (U–Th, K–Ar, and 40Ar/39Ar) or cosmogenic nuclides (14C) is commonly precluded by the lack of adequate material in distal or deep-sea deposits (e.g., tephra). Radioisotopic dating of authigenic or biogenic phases occurring in sedimentary sequences is also compromised by problems of age-reservoir correction (14C), detrital contribution (U-series), and the issue of the calendar calibration of the 14C time scale [1], [2].

Dating geomagnetic excursions recorded on land and identified in marine sediments provides a unique means to calibrate the time scale of the δ18O climate signal, independent of orbital tuning theory. Through their magnetic properties, deep-sea sediments carry information on temporal variations of the geomagnetic field that occurred during deposition [3], [4], [5]. These variations, tied directly to climate-driven δ18O oscillations in deep-sea records, can be correlated to geomagnetic excursions recorded on land by volcanic rocks. The unstable behaviour of the Earth's magnetic field during the Brunhes chron is characterized by many short-lived geomagnetic excursions, corresponding to low intensity of the Earth's magnetic field [3], [4], [5], [6], [7], [8], that can potentially provide a unique series of tie-points for the period between the present day and 790 ka. Modulation of incoming cosmic rays induced by paleofield variations generates distinct perturbations of cosmogenic nuclide flux rates archived in sediments [9], [10], [11], [12] and in polar ice caps [13], [14], [15], [16], [17], [18]. Intervals of enhanced cosmogenic flux corresponding to brief lows in paleomagnetic field intensity can therefore be used to synchronize marine sediment and ice core proxy records. Moreover, the precise timing of changes in the cosmogenic flux rate is a key for calibrating the radiocarbon time scale. Specifically, difficulties in measuring residual 14C in older materials leave the radiocarbon time scale between 35 and 50 ka inadequately constrained, thus independent tie-points in this interval are highly desirable [19].

The classic example of a geomagnetic excursion recorded in late Pleistocene lava flows of the French Massif Central [20], [21] and in reference deep-sea cores worldwide [3], [4] is the Laschamp excursion. This excursion provides a critical tie-point for the period between 35 and 50 ka that is widely used as a global correlation tool in marine sediments [3], [5], [22], [23], GISP and GRIP polar ice cores [15], [16], [17], [18] and volcanic sequences [20], [24], [25], [26], [27]. Early K–Ar and 40Ar/39Ar ages determined from the Laschamp and Olby flows [21], [25], [28], [29] gave a wide range of ages which on average disagree with the age of the Laschamp excursion deduced from the NAPIS-75 (North Atlantic Paleointensity Stack for the last 75 ky [3]) transferred to the GISP2 ice core time scale (using coeval δ18O variations in ice and magnetic properties in sediments) and from the occurrence of the peak in cosmonuclides production in the Summit ice cores in Greenland [18], [15], [16], [17]. Despite three decades of study, the timing of the Laschamp excursion and its temporal and spatial distinction from other geomagnetic excursions, including the Mono Lake event, remain controversial [30]. Here, we combine new K–Ar and 40Ar/39Ar results from two basaltic lava flows in the type locality to better resolve the age of the Laschamp excursion.

This is possible in part due to advances in 40Ar/39Ar incremental heating methods using a resistance furnace which can yield ages for basaltic lava flows between 100 and 20 ka to a precision of better than 5% at the 95% confidence level [31], [32], [33]. Similarly, the unspiked K–Ar dating technique facilitates accurate and precise detection of minute quantities of radiogenic 40Ar from basaltic samples well within the range of the 14C and U-series time scales [34]. Used in combination, these methods have been effective in calibrating terrestrial paleoclimate proxy records for the past 110 ky [33], [35].

Section snippets

Site locations

The “Puy de Laschamp” is part of the Chaîne des Puys (French Massif Central) a narrow, 30-km-long chain of about 100 volcanic edifices, mostly strombolian cones with a few maars and domes, built up during the last 70 ky. The “Puy de Laschamp” gave rise to an eastward directed flow on top of which the village of Laschamp is built. The lava flow was sampled at two different sites, Laschamp-1 and Laschamp-2 (Fig. 1, Table 1) within the village. This 2–5-m-thick flow is mugearitic in composition,

Paleomagnetic results

As noted by Bonhommet et al. [20] and Roperch et al. [24], a secondary remanent component is present in the three sites investigated and is removed at about 5 mT. Beyond this step, a single component of magnetization is observed with a well-defined direction for most samples. A representative example is reported in Fig. 2. The mean angular deviation values (MAD) are smaller than 5°. One sample from the Olby site (Olby-01) was unstable as shown by the MAD value (11°) and was thus rejected from

Discussion

The two lava flows from the Laschamp type locality were previously dated using K–Ar [21], [25], [28], [29], 40Ar/39Ar [28] and U–Th [50] methods. Several ages were also obtained using thermoluminescence and 14C methods on baked sediments below the Laschamp lava flow [29]. The previous 22 K–Ar ages, that range between 21.5±40.0 and 61.5±26.0 ka [21], [25], [28], [29], are summarised in Fig. 4. When pooled together with earlier 40Ar/39Ar plateau ages between 42.9±18.0 and 60.1±15.4 ka [28], the

Conclusions

Six unspiked K–Ar and 13 40Ar/39Ar incremental heating experiments on samples from the Laschamp and Olby basalt flows yield a remarkably concordant age for the Laschamp excursion of 40.4±1.1 ka. When uncertainty in the 40K decay constant is considered, the absolute age of the Laschamp excursion becomes 40.4±2.0 ka. Earlier K–Ar and 40Ar/39Ar ages vary widely, but suggested an age of more than 46 ka for this excursion. Our age acquired on small samples of purified groundmass is 10% younger,

Acknowledgments

The authors are grateful to J.F. Tannau (LSCE) and Lee Powell for sharing their technical expertise and to A. Mazaud (LSCE) for developing the software used for the paleomagnetic measurements and interpretations. A. Gourgaud provided the major element compositions of the Laschamp and Olby flows. D. Kent, N. Thouveny and an anonymous referee provided constructive and helpful comments. C.L. is particularly grateful to Jacques Labeyrie for introducing him about 30 years ago to the mysteries of the

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