40Ar/39Ar, K–Ar and 230Th–238U dating of the Laschamp excursion: A radioisotopic tie-point for ice core and climate chronologies
Introduction
Accurate and precise geochronology is fundamental to understanding the past 50,000 years of Earth history during which climate frequently oscillated on a millenial time scale while the major transition into and out of the last global glacial maximum took place. The main chronometer for this period is radiocarbon (14C), but because the 14C/12C ratio in the atmosphere varies over time, radiocarbon dates depend upon a precise knowledge of this temporal variation (Bard et al., 2004). The calibration curve most widely adopted by the radiocarbon community – known as INTCAL04 (Reimer et al., 2004) – uses 14C dates of rings in fossil trees, extended by way of paired 230Th and 14C dates from tropical corals to 26,000 y BP (= before year 1950 AD; radiocarbon convention). However, between 26,000 and 50,000 y BP coral data are sparse and there is no consensus curve (Reimer et al., 2006). Several approaches, including varved lake sediments dated by 230Th (Kitigawa and van der Plicht, 1998), a set of carefully selected, pristine corals that have been dated using 230Th and 14C methods (Fairbanks et al., 2005), and the “stratigraphic” method of tuning climate-sensitive signals in 14C-dated marine sediment cores to the δ18O stratigraphy of the Greenland Ice Sheet Project II (GISP2) ice core (Hughen et al., 2004), yield different curves from which the calibration of a single 14C age can vary by more than ± 16% in calendar age (e.g., Bard et al., 2004). A more recent approach by Hughen et al. (2006) to reduce uncertainty in the 14C calibration curve ties the marine record in the Cariaco Basin to the 230Th chronology of the Hulu Cave speleothem record (Wang et al., 2001).
On one hand, the stratigraphic method in which the chronology is based on the counting of annual layers in ice offers perhaps the most continuous and straightforward template for calibrating 14C ages between 26,000 and 50,000 y b2k. On the other hand, it has proven difficult to establish a precise chronology for the GISP2 and Greenland Ice core Project (GRIP) ice cores using annual layer counting beyond the Holocene (Southon, 2004). For example, uncertainties in the most widely used timescale for the GISP2 ice core rise dramatically from ± 700 y to ± 1700 y in the period between about 40 and 50 ka (Alley et al., 1997, Meese et al., 1997). No single or consensus chronology arose until the recent multi-parameter counting of annual layers, including visual stratigraphy, conductivity of ice and melt-water and concentrations of Na+, Ca2+, SO42−, NO3−, and NH4+ at the NorthGRIP core site correlated to previous results from the GRIP ice core, resulting in the Greenland Ice Core Chronology 2005 or GICC05 (Andersen et al., 2006). Although the GICC05 timescale is a major step forward, 2σ precision estimated from layer counting degrades with depth from < 1% at 15 ka to > 4%, or more than ± 1600 y, at 40 ka, near the useful limit of 14C (Andersen et al., 2006, Svensson et al., 2006). Many types of proxy records of climate depend on correlation to the GISP2 or GRIP ice cores. For example, interpretations of high-resolution 230Th-dated speleothem records of climate and rainfall in temperate regions far removed from the polar ice caps rely on matching δ18O records of polar ice and cave carbonate that, particularly for the period of marine isotope stage 3 between 35 and 50 ka, would benefit from independently-dated tie points (Wang et al., 2001, Genty et al., 2003, Yuan et al., 2004). Thus, it remains desirable to find an alternative, independent method with which to calibrate the ice core chronologies, rather than relying on the ice cores themselves to calibrate the 14C chronometer (Southon, 2004).
The production of radiocarbon and other cosmogenic nuclides such as 10Be and 36Cl in the atmosphere is strongly modulated by changes in the Earth's magnetic field (Wagner et al., 2000, Laj et al., 2002, Muscheler et al., 2004). The most significant fluctuation of the past 50,000 years occurred during the Laschamp excursion as the strength of the magnetic field recorded by lava flows and marine sediments dropped to less than about 10% of its prior value (Laj et al., 2002, Laj et al., 2004, Channell, 2006). Cosmogenic nuclide flux into polar ice and marine sediments increased significantly at this time (Raisbeck et al., 1992, Yiou et al., 1997, CiniCastagnoli et al., 1995), and several cooling lava flows captured “snapshots” of the changing field geometry and intensity (Roperch et al., 1988, Chauvin et al., 1989, Guillou et al., 2004, Cassata et al., 2008). We report new 40Ar/39Ar data from the Laschamp lava flow, 40Ar/39Ar and unspiked K–Ar data from the la Louchadière flow, and 230Th–238U data from the Olby flow that had been previously dated using combined 40Ar/39Ar and unspiked K–Ar methods (Guillou et al., 2004). Moreover, we re-calculate the 40Ar/39Ar ages obtained previously from the Laschamp and Olby flows (Guillou et al., 2004) and from a transitionally magnetized flow in New Zealand (Cassata et al., 2008), using a newly proposed standard age in order to combine these data into a single, robust, accurate date for the Laschamp excursion. Given the effect of the changes in geomagnetic dipolar intensity on the production of cosmogenic nuclides, including 10Be and 36Cl, our new radioisotopic age determination provides a tie-point that can be used to constrain correlations between proxy climate records from speleothems (e.g., Wang et al., 2001, Genty et al., 2003) or marine sediments (e.g., Hughen et al., 2004), and the ice cores from Greenland and Antarctica.
Section snippets
Lava flows that record the Laschamp excursion
Trachyandesitic lava flows that crop out near the villages of Laschamp and Olby in the Chaîne des Puys, Massif Central, France (Fig. 1), were discovered by Bonhommet and Babkine (1967) to record a nearly reversed magnetic field direction relative to that of the modern field. Until 2004, however, the ages of these lava flows, and another flow issued from the Puy de Louchadière that records a different transitional field direction, remained poorly constrained despite efforts over four decades
40Ar/39Ar incremental heating experiments
For this study, new samples were collected from outcrops of the Laschamp and Louchadière flows (Fig. 1). Sample CP-06-04 from the Laschamp flow was taken from the easternmost of two outcrops, about 2 m from where a previous sample (Laschamp LOL-01) had been collected for 40Ar/39Ar and K–Ar dating by Guillou et al. (2004). It brings to three the number of independent samples that we have dated from the two outcrops of this lava flow. 40Ar/39Ar incremental heating analyses at the University of
Radioisotopic age of the Laschamp excursion
The new 40Ar/39Ar, K–Ar, and 238U–230Th data from the Laschamp, Olby and Louchadière flows – together with data reported by Guillou et al. (2004) – bring the total number of independent radioisotopic age determinations of these transitionally magnetized lavas in our laboratories to nine. In Table 4 we have re-calculated the 40Ar/39Ar ages of Guillou et al. (2004) to be consistent with a 28.201 Ma age for the FCs fluence monitor (Kuiper et al., 2008). A fourth transitionally magnetized lava, the
Discussion
The radioisotopic age of 40,700 ± 950 y b2K of the Laschamp excursion coincides with the position of the weakest geomagnetic field intensity recorded globally in marine sediments comprising the GLOPIS-75 record. Given the assumption made in calculating the mean radioisotopic age, the ± 950 y of uncertainty clearly does not constrain the maximum possible duration of the Laschamp excursion. For this we must turn to the marine sediment and ice core records. In both, the geomagnetic excursion –
Conclusions
The new radioisotopic age of the Laschamp excursion, based on 40Ar/39Ar, unspiked K–Ar and 230Th–238U data from the Laschamp, Olby, and Louchadière lava flows, France, together with the 40Ar/39Ar age of the Mclennan's Hill lava flow, New Zealand, is 40,700 ± 950 y b2k including both analytical and systematic sources of uncertainty. The internal 230Th–238U isochron of 40.8 ± 4.7 ka obtained from the Olby flow using ICP-MS methods is: (a) in excellent agreement, but nearly three times more precise
Acknowledgements
We thank Lee Powell for his many years of advice on automation of the lab at UW-Madison. Raimund Muscheler and Anders Svensson are thanked for providing ice core data. Discussions with Grant Raisbeck helped stimulate us to make this contribution and are greatly appreciated. Thoughtful review comments by Anders Svensson and Michel Condomines helped us to clarify key points and also are greatly appreciated along with editor Rick Carlson's efficiency. Singer's research on the geomagnetic field was
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<sup>10</sup>Be depositional flux variation in the central Indian Ocean during the last 43 ka
2022, Science of the Total EnvironmentTiming of Quaternary geomagnetic reversals and excursions in volcanic and sedimentary archives
2020, Quaternary Science ReviewsCitation Excerpt :The most thoroughly documented magnetic excursion is undoubtedly the Laschamp excursion at ∼41 ka with aberrant magnetic directions that have sub-millennial duration (see reviews of Laj et al., 2014 and Singer, 2014). Volcanic rocks close to the village of Laschamps in the Chaîne des Puys region of the Massif Central (France) provided the first credible record of any excursion (Bonhommet and Babkine, 1967), supported by subsequent studies in the same region (e.g., Bonhommet and Zahringer, 1969; Roperch et al., 1988; Guillou et al., 2004; Singer et al., 2009; Laj et al., 2014). The geomagnetic origin of the Laschamp excursion was debated well into the 1980s when its recording in the Chaîne des Puys was interpreted by some authors in terms of a self-reversal process due to negative magnetostatic interactions of two titanomagnetite phases (Heller, 1980; Heller and Petersen, 1982).
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2019, Earth and Planetary Science Letters