40Ar/39Ar, K–Ar and 230Th–238U Dating of the Laschamp Excursion: a Radioisotopic Tie- Point for Ice Core and Climate Chronologies

Earth and Planetary Science Letters 286 (2009) 80–88

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Earth and Planetary Science Letters

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40Ar/39Ar, K–Ar and 230Th–238U dating of the Laschamp excursion: A radioisotopic tie- point for ice core and climate chronologies

Brad S. Singer a,⁎, Hervé Guillou b, Brian R. Jicha a, Carlo Laj b, Catherine Kissel b, Brian L. Beard a, Clark M. Johnson a a Department of Geoscience, University of Wisconsin-Madison, 1215 West Dayton St., Madison, WI 53706, USA b Laboratoire des Sciences du Climat et de l'Environnement/IPSL, CEA-CNRS-USVQ, Domaine du CNRS Bat.12, Avenue de la Terrasse, 91198 Gif sur Yvette, France article info abstract

Article history: A brief period of enhanced 10Be flux that straddles the interstadial warm period known as Dansgaard– Received 10 March 2009 Oeschger event 10 in Greenland and its counterpart in Antarctica, the Antarctic Isotope Maximum 10 is but Received in revised form 11 June 2009 one consequence of the weakening of Earth's magnetic field associated with the Laschamp excursion. This Accepted 12 June 2009 10Be peak measured in the GRIP ice core is dated at 41,250 y b2k (= before year 2000 AD) in the most recent Available online 18 July 2009 GICC05 age model obtained from the NorthGRIP core via multi-parameter counting of annual layers. 10 fi Editor: R.W. Carlson Uncertainty in the age of the Be peak is, however, no better than ±1630 y at the 95% con dence level, reflecting accumulated error in identifying annual layers. The age of the Laschamp excursion [Guillou, H., Keywords: Singer, B.S., Laj, C., Kissel, C., Scaillet, S., Jicha, B., 2004. On the age of the Laschamp geomagnetic excursion. 40 39 238 230 magnetic field Earth Planet. Sci. Lett. 227, 331-343.] is revised on the basis of new Ar/ Ar, unspiked K–Ar and U– Th excursion data from three lava flows in the Massif Central, France, together with the 40Ar/39Ar age of a transitionally geochronology magnetized lava flow at Auckland, New Zealand. Combined, these data yield an age of 40,700±950 y b2k, ice core where the uncertainty includes both analytical and systematic (40K and 230Th decay constant) errors. Taking argon the radioisotopic age as a calibration tie point suggests that the layer-counting chronologies for the thorium NorthGRIP and GISP2 ice cores are more accurate and precise than previously thought at depths lava corresponding to the Laschamp excursion. © 2009 Elsevier B.V. All rights reserved.

1. Introduction the δ18O stratigraphy of the Greenland Ice Sheet Project II (GISP2) ice core (Hughen et al., 2004), yield different curves from which the Accurate and precise geochronology is fundamental to under- calibration of a single 14C age can vary by more than ±16% in calendar standing the past 50,000 years of Earth history during which climate age (e.g., Bard et al., 2004). A more recent approach by Hughen et al. frequently oscillated on a millenial time scale while the major (2006) to reduce uncertainty in the 14C calibration curve ties the transition into and out of the last global glacial maximum took place. marine record in the Cariaco Basin to the 230Th chronology of the Hulu 14 The main chronometer for this period is radiocarbon ( C), but because Cave speleothem record (Wang et al., 2001). 14 12 the C/ C ratio in the atmosphere varies over time, radiocarbon dates On one hand, the stratigraphic method in which the chronology is depend upon a precise knowledge of this temporal variation (Bard based on the counting of annual layers in ice offers perhaps the most et al., 2004). The calibration curve most widely adopted by the continuous and straightforward template for calibrating 14C ages radiocarbon community – known as INTCAL04 (Reimer et al., 2004) – between 26,000 and 50,000 y b2k. On the other hand, it has proven 14 230 uses C dates of rings in fossil trees, extended by way of paired Th difﬁcult to establish a precise chronology for the GISP2 and Greenland 14 and C dates from tropical corals to 26,000 y BP (= before year Ice core Project (GRIP) ice cores using annual layer counting beyond 1950 AD; radiocarbon convention). However, between 26,000 and the Holocene (Southon, 2004). For example, uncertainties in the most 50,000 y BP coral data are sparse and there is no consensus curve widely used timescale for the GISP2 ice core rise dramatically from (Reimer et al., 2006). Several approaches, including varved lake ±700 y to ±1700 y in the period between about 40 and 50 ka (Alley 230 sediments dated by Th (Kitigawa and van der Plicht, 1998), a set of et al., 1997; Meese et al., 1997). No single or consensus chronology 230 carefully selected, pristine corals that have been dated using Th and arose until the recent multi-parameter counting of annual layers, 14 C methods (Fairbanks et al., 2005), and the “stratigraphic” method of including visual stratigraphy, conductivity of ice and melt-water and tuning climate-sensitive signals in 14C-dated marine sediment cores to + 2+ 2− − + concentrations of Na ,Ca ,SO4 ,NO3 , and NH4 at the NorthGRIP core site correlated to previous results from the GRIP ice core, ⁎ Corresponding author. resulting in the Greenland Ice Core Chronology 2005 or GICC05 E-mail address: bsinger@geology.wisc.edu (B.S. Singer). (Andersen et al., 2006). Although the GICC05 timescale is a major step

0012-821X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2009.06.030 B.S. Singer et al. / Earth and Planetary Science Letters 286 (2009) 80–88 81 forward, 2σ precision estimated from layer counting degrades with depth from b1% at 15 ka to N4%, 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, 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 Fig. 1. Location of sample sites in lava flows of the Chaîne des Puys, France. Map simplified from Boivin (2004). 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 relative to a Cretaceous glauconite standard (GL-O) that has not determination provides a tie-point that can be used to constrain been calibrated against modern 40Ar/39Ar geochronology standards, correlations between proxy climate records from speleothems hindering comparison with results obtained in other laboratories. (e.g., Wang et al., 2001; Genty et al., 2003) or marine sediments Moreover, K–Ar ages assume an atmospheric initial ratio of 40Ar/36Ar (e.g., Hughen et al., 2004), and the ice cores from Greenland and in the lava at the time of eruption, and the method does not provide Antarctica. the tests for inherited argon or argon loss that are possible with the 40Ar/39Ar method. Curiously, Plenier et al. (2007) found that their 2. Lava flows that record the Laschamp excursion sample of the Louchadière lava flow was too vesicular and hydro- thermally altered to obtain a K–Ar age, yet they correlate the nearby Trachyandesitic lava flows that crop out near the villages of Royat flow, K–Ar dated at 34.0±1.4 ka, with the Olby age, and hence Laschamp and Olby in the Chaîne des Puys, Massif Central, France the Laschamp excursion. We question the accuracy of both the K–Ar (Fig. 1), were discovered by Bonhommet and Babkine (1967) to record ages and the long 6000 y duration of the Laschamp excursion a nearly reversed magnetic field direction relative to that of the proposed by Plenier et al. (2007); instead we rely on data acquired modern field. Until 2004, however, the ages of these lava flows, and using multiple techniques from the same rock samples of the Olby, another flow issued from the Puy de Louchadière that records a dif- Laschamp, and Louchadière lava flows. ferent transitional fielddirection,remainedpoorlyconstrained Combined 40Ar/39Ar incremental heating and unspiked K–Ar despite efforts over four decades using K–Ar, thermoluminescence, measurements on highly purified groundmass samples, using well- 40Ar/39Ar, and 230Th–238U methods (Bonhommet and Zähringer, 1969; characterized mineral standards and more sensitive mass spectro- Condomines, 1978; Hall and York, 1978; Gillot et al., 1979; Chauvin metry than in previous studies, yielded ages of 41.1±1.8 and 40.0± et al., 1989). The earlier age determinations vary widely from N160 ka 1.3 ka for the Olby and Laschamp flows, respectively, thereby to b20 ka and are associated with large analytical uncertainty. For establishing a very precise correlation (Guillou et al., 2004). These example, the first 40Ar/39Ar age etermination by Hall and York (1978) two lava flows record nearly identical paleomagnetic field directions yielded plateau ages of 47±4 and 60±8 ka (here and elsewhere, with a virtual geomagnetic pole (VGP) in the south Pacific Ocean uncertainties reflect ±2σ analytical errors, unless otherwise speci- (Guillou et al., 2004). Moreover, the two lavas are weakly magnetized fied) from the Olby and Laschamp flows, respectively. Condomines at an average of 7.7 µT, about one sixth of the present day field (1978) determined by alpha counting a 238U–230Th internal isochron strength, suggesting that they record a weak excursional field, not a for the Olby flow of 39±12 ka. fully reversed polarity state (Roperch et al., 1988). Recently, Plenier et al. (2007) reported an unspiked K–Ar age of The similar VGPs and paleointensity of the Laschamp and Olby 37.0±1.4 ka for the Olby flow. This age, however, was calculated flows allow an inverse-variance weighted mean age to be calculated at 82 B.S. Singer et al. / Earth and Planetary Science Letters 286 (2009) 80–88

40.4±1.1 ka, which was taken as the most precise age for the an anomalous direction (Fig. 2), virtually identical to the one reported Laschamp excursion (Guillou et al., 2004). The 40Ar/39Ar-based age by Chauvin et al. (1989) that corresponds to a transitional VGP which was calculated relative to an age of 28.02 Ma for the Fish Canyon falls near the equator over east Africa. This lava is weakly magnetized sanidine (FCs) standard and includes only analytical contributions to at 12.9 µT, about 30% of the strength of the modern field (Chauvin its uncertainty (Renne et al., 1998). Compared to ages for the et al., 1989), but it has yet to be precisely dated using the 40Ar/39Ar or Laschamp excursion that are estimated using chronometers other unspiked K–Ar methods. than 40Ar/39Ar, including for example 14C, 238U–230Th, or layer- We present new 40Ar/39Ar data for two samples: CP-06-04 from counting ages, the systematic uncertainty arising primarily from the the Laschamp flow and LOL-05 from the Louchadière flow near 40K decay constant must also be propagated. Guillou et al. (2004) used Beauloup; new unspiked K–Ar data are also reported from the a conservative value of 2.4% for the systematic error based on com- latter. In addition, we present new 230Th–238U isotope data from parisons between U–Pb and 40Ar/39Ar ages of ancient coeval volcanic sample CP-06-05 collected from the Olby flow (Fig. 1). zircon and sanidine (Min et al., 2000). Thus, the age of the Laschamp excursion was constrained to 40.4±2.0 ka, which agrees remarkably 3. Methods and results well with the age of the excursion expressed in the NAPIS-75 and GLOPIS-75 paleointensity stacks of marine sediment that is tempo- 3.1. 40Ar/39Ar incremental heating experiments rally calibrated to the δ18O record of the GISP2 ice core (Laj et al., 2000, 2004; Guillou et al., 2004). At depths corresponding to the period For this study, new samples were collected from outcrops of the between 40 and 44 ka, precision of the GISP2 layer counting chro- Laschamp and Louchadière flows (Fig. 1). Sample CP-06-04 from the nology on which GLOPIS-75 is based is estimated to degrade from Laschamp flow was taken from the easternmost of two outcrops, about ±800 y to greater than ±2000 y as it becomes more difficult to about 2 m from where a previous sample (Laschamp LOL-01) had match visual stratigraphy with laser light scattering measurements of been collected for 40Ar/39Ar and K–Ar dating by Guillou et al. (2004).It dust in annual layers (Meese et al., 1997). brings to three the number of independent samples that we have 40 39 Motivation for obtaining new radioisotopic age control for the dated from the two outcrops of this lava flow. Ar/ Ar incremental Laschamp excursion is three-fold: First, the age of the FCs standard heating analyses at the University of Wisconsin-Madison used a used in 40Ar/39Ar geochronology has been recently calibrated to an resistance furnace to degas phenocryst-free groundmass separates astrochronologically-dated sequence of Miocene sediments, the from the Laschamp and Louchadière samples following procedures in implications of which are: (1) the age of FCs is best constrained at Singer et al. (2004) and Cassata et al. (2008); complete analyses, 28.201±0.046 Ma, which is 0.65% older than previously adopted including reactor constants, mass discrimination, and J values are in values, and (2) systematic uncertainty of the method reduces by an the Supplementary materials. Replicate experiments on each sample order of magnitude to ±0.25% (Kuiper et al., 2008). Second, yield concordant age spectra that, when calculated using an age of inductively coupled plasma mass spectrometry (ICP-MS) techniques 28.201 Ma for the FCs standard (Kuiper et al., 2008), give weighted may be used, rather than alpha counting or thermal ionization, to mean isochrons of 40.4±5.9 and 41.9±2.7 ka for the CP-06-04 obtain exceptionally precise 230Th–238U mineral isochron ages from sample from Laschamp and Louchadière flows, respectively (Table 1; lava flows on very small, highly purified, mineral and groundmass Fig. 3). Isochrons are preferred, rather than apparent ages using samples (Jicha et al., 2005, 2007, 2009). Third, near Beauloup, 8 km plateau criteria, because they take into account potential non- north of the Laschamp and Olby flows (Fig. 1), we sampled another atmospheric trapped components that may otherwise bias the age trachyandesitic lava flow from a very fresh outcrop, obviously a of young, relatively unradiogenic samples such as these (Singer et al., different site from that described by Plenier et al. (2007). This flow has 2004; Sharp and Renne, 2005). The isochron age from the Laschamp

Fig. 2. Paleomagnetic data from La Louchadière flow (LOL-05). a) Representative example of demagnetization diagram using alternating field. The units of the axes are in A/m, full (open) dots correspond to the projection onto the horizontal (vertical) plane. The direction of the Characteristic Remanent Magnetization (ChRM) of this specific sample is reported in declination, inclination and Mean Angular Deviation (MAD). b) Stereoplot (lower hemisphere) of the direction of ChRM from the different samples together with the mean (grey star) and the confidence ellipse (grey). The site mean direction, based on 6 determinations is given in declination, inclination and confidence angle (α95). B.S. Singer et al. / Earth and Planetary Science Letters 286 (2009) 80–88 83

Table 1 Summary of 40Ar/39Ar incremental heating results for the Louchadière and Laschamp lava ﬂows.

Sample K/Ca Age spectrum Isochron analysis experiment # total 39 40 36 a Increments Ar % MSWD Age (ka) Steps used Ar/ Ari MSWD Age (ka) (°C) ±2σ ±2σ ±2σ CP-06-04; Laschamp UW68A03 0.55 675–1225 100.0 0.52 39.9±4.4 8 of 8 295.2±1.5 0.57 41.5±7.8 UW68A04 0.52 675–1015 100.0 0.28 41.3±4.6 6 of 6 296.0±1.8 0.27 38.9±9.4 Weighted mean plateau age: 40.6±3.1 Isochron: 295.5±1.1 0.17 40.4±5.9

LOL 05; Louchadière UW30K46 0.42 750–1360 100.0 0.53 43.5±3.5 8 of 8 296.6±1.4 0.22 42.0±3.9 UW30K47 0.63 750–1290 100.0 0.51 42.1±3.3 9 of 9 295.8±1.7 0.57 41.8±3.9 Weighted mean plateau age: 42.5±2.4 Isochron: 296.3±1.1 0.01 41.9 ±2.7

Isochron ages in bold give the preferred age of each sample. a Ages calculated using the decay constants and isotope abundances of Steiger and Jäger (1977) (λ40K=5.543×10− 10 yr− 1),with J values determined using 28.201 Ma Fish Canyon sanidine (FCs) (Kuiper et al., 2008). sample is indistinguishable from previous 40Ar/39Ar and K–Ar ages we The determination of K was carried out by atomic and flame have determined for this lava. The isochron age from the Louchadière emission spectrophotometry with a relative precision of 1%. Argon flow is also indistinguishable from the 40Ar/39Ar and K–Ar ages extracted by radio frequency heating of 2.1–2.8 g of sample was determined for the Laschamp and Olby flows (Table 1). transferred to an ultra-high-vacuum glass line and purified on a titanium sponge and Zr–Ar getters. The isotopic composition of gas samples comprising between 1.7 and 2.2×10− 11 mol of 40Ar was 3.2. Unspiked K–Ar experiments measured on dual Faraday detectors using a 180°, 6 cm radius mass spectrometer with an accelerating potential of 620 V. Manometric As an independent test of the 40Ar/39Ar results, replicate unspiked calibration, based on periodic measurements of mineral standards K–Ar measurements were done on subsamples of the same ground- using the same procedure, allows the total 40Ar content of a sample to mass separate prepared from the LOL-05 whole-rock sample of the be determined with a precision of about ±0.2% (2σ)(Charbit et al., Louchadière flow at the LCSE laboratory in Gif sur Yvette, France 1998). Mineral standards used include LP-6 biotite (127.8±0.7 Ma) following procedures in Guillou et al. (2004). The unspiked technique which has been intercalibrated against several 40Ar/39Ar standards differs from the conventional isotope dilution method in that argon (Spell and McDougall, 2003), including GA-1550 biotite (Renne et al., extracted from the sample is measured in sequence with purified 1998). Uncertainties for the K and Ar data are 1σ analytical only, and aliquots of atmospheric argon at the same pressure in the mass consist of propagated and quadratically averaged experimental spectrometer. This suppresses mass discrimination effects between uncertainties arising from the K, 40Ar (total), and 40Ar⁎ determina- the atmospheric reference and the unknown, and allows quantities tions. Uncertainties on the ages are given at 2σ. of radiogenic 40Ar⁎ as small as 0.14% to be detected (Scaillet and The two unspiked K–Ar experiments on sample LOL-05 yield Guillou, 2004). an inverse-variance weighted mean age of 41.1±2.2 ka that is

Fig. 3. 40Ar/39Ar age spectra and isochron diagrams of replicate incremental heating experiments on groundmass separated from samples of the Laschamp and Louchadière lava ﬂows. 84 B.S. Singer et al. / Earth and Planetary Science Letters 286 (2009) 80–88

Table 2 Table 3 Unspiked K–Ar results from groundmass of the Louchadière lava ﬂow. U–Th isotope data from Olby ﬂow sample CP-06-05.

Sample ID Weight K 40Ar⁎ 40Ar⁎ 40Ar⁎ Agea (ka) Material (238U/232Th) ±2SE (230Th/232Th) ±2SE Th U experiment # (g) (wt. %) (%) (10− 13 mol/g) Weighted ±2σ (ppm) (ppm) ±1σ mean±1σ Whole rock 0.783 0.001 0.793 0.002 8.398 2.167 LOL-05 Groundmass 0.764 0.001 0.788 0.003 8.486 2.137 b 5823 2.05536 1.764±0.018 1.489 1.200±0.047 Magnetite 180 µm 0.699 0.002 0.766 0.004 3.997 0.921 N 5847 2.77685 “…………..” 1.634 1.299±0.040 1.257±0.031 41.1±2.2 Magnetite 250 µm 0.863 0.002 0.818 0.003 1.215 0.345 Clinopyroxene 0.760 0.002 0.789 0.005 0.122 0.031 The preferred age in bold. 230 232 238 232 a Calculated using decay constants and isotope abundances of Steiger and Jäger ( Th/ Th) and ( U/ Th) uncertainties reported as internal 2 standard errors of 229 232 (1977) (λ40K=5.543×10−10 yr− 1). the mean and include uncertainties associated with measurement of Th/ Th and 238U/235U ratios. Activity ratio calculated using λ230Th=9.158 (±0.028)×10−6 yr−1 (Cheng et al., 2000). indistinguishable from the 40Ar/39Ar isochron age of the same material (Table 2). Moreover, the new K–Ar and 40Ar/39Ar ages are in agreement with the conventional K–Ar age of 42±8 ka (2σ) (230Th decay constant) contributions. Isotope-dilution thermal ioni- determined from the Louchadière lava flow by Chauvin et al. (1989). zation mass spectrometry (ID-TIMS) to determine U and Th contents, coupled with alpha counting of the activities of U and Th isolated from 3.3. 230Th–238U chemistry and mass spectrometry whole rock, pyroxene, plagioclase and two magnetite fractions of the Olby lava by Condomines (1978) yielded an internal isochron of Sample CP-06-05 was collected from the Olby flow for 238U–230Th 39±12 ka (2σ), with a (230Th/232Th) ratio of 0.805±0.002. Our analysis about 1 m from the site in the same outcrop where sample results are remarkably consistent with those obtained by Condomines LOL-03 was collected for paleomagnetic study and the 40Ar/39Ar and (1978) who, using mineral separates of several grams each, achieved unspiked K–Ar age determinations by Guillou et al. (2004). Whole- an uncertainty in age of ±30.8%, compared to an uncertainty of rock (100 mg), groundmass (120 mg), clinopyroxene (110 mg) and ±11.6% for the ICP-MS age — a nearly three-fold improvement. magnetite separates (125–225 mg) were powdered, spiked with a Moreover, our 230Th–238U isochron is indistinguishable from both the 235 229 40 39 mixed U– Th tracer, and dissolved in HF+HNO3, then HCl in Ar/ Ar isochron and the unspiked K–Ar age determined for the Olby Savillex Teflon beakers at 110 °C. Upon dissolution and spike flow (Table 4). equilibration, U and Th were co-precipitated with Fe(OH)3 using NH4OH, and separated into Th and U fractions using BioRad AG1x8 200–400 mesh anion exchange resin using 1.2 ml Teflon columns 4. Radioisotopic age of the Laschamp excursion (Asmerom and Edwards, 1995; Jicha et al., 2007). Th was further purified by a second pass through 700 µl columns. The new 40Ar/39Ar, K–Ar, and 238U–230Th data from the Laschamp, Isotope ratio measurements were done at UW-Madison using a Olby and Louchadière flows – together with data reported by Guillou Micromass IsoProbe MC-ICP-MS following procedures in Jicha et al. et al. (2004) – bring the total number of independent radioisotopic (2009). Solutions were aspirated using a 50 µl/min self-aspirating, age determinations of these transitionally magnetized lavas in our concentric-flow nebulizer tip and an Aridus® desolvating nebulizer laboratories to nine. In Table 4 we have re-calculated the 40Ar/39Ar system. U and Th fractions were analyzed separately. U was measured ages of Guillou et al. (2004) to be consistent with a 28.201 Ma age for on Faraday detectors, whereas Th was measured using both Daly the FCs fluence monitor (Kuiper et al., 2008). A fourth transitionally (230Th) and Faraday (229Th and 232Th) detectors. Whole-rock stan- magnetized lava, the Mclennan's Hill flow, Auckland Volcanic Field, dards and samples were analyzed using a standard-sample-standard New Zealand, yields a VGP in the south Atlantic Ocean that is technique. For Th isotope analyses, a U500 (238U/235U=1.0003) solution was mixed with the Th analyte to simultaneously determine instrumental mass fractionation during Th isotope analyses. Daly– Faraday gain calibration was accomplished by measuring a standard of known Th isotope composition (IRMM-035), which was also mixed with U500. The ratio of the mass-bias corrected 232Th/230Th ratio to the true ratio is the Daly–Faraday gain, which is interpolated for each sample by bracketing standards. The isotopic composition of IRMM- 035 that was used for Daly–Faraday gain calibration was independently confirmed relative to NBL 114 (238U/235U=137.88) at the beginning of the analytical session and yielded a 232Th/230Th ratio of 87,859±0.47% (uncertainties reported as % 2SD), which is nearly identical to the consensus value of Sims et al. (2008). External precision, reproducibility, and accuracy of Th and U isotope measurements were evaluated through repeated analyses of spiked and unspiked rock standards (ATHO, AGV-1, BCR-1) and thorium reference solutions (IRMM-035 and IRMM-036). 232Th/230Th ratios determined for standards were IRMM-035: 87,853±0.49% (n=96), IRMM-036: 325,720±0.70% (n=124), AGV-1: 199,587±

0.74% (n=11), A-THO: 182,008±0.52% (n=26), BCR-1: 210,789± Fig. 4. U–Th isochron diagram for phases separated from the Louchadière lava flow. 0.54% (n = 16). Our measured ratios for each standard are in Abbreviations: wr = whole rock, gm = groundmass, cpx = clinopyroxene, and mt = agreement with the consensus ratios of Sims et al. (2008). titanomagnetite. The uncertainty in the isochron age reflects both analytical errors and 230 Activity ratios (Table 3) calculated using a 230Th decay constant of uncertainty in the Th decay constant (Cheng et al., 2000). Error ellipses repre- − − sent internal 2σ uncertainties associated with mass spectrometer analysis where the (9.158±0.028)×10 6 yr 1 (Cheng et al., 2000) yield an internal (230Th/232Th) and (238U/232Th) reflect the 2 standard errors measured in 232Th/230Th 230 232 isochron of 40.8±4.7 ka, with a ( Th/ Th) ratio of 0.797±0.001 and 229Th/232Th ratios, respectively. These internal errors are comparable to the exter- (Fig. 4); uncertainties reflect both analytical and minor systematic nal reproducibility of rock standards. B.S. Singer et al. / Earth and Planetary Science Letters 286 (2009) 80–88 85

Table 4 Summary of paleomagnetic data, 40Ar/39Ar isochron, K–Ar, and U–Th ages from the Laschamp, Olby, Louchadière and McLennan's Hill lava ﬂows.

Lava ﬂow sample ID # Location Dec (°) Inc (°) VGP VGP Intensity VADM Dating method n Agea ±2σ lat (°) long (°) lat (°) long (°) (μT) (1022Am2) (ka) Laschamp Site 1 (LOL-01)b 45.74 2.97 241.0 −66.0 −49.0 246.0 8.1±0.6 1.77±0.13 40Ar/39Ar 6 39.7±2.6 Site 1 (LOL-01)b K–Ar 3 41.5±2.0 Site 2 (LOL-02)b 45.74 2.97 227.0 −55.5 −53.5 272.2 40Ar/39Ar 3 38.5±2.5 Site 2b (LOL-06-04) 40Ar/39Ar 2 40.4±5.9 Weighted mean (MSWD=1.2) 40.2±1.3

Olby LOL-03b 45.74 2.88 239.0 −65.0 −46.0 245.0 7.9±2.5 1.73±0.55 40Ar/39Ar 4 39.5±5.2 LOL-03b K–Ar 3 41.4±2.0 CP-06-05 238U–230Th 1 40.8±4.7 Weighted mean (MSWD=0.3) 41.1 ± 1.7

Louchadière LOL-05c 45.83 2.90 114.1 58.2 13.2 49.8 12.9±0.13 2.82±0.30 40Ar/39Ar 2 41.9±2.7 LOL-05 K–Ar 2 41.1±2.2 Weighted mean (MSWD=0.2) 41.4±1.7

McLennan's Hill NZ-06-05d −36.92 174.85 162.7 −20.8 −39.4 332.5 2.5±0.5 0.39±0.08 40Ar/39Ar 8 39.4±4.1 Weighted mean age of four lava ﬂow ages (MSWD=0.7) 40.69±0.85 Including 0.25% systematic uncertainty 40.69±0.95

Abbreviations: Dec, Inc = declination and inclination; VGP = virtual geomagnetic pole; VADM = virtual axial dipole moment, n = number of independent experiments conducted. Ages in bold are preferred mean ages of each lava flow. a40Ar/39Ar ages relative to 28.201 Ma Fish Canyon sanidine standard age (Kuiper et al., 2008). b Magnetic data from Roperch et al. (1988) and Guillou et al. (2004); ages from Guillou et al. (2004). c Magnetic data from Chauvin et al. (1989) and this study; ages from this study. d Magnetic data from Mochizuki et al. (2006); age from Cassata et al. (2008). associated with an exceptionally low paleointensity of 2.5 µT al., 2000), already propagated into the activity ratios, and hence the (Mochizuki et al., 2006). Six 40Ar/39Ar incremental heating experi- isochron regression. Our use of 28.201± 0.046 Ma for the FCs ments on groundmass from the Mclennan's Hill flow – obtained at standard effectively reduces the systematic error associated with the UW-Madison using methods identical to the present study – yield an 40Ar/39Ar age to between 0.2 and 0.3% (Kuiper et al., 2008). isochron of 39.1±4.1 ka, indicating that this southern hemisphere Accordingly, the radioisotopic age of the Laschamp excursion, lava also records a very weak magnetic field corresponding to the including both analytical and systematic errors, is 40,700 ±950 y Laschamp excursion (Cassata et al., 2008). The age determined by b2k (Table 4); this age and uncertainty are appropriate when Cassata et al. (2008) may be recast relative to an age of 28.201 Ma for comparing to other independent chronometers, including radio- the FCs fluence monitor (Kuiper et al., 2008), which produces an age carbon or layer-counting. of 39.4±4.1 ka (Table 4). Each of the four lavas records a very weak local magnetic field and 5. Discussion yields statistically indistinguishable ages that, when compared to the GLOPIS-75 paleointensity curve reported on the GISP2 time scale The radioisotopic age of 40,700±950 y b2K of the Laschamp (Laj et al., 2004), indicate eruption during the pronounced intensity excursion coincides with the position of the weakest geomagnetic minimum at ca. 41 ka associated with the Laschamp excursion field intensity recorded globally in marine sediments comprising the (Fig. 5). However, noting that the magnetic inclination and declina- GLOPIS-75 record. Given the assumption made in calculating the tion of the Laschamp and Olby flows are identical to one another, the mean radioisotopic age, the ±950 y of uncertainty clearly does not paleofield directions recorded by the four lava flows suggest that constrain the maximum possible duration of the Laschamp excursion. they record “snapshots” of differing field geometry at three distinct For this we must turn to the marine sediment and ice core records. In instances during the evolution of the Laschamp excursion. A question both, the geomagnetic excursion – defined by the intensity low and then arises: can the order in which these snapshots were captured be 10Be peak at mid-height – covers Dansgaard–Oeschger stages 9 and resolved given the uncertainties of the dating methods? or, were 10, giving a duration of about 1.5 to 2.0 kyr (Laj et al., 2000, 2004; these snapshots so close together in time that they cannot be Svensson et al., 2006). Although uncertainties in the calibration of resolved from one another? The answer is they cannot be resolved GLOPIS-75 to absolute paleointensity are perhaps no better than when one considers that random analytical errors – and thus the ±15% (Fig. 5), it is noteworthy that the four lava flows have uncertainties for individual age determinations – are large relative to VADMsb2.8×1022 Am2, suggesting that they erupted when the mag- both the age differences that might exist between the flows (Table 4), netic field was weaker than at any other time during the last 50,000 y. and to the duration of the ca. 41 ka global intensity minimum in the Close inspection of the GLOPIS record suggests that the field was in GLOPIS-75 record (Fig. 5). Assuming that the paleofield directions this remarkably weakened state for only about 1000 y (Laj et al., record the shortest possible period of time, the age of the excursion 2004). The GLOPIS-75 calendar-age model is based on that of the can be estimated from the inverse-variance weighted mean NAPIS-75 record, itself resulting from correlation of δ18O and other (Taylor, 1982) of the ten independent age determinations, which climate-proxy data with the GISP2 ice core climate record (Laj et al., is 40,700±850 y b2k at the 95% confidence level. The 40Ar/39Ar and 2000, 2004). The paleointensity stack was used to calculate the K–Ar ages are also associated with non-random, systematic error geomagnetic modulation of 14C production and to provide a first order that arises mainly from uncertainty in the 40K decay constant modeled correction for conventional radiocarbon ages older than (e.g., Renne et al., 1998; Min et al., 2000) with very small, essentially 25 kyr (Laj et al., 2002). A similar approach was later used by Hughen insignificant, contribution from the 230Th decay constant (Cheng et et al. (2004) and applied to calculate atmospheric Δ14C activity from 86 B.S. Singer et al. / Earth and Planetary Science Letters 286 (2009) 80–88

Fig. 5. Radioisotopic age determinations of lava flows that record the Laschamp excursion, with ±2σ analytical uncertainties and corresponding probability density curve (● 40Ar/39Ar; ○ K–Ar; ☆ 238U–230Th). The mean of ten 522 524 526 528 530 age determinations (Table 4) is the vertical line with grey band denoting the ±2σ uncertainty including systematic errors. For comparison, the GLOPIS-75 stacked record of paleointensity, calibrated to absolute values of archeologic and volcanic materials for the 0–19.5 ka interval (Laj et al., 2004), is shown on the GISP2 time scale (grey lines show ±1σ uncertainty envelope on the VADM). The GISP2 δ18O record on the Meese et al. (1997) time scale is shown for reference. Also shown are the 10Be flux and δ18O records from the NorthGRIP ice core on the GICC05 timescale (10Be data from Muscheler et al., 2004; δ18O data and timescale calibration from Svensson et al., 2006, 2008). Paleointensity values expressed as Virtual Axial Dipole Moments (VADM) of the four dated lava flows (Table 4) are shown as filled circles at 40.7 ka along the GLOPIS record. Note that these lavas have VADMs that correspond to the weakest magnetic field recorded at any time during the last 50 ka. The GLOPIS record implies that VADMs this low only occur during an approximately 1000 y period of the Laschamp excursion (Laj et al., 2004). the Cariaco Basin sediment sequence. The exceptionally precise match record (Hughen et al., 2006). The accuracy of the Laschamp excursion between our age for the lava flows and the intensity minimum in age relative to this and other 230Th-dated speleothem records, is GLOPIS-75 provides an independent radioisotopically-dated tie point. discussed below. The 40,700±950 y b2k age for the Laschamp excursion implies that When placed on the GICC05 time scale the 10Be flux within the the Hughen et al. (2004) 14C data from Cariaco Basin sediment may be GRIP ice core exhibits a short-lived maximum centered at 41,250 y b2k used to calibrate radiocarbon ages to better than 1000 y precision (Fig. 5). Although the duration of the period of peak 10Be flux is about during the period around 41 ka when the calibration correction 1000 y, the precision of the layer-counted age is only ±1630 y at the becomes large and imprecise. Moreover, this latter finding is 95% confidence level (Andersen et al., 2006). Our new radiosotopic consistent with the uncertainty associated with the Cariaco Basin age of 40,700±950 y b2k for the Laschamp excursion lends calibration curve – about ±700 y (2σ)at41ka– when it is tied to the considerable independent support to the GICC05 time scale and, Wang et al. (2001) 230Th chronology of the Hulu Cave speleothem given the comparison of the ages and paleointensities of the lava flows B.S. Singer et al. / Earth and Planetary Science Letters 286 (2009) 80–88 87 to the GLOPIS-75 record (Fig. 5), implies that the uncertainty appreciated along with editor Rick Carlson's efficiency. Singer's estimates of N1600 y at 40 ka (Andersen et al., 2006) are perhaps research on the geomagnetic field was supported by the U.S. National pessimistic by a factor of two. In detail, the rise in 10Be flux comprises a Science Foundation grants EAR-0337684 and EAR-0516760. Analyses pair of peaks in both the GRIP and the EPICA dome C ice cores that at LSCE were supported by the French Atomic Energy Commission have been used in parallel with stable isotope and gas content records (CEA) and by the Centre National de la Recherche Scientifique (CNRS). to argue for a bipolar see-saw connection between millenial scale This is LSCE contribution no. 4006. climate shifts in the opposing polar hemispheres (EPICA Community members, 2006; Raisbeck et al., 2007). Given the uncertainties, Appendix A. Supplementary data however, our radioisotopic age of 40,700 y b2k for the Laschamp excursion cannot be matched definitively to one or the other of these Supplementary data associated with this article can be found, in 10Be flux peaks (Fig. 5). Accordingly, we propose that the radioisotopic the online version, at doi:10.1016/j.epsl.2009.06.030. age of 40,700±950 y b2k for the Laschamp excursion be used as a tie- point that demarcates the center of Daansgaard–Oeschger event 10 (GIS-10) in Greenland and its counterpart in AIM-10 in Antarctica, as References defined by shifts in δ18O recorded in the GRIP and EPICA dome C ice cores (Fig. 5). Alley, R.B., Shuman, C.A., Meese, D.A., Gow, A.J., Taylor, K.C., Cuffey, K.M., Fitzpatrick, J.J., fi 234 230 Grootes, P.M., Zielinski, G.A., Ram, M., Spinelli, G., Elder, B.,1997. Visual-stratigraphic Our ndings are in excellent agreement with U/ Th ages of dating of the GISP2 ice core: basis, reproducibility, and application. J. Geophys. Res. speleothem carbonate that records the Dansgaard–Oscheger 10 102, 26367–26381. interstadial at mid-latitude sites. For example, Genty et al. (2003) Andersen, K.K., Svensson, A., Rasmussen, S.O., Steffensen, J.P., Johnsen, S.J., Bigler, M., δ13 δ18 – Röthlisberger, R., Ruth, U., Siggaard-Andersen, M.-L., Dahl-Jensen, D., Vinther, B.M., tightly bracketed C and O records of Dansgaard Oescheger Clausen, H.B., 2006. The Greenland Ice Core Chronology 2005, 15–42 ka. Part 1: event 10 to between 40.43±0.25 and 40.61±0.31 ka in the Villars constructing the time scale. Quat. Sci. Rev. 25, 3246–3257. Cave, France, implying an age of ~40.5 ka. Similarly, the δ18O record of Asmerom, Y., Edwards, R.L., 1995. U-series isotope evidence for the origin of continental – basalts. Earth Planet. Sci. Lett. 134, 1–7. Dansgaard Oeschger event 10 in two stalagmites from Hulu Cave, Bard, E., Rostek, F., Ménot-Combes, G., 2004. A better radiocarbon clock. Science 303, China is bracketed between 38.8±0.9 and 42.7±0.54 ka in one, and 178–179. 39.3±0.3 and 42.8±0.2 ka in the other, suggesting an age of ~41.2 ka Boivin, P., 2004. Volcanologie de la Chaîne des Puys, 4th ed. Parc Naturel régional des ’ (Wang et al., 2001). The 234U/230Th chronology of Hulu cave Volcans d Auvergne. 179 pp., with 1:25,000 geologic map. Bonhommet, N., Babkine, J., 1967. Sur la presence d’aimantations inversees dans la stalagmites – key to establishing a long-term record of the Asian Chaine des Puys. C. R. Acad. Sci. Paris B264, 92–94. monsoon – allows for two correlations with the GISP2 δ18O record Bonhommet, N., Zähringer, J., 1969. Paleomagnetism and potassium argon age determinations of the Laschamp geomagnetic polarity event. Earth Planet. Sci. between 38 and 50 ka (Wang et al., 2001). A radioisotopic age of – – Lett. 6, 43 46. 40,700 y b2k for Dansgaard Oeschger event 10 is consistent with both Cassata, W.S., Singer, B.S., Cassidy, J., 2008. 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Nature 274, 462–464. adoption of the Laschamp excursion age at 40,700±950 y b2k as a Hughen, K.A., Lehman, S., Southon, J., Overpeck, J., Marchal, O., Herring, C., Turnbull, J., global radioisotopic tie-point for the midpoint of polar isotope 2004. 14C activity and global carbon cycle changes over the past 50,000 years. warming event 10. Science 303, 202–207. Hughen, K.A., Southon, J., Lehman, S., Bertrand, C., Turnbull, J., 2006. Marine-derived 14C calibration and activity record for the past 50,000 years updated from the Cariaco Acknowledgements Basin. Quat. Sci. Rev. 25, 3216–3227. Jicha, B.R., Singer, B.S., Beard, B.L., Johnson, C.L., 2005. Contrasting timescales of We thank Lee Powell for his many years of advice on automation of crystallization and magma storage beneath the Aleutian Island arc. Earth Planet. Sci. Lett. 236, 195–210. the lab at UW-Madison. Raimund Muscheler and Anders Svensson are Jicha, B.R., Singer, B.S., Beard, B.L., Johnson, C.M., Moreno-Roa, H., Naranjo, J.A., 2007. thanked for providing ice core data. Discussions with Grant Raisbeck Rapid magma ascent and the generation of 230Th excesses in the lower crust at helped stimulate us to make this contribution and are greatly Puyehue–Cordón Caulle, Southern volcanic zone, Chile. Earth Planet. Sci. Lett. 255, 229–242. appreciated. Thoughtful review comments by Anders Svensson and Jicha, B.R., Johnson, C.M., Hildreth, W., Beard, B.L., Hart, G.L., Shirey, S.B., Singer, B.S., Michel Condomines helped us to clarify key points and also are greatly 2009. Discriminating assimilants and decoupling deep- vs. shallow-level crystal 88 B.S. Singer et al. / Earth and Planetary Science Letters 286 (2009) 80–88

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