40Ar/ 39Ar dating links Albuquerque Volcanoes to the Pringle Falls excursion and the Geomagnetic...

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Earth and Planetary Science Let

40Ar/39Ar dating links Albuquerque Volcanoes to the Pringle Falls excursionand the Geomagnetic Instability Time Scale

Brad S. Singer a,⁎, Brian R. Jicha a, Benjamin T. Kirby a,John W. Geissman b, Emilio Herrero-Bervera c

a Department of Geology and Geophysics, University of Wisconsin—Madison, 1215 West Dayton, St. Madison, WI 53706, USAb Department of Earth and Planetary Sciences, MSC03 2040, University of New Mexico, Albuquerque, NM 87131, USA

c Hawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa, Honolulu, HI, 96822, USA

Received 1 October 2007; received in revised form 29 November 2007; accepted 4 December 2007
Available online
Editor: R.W. Carlson23 December 2007

Abstract

Incremental heating of basaltic lava groundmass from the Albuquerque Volcanoes, New Mexico, and plagioclase from Ash D at Pringle Falls,Oregon, yield 40Ar/39Ar isochron ages of 218 ± 14 and 211 ± 13 ka, respectively. Sediments in which Ash D was deposited and the eight lavaflows of the Albuquerque Volcanoes display excursional paleomagnetic data with virtual geomagnetic poles (VGPs) in the southern hemisphereand together with the statistically indistinguishable 40Ar/39Ar dates, establish that both sites record the Pringle Falls excursion.

This excursion is also recorded by the 227 ± 8 ka Mamaku Ignimbrite, New Zealand, and by high deposition rate sediments at ODP site 919 inthe North Atlantic Ocean that are dated astrochronologically at 209–207 ka. We propose that the names “Albuquerque” and “Jamaica” excursionbe abandoned and that a radioisotopic age of 211 ± 13 ka be adopted for the Pringle Falls excursion, which is one of five globally expressed, well-documented excursions in marine sediment cores or dated by 40Ar/39Ar methods that took place from 220 to 30 ka. Together with at least fiveother well-dated excursions between 730 and 520 ka, some ten excursions define the Geomagnetic Instability Time Scale (GITS) for the BruhnesChron. Eight of these excursions have been dated using 40Ar/39Ar methods. If the temporal clustering of excursions in the GITS is not a samplingartefact, this suggests that the geodynamo is not intrinsically unstable, but rather that instability in the flow pattern of the outer core fluid flowcauses the main dipole field to be considerably weakened such that it becomes unstable with a 200 to 300 ka recurrence interval. Excursions are animportant part of dynamo behavior over geologic time scales that, in addition to reversals of the main dipole field, need to be fully consideredwhen assessing whether theoretical and numerical simulations of the dynamo produce earth-like results.© 2007 Elsevier B.V. All rights reserved.

Keywords: paleomagnetism; excursion; geochronology; 40Ar–39Ar; geodynamo

1. Introduction

The numerous reversals of the Earth's magnetic field haveproduced stratigraphic markers of global extent that provide awidely used time scale inwhich stable polarity chrons have an ageresolution of 105 to 106 a for the entire Cenozoic (Cande andKent, 1995; Opdyke and Channell, 1996). Excursions, on theother hand, are brief, b 104 a, periods characterized by largeamplitude shifts in direction of the geomagnetic field during

⁎ Corresponding author. Tel.: +1 608 265 8650; fax: +1 608 262 0693.E-mail address: [email protected] (B.S. Singer).

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intervals of diminished field intensity that punctuate otherwisestable polarity chrons (Laj and Channell, 2007). Despiteskepticism about the veracity of excursions (Merrill andMcFadden, 1994) and their use as stratigraphic markers (Verosub,1975; Merrill and McFadden, 2005), the improved sampling ofmarine sediments deposited at high rates and precise geochronol-ogy applied to both sediments (oxygen isotope-based astro-chronology) and lava flows (40Ar/39Ar dating) has led to broadacceptance that many globally expressed excursions characterizethe Brunhes and Matuyama polarity chrons, and are likely anintrinsic feature of geodynamo behavior during pre-Quaternarytime as well, albeit less well resolved. The remarkable progress of

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585B.S. Singer et al. / Earth and Planetary Science Letters 267 (2008) 584–595

the past decade using both quasi-continuous sedimentary records(Guyodo and Valet, 1999; Channell, 1999, 2006; Laj et al., 2004,Channell et al., 2004; 2006) and Quaternary lava flows (Singeret al., 1999, 2004a, 2002, 2005; Singer and Brown, 2002;Hoffman and Singer, 2004; Coe et al., 2004; Guillou et al., 2004;Singer, 2007) has begun to resolve a complex time series ofintensity changes during which perhaps two dozen excursionsalso took place. Knowing with precision the number, timing andduration of these excursions is now considered essential, both tounderstanding geodynamo processes and to establishing a globalhigh resolution correlation of relative paleointensity records inmarine sediment as well as establishing chronologies in Quate-rnary continental sequences.

For example, geomagnetic field reversals and excursions mayreflect different yet complementary processes within the core,with excursions reflecting a 5-fold weakening of the magneticflux within the liquid outer core dynamo on a characteristic timescale of 500 a and reversals requiring diffusion of magnetic fluxnot only out of the liquid outer core but from the solid inner core aswell over a much longer characteristic time of 3000 a (Gubbins,1999). Thus, the timing and frequency of dynamo instabilities thatproduce excursions and associated intensity minima mayilluminate the role of the inner core in stabilizing the field andthus preventing more frequent reversals (Gubbins, 1999; Singeret al., 2005; Channell, 2006). A complete record of excursions isalso essential to evaluate whether numerical simulations of thegeodynamo (e.g., Glatzmaier et al., 1999) produce truly earth-likemagnetic field variations over geologic time.

A major challenge is therefore to integrate observations fromboth sediments and lava flows into a unified time scale ofgeomagnetic field instability (Singer, 2007; Laj and Channell,2007). Unfortunately, the brief duration of excursions posesmajorobstacles. First, many excursions are not recorded, even in sedi-ments deposited at rates higher than 10 cm/ka due to smoothingduring remanence acquisition (Verosub, 1975; Roberts andWink-lhofer, 2004; Channell, 2006). Second, precise geochronology isunavailable for many key sequences in both sediments and lavaflows rendering correlations uncertain. Isolated excursion recordsthat cannot be confidently correlated to other independent re-cordings, or are otherwise poorly-dated, are rightfully viewedwith skepticism. Here we present new 40Ar/39Ar age data fromtwo sites in the western U.S.A., the Albuquerque Volcanoes, NewMexico, and Pringle Falls, Oregon, that record excursionalpaleofield directions. Although the paleodirections and ageestimates from these two sites are similar, a firm correlation andlink to paleofield behavior observed elsewhere in marine sedi-ments has been hampered by the low resolution of the geo-chronology (Geissman et al., 1990; Herrero-Bervera et al., 1994;Peate et al., 1996). Our new age determinations are an order ofmagnitude more precise than previous K–Ar, 238U–230Th, and40Ar/39Ar dates obtained from these materials and, together withthe paleomagnetic data, imply that a common period of geo-magnetic field instability has been recorded at about 211 ka.These terrestrial records of the Pringle Falls excursion provide anew well-defined tie point in a Geomagnetic Instability TimeScale (GITS) for the Brunhes Chron (Singer et al., 2002; Singer,2007).

2. Geochronologic and paleomagnetic background

The Pringle Falls excursion is expressed as three distinctswings in inclination measured within a 7.5 meter interval oflaminated diatomaceous lacustrine sediment in parallel sections1.5 km apart, east of the Cascade Range at 43.7°N, 238.5°E(Herrero-Bervera et al., 1989, 1994). Ash D, a 75 cm thickdacitic lapilli tuff, crops out near the bottom of these sections,bisecting the earliest inclination swing, thus it was depositedduring the onset of this brief period of field instability. An40Ar/39Ar step heating experiment on a single 169.6 mg sampleof plagioclase crystals separated from Ash D pumice yieldedisochron andweightedmean plateau ages of 195 ± 118 and 218 ±20 ka, respectively (Herrero-Bervera et al., 1994; all ages hereinreported with ± 2σ analytical and intercalibrartion uncertainties).These ages were obtained using the Fish Canyon Tuff sanidine(FCTs) with an age of 27.84 Ma as the neutron fluence standard.FCTs has since been calibrated to the GA-1550 biotite primarystandard (98.79 ± 0.96 Ma), yielding an age of 28.02 ± 0.16 Ma(Renne et al., 1998). Adopting the latter age for FCTs leads to arecalibration of the isochron and plateau ages determined byHerrero-Bervera et al. (1994) to 198 ± 118 ka and 221 ± 20 ka,respectively.

Virtual geomagnetic poles (VGPs) from the Pringle Fallssediments, as well as sediments from Long Valley, California,define a clockwise looping path crossing the equator throughthe Americas and returning over the western Pacific (Fig. 1A;Herrero-Bervera et al., 1994). From the Mamaku ignimbritein New Zealand, 40Ar/39Ar dated at 227 ± 8 ka (Houghton et al.,1995; McWilliams, 2001; calibrated to 28.02 Ma FCTs with± 2σ uncertainty), Shane et al. (1994) and Tanaka et al. (1996)obtained 26 VGPs that fall over the North Atlantic Ocean andthus are remarkably similar to those from the earliest part of thePringle Falls record (Fig. 1A). This finding led McWilliams(2001) to propose that, rather than an excursion of global extent,the Pringle Falls record may represent a reversed polaritysubchron during which the transitional field was largely dipolar.

The Albuquerque Volcanoes are a chain of cinder cones andat least eight related olivine tholeiite basalt lava flows covering30 km2 in the Rio Grande rift, west of Albuquerque, NewMexico at 35.2°N, 253.2°E (Geissman et al., 1990). A tightlyclustered mean direction with an inclination of −36.1° anddeclination of 101.1°, (α95 = 1.2°) was obtained from 63 sites inthese eight flows, yielding a VGP of 354.1°E, 20.2°S over theSouth Atlantic Ocean within a few degrees of the VGP loopdefined by the Pringle Falls sediment (Brown et al., 1985;Geissman et al., 1990; Fig. 1A). The exceptionally low K2Ocontent of the basalt flows, b 0.5 wt.%, is reflected in the widelyvarying concentrations of radiogenic argon obtained from a setof six whole-rock samples from three of the flows, eachweighing between five and seven grams, that gave an impreciseweighted mean conventional K–Ar age of 155 ± 94 ka (Geiss-man et al., 1990). A 238U–230Th isochron based on three whole-rock samples and two magnetite separates from three Albu-querque Volcanoes lava flows is 156 ± 29 ka (Peate et al., 1996).Although the K–Ar age is permissive of correlating theexcursional paleodirection of the Albuquerque Volcanoes with

Fig. 1. Virtual Geomagnetic Pole (VGP) positions and paths determined from: A. Terrestrial sites that have been 40Ar/39Ar-dated, including lacustrine sediment atPringle Falls, Oregon (filled circles, only the lowermost inclination swing “A” that includes Ash D is illustrated; Herrero-Bervera et al., 1989, 1994), eight lava flows ofthe Albuquerque Volcanoes (filled diamonds; Geissman et al., 1990) and 26 sites in the Mamaku Ignimbrite, New Zealand (filled squares; Shane et al., 1994; Tanakaet al., 1996). B. VGP path determined from ODP site 919 marine sediments that have been astrochronologically dated to between 245 and 201 ka (Channell, 2006).Note the similarity of these terrestrial and marine records that each contain a clockwise VGP loop through the North Atlantic Ocean and back through the South PacificOcean and Kamchatka.

586 B.S. Singer et al. / Earth and Planetary Science Letters 267 (2008) 584–595

the Pringle Falls excursion, there are several other possibleexcursions between 100–250 ka that are also candidates (Fig. 2;Langereis et al., 1997; Channell, 2006; Laj and Channell, 2007).The 238U–230Th isochron is three times more precise than theK–Ar age, but also falls well within the uncertainties of the 198 ±118 ka isochron age obtained from Ash D at Pringle Falls(Herrero-Bervera et al., 1994). Thus, unequivocal correlation ofthe geomagnetic field instability recorded by the AlbuquerqueVolcanoes to these or other excursions (Fig. 2) requires far moreprecise geochronology.

3. 40Ar/39Ar geochronology samples and methods

A visual survey of several hundred paleomagnetic coresobtained at 63 sites by Geissman et al. (1990) suggested that thematerial from sites AV-1, -40, -41, -44, -58, and -59 contains themost crystalline groundmass and relatively low abundance ofphenocrysts. Approximately 25 g of material (a one inchpaleomagnetic core specimen from each site) was crushed, the180–250 micron size fraction isolated for further cleaning.Groundmass was separated from phenocrysts of olivine andplagioclase using a Sm–Co magnet and density separation in

methylene iodide, followed by hand-picking under a binocularmicroscope. Several subsamples of 93–200 mg from eachgroundmass separate were irradiated for 20, 60 or 90 min in Cupackets adjacent to 1.194 ± 0.012 Ma Alder Creek rhyolitesanidine neutron fluence standards (Renne et al., 1998) in 2.5 cmdiameter Al discs. Correction factors for undesirable nucleogenicreactions at the Oregon State University TRIGA reactor'scadmium-lined in-core irradiation tube (CLICIT) are based onmeasurements of Ca- and K-free salts, including data collected atUW—Madison, and are: [40Ar/39Ar]K = 0.00086; [36Ar/37Ar]Ca =0.000264; [39Ar/37Ar]Ca= 0.000673 (Singer et al., 2004b).

Single crystals of the ACs standard were fused with a 25 WCO2 laser and their argon isotope composition was measuredstatically on a Balzers SEV-217 electron multiplier; mean valuesfrom a dozen or more measurements from each Al disk wereused to calculate J values. Using a resistance furnace, thegroundmass aliquots were degassed at 450–480 °C for 50 min toreduce atmospheric contamination (Singer and Pringle, 1996),then incrementally heated to release the argon in 30–100° stepsbetween 540° and 1300 °C and measured on the MAP 215-50mass spectrometer operated at a sensitivity of 1.4×10−14 mol/Vat UW—Madison using procedures detailed in Singer et al.

Fig. 2. Geomagnetic field excursions of the Brunhes Chron proposed by various authors. A. Composite of recordings in Langereis et al. (1997) from theMediterranean Sea, other marine sediment cores and terrestrial lava flows, including the summary of Champion et al. (1988). B. Inclination, declination,and paleointensity proxy record from ODP site 1072 sediment core (Lund et al., 2001, 2006). C. Inclination and declination in ODP site 919 sediment core(Channell, 2006). D. The stacked record of paleointensity from 33 globally-distributed marine sediment cores known as SINT-800 (Guyodo and Valet,1999). E. The Geomagnetic Instability Time Scale (GITS) based on excursions and reversals that have been either 40Ar/39Ar dated, astrochronologicallydated using oxygen isotope records, or both (see Singer, 2007). The 40Ar/39Ar-dated instabilities include the Laschamp (40.4 ka), Pringle Falls (211 ka),Big Lost (579 ka), West Eifel-1 (722 ka), -2 (626 ka), -4 (555), and -5 (528 ka) excursions and the Matuyama–Brunhes reversal (776 ka) (Singer et al.,2005, 2006).


(2004b). As experience with these samples grew, the getteringtime after heating was extended to as long as 24 min to improvegas clean-up and suppress build-up of memory effects in themass spectrometer. Five or more blanks spanning the tempera-ture range utilized were measured between each sample andwere at least an order of magnitude smaller than the samples.Mass discrimination was monitored by measuring the isotopiccomposition of purified atmospheric argon several dozen times

during each of the three main analytical sessions and rangedfrom 1.0039 ± 0.0004 to 1.0070 ± 0.0003 (2σ) per atomic massunit. Analytical uncertainties are reported at ± 2σ, i.e., the 95%confidence level, and the criteria for calculating plateau ages andthe isochron from the plateau data are from Singer et al. (2004b).As many as seven subsamples were measured from each lavaflow with the inverse-variance weighted mean plateau andisochron ages calculated from the pooled results.


A widely used criteria for reliable 40Ar/39Ar ages is that aplateau of concordant ages comprises the majority of gasincrements (e.g., McDougall and Harrison, 1999). However,in this age spectrum approach the ages of each gas incre-ment, including those of any age plateau, are only model ages(like K–Ar ages) that assume trapped Ar has the atmospheric40Ar/36Ar ratio of 295.5. Young tholeitic basalt, including lavaflows at Hawaii and the Albuquerque Volcanoes, typicallycontain Ar with very low 40Ar/36Ar ratios, making age calcu-lations extremely sensitive to the composition of the trapped Ar(Sharp and Renne, 2005). An alternative to relying on plateauages is to use the isochron which allows for simultaneousdetermination of both the age of the sample and the compositionof the trapped Ar, provided the latter has a homogeneous40Ar/36Ar ratio (e.g., Singer and Pringle, 1996; Singer et al.,2004b). A further advantage of the isochron approach is that theuncertainty in the composition of trapped Ar, ignored in theplateau age, is propagated into the isochron age (Sharp andRenne, 2005). For these reasons, we believe the isochronscalculated from gas increments that define an age plateau yieldthe most reliable estimates of time elapsed since theAlbuquerque Volcanoes lava flows cooled through theirremanence blocking temperatures.

Wavelength dispersive electron probe microanalyses indi-cates that plagioclase in Pringle Falls Ash D averages An35 andcontains about 0.3 wt.% K2O. Plagioclase crystals 250–500 µmin diameter were separated from the pumice lapilli by crushing,magnetic susceptibility, gravitative settling in methylene iodideand hand-picking under a binocular microscope. The crystalswere washed ultrasonically in acetic acid and deionized water toremove carbonate. Several aliquots of ~ 200 mg each werepackaged in Cu foil and irradiated 20 min using the sameprocedures and standards outlined above. Sixteen aliquots ofplagioclase crystals, 20 mg each, were loaded as layers onecrystal thick into 3 mm deep troughs in a copper planchet thatare 15 mm long and 3 mm wide. These aliquots wereincrementally heated to fusion using an automated CO2 laserdefocused to deliver a 3×3 mm beam in four successive steps of0.75, 0.9, 2.1 and 9.0 W power. A traverse speed of 40 μm/syielded a total heating time of 4 min per trough. Following eachheating step, the released gas was cleaned on two SAES C50Zr–Al Getters attached to the 300 cm3 extraction line for 5 min.The first nine experiments involved incrementally heatingindividual troughs, but in six additional experiments twotroughs were lased during each heating step, and in a finalexperiment three troughs were lased together. These latterexperiments, wherein 40 or 60 mg of plagioclase weredegassed, required a traverse speed of 75 μm/s to minimizetotal lasing time and build-up of blanks. System blanks weremeasured prior to each analysis with average values in volts of36Ar = 0.000196, 37Ar = 0.000015, 38Ar = 0.000039, 39Ar =0.000066 and 40Ar = 0.05500. These blanks are typically one totwo orders of magnitude smaller than the sample signals thusuncertainties arising from blanks are minimized. Mass dis-crimination was 1.0069 ± 0.0005 per atomic mass unit duringthe three day analytical period. Each of the 16 incremental-heating experiments on Ash D yielded both a 100% concordant

age plateau and well-defined isochron; as argued above weprefer the isochron as the most reliable estimate of time sinceeruption and deposition of the plagioclase.

4. 40Ar/39Ar results

Table 1 summarizes the complete argon isotope data from the 26experiments on Albuquerque Volcanoes samples (Appendix A).These experiments yield mostly concordant age spectra(Fig. 3); 18 have age plateaux that comprise 100% of the39Ar released, whereas only two contain less than 90%(Table 1). As expected from the low-K tholeiitic composition,radiogenic argon contents are typically only 2–3% in each gasstep, hence the plateau and isochron age determined from eachsubsample are typically only precise to about ± 30% (Table 2).In every subsample, the isochron regression revealed that thecomposition of trapped Ar is indistinguisable from that of theatmosphere (Table 1). In two samples, AV-1 and AV-40, theuncertainties on the isochron ages are about 50–75%, reflectingtotal concentrations of radiogenic argon that are significantlylower than in the other samples (Appendix A). At face value therange in isochron ages in Table 1, from 163 to 234 ka, is large,possibly reflecting variable amounts of atmospheric argonincorporated into each lava during eruption and flow at thesurface, or perhaps subtle weathering-induced loss of K or Arthat varied from flow to flow (e.g., Singer et al., 2004b).Notwithstanding, each of the six samples yield isochrons thatare indistinguishable from one another at the 95% confidencelevel, hence there is no statistical or analytical justification toexclude any of the data in Table 1. The inverse-varianceweighted mean of the combined isochrons from the six lavaflow samples, 218.0 ± 14.0 ka, is precise to ± 6.4% (Table 1).

The complete argon isotope data from Pringle Falls Ash Dplagioclase (Appendix A) are summarized in Table 2;representative age spectra and isochrons for each sample sizeare shown in Fig. 4. The gas steps contain between 4 and 40%radiogenic argon, with the 3 highest temperature steps fromeach aliquot averaging about 30%. All experiments yieldconcordant spectra with 100% of the 39Ar defining the ageplateaux. The 40Ar/36Ar intercepts of isochron regressions areindistinguishable from atmosphere, indicating that excess argonis not present in levels that may bias the age. The precision ofthe measured 40Ar/39Ar ratio for each gas analysis scales withthe absolute size of signal relative to the system blank, hence the20 mg aliquots yield plateau and isochron ages that are a factorof two less precise than those obtained from the 40 mg aliquots.The increase to a 60 mg subsample produces only a slightimprovement in uncertainty (Fig. 4). The 16 isochrons arecolinear and statistically indistinguishable from one another,therefore the weighted mean of these 16 ages gives the bestestimate of time since deposition of Ash D, which is 211.0 ±12.8 ka with an MSWD of 1.66 (Fig. 5).

5. Correlation and paleomagnetic significance

The isochron ages of 218.0 ± 14.0 and 211.0 ± 12.8 kadetermined for the Albuquerque Volcanoes and Pringle Falls

Table 1Summary of 40Ar/39Ar experiments on groundmass from Albuquerque Volcanoes basalt flows

Sample site Wt. K/Ca Total fusionage

Age spectrum Isochron analysis

Increments used 39Ar Age±2σ MSWD N MSWD 40Ar/36Ar±2σ Age±2σExperiment no. (mg) (total) (ka) (°C) (%) (ka) intercept (ka)

AV-1UW38J01 200 0.11 192.1±48.9 870–1250 59.6 225.4±56.6 0.86 8 of 14 1.02 295.8±10.0 212.2±158.7

AV-40UW38J05 200 0.11 200.0±60.4 875–1300 92.1 208.2±56.1 1.55 8 of 10 1.81 295.3±4.0 216.4±133.0UW38J06 200 0.12 208.6±42.1 995–1235 50.5 223.9±50.5 0.26 7 of 12 0.27 293.7±7.6 302.7±286.6

Weighted mean isochron age from 2 experiments: 15 of 22 0.32 295.0±3.5 232.0±120.0

AV-41UW43A10 107 0.11 209.9±33.2 580–1130 100.0 213.2±24.1 0.90 12 of 12 0.45 295.2±1.6 224.3±55.2UW43A12 103 0.10 187.0±40.7 550–1150 100.0 188.6±23.4 0.62 10 of 10 0.42 295.0±1.8 203.8±57.8UW43A9 107 0.07 188.8±45.4 575–1200 100.0 199.9±30.5 0.30 9 of 9 0.30 295.1±1.6 214.2±62.1UW43A11 93 0.06 275.5±45.3 620–1225 100.0 267.8±34.6 0.76 7 of 7 0.78 296.2±1.8 240.4±71.0UW50F65 97 0.05 226.7±39.0 540–1250 100.0 233.6±27.2 0.31 8 of 8 0.21 296.5±2.1 191.8±90.8UW50F66 106 0.05 187.9±95.3 540–1250 100.0 200.6±29.7 0.56 8 of 8 0.61 294.9±2.6 217.8±76.0UW50F67 101 0.05 245.1±34.6 550–1125 94.1 203.0±27.9 1.27 8 of 9 0.58 294.1±1.2 258.5±52.7


AV-44UW43B18 97 0.06 228.2±58.3 570–1200 100.0 223.2±33.3 0.18 12 of 12 0.07 295.0±1.8 242.9±77.5UW43B20 101 0.09 226.1±47.1 550–1150 100.0 222.5±28.6 0.39 10 of 10 0.43 295.3±1.2 228.1±48.3UW43B17 96 0.10 228.1±62.6 550–1175 100.0 238.5±34.4 0.40 10 of 10 0.45 295.5±1.7 237.2±77.9UW43B19 104 0.10 221.1±47.5 540–1150 100.0 226.2±40.0 0.89 9 of 9 0.94 294.8±1.7 255.7±79.9UW50F68 103 0.05 223.1±46.6 550–1125 99.0 209.5±28.7 0.24 8 of 9 0.15 294.8±1.6 237.2±67.3UW50F69 101 0.04 252.6±53.0 550–960 97.2 239.7±32.1 0.29 8 of 10 0.27 294.8±2.2 262.0±75.6UW50F70 99 0.05 218.9±43.9 530–1250 100.0 215.8±32.7 0.23 11 of 11 0.25 295.6±1.5 212.8±63.4


AV-58UW38K09 200 0.11 240.1±59.2 800–1305 100.0 225.2±38.6 0.43 14 of 14 0.20 297.9±2.7 153.2±82.0UW38K10 200 0.10 216.5±40.7 765–1320 100.0 207.9±33.1 0.25 13 of 13 0.09 297.1±2.2 165.0±67.5


AV-59UW43C30 100 0.07 239.0±45.0 540–1200 100.0 230.9±30.0 0.40 11 of 11 0.33 296.8±2.6 176.1±96.9UW43C32 102 0.06 180.6±44.9 540–1200 100.0 183.8±41.5 0.28 9 of 9 0.29 296.1±2.4 156.7±105.8UW43C29 98 0.08 218.7±34.6 540–1200 100.0 217.7±29.7 0.53 10 of 10 0.41 297.0±2.5 161.8±85.4UW43C31 99 0.09 266.4±67.2 540–920 94.6 240.9±40.1 0.22 7 of 9 0.26 294.9±6.7 264.7±241.7UW50E62 100 0.10 197.2±55.6 550–1225 100.0 200.0±44.6 0.11 11 of 11 0.12 295.0±5.8 217.7±204.1UW50E63 105 0.06 218.0±163.0 530–1220 100.0 228.2±62.6 1.11 8 of 8 1.04 296.0±3.4 196.4±130.0UW50F64 101 0.05 228.5±36.5 540–1250 100.0 222.9±29.3 0.48 8 of 8 0.53 295.7±1.1 214.6±51.8

Weighted mean isochron age from 7 experiments: 64 of 66 0.37 296.0±0.8 194.0±35.0Weighted mean isochron ages from 6 lava flow sites: 218.0±14.0

All ages calculated relative to 1.194 Ma Alder Creek Rhyolite sanidine standard (Renne et al., 1998).


Ash D, respectively, are indistinguishable from one another atthe 95% confidence level. Moreover, these age determinationscannot be distinguished from the 227 ± 8 ka age determined forthe transitionally magnetized Mamaku ignimbrite by Houghtonet al. (1995) and McWilliams (2001). The VGPs of theAlbuquerque Volcanoes (Fig. 1A) fall over the South AtlanticOcean within 20° latitude of those from the Mamaku ignimbrite(Fig. 1A). The finding of a common age estimate for these threerecordings, coupled with the similarity of the VGPs, isremarkable and leads to the conclusion that a single excursionis expressed at each of the sites in Fig. 1A. We therefore proposethat the name Pringle Falls be used for this excursion based onthe initial discovery of distinctive VGP behavior at this site

(Herrero-Bervera et al., 1989, 1994). We recommend thatnames Albuquerque and Jamaica excursion (Fig. 2; Langereiset al., 1997; Channell, 2006) be abandoned.

It is beyond the scope of this paper to address in detail whythe 238U–230Th isochron determined by Peate et al. (1996) issignificantly younger than our 40Ar/39Ar age determination forthe Albuquerque Volcanoes. We note, however, that the fivesamples of whole-rock and magnetite represent three spatiallydistinct lava flows and that the isochron is essentially acomposite of a pair of two-point whole-rock + magnetite“isochrons” that separately give values of 149 ± 28 and 180 ±49 ka. Although the older of these two “isochrons” is withinerror of our 40Ar/39Ar determination for the Albuquerque

Fig. 3. 40Ar/39Ar age spectrum and isochron diagrams for Albuquerque Volcanoes flow site samples AV-44 and AV-59 of Geissman et al. (1990). The weighted mean ofthese two isochrons, combined with the four others in Table 1, gives the preferred age of these basaltic lavas of 218.0±14.0 ka (±2σ analytical and intercalibrationuncertainty).


Volcanoes, the magnetite samples that anchor these isochronsshow unusually large differences in U/Th ratio that most likelyreflect the occlusion of apatite or other accessory mineralswithin the magnetite. In summary, until the 238U–230Thcomposition of several additional phases including plagioclase,olivine, apatite and the groundmass are measured from at leastone of these lava flows, the composite whole-rock + magnetiteisochron of (Peate et al., 1996) should be viewed with caution.The 40Ar/39Ar isochron of 211.0 ± 12.8 ka obtained from Ash D(Fig. 4) is an order of magnitude more precise than the isochrondetermined by Herrero-Bervera et al. (1994), thus werecommend that it be adopted as the radioisotopic age for thePringle Falls excursion.

The number of excursions thought to have occurred duringthe Brunhes Chron has proliferated in the last decade or so, inlarge part as sediments deposited at high rates in the NorthAtlantic Ocean have been cored by the Ocean Drilling Program(ODP) and high resolution magnetic data acquired from them(e.g., Lund et al., 2001, 2006; Channell, 2006; Laj andChannell, 2007). For example, Lund et al. (2001, 2006) foundevidence from directional measurements in sediments coredduring ODP leg 172 at Sites 1060, 1061, 1062, and 1063 that at

least five, and perhaps seven, excursions occurred during thelast 300 ka (Fig. 2B). In contrast, paleodirectional andpaleointensity measurements by Channell (2006) in ODP site919 sediments reveal evidence for only four excursions in thelast 300 ka (Fig. 3C). These examples highlight the fact that theextent to which a particular excursion is recorded in marinesediment reflects several factors, including grain size, miner-alogy, diagenesis and post-depositional compaction andsmoothing of the signal and these vary from site to site andover time. Another issue is the quality of the age models used tocorrelate the excursions both among marine sediment cores andwith terrestrial volcanic rock or polar ice core records. Acontinuous oxygen isotope-based astrochronology has yet to bedeveloped for the leg 172 cores making it difficult to determineprecise ages for these excursions and to correlate them withother records. The age model for site 919 magnetic data, thoughimperfect, is based on the planktonic oxygen isotope recordsthat have been compared to a global benthic target record with atie point based on the occurrence of Ash Layer 2 at −55 ka(Channell, 2006). Thus for site 919 the excursional directionsare most clearly expressed during intervals of low relativepaleointensity at 34–32, 41–39, 188–180, and 209–207 ka and

Table 2Summary of 40Ar/39Ar laser incremental-heating experiments on plagioclase from Ash D

Sample weight Total fusion Age spectrum Isochron analysis

Age±2σ Age±2σ MSWD N MSWD 40Ar/36Ar±2σ Age±2σExperiment no. (ka) (ka) intercept (ka)

20 mgUW46C001 263.6±43.5 253.7±38.6 0.49 5 of 5 0.51 302.5±21.2 234.1±70.7UW46C002 218.0±51.4 214.7±43.7 0.53 4 of 4 0.26 307.0±22.9 181.7±78.6UW46C003 243.0±29.9 214.3±26.3 0.34 4 of 4 0.49 301.7±27.7 210.6±44.0UW46C004 205.8±69.8 208.2±44.1 0.91 4 of 4 1.33 300.1±40.2 197.0±102.8UW46C005 210.4±63.4 210.4±63.4 0.24 4 of 4 0.36 295.8±32.4 200.2±84.4UW46C006 239.5±56.5 233.0±52.4 0.34 4 of 4 0.32 304.1±27.4 211.3±86.4UW46C007 206.7±50.9 201.2±43.5 0.41 4 of 4 0.60 299.0±26.9 194.3±71.1UW46C008 218.2±57.5 215.3±50.9 0.11 4 of 4 0.13 299.4±27.1 205.7±83.8UW46C009 206.0±54.8 207.0±48.6 0.08 4 of 4 0.11 296.4±24.0 204.7±81.4

40 mgUW46C010 215.1±26.4 219.7±23.3 1.15 4 of 4 0.60 284.0±14.9 245.1±38.6UW46C011 199.3±26.9 199.2±25.5 0.05 4 of 4 0.02 293.8±11.2 204.2±41.5UW46C102 190.9±35.1 193.6±31.1 0.15 4 of 4 0.22 295.5±12.3 193.5±48.0UW46C103 219.7±37.0 222.4±31.4 0.04 4 of 4 0.02 292.7±18.5 228.8±52.9UW46C107 217.4±42.5 222.2±38.2 0.13 4 of 4 0.19 294.9±21.4 223.7±64.4UW46C108 207.3±32.7 210.6±28.1 0.07 4 of 4 0.05 292.3±18.3 217.3±47.2

60 mgUW46C012 205.1±25.6 201.3±23.0 0.18 4 of 4 0.01 300.2±13.1 188.8±41.6Weighted means from 16 experiments: 211.1±7.8 0.88 65 of 65 1.66 296.5±1.8 211.0±12.8

All ages calculated relative to 1.194 Ma Alder Creek rhyolite sanidine standard (Renne et al., 1998).


these have been correlated with the Mono Lake, Laschamp,Iceland Basin, and Pringle Falls excursions, respectively(Channell, 2006). The Pringle Falls excursion occurs near theend of a period of low field intensity and unstable field behaviorthat began at ca. 225 ka and ended at ca. 205 ka (Channell,2006; Fig. 2C). Our preferred age of 211 ± 13 ka for the PringleFalls excursion is distinctly older than the Iceland Basinexcursion, yet agrees remarkably well with the astrochronologicestimate of 209–207 ka of Channell (2006), and may becorrelative to excursion 7$ in the Leg 172 sites, to which Lundet al. (2001) assigned an age of −236 ka (Fig. 2B). Theclockwise loopings of the VGP path defined by the site 919sediments during the 225–205 ka period of field instability,including the final excursional swing through Africa to aposition identical to the VGPs of the Albuquerque Volcanoes,then on to Antarctica, Australia and Kamchatka is very similarto that of the Pringle Falls sediments, thereby bolstering thetemporal correlation between these terrestrial and marinerecordings (Fig. 1; Channell, 2006).

The Albuquerque Volcanoes and Mamaku Ignimbrite VGPsfall either along or between VGP paths found at Pringle Fallsand ODP site 919, and may therefore be interpreted asfragmentary “snap-shots” of the same excursion, perhapsrecorded a few hundred or thousand years apart given theuncertainties in the 40Ar/39Ar age determinations. The repeatedexcursional loops during a nearly 20 ka period of fieldinstability argue that, although the Pringle Falls is among themore complex excursions of the Brunhes Chron, it is not areversed polarity subchron as suggested by McWilliams (2001).Similarity between the initial clockwise looping progression of

VGPs in the Pringle Falls sediments, including “loitering” andbacktracking of poles along paths through the North Atlantic,Southern Ocean/Antarctica and Kamchatka and the final VGPloop discovered in the ODP site 919 sediments might be takento suggest that the Pringle Falls, and other Brunhes chronexcursions, reflect simple rotation of a mainly dipolar field (e.g.,Herrero-Bervera et al., 1994; Laj et al., 2006). Alternatively, thebroad looping VGP path over the Americas and eastern Asiacommon to many records of full polarity reversals mightsuggest that long-lived thermal gradients in the lowermostmantle – identified by seismic tomography – control fluid flowin the outer core and hence field geometry during successivedynamo instabilities (e.g., Laj et al., 1991; Herrero-Bervera andRuncorn, 1997; Laj et al., 2006). If true, the initiation ofexcursions and full polarity reversals may share a mechanismthat nucleates field instability in the outermost part of the core,but only on occasion does the instability last long enough topermit the field held by the inner core to fully reverse (Gubbins,1999; Singer et al., 2005; Laj et al., 2006).

6. Geomagnetic Instability Time Scale (GITS) andthe geodynamo

One view of the sudden directional changes that take placeduring periods when the field is relatively weak is that theseexcursions are interruptions of the “normal” geomagnetic field,which is dominated by a relatively strong axial dipole (Gubbins,1999; Zhang and Gubbins, 2000). As the number and frequencyof excursions found in numerous sediment cores and terrestrialsites has increased, the early paradigm of stable polarity chrons

Fig. 4. 40Ar/39Ar age spectra and isochrons from laser incremental-heating experiments on 20, 40, and 60 mg subsamples of plagioclase from Pringle Falls Ash D.


during which normal or reversed fields persisted for severalhundreds of thousands of years has given way to the hypothesisthat the geodynamo may be intrinsically unstable for as much as20% of the time (Lund et al., 1998) perhaps owing in part to acritical Reynolds number in the outer core fluid (Zhang andGubbins, 2000). It has also been proposed that excursionsreflect frequent, rapid changes in the convecting outer core fluidwherein the diffusion of magnetic flux as the dipole field decaysis governed by a 500 a timescale, but that an order of magnitudelonger time constant of 3 ka for flux to diffuse from the solid

inner core prevents excursions originating in the outer core fromlasting long enough that the field held by the inner core can fullyreverse (Gubbins, 1999). Gubbins (1999) suggested that a 10:1ratio of excursions to reversals over time is consistent in aprobabilistic sense with the inferred order of magnitude dif-ference in flux diffusion time constants for the inner and outercore. Numerical models of geodynamo behavior over severalhundred thousand year periods have been used to investigatethe strength and configuration of heat flux across the core–mantle boundary (Glatzmaier et al., 1999). The interpretation by

Fig. 5. Isotopic analyses acquired in 16 laser incremental-heating experimentsdefine a 65 point isochron that gives the preferred age of Ash D, and hence thePringle Falls excursion, of 211.0±12.8 ka (±2σ analytical and intercalibrationuncertainty).


Glatzmaier et al. (1999) that a uniform and perhaps subduedheat flux may prevail is based on a particular model thatproduces earth-like reversals, but no excursions.

A complete and precise calendar of excursions and reversalsfor the past 2–3 Ma, coupled with detailed knowledge of thefield directions and intensity changes preceding and post-datingexcursions, are critical observations necessary to: (1) refineestimates of diffusion constants for flux out of the inner andouter core, (2) address the hypothesis that the outer coregeodynamo is intrinsically unstable, and (3) determine whichnumerical simulations and lower mantle heat flux configura-tions produce the most “earth-like” field behavior over time.

Considering the excursions for which a precise oxygenisotope-based astrochronologic age estimate, 40Ar/39Ar age, orboth, have been determined, leads us to a refined GeomagneticInstability Time Scale (GITS) for the Brunhes Chron (Fig. 2E).One of the most striking features of the GITS is that the ten well-dated excursions are not evenly distributed during the past776 ka. Five excursions (Pringle Falls, Iceland Basin, Blake,Laschamp, and Mono Lake) occurred during the 190 ka periodbetween about 220 and 30 ka, whereas five others (Stage 17,West Eifel 2, Big Lost, West Eifel 4 and West Eifel 5) occurredduring a 160 ka period between 680 and 520 ka (Fig. 2E). Notethat although the Blake excursion has not been dated directlyusing astro- or radioisotope chronology, it has been found insediment cores from several oceans and is constrained to marineisotope stage 5.5 at −120 ka (Channell, 2006; Lund et al.,2006). Evidence that other excursions occurred between 520and 220 ka is based on limited sampling and imprecisely datedmarine records and therefore remains highly speculative. One ofthese sites is in the Mediterranean Sea (Langereis et al., 1997;Fig. 2A), where excursions in this interval are each defined byonly a single specimen and the chronology is based on sapropeldeposition rather than a continuous oxygen isotope-basedastrochronology. The other site is ODP 1062 which, althoughit reveals the largest number of potential excursions (Lund et al.,2001, 2006; Fig. 2B), many of these have not been found in

more precisely dated Atlantic sediments cored at ODP sites 919,983 and 984 (Channell et al., 2004; Channell, 2006; Laj andChannell, 2007; Fig. 2C). The West Eifel volcanic field, oncethought to record a single poorly-dated excursion (Schnepp andHradetzky, 1994), has recently been shown to record as many asfive excursions between 730 and 530 ka on the basis of40Ar/39Ar geochronology (Singer et al., 2006; Fig. 2E). At WestEifel one of the excursions recorded in at least two separate lavaflows coincides with the Big Lost excursion dated on La Palma(Singer et al., 2002) and Tahiti (Hoffman and Singer, 2004) at579 ka; two others at 555 and 626 ka are recorded by four andthree flows, respectively, leaving little doubt that these are non-unique recordings of excursional field behavior (Singer et al.,2006). To our knowledge, however, there are no terrestrialvolcanic records of excursions in the 520 to 220 ka period.

Interestingly, the two periods during which well-datedexcursions are concentrated coincide with intervals when theintensity of the geomagnetic field measured by the virtual axialdipole moment (VADM) drops to below half the present dayaverage (Fig. 2D; Guyodo and Valet, 1999). For example, thisoccurs five times during the earlier period and three timesduring the later interval. The global stack of paleointensityrecords (Guyodo and Valet, 1999) suggests that during the300 ka period between 520 and 220 ka, field intensity, thoughoscillatory, did not fall below half of today's value (Fig. 2D).These observations support previous findings (summarized inLaj and Channell, 2007) that directional changes of the fieldhave a greater probability during periods of low field intensity.If the temporal clustering of excursions in the GITS into twointervals of the Brunhes Chron is real and not a samplingartefact, this suggests that the geodynamo may not beintrinsically unstable, but rather that core fluid dynamics aresuch that the main dipole field oscillates between weakened andstronger states with a 200–300 ka periodicity and thatexcursions are expected to occur in bundles during periods oflow field intensity.

Numerical simulations have investigated the role of thelowermost mantle in controlling the geodynamo by way ofimposing various configurations of heat flux across the core–mantle boundary as a starting condition (Glatzmaier et al.,1999). It has been suggested by Glatzmaier et al., 1999) thatbecause the most “earth-like” field reversals occur during asimulation in which the heat flux is spatially uniform, theseismic low velocity anomalies detected in the “D” layer mayreflect compositional rather than thermal contrasts. We notehere that the simulation favored by Glatzmaier et al. (1999)produced full field reversals, but no excursions, therefore it is arather poor match to the GITS (Fig. 5). A simulation thatimposes a tomographically defined variation in radial heat fluxacross the core–mantle boundary not only produces reversals,but also four directional excursions over 300,000 model years(Glatzmaier et al., 1999). Although the dipole moment in thislatter simulation varies in a less “earth-like” manner than in themodel with uniform heat flux, this simulation produces a farcloser match to the observed paleomagnetic record of dynamoinstabilities for the last 776 ka than Glatzmaier et al. (1999)anticipated. We suggest that laterally variable heat flux into the


lowermost mantle, perhaps with smaller temperature contraststhan those implied by seismic tomography, remains a viablecandidate to modulate convective flow, and hence geodynamoinstabilities, in the outermost core over geologic time scales.

7. Conclusions

The determination of 40Ar/39Ar isochron ages of 218 ± 14and 211 ± 13 ka from the Albuquerque Volcanoes and PringleFalls Ash D, respectively, in combination with transitionalpaleomagnetic directions from these sites, establish that theyboth record the Pringle Falls excursion. This excursion is alsorecorded by the 227 ± 8 ka Mamaku Ignimbrite, New Zealand,and by sediments deposited rapidly at ODP site 919 in the NorthAtlantic Ocean that are dated astrochronologically at 209–207 ka. We propose that the names “Albuquerque” and“Jamaica” excursion be abandoned and that the radioisotopicage of 211 ± 13 ka obtained at the namesake locality be adoptedfor the Pringle Falls excursion.

The Pringle Falls excursion is one of five globally expressedexcursions between 220 and 30 ka that have been well-documented in several marine sediment cores or dated using40Ar/39Ar methods on terrestrial rocks. Together with at leastfive other well-dated excursions between 730 and 520 ka, thereare ten excursions that define the Geomagnetic Instability TimeScale (GITS) for the Bruhnes Chron. Eight of these are recordedby terrestrial lava flows, or in the case of the Pringle Fallsexcursion by volcanic ash, that have been dated using 40Ar/39Armethods. If the temporal clustering of excursions in the GITSinto two periods of the Brunhes Chron is not a samplingartefact, this suggests that instabilities of the geodynamo are notan intrinsic feature, but rather that outer core fluid flowoscillates such that the main dipole field weakens and becomesunstable with recurrence interval of some 200–300 ka. Excur-sions are an important part of dynamo behavior over geologictime scales that, in addition to reversals of the main dipole field,need to be fully considered in the next generation of numericaldynamo simulations.

Acknowledgements

Supported by NSF grants EAR-033768403 and EAR-0516760 to Singer. We thank Xifan Zhang and Lee Powellfor laboratory assistance and advice. Comments by Carlo Lajand an anonymous reviewer helped clarify our presentation ofthe geochronologic results.

Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version, at doi:10.1016/j.epsl.2007.12.009.

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