Earth and Planetary Science Letters 405 (2014) 25–38

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Earth and Planetary Science Letters

www.elsevier.com/locate/epsl

Precise ages of the Réunion event and Huckleberry Ridge excursion:

Episodic clustering of geomagnetic instabilities and the dynamics

of flow within the outer core

Brad S. Singer a,∗, Brian R. Jicha a, Daniel J. Condon b, Alexandra S. Macho a, Kenneth A. Hoffman a,c, Joseph Dierkhising c, Maxwell C. Brown d, Joshua M. Feinberg e, Tesfaye Kidane f

a Department of Geoscience, University of Wisconsin–Madison, 1215 West Dayton St., Madison, WI 53706, United Statesb NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham NG12 5GG, UKc Department of Physics, California Polytechnic State University, San Luis Obispo, CA, United Statesd Helmholtz-Zentrum Potsdam, Deutsches GeoForschungsZentrum, Telegrafenberg, 14473 Potsdam, Germanye Department of Earth Sciences and Institute for Rock Magnetism, University of Minnesota, United Statesf School of Earth Science, College of Natural Science, Addis Ababa University, P.O. Box 1176, Addis Ababa, Ethiopia

a r t i c l e i n f o a b s t r a c t

Article history:Received 22 August 2013Received in revised form 16 July 2014Accepted 5 August 2014Available online xxxxEditor: T. Elliott

Keywords:40Ar/39ArgeochronologyU–Pbpaleomagnetismgeodynamo

The Réunion event is one of the earliest recognized periods of normal polarity within the reversed Matuyama chron. Named for the site at which it was first discovered on Réunion Island, it has since purportedly been found globally in both volcanic rocks and sediments, and thus has become a key chronostratigraphic marker. However, geochronologic results from several locations thought to have recorded this event have caused considerable confusion regarding not only its age and duration, but also the number of Réunion events. New 40Ar/39Ar ages from eight Réunion Island lavas in three distinct sections are indistinguishable from one another, thereby placing the event at 2.200 ± 0.007/0.010 Ma(±2σ analytical/total uncertainty, note this format is used throughout the paper). The paleomagnetic behavior recorded at two of the island sites shows that the opposite (normal) polarity was reached and sustained for a period during which several lava flows were erupted. Whether this can be classified as a very short subchron bounded by a rapid set of back-to-back reversals, or as a special case of a geomagnetic excursion, is unclear. Hence, we choose to continue labeling the dynamo activity recorded by these Réunion Island lavas as an “event”. This event preceded a ∼38 kyr period of normal polarity that we name the Feni subchron after its locality of discovery at ODP site 981. The Feni subchron was succeeded by the Huckleberry Ridge excursion for which 40Ar/39Ar sanidine and U–Pb zircon ages of 2.077 ± 0.001/0.003 Ma and 2.084 ± 0.012/0.013 Ma, respectively, from member B of the Huckleberry Ridge tuff in Idaho, are in agreement. These findings suggest that the full normal polarity recorded on Réunion Island is a singular brief period of unstable field behavior at the onset of a ∼125 kyr bundling of dynamo instabilities from 2.20 to 2.07 Ma. Disturbances to the axial dipole component of earth’s magnetic field during this period, and by analogy similar periods of temporally-clustered excursions during the early and late portions of the Brunhes chron, may reflect disruptions to convective flow arising from parcels of material introduced into the outer core from either the inner-core or core–mantle boundaries; a proposition that might be tested by future numerical dynamo simulations.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Poorly understood physical processes within earth’s core cause the geomagnetic field to undergo reversals of polarity, excursions, or events. The timing of these phenomena, particularly the number

* Corresponding author.E-mail address: bsinger@geology.wisc.edu (B.S. Singer).

http://dx.doi.org/10.1016/j.epsl.2014.08.0110012-821X/© 2014 Elsevier B.V. All rights reserved.

and sequence of short-lived excursions or events relative to com-plete polarity reversals, has long been central to debate about conditions and processes originating either in deep or shallow re-gions of the core that may modulate geomagnetic field stability on time scales of 105–106 yr (e.g., McFadden and Merrill, 1986;Gubbins, 1999; Glatzmaier et al., 1999; Hoffman and Singer, 2008;Davies and Gubbins, 2011). The dramatic improvement during the last two decades in the radioisotopic dating of many short-lived

26 B.S. Singer et al. / Earth and Planetary Science Letters 405 (2014) 25–38

excursions and events has led to a nascent geomagnetic insta-bility time scale (GITS) for the Quaternary (Singer et al., 1999;Singer, 2014). Notably, this chronology has revealed a temporal clustering of excursions during the early (730–530 ka) and late (212–17 ka) portions of the Brunhes chron (Singer et al., 2008a, 2008b; Singer, 2014). Here, we present new geochronologic data from volcanic rocks that, together with published data from marine sediments, record a sequence of excursions, reversals, and events during 125 kyr of the early portion of the Matuyama chron. The chronology strongly suggests that dynamo instability is temporally modulated by a process that operates on a 100–200 kyr time scale. We offer one possible explanation for directional instability in the axial dipole component of the geomagnetic field for such periods of time that is based on thermodynamic analysis and modeling the effects of crystallization of the mush zone atop the outer core (Braginsky, 1963; Moffatt and Loper, 1994).

2. Previous work on the Réunion event

There is a half-century-long history related to paleomagnetic investigations on Réunion Island. The first paleomagnetic and K–Ar dating experiments on Réunion lavas revealed a normal-to-reversed polarity transition at 2.0 ± 0.1 Ma (Chamalaun and Mc-Dougall, 1966; K–Ar ages are calculated using the updated decay constants of Steiger and Jaeger (1977). Although initially consid-ered to be part of the Olduvai subchron (Grommé and Hay, 1971), this distinction was later changed to the “Réunion normal polar-ity event(s)”, characterized by one or two shifts to normal polarity around 2.0 Ma, based on several statistically indistinguishable K–Arages of Réunion lavas. McDougall and Watkins (1973) re-dated lavas from the Rivière St. Denis (RSD) and Grand Chaloupe (GC) sections on Réunion Island and assigned a K–Ar age of 2.07 ±0.04 Ma for a single Réunion normal subchron (Fig. 1). The first 40Ar/39Ar experiments conducted on lavas from three sections on Réunion Island—RSD, GC, and LM, a road section near the village of La Montagne—indicated that a single normal polarity event of un-certain duration occurred at 2.186 ± 0.040 Ma (Baksi et al., 1993;Baksi and Hoffman, 2000; note that all 40Ar/39Ar ages are relative to the 28.201 Ma calibration of Fish Canyon sanidine—FCs; Kuiper et al., 2008; with ±2σ analytical uncertainties reported unless to-tal uncertainties also given). However, recent unspiked K–Ar dating has been used to argue that the RSD and GC sections may have recorded two separate reversals at 2.15 ±0.04 and 2.04 ±0.04 Ma, named the Réunion I and II events (Quidelleur et al., 2010).

Normally and transitionally magnetized rocks of similar age found at several other localities have been correlated to the Réu-nion event, adding further to the confusion. Fleck et al. (1972)K–Ar dated a sequence of three lavas at Cerro del Fraile, Ar-gentina with T–N–R magnetization and assigned an age range of 2.12–1.91 Ma to paleomagnetic behavior associated with the Réu-nion event. Singer et al. (2004) 40Ar/39Ar dated the same three lavas and calculated a weighted mean age of 2.151 ± 0.016 Mafor the event. The Huckleberry Ridge Tuff (HRT), the oldest and largest ash-flow tuff of the Yellowstone Plateau volcanic field, has long been associated with the Réunion event based on a common transitional paleomagnetic direction determined from tuff mem-bers A, B, and C (Reynolds, 1977). Numerous K–Ar and 40Ar/39Ar ages between 2.0 and 2.1 Ma have been obtained from HRT mem-bers A, B, and C (Gansecki et al., 1998; Lanphere et al., 2002, and references within). Gansecki et al. (1998) measured 21 sanidine grains from sample 2R577 collected near the middle of mem-ber B, which is the most voluminous of the three members. The weighted mean 40Ar/39Ar age (n = 9), wherein we have excluded a 2.25 Ma xenocryst and analyses that have <90% radiogenic 40Ar∗or K/Ca <10, is 2.059 ±0.031 Ma (Fig. 1, Table A.1). Lanphere et al.(2002) report an 40Ar/39Ar age of 2.103 ± 0.008 Ma for the HRT,

based on combined total fusion and incremental heating experi-ments done on members A and B, and proposed that a separate excursion, named for the Huckleberry Ridge Tuff locality, occurred several thousand years after the Réunion event. More recently, Ellis et al. (2012) report 40Ar/39Ar ages for the three members of the HRT (Fig. 1), arguing that members A and B record the Réunion event as defined by the Geologic Time Scale 2004 (Gradstein et al., 2004), and that member C was erupted some 20 kyr later. How-ever, Rivera et al. (2014) determined 40Ar/39Ar and U–Pb ages from the HRT that are significantly younger than those of Ellis et al.(2012). The average of several imprecise unspiked K–Ar dates from normally magnetized lavas at Gamarri, Ethiopia, led Kidane et al.(1999) to propose that they also record the Réunion event at about 2.07 ± 0.10 Ma (Fig. 1).

In addition to the volcanic record, magnetostratigraphic and as-trochronologic studies of deep sea sediment cores reveal polarity reversals between 2.2 and 2.1 Ma (Fig. 1). Using sediment core MD97423 from the Philippine Sea, Horng et al. (2002) places a normal polarity subchron at 2.133–2.118 Ma, but the very low sedimentation rate of 1.5 cm/kyr, coarse spacing of the discrete samples, and a sandy layer near the base of the core limit the temporal resolution around the time of the Réunion event. Similar ages were assigned to normal polarity subchrons found in cores from the North Atlantic (Channell and Guyodo, 2004; Ohno et al., 2012). The most highly resolved normal polarity chronozone in sediments, occurring from 2.153 to 2.115 Ma, is recorded in the Feni Drift at ODP site 981, where high sedimentation rates (peak-ing at 18 cm/kyr) give an expanded, high-fidelity record (Channell et al., 2003; Fig. 1). The astronomical age model was produced by matching the site 981 δ18O record to an orbitally-tuned reference record of Shackleton et al. (1990); details are in Channell et al.(2003).

Lacustrine sediment cored from the Senèze maar, France, that exhibits an N–R polarity transition contains a conspicuous sanidine-bearing tephra bed in the uppermost of the normal po-larity zone (Roger et al., 1999). Although the individual measure-ments show considerable scatter and the neutron fluence mon-itors used have led to questions about accuracy (Baksi, 2001;Roger et al., 2001), 40Ar/39Ar dating of sanidine from this tephra using both incremental heating and single-crystal fusion methods yields a weighted mean age of 2.11± 0.02 Ma (2σ analytical un-certainty; re-calibrated relative to 28.201 Ma FCs) leading Roger et al. (1999) to correlate the normal polarity sediment with the Réunion event.

Our findings from new 40Ar/39Ar and U–Pb dating of magnet-ically salient lava flows and tuff help to clear up confusion re-garding the timing of the excursions and reversals that occurred between about 2.2 and 2.0 Ma. The improved chronology of dy-namo behavior during the early Matuyama chron, coupled with similar findings from the Brunhes chron, helps illuminate a time scale for a physical process that may operate deep within the outer core to weaken the axial dipole portion of the geomagnetic field. Moreover, the new chronology provides an updated template for interpreting sedimentary deposits and their fossils.

3. Methods

3.1. Magnetic measurements

In parallel to new geochronologic experiments, new paleomag-netic directions were determined through alternating field demag-netization of standard specimens from the more than 100 lava flows previously studied from three distinct sections on Réunion Island (RSD, GC, and LM) and combined with the original dataset of Baksi and Hoffman (2000). All measurements, new and old, were conducted at California Polytechnic State University, San Luis

B.S. Singer et al. / Earth and Planetary Science Letters 405 (2014) 25–38 27

Fig. 1. Comparison of 40Ar/39Ar, K–Ar, and U–Pb ages of volcanic rocks and astrochronologically-dated sediments. RSD: Riviere St. Denis lava section; GC: Grande Chaloupe section, LM: La Montagne section. K–Ar ages of Fleck et al. (1972) and McDougall and Watkins (1973) updated to decay constants of Steiger and Jaeger (1977). K–Ar ages reflect weighted means as reported in original papers. 40Ar/39Ar ages calibrated to 28.201 Ma FCs, with 2σ analytical uncertainties. Lanphere et al. (2002) data for HRT member B is the total fusion data of devitrified tuff sample 79Y-182I. The incremental heating data for this sample gave a discordant spectrum with an MSWD of 4.38 and therefore is not shown for comparison. Ages for samples 79Y-182A (member A) and 68-O-46 (member A, Mt. Everts) of Lanphere et al. (2002) are the plateau ages from an incremental heating experiments on 15–52 mg of sanidine. Data of Gansecki et al. (1998) has been filtered to remove xenocrysts, analyses with <90% 40Ar∗ , and grains that were not completely fused (see text for details). The 40Ar/39Ar and U–Pb ages from Rivera et al. (2014) for the Huckleberry Ridge tuff are mean values of the youngest modes of populations of single crystal sanidine and zircon dates. Magnetic polarity of lava indicated by: open bars reversed; filled bars normal; striped bars transitional. Astrochronologic age models from Channell et al. (2003), Ohno et al. (2012), and Channell and Guyodo (2004) show VGP latitude vs. age. VGP data from Horng et al. (2002)is uncertain due to anomalous declination measurements arising from deformation of the core during its retrieval; their reported age of the “Réunion subchron” is highly uncertain at the older limit. The proposed Geomagnetic Instability Time Scale (GITS) for the period between 2.3–1.9 Ma is shown at the top of the figure. OS: Olduvai subchron, from Channell et al. (2003); POE: Pre-Olduvai excursion, from Channell et al. (2002); HRE: Huckleberry Ridge excursion, 2.077 ± 0.001 Ma (this study); FDS: Feni Drift subchron, based on Channell et al. (2003); RS: Réunion excursion, weighted mean of 2.200 ± 0.007 Ma.

Obispo. The primary remanence directions were identified using principal component analysis (Kirshvink, 1980; see Table A.2, Sup-plementary Documents). Sample locations and details can also be found in the Supplementary Documents.

3.2. 40Ar/39Ar experiments

40Ar/39Ar ages were determined for lavas from Réunion Is-land, Cerro del Fraile, and Gamarri, and the HRT at the WiscAr

28 B.S. Singer et al. / Earth and Planetary Science Letters 405 (2014) 25–38

Laboratory at the University of Wisconsin–Madison. Groundmass separates (∼200 mg) were prepared from the lavas following Jicha et al. (2012). Large sanidine phenocrysts (∼2 mm diameter) were separated from sample HRT-1 of member B of the Huckleberry Ridge tuff via crushing and hand-picking. Groundmass and sani-dine separates were irradiated along with the 28.201 Ma FCs stan-dard (Kuiper et al., 2008) at the Oregon State University TRIGA reactor in the Cadmium-Lined In-Core Irradiation Tube. For the furnace incremental heating experiments on groundmass, the an-alytical protocol including blank analysis, gas cleanup, and iso-tope measurements follow Jicha et al. (2012). Multiple experiments were conducted on several lavas to improve precision. Single sani-dine crystals were fused and incrementally heated using a 25 WCO2 laser and analyzed using a MAP 215-50 mass spectrometer operated in single collector mode following procedures outlined in Meyers et al. (2012). The gas was cleaned during and after the heating period with two SAES C50 getters, one of which was oper-ated at ∼450 ◦C and the other at room temperature. Blanks were analyzed after every second laser fusion or heating step, and were less than 2.5 ×10−17, 2.0 ×10−19, 6.0 ×10−20, and 1.0 ×10−19 molfor 40Ar, 39Ar, 37Ar, 36Ar, respectively.

Single sanidine phenocrysts were also incrementally heated us-ing a 60 W CO2 laser and analyzed with a Nu Instruments Noblesse five-collector mass spectrometer, which features 4 ion counting electron multipliers and one axial Faraday detector. The Noblesse offers three distinct advantages relative to the MAP 215-50 spec-trometer: (1) sensitivity is improved by more than an order of magnitude, (2) counting statistics for each analysis are also much better, thus each m/e measurement is more precise, and (3) im-proved resolution of hydrocarbon species on the high mass side of each Ar isotope (e.g., Rivera et al., 2011). The analytical pro-tocol followed a blank–standard–blank-sample routine where Ar isotope ratios for the samples were corrected for source mass bias and detector efficiency based on bracketing analyses of an in-house standard gas of known isotope composition. Data reduction was performed using a modified version of the ArArCalc software (http :/ /earthref .org /ArArCALC/). Further analytical details are pro-vided in the Supplementary Documents.

3.3. U–Pb analyses

Single zircon crystals or fragments were extracted from hand sample HRT-1 of Huckleberry Ridge tuff; the same sample from which sanidine for the 40Ar/39Ar dating was separated. U–Pb Iso-tope Dilution Thermal Ionization Mass Spectrometry (ID-TIMS) was conducted on 15 zircons that were chosen on the basis of exter-nal form and internal growth zonation revealed by cathodolumi-nescence imaging (Supplementary Documents, Fig. A.5). Prior to complete dissolution, zircons were chemically annealed and par-tially dissolved to preferentially remove radiation damaged areas within the crystal lattice (Mattinson, 2005). Solutions were spiked with the ET535 tracer solution (Condon et al., 2007), dissolved and analyzed at the National Environment Research Council Iso-tope Geosciences Laboratory (NIGL) on a Thermo-Electron Triton TIMS instrument. The accuracy of the 206Pb/238U determinations is controlled by the accuracy of the tracer calibration (Condon et al., 2007) and the uncertainty of the 238U decay constant (Jaffey et al., 1971). 206Pb/238U data for calculation of 206Pb/238U dates were corrected for initial 230Th disequilibrium and initial Pb contained in mineral inclusions visible within the zircons. A Th/U value of 4.76 has been determined for member B of the Huckleberry Tuff (Hildreth et al., 1991) and we use a value of 4.76 ±0.50 for correc-tion of the 206Pb/238U dates using the algorithm of McLean et al. (2011). 230Th disequilibrium correction using a Th/Umagma value of 4.76 results in a correction of ∼93 kyr that is close to the maxi-mum correction of 109.2 kyr. Initial Pb in excess of the assumed

total procedural blank was corrected using the isotopic composi-tion of feldspar from HRT member B (Doe et al., 1982). Analytical details, including the 230Th disequilibrium correction, are in the Supplementary Documents.

4. Results

New 40Ar/39Ar ages were obtained for eight of the lava flows in the three sections previously analyzed on Réunion Island by Baksi et al. (1993) and Baksi and Hoffman (2000) (Table 1; Fig. 2; Com-plete argon data in Table A.3). Lavas of the RSD, GC, and LM sec-tions record R–N–T, N–T–N, and N–T–R field behavior, respectively (Fig. 3). For most lavas, demagnetization behavior was typical for Pleistocene basalts, the process having removed the only signif-icant secondary component of magnetization, viscous in origin, often after peak alternative fields of only 10 mT (Fig. 4). The mag-netic carriers in these flows comprise a wide grain size distribution of titanomagnetite and magnetite. For a few flows only one pris-tine specimen was available for demagnetization. For these cases the determined primary remanence cannot be considered precise; however, in all such instances the resulting direction is seen to be similar to the more robust determinations associated with at least one of the two adjacent flows (Fig. 3). Hence, these few sin-gle specimen findings have no effect on the magnetostratigraphy of the sections.

Although the lavas from each section cannot be unequivocally correlated stratigraphically with one another, they all show mag-netic behavior through a slightly different window during the full R–N–R event. The eight 40Ar/39Ar ages, including two from re-versely magnetized lavas, three from normally magnetized lavas, and three from transitionally magnetized lavas, are indistinguish-able from one another at the 95% confidence level with a mean of 2.200 ± 0.007/0.010 Ma, in close agreement with the recali-brated 40Ar/39Ar ages of the same lavas dated in previous studies (Fig. 1; Baksi et al., 1993; Baksi and Hoffman, 2000). Moreover, these 40Ar/39Ar results are tightly calibrated to an astrochronologic age model such that systematic uncertainties are <3� (Kuiper et al., 2008; Singer et al., 2009; Schmitz, 2012), thus they super-sede less accurate K–Ar dates for the Réunion lavas (McDougall and Watkins, 1973; Quidelleur et al., 2010). We conclude that a single short period of normal polarity is recorded on Réunion Is-land.

New 40Ar/39Ar experiments were also done on four previously dated lavas from Cerro del Fraile with R–T–N–R magnetization (Fig. 5, Table 1). The evidence of two full polarity reversals in the section is consistent with a subchron (Fleck et al., 1972;Singer et al., 2004). The transitionally magnetized flow CF-02 marks the onset of this subchron at 2.155 ± 0.008/0.009 Ma, and the upper normal-to-reversed transition is bracketed between 2.133 ±0.009/0.011 Ma and 2.108 ±0.006/0.008 Ma based on the ages of the upper two flows (Fig. 1). These new ages supersede the results of Singer et al. (2004) because of lower blank levels and more accurate and precise measurements of 36Ar (Singer, 2014, provides a complete discussion).

40Ar/39Ar experiments on four normally-magnetized tholei-itic basalt flows from Gamarri, Ethiopia, give ages ranging from 2.118 ± 0.057 to 2.063 ± 0.044 Ma (Table 1). Although the un-certainties are large due to exceptionally low K2O concentrations, these ages are distinctly younger than those of the Réunion Island lavas (Fig. 1).

Fusions of single sanidine crystals from sample HRT-1 from member B of the Huckleberry Ridge Tuff give a weighted mean age of 2.087 ± 0.006 Ma (Table 2). Lanphere et al. (2002) sug-gested that there is a small amount of excess Ar present in HRT sanidine, which results in total fusion ages that are older than the eruption age. Rivera et al. (2014) suggest that sanidine and

B.S. Singer et al. / Earth and Planetary Science Letters 405 (2014) 25–38 29

Table 140Ar/39Ar furnace incremental heating results from Réunion Island, Cerro del Fraile, and Gamarri lavas.

Sample # Polarity K/Ca total N 39Ar (%)

MSWD 40Ar/36Ari ± 2σ (Ma) ± 2σ # ±2σ full

Isochron age Plateau age

Réunion Island lavasLM 41-1 R 0.07 7 of 9 91.1 0.52 303.3 ± 22.3 2.182 ± 0.070 2.204 ± 0.028

0.07 10 of 10 100.0 0.66 298.6 ± 7.3 2.204 ± 0.042 2.214 ± 0.034Weighted mean plateau and isochron ages: 2.198 ± 0.036 2.208 ± 0.022 ±0.023

GC 42-2 T 0.10 9 of 9 100.0 1.02 294.4 ± 4.6 2.196 ± 0.022 2.194 ± 0.0180.11 6 of 10 71.3 0.57 293.9 ± 10.0 2.189 ± 0.031 2.186 ± 0.024

Weighted mean plateau and isochron ages: 2.194 ± 0.018 2.191 ± 0.014 ±0.016

RSD 53-1 T 0.11 10 of 10 100.0 0.73 292.7 ± 8.0 2.213 ± 0.017 2.209 ± 0.014 ±0.016

RSD 34-1 T 0.09 8 of 10 86.3 1.11 295.2 ± 9.9 2.212 ± 0.072 2.210 ± 0.0280.10 10 of 12 91.6 0.98 291.5 ± 5.7 2.223 ± 0.046 2.197 ± 0.025

Weighted mean plateau and isochron ages: 2.220 ± 0.038 2.203 ± 0.018 ±0.019

RSD 38-2 N 0.07 9 of 9 100.0 0.65 294.9 ± 2.1 2.187 ± 0.054 2.178 ± 0.0450.04 7 of 10 79.7 0.37 298.6 ± 13.6 2.183 ± 0.102 2.203 ± 0.049

Weighted mean plateau and isochron ages: 2.186 ± 0.048 2.189 ± 0.033 ±0.034

RSD 42-6 N 0.03 8 of 9 93.6 0.25 289.8 ± 15.8 2.205 ± 0.073 2.187 ± 0.056 ±0.056

RSD 43-3A R 0.08 8 of 10 98.3 0.82 289.2 ± 18.8 2.207 ± 0.038 2.197 ± 0.027 ±0.028

RSD 47-3A R 0.10 10 of 11 96.0 0.17 297.1 ± 9.1 2.191 ± 0.027 2.194 ± 0.021 ±0.022

Cerro del Fraile lavasCF-04 R 0.31 7 of 9 95.9 0.70 294.7 ± 0.8 2.112 ± 0.007 2.108 ± 0.009

0.33 11 of 13 94.0 0.69 294.8 ± 1.0 2.111 ± 0.007 2.108 ± 0.009Weighted mean plateau and isochron ages: 2.111 ± 0.005 2.108 ± 0.006 ±0.009

CF-03 N 0.17 9 of 9 100.0 0.34 295.1 ± 1.5 2.134 ± 0.009 2.132 ± 0.0130.33 8 of 9 96.8 0.24 296.1 ± 3.7 2.132 ± 0.014 2.134 ± 0.015

Weighted mean plateau and isochron ages: 2.133 ± 0.008 2.133 ± 0.009 ±0.011

CF-02 T 0.18 8 of 8 100.0 0.54 295.5 ± 1.2 2.152 ± 0.009 2.152 ± 0.0120.34 9 of 9 100.0 0.44 294.2 ± 1.5 2.164 ± 0.010 2.159 ± 0.012

Weighted mean plateau and isochron ages: 2.157 ± 0.075 2.155 ± 0.008 ±0.010

CF-01 R 0.05 9 of 9 100.0 0.41 290.8 ± 4.7 2.185 ± 0.040 2.153 ± 0.0520.08 8 of 10 90.3 1.05 292.5 ± 4.1 2.160 ± 0.041 2.136 ± 0.045

Weighted mean plateau and isochron ages: 2.173 ± 0.038 2.144 ± 0.033 ±0.034

Gamarri lavasGA-11-16 (GB21)1

R 0.03 10 of 10 100.0 1.73 294.6 ± 1.5 2.057 ± 0.083 2.032 ± 0.0680.06 8 of 11 80.8 0.19 295.5 ± 7.5 2.028 ± 0.153 2.028 ± 0.052

Weighted mean plateau and isochron ages: 2.050 ± 0.072 2.029 ± 0.041 ±0.041

GA-11-13 (GB18)

N 0.01 9 of 9 100.0 1.86 295.0 ± 1.3 2.108 ± 0.055 2.097 ± 0.0460.10 11 of 11 100.0 1.26 294.1 ± 1.3 2.086 ± 0.050 2.054 ± 0.0460.06 11 of 11 100.0 1.13 294.8 ± 0.9 2.082 ± 0.041 2.064 ± 0.0360.06 11 of 11 100.0 1.32 295.2 ± 1.1 2.065 ± 0.060 2.055 ± 0.047

Weighted mean plateau and isochron ages: 2.085 ± 0.024 2.067 ± 0.021 ±0.022

GB15-3 (GB15)

N 0.01 9 of 9 100.0 0.66 295.3 ± 1.7 2.066 ± 0.135 2.052 ± 0.0680.01 10 of 10 100.0 0.40 295.0 ± 1.6 2.096 ± 0.103 2.071 ± 0.058

Weighted mean plateau and isochron ages: 2.085 ± 0.082 2.063 ± 0.044 ±0.044

GA-11-12 (GB12 = GA22)

N 0.03 7 of 10 77.9 0.45 293.0 ± 6.6 2.200 ± 0.231 2.117 ± 0.0710.06 6 of 11 74.5 1.59 288.7 ± 8.3 2.338 ± 0.275 2.119 ± 0.100

Weighted mean plateau and isochron ages: 2.257 ± 0.176 2.118 ± 0.057 ±0.057

GA-11-11 (GB11 = GA21)

N 0.04 6 of 10 74.3 0.69 293.9 ± 6.7 2.138 ± 0.168 2.099 ± 0.0540.06 8 of 10 92.4 0.63 294.6 ± 2.5 2.136 ± 0.097 2.106 ± 0.0560.05 11 of 11 100.0 1.43 293.4 ± 2.2 2.167 ± 0.087 2.102 ± 0.062

GA-11-10 (GA21)

N 0.03 6 of 9 68.7 1.03 297.9 ± 5.7 2.013 ± 0.237 2.112 ± 0.0650.05 8 of 9 91.4 0.51 295.1 ± 2.7 2.126 ± 0.159 2.106 ± 0.074

Weighted mean plateau and isochron ages: 2.141 ± 0.056 2.105 ± 0.027 ±0.028

(GA17) R 0.05 9 of 9 100.0 1.79 295.0 ± 2.3 2.227 ± 0.159 2.195 ± 0.0790.05 6 of 9 61.1 0.35 294.0 ± 2.8 2.294 ± 0.176 2.209 ± 0.082

Weighted mean plateau and isochron ages: 2.257 ± 0.118 2.202 ± 0.056 ±0.056

Ages calculated relative to 28.201 Ma Fish Canyon sanidine (Kuiper et al., 2008) using decay constants of Min et al. (2000). Preferred age for each sample is given in boldfont. Sample labels in parentheses for Gamarri lavas are from Kidane et al. (1999).

zircon antecrysts in HRT member B may record the protracted pre-eruptive crystallization of the large silicic magma body. To evaluate the role of excess Ar and antecrysts in the HRT, we conducted twenty nine single-crystal incremental heating experiments (Ta-

ble 2; Fig. 6). Six of the seven experiments using the MAP 215-50 mass spectrometer yield statistically acceptable plateaus that give a weighted mean age of 2.071 ± 0.006/0.008 Ma. Twenty-two sin-gle crystal heating experiments were performed using the Noblesse

30 B.S. Singer et al. / Earth and Planetary Science Letters 405 (2014) 25–38

Fig. 2. Results of 40Ar/39Ar incremental heating experiments on basalts from Réunion Island. Plateau and inverse isochron diagrams for eight lavas – six from RSD, one from GC, and one from LM. Samples with multiple incremental heating experiments shown on same axes are annotated with ages for individual experiments. Ages calibrated to 28.201 Ma Fish Canyon sanidine with 2σ analytical uncertainties.

five-collector mass spectrometer. Seventeen of these experiments produced plateau ages, but five were discordant with concave up-ward shaped spectra trending towards older ages in the higher temperature heating steps (Table 2; Fig. 6). Thirteen of the seven-teen plateau ages constitute the youngest mode with a weighted mean 40Ar/39Ar age of 2.0773 ± 0.0014/0.0034 Ma (Fig. 7). Four

concordant plateaus ranging in age from 2.084 ± 0.005/0.006 to 2.097 ±0.004/0.006 Ma were generated from what we interpret to be antecrysts, thereby confirming the observations of Rivera et al.(2014) and Gansecki et al. (1998), but at a scale that the latter study could not resolve owing to large single-crystal uncertain-ties.

B.S. Singer et al. / Earth and Planetary Science Letters 405 (2014) 25–38 31

Fig. 3. Paleomagnetic stratigraphy represented by Virtual Geomagnetic Pole (VGP) latitude for the RSD, GC, and LM lava flow sections. The LM section was originally sampled for paleomagnetic study by Hoffman in the early 1990’s and the evolution of VGPs is documented in Baksi and Hoffman (2000). Sample LM-41 that we have dated using modern 40Ar/39Ar methods is from the original section sampled by Hoffman. The VGPs in this figure are from lavas in a slightly different location than Baksi and Hoffman(2000) due to road construction, and are thus labeled LMN for “La Montagne (new)”. Both the location along the section as well as the transitional paleomagnetic directional findings place LM-41 sample as having come from either flow LMN12 or adjacent flow LMN13. GC42, a particularly massive lava adjacent to GC41 and GC43 that also was newly dated, records a range of transitional field remanence directions (from D = 271.8, I = −46.8 to D = 231.5, I = 7.0) that may have resulted from the combination of slow cooling and rapid field behavior.

206Pb/238U (230Th corrected) dates have been obtained from 15 individual zircons in sample HRT-1. The accuracy of these U–Pbzircon dates is controlled by the tracer calibration, U and Th decay constants, and corrections made to account for any initial disequi-librium in the U–Pb decay chain (details in Supplementary Docu-ments). The span of ∼200 kyr within this dataset (MSWD = 13.2) is in excess of the single data point uncertainties which range from 10.4 to 96.6 kyr (2σ ) (Fig. A.4). We interpret this age vari-ation to reflect both eruptive and pre-eruptive crystallization of zircon, and the youngest seven dates, which define a coherent pop-ulation (MSWD = 0.93) yield a weighted mean (230Th-corrected) 206Pb/238U age of 2.084 ± 0.012/0.013 Ma. Rivera et al. (2014)present a data set of U–Pb ID-TIMS data for zircon from a split of our HRT-1 sample and obtained similar range in 206Pb/238U (230Th corrected) dates and derive a weighted mean date of 2.0915 ±0.0034/0.0042 Ma (n = 13, MSWD = 0.91) on a subset of youngest dates. The difference in the U–Pb age uncertainties estimated by the two laboratories is primarily due to the uncertainty assigned to the procedural blanks. Both U–Pb ID-TIMS data sets were ob-tained using the EARTHTIME tracers, thus are not fully indepen-dent, however the ability to reproduce the same interpreted date serves to verify laboratory specific protocols, such as blank Pb cor-rection.

We note that the inferred eruption age based on the U–Pbzircon dates, which is independent of the 40Ar/39Ar system, is within uncertainty of the preferred 40Ar/39Ar age of 2.077 ±0.001/0.003 Ma (Fig. 7; Supplementary Documents; Fig. A.4; com-plete data in Table A.5) and therefore serves to verify the accuracy of the 40Ar/39Ar dates for HRT-1 produced in this study (see be-low). We take the more precise 40Ar/39Ar age of the HRT as the best estimate of time elapsed since its eruption and for compari-son to the other 40Ar/39Ar ages in Tables 1 and 2.

5. Untangling geomagnetic field behavior between 2.2 and 2.0 Ma

The duration of the field behavior recorded in the RSD/Réunion lavas is difficult to evaluate as lavas with normal, reversed and

transitional polarity are bounded by statistically indistinguishable ages. Nevertheless, the level of uncertainty indicates a maximum duration of ∼10 kyr. Ohno et al. (2012) observed a number of lows in relative paleointensity between 2.8 and 2.1 Ma in a sedimen-tary record from the North Atlantic. These lows were accompanied by directional changes that lasted between 1 and 4 kyr. How-ever, no substantial low was seen at the time the RSD lavas were erupted. If we assume significant changes in direction are related to times of low field intensity, then the absence of either intensity or directional changes in the Ohno et al. (2012) record suggests that the RSD lavas were erupted in less than a few kyr, an inter-val of time too short to be immune to smoothing effects caused by the recording process in low to mid-range sedimentation rate environments (e.g., Roberts and Winklhofer, 2004). If correct, the rapidity of dynamic field changes as recorded with such resolution by these Réunion Island lavas is remarkable.

In regards to geodynamo behavior it is unclear how the Réu-nion event should be classified. One may choose to refer to the recorded field behavior found in the Réunion Island lavas as a geomagnetic excursion for similar behavior observed in sediment cores (Laj and Channell, 2007; Singer, 2014). However, the demag-netized directional data from both the RSD and GC sections clearly show that normal (opposite) polarity was both attained and held, albeit for an undetermined short interval of time. Moreover, the RSD section contains some 35+ flow sites in succession that pos-sess not only normal polarity, but also behavior suggestive of typi-cal secular variation. If normal polarity had been held for a longer time, the Réunion might be considered a normal polarity subchron. Given the observed field behavior one could argue that the Réu-nion is a documented case of an aborted reversal. We suggest that a more robust set of definitions for dynamo field behaviors is needed. At present we choose to continue to refer to the field behavior as the “Réunion event”, consistent with its earlier desig-nation (e.g., Jacobs, 1994).

The new 40Ar/39Ar ages from this study indicate that the Réunion event is temporally distinct from normal polarity fields recorded at Cerro del Fraile, Argentina; Gamarri, Ethiopia; several

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transitional directions (GC42-2T, RSD53-1B), one is normal

Fig. 4. Demagnetization plots of four Réunion Island samples that were 40Ar/39Ar dated for this study including at least one from each of the three sections. Two have(RSD38-3T), and one is “transitional-reverse” (LMN12-5A).

B.S. Singer et al. / Earth and Planetary Science Letters 405 (2014) 25–38 33

Fig. 5. Results of 40Ar/39Ar incremental heating experiments on basalts from Cerro del Fraile. Plateau and inverse isochron diagrams for four lavas, in stratigraphic order. Ages calibrated to 28.201 Ma Fish Canyon sanidine with 2σ analytical uncertainties.

marine sediment sites; and from the transitional field recorded by the HRT. The new 40Ar/39Ar ages of lavas from Cerro del Fraile are significantly younger than lavas from Réunion Island, but they are in excellent agreement with the normal polarity subchron recorded at ODP site 981 from 2.153 to 2.115 Ma (Channell et al., 2003; Fig. 1). In particular, the transitionally magnetized flow from Cerro del Fraile is indistinguishable in age to the onset of the subchron

recorded in the sediments, and the upper age constraint from ODP site 981 falls within the age range of the upper normal-to-reversed transition recorded in the lavas. Given the uncertainty, it is dif-ficult to ascertain whether the period of normal field behavior recorded at Gamarri corresponds with the Huckleberry Ridge ex-cursion, or with the late stage of the subchron recorded in the ODP site 981 sediments (Fig. 1). Our preferred interpretation, based on

34 B.S. Singer et al. / Earth and Planetary Science Letters 405 (2014) 25–38

Table 2Summary of 40Ar/39Ar results from Huckleberry Ridge Tuff sanidine.

Sample # Polarity K/Ca total N 39Ar (%)

MSWD 40Ar/36Ari ± 2σ (Ma) ± 2σ # ±2σ full

Isochron age Plateau age

Laser – single crystal incremental heating; MAP 215-50 mass spectrometerHRT-1 B T 146 5 of 5 100.0 0.53 342 ± 85 2.064 ± 0.018 2.071 ± 0.012

41 5 of 5 100.0 0.27 307 ± 194 2.070 ± 0.019 2.071 ± 0.012153 3 of 6 76.8 0.16 342 ± 272 2.068 ± 0.025 2.072 ± 0.010

51 5 of 6 86.3 1.03 267 ± 407 2.073 ± 0.037 2.070 ± 0.02552 5 of 5 100.0 0.06 304 ± 67 2.057 ± 0.043 2.062 ± 0.02547 4 of 5 71.6 0.37 390 ± 427 2.015 ± 0.204 2.065 ± 0.039

Weighted mean plateau and isochron ages: 2.067 ± 0.011 2.071 ± 0.006 ±0.008

Laser – single crystal fusion; MAP 215-50 mass spectrometerHRT-1 B 21.9 23 of 24 0.54 305.0 ± 9.5 2.081 ± 0.013 2.087 ± 0.007 ±0.010

Laser – single crystal incremental heating; Noblesse 5-collector mass spectrometerHRT-1 B youngest 13

T 25.7 7 of 10 68.4 0.91 299 ± 8 2.073 ± 0.006 2.0725 ± 0.004425.8 6 of 8 68.7 0.75 409 ± 250 2.074 ± 0.005 2.0747 ± 0.004322.3 4 of 9 88.3 0.62 304 ± 9 2.073 ± 0.005 2.0748 ± 0.004123.3 7 of 8 97.2 1.05 296 ± 14 2.075 ± 0.004 2.0749 ± 0.004123.0 10 of 10 100.0 1.50 301 ± 7 2.078 ± 0.009 2.0765 ± 0.004016.5 7 of 10 68.3 1.30 297 ± 10 2.077 ± 0.004 2.0772 ± 0.004322.4 8 of 10 76.3 1.70 453 ± 190 2.071 ± 0.008 2.0779 ± 0.003916.2 8 of 10 98.7 1.70 307 ± 15 2.077 ± 0.005 2.0784 ± 0.003915.9 8 of 8 100.0 1.70 299 ± 7 2.079 ± 0.004 2.0791 ± 0.004019.5 9 of 9 100.0 1.60 289 ± 10 2.080 ± 0.004 2.0792 ± 0.003915.3 6 of 9 66.4 1.17 298 ± 51 2.079 ± 0.004 2.0794 ± 0.004116.2 7 of 10 93.1 1.30 278 ± 20 2.083 ± 0.006 2.0795 ± 0.004019.3 5 of 9 77.0 1.30 321 ± 24 2.078 ± 0.005 2.0796 ± 0.0042

Weighted mean plateau and isochron ages: 2.0771 ± 0.0019 2.0773 ± 0.0014 ±0.0034

HRT-1 B antecrysts

27.3 7 of 10 66.8 1.40 252 ± 52 2.093 ± 0.009 2.0841 ± 0.004119.6 7 of 10 93.6 1.30 301 ± 84 2.092 ± 0.004 2.0919 ± 0.003819.6 7 of 9 85.5 1.40 306 ± 22 2.094 ± 0.004 2.0940 ± 0.003917.1 5 of 9 86.3 1.01 349 ± 120 2.095 ± 0.010 2.0970 ± 0.0040

Ages calculated relative to 28.201 Ma Fish Canyon sanidine (Kuiper et al., 2008) using decay constants of Min et al. (2000). Preferred age for each sample is given in boldfont.

Fig. 6. Results from 40Ar/39Ar single sanidine incremental heating experiments of Huckleberry Ridge Tuff member B. (A) Age spectrum diagram representative of typical gas release pattern with ages and uncertainties obtained using MAP 215-50 mass spectrometer. (B–D) Age spectra showing various results obtained using Nu Instruments Noblesse multi-collector mass spectrometer including (B) concordant spectrum from a juvenile phenocryst, (C) a 2.091 Ma antecryst, and (D) discordant spectrum.

B.S. Singer et al. / Earth and Planetary Science Letters 405 (2014) 25–38 35

Fig. 7. Results from 40Ar/39Ar single crystal fusion and incremental heating experiments relative to 28.201 Ma Fish Canyon sanidine and U–Pb zircon dating of the Huckleberry Ridge Tuff member B. The 7 single sanidine incremental heating experiments performed using the MAP 215-50 mass spectrometer give plateau ages that have a weighted mean of 2.070 ± 0.005/0.008 Ma, where the uncertainties given by the horizontal filled bar and open bar correspond to the analytical and total uncertainties, respectively. The weighted mean of the 13 plateau ages obtained using the Noblesse five-collector mass spectrometer is 2.0773 ± 0.0014/0.0034 ka. Similarly, the youngest 7 zircons shown by filled vertical bars give a weighted mean age of 2.084 ± 0.011/0.013 Ma, where the uncertainties given by the horizontal filled bar and open bar correspond to the analytical and total uncertainties. The U–Pb dates are based on a Th/U ratio of the melt of 4.76 ± 0.50 (see text and Supplementary Documents for explanation). Gansecki et al. (1998), Lanphere et al. (2002), Ellis et al. (2012), and Rivera et al. (2014) data for member B are the same as in Fig. 1.

a weighted mean age of 2.082 ± 0.037 Ma, is that the Gamarri lavas record field behavior which post-dates the normal subchron at ODP site 981 (Fig. 1). The new 40Ar/39Ar age for the HRT, along with the U–Pb age, defines the Huckleberry Ridge excur-sion as unequivocally younger than the normal polarity behavior recorded by the lava flows at Réunion Island and Cerro del Fraile (Fig. 1). Based on all of these observations, we recognize that the following geomagnetic instabilities occurred between 2.20 and 2.07 Ma: (1) the Réunion event, (2) the two polarity reversals that bound the newly proposed Feni subchron, which is most com-pletely recorded at ODP site 981, and (3) the Huckleberry Ridge excursion (Fig. 1).

Given our findings, the normally magnetized sediment that is 40Ar/39Ar-dated at 2.11 ± 0.02 Ma in the Senèze maar sediments (Roger et al., 1999) is best correlated with the uppermost N–R transition at the end of the Feni subchron, and not the Réunion event as originally proposed (Fig. 1). The magnetostratigraphy of sediment in the Turkana Basin, East Africa, is also consistent with two periods of normal polarity between about 2.3 and 2.0 Ma, however the ages for Réunion I and Réunion II subchrons were interpolated assuming sedimentation occurred at a constant rate between dated tuffs (McDougall et al., 1992). Thus a firm correla-tion to the geomagnetic instability time scale (GITS) in Fig. 1 is not possible.

6. New vs. published ages for the Huckleberry Ridge Tuff

Our new 40Ar/39Ar age for the HRT member B is within er-ror of the 40Ar/39Ar data of Gansecki et al. (1998), Rivera et al.(2014), and Lanphere et al. (2002). However, the Lanphere et al.(2002) age is based on sanidine total fusions and is ∼17 kyr older than the weighted mean of our plateau ages obtained using the Noblesse five collector mass spectrometer. This is not surprising given the presence of antecrysts in the HRT member B, coupled with the fact that Lanphere et al. (2002) performed total fusions on either large single crystals, or aliquots of three to four crystals at a time.

The ∼50 kyr discrepancy between the older 40Ar/39Ar ages of Ellis et al. (2012) relative to these four independent sets of 40Ar/39Ar data, as well as the U–Pb zircon data (this study and Rivera et al., 2014) cannot be accounted for by the age of the neu-tron fluence monitor or decay constants used. The source of bias toward significantly older apparent ages is unknown, but may be related to the analytical blanks or the purity of the sanidine sep-arate measured by Ellis et al. (2012). The 40Ar and 36Ar blanks in the present study are 40–100 times lower, respectively, than those reported in Ellis et al. (2012). Moreover, the 40Ar/36Ar of our blanks during analysis of the HRT sanidine average ∼251 ±97 (2σ ) for the MAP 215-50 and 248 ± 51 (2σ ) for the Noblesse, which are within error of the atmospheric value, whereas the 40Ar/36Ar ra-tios of the blanks in Ellis et al. (2012) average ∼74, suggesting the presence of organic species at nominal mass 36 in their ana-lytical system. Additionally, the HRT member B sanidine analyzed by Ellis et al. (2012) has an average K/Ca of 9.8, which is low for sanidine and distinctly lower than the average K/Ca ratios for sani-dine fused in this study (K/Ca = 21.9), as well as by Rivera et al.(2014) (K/Ca = 22.4) and Gansecki et al. (1998) (K/Ca = 22.7). With respect to the accuracy of the U–Pb zircon data, the Th/U dis-equilibrium correction adopted here and by Rivera et al. (2014) is based upon empirical whole-rock data, however the maximum cor-rection results in a 206Pb/238U (230Th corrected) date of ∼2.09 Maand cannot be made to become coincident with the Ellis et al.(2012) interpreted 40Ar/39Ar date (Fig. A.4).

It is also worth noting the claim of Ellis et al. (2012), based on their 40Ar/39Ar dating of the HRT, that correlations among astronomically-tuned O-isotope climate proxy records may be inac-curate. This inference is hardly surprising because Ellis et al. (2012)chose to compare their results to an astronomical age model from the deep sea sediment core of Horng et al. (2002) that is infe-rior due to very low sedimentation rates that compromise mag-netic lock-in processes associated with short-lived events (Roberts and Winklhofer, 2004). Moreover, the interpretations of Ellis et al.(2012) involve comparisons to K–Ar ages from Réunion Island lavas

36 B.S. Singer et al. / Earth and Planetary Science Letters 405 (2014) 25–38

(Quidelleur et al., 2010) that are likely less accurate than our 40Ar/39Ar incremental heating ages.

The magnetostratigraphy of lacustrine beds of the Beaver basin in Utah (Honey et al., 1998), and two parallel sections in Death Valley (Holt and Kirshvink, 1995) further emphasize the temporal break, recorded by several meters of reversely magnetized sedi-ment, between an older normal polarity subchron and the Huckle-berry Ridge tuff and excursion. Although the geochronologic con-trol is poor, the sedimentary records from the Beaver basin and Death Valley are remarkably consistent with our interpretation of a short Réunion event, followed by a longer (Feni) normal sub-chron, then deposition several thousand years later of a singular, transitionally magnetized, HRT (Holt and Kirshvink, 1995).

7. Implications for the geodynamo and beyond

Singer et al. (2008a, 2008b) proposed that there are two peri-ods during the Brunhes chron in which the dynamo is relatively weak, each lasting about 200 kyr and encompassing at least five excursions. These two periods are separated by ∼300 kyr during which there is little compelling evidence for excursional behav-ior (Singer, 2014). Singer et al. (2008a, 2008b) hypothesized that the pattern of fluid flow within the outer core may be such that there is a 200–300 kyr oscillation between stable states of the main dipole and weaker states during which excursions or rever-sals can take place. The possibility that the dynamo was relatively weak and therefore unstable between 2.20 and 2.07 Ma may ex-plain the occurrence of a short-lived event, two successive polarity reversals bounding a subchron, and another short-lived excursion during this interval. If true, our results demonstrate that rapidly erupted lava flows are capable of capturing very brief geodynamo instabilities that may not be well-recorded in sediments (Roberts and Winklhofer, 2004), including those deposited at a high rate. A paleointensity record from a stack of six Pacific Ocean sediment cores (Yamazaki and Oda, 2005) shows a prolonged period of low paleointensity between about 2.2 and 2.1 Ma. Since the accuracy of the age model for this stack is only ∼40 ka (Yamazaki and Oda, 2005), this period of low intensity overlaps with our ages for the Réunion event and the Feni Drift subchron. Additional paleointen-sity studies for the period between 2.2 and 2.0 Ma, coupled with more accurate astrochronologic age models on deep sea sediment cores, may help clarify whether geomagnetic instabilities preferen-tially occur in bundles when the dipole component of the field is relatively weak.

There is paleomagnetic evidence consistent with the contention that the morphology of transitional fields is largely controlled by long-lived heterogeneities in the lower-most mantle (see Laj et al., 1991; Hoffman, 1992). Somewhat less controversial, there is also evidence that the global field present during reversals and events is dominated by the structure of the so-called non-axial dipole (NAD) field, the field remaining after removal of the axial dipole (AD) (Constable, 1992; Hoffman, 1992). Such findings have given way to the conjecture, the “SCOR-field hypothesis” of Hoffman and Singer (2008) that the source of the axial dipole field is quasi-independent from, and in poor communication with, the source of the remainder of the field, that is, that there exists a physical separation of sources responsible for the AD, generated at depth within the outer core, and the NAD, originating by complex fluid movements within the shallow core.

At present we can only speculate how it is that the dynamo process involves periods both of relative strength and stability, and of relative weakness and instability of the generated magnetic field. Nonetheless, we suggest that exploration of the SCOR-field hypothesis may offer insights into such workings of the geody-namo. The model implies that times of relative field instability need not involve conditions throughout the outer core fluid, but

rather involves only that portion of the core dynamo responsible for AD-field generation (i.e. the deeper residing fluid). If so, con-ditions along the inner core–outer core boundary will have a sub-stantial impact on AD-field generation. The process of the freezing of the inner core will produce upwellings of lighter material into the fluid outer core (Braginsky, 1963). This process may produce parcels, or “blobs” of material (see e.g., Moffatt and Loper, 1994) that may disrupt convective fluid motions at depth (McFadden and Merrill, 1986) and ultimately cause transitional fields by way of the weakening, if not the demise, of the axial dipole. The paleo-magnetic data presented here, along with those from the Brunhes Chron, are consistent with this model: Dislodged parcels of lighter material from the inner core surface may be the cause of long-term disruptions of fluid motions in the deep outer core, and by extension have a significant impact on the generated strength of the axial dipole field. Each disturbance may require on order of 105 yr for full recovery of previous fluid activity. If so, these paleo-magnetic findings will aid computer simulations of the geodynamo that attempt to include temporal aspects of the thermodynamic properties at the surface of the inner core.

The magnetostratigraphy of terrestrial and cave sediments pro-vides an important means of dating of hominin fossils, particularly across South Africa (Pickering et al., 2011). For example, Herries and Shaw (2011) used, in part, the identification of two closely spaced normal polarity zones recorded in speleothem and flow-stone deposits at Sterkfontain Cave, South Africa, and previously published ages for periods of normal polarity to estimate the ages of A. africanus fossil Sts 5 and Australopithecus fossil StW 573 at between 2.6 and 2.0 Ma. Fossil Sts 5, Mrs Ples, is the youngest A. africanus fossil yet described. The flowstone contemporaneous with the Sts 5 fossil records two brief periods of normal polar-ity. Herries and Shaw (2011) suggested these two normal polarity phases correspond to the Réunion event (as previously defined) and Huckleberry Ridge excursion and therefore Sts 5 dates be-tween 2.16 and 2.05 Ma. Our new dating indicates three normal polarity events between 2.2 Ma and 2.07 Ma. Therefore the flow-stone may alternatively record the Réunion event (as we have de-fined it) and the Feni Subchron. If this interpretation is correct, it suggests a date between 2.2 and 2.15 Ma for the Sts 5 fossil. Improved dating of the Sterkfontain Cave deposits, in light of the chronology we have determined, may help refine the hominin his-tory of South Africa.

8. Conclusions

Based on new 40Ar/39Ar age determinations from Réunion Is-land, Cerro Fraile in Argentina, and Gamarri in Ethiopia, plus equiv-alent U–Pb zircon and 40Ar/39Ar sanidine ages for the Huckleberry Ridge Tuff, we conclude that at least two field excursions—one designated as an event having recorded full normal polarity on Réunion Island—and two polarity reversals bounding a normal sub-chron, occurred between 2.20 and 2.07 Ma. Specifically, these in-clude: (1) the Réunion event at 2.200 Ma, (2) the much longer Feni Drift subchron with an onset at 2.153 Ma and termination at 2.118 Ma, and (3) the 2.077 Ma Huckleberry Ridge excursion. Importantly, only a single short-lived event is recorded in the lava flows at Réunion Island. Thus, we recommend that the terms “Réu-nion I” and “Réunion II” be abandoned. Not unlike certain time intervals within the Brunhes normal chron, this ∼125 kyr period within the Matuyama reversed chron may represent a bundling of instabilities signaling a weakened geodynamo as proposed by Singer et al. (2008a, 2008b). The mechanism that produces this weakening and magnetic field instability remains elusive but may reflect disruption of fluid flow in the deepest part of the outer core by buoyant liquid blobs released from the crystallizing zone atop the inner core (Moffatt and Loper, 1994). As the radioisotopic

B.S. Singer et al. / Earth and Planetary Science Letters 405 (2014) 25–38 37

dating of early Quaternary hominin sites in South Africa improves, the geomagnetic instability time scale (GITS) for microchrons and excursions presented here will help refine the timing of key fossil evidence for human evolution.

Acknowledgements

We are grateful for financial support from U.S. NSF grants EAR-1250446, EAR-0943584, and EAR-0943770. Condon was sup-ported by the European Community’s Seventh Framework Pro-gramme (FP7/2007–2013) grant agreement 215458 (GTSnext) and NIGFSC award IP/1011/1107. The Shell Corporation sponsored the Senior Honors thesis research of Macho at UW–Madison. Brown was partially supported by German DFG grant SPP 1488.

Appendix A. Supplementary material

Supplementary material related to this article can be found on-line at http://dx.doi.org/10.1016/j.epsl.2014.08.011.

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