Konishi and Okada Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-020-00352-0(2020) 7:35

RESEARCH ARTICLE

Progress in Earth andPlanetary Science

Open AccessA paleomagnetic record of the early
Matuyama chron including the Réunion
subchron and the onset Olduvai boundary:High-resolution magnetostratigraphy andinsights from transitional geomagnetic
fieldsTakumi Konishi1 and Makoto Okada2*

Abstract
We present a novel paleomagnetic record for the lower Matuyama chronozone, which includes the Réunion
subchronozone and the lower Olduvai polarity reversal, from a continuous section of a 168-m-thick on-land marine
succession in the southernmost part of the Boso Peninsula, central Japan. In this section, the Réunion subchronozone
and the lower Olduvai reversal are observed at 38.6–44.6 m and 142.0 m, respectively. The average sedimentation rates
between the lower and upper Réunion boundaries and between the upper Réunion boundary and lower Olduvai
boundary are calculated as 25 cm/ky and 57 cm/ky, respectively. The virtual geomagnetic pole (VGP), observed in the
Boso Peninsula, at both the upper and lower Réunion boundaries passed across the equator within a similar
longitudinal band over Africa. Immediately below the upper boundary, between 43.0 and 43.5 m, the VGP settled in acluster area around China. Relative paleointensity (RPI) values for the entire Réunion interval are generally lower than
the average for the entire interval from the Réunion to the lower Olduvai subchronozone. Conversely, the VGP for the
lower Olduvai reversal boundary did not pass across the equator within a narrow longitudinal band but settled inseveral cluster areas; i.e., the southern Indian Ocean, North America, and the southern South Pacific Ocean off SouthAmerica. The VGP then moved rapidly between the clusters. The locations of VGP cluster areas in the lower Olduvai
reversal seem to coincide with areas where a vertical component of the present geomagnetic non-axial dipole (NAD)field is dominant. During the reversal, the RPI declined rapidly and recovered slowly as the VGP moved rapidly
between cluster areas. Our new paleomagnetic data are one of the most detailed records for those geomagnetic
reversals from marine sediments, and will, therefore, help to understand the dynamics of the geomagnetic reversals.
Keywords: Paleomagnetism, Rock-magnetism, Geomagnetic reversals, Lower Olduvai reversal, Réunion subchron,
Relative paleointensity, Boso Peninsula* Correspondence: makoto.okada.sci@vc.ibaraki.ac.jp
2
Department of Earth Sciences, Ibaraki University, 2-2-1 Bunkyo, Mito, Ibaraki
310-8512, Japan
Full list of author information is available at the end of the article© The Author(s). 2020 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License,
which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give
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Konishi and Okada Progress in Earth and Planetary Science(2020) 7:35

Introduction
Geomagnetic reversals are one of the most significant fluctuations in the Earth's magnetic field, during which themagnetic intensity decreases by an order of magnitude as
the polarity transitions (e.g., Merrill and McFadden 1999;
Valet and Fournier 2016). Moreover, studies based on several high-resolution reversal records suggest that the behavior of the geomagnetic field during polarity transitions
could be related to the present non-axial dipole (NAD) field(e.g., Channell et al. 2003, 2004; Hoffman et al. 2008).
Therefore, determining the behavior of the magnetic field
during a geomagnetic reversal could be the key to revealing
the mechanisms governing geomagnetic field generationand outer core dynamics (e.g., Ingham and Turner 2008;
Amit et al. 2010). Hoffman (1992) attempted to determine
the characteristics of geomagnetic variations during reversals using paleomagnetic records from lava flow sequences
around the world. Although paleomagnetic records fromvolcanic rocks, such as lava flows, can provide stable magnetic signals associated with the absolute intensity of pastgeomagnetic fields, the resulting records are intermittent
because volcanic eruptions do not occur continuously (e.g.,
Jarboe et al. 2011; Valet et al. 2012).
Conversely, despite only recording relative and not absolute magnetic intensities, sedimentary paleomagnetic records reveal continuous geomagnetic field variations,
including geomagnetic reversals (e.g., Channell 2017; Valetand Fournier 2016). Previous sedimentary records reconstructing long-term variations in the geomagnetic dipole
moment have successfully revealed the general features ofgeomagnetic dipole fluctuations (e.g., Valet et al. 2005;
Yamazaki and Oda 2005; Channell et al. 2009). However,the details of paleointensity variations during polarity
transitions are often not recorded in standard deep-sea
cores because the sedimentation rates are generally relatively low, i.e., approximately a few centimeters per thousand years. Such a low sedimentation rate prevents theaccurate recording of rapid geomagnetic variations due tothe smoothing of the magnetic signal associated with thepost-depositional remanent magnetization (pDRM)
process (e.g., Roberts and Winklhofer 2004; Suganuma
et al. 2011; Valet et al. 2016). Therefore, for a more detailed description of geomagnetic field characteristics during polarity transitions, on-land marine successions with
sufficiently high sedimentation rates, such as reported by
Okada et al. (2017) and Simon et al. (2019), are crucial.
The Kazusa Group, a forearc basin-fill deposit formed
from the Calabrian to the early Chibanian age, is widelydistributed in the central part of the Boso Peninsula (e.g.,
Kazaoka et al. 2015) (Fig. 1b, c). Due to good exposure of
the geologic strata, the biostratigraphy, magnetostratigraphy, and oxygen isotope stratigraphy of the Kazusa Group
have been comprehensively studied (e.g., Niitsuma 1971;
Oda 1977; Niitsuma 1976; Okada and Niitsuma 1989). In

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recent years, many chronostratigraphic studies have been
conducted for the upper part of the Kazusa Group(Kazaoka et al. 2015; Suganuma et al. 2015; Nishida et al.
2016; Okada et al. 2017; Suganuma et al. 2018; Simon
et al. 2019; Haneda et al. 2020). As well as the Kazusa
Group, the middle Miocene-upper Pliocene Awa Group,
distributed in the middle part of the Boso Peninsula, hasbeen well documented using magnetostratigraphy (Niitsuma 1976), and magnetic and oxygen isotope stratigraphy (Haneda and Okada 2019) (Fig. 1b, c).
In contrast, the Chikura Group, a small basin-fill deposit on the trench slope in the southernmost part ofthe Boso Peninsula formed during the Piacenzian toearly Calabrian (Kotake 1988; Kotake et al. 1995; Kawakami and Shishikura 2006) (Fig. 1c), exhibits complex
geology and developed folds and faults (see Fig. 1 d). As
a result, the construction of a continuous chronostratigraphic record has been hindered for a long time. However, Okada et al. (2012) established the paleomagneticoxygen isotopic stratigraphy between 3.15 and 2.35 Ma
in the Chikura Group by selecting suitable routes for recovering a continuous stratigraphic record (see Fig. 1c,e)
In this study, we aim to provide a paleomagnetic record
during the early Matuyama chron, which includes the
Réunion subchron and the onset Olduvai boundary where
the previous study was not conducted, through the establishment of the detailed magnetostratigraphy of the upper
part of the Chikura Group. Oxygen isotopic analysis of fossil foraminiferal tests from this succession is ongoing; therefore, this will be reported elsewhere. Note that the Réunion
subchron used in this study is the same one that has been
redefined as “Feni” subchron by Channell et al. (2020).

Geologic setting
The Boso Peninsula is located on the Pacific side of central Japan, close to the trench-trench-trench type triple
junction (Fig. 1a). The southernmost part of the Boso
Peninsula contains sediments deposited in deep-sea
trench and trench-slope basins; this sedimentary sequence was named as the Chikura Group (Naruse et al.
1951; Kotake 1988) (Fig. 1b). The central part of the
Chikura Group consists of, in ascending order, the Shirahama, Shiramazu, Mera, Minamiasai, and Hata Formations, in which the sediments are predominantly
siltstone, sandy siltstone, and alternating sandstone andsiltstone layers, with frequently intercalated tephra beds
(Kotake 1988). Kotake et al. (1995) reconstructed thechronostratigraphy of the Chikura Group based on a detailed field survey and a paleomagnetic study. They also
revealed that the Matuyama-Gauss boundary is contained in the Minamiasai Formation (Fig. 1e). By conducting a detailed correlation of the tephra beds,
Kawakami and Shishikura (2006) redefined parts of theKonishi and Okada Progress in Earth and Planetary Science(2020) 7:35

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Fig. 1 Location map of this study sites. a Location of the Japanese archipelago. The tectonic setting is drawn after Tatsumi et al. (2016). b Distributionof the Awa Group, the Chikura Group, and the Kazusa Group, which are Miocene-Pleistocene marine successions on the Boso Peninsula based on
Mitsunashi and Suda (1980), Unozawa et al. (1983), and Kawakami and Shishikura (2006). c Chronological positions for the last 5 Ma of the Awa Group,
the Chikura, and the Kazusa Group associated with the magnetostratigraphy. d Geologic map of the southernmost part of the Boso Peninsula,modified from Kawakami and Shishikura (2006). e Stratigraphic correlation of the Chikura Group associated with the magnetostratigraphy and thecalcareous nannofossil datums. *1 Kotake et al. (1995), *2 Kotake (1988), *3 Kawakami and Shishikura (2006), *4 Ogg (2012)

Chikura Group, namely that the Minamiasai Formation
is a part of the Mera Formation and the Mera Formation
is partly inter-fingered with the Hata Formation (Fig.
1e). However, since most of the previous chronostratigraphic studies conducted on the Chikura Group (e.g.,
Kotake et al. 1995; Okada et al. 2012; Tamura et al.
2016; Kameo and Okada 2016) have followed the stratigraphic classification by Kotake (1988), we use the Hata

Formation defined by Kotake (1988)
consistency with the previous studies.

to

keep

Methods/Experimental
Collection of paleomagnetic core samples

We conducted a geologic survey and collected paleomagnetic
mini-core samples from the lower to the middle part of the
Hata Formation that is exposed along a route consisting of a

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Fig. 2 Location of the study area. a Location of the study area and its vicinity. The box indicates the KJ route area used in this study. b Locationsof sampling sites and lithologies along a branch creek of the Seto river. Low-resolution sampling sites are shown by black dots, and highresolution sites corresponding to the geomagnetic reversal boundaries are shown by black bars. Detailed descriptions for the horizons of thehigh-resolution sites are shown in S. Figure 1

small creek running to near Komatsu-Ji (Komatsu Temple),
Chikura town, Minamiboso City, in the southernmost part ofthe Boso Peninsula (Fig. 2a, b). We named this route as the“KJ route” (34.9668° N, 139.9114° E–34.9649° N, 139.9117°
E). This part of the Hata Formation contains the last appearance horizon of Discoaster brouweri, which is a calcareous nannoplankton indicating a horizon just below the
Olduvai subchronozone (Sato et al. 1988), and a normal
polarity zone above the datum horizon (Kotake et al.
1995) (Fig. 1e). The KJ route follows a small creek, which
is a branch of the Seto River that runs beside the Komatsu
Temple along the Komatsu forest road (Fig. 2 a). In thisstudy, we described a continuous section along the KJ
route measuring 168 m thick (Fig. 2 b and S.Fig. 1).
From the KJ route, we collected 25-mm-diameter drilledcore samples for rock-magnetic and paleomagnetic measurements using a portable engine drill. The cores were collected from 218 horizons, typically at intervals of 1–4 m;
however, intervals of less than 10 cm were employedaround the magnetozone boundaries assigned with thelower Olduvai and upper and lower Réunion boundaries.

At each horizon, we collected more than three pieces ofcore. The drilled core samples were oriented with a magnetic compass before being extracted from the holes. The
collected core samples were cut into 2-cm-long specimens
for subsequent measurements.
Paleomagnetic measurements

The natural remanent magnetization (NRM) of each specimen was measured using a two-axis cryogenic magnetometer (SRM-750R; 2G Enterprises, USA) in a magnetic shieldroom at Ibaraki University. To extract the primary magnetic component, which was acquired when the sample
rock was formed, we removed the secondary remanencefrom the NRM by conducting three types of treatment:
progressive alternating field demagnetization (pAFD), progressive thermal demagnetization (pThD), and a hybrid
method that is a combination of thermal demagnetization
at 250 °C and subsequent pAFD. To minimize the thermal
alternation of specimens, the minimum temperature towhich the secondary components are effectively removed
was selected as the appropriate temperature. The hybrid

Konishi and Okada Progress in Earth and Planetary Science(2020) 7:35

method was used in Okada et al. (2017), in which 300 °C
was chosen as the appropriate temperature, whereas 250 °C
was used in this study. Alternating field and thermal
demagnetization treatments were conducted at Ibaraki University using an alternating field demagnetizer (DEM-8601;
Natsuhara-Giken, Japan) and a thermal demagnetizer (TD48; ASC Scientific, USA), respectively. The pAFD treatment
was performed up to 80 mT every 5 mT for 45 specimensfrom 45 horizons and the pThD from 150–600 °C every 50°C for 77 specimens from 77 horizons. The hybrid treatment, performed for 216 specimens from 154 horizons, wasapplied to remove the secondary remanence, which holds asimilar or higher coercive force, but a lower unblocking
temperature than the primary remanence.
We extracted the characteristic remanent magnetization(ChRM) using more than five remanent vector endpoints,
which were aligned linearly toward the origin, by principal
component analysis (Kirschvink 1980). In the pAFD and
pThD treatments, the data points used to deduce ChRMs
were selected from appropriate demagnetization intervals
at each specimen. In contrast, for the hybrid treatment,
the data points were from the demagnetization interval
between 20 and 50 mT for all the specimens. The virtual
geomagnetic pole (VGP) at each sampling site was calculated from a site-mean direction obtained from multiple
ChRM directions when multiple ChRMs at a site wereavailable. However, if the only single ChRM was available
at a site, the VGP was calculated from a single ChRM. In
any case, the VGPs were deduced from the ChRMs whose
maximum angular deviations (MADs) were equal to orless than 15°. We identified the magnetic polarities basedon VGP latitudes, where latitudes of greater than + 45° are
defined as normal polarity, latitudes of less than − 45° are
defined as reversed polarity, and latitudes of less than theabsolute value of 45° are defined as intermediate polarity.
Proxies of magnetic grain concentration; i.e., kLF, anhysteretic remanent magnetization (ARM), and isothermal
remanent magnetization (IRM), are typically employed tonormalize NRM when calculating the relative paleointensity (RPI) index (e.g., Valet 2003; Suganuma et al. 2008).
Following the methodology of Okada et al. (2017), we calculated the NRM30–50/ARM30–50 ratio as the RPI
index; i.e., NRM30–50 and ARM30–50 are coercivity fractions between 30 and 50 mT for the NRM and ARM afterthermal demagnetization at the appropriate temperature(250 °C) used in the hybrid demagnetization method.
Rock-magnetic measurements

To evaluate the magnetic properties (magnetic grain concentration, magnetic grain size, domain state of magnetic
grains, magnetic carrier minerals) of samples from the KJ
route, we performed the following rock-magnetic experiments. Before any other paleomagnetic and rock-magnetic
measurements were performed, we measured the low-

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field magnetic susceptibility (kLF) of each specimen usinga Kappabridge susceptibility meter (KLY-3S; AGICO,
Czech Republic) at Ibaraki University. The anhysteretic
remanent magnetization (ARM) was applied for specimens in a 0.03-mT DC field with an 80-mT alternating
field using the DEM 8601 demagnetizer at Ibaraki University. The ARM susceptibility (kARM) was calculated fromthe initial intensity of ARM. These magnetic susceptibilities represent magnetic grain concentration. The kARM/
kLF ratio could be used as a proxy for the magnetic grain
size only under certain conditions, including a constantconcentration of magnetic grains (e.g., Yamazaki and Ioka
1997). Although, we exhibit the ratio as a reference for themagnetic grain size due to that the variations of both concentration proxies (kARM and kLF) are well within an orderof magnitude in this study (Fig. 5a).
Magnetic hysteresis was measured with a maximum
magnetic field of 1.0 T for selected specimens using analternating gradient magnetometer (PMC MicroMag
2900 AGM; Lake Shore Cryogenics Inc., USA) at theCenter for Advanced Marine Core Research, Kochi University (KCC). By using both the ratios of saturation
remanence to saturation magnetization (Mrs/Ms) andcoercivity of remanence to coercivity (Hcr/Hc) deduced
from a hysteresis curve, a Day diagram (Day et al. 1977)
was generated to illustrate the broad distribution of themagnetic domain states of ferrimagnetic particles.
Thermomagnetic experiments were performed using athermomagnetic balance (NMB-89; Natsuhara Giken,
Japan) at the KCC. The specimens were heated to 700 °C
then cooled to room temperature in air or a vacuum in an
applied field of 0.3 T. The J-T curves obtained by these experiments allowed us to infer the magnetic minerals
present in the specimens based on their Curie/Néel temperatures, blocking temperatures, and temperatures of thermal alteration. Low-temperature magnetic experiments
were performed using a low-temperature SQUID magnetometer (MPMS XL-5; Quantum Design, USA) at the KCC.
The specimens were magnetized in a 1-T field and warmed
to 300 K after cooling to 10 K in a zero-T field. These experiments reveal the magnetic phase transition temperatures of the magnetic minerals included in a specimen
without alteration due to heating and/or oxidation.

Results and discussion
Rock-magnetic characteristics

The results of the thermomagnetic and low-temperaturemagnetic analyses of the two selected specimens are shownin Fig. 3. The thermomagnetic experiments, performed inboth air and a vacuum, demonstrate that the specimens
have a single Curie/Néel temperature at approximately 580°C (Fig. 3a). The thermomagnetic curve for specimen KJ100
shows no abrupt changes throughout the heating and cooling processes, but specimen Rhr44 exhibits a small plateau

Konishi and Okada Progress in Earth and Planetary Science(2020) 7:35

at approximately 450 °C during the heating process. The
normalized induced magnetization (J/J0) during cooling in
air is slightly smaller than that during heating for both
specimens. In contrast, J/J0 during cooling in a vacuum isslightly larger for KJ100 but substantially larger than thatduring heating for Rhr44 (Fig. 3a). The results of KJ100 indicate that the specimen contains (titano) magnetite with aCurie temperature close to that of magnetite at 585 °C (e.g.,
Hunt et al. 1995). The cooling curves are irreversible because of the degradation of iron oxyhydroxides within thespecimens into ferrimagnetic minerals due to the heatingor minor thermal alteration of titanomagnetite (Bowles
et al. 2013).
Conversely, the results of Rhr44, in which the specimen
was heated in air, exhibit a small plateau at approximately
450 °C, indicating the creation of a ferromagnetic mineralphase. The thermomagnetic curve of the cooling process
in air has a lower magnetization than the equivalent heating curve, indicating that the ferromagnetic or ferrimagnetic minerals created at approximately 450 °C were
further altered by continued heating and oxidized to forma higher Curie/Néel temperature mineral, such ashematite. In contrast, the specimen heated in a vacuum
has a single Curie/Néel temperature at 580 °C, the sameas that heated in air, but almost no increase inmagnetization at approximately 450 °C. Moreover, unlike
during the heating process, the curve exhibits a drastic increase in magnetization throughout the cooling process.
The thermomagnetic behavior seen in Rhr44 was also observed in the Kazusa Group (Okada et al. 2017) and has
been reported as one of the typical behaviors of greigitebearing samples (Roberts et al. 2011).
The low-temperature remanence curves (Fig. 3b) indicate the presence of a Verwey transition, where the magnetite transforms from a monoclinic to cubic spinel
structure between 110 and 120 K (Verwey 1939; Özdemir et al. 1993). The curves, which indicate a declining
ratio of magnetization per unit temperature, are characterized by a rapid remanence decline from 10 to 50 K,
with a broad Verwey transition from 100 to 120 K.
Özdemir et al. (1993) reported that the Verwey transition tends to shift to lower temperatures with a broadertransition temperature range due to maghemitization(oxidation) on the surface of magnetite. The rapid decrease in remanence between 10 and 50 K is considered
to be due to the influence of superparamagnetic particles(Özdemir et al. 1993; Özdemir and Dunlop 2010). The
Day diagram of selected specimens is shown in Fig. 4.
The boundaries of the magnetic domain state regions
for the Day diagram follow the work of Dunlop (2002).
All specimens fall into the vortex state (PSD) region(Roberts et al. 2017), which can also be interpreted as a
mixture of particles in the SD and MD states, within arelatively small area where the data are distributed.

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Although, as Roberts et al. (2018) recently discussed, theDay diagram is fundamentally ambiguous for domainstate diagnosis; therefore, our diagnosis described above
should be treated as just a reference for the domainstates.
To evaluate the variation in rock-magnetic properties,
the low-field magnetic susceptibility (kLF) and the ARM
susceptibility (kARM), as well as the ratio of both parameters throughout the stratigraphic sequence, are shown in
Fig. 5a, b. As the ARM experiment was only conducted
for the upper part above 36 m, i.e., where the hybrid
method was applied, the kARM and the kARM/kLF ratio
are not plotted below 36 m. Although several spikes areobserved in the kARM/kLF ratio, which likely corresponds
to tephra and/or sandy layers, these data indicate thatthe rock-magnetic characteristics are relatively homogeneous for this interval (Fig. 5b). The kLF and kARM
curves, which exhibit variations well within an order ofmagnitude, suggest that the specimens from the KJ routeare suitable for estimating RPIs (Tauxe 1993).
Remanent magnetization

Typical orthogonal vector diagrams (Zijderveld 1967) for the
NRMs obtained from pAFD, pThD, and the hybrid method
are shown in Fig. 6. The results of the pAFD (Fig. 6a, d) indicate that the NRMs consist of two magnetic components.
The low-coercivity (LC) components were removed at a peakfield of 15–20 mT and the remaining high-coercivity (HC)remanences demagnetized almost linearly, but slightly curvilinearly toward the origin (grey arrows in Fig. 6a, d). The
demagnetization paths for the pThD (Fig. 6b, e) also indicate
the existence of two components, among which the lowtemperature (LT) components are demagnetized by 250 °C.
The remaining high-temperature (HT) component of KJ85
(Fig. 6b) is almost linearly demagnetized toward the origin
but slightly digresses at 600 °C, whereas that of KJ82 (Fig. 6f)
demagnetized with a curvilinear path (green arrows in Fig.
6b, e), indicating that the HT component in KJ82 consists ofa mixture of two different components. One of those components in HT had the same direction with LT but might have
had blocking temperatures overlapped with the other component in HT. Moreover, the demagnetization path digresses
substantially from 450 °C (red arrow in Fig. 6e), probably affected by magnetic minerals newly created during the ThD
process. Although the secondary components seem to be removed more from the pThD paths than from the pAFD
paths, the HT component in pThD may not be a primary
component (Fig. 6e). In contrast, the hybrid results (Fig. 6c, f)
for both specimens indicate that the secondary components
are effectively removed by 15–20 mT of AFD after 250 °C of
ThD with no curvilinear demagnetization paths (orange
arrow in Fig. 6f). Here, we assume that both the LC component and HC component in pAFD consist of a low unblocking temperature component and a high unblocking

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Fig. 3 Typical results of thermomagnetic and low-temperature magnetic experiments. a Results of thermomagnetic experiments. Left (right)
diagrams indicate results from specimen KJ100 (Rhr44). Upper (lower) diagrams indicate results in air (a vacuum). b Results of low-temperatureexperiments. The left (right) diagram indicates results from specimen KJ100 (Rhr44)

Konishi and Okada Progress in Earth and Planetary Science(2020) 7:35

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Fig. 4 Rock-magnetic results. Day diagram of hysteresis parameters from selected specimens. SD single domain, PSD pseudo-single domain, MD
multi-domain. Domain state boundaries are drawn after Dunlop (2002)

temperature component, respectively. We can express the
low and the high unblocking temperature component in the
LC as LTLC and HTLC, and those in the HC as LTHC and
HTHC, respectively. As above, we assume that both the LT
and HT components in pThD consist of a low-coercivity anda high-coercivity component, respectively. Here, too, we can
express the low- and the high-coercivity component in the
LT as LTLC and LTHC, and those in the HT as HTLC and
HTHC, respectively. In the hybrid results, the component
below 250 °C corresponds to the LT in pThD, and theremaining two components from 250 °C to 15 mT (bluearrow in Fig. 6f) and higher than 15 mT (orange arrow in
Fig. 6f) correspond to HTLC and HTHC, respectively. The
comparison between the hybrid and the pThD results(Fig. 6f, e) clearly shows that the HT component (green
arrow in Fig. 6e) consists of the sum of HTLC and HTHC
(dashed blue and orange arrows in Fig. 6e), indicatingthat the HT component includes the HTLC component as a secondary magnetization. The HTLC component is not able to be separated by the pThD method
alone since the spectrum of unblocking temperatureof HTLC is comparable with that of the HTHC component, which probably corresponds to the primarymagnetization.
In the reversals test, if the ChRMs indicating reversed
polarity are distributed symmetrically with those indicating normal polarity, the ChRMs can be treated as a primary paleomagnetic record without any secondary
component. For the reversals test, we used all the ChRMs,
excluding the intervals of the reversal boundaries and thelower normal polarity zone (40–47 m), where the RPIs indicate much lower than the average value. In Fig. 7a, the
α95 circle of reversed polarity ChRMs does not overlap
with the α95 of normal polarity, indicating that the secondary components are not removed from the pAFD results. The same results are observed for pThD in Fig. 7b.
This situation, whereby both pAFD and pThD failed to remove the secondary components, was also observed in thelower Chikura Group (Okada et al. 2012). In contrast, thehybrid result (Fig. 7c) shows that the α95 of reversed polarity includes the average of the normal polarity, and viceversa, which indicates that the ChRMs deduced by the hybrid method can be treated as the primary paleomagnetic
record with a level of significance of 5% or less. This result
indicates, for the first time, that the hybrid method is statistically confirmed to be able to successfully remove thesecondary components since Okada et al. (2017), which
did not conduct any field tests, including the reversals test.
This result also indicates that the interpretation that the
HTHC corresponds to the primary magnetization isproven.
According to the results of the hybrid method (Fig. 7c),
we calculated the average declination value and the associated α95 interval of the hybrid ChRMs as 8.6 ± 3.3°, indicating that a clockwise tectonic rotation of ~ 10° occurredin the KJ route region, which is similar to the results of aprevious study (Kotake et al. 1995). We subtracted thisvalue from the declinations to remove the rotation effectfor subsequently determining the VGPs. All the ChRM directions, MADs, VGP latitudes, and RPIs, which wereKonishi and Okada Progress in Earth and Planetary Science(2020) 7:35

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Fig. 5 Stratigraphic profiles for paleomagnetic directions and rock-magnetic properties. Note that the data plotted below 36-m thickness are
obtained by the pThD method, not the hybrid method. a kARM and kLF values. b kARM / kLF values. c MADs associated with calculations used todeduce the ChRMs. The green dashed line indicates 15°, i.e., the maximum limit for using the deduced ChRMs. d Inclination values. The dashedlines indicate the geocentric axial dipole inclinations for normal and reversed polarities. e Declinations with the corrected tectonic rotation of 8.6°
are shown in circles. f VGP latitude. g RPI values. The green dashed line indicates the average RPI value for the entire interval of this study. For (d,e, and f), the solid square shows a site-mean value, or a single value if the only ChRM is available at a site, whereas the open square shows asingle value used to deduce a site-mean value

used following the discussion of geomagnetic field variations, were deduced by the hybrid method, which was
conducted for all horizons above 36 m (Fig. 5c–g). However, we also plotted the ChRM directions, MADs, and
VGP latitudes deduced by the pThD below 36 m as reference data (Fig. 5c–f). The ChRM inclinations are in close
agreement with the expected values for the geocentric
axial dipole (dashed lines in Fig. 5d). The MADs generally
remain below 5°, while they exhibit much higher values insome intervals, mainly associated with geomagnetic reversals. We avoided using ChRMs with MADs greater than
15° (Fig. 5c) in the subsequent discussion.

Magnetostratigraphy

The VGP latitudes plotted in Fig. 5f indicate that two
normal polarity zones are identified in the stratigraphic
intervals above 142 m and between 38.6 and 44.6 m, respectively. An intermediate polarity zone identified from12 to 15 m will be discussed in a future study after applying the hybrid method. Kotake et al. (1995) reported
that the Olduvai subchronozone was identified in the
middle part of the Hata Formation, and a marker tephra
bed named the KO tephra was intercalated in the Olduvai interval (Fig. 1e). The KO tephra is described as HF
tephra in Kawakami and Shishikura (2006). They

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Fig. 6 Representative orthogonal vector diagrams. a–c Results from horizon KJ85. d-f Results from horizon KJ82. a, d Show the pAFD results. b, e
Show the pThD results. c, f Show the hybrid results (pAFD after ThD at 250 °C). Closed and open circles indicate horizontal and verticalcomponents, respectively. The grey arrows indicate the high-coercivity (HC) components in pAFD, whereas the green arrows indicate the highunblocking temperature (HT) components in pThD. The blue and orange arrows in hybrid indicate the low-coercivity components in HT (HTLC)
and the high-coercivity components in HT (HTHC), respectively

reported that the HF tephra was identified on a horizon
slightly above the interval of this study. On the other
hand, Okada et al. (2012) reported that the Gauss/
Matuyama boundary was identified in the Minamiasai
Formation, which is just below the Hata Formation. Theseobservations confirm that the upper and lower normal polarity zones identified in this study correspond to the Olduvai and Réunion subchronozones, respectively. The
average sedimentation rates between the lower Réunion
boundary at 39.3 m and upper Réunion boundary at 44.6m, and between the lower Olduvai boundary at 142.0 m
and the upper Réunion boundary, are calculated as 25

cm/ky and 57 cm/ky respectively, using boundary ages of2137 ka and 2116 ka for the lower and upper Réunion, respectively (Channell et al. 2016), and 1945 ka for the lower
Olduvai (Ogg 2012). Since these sedimentation rates arehigher than those in any deep-sea core (e.g., Channell
et al. 2002, 2016; Valet et al. 2014), this sedimentary sequence has the potential to provide a paleomagnetic record that will play an important role in finding unknown
field behaviors during the geomagnetic reversals.
In addition, we found a very characteristic tephra bed, inwhich cummingtonite is exclusively predominant among
the heavy minerals, at the 43.55 m horizon that is 1 m

Konishi and Okada Progress in Earth and Planetary Science(2020) 7:35

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Fig. 7 Reversals test results. The equal-area stereo-net diagrams (a–c) show the ChRMs from the pAFD, pThD, and hybrid treatments, respectively.
Closed and open circles indicate projections on the lower and upper hemispheres, respectively. The blue closed and open squares indicate theaverage directions of ChRMs on the lower and upper hemisphere, respectively. The blue circle indicates the 95% confidence limits (α95) of each
average direction. The red closed square and the surrounding circle indicate the symmetric average direction and its α95 on the lower
hemisphere, which are transposed from those on the upper hemisphere in each diagram. The green dashed line on the right diagram indicatesthe average declination of all the ChRMs, excluding the intervals of the reversal boundaries and the lower normal polarity zone (40–47 m), where
the RPIs indicate much lower than the average value

below the upper Réunion boundary (see S. Figure 1). This
tephra bed could be useful as a local marker tephra bed toindicate a horizon of the upper Réunion boundary.
Variations of VGP and RPI during the reversals

The VGP paths, latitudes, and RPIs between 36 and 47
m, which cover the entire Réunion subchronozone, areshown in Fig. 8a–c, respectively. In the same manner,
those for the Réunion subchron from ODP Site 981(Channell et al. 2003) are shown in Fig. 8d–f as a corresponding record. The VGPs of this study passed acrossthe equator within a similar longitudinal band over Africa during both the upper and lower Réunion reversals,
which is also observed in the Site 981 record, although
the longitude is different (Fig. 8d). Moreover, the VGPsof this study settled in a VGP cluster area around China,termed “A” in this study (Fig. 8a) and is identified between 43.0 and 43.5 m immediately below the upperboundary (Fig. 8b). A similar cluster area with the sametiming, but on a different longitudinal band from thisstudy, is also observed in the Site 981 record (marked“A” in Fig. 8d, e). These observations suggest that these
two records might reflect the nondipole feature of thegeodynamo during the Réunion subchron. The VGPs in
this study settled in a VGP cluster area around
Argentina during the upper boundary reversal, termed“B” in this study (Fig. 8a, b). However, this is notapparent in Site 981 (Fig. 8d). In Fig. 8c, although the
RPI values indicate minima at both the upper and lower
Réunion boundaries, the RPI values for the entire
Réunion interval are generally lower than the average ofthe entire interval of this study (Fig. 5g). This observation suggests that the dipole moment during the
Réunion subchron never recovered to the average level
after the first reversal ended, which was also observed in
the RPI record from deep-sea sediments of the North
Atlantic Ocean (Channell et al. 2016; Channell et al.
2020), and authigenic 10Be/9Be record from a deep-sea
core of the western Pacific Ocean (Simon et al. 2018).
In the same manner in Fig. 8, we show our record
from the stratigraphic interval between 138 and 145 m
covering the lower Olduvai reversal in Fig. 9a–c, and theSite 984 record (Channell et al. 2002, 2013a,
2013b) in Fig. 9d–f. In the lower Olduvai polarity transition, the VGP did not move across the equator within anarrow longitudinal band, unlike in the Réunion subchron, but instead settled in several VGP cluster areas.
The VGP can be observed in three areas (Fig. 9a): (A)the southern Indian Ocean, (B) North America, and (C)the southern South Pacific Ocean off South America.
The VGP moved rapidly between these clusters. The
RPI started to decline at 139.4 m immediately before theonset of the paleomagnetic directional change, where
the VGP settled in cluster A in the southern hemisphereKonishi and Okada Progress in Earth and Planetary Science(2020) 7:35

Page 12 of 16

Fig. 8 VGP paths and horizontal plots of the VGP latitude and RPI covering the Réunion subchron that correlates with a North Atlantic record. a–c Show the results of this study. d–f Show the results from ODP Site 981 (Channell et al. 2003). The letter “A” and “B” in (a) and (b), and “A” in (d)and (e) indicate parts of the characteristic VGP clusters identified on (a) and (d), respectively. The blue shaded areas in (a) and (d), indicate thatthe vertical component of the NAD field at the Earth’s surface is less than − 15 μT (Constable 2007). The yellow stars indicate the sample/coresites. The green dashed line in (c) indicates the average RPI value for the entire interval of this study(Fig. 9b, c). Immediately following this, the RPI declined
rapidly, reaching a minimum at 140.1 m, which corresponds to the VGP settling in cluster B in the northernhemisphere. Subsequently, the RPI gradually recovered
to a peak at 144.2 m. During this recovery, the VGP settled in cluster C in the southern hemisphere at approximately 141 m, before moving back again to cluster B in
the northern hemisphere.
Similar features to these, particularly for the asymmetric RPI profile consisting of a rapid decline and slow recovery and the locations of the VGP cluster areas, arewell documented in the Site 984 record (Channell et al.
2002, 2013a, 2013b) shown in Fig. 9d–f. The cluster
areas A, B, and C are observed in almost the same locations in both records. Similar VGP cluster areas havealso been identified in lava records (e.g., Hoffman 1991,
1992) as well as other sedimentary records (e.g., Channell et al. 2002; Kusu et al. 2016). These cluster positions
have been discussed to be related to the vertical flux
patches in the present non-axial dipole (NAD) field (e.g.,

Channell et al. 2003, 2004; Hoffman et al. 2008). Kusu
et al. (2016) reported that the VGP cluster areas in theupper Olduvai boundary were observed in similar locations to downward flux patches of the NAD, which
occur in North America and the southern South Pacific,
where the vertical component is less than − 15 μT according to the average geomagnetic field for the past
400 years (Constable 2007). The locations of these flux
patches of the NAD correlate relatively well with ourrecord, especially for cluster B in the Réunion subchron
and clusters A and B in the lower Olduvai reversal,
which suggests that the VGPs during geomagnetic reversals associated with low RPIs were strongly affected by
the NAD field.

Conclusions
This study presented a reliable paleomagnetic recordthat passed the reversals test for the lower Matuyama
chronozone, which includes the Réunion subchronozone and the lower Olduvai polarity reversal from aKonishi and Okada Progress in Earth and Planetary Science(2020) 7:35

Page 13 of 16

Fig. 9 VGP paths and horizontal plots of the VGP latitude and RPI covering the Olduvai bottom reversal of this study correlated with a North Atlantic
record. a–c Show the results of this study. d–f Show the results from ODP Site 984 (Channell et al. 2002, 2013a, 2013b). Letters “A” to “C” in (a) and (b) and(d) and (e) indicate the locations of the characteristic VGP clusters identified on (a) and (d), respectively. The blue shaded areas in (a) and (d) indicate thatthe vertical component of the NAD field at the Earth’s surface is less than − 15 μT (Constable 2007). The yellow stars indicate the sample/core sites. The
green dashed line in (c) indicates the average RPI value for the entire interval of this study. The red square shows the site mean value at a site in (a) and(b) if multiple ChRMs are available at a site. The grey circle in (a) and (b) shows a single value used to deduce a site-mean value, while the red circle in (a)and (b) shows a representative value at a site if a single ChRM is only available at a site. The lines are connected among the red squares or the red circles

continuous section along the KJ route, with a stratigraphic thickness of 168 m. In this section, theRéunion subchronozone and lower Olduvai reversal
were identified at 38.6–44.6 m and 142 m, respectively. The average sedimentation rates between thelower and upper Réunion boundaries and between theupper Réunion boundary and the lower Olduvai boundary
were 25 cm/ky and 57 cm/ky, respectively, whose sedimentation rates are higher than those recorded in any
deep-sea core. The VGP at both the upper and lower
Réunion boundaries passed across the equator within asimilar longitudinal band over Africa. Immediately below
the upper boundary, between 43.0 and 43.5 m, the VGP
settled in a cluster area around China. Additionally, the
RPI values for the entire Réunion interval were generally
lower than the average of the entire interval of this study.

Conversely, the VGP for the lower Olduvai reversal
boundary did not pass across the equator within a narrow
longitudinal band and instead settled in several cluster
areas, i.e., the southern Indian Ocean, North America, andthe southern South Pacific Ocean off South America. The
VGP then moved rapidly between these clusters. The RPI
declined rapidly and recovered slowly as the VGP moved
rapidly between the cluster areas. The geomagnetic reversal features observed in our record might be affected by
the NAD field. The ongoing study, which is for an oxygenisotopic analysis of fossil foraminiferal tests from this formation, will provide us a reliable age model for the section
along the KJ route. That will facilitate a further discussion
on the timings and durations of the individual geomagnetic field variations during the polarity reversals observedin this study.

Konishi and Okada Progress in Earth and Planetary Science(2020) 7:35

Supplementary information
Supplementary information accompanies this paper at https://doi.org/10.
1186/s40645-020-00352-0.
Additional file 1:. Figure S1. Detailed geologic column and samplinghorizons for the geomagnetic reversal intervals; i.e., a: the upper andlower boundaries of the Réunion subchronozone, b: the lower boundaryof the Olduvai subchronozone.
Additional file 2:. Table S1. Paleomagnetic and rock-magnetic results
for the specimens after the hybrid treatment. Note that the results ofthe specimens below 36 m, after the pThD treatment, are not shown inthis table.
Abbreviations
AFD: Alternating field demagnetization; ARM: Anhysteretic remanent
magnetization; pDRM: Post-depositional remanent magnetization;
IRM: Isothermal remanent magnetization; MAD: Maximum angular deviation;
M–B boundary: Matsuyama–Brunhes boundary; MIS: Marine isotope stage;
NAD: Non-axial dipole; NRM: Natural remanent magnetization; RPI: Relative
paleointensity; ThD: Thermal demagnetization; VGP: Virtual geomagnetic
pole; kARM: ARM susceptibility; kLF: Low-field magnetic susceptibility
Acknowledgements
We thank Toru Maruoka for collecting the paleomagnetic samples andhelping with the paleomagnetic measurements. We also thank Prof. Yuhji
Yamamoto, who supported the rock-magnetic measurements at the Center
for Advanced Marine Core Research, Kochi University through a joint-use system (Grants 16A023/16B021, and 17A038/17B038). We want to thank the editor, Prof. Ryuji Tada, and the anonymous reviewers for their constructive
comments, and Editage (www.editage.com) for English language editing.
Availability of data and material
Please contact the author for data requests.
Authors’ contributions
TK performed the experimental study, analyzed the data, and helped prepare
the manuscript. MO proposed the topic, designed the study, analyzed thedata, and wrote the manuscript. Both authors read and approved the final
manuscript.
Authors’ information
TK is an employee of Yachiyo Engineering Co., Ltd., Japan. This study isbased on part of his Master thesis at Ibaraki University. MO was thesupervisor of TK and a professor at Ibaraki University, Japan.
Funding
This study was supported by JSPS KAKENHI grants (16H04068 and 19H00710)
awarded to MO.
Competing interests
The authors declare that they have no competing interests.
Author details
1
Yachiyo Engineering Co., Ltd., Tokyo, Japan. 2Department of Earth Sciences,
Ibaraki University, 2-2-1 Bunkyo, Mito, Ibaraki 310-8512, Japan.
Received: 1 November 2019 Accepted: 15 July 2020

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