Head Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-021-00439-2(2021) 8:50

Progress in Earth andPlanetary Science

REVIEW

Open Access

Review of the Early–Middle Pleistocene
boundary and Marine Isotope Stage 19
Martin J. Head

Abstract
The Global Boundary Stratotype Section and Point (GSSP) defining the base of the Chibanian Stage and Middle
Pleistocene Subseries at the Chiba section, Japan, was ratified on January 17, 2020. Although this completed aprocess initiated by the International Union for Quaternary Research in 1973, the term Middle Pleistocene had beenin use since the 1860s. The Chiba GSSP occurs immediately below the top of Marine Isotope Substage (MIS) 19c
and has an astronomical age of 774.1 ka. The Matuyama–Brunhes paleomagnetic reversal has a directional midpoint
just 1.1 m above the GSSP and serves as the primary guide to the boundary. This reversal lies within the Early–
Middle Pleistocene transition and has long been favoured to mark the base of the Middle Pleistocene. MIS 19
occurs within an interval of low-amplitude orbital eccentricity and was triggered by an obliquity cycle. It spans two
insolation peaks resulting from precession minima and has a duration of ~ 28 to 33 kyr. MIS 19c begins ~ 791–
787.5 ka, includes full interglacial conditions which lasted for ~ 8–12.5 kyr, and ends with glacial inception at ~ 774–
777 ka. This inception has left an array of climatostratigraphic signals close to the Early–Middle Pleistocene
boundary. MIS 19b–a contains a series of three or four interstadials often with rectangular-shaped waveforms andmarked by abrupt (< 200 year) transitions. Intervening stadials including the inception of glaciation are linked to thecalving of ice sheets into the northern North Atlantic and consequent disruption of the Atlantic meridional
overturning circulation (AMOC), which by means of the thermal bipolar seesaw caused phase-lagged warming
events in the Antarctic. The coherence of stadial–interstadial oscillations during MIS 19b–a across the Asian–Pacificand North Atlantic–Mediterranean realms suggests AMOC-originated shifts in the Intertropical Convergence Zone
and pacing by equatorial insolation forcing. Low-latitude monsoon dynamics appear to have amplified responses
regionally although high-latitude teleconnections may also have played a role.
Keywords: Early–Middle Pleistocene, Quaternary, GSSP, MIS 19, Chiba

1 Introduction
On January 17, 2020, the Executive Committee of theInternational Union of Geological Sciences ratified the
Global Boundary Stratotype Section and Point (GSSP) defining the base of the Chibanian Stage and Middle Pleistocene Subseries at the Chiba section, Japan (Suganuma
et al. in press), with an astronomically calibrated age of
774.1 ± 5.0 ka (Suganuma et al. 2018). This gave official
recognition to the Middle Pleistocene, a term in use sincethe 1860s. The primary guide to this boundary is theCorrespondence: mjhead@brocku.ca
Department of Earth Sciences, Brock University, St. Catharines, Ontario L2S
3A1, Canada

Matuyama–Brunhes (M–B) paleomagnetic reversal whichfalls within Marine Isotope Stage (MIS) 19. Not only does
MIS 19 allow the base of the Middle Pleistocene to be recognized independently of the M–B reversal and at
millennial-scale resolution, but its earliest substage, MIS
19c, also serves as an orbital analogue for our own interglacial (e.g. Pol et al. 2010; Tzedakis et al. 2012a, 2012b;
Yin and Berger 2015). This review examines the history
behind the use of the term Middle Pleistocene, documents
the procedure leading to the selection and ratification of
the GSSP, examines and critiques the development of terminology used for MIS 19 and its subdivision, and synthesizes its climatic evolution on a global scale.

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Head Progress in Earth and Planetary Science(2021) 8:50

2 The Middle Pleistocene and formal
chronostratigraphy
A GSSP is an internationally designated point within astratotype. It serves as a global geostandard to define thebase of an official unit (or coterminous units) within theInternational Chronostratigraphic Chart (Cohen et al.
2013 updated). This chart is administered by the International Commission on Stratigraphy (ICS), a constituent body of the International Union of Geological
Sciences (IUGS), and it provides an officially approved
framework for the geological time scale. The International Chronostratigraphic Chart is hierarchical in
topology, with the base of each unit of higher rank
defined by the base of the unit of next lower rank, a pattern that repeats down to the lowest unit definable by a
GSSP, the stage (Salvador 1994; Remane et al. 1996).
Accordingly, the Chiba GSSP defines both the ChibanianStage and Middle Pleistocene Subseries (Fig. 1). A GSSP
technically defines only the base of a chronostratigraphic
unit, but in practice it marks the termination also of the
top of the subjacent unit, in this case the Calabrian Stageand Lower Pleistocene Subseries. The top of the Chibanian Stage and Middle Pleistocene Subseries are presently not officially defined, except nominally by
ratification of the term Upper Pleistocene Subseries
which awaits official definition by a GSSP. The base of
the Upper Pleistocene has a provisional age of ~ 129 ka(Head et al. in press).
It remains to be determined whether the ChibanianStage will always precisely equate in extent with the
Middle Pleistocene Subseries. There are good grounds
for introducing a second stage for the Middle Pleistocene Subseries defined at its base by the onset of a major
climatic event known as the “Mid-Brunhes Event” (Jansen et al. 1986) or “Mid-Brunhes Transition” (Yin 2013)
which marks a step change in Quaternary climate. This
climatic shift corresponds to an increase in the amplitude of quasi-100 kyr glacial–interglacial cycles and ismarked by increases in interglacial sea-surface andPage 2 of 38

Antarctic temperatures, atmospheric CO2, and CH4levels, all beginning with Marine Isotope Stage (MIS) 11(Barth et al. 2018). The onset of this transition is globally
synchronous and corresponds to that between MIS 12
and MIS 11 (Termination V), dating to ~ 430 ka (Barthet al. 2018) (Fig. 2). It is readily identified in successions
where astrochronology can be applied, including deepocean, ice-core, and European and Chinese loess records, and coincides with the base of Holsteinian Northwest European Stage, Likhvinian Russian Plain Stage andZavadivian Ukrainian Loess Plain Stage (Cohen and Gibbard 2019). The Bermuda geomagnetic excursion, whichlies at a prominent relative paleointensity minimum at~ 412 ka in MIS 11c (Channell et al. 2020), could serveas an additional stratigraphic marker (Fig. 2). However,for now the Chibanian Stage extends upwards to thebase of the Upper Pleistocene Subseries (Fig. 1).
The International Stratigraphic Guide distinguishes
only between formal and informal stratigraphic terms.
Formal terms “are properly defined and named according to an established or conventionally agreed scheme ofclassification ... The initial letter of the rank- or unitterm of named formal units is capitalized” (Salvador
1994, p. 14, see also p. 24). Those unit-terms appearing
in the International Chronostratigraphic Chart are notmerely formal terms but have also been approved by the
ICS following extensive deliberation and then ratified by
the Executive Committee of the IUGS (see Head 2019
for details of this process). These terms are here treatedas “official” or “ratified” to distinguish them from formal
terms lacking this approval (Head and Gibbard 2015a).
2.1 History of the term Middle Pleistocene

Charles Lyell in 1839 introduced the term Pleistocene(Greek, pleīstos, most; and kainos, recent) as a substitutefor his Newer Pliocene (Lyell 1839, p. 621), but unlike
his other series of the Cenozoic (Head et al. 2017), he
refrained from dividing it into subseries. Indeed in 1863,
Lyell proposed abandoning Pleistocene altogether on

Fig. 1 The Quaternary System/Period and its official subdivision as currently approved by the ICS and ratified by the IUGS EC. Stage 4 corresponding
to the Upper Pleistocene Subseries has yet to be officially defined. GSSP = Global Boundary Stratotype Section and Point (from Head et al. in press)

Head Progress in Earth and Planetary Science(2021) 8:50

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Fig. 2 Stratigraphic correlation table and orbital parameters for the last 1.9 million years, including the Early–Middle Pleistocene transition (1.4–
0.7 Ma, Sánchez-Goñi et al. 2019; or 1.4–0.4 Ma, Head and Gibbard 2015b). The time scale is based on Fig. 1; geomagnetic polarity reversals andfield paleointensity data are from Cohen and Gibbard (2019) and Channell et al. (2016, 2020) with ages of reversals based on orbital tuning of thesedimentary record (Channell et al. 2020); marine isotope record and numbering of marine isotope stages is from Lisiecki and Raymo (2005), withages of terminations from Lisiecki (undated) and selected substages from Railsback et al. (2015); orbital parameters representing precession(Laskar et al. 2004), obliquity (Laskar et al. 2004), and eccentricity (Laskar et al. 2011) are from Head and Gibbard (2015b). Updated from Cohen
and Gibbard (2019)

grounds that Forbes (1846) had popularized this term
not in the sense of Lyell’s Newer Pliocene but almost
precisely with reference to the subsequent interval oftime for which Lyell was now introducing the term Postpliocene (Lyell 1863, p. 6). By 1865, Lyell had concededthat if the term Pleistocene continued to be used, then it
should not be as originally intended but in place of his“Post-pliocene” (Lyell 1865, footnote to p. 108). By thetime Lyell had unconditionally accepted the Pleistocene
in place of his Post-pliocene (Lyell 1873, p. 3, 4), thissuggestion had already been generally adopted, with subdivision quickly following. The term “middle Pleistocene” for instance was employed informally by Harkness
as early as 1869 (Harkness 1869), and the positional
modifiers “early”, “middle”, and “late” have been used for
the Pleistocene since at least the 1870s (e.g. Dawkins

1878). By 1900, this tripartite subdivision had become
formalized in the English literature, with Osborn using
the terms Lower Pleistocene (preglacial), Middle Pleistocene (glacial and interglacial, itself subdivided into lower,
middle and upper), and Upper Pleistocene (postglacial
and Recent) (Osborn 1900, p. 570, charts I and II)(Fig. 3).
This use of subseries for the Pleistocene had become
entrenched by the time of the Second International Conference of the Association pour l’étude du Quaternaire
européen (the forerunner of the International Union for
Quaternary Research [INQUA] meetings) held in Leningrad in 1932, and subseries terms were used in a formal
sense by Zeuner (1935, 1945) who in 1935 was alreadyapplying Milankovitch cyclicity and insolation curves toprovide absolute dates for Pleistocene successions. In

Head Progress in Earth and Planetary Science(2021) 8:50

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Fig. 3 Reproduction of Chart 1 of Osborn (1900), an early example of the tripartite subdivision of the Pleistocene along with other Cenozoic
series and their subdivision. The word Pleiocene, from the Greek pleiōn (Latinized as plio-), is a rare variant spelling of Pliocene

1945, he considered the base of the Middle Pleistocene
to have an age of ~ 425 ka.
The Japanese geophysicist Motonori Matsuyama (1884–
1958, as spelled and pronounced but mistransliterated in
his own publications and others as Matuyama) was thefirst to document clearly from basalts in the Genbudō(basalt caves), Japan (Matuyama 1929), the reversed magnetic polarity interval from 2.58 to 0.773 Ma that we now
call the Matuyama Reversed Polarity Chron. However, it
was the emergence of the geomagnetic polarity reversal
time scale for the Pleistocene in the 1960s (Cox et al.

1963, 1964; Opdyke et al. 1966; Ninkovich et al. 1966;
Glass et al. 1967; see Watkins 1972 for historical review),
and particularly the recognition and radiometric dating of
the M–B reversal and Jaramillo “event” (Doell and Dalrymple 1966), that created new possibilities for global
stratigraphic correlation and Pleistocene time scale calibration. Accordingly, participants at the Burg Wartenstein
Symposium “Stratigraphy and Patterns of Cultural Change
in the Middle Pleistocene”, held in Austria in 1973, recommended that “The beginning of the Middle Pleistocene
should be so defined as to either coincide with or beHead Progress in Earth and Planetary Science(2021) 8:50

closely linked to the boundary between the Matuyama Reversed Epoch and the Brunhes Normal Epoch of
paleomagnetic chronology” (Butzer and Isaac 1975, appendix 2, p. 901), as noted by Pillans (2003). In the same
year, the INQUA Working Group on Major Subdivisionsof the Pleistocene was established at the IX INQUA Congress in Christchurch, New Zealand, 1973, with its primary aim to define globally recognizable boundaries forthe lower, middle, and upper Pleistocene subseries (Richmond 1996). The rank of subseries was adopted in preference to stage as the latter term was already used widely in
Quaternary stratigraphy for locally and regionally definedunits. At the XIIth INQUA Congress in Ottawa in 1987,
the Working Group submitted a proposal, which was
accepted by INQUA’s stratigraphic commission and approved by the congress, that “As evolutionary biostratigraphy is not able to provide boundaries that are as globally
applicable and time-parallel as are possible by othermeans, the Lower–Middle Pleistocene boundary should
be taken provisionally at the Matuyama–Brunhes palaeomagnetic reversal ...” (Anonymous 1988, p. 228; Richmond
1996, p. 320). From then on, the M–B reversal became thepreferred and indeed de facto marker for the Early–Middle Pleistocene boundary (e.g. Bowen 1988; Berggren et al.
1995; Pillans 2003; Gradstein et al. 2005; Head and Gibbard 2005, 2015a, 2015b; Cita et al. 2006, 2008, Cita 2008;
Head et al. 2008). Nonetheless, the Early–Middle Pleistocene boundary did not have official standing because thisrequired the selection and approval of a GSSP.
2.2 Selecting a primary guide for the base of the Middle
Pleistocene Subseries

The XIVth INQUA Congress in Berlin in 1995 focusedon three potential candidate GSSPs: Chiba in Japan,
Montalbano Jonico in Basilicata, Italy, and the WanganuiBasin in New Zealand (Pillans 2003), the latter being discounted because it contained unconformities (Headet al. 2008). Meanwhile, the ICS Subcommission on
Quaternary Stratigraphy (SQS), in 2002 after a period ofinactivity, established a Working Group to review all aspects of the Early–Middle Pleistocene boundary including the selection of a suitable GSSP (Head et al. 2008).
At the 32nd International Geological Congress in Florence in 2004, the Early–Middle Pleistocene boundary
Working Group recommended that (1) The boundary be
defined in a marine section at a point “close to” theMatuyama–Brunhes palaeomagnetic reversal, where thedefinition of “close” was agreed to mean within plus orminus one isotope stage of the reversal; and (2) the
GSSP should be located in a marine section exposed onland, not in a deep sea core (Head et al. 2008). A third
potential candidate GSSP emerged at the Florence congress: the Valle di Manche section in Calabria, Italy(Capraro et al. 2004, 2005) (Fig. 4).

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Deciding upon the primary guide to the boundary
should be made prior to the consideration of candidate
sections because the expression of this guide in the
GSSP must be exemplary (Remane et al. 1996). The
Working Group’s decisions at Florence were therefore
crucial in moving the process forward. The M–B reversal with an age of ~ 773 ka (Singer et al. 2019; Channell
et al. 2020; Haneda et al. 2020a; and earlier reviews by
Head and Gibbard 2005, 2015b) was chosen in part because it (1) has an isochronous expression in most marine and terrestrial sediments and even in ice cores, (2) is
the most prominent geomagnetic field reversal in thepast 773 kyr, and (3) occurs within the Early–Middle
Pleistocene transition (1.4–0.7 or 1.4–0.4 Ma; Fig. 2),aligning the Early–Middle Pleistocene boundary with afundamental shift in Earth’s history. This shift from a 41
ky to quasi-100 ky orbital rhythm was marked by increases in the amplitude of climate oscillations and inlong-term average global ice volume, and by strong
asymmetry in global ice volume cycles. It resulted inprogressive and fundamental physical, chemical, climatic, and biotic adjustments to the planet (Head andGibbard 2015b).
2.3 Voting on candidates for the Middle Pleistocene
Subseries GSSPThe three final candidates for the Early–Middle Pleistocene GSSP were the Valle di Manche section in Calabria
and the Ideale section at Montalbano Jonico in Basilicata, both in Italy, and the Chiba section in Japan (Headand Gibbard 2015a) (Fig. 4). Following field trips thatallowed members of the Working Group to visit all threesites in advance of voting (Ciaranfi et al. 2015; Okada
and Suganuma 2018), the vigorous and exhaustive
process of selecting a GSSP began on July 11, 2017, withthe circulation of proposals to the membership of theWorking Group (Table 1). It had been decided by all
three proponents in advance that the proposals should
remain confidential because they contained unpublishedmaterial. This confidentiality was respected through theentire selection process. Discussions started on July 25
and ended at the close of October 3, 2017, allowing an
extended opportunity to exchange views. Discussions
were wide-ranging, in acknowledgement that a GSSP
must record an array of stratigraphic markers, but inevitably focused on the M–B reversal. A detailed commentary on these discussions is given in Head (2019) andonly key aspects will be presented here.
The M–B reversal in the Chiba composite section (CbCS)
is expressed by directional changes (virtual geomagnetic pole
[VGP] latitudes) and changes in the geomagnetic field intensity based on both the paleomagnetic record and a coherent
record of its proxy, the authigenic 10Be/9Be record (Suganuma et al. 2015; Okada et al. 2017; Simon et al. 2019;

Head Progress in Earth and Planetary Science(2021) 8:50

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Fig. 4 Location of sites discussed in the text and present atmospheric features. ODP Site 983 Gardar Drift, Iceland Basin; IODP Site U1313 upper western
flank of the Mid-Atlantic Ridge, central North Atlantic; IODP Site U1385 southwest Portuguese margin; ODP Site 976 Alboran Sea; ODP Site 975 western
Mediterranean Sea; Core KC01B Ionian Sea; Core MD900963 tropical Indian Ocean; Lake Baikal, SE Siberia; Yimaguan and Luochuan, Chinese Loess
Plateau; Chiba composite section, Japan; Vema 28-238 and RC11-209 cores, western equatorial Pacific Ocean; ODP Site 1123 Chatham Ridge, South
Pacific; EPICA Dome C ice core (75° 06′ S, 123° 21′ E, location is off the map); Core 58 of Arrhenius (1952), eastern equatorial Pacific Ocean (6° 44′ N, 129°
28′ W; location is off the map); inset shows important Italian sites. The Westerly Jet (WJ) during summer (S), East Asian Summer/Winter Monsoon (EASM/
EAWM), and summer/winter variation in the position of the Intertropical Convergence Zone (ITCZ) adapted from Cheng et al. (2012) and Liu et al.
(2015); the Siberian High (SH) and Aleutian Low (AL) are primarily winter atmospheric pressure systems; the AL and Pacific High (PH) form the North
Pacific Oscillation; the Islandic Low (IL) and Azores High (AH) form the North Atlantic Oscillation

Haneda et al. 2020a). These studies are based on an astronomical age model introduced by Suganuma et al. (2015)
and refined by Okada et al. (2017) and again by Suganuma
et al. (2018). Okada et al. (2017) determined the directional
midpoint at 771.7 ka with a duration of 2.8 kyr; these values
revised to 772.9 ka and 1.9 kyr on the age model of Suganuma et al. (2018). Simon et al. (2019) using newpaleomagnetic data reported a directional switch between
773.9 and 771.9 ka, with a duration therefore of 2.0 kyr.
Haneda et al. (2020a) using new paleomagnetic data combined with earlier studies (Suganuma et al. 2015; Hyodo
et al. 2016; Okada et al. 2017) determined the average directional midpoint at 772.9 ka with a duration of 1.1 kyr basedon the age model of Suganuma et al. (2018). Allowing for a 5
kyr chronological uncertainty in the orbital tuning of the
CbCS (4 kyr from Lisiecki and Raymo 2005, and 1 kyr from
Elderfield et al. 2012; see Suganuma et al. in press) and astratigraphic uncertainty of 0.4 ka (Haneda et al., 2020a), theastronomical age of the directional midpoint of the M–B

reversal is 772.9 ± 5.4 ka, with a duration of up to ~ 2 kyr.
The close match between the geomagnetic field intensity andthe 10Be/9Be record confirms that any lock-in depth offset(Roberts and Winklhofer 2004; Suganuma et al. 2010, 2011)at this high sedimentation rate site (89 cm/kyr across theboundary) is minimal. This age closely accords with ages ofaround 773 ka from other well constrained sites(Channell 2017; Channell et al. 2020; Singer et al.
2019; Valet et al. 2019; Haneda et al. 2020a; earlier
records reviewed in Head and Gibbard 2015b). The
geomagnetic field intensity record shows two pronounced minima, one at 772 ka near the polarity
switch and the other at 764 ka (Simon et al. 2019). It
is therefore evident that the position of the VGP
switch cannot be precisely predicted using geomagnetic field intensity data alone.
Montalbano Jonico lacks a paleomagnetic record
owing to late diagenetic remagnetization associated withthe growth of greigite (Sagnotti et al. 2010). A 10Be/9Be

Head Progress in Earth and Planetary Science(2021) 8:50

Table 1 Members of the SQS Working Group on the Early–
Middle Pleistocene Boundary in 2017 at the time of voting on
the GSSP• Luca Capraro, Dipartimento di Geoscienze, Università degli Studi di
Padova, Padova, Italy• Bradford M. Clement, Integrated Ocean Drilling Program and
Department of Geology and Geophysics, Texas A&M University, College
Station, USA• Mauro Coltorti, Dipartimento di Scienze Fisiche, della Terra edell’Ambiente, Università di Siena, Siena, Italy• Craig S. Feibel, Department of Earth and Planetary Sciences, Rutgers
University, Piscataway, New Jersey, USA• Martin J. Head, Department of Earth Sciences, Brock University, St.
Catharines, Ontario, Canada (Co-Convener)• Lorraine E. Lisiecki, Department of Earth Science, University ofCalifornia, Santa Barbara, USA• Jiaqi Liu, Institute of Geology and Geophysics, Chinese Academy of
Sciences, Beijing, China• Maria Marino, Dipartimento di Scienze della Terra e Geoambientali,
Università degli Studi di Bari Aldo Moro, Bari, Italy• Anastasia K. Markova, Institute of Geography, Russian Academy of
Sciences, Moscow, Russia• Brad Pillans, Research School of Earth Sciences, Australian National
University, Canberra, ACT, Australia (Co-Convener)• Yoshiki Saito, Geological Survey of Japan, AIST, Tsukuba, Ibaraki, Japan;
and Estuary Research Center, Shimane University, Matsue, Shimane,Japan• Brad S. Singer, Department of Geoscience, University of Wisconsin–
Madison, USA• Yusuke Suganuma, National Institute of Polar Research, Tachikawa,Tokyo, Japan and Department of Polar Science, School ofMultidisciplinary Sciences, The Graduate University for Advanced
Studies (SOKENDAI), Tachikawa, Tokyo, Japan• Charles Turner, Department of Earth Sciences, The Open University,
Milton Keynes, UK• Chronis Tzedakis, Department of Geography, University College
London, London, UK• Thijs van Kolfschoten, Faculty of Archaeology, Leiden University,
Leiden, The Netherlands

record at this site serves as a proxy for the geomagnetic
field intensity site and reveals a peak (field intensity
minimum) at the approximate position of the M–B reversal as determined by the marine isotope record (Simon et al. 2017; Nomade et al. 2019) and dated by an
40
Ar/39Ar age of 774.1 ± 0.9 ka for tephra layer V4 whichcoincides with the 10Be/9Be peak (Nomade et al. 2019).
While this corroborates the position and age of the M–B
reversal in this part of the Mediterranean, the geomagnetic field intensity alone is insufficient to identify theprecise position of the polarity switch (see Channell et al.
2020), as demonstrated for Chiba and elsewhere.
The M–B reversal as recorded at the Valle di Manche(Capraro et al. 2017) has been astronomically dated at
786.9 ± 5 ka (Macrì et al. 2018), an anomalously old age

Page 7 of 38

when compared with most global records including the10
Be/9Be proxy record of Montalbano Jonico section just
135 km to the north (Head 2019). A 10Be/9Be record at
the Valle di Manche section gives a peak in 10Be concentration ∼3.5 m above the reported M–B reversal. Thistranslates to a difference of ∼12 kyr (Capraro et al. 2018)
and is coincident with the age of this reversal elsewhere.
Lock-in depth seems unable to explain the spuriously
low position of the reversal because sedimentation rates
at ∼27 cm/kyr in this part of the Valle di Manche section
are reasonably high (Macrì et al. 2018). When the10
Be/9Be curves for the Valle di Manche and Montalbano Jonico sections are compared, they show strong
agreement (Capraro et al. 2019). The 10Be/9Be peaktherefore most likely marks the true position of the M–
B Chron boundary at both sections, with the Valle diManche paleomagnetic reversal ∼3.5 m below representing diagenetic overprinting and remagnetization (Head2019; but see Capraro et al. 2019 for an alternative interpretation). This explanation would also account for theunusually rapid directional transition of this reversal in
the order of 100 years or less at the Valle di Manche
section (Macrì et al. 2018). A similar relatively old (786.1± 1.5 ka) M–B reversal, perhaps with an even more rapidtransition, reported from the Sulmona basin in central
Italy (Sagnotti et al. 2014; Sagnotti et al. 2016) has been
restudied and appears to carry an unreliable signal (Evans and Muxworthy 2018; but see Sagnotti et al. 2018).
Another relatively old age (~ 779 ka) for the reversal has
been reported from Site IODP U1385 off Portugal (Sánchez-Goñi et al. 2016). The position of this reversal has
since been revised, and it is now provisionally placed
higher in the core than reported from shipboard analysis(Xuan Chuang, pers. comm. 2018). Moreover, a reported
M–B reversal age of 783.4 ± 0.6 ka at ODP Site 758 in
the Indian Ocean (Mark et al. 2017) has been challenged
on grounds that the sedimentation rates and hence resolution of the isotope and magnetic stratigraphies are all
too low for precise age determination (Channell and
Hodell 2017).
It had been decided in advance that the choice individual members made when voting within the Working
Group would not be revealed, contrary to usual practicewithin ICS. Because of active and potential research collaborations within the group, to do otherwise might havecompromised the vote. Voting by the SQS Working
Group commenced on October 10, 2017, and concludedon November 10, 2017. As noted in Head (2019), theChiba proposal was passed by supermajority, gaining
73% of the total votes cast.
2.4 Final approval and ratification of the Chiba GSSP

Following minor revision, the Chiba proposal was submitted to the SQS voting membership for discussion andHead Progress in Earth and Planetary Science(2021) 8:50

voting, this process concluding on 16 November 2018with a supermajority of 86% in favour of the Chiba proposal. Discussion within the ICS voting membershipbegan on August 16, 2019, and closed on October 28,
2019. Voting concluded on November 28, 2019, with theresults as follows: 17 in favour, 2 against, no abstentions,
all ballots returned. The proposal was therefore carried
with a supermajority of 89.5%. This ICS-approved proposal for the Chibanian Stage/Age and Middle Pleistocene
Subseries/Subepoch was ratified in full by the IUGS EC
on January 17, 2020, drawing to a close a process initiatedby INQUA in 1973, some 47 years earlier.
The GSSP is placed at the base of a regional lithostratigraphic marker, the Ontake-Byakubi-E (Byk-E) tephrabed (Takeshita et al. 2016), in the Chiba section. It has
an astronomical age of 774.1 ka (Suganuma et al. inpress) and a zircon U-Pb age of 772.7 ± 7.2 ka (Suganuma et al. 2015), occurring immediately below the topof Marine Isotope Substage 19c. The directional midpoint of the M–B reversal, serving as the primary guideto the boundary, is just 1.1 m above the GSSP and has
an astronomical age of 772.9 ± 5.4 ka (Haneda et al.
2020a; Suganuma et al. in press). The numerous climatostratigraphic signals associated with the MIS 19c/btransition, which represents the inception of glaciation
for MIS 19 (see below), provide additional means to
identify this boundary precisely on a global scale.
IUGS ratification of the Middle Pleistocene Subseries officially legitimized a unit-rank term already in wide andformal use within the Quaternary community (Head et al.
2017), and the ratification of an accompanying stage complied with the requirements of the International Commission on Stratigraphy. INQUA fully supported ratificationof both stage and subseries (van Kolfschoten 2020). This
also provided Japan with its first GSSP, coincidentally
based on a paleomagnetic reversal first clearly documented in Japan by Motonori Matsuyama, an early Japanese pioneer of magnetostratigraphy. The achievements ofJapanese geophysicist Naoto Kawai may also be recalled,
as he was the first to record a paleomagnetic reversal in
sedimentary rocks (Kawai 1951).

3 Marine Isotope Stage 19
MIS 19 has long been associated with the M–B reversal,
and this interglacial stage therefore provides a welldocumented cluster of additional stratigraphic signals to
identify the base of the Chibanian Stage on a global
scale. Its climatic evolution is also significant because
MIS 19c serves as an orbital analogue for the presentinterglacial (e.g. Berger and Loutre 1991; Pol et al. 2010;
Tzedakis et al. 2012a, 2012b; Yin and Berger 2015) andtherefore provides a natural baseline for assessing our
future climate.

Page 8 of 38

3.1 History of MIS 19

In labelling fluctuating percentages of carbonate in marine cores from the equatorial Pacific Ocean, Arrhenius(1952) introduced a numbering system in which even/odd numbers represent glacial/interglacial cycles. Arrhenius correctly surmised that carbonate-rich layers represent increased productivity linked to upwelling driven bystrengthened trade winds during glacial intervals. Arrhenius labelled 18 carbonate cycles, recording although notlabelling older cycles including the equivalent of what
was to be known as MIS 19 (Fig. 5). Hays et al. (1969)
continued this research through additional cores in the
Pacific. They labelled as B17 (where B = Brunhes) a
carbonate-poor interglacial cycle coinciding with thebase of the Brunhes Chron (Fig. 5). Emiliani’s (1955,
1966) original oxygen isotope stages followed the numbering scheme of Arrhenius. Shackleton and Opdyke(1973) in their now famous oxygen isotope and magnetostratigraphic analysis of the Vema 28-238 core fromthe western equatorial Pacific Ocean (V28-238 in Fig. 4)
extended to Stage 22 Emiliani’s original oxygen isotope
stages 1–14 (Emiliani 1955) and then 1–17 (Emiliani1966). In doing so, Shackleton and Opdyke (1973) were
the first to label MIS 19 (Fig. 6). They equated cycle B17
of Hays et al. (1969) with their MIS 19, confirming theassociation of this interglacial stage with the M–B
reversal.

3.2 Subdivision of MIS 19

The division of marine isotope stages into substages has
a long history beginning with Shackleton (1969) who
subdivided MIS 5 into five lettered substages, a–e (Railsback et al. 2015). As noted by Railsback et al. (2015), a
parallel system of subdividing marine isotope stages into
decimal-style numbered “events” has its roots in the labelling system of Arrhenius (1952) and was first applied
to marine isotope stages by Prell et al. (1986; but see
Railsback et al. 2015 for historical development) who
reasoned that defining events (peaks and troughs) rather
than stages (intervals of sediment or time) was more
useful in applying tie points for age models. Although
the two approaches tended to be used rather indiscriminately and interchangeably, Shackleton (2006) remarked
that conceptually they are different and not interchangeable. He noted that “events” relied upon peak values inanalyses that are more difficult to replicate in practice,and hence reliably correlate, than the midpoints of transitions that define substage boundaries. This midpoint
approach is indeed is how stages themselves are defined
following Emiliani (1955). Accordingly, Shackleton(2006), Railsback et al. (2015) in their extensive review,and the present study, have all favoured contiguous lettered subdivisions for marine isotope stages.

Head Progress in Earth and Planetary Science(2021) 8:50

Page 9 of 38

Fig. 5 Reproduction of fig. 14 in Hays et al. (1969) showing correlation between carbonate percentage in equatorial Pacific core RC11-209 andthat of east equatorial Pacific core 58 of Arrhenius (1952). Carbonate cycle B17 in core RC11-209 corresponds to an unlabelled cycle in Arrhenius’core 58. This would have been cycle 19 had Arrhenius continued labelling. Cycle B17 aligns with the Matuyama–Brunhes paleomagnetic reversal
and represents MIS 19

3.2.1 Subdivision used in the present study

The scheme used here is illustrated by its application to
the CbCS record (Fig. 7). Three substages, 19c, 19b, and19a, are recognized. MIS 19c comprises full interglacialconditions together with the rise to lighter foraminiferal
δ18O values at the beginning of MIS 19 (Termination
IX) and the decline to heavier values towards the end of

MIS 19c, terminating with a glacial inception (Tzedakis
et al. 2012a, 2012b). MIS 19b represents a single intervalof heavier foraminiferal isotopic values (Nomade et al.
2019) which is recognized at the CbCS within the benthic record (Haneda et al. 2020b). The benthic foraminiferal δ18O record of MIS 19a is represented by a series ofmillennial-scale oscillations, with as many as four peaks

Fig. 6 Reproduction of fig. 9 in Shackleton and Opdyke (1973) showing the δ18O record of the planktonic foraminifera Globigerinoides sacculiferafrom core V28-238, western equatorial Pacific, from which MIS 19 was defined for the first time (Shackleton and Opdyke 1973). This study
confirms the links between MIS 19, carbonate cycle B17 of Hays et al. (1969), and the Matuyama–Brunhes paleomagnetic reversal

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Page 10 of 38

Fig. 7 Differing subdivisions of the interval comprising Marine Isotope Substages 19b and 19a illustrated using the isotope record of the Chiba
composite section (CbCS; Haneda et al. 2020b). (a) Four millennial-scale benthic isotope oscillations (MIS 19a-o1 to MIS 19a-o4) represented in thebenthic foraminiferal record by lighter values exclusively within MIS 19a. Four stadials (MIS 19-s1 to MIS 19-s4) and four interstadials (MIS 19-i1 to
MIS 19-i4) reflect more localized millennial-scale paleoenvironmental alternations across MIS 19b–a, in this case characterized by the planktonic
foraminiferal record. (b) The labelling scheme of Haneda et al. (2020b) based on the planktonic foraminiferal record of the CbCS. (c) The labelling
scheme of Nomade et al. (2019) based on the benthic foraminiferal record of Montalbano Jonico, Italyof lighter isotopic values here labelled as MIS 19a-o1 to
MIS 19a-o4, where “o” refers to benthic isotope oscillation. MIS 19a begins with MIS 19a-o1 (Fig. 7).
Superimposed on this benthic foraminiferal isotopic
record through MIS 19b–a is as many as four stadial–interstadial alternations, here labelled MIS 19-s1 to MIS
19-s4 (stadials) and MIS 19-i1 to MIS 19-i4 (interstadials). MIS 19-s1 is the first of these millennial-scale climatic episodes and broadly coincides with the glacial
inception marked by MIS 19b. They are recognized primarily in planktonic records including planktonic foraminiferal δ18O, but may be observed in pollen spectra
and other terrestrial proxies.
Figure 7 shows how the labelling scheme presented here
differs from those of Nomade et al. (2019) and Haneda et al.
(2020b) as applied to the isotopic record of the CbCS. The
present scheme does not preclude the use of additional biozones and informal event stratigraphy through all or part of
MIS 19 where such detail is needed. The rationale for thissubdivision is discussed in Section 3.2.3.
3.2.2 Division into substages

Bassinot et al. (1994) were the first to subdivide MIS 19
formally, defining MIS 19.1, 19.2, and 19.3 (Fig. 8b) on

the basis of two pronounced planktonic foraminiferal
δ18O peaks recorded from Core MD900963 in the tropical Indian Ocean (Fig. 4). No explanation was given forthese two peaks although precession is strongly
expressed in this core.
Tzedakis et al. (2012a, 2012b) seem to have initiatedthe application of lettered substages for MIS 19, with
Tzedakis et al. (2012b, their fig. 4) applying MIS 19a,
19b, and 19c to the foraminiferal δ18O and ice-rafted
debris (IRD) record from ODP Site 983 on the Gardar
Drift, North Atlantic (Figs. 4 and 8c) and correlating thisto the Antarctic ice-core record of EPICA Dome C(Figs. 4 and 8c). Tzedakis et al. (2012b) did not define
the boundaries of their substages but MIS 19c clearly
represents the rise to lightest isotopic values and the ensuing peak or plateau followed by a gradual decline toheavier values in the upper part of MIS 19c. MIS 19b
and MIS 19a together represent three succeeding peaksof light isotopic values, here labelled interstadials i1, i2,
and i3, that characterize the upper part of MIS 19. MIS
19b includes the two lowest interstadials and MIS 19a
includes the third. Tzedakis et al. (2012b) correlated
these three interstadials in the upper part of MIS 19
at Site 983 with three Antarctic Isotope Maxima

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Page 11 of 38

Fig. 8 North Atlantic and global records of Marine Isotope Stage 19 (see also Fig. 9). (a) Insolation at 65° N in June, and precession and obliquity parameters.
(b) Designation of events by Bassinot et al. (1994) based on the planktonic foraminiferal δ18O record of Indian Ocean core MD900963. (c) Lettered substages of
Tzedakis et al. (2012b) applied to the foraminiferal δ18O and other records of ODP Site 983 Iceland Basin, with peaks correlated to Antarctic Isotope Maxima(AIMs) reflected in the Antarctic ice-core methane record from EPICA Dome C. Also included are the ice-rafted debris and sortable silt mean size (mean of 10–
63 μm fraction) records (Kleiven et al. 2011). (d) Lettered substages of Railsback et al. (2015) as applied to the LR04 benthic δ18O foraminiferal stack of Lisiecki
and Raymo (2005). (e) The MIS 19 subdivisional scheme used here (Fig. 7): interstadials i1, i2, and i3 are labelled in red. All records are plotted on their own
published time scales and use the original substage designations

Head Progress in Earth and Planetary Science(2021) 8:50

(AIMs) documented in the EPICA Dome C ice-corerecord (EPICA Community Members 2006). Hence,
the two lowest interstadials, assigned to MIS 19b,
were correlated to AIM C and B, and the uppermost
interstadial, assigned to MIS 19a, was correlated to
AIM A (Fig. 8c).
Railsback et al. (2015) similarly subdivided MIS 19
into substages a, b, and c, but defined them with respect to the LR04 global benthic foraminiferal δ18O
stack of Lisiecki and Raymo (2005) which only clearly
distinguishes two peaks in the upper part of MIS 19.

Page 12 of 38

Railsback et al. (2015) assigned both peaks to MIS
19a and the preceding trough to MIS 19b (Fig. 8d).
This scheme therefore differed significantly from thatof Tzedakis et al. (2012b).
Ferretti et al. (2015) published detailed benthic andplanktonic foraminiferal δ18O records from IODP
Site U1313 in the central North Atlantic (Fig. 4), although the upper part of MIS 19 could not be unambiguously resolved into three interstadials. MIS
19c and MIS 19a were therefore labelled only approximately and MIS 19b was omitted (Fig. 9b).

Fig. 9 North Atlantic records of Marine Isotope Stage 19 (see also Fig. 8). (a) Insolation at Equator in spring and autumn, and absolute maximumof mean irradiance (Laskar et al. 2004; Ferretti et al. 2015; Haneda et al. 2020b). (b) IODP Site U1313 central North Atlantic: foraminiferal δ18O
(Ferretti et al. 2015). (c) IODP Site U1385 southwest Portuguese margin: foraminiferal δ18O, alkenones (C37:4), and pollen records with grey bars
indicating major contractions of the Mediterranean forest; the Tajo Interglacial occurs within MIS 19c, and a dark blue triangle marks a briefcooling event just before it (Sánchez-Goñi et al. 2016); stadials (s1–s3) and interstadials (i1–i3) are labelled following the correlations of Regattieri
et al. (2019) and Nomade et al. (2019). (d) The MIS 19 subdivisional scheme used here (Fig. 7). All records are plotted on their own published timescales and use the original substage designations

Head Progress in Earth and Planetary Science(2021) 8:50

Sánchez-Goñi et al. (2016) in their study of IODP Site
U1385 off southwest Portugal (Fig. 4) extended theupper boundary of MIS 19c to the top of the plateau oflightest δ18O values. The progressive decline to heavier
values as well as the first of three conspicuous peaks in
the upper part of MIS 19 were assigned to MIS 19b. The
second and third peaks were assigned to MIS 19a(Fig. 9c). This scheme essentially follows that of Tzedakis et al. (2012b). Sánchez-Goñi et al. (2016) determined
the MIS 20/19 and MIS 19/18 boundaries at the midpoints between highest/lowest and lowest/highest values
in the δ18O benthic foraminiferal record, an approach
following that of Shackleton et al. (2003) for establishing
the MIS 6/5e and MIS 5e/d boundaries. They then applied the same method to determine the positions of the
MIS 19c/b and MIS 19b/a boundaries, finding that the
midpoints were broadly similar to positions they had statistically identified by the “Change point method” of Zeileis et al. (2002, 2003). Therefore, these limits and thesubstage classification they embody may reflect significant changes in global ice volume (Sánchez-Goñi et al.
2016). This method does not appear to have been usedon other foraminiferal benthic records of MIS 19 to test
such a possibility, but the shape of the δ18O benthic foraminiferal record at the CbCS in Japan (Fig. 7), for example, is rather different from that at Site U1385
especially across the MIS 19b–a interval.
Regattieri et al. (2019) in their study of the lacustrine
Sulmona basin in central Italy (Figs. 4 and 10b) broadly
followed the lettered substage classification of SánchezGoñi et al. (2016). Their MIS 19b, the base of which isplaced between two reduced-precipitation events (V andVI), includes stadial s1, interstadial i1 and part of the following stadial, s2. The MIS 19b–a boundary is drawn
midway through their event IIX, here labelled stadial s2.
In total, three interstadials are recognized within the
MIS 19b–a interval at Sulmona (Fig. 10b), as with IODP
Site U1385.
The lacustrine Piànico-Sèllere basin of northern
Italy (Fig. 4) contains a finely resolved pollen record
established by Moscariello et al. (2000). Although previously assigned to MIS 11, it most likely represents
MIS 19 based on tephrochronology (Pinti et al. 2001,
2007; Roulleau et al. 2009) and paleoclimatic concordance with other Italian sites (Nomade et al. 2019).
Nomade et al. (2019) labelled the Piànico-Sèllere
pollen record (Fig. 10c) following their new classification of the Ideale section of Montalbano Jonico which
recognizes at least three interstadials for MIS 19a andrestricts MIS 19b to a single stadial, here labelled s1.
Although based on pollen percentages, the record of
the Piànico-Sèllere basin allows precise comparison
with the marine isotope substages and recognition ofat least interstadials i1 and i2 in MIS 19a.

Page 13 of 38

The Ideale section of Montalbano Jonico, Basilicata,
Italy (Fig. 4), yields one of the most detailed δ18O records of MIS 19 available (Simon et al. 2017; Nomade
et al. 2019). MIS 19b is restricted to a brief cooling event
following MIS 19c, and MIS 19a includes discrete intervals of lighter isotopic values labelled by Nomade et al.
(2019) as interstadials 1 through 3, with a fourth labelledin the present study (fig. 8b in Nomade et al. 2019;
Fig. 10d).
A somewhat truncated planktonic foraminiferal
δ18O record of ODP Site 976, Alboran Sea, western
Mediterranean (Toti et al. 2020; Fig. 4) does not distinguish between MIS 19b and MIS 19a, although twointerstadials and the onset of a third in the upper
part of MIS 19 can be referred to interstadials i1–i3(Fig. 10e).
The highly resolved δ18O record from the CbCS, Japan(Haneda et al. 2020b; Suganuma et al. in press; Fig. 4),
has been subdivided into lettered substages such thatfour peaks identified in the upper part of MIS 19 are allassigned to MIS 19a (Fig. 11). This scheme follows theclassification of Nomade et al. (2019).
It is evident from the foregoing that most records of
MIS 19 naturally allow subdivision into two parts, anearlier relatively stable phase representing MIS 19c andoccurring within one precession cycle, and a later phase(the inconsistently applied MIS 19b and MIS 19a) usually featuring three or four millennial-scale isotopicallylight peaks and occurring within a second precession
cycle. There might be merit in dividing MIS 19 into justtwo substages separated by the current MIS 19c/b
boundary as this most reasonably represents the inception of glaciation (Tzedakis et al. 2012a), and indeed
Nomade et al. (2019) considered MIS 19b as the first bipolar seesaw oscillation. However, the tripartite subdivision first introduced by Bassinot et al. (1994) has
become entrenched. The approach used here is therefore
to follow the substage classification of Nomade et al.
(2019) and Haneda et al. (2020b) in which MIS 19b is
restricted to the first interval of high benthic isotopic
values following MIS 19c.
3.2.3 Fine-scale subdivision of MIS 19

Millennial to centennial changes occurring within
MIS 19 include both local and global signals. Recognizing events, for example of warming or drying, andnumbering them consecutively without reference totheir substage is the simplest approach. Sánchez-Goñi
et al. (2016) applied constrained cluster analysis to
subdivide their pollen record of Site U1385 into 20
numbered pollen biozones through MIS 19, and thenused the relative abundances of Mediterranean pollen
taxa to indicate intervals of Mediterranean forest contraction (Fig. 9c). Regattieri et al. (2019) in their study

Head Progress in Earth and Planetary Science(2021) 8:50

Page 14 of 38

Fig. 10 Mediterranean records of Marine Isotope Stage 19. (a) Insolation at Equator in spring and autumn, and absolute maximum of mean irradiance(Laskar et al. 2004; Ferretti et al. 2015; Haneda et al. 2020b). (b) Lacustrine Sulmona basin, central Italy: endogenic calcite δ18O, events I–IX of reduced
precipitation based on δ18O, and tephra beds (positions based on modelled ages with 40Ar/39Ar ages shown separately) used to construct the age
model, all from Regattieri et al. (2019); Younger Dryas-like event (YDt) from Giaccio et al. (2015). (c) Lacustrine Piànico-Sèllere basin, northern Italy:pollen (Nomade et al. 2019). (d) Ideale section, Montalbano Jonico, Italy: benthic foraminiferal δ18O and supporting age control using 40Ar/39Ar-dated
tephra (volcaniclastic) beds V3 and V4 from Nomade et al. (2019). Ghost sapropel assigned to insolation cycle 74, alkenone sea-surface temperature(SST) with Heinrich-type (Med-HTIX) Mediterranean Bølling-Allerød-type (Med-BATIX), and Younger-Dryas-type (Med-YDTIX) episodes associated withTermination IX, and total coccolith abundance and mesothermic arboreal pollen records showing phases I–III of climatic amelioration; from Marino
et al. (2020). (e) ODP Site 976 Alboran Sea, western Mediterranean: planktonic foraminiferal δ18O and pollen (first component of Principal Component
Analysis); light blue triangles show temperate forest contractions within MIS 19c and correspond to increases in total coccolithophore abundance; dark
blue triangle represents a climate event marked by abundant Asteraceae pollen (Toti et al. 2020). The subdivisional scheme of Nomade et al. (2019) is
given. (f) The MIS 19 subdivisional scheme used here (Fig. 7): benthic marine isotope oscillations o1–o4 are labelled in red, interstadials i1–i4 arelabelled in red and orange. All records are plotted on their own published time scales and use the original substage designations

Head Progress in Earth and Planetary Science(2021) 8:50

Page 15 of 38

Fig. 11 Chiba composite section (CbCS), Japan; records of Marine Isotope Stage 19 based on the age model of Suganuma et al. (2018). (a) Insolation
at Equator in spring and autumn, and absolute maximum of mean irradiance (Laskar et al. 2004; Ferretti et al. 2015; Haneda et al. 2020b). (b) Totalorganic carbon wt.% (Izumi et al. 2021). (c) Percentage abundance of Protoceratium reticulatum cysts and total dinoflagellate cyst concentrations
interpreted as a southward shift of the Kuroshio Extension during stadial s1 and early interstadial i1 (Balota et al. 2021). (d) Percentage abundance of
calcareous nannofossil Florisphaera profunda, an indicator of near-surface ocean stratification (Kameo et al. 2020). (e) Profile of ΔT (δ18Obenthic minus
δ18OGlobigerina bulloides) as a measure of the vertical temperature gradient between surface and bottom waters (thin grey line) and 1000-year moving
average (thicker black line) (Haneda et al. 2020b, 2020c). (f) Profile of Δδ18Oinf-bul (δ18OGloborotalia inflata minus δ18OGlobigerina bulloides) as an indicator ofnear-surface ocean stratification (Kubota et al. 2021). (g) Planktonic and benthic foraminiferal δ18O records (Haneda et al. 2020b). (h) The MIS 19
subdivisional scheme used here (Fig. 7): benthic marine isotope oscillations o1–o4 and interstadials i1–i4 are labelled in red, stadials s1–s4 are labelledin blue. (i) Substage classification and interstadial and stadial classification and labelling of Haneda et al. (2020b) where MIO = Millennial Isotopic
Oscillation, showing MIO-Stadial 1 to 4 (MIO-S1 to MIO-S4), and MIO-Interstadial 1 to 4 (MIO-I1 to MIO-I4); YDt = Younger Dryas-type cooling subevent

Head Progress in Earth and Planetary Science(2021) 8:50

of the lacustrine Sulmona basin in Italy recognizednine events of reduced precipitation inferred frommultiple paleoenvironmental proxies. These eventswere labelled I–IX and occur throughout MIS 19(Fig. 10b).
An additional approach is the recognition of numberedstadials and interstadials. These have been used traditionally to describe cooler and warmer episodes within glacial
cycles and are therefore climatic subdivisions. Their use in
the Pleistocene (Penck and Brückner 1909) considerably
predates that of marine isotope stages and substages. The
two systems while obviously complementary are often
based on different criteria. Stadials and interstadials arethen more logically used alongside marine isotope substages than to subdivide them.
The benthic marine isotope record at Montalbano
Jonico, Italy (Fig. 10d) shows at least three sharply defined lighter isotopic phases within MIS 19a, andNomade et al. (2019) defined these as interstadial 19a-1,
19a-2, and 19a-3 (Fig. 10d). These interstadials arenumbered in stratigraphically ascending rather thandescending order, allowing them to be labelled consistently since a “fourth” interstadial at the end of MIS
19a is less pronounced and may not always be recognized. The Montalbano Jonico succession was depositedon the shelf in relatively shallow (~ 100–200 m) waters,
and the benthic isotope record closely resembles otherclimatic proxies (Marino et al. 2020). Hence, in this case,the benthic isotopic record incorporates a localized
climatic signal and serves to indicate stadial and interstadial conditions. Nomade et al. (2019) did not number
adjacent stadials.
Haneda et al. (2020b) extended the scheme of Nomade
et al. (2019) by subdividing the latter half of MIS 19 at
the CbCS in Japan into both stadials and interstadials,
labelling them as MIO-Stadial 1 to 4 (MIO-S1 to MIOS4) and MIO-Interstadial 1 to 4 (MIO-I1 to MIO-I4),
where MIO stands for Millennial Isotopic Oscillation(Fig. 11i). The first stadial was understandably assigned
to the cooling event of MIS 19b that marks the inceptionof glaciation (Tzedakis et al. 2012a). These stadial–interstadial designations are based on the planktonic isotopic
record which is largely a sea-surface temperature signal.
It is essentially a climatic subdivision for which a stadial–interstadial designation is indeed appropriate. However, the MIS stage and substage boundaries at Chiba
are based on the benthic foraminiferal δ18O record following the approach of Lisiecki and Raymo (2005) andRailsback et al. (2015), and consequently may not alignprecisely with the stadial–interstadial boundaries which
are based on surface to near-surface (planktonic) properties. Hence, the first stadial (MIO-Stadial 1) begins justafter the start of MIS 19b, and the first interstadial straddles the MIS 19b–a boundary (Fig. 11i).

Page 16 of 38

The labelling scheme proposed here (Section 3.2.1;
Fig. 7) extends and modifies the schemes of Nomade
et al. (2019) and Haneda et al. (2020b). It treats separately the benthic isotopic record in which as many asfour millennial-scale oscillations (MIS 19a-o1 to MIS
19a-o4) may be discerned, and the planktonic / terrestrial record in which as many as four stadials (MIS 19-s1
to MIS 19-s4) and four interstadials (MIS 19-i1 to MIS
19-i4) may be identified. It resolves the incompatibility
between the benthic isotopic record which may containa strong regional to global signal especially at deepocean sites and upon which MIS substages are often
based, and the planktonic and terrestrial signals thatemphasize more localized climatic variations and permit
the most reliable characterization of stadials and interstadials. This approach can be used alongside informal
climatic schemes that in some cases already facilitate therecognition of stadials and interstadials in the latter partof MIS 19 (e.g. Sánchez-Goñi et al. 2016; Regattieri et al.
2019; Figs. 9c and 10b).
Climatic fluctuations associated with deglaciation
across Termination IX have also been labelled as stadials
and interstadials using terminology developed for thelast deglaciation (Mangerud et al. 1974; Björck et al.
1998). Giaccio et al. (2015) labelled an abrupt cold anddry interval from the Sulmona basin as a Younger
Dryas-like event (Fig. 10b). Maiorano et al. (2016) applied the terms Heinrich-like, Bølling–Allerød-like, andYounger Dryas-like to similar climate oscillations recorded at the Montalbano Jonico section, and this terminology(Med-HTIX,
Med-BATIX,
Med-YDTIX,
referencing Termination IX in the Mediterranean) was
continued by Marino et al. (2020; Fig. 10d). With respect
to the CbCS in Japan, Suganuma et al. (2018) documented a single cooling phase they labelled as a Younger
Dryas-type cooling event, and Haneda et al. (2020b) distinguished two closely separated cooling episodes theylabelled as Younger Dryas-type cooling sub-events 1 and
2, abbreviated to YDt-1 and YDt-2 (Fig. 11i). These are
effectively stadial–interstadial alternations but their precise expression is perhaps too uncertain at present towarrant a standardized terminology.
3.3 Age model calibration of MIS 19 records

Lisiecki (undated) gives the bounding ages for MIS 19 as
790 and 761 ka, based on the Lisiecki and Raymo (2005)
global benthic foraminiferal δ18O stack (LR04) which is
tuned to the insolation curve for 65° N (Laskar et al.
1993). Several studies of MIS 19 have used the LR04 record as the tuning target for their age models, including
Ferretti et al. (2015) for the central North Atlantic Site
U1313 (Fig. 9b) and Sánchez-Goñi et al. (2016) for IODP
Site U1385 off Portugal (Fig. 9c). However, a primary
limitation of LR04 in this regard is its weak expression

Head Progress in Earth and Planetary Science(2021) 8:50

of millennial-scale oscillations that characterize MIS
19b–a (Fig. 13). It should also be noted that the LR04
stack while ostensibly a globally averaged record is infact heavily biased towards the Atlantic and also contains a significant temperature component (Elderfield
et al. 2012). Moreover, Pacific records lag the Atlantic byas much as ~ 4 kyr (Lisiecki and Raymo 2009; see Head2019 for discussion). Suganuma et al. (2018) therefore
used the sea-level proxy curve of ODP Site 1123 locatedon Chatham Ridge in the South Pacific (Elderfield et al.
2012; Fig. 4) as their tuning target for the CbCS in Japan,
although this itself is tuned to the LR04 stack. An updated benthic foraminiferal δ18O data set (Haneda et al.
2020b; Suganuma et al. in press; Fig. 11g) prompted aslight readjustment of the stratigraphic positions of thestage and substage boundaries for the CbCS, as follows:
787.5 ka (MIS 20–19c), 773.9 ka (MIS 19c–19b), 770.1 ka(MIS 19b–19a), and 756.9 ka (MIS 19a–18) (Table 2).
Nonetheless, the original astrochronological age modelof Suganuma et al. (2018) was used in Haneda et al.
(2020b) and Suganuma et al. (in press). This age model
is subject to an uncertainty of about 5 kyr, allowing for
an uncertainty of 4 kyr in the Lisiecki and Raymo (2005)
target curve used for ODP Site 1123 (supplementary material in Elderfield et al. 2012), and an estimated 1 kyr
uncertainty in tuning the Chiba record to the ODP Site
1123 sea-level curve.
In addition to the limitations discussed above ofusing a single global stack such as LR04 as an alignment target, Lisiecki and Stern (2016) cautioned that
the LR04 stack appears to be 1 to 2 kyr too young
throughout the Pleistocene. The radiometric datingof interbedded tephras, correlation to radiometrically
dated regional climatic events in speleothem records,
and for the Mediterranean the use of sapropels andsapropel-like beds, should therefore be incorporated
into age model construction wherever possible.
Varve counting where available is also invaluable forPage 17 of 38

precisely estimating the duration of events within
MIS 19.
Nomade et al. (2019) implemented a hybrid chronology for the Ideal section of Montalbano Jonico insouthern Italy that integrates both astronomical tie
points, including a ghost sapropel tentatively representing insolation cycle 74 (Maiorano et al. 2016; Marino
et al. 2020), and 40Ar/39Ar-dated tephra layers (Fig. 10d).
Rigattieri et al. (2019) for the lacustrine Sulmona succession in central Italy used an age model based exclusively
on 40Ar/39Ar-dated tephra layers, six of these occurring
through an 805–753-ka interval spanning MIS 19(Fig. 10b). The lacustrine deposits at Piànico-Sèllere in
northern Italy allow a floating varve chronology to be
combined with a K/Ar-dated tephra layer (Pinti et al.
2001; Roulleau et al. 2009; Nomade et al. 2019; Fig. 10c).
All these approaches are subject to uncertainties, someof which cannot presently be estimated.
Table 2 shows the age and duration of MIS 19c, 19b,
and 19a for each of the sites discussed using their own
time scales to illustrate the variation recorded, which reflects tuning uncertainties as well as local and regional
influences superimposed on a global ice volume signal.
One example serves to illustrate these uncertainties.
Interstadial i1 (MIS 19a-1 in the scheme of Nomade
et al. 2019) is sharply defined in Italy with its boundingtransitions concluded within 200 years (Fig. 10b–d).
Comparing its duration based on the published timescale of each site, it is ~ 4.3 kyr at Sulmona (Regattieri
et al. 2019, fig. 5), 3.2 kyr at Montalbano Jonico(Nomade et al. 2019), and 4.9 kyr at Piànico-Sèllere(Nomade et al. 2019). As noted by Nomade et al. (2019),
these discrepancies illustrate some of the challenges inrefining the age model for the latter part of MIS 19.
Other sites have been studied at lower stratigraphic
resolution through MIS 19. Lake Baikal in southeastern
Siberia (~ 53°N, Figs. 4 and 12d) represents an area withthe highest sensitivity to insolation forcing on Earth,

Table 2 Extent and duration of MIS 19 and its substages and of full interglacial conditions. Ages and durations (in ka/kyr) are basedon the age models published for each site
AuthorLocation

Lisiecki (undated)

LR04 global stackTzedakis et al. (2012a)

ODP 983

Ferretti et al. (2015)

IODP U1313

Sánchez-Goñi et al. (2016)

U1385

MIS19
duration

MIS19a
ends

29

761

MIS19bends

MIS19cends

MIS19cduration

MS19cbeginsFull Interglacial
ends

duration

begins~ 777.5

12.5–10.5

788

~12.5

787.5

790

~ 779
33

758

765

774

17

791

775

Nomade et al. (2019)

Ideale section MBJ

30.5

757.0

772.7

773.9

13.7 ± 5

787.6

773.8

11.5 ± 3.4

785.3

Regattieri et al. (2019, fig. 9)

Sulmona

31

760

765

777

14

791

777

11

788

Haneda et al. (2020b)

Chiba

28.1

756.9

770.1

773.9

11.1

787.5

Toti et al. (2020)

ODP 976

775.1

9.9

785.0

~ 774

~8

~ 782

Head Progress in Earth and Planetary Science(2021) 8:50

Page 18 of 38

Fig. 12 Selected Asian records of Marine Isotope Stage 19. (a) Insolation at 65° N in June, and precession and obliquity parameters. (b) Insolation
at Equator in spring and autumn, and absolute maximum of mean irradiance (Laskar et al. 2004; Ferretti et al. 2015; Haneda et al. 2020b). (c) Lake
El’gygytgyn, northeast Siberia: XRF core scanning-derived Si/Ti ratio (Wennrich et al. 2014). (d) Lake Baikal, southern Siberia: Biogenic silica
contents (Prokopenko et al. 2006). (e) Normalized Yimaguan and Luochuan (China) stacked loess–palaeosol proxy records for East Asian Summer
Monsoon (EASM; frequency-dependent magnetic susceptibility, orange line) and East Asian Winter Monsoon (EAWM; > 32 μm particle content,
blue line) (suppl. fig. 12 of Hao et al., 2012). (f) The MIS 19 subdivisional scheme used here (Fig. 7): interstadials i1 and i2 are labelled in red. All
records are plotted on their own published time scales

owing to its central position within Asia. By correlating
biogenic silica peaks, representing lake productivity maxima, with precessional cycles (Laskar et al. 2004), an
astronomically tuned composite record of the biogenic
silica was obtained over the entire Pleistocene. Magnetostratigraphic boundaries enabled the cross-checking of
this chronology (Prokopenko et al. 2006). The Holocene–Pliocene record of Lake El’gygytgyn (67° 30′ N,
172° 05′ E; Figs. 4 and 12c), located in the Far East Russian Arctic, was dated using a combination of magnetostratigraphic reversals and palaeoclimatic records tuned
to summer insolation at 65° N (Laskar et al. 2004) andto the Lisiecki and Raymo (2005) LR04 global stack(Nowaczyk et al. 2013). It is worth reiterating here concerns about using the LR04 global stack for this kind oftuning (Lisiecki and Stern, 2016).

3.4 The climatic evolution of MIS 19

MIS 19 has been studied intensively owing to the close
similarity between its substage c and the present interglacial with respect to orbital configuration, rapid deglaciation history, and early peak Antarctic temperatures(Tzedakis et al. 2012b; Fig. 13). This similarity allows unambiguous alignment of MIS 19c with the present interglacial, thereby offering insights into our future climate(Tzedakis et al. 2012b).
Earlier overviews of the climatic evolution of MIS 19
are given by Tzedakis et al. (2012a, b) and most recently
with inter-site comparisons by Suganuma et al. (2018, inpress), Nomade et al. (2019), Regattieri et al. (2019), andHaneda et al. (2020b). Sites yielding highly resolved
paleoclimatic records are listed in Table 3 and show aconcentration of sites in the northern hemisphere. The

Head Progress in Earth and Planetary Science(2021) 8:50

Page 19 of 38

Fig. 13 Orbital and climatic characteristics of MIS 19 (blue) and the present interglacial MIS 1 (red) compared. (a–c) Orbital configurations and (b)insolation for MIS 19 and past and future MIS 1 (Laskar et al. 2004). (d) δD composition of ice in the EPICA Dome C ice core as a proxy for
Antarctic surface temperature (Jouzel et al. 2007). (e) Antarctic ice-core CO2 values from the 0–800 kyr BP composite record of Bereiter et al.
(2015, supplementary data) and show MIS 1 and MIS 19 in close alignment through the Early Holocene. (f) LR04 benthic stack of Lisiecki andRaymo (2005) showing lighter values for MIS 1, beginning ~ 11,000 years before present. Adapted from fig. 3 of Tzedakis et al. (2012b)

modelling study of Vavrus et al. (2018) adds spatial detail to this picture.
MIS 19 compares with both MIS 11 and the presentinterglacial in having a reduced-amplitude 400 ky eccentricity cycle and consequent suppression of precessional
forcing (Fig. 2). Precession is in phase for all three interglacials. However, whereas the obliquity peak closelyaligns with the precession minimum for both MIS 19c
and the present interglacial, it leads the precession minimum by about 9 kyr in MIS 11. As a result, June insolation at 65° N increases more slowly for MIS 11 than for
MIS 19c and the present interglacial (fig. 6 of Tzedakis2010). MIS 19c is therefore the closest orbital analogue
for the present interglacial and even though the amplitude of obliquity is lower for MIS 19c the alignment of
their onset is unambiguous (Tzedakis et al. 2012b). This
close similarity will begin to diverge in the future, as theamplitude of precession will decline more strongly thanfor late MIS 19, and June insolation at 65° N will belower (Fig. 13).

Although the phasing between precession and obliquity are closely similar for MIS 19c and the presentinterglacial, with the obliquity maximum close to theprecession minimum, obliquity during MIS 19c increases
less rapidly and hence to a lower amplitude than during
the beginning of the present interglacial. Moreover, theLR04 foraminiferal isotopic record shows lighter peakvalues for MIS 1 (figs. 6 and 7 of Tzedakis 2010; Fig. 13)
and agrees with observations from the CbCS that temperatures were cooler during MIS 19 than today (Suganuma et al. 2018). Ganopolski et al. (2016) proposedthat higher CO2 levels of around 280 ppm during the
pre-industrial Late Holocene explain this temperature
difference, and Studer et al. (2018) discussed reasons forthe exceptional rise in CO2 from 8 ka (Middle Holocene)
onwards (Fig. 13). However, for the Early Holocene, CO2
levels reached a maximum of only 270 ppm which isvery close to the 269 ppm maximum for MIS 19 basedon corrected CO2 records for the EPICA Dome C core
(Bereiter et al. 2015). Indeed, Early Holocene CO2 levels

Head Progress in Earth and Planetary Science(2021) 8:50

Page 20 of 38

Table 3 Sites from which highly resolved paleoclimatic records of MIS 19 are available
Site

Location

Latitude, longitude

Deposition

Selected publications

ODP Site 983

Gardar Drift, North Atlantic

60.4° N, 23.6° W

MarineKleiven et al. (2011), Tzedakis et al. (2012b)

IODP Site U1313

Central North Atlantic

41° 00′ N, 32° 58′ W

MarineFerretti et al. (2015), Emanuele et al. (2015)

IODP Site U1385

Eastern North Atlantic off
SW Portugal

37° 34.285′ N, 10° 7.562′ W

MarineSánchez-Goñi et al. (2016)

ODP Site 976

Alboran Sea, western
Mediterranean

36° 12.3′ N, 4° 18.8′ W

MarineToti et al. (2020)

Valle di Manche

Calabria, southern Italy

39° 05′ 39″ N, 16° 55′ 13″ E

MarineCapraro et al. (2004, 2005, 2019),
Azzarone et al. (2018),
Macrì et al. (2018), Rossi et al. (2018)

Montalbano Jonico

Basilicata, southern Italy

40° 17′ 30″ N, 16° 33′ 11″ E

MarineBertini et al. (2015), Marino et al. (2015, 2020),
Maiorano et al. (2016), Simon et al. (2017),
Nomade et al. (2019)’

Sulmona basin

Central Italy

42° 09′ 07″ N, 13° 49′ 15″ E

Lacustrine

Giaccio et al. (2013, 2015),
Regattieri et al. (2019)

Piànico-Sèllere basina

Northern Italy

45° 48′ N, 10° 02′ E

Lacustrine

Moscariello et al. (2000),Scardia and Muttoni (2009),
Roulleau et al. (2009)

Chiba composite section

East-central Japan35° 16′ 51″ N, 140° 07′ 28″ E to
35° 18′ 38″ N, 140° 11′ 50″ E

MarineSuganuma et al. (2015, 2018, in press),
Haneda et al. (2020b), Kameo et al. (2020),
Balota et al. (2021), Izumi et al. (2021),
Kubota et al. (2021)

EPICA Dome C

Antarctica

75° 06′ S, 123° 21′ E

Ice sheet

Jouzel et al. (2007), Bazin et al. (2013),
Bereiter et al. (2015)a

The Piànico-Sèllere site was assigned to MIS 19 only in 2001 (Pinti et al. 2001, 2007), having previously been considered much younger

track those of MIS 19 closely and do not provide an explanation for a pronounced trend to lighter values seenin the δ18O stack of Lisieck and Raymo (2005) (Fig. 13).
MIS 19c therefore presents a good analogue at least for
the Early Holocene based on CO2 as well as orbital criteria. It should be noted that residual global ice volume
might have been higher during MIS 19 than MIS 1(Elderfield et al. 2012; Regattieri et al. 2019; Vavrus et al.
2018). This may have increased climate sensitivity during
MIS 19 given the nonlinear relationship between astronomical forcing and ice volume during the Quaternary(Past Interglacials Working Group of PAGES 2016) andthe fact that polar ice volume provides one of the mostimportant feedback mechanisms in the climate responseto radiative forcing (Berger et al. 2017; Westerhold et al.
2020).
3.4.1 MIS 20–19 transition

There is widespread evidence of climatic and oceanographic instability during late MIS 20 and across Termination IX. A Younger Dryas-type cooling event
interrupts the deglaciation of Termination IX in several
records, notably at Montalbano Jonico (Maiorano et al.
2016; Simon et al. 2017; Marino et al. 2020) and Sulmona (Giaccio et al. 2015; Regattieri et al. 2019) in Italy,
and the CbCS in Japan (Suganuma et al. 2018; Haneda
et al. 2020b), and is dated at around 785–790 ka. A similar cooling event is also recorded in the Lake Baikal

succession in Russia (Prokopenko et al. 2006; Fig. 12d)and elsewhere. At the CbCS, the dinoflagellate cyst record registers a rapid retreat of the Subpolar Front at
789.3 ka, indicating warming around 2000 years before
the end of MIS 20 (Balota et al. 2021). At IODP Site
U1313 in the central North Atlantic, the calcareous
nannofossil record indicates a return of the North Atlantic Current to this site at 793 ka in association with Termination IX (Emanuele et al. 2015).
At Montalbano Jonico, a cool–warm–cool succession begins with a stadial, labelled a Heinrich-like episode, near the end of MIS 20 (Med-HTIX of Marino
et al. 2020; Fig. 10d) that may be coherent with afreshwater pulse at IODP Site U1385 off SW Portugal(Sánchez-Goñi et al. 2016; Fig. 9c). This suggests a
direct Mediterranean response to climate dynamics in
the high-latitude North Atlantic (Marino et al. 2020).
Cooling at Montalbano Jonico is followed by a
Bølling-Allerød-like warming phase (Med-BATIX) andthen Younger Dryas-like cooling (Med-YDTIX) prior
to final rapid warming early in MIS 19c. This same
succession is observed at Sulmona (Regattieri et al.
2019; Fig. 10b) and in Core KC01B in the central
Mediterranean Sea (Trotta et al. 2019; Marino et al.
2020; Fig. 4), suggesting a pattern of oscillations thatis at least regional in extent.
A subsequent brief climatic reversal at ~ 785 ka observed in the pollen record of ODP Site 976 in the

Head Progress in Earth and Planetary Science(2021) 8:50

western Mediterranean (Toti et al. 2020; Fig. 10e) and acooling event documented at IODP Site U1385 off
Portugal (Sánchez-Goñi et al. 2016; Fig. 9c) both occur
just before the plateau of lightest isotope values for MIS
19c and may reflect the same event.
A combination of obliquity phasing with low precessional forcing amplitude may have been a precondition for the instability seen across the MIS 20–19
transition (Suganuma et al. 2018). The actual trigger
likely reflects a short-term disruption of the Atlantic
meridional overturning circulation (AMOC) andwould have connected to the Pacific through shifts in
the Intertropical Convergence Zone (ITCZ) (Haneda
et al. 2020b; Fig. 4). The presence of two closely separated Younger Dryas-type subevents at the CbCS
(Haneda et al. 2020b; Fig. 11i) attests to the complexity of processes in operation at the global scale.
3.4.2 MIS 19c

MIS 19c has among the lightest isotopic values andspans full interglacial conditions, the duration of whichholds considerable interest in assessing the naturallength of our own interglacial. MIS 19c extends fromaround 791–787.5 ka to the expansion of ice sheets (glacial inception) at 774–777 ka and represents a morestable episode mostly coinciding with full interglacialconditions (Table 2).
In spite of weak eccentricity at this time, planktonic
and benthic foraminiferal isotope data for central North
Atlantic Site U1313 (Fig. 9b) reflect variation concentrated in the half-precession bandwidth (~ 11 kyr), andalso quarter-precession bandwidth for the benthic foraminiferal isotope data, indicating low-latitude insolation
forcing particularly when the amplitude of precession isat its greatest, which is during MIS 19c (Ferretti et al.
2015). The second harmonic of precession occurs when
the perihelion of Earth’s orbit coincides with the springor autumn equinox (fig. 10 of Ferretti et al. 2015; Fig. 9a).
Dinoflagellate cyst analysis indicates peak interglacialconditions between 790.5 and 784.0 during which time
Site U1313 was fully under the influence of the subtropical gyre (Abomriga 2018). Coccolithophore assemblages
reflect modern-type warmer North Atlantic Transitional
Waters between ~ 788 and 782 ka, with glacial inception
occurring at ~ 779 ka (Ferretti et al. 2015; Emanuele
et al. 2015). Glacial inception appears to have occurred
quite early but with enhanced iceberg discharges from ~776 ka onwards suggesting multiple ice-sheet calving
events (Ferretti et al. 2015).
IODP Site U1385 off southwestern Portugal (SánchezGoñi et al. 2016; Fig. 9c) has a detailed pollen record ofvegetation development across MIS 19. Full interglacialconditions (the Tajo interglacial) as determined by thispollen record extend from 787.5 to 775 ka. MIS 19c

Page 21 of 38

extends to stadial s1 (Fig. 9c) as identified by correlating
to the Sulmona basin record (Regattieri et al., 2019; fig. 9i
in Nomade et al. 2019; Fig. 10b). Significantly, Mediterranean forest growth during MIS 19c was interrupted bythree pronounced contractions (fig. 5 of Sánchez-Goñi
et al. 2016; Fig. 9c) that represent cooling and drying
events not significantly reflected in the proxies for seasurface freshwater input or temperature, which are relatively stable and warm through MIS 19c. This decoupling of terrestrial and marine climate through MIS 19c
implies that the westerlies supplying moisture to southern Portugal through most of MIS 19c were periodicallydiverted northwards along with moisture that would
have contributed to the growth of high-latitude ice
sheets. These were expanding progressively through MIS
19c, aided by decreasing boreal summer insolation.
These forest contraction events occur throughout MIS
19 with a 5 kyr periodicity and appear to represent a response to the fourth harmonic of precession, implying aswith central North Atlantic Site U1313 the influence of
low-latitude insolation forcing at this location. These decoupling events are not restricted to MIS 19 as theyoccur at other times during the Quaternary along theEuropean margin (Sánchez Goñi et al., 2018).
Within the Mediterranean region, ODP Site 976 in the
Alboran Sea (Fig. 4) is influenced directly by Atlantic inflow and has yielded detailed pollen, coccolithophore, foraminiferal assemblage, and planktonic foraminiferal
δ18O records through MIS 19 (Toti et al. 2020; Fig. 10e).
Millennial-scale climate oscillations occur throughout
MIS 19c and are registered synchronously in both marine and terrestrial proxies. Brief episodes of forest contraction centred at 781, 780, 777, and 775 ka throughMIS 19c attest to short-term climate fluctuations (lesshumid winter conditions) presumably caused by theperiodic northward deflection of westerlies as proposedby Sánchez-Goñi et al. (2016) for Site U1385 off
Portugal. Similar fluctuations are observed in the Sulmona succession in Italy, suggesting that the entire western Mediterranean was affected by precession-driven
episodes of drought through MIS 19c (Toti et al. 2020,and see below).
In Italy, the marine Montalbano Jonico composite section and the lacustrine Sulmona basin succession haveprovided exceptionally detailed and well constrained records through MIS 19. Montalbano Jonico includes foraminiferal isotopes, various marine proxies, and pollen(Bertini et al. 2015; Nomade et al. 2019; Marino et al.
2020; Fig. 10d). The onset of full interglacial conditions
shortly after the start of MIS 19c is marked by a faintsapropel-like feature (“ghost sapropel”) assigned tentatively to i-cycle 74 by Maiorano et al. (2016) which has a
midpoint age of 784 ka based on a phase lag of 3 kyr
relative to maximum insolation in June at 65° N

Head Progress in Earth and Planetary Science(2021) 8:50

(Lourens 2004). Emeis et al. (2000) assigned a comparable red horizon from the eastern Mediterranean to icycle 74 although this might represent an obliquity maximum rather than a precession minimum (Konijnendijk
et al. 2014), although the two are in phase for MIS 19c.
This recalls the often overlooked influence of highlatitude obliquity on Mediterranean climate (Konijnendijk et al. 2014) even in the absence of high-latitude ice
sheet dynamics (Bosmans et al. 2015). The ghost sapropel at Montalbano Jonico lasted for about 2.5 kyr, andrepresents an interval of water-column stratification
resulting from freshwater inflow related to strengthened
North African summer monsoon conditions during theinsolation maximum (Nomade et al. 2019; Marino et al.
2020). The presence at ODP Site 975 in the western
Mediterranean (Fig. 4) of an organic-rich layer within
MIS 19c suggests this to be a basin-wide phenomenon(Quivelli et al. 2020).
Pollen records from the Ideale section at Montalbano
Jonico evidence an interval of full interglacial conditions(climatic optimum) marked by the expansion of temperate forests. These forests were dominated by broadleaved trees and indicate a warm and relatively humid
climate. This climatic optimum extends from thesapropel-like layer to the top of MIS 19c at ~ 774 ka,
with a duration of 11.5 ± 3.4 kyr (Nomade et al. 2019;
Fig. 10d). Three mesothermic forest expansions are recognized and are almost concurrent with higher seasurface temperature phases as reflected by alkenone SST
reconstructions and the abundance of calcareous nannofossils. These phases are labelled I, II, and III in Marino
et al. (2020; Fig. 10d). Spectral analysis also shows climate oscillations occurring with a periodicity of about
5.4 kyr throughout MIS 19 although these are dampened
in MIS 19c (Nomade et al. 2019). The increasing benthic
foraminiferal isotope values towards the end of MIS 19c
suggest a strong global component in spite of the shallow (not more than 180–200 m deep) marine setting.
The benthic foraminiferal record registers a maximum
depth between ~ 778.1 and 773.4 ka which spans the774-ka timing of maximum global sea level given by
Elderfield et al. (2012).
The lacustrine Sulmona basin record (Regattieri et al.
2019; Fig. 10b) is just 295 km to the northwest of Montalbano Jonico (Nomade et al. 2019) and is based on a40
Ar/39Ar timescale fully independent of orbital tuning
and with a mean uncertainty of ± 2.6 kyr. A duration of
11 kyr for full interglacial conditions agrees with the11.5 ± 3.4 kyr duration recorded for the Ideale section atMontalbano Jonico. The stable isotope record at Sulmona has a temporal resolution of ~ 60 years, allowing
MIS 19c to be analysed in exceptional detail. Rapidly increasing precipitation after 788 ka, reflecting deglaciation, reached a peak at ~ 786 ka but was interrupted byPage 22 of 38

a prominent 0.8 kyr-long drier interval starting at 785 ka(event I of Regattieri et al. 2019) and speculated to beanalagous with the 8.2 ka event in the Holocene. This isfollowed by additional events of increasing dryness.
Regattieri et al. (2019) linked event I with a reduction in
deep-water ventilation at ODP Site 983 (Fig. 8c), implying a connection with a brief interruption of the AMOC.
These authors therefore proposed that these drying
events in MIS 19c were causally linked to deep hydrography in the northern North Atlantic although were notable to tie them specifically to precession forcing.
A detailed pollen record from the CbCS in Japanshows a well-defined rise and fall in deciduous broadleaved trees between 785.0 and 775.1 ka (pollen subzone
CbCS-2a in fig. 7 of Suganuma et al. 2018), suggesting
9.9 kyr for the duration of full interglacial conditions at
this site. The benthic foraminiferal δ18O record (Fig. 11g)
shows a steep rise to lighter values at the beginning of
MIS 19c and a more gradual decline towards the end,
with otherwise relatively little variation (Haneda et al.
2020b; Suganuma et al. in press). In contrast, G. bulloides (planktonic) foraminiferal δ18O values show considerable fluctuations (Fig. 11g), and a high-resolution
study of the dinoflagellate cysts across MIS 19c reveals
instability and latitudinal shifts in the Kuroshio Extension system at this site (Balota et al. 2021). This instability presumably reflects the close proximity of the CbCS
to the convergence of the warm Kuroshio and cold Oyashio currents that forms an extreme hydrographic gradient in this part of the western North Pacific (Kubota
et al. 2021). A spectral and wavelet analysis of the planktonic foraminiferal δ18O record and an index for watercolumn stratification reveal 9.6 kyr cycles throughout
MIS 19 but expressed particularly strongly through MIS
19c (Haneda et al. 2020b). This is interpreted as the second harmonic of precession. As with North Atlantic Site
U1313 (Ferretti et al. 2015), it seems related to equatorial insolation forcing and similarly appears to have been
greatest during MIS 19c when precession at the Equator
was at its highest amplitude (Fig. 11a).
3.4.3 MIS 19b and a

A critical feature of MIS 19b and 19a, together representing approximately the second half of MIS 19, is theestablishment of three or four stadial/interstadial alternations broadly coinciding with a second precessional
minimum that is in antiphase with obliquity, resulting ina damped insolation peak (Fig. 7a). This is transposed
onto a longer-term trend of increasing global ice volume. Several mechanisms have been proposed for thebistability in MIS 19b–a.
Tzedakis et al. (2012b) posited that the termination offull interglacial conditions at the end of MIS 19c would
have coincided with the expansion of ice sheets leading

Head Progress in Earth and Planetary Science(2021) 8:50

to iceberg discharges into the North Atlantic and hence
disruption of the AMOC. This in turn will have led, aftersome delay, to warming over Antarctica by means of thethermal bipolar seesaw mechanism (Stocker and Johnsen
2003). This happens when heat normally transported
northward into the North Atlantic Ocean instead accumulates to the south in the global interior ocean, resulting after a short delay in its advection southwards into
the Southern Ocean (Pedro et al. 2018). From here it
then causes warming over Antarctica. Our understanding of the thermal bipolar seesaw derives from the studyof Dansgaard–Oerschger events during the last glaciation (and their Holocene equivalents called Bond cycles) in Greenland ice cores and their antiphaserelationship with their equivalent Antarctic Isotope
Maxima (AIMs) in Antarctic ice cores. While this teleconnection is well documented, the mechanism controlling the frequency of Dansgaard–Oeschger events is notknown. Tzedakis et al. (2012b) examined published records of MIS 19 from ODP Site 983 on the Gardar Drift,
northern North Atlantic (Channell and Kleiven 2000;
Kleiven et al. 2011; Fig. 8c). They noted that for the MIS
19b–a interval, three minima in the planktonic foraminiferal δ18O records match temperature peaks in the
Antarctic ice-core record and that minima in the planktonic and benthic foraminiferal δ18O records are phaseshifted in a manner that invokes the thermal seesaw.
Moreover, peaks of ice-rafted debris at ODP Site 983 occurring after the end of MIS 19c (Fig. 8c) seem to support the contention that ice-sheet calving had triggered
AMOC disruption, leading to the proposal that thermal
seesaw bistability caused the pronounced minima in thesecond half of MIS 19 at ODP Site 983. Further support
for this mechanism comes from the sortable silt andbenthic δ13C records at Site 983 (Fig. 8c) that point to a
slowdown of North Atlantic Deepwater Formation
which is an essential component of the AMOC (Kleiven
et al. 2011). A characteristic feature of the planktonic foraminiferal δ18O record is the rectangular-shaped waveform that seems to result from abrupt ice-sheet calving
followed by the rapid return to strength of the AMOC.
This contrasts with the much slower response of Antarctica via AMOC perturbations and the bipolar seesaw
which translates via deep-water advection to a time
lagged and v-shaped corresponding benthic record(Shackleton et al. 2000; Tzedakis et al. 2012b).
Ferretti et al. (2015) in their analysis of foraminiferal
isotopes (Fig. 9b) and alkenones from IODP U1313 in
the central North Atlantic found strong variability at frequencies of ~ 11 kyr, in both surface and deep-water records, and 5.8 kyr in the benthic oxygen signal
suggesting forcing mediated by the second and fourthharmonics of precession. Because the harmonics of precession are important features of insolation in the

Page 23 of 38

tropics, resulting in two insolation peaks for every precessional cycle (Fig. 9a), this implies that low-latitudeastronomical forcing and other processes are importantdrivers of millennial-scale climate variability even at atime when the effects of precession on insolation are
subdued (Ferretti et al. 2015).
At IODP Site U1385 off southwestern Portugal, a forest contraction beginning at 775 ka marks the end of the
Tajo interglacial (Sánchez-Goñi et al. 2016; Fig. 9c). Glacial inception terminating MIS 19c is marked by a significant marine cooling event at 769 ka on the time scaleof Sánchez-Goñi et al. (2016), based on correlation with
the independently dated Sulmona basin record in Italy
by Regatierri et al. (2019). This event, which also represents a forest contraction, is dated at ~ 772 ka on the
Sulmona time scale. It represents MIS 19b and is labelled as stadial s1 on Fig. 9c. This stadial is followed bya further two. All three stadials represent forest contractions and reflect cooling and drying episodes as withMIS 19c but are accompanied by heavier planktonic andbenthic foraminiferal δ18O values. Moreover, alkenone
results indicate both cooling and freshening of the surface waters (Fig. 9c) and may align with three IRD
peaks at ODP Site 983 on the Gardar Drift in the subpolar North Atlantic (Fig. 8c). With the expansion ofice sheets, a threshold must have been crossed allowing the triggering of successive iceberg discharges andaccompanying freshening and cooling of the surface
waters off Portugal (Sánchez-Goñi et al. 2016). Spectral
analysis of records from Site U1385 reveals ~ 5 kyr
periodicity throughout MIS 19, suggesting the fourthharmonic of precession and hence the influence ofequatorial insolation. The relatively high ice-volume
baseline conditions for MIS 19 may have increased thesensitivity of this interglacial to high-frequency climate
oscillations. Sánchez-Goñi et al. (2016) contrasted thedominant ~ 5 kyr cyclicity in global records of MIS 19
with the dominant 2.5 kyr cyclicity extracted from theMediterranean forest pollen record of MIS 1 (SánchezGoñi et al. 2016, fig. 8a), calling into question some of
the accepted similarities between the two marine isotope stages.
At Montalbano Jonico, MIS 19c ends at ~ 774 ka andis succeeded by four abrupt oscillations (o1–o4) that define two stable states between higher and lower benthic
foraminiferal δ18O values (Nomade et al. 2019; Fig. 10d).
The transitions from one state to the other took lessthan 200 years. These oscillations almost precisely coincide with alkenone SST reconstructions and are supported by simultaneous expansions and contractions of
the mesothermic forest (warm/wet versus cool/dry climates), allowing the recognition of three interstadials in
Nomade et al. (2019) and Marino et al. (2020) and four
here (i1–i4; Fig. 10d).

Head Progress in Earth and Planetary Science(2021) 8:50

These interstadials and their intervening stadials aresuperimposed on a longer-term trend of dryer andcooler climates through the latter part of MIS 19. Theyarise and decline with the same distinctive rapidity observed in the high-resolution records at Sulmona andPiànico-Sèllere, also in Italy. They occur within a single
precession cycle and are the amplified part of a 5–6 kyr
cyclicity detectable throughout MIS 19. Nomade et al.
(2019) emphasized that while a direct linkage of theseoscillations to Northern Hemisphere ice sheet dynamics
and North Atlantic IRD events is clear, local factors arealso needed to explain the large amplitude of these oscillations and their abrupt nature. The influence of the African monsoon on Mediterranean climate is already wellillustrated by the development of sapropels during precession minima (insolation maxima). Nomade et al.
(2019) therefore raised the possibility that interstadials
might reflect oscillatory northward shifts of the ITCZ
(Fig. 4) over the Mediterranean region. This would havebrought increased summer moisture and temperatureduring the African monsoon. Two lines of evidence support this connection. Firstly, the interstadials, along withMIS 19c, are represented by dark grey silty clays,
whereas the intervening stadials and MIS 19b are lightgrey. The darker clays have more negative δ13C values
and indicate reduced water-column ventilation which
may be explained by freshwater input through monsoon
rains in the same way that sapropels are formed. As
noted by Nomade et al. (2019), the three most pronounced interstadials closely match the three methane
peaks in the EPICA Dome C ice-core record. The West
African monsoon has a major effect on global methane
production (Kleinen et al. 2020) and provides a potential
link between the Mediterranean interstadials and Antarctic ice-core methane during MIS 19a. Nomade et al.
(2019) therefore suggested that the wet/warm oscillations found at Montalbano Jonico but also at Sulmonaand Piànico-Sèllere correspond to worldwide climatic
phenomena associated with the tropical monsoon regime modulated by latitudinal shifts in the ITCZ.
In the Sulmona basin record (Giaccio et al. 2013,2015; Regattieri et al. 2019), at least three interstadials
can be recognized within the second half of MIS 19, andthese clearly correlate to interstadials i1, i2, and i3 at
Montalbano Jonico, with a fourth possibly also
recognizable at Sulmona (Fig. 10b). Regattieri et al.
(2019) noted the concordance between their reducedprecipitation events VII, IIX [sic], and IX (stadials s1, s2,
and s3) and the subpolar record of IRD and linked these
events to disruptions of the AMOC. Hence, whereas
Nomade et al. (2019) proposed northward shifts in the
ITCZ and increased influence of the monsoon to explain
millennial-scale shifts in climate at Montalbano Jonico,Regattieri et al. (2019) invoked the direct influence ofPage 24 of 38

AMOC weakening on the Sulmona record, noting numerous examples of high-latitude forcing on the Mediterranean climate during the Quaternary. The influenceof the African monsoon on Mediterranean hydrography
nonetheless remains uncontestable.
ODP Site 976 in the Alboran Sea (Toti et al. 2020;
Fig. 4) is situated close to the Strait of Gibralter and isstrongly influenced by hydrographic exchanges with theNorth Atlantic. Its pattern of stadial–interstadial alternations compares closely with that recorded in other Mediterranean sites (Fig. 10) and with Site U1385 offsouthwestern Portugal (Fig. 9c). Cyclic northern shifts of
the ITCZ explain the increased winter precipitationneeded for the expansion of Mediterranean forests during interstadials (Nomade et al. 2019; Toti et al. 2020).
These northern shifts also facilitated the inflow of warm
waters from the Azores Current into the Alboran Sea, as
evidenced by increases in the abundances of warm watercoccolithophore and foraminiferal taxa. Stadials conversely are characterized by increases in polar to subpolar foraminifera indicating the inflow of reduced-salinity
subpolar waters from the North Atlantic. ODP Site 976
has therefore recorded the combined influences of North
Atlantic inflow and atmospheric processes throughout
MIS 19.
Sites located beyond the direct influence of the North
Atlantic circulation and the AMOC are crucial in determining additional factors that might have driven the development of MIS 19, especially the stadial/interstadial
oscillations during MIS 19b–a. The CbCS provides themost detailed MIS 19 record in the Pacific realm (Suganuma et al. 2018, in press; Haneda et al. 2020b; Kameo
et al. 2020; Izumi et al. 2021, Balota et al. 2021; Kubota
et al. 2021; Fig. 11). Here, climatic oscillations characterizing MIS 19b–a are well defined and reveal harmonics
of precession that point to low-latitude forcing. Latitudinal shifts of the ITCZ (Haneda et al. 2020b) along withfluctuations in the Siberian High–Aleutian Low atmospheric system that controls the East Asian winter monsoon and Westerly Jet (Suganuma et al. 2018; Kubota
et al. 2021) seem to have been driving factors. Oscillations of the ITCZ in particular would explain the similar
stadial–interstadial pattern recorded at the CbCS andthe North Atlantic–Mediterranean sites (Haneda et al.
2020b). The CbCS is discussed in detail in the next
section.
Further evidence for climatic oscillations in the latterpart of MIS 19 is found in the higher-latitude biogenic
silica records of Lake Baikal (Prokopenko et al. 2006)
and Si/Ti records of Lake El’gygytgyn (Wennrich et al.
2014). These records reflect elevated diatom production
during the spring–fall and show pronounced interstadials i1 and i2 (Fig. 12c, d), recalling similar oscillations
from the CbCS, North Atlantic Ocean, Mediterranean

Head Progress in Earth and Planetary Science(2021) 8:50

region, and Antarctic ice-core records. The oscillationsat Lake Baikal are paced by the harmonics of precession,
the influence of precession reaching Lake Baikal at 51°–53° N (although declining with increasing latitude; Prokopenko et al. 2006). The pronounced oscillations in
MIS 19b–a may represent amplifications connected to
global ice volume that had been increasing since the latter part of MIS 19c.
The lake El’gygytgyn record of northeastern Siberia(Wennrich et al. 2014; Fig. 12c), while showing climate
oscillations similar to those at Chiba during MIS 19b–a,should have been influenced directly by the northern Siberia ice sheet at this time (Vavrus et al. 2018). As with
the IRD record of ODP Site 983 just south of Iceland(Fig. 8c), the mechanisms driving these high-latitude oscillations are not well understood but appear phaselinked across the northern hemisphere and suggest highlatitude atmospheric teleconnections, as discussed in
Section 4.2 below.

4 The Chiba composite section and GSSPThe Chiba section, located in the central part of the
Boso Peninsula and within the Chiba Prefecture, contains the GSSP (35° 17′ 39.6″ N, 140° 08′ 47.6″ E) for
the Middle Pleistocene Subseries/Subepoch and Chibanian Stage/Age (Fig. 14). The Chiba section is a segmentof the Yoro River section which itself is one of five outcrops that comprise the CbCS (west to east): the Urajiro,
Yanagawa, Yoro River, Yoro-Tabuchi, and Kokusabata
sections (Table 1 of Haneda et al. 2020a; Suganuma
et al. in press). A borehole, TB-2, near the Yoro-Tabuchi
outcrop and 190 m northeast of the Chiba section(Hyodo et al. 2016, 2017) contributes to this composite
section. Collectively, these sections span a distance of
7.4 km along strike and are stratigraphically linked by aseries of tephra beds (Kazaoka et al. 2015). The GSSP is
located at the base of the Byk-E tephra bed (Fig. 14d), a
conspicuous regional marker 1 to 3 cm thick in the
Chiba section (Nishida et al. 2016) that has been correlated with the YUT5 bed erupted from the Older Ontake
volcano in the central part of Honshu approximately
250 km to the west (Takeshita et al. 2016).
4.1 Geological background

The Chiba section exposes the middle of the Kokumoto Formation which itself is within the middle
part of the Kazusa Group. The Kazusa Group is approximately 3000 m in thickness and represents theLower and Middle Pleistocene infill of a forearc
basin, the Kanto Tectonic Basin (Fig. 14b), resulting
from the west–northwestward subduction of the
Pacific plate beneath the Eurasian plate at the Izu–
Bonin trench (Ito and Katsura 1992). Uplift began
about one million years ago, resulting in the deeply

Page 25 of 38

incised gorges that characterize the Boso Peninsula.
Interbedded sandstones and siltstones dominate thelithology, with the sandstones typically being turbiditic. The depositional environment has been variouslyinterpreted as basin plain, lower fan and base ofslope in the lower part of the Kazusa Group(Fig. 14b) but gradually shallowing upwards to upper
slope and shelf environments at the top (Kazaoka
et al. 2015). Deep-water massive sandstones in the
middle and upper parts of the Kazusa Group represent hyperpycnal and sediment gravity flows originating from shelf-margin deltas or fan deltas. Theseflows were activated during the falling and lowstandstages of sea-level oscillations controlled primarily byglacioeustacy (Takao et al. 2020).
The Kokumoto Formation is approximately 350 m thickalong the Yoro River and represents MIS 21–18 (~ 860–
720 ka). It comprises thick silty beds along with alternating
sand and thinner silt beds (Kazaoka et al. 2015). Where
exposed along the CbCS, it is a muddy unit depositedfrom suspension under stable and calm bottom-waterconditions, with sedimentary structures and trace fossil assemblages together indicating a continental slope setting(Nishida et al. 2016). In particular, the presence of the ichnogenus Zoophycos in the CbCS (Nishida et al. 2016) implies a water depth of more than 800–1000 m based on its
modern bathymetric distribution (fig. 4 in Löwemark andWerner 2001), as noted by Izumi et al. (2021). Parallel
bedding observed at the CbCS (Fig. 14c) attests to continuous deposition without slumping. This muddy unit,
underlain and overlain by deep-water massive sandstones,
is a relatively condensed section formed in the uppermost
part of a transgressive systems tract (Takao et al. 2020) on
the continental slope.
4.2 Modern oceanography and climate

The area off the CbCS today experiences the highest
oceanographic gradients in the western Pacific owing to
the confluence of two major western boundary currents:
the warm, nutrient-deficient north-flowing Kuroshio
Current and the cold south-flowing nutrient-rich andless saline Oyashio Current. After converging, the Kuroshio Current flows eastwards as a jet known as theKuroshio Extension along a frontal zone that separates
the North Pacific Subpolar Gyre from the North Pacific
Subtropical Gyre. This frontal system consists of the
Subarctic Front to the north and Kuroshio Extension
Front to the south, forming an intervening zone called
the Kuroshio–Oyashio Interfrontal Zone (KOIZ)(Komatsu and Hiroe, 2019; Fig. 15). This area is therefore highly sensitive to changes in the strength and latitudinal position of the Kuroshio Extension which itself isdriven by North Pacific, East Asian, and global climate
oscillations on seasonal, decadal, and orbital time scales.

Head Progress in Earth and Planetary Science(2021) 8:50

Page 26 of 38

Fig. 14 The Chiba section, Japan. (a) Location of the Chiba section on the Boso Peninsula showing major cities. (b) Schematic geology of the
Kanto area showing the depth (metres below sea level) of the base of the Kazusa Group (from fig. 1 of Kazaoka et al. 2015). (c) Chiba section
showing faint but discernible parallel bedding in this massive siltstone unit; the location of the GSSP is indicated by a red star (from fig. 10 ofSuganuma et al. in press). (d) Detail of the Ontake-Byakubi-E (Byk-E) tephra bed in the Chiba section at the position of the GSSP, showing
bioturbation. The GSSP is located at the base of the tephra bed (photo by the author). The GSSP has an astronomical age of 774.1 ± 5 ka and is
1.1 m below the directional midpoint of the Matuyama–Brunhes paleomagnetic reversal (Suganuma et al. in press)

The CbCS then is well positioned to record the evolving
behaviour of this major oceanographic boundary systemthroughout MIS 20–18 (Suganuma et al. 2018; Haneda
et al. 2020b; Kameo et al. 2020; Izumi et al. (2021),
Balota et al. 2021; Kubota et al. 2021).
Monthly satellite imagery of the western North Pacificfor 2019 shows the position of the Kuroshio Extension
Front and pronounced seasonal changes affecting seasurface temperature and primary productivity (Fig. 16).
Although the latitudinal position and flow speed of theKuroshio Extension change little through the year, themagnitude of the Kuroshio Extension Front strength, asmeasured by the horizontal temperature gradient, isgreatest during the cold season and least during thewarm season (Chen 2008, Kida et al. 2015, Wang et al.
2016, Yu et al. 2020). Mesoscale perturbations along theKuroshio Extension Front are also greater in winter(Wei et al. 2017). Seasonal variation in frontal strengthis greatest off Japan and hence relevant to the CbCS.
The Oyashio Current as observed along the continental

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Page 27 of 38

Fig. 15 Present oceanography of the northwestern Pacific Ocean, showing annual sea-surface temperature from the World Ocean Atlas 2013
(Locarnini et al. 2013) drawn using Ocean Data View software (Schlitzer 2015). Major oceanographic features are included. TWC = Tsugaru Warm
Current. SF = Subarctic Front formed by the Subarctic Current, an eastward extension of the Oyashio Current. KEF = Kuroshio Extension Front.
KOIZ = Kuroshio–Oyashio interfrontal zone, a zone of mixing between the SF and KEF. TS = Tokara Strait. FOI = first Oyashio intrusion. Adaptedfrom fig. 1 of Suganuma et al. (2018)

slope off Hokkaido flows more weakly in summer andautumn, its total volume transport reaching 20–30 Sv in
winter and spring but only 3–4 Sv in summer andautumn (Qiu 2019). As a result of these seasonalvariations, the mixed layer in the Kuroshio region is deep(> 100 m) during winter, promoting the supply ofnutrients to the surface, whereas in summer it is shallower(< 15 m) and nutrient depleted (Komatsu and Hiroe 2019).
These seasonal differences are significant for the interpretation of climate proxies in the CbCS.
The Kuroshio Extension Front varies significantly at interannual to decadal frequencies with respect to strength,
latitude, and elongated versus convoluted pathway (Wanget al. 2016; Yu et al. 2020), and its annual mean position
off Japan has shifted between 33° and 37° N over theperiod 1993 to 2013 (Wang et al. 2016). This variability
strongly correlates with the North Pacific Oscillation, anorth–south seesaw between the Aleutian Low and thePacific High below it (Fig. 4). The Aleutian Low intensifies
during its positive phase and shifts northwards (Sugimotoand Hanawa 2009; Yu et al. 2020), favouring a strengthened and northward-moving Kuroshio Extension Front.
Changes in the latitude of the Aleutian Low, a cold-season
phenomenon that dissipates almost entirely in summer,
may therefore be linked to the position of the Kuroshio
Extension Front during MIS 19.
The Siberian High, North Pacific Oscillation, Arctic
Oscillation, and North Atlantic Oscillation are majorinterlinked sea-level pressure systems in the northernhemisphere. The Arctic Oscillation (Thompson andWallace 1998) is a large-scale surface-pressure systemlinked to the stratospheric polar vortex and alternating
between a negative and positive mode. In its positivemode, surface pressure is high in the polar region whichcauses the encircling mid-latitude jet stream to intensify
and confine cold air within this region. In negativemode, the surface pressure falls and the resulting zonal
winds become weaker and more distorted, allowing cold
arctic air masses to flow into the mid-latitudes. A coupling between the Siberian High and Aleutian Low has
been proposed by Huang et al. (2016) and Kumar et al.
(2019). Climate modelling experiments have also revealed linkages between the EAWM, which reflects theintensity of the Siberian High, and both the North
Pacific Oscillation and Arctic Oscillation (Miao et al.
2020). Moreover, the coupling strength between the
Arctic Oscillation and the EAWM is enhanced by increased ice cover over the East Siberian Seas (Wie et al.
2019). The Arctic Oscillation is also strongly linked to
the North Atlantic Oscillation comprising the Azores
High and the Iceland Low (Hamouda et al. 2021; Fig. 4).
The connections between these major systems are notstraightforward and may vary with global temperatures(Hamouda et al. 2021). Nonetheless, the Arctic Oscillation represents a plausible high-latitude link between
North Atlantic and North Pacific climate processes

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Page 28 of 38

Fig. 16 Monthly chlorophyll-a (Chl-a) concentrations and monthly sea-surface temperature (SST) for the western North Pacific in 2019 based onsatellite observations. a–f Chlorophyll-a concentrations as a measure of primary (phytoplankton) productivity. The generalized positions of theKuroshio Current (KC) and Kuroshio Extension (KE) are given in a. The position of the Kuroshio Extension Front (KEF; Fig. 15) is evident fromsharply enhanced productivity around and north of ~ 30° N; greatest productivity in the mid-latitudes east and north of Chiba is during theboreal spring (March–May) with a subdued rise in the autumn (September–October). g–l Sea-surface temperature; the KEF moves only slightly
northwards during summer–autumn but the latitudinal gradient across the KEF lessens significantly in summer with considerable northward
diffusion of warm water during July–September. Note mesoscale eddies pinching off along the KEF and advecting heat northwards. NASA Earth
Observations (2020)

Head Progress in Earth and Planetary Science(2021) 8:50

during MIS 19 especially at times of increased ice cover
over Siberia. These high-latitude linkages, along with themodelled (e.g. Moreno-Chamarro et al. 2020) and historical (Chen et al. 2019, Liu et al. 2020) relationship between the North Atlantic Oscillation, AMOC andposition of the ITCZ, illustrate the tightly integrated nature of the global climate system.
4.3 Chiba paleoceanography and paleoclimate throughMIS 19The CbCS has sedimentation rates of ~ 89 cm/ky acrossthe GSSP (Suganuma et al. in press) and represents oneof the most intensely researched intervals available tounderstand the climatic development of MIS 19. A benthic and planktonic foraminiferal isotope record at highstratigraphic resolution (Haneda et al. 2020b) has astructure remarkably similar to that of North Atlanticand Mediterranean records throughout MIS 19. Similarities include power spectra containing the harmonics ofprecession which implies low-latitude forcing throughout, and a gradual trend to higher benthic foraminiferal
δ18O values in the latter part of MIS 19c that reflects a
progressive increase in global ice volume. At the CbCS,a sharp increase in benthic and especially planktonic foraminiferal δ18O values marks the onset of MIS 19b andis taken to reflect the inception of glaciation. As withsome Mediterranean records, MIS 19a is represented byfour interstadials, i1–i4, and by four corresponding benthic marine isotope oscillations (MIS 19a-o1–4) (Fig. 11g,h). Haneda et al. (2020b) invoked shifts in the ITCZ to
explain the coherence between the Chiba and Mediterranean records, as noted above.
The CbCS documents changes in the Kuroshio Extension system through MIS 19, including rapid north–south
shifts associated with stadial/interstadial alternations during MIS 19b–a (Haneda et al. 2020b). Foraminiferal isotopes and other proxies reflect both warming andincreased surface and near-surface water-column stratification during the interstadials (Fig. 11d–f). Because theseproxies represent winter oceanographic conditions, they
imply weaker mixing of the water column during winter.
The East Asian Winter Monsoon (Fig. 4) largely controls
winter wind strength today and was therefore likely
weaker during the warmer intervals of MIS 19a (Suganuma et al. 2018). The intensity of the East Asian WinterMonsoon system is driven by the thermal contrast between the Siberian High (cold) and Aleutian Low (warm)pressure systems, both of which develop primarily during
winter. A weak EAWM occurs when winter conditions in
Siberia are relatively warm (low pressure) and those overthe northwestern Pacific Ocean are relatively cold. A weak
EAWM during the latter part of MIS 19 is supported byevidence from the Chinese loess record and has been attributed to a weak minimum in summer insolation at 65°

Page 29 of 38

N resulting in reduced ice accumulation over Siberia (Haoet al. 2012; Fig. 12e). Suganuma et al. (2018) proposed that
under ice-free conditions, enhanced winter insolation at
50° N might also have contributed to a weak SiberianHigh, and consequently a weak EAWM in the loweccentricity configuration of MIS 19. A weak EAWM willthen have caused the position of the westerly jet to advance northwards. The latitudinal position of the Kuroshio Extension system is strongly influenced by that of thewesterly jet and will have shifted northwards accordingly.
During interstadials, therefore, the Kuroshio Extension
system would have migrated northwards, bringing warm,
stratified water masses to the CbCS (fig. 15 in Suganuma
et al. 2018; Fig. 17).
While this mechanism explains the continued presence of warm, stratified waters at the CbCS late in MIS
19, it does not address the numerous abrupt stadial–interstadial alternations that characterize MIS 19b–a atthis site or the similarity with North Atlantic–Mediterranean records, and it does not account for potential variation in the intensity of the Aleutian Low.
The loess records of China show a continually weak
EAWM and EASM through the latter half of MIS 19(Hao et al. 2012; Peng et al. 2020) and while this provides evidence for the persistence of unusually warm
conditions late in MIS 19, the millennial-scale stadial–interstadial alternations that characterize MIS 19b–a are
not clearly expressed. A combination of pedogenic processes and relatively low sedimentation rates likely accounts for this absence of abrupt changes. This itself
introduces uncertainty into age models for the latter partof MIS 19 and hinders insights into the mechanisms
driving stadial–interstadial alternations. The S7 palaeosol, now widely accepted as equating with MIS 19, hassince been studied in detail by Zhang et al. (2020) who
examined successions on the Chinese Loess Plateau for
their mollusc content. They determined the earlier partof MIS 19 to be slightly warmer than at present, and thelater part similar to present but with stronger climatic
variability. In the east, warm conditions continued ~ 15
kyr into MIS 18, suggesting a relatively strong summer
monsoon through MIS 19 and well into MIS 18, contrary to Hao et al. (2012, suppl. fig. 12) who showed progressive weakening through MIS 19. Again, however,clear coherent oscillations in the latter part of MIS 19
are not apparent.
The CbCS planktonic foraminiferal isotope data
through MIS 19b–a (Haneda et al. 2020b) show four
pronounced stadials (MIS 19-s1 to s4) and interstadials(MIS 19-i1 to i4) (Fig. 11). Less than 1000 years after the
beginning of MIS 19b, the onset of stadial s1 is identified
by a rapid and significant increase in planktonic δ18O
values accompanied by increases in dinoflagellate cyst
concentrations and especially a sharp (within ~ 300

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Page 30 of 38

Fig. 17 A schematic representation of the northwest Pacific showing the north/south migration of the westerly jet in winter and associated shifts of
the Subarctic Front. A weaker east Asian winter monsoon (EAWM) (a) causes the Subarctic Front to move northwards (interstadial conditions), and astronger EAWM (b) deflects it southwards (stadials). The EAWM is controlled by the thermal contrast between the Siberian High and Aleutian Low
atmospheric pressure systems: when both are intensified (b), the EAWM is strongest. Locations of sites discussed in the text are shown. Adapted fromfig. 15 of Suganuma et al. (2018)

years) increase in the relative abundance of the dinoflagellate Protoceratium reticulatum (Balota et al. 2021;
Fig. 11). The dinoflagellate cyst record also shows arapid transition out of stadial s1 as represented by a decrease in Protoceratium reticulatum. If this interval ofabundance (biozone Df7 in Balota et al. 2021; Fig. 11c)
indeed reflects the true duration of stadial s1, then at
2.48 kyr (772.85–770.37 ka) it is longer than the 1.0 and
1.2 kyr durations at Piànico-Sèllere and Montalbano
Jonico, respectively (Nomade et al. 2019). The abrupt
rise in Protoceratium reticulatum coincides with a sharp
shift to heavier planktonic foraminiferal δ18O values,
reasonably marking the onset of stadial s1, but this species remains abundant nearly half-way into the followinginterstadial i1. Indeed, its abrupt decline coincides withthe sharp change to heavier benthic δ18O values marking

the end of benthic isotope oscillation o1 (Fig. 11). The
abundance of Protoceratium reticulatum appears to indicate the influence of cooler, mixed, nutrient-rich watersof the Kuroshio–Oyashio Interfrontal Zone resulting
from a southward shift of the Kuroshio Extension(Balota et al. 2021; Fig. 15). Future ultra-high-resolution
multiproxy studies are needed to fully understand these
rapid climate changes and their respective planktonic
and benthic expressions through MIS 19b–a. In any
case, these stadials in the Chiba record appear to correspond well with cool/dry stadials in the Mediterranean
and with intervals in the North Atlantic record characterized by IRD, meltwater, cooling, and reduced ventilation, all indicative of AMOC disruptions. This suggests a
teleconnection between the North Atlantic and the western Pacific at this time.

Head Progress in Earth and Planetary Science(2021) 8:50

Haneda et al. (2020b) used very high-resolution benthic and planktonic foraminiferal δ18O analyses of the
CbCS throughout MIS 19 to estimate water-column
stratification by comparing δ18O values between benthic
foraminifera, the deep-dwelling (~ 300–400 m depth in
the subtropical North Pacific) Globorotalia inflata, andthe surface-dwelling Globigerina bulloides. While all records show millennial-scale alternations during MIS
19b–a, the amplitude of the G. bulloides record reasonably suggests that the source of these alternations wasfrom surface-water processes (Haneda et al. 2020b).
However, the onset of benthic isotope oscillation o2
appears to lead slightly that of interstadial i2 (Fig. 11g).
Detailed multiproxy studies at ultra-high stratigraphic
resolution are needed to resolve these small discrepancies. Variations in δ18O values have been consideredprimarily to reflect temperature, with ΔT (δ18Obenthic
minus δ18Obulloides; Fig. 11e) representing the difference
between bottom and surface water temperature (Haneda
et al. 2020b, 2020c). The difference between δ18O values
for Globorotalia inflata and Globigerina bulloides (Suganuma et al. 2018; fig. 2d of Kubota et al. 2021; Fig. 11f)
and the relative abundance of the calcareous nannofossil
Florisphaera profunda (Kameo et al. 2020; Fig. 11d) provide additional measures of surface and near-surface
water-column stratification. Haneda et al. (2020b)
showed that increasing stratification occurred during interstadials and vice versa for stadials, supporting the notion that stadial–interstadial oscillations represent
latitudinal displacements of the Kuroshio Extension
Front at the CbCS. Total organic carbon (TOC) values
generally mirror the stadial–interstadial alternations of
the latter part of MIS 19a, with increased values coinciding with interstadials i2, i3, and i4 (Izumi et al. 2021;
Fig. 11b). Izumi et al. (2021) tentatively proposed thatpeaks at interstadials i2 and i3 represent enhanced organic matter preservation owing to water-column stratification rather than increased surface water productivity.
This follows the reasoning that surface water productivity should be lower during interstadials as they mark thenorthward shift of the Kuroshio Current which is relatively poor in nutrients. Enhanced organic matter preservation is supported by geochemical data and evidence
from trace fossils that indicate reduced oxygen levels inthe bottom waters. However, among the highest valuesof TOC occur within stadial s1 which was influenced by
the nutrient-rich Oyashio Current, as evidenced by the
dinoflagellate cyst record (Balota et al. 2021; Fig. 11c).
Izumi et al. (2021) acknowledged that more research isneeded to understand the important relationship between surface water productivity, organic matter preservation, and stadial–interstadial alternations.
Latitudinal changes in the ITCZ best explain themillennial-scale oscillations at the CbCS and theirPage 31 of 38

teleconnection to the North Atlantic and Mediterranean
records (Haneda et al. 2020b; Fig. 4). Disruptions to
AMOC by meltwater release into the North Atlantic,
and consequent triggering of the thermal bipolar seesaw,
creates a strong thermal contrast between the northernand southern hemispheres. This causes the ITCZ tomove southwards and the trade winds to intensify in association with a deepened Aleutian Low. The midlatitude prevailing westerlies move in parallel and theconsequent atmospheric reorganization causes the Kuroshio Extension also to shift southwards during stadials.
Using a combination of Antarctic ice-core records andclimate modelling over the past 720 kyr, Kawamura
et al. (2017) showed that Antarctic warming events,
linked to activation of the thermal bipolar seesaw, aremost frequent when the climate is intermediate between
glacial and interglacial states, as occurs at the beginningof MIS 19 (the Younger Dryas-type interruption of Termination IX) and the stadial–interstadial states of MIS
19b–a. These instabilities recorded at the CbCS andelsewhere therefore appear to reflect an intrinsic oscillation within the Earth system amplified during an intermediate glacial–interglacial state. Modelling results
suggests that reduced CO2 concentrations along with extended Northern Hemisphere ice sheets, as must havebeen developing after the glacial inception near the endof MIS 19c (see above, also Vavrus et al. 2018), are prerequisites for such instability (Kawamura et al. 2017). To
understand the rapid warming from stadial to interstadialstates during MIS 19a, it may be relevant to examine thesimilarly rapid transition from the Heinrich 1 stadial to the
Bølling-Allerød interstadial during the last deglaciation.
Modelling studies show that gradually changing conditions
can lead to an abrupt recovery of the AMOC and consequent rapid warming that marks the onset of the BøllingAllerød interstadial (Obase and Abe-Ouchi 2019).
Time series analyses of planktonic foraminiferal δ18O
records (Fig. 11g) including records of water-column
stratification (ΔT; Fig. 11e) for the ChCS have revealed
periodicities of a half-precession cycle throughout MIS
19 but also higher frequencies including those of the
fourth harmonic of precession for MIS 19b–a (Haneda
et al. 2020b). These harmonics of precession in the timeseries are derived from equatorial insolation (Fig. 11a).
The influence of precession is strong at Lake Baikal at
51°–53° N, although it declines with increasing latitude(Prokopenko et al. 2006). The half-precession cycle at
the CbCS is strongest within MIS 19c presumably owing
to the absence of major ice melting events and AMOC
disruption in the North Atlantic at this time. These
high-latitude North Atlantic events then account for thestadial–interstadial alternations of MIS 19b–a via latitudinal displacements of the ITCZ, although paced by
low-latitude insolation variations (Haneda et al. 2020b).

Head Progress in Earth and Planetary Science(2021) 8:50

Despite the significant advances reviewed above (Suganuma et al. 2021), several aspects of the CbCS
paleoenvironmental record remain incompletely known.
Most proxies available at high stratigraphic resolution,
including the foraminiferal isotopes, reflect the cooler
seasons (Suganuma et al. 2018; Haneda et al. 2020b).
Summer sea-surface temperatures and hence seasonal
contrasts through MIS 19 are poorly understood but were
presumably considerable then as they are now (Fig. 16).
Over the western Pacific at present, a significant divergence exists between winter and summer positions of
the ITCZ owing to summer heating across Asia. For example, the ITCZ occurs over northern India in summer
but shifts to just north of Australia in winter (Fig. 4)
along with its associated trade winds. However, northernhemisphere ice sheets will have depressed this divergence and reduced the northward shift of the ITCZ insummer (Chiang and Friedman 2012; Schneider et al.
2014). In the CbCS, sea-surface temperature and nearsurface stratification proxies are based on calcareous microfossils that accumulate calcite during winter (Suganuma et al. 2018; Haneda et al. 2020b). These will havediffered substantially from proxies representing the summer months, although both will have been affected by
the bipolar thermal seesaw. More research is needed tocharacterize summer land and ocean temperatures in
the CbCS, especially during MIS 19b–a.
The dinoflagellate cyst record may represent inpart a late spring–early autumn signal and shows significant fluctuations through MIS 19c and across stadial s1 into interstadial i1 within MIS 19a (zones Df7
and Df8 in Balota et al. 2021; Fig. 11c). These indicate an abrupt southward shift of the Kuroshio extension during glacial inception (stadial s1) and asimilarly abrupt northward shift within interstadial i1(biozone Df7 of Balota et al. 2021). For the latter partof MIS 19, it would then appear that the dynamics of
the westerly jet and ITCZ are also reflected by warmseason as well as cold-season proxies. While this is to
be expected, the remaining alternations of MIS 19a
have yet to be analysed in detail for indicators ofwarm-season sea-surface conditions, and therefore
variations in seasonality are not known.
A high-resolution pollen record at the CbCS similarly
extends only to the MIS 19b–a boundary (Suganuma
et al. 2018), although the terrestrial vegetation will havebeen sensitive to changes in spring–summer warmthand precipitation as well as winter frost during the stadial–interstadial oscillations of MIS 19a. An extension ofthis high-resolution pollen record through MIS 19ashould yield information about variations in strength of
the EASM across stadial–interstadial alternations. Indeed,
many proxy records lack the ultra-high stratigraphic resolution needed to fully characterize changes through MIS

Page 32 of 38

19b–a, although abundant opportunities are afforded by
the high sedimentation rates (> 89 cm/kyr, Suganuma
et al. 2018) in this part of the succession.
Further evidence for climatic oscillations in the latterpart of MIS 19 during summer is found in the higherlatitude biogenic silica records of Lake Baikal which wasstrongly influenced by precessional variation (Prokopenko
et al. 2006) and Si/Ti records of Lake El’gygytgyn (Wennrich et al. 2014) which seem to represent direct highlatitude teleconnections with the northern North Atlantic.
These records reflect warm-season productivity and showpronounced interstadials i1 and i2 (Fig. 12c, d).

5 Summary and conclusions
The GSSP defining the base of the Chibanian Stage andMiddle Pleistocene Subseries at the Chiba section, Japan,
was ratified on January 17, 2020, by the Executive Committee of the IUGS (Suganuma et al. in press). The GSSP
occurs immediately below the top of MIS 19c and has
an astronomical age of 774.1 ± 5.0 ka. The M–B reversal,
with a directional midpoint just 1.1 m above the GSSP,
an astronomical age of 772.9 ± 5.4 ka, and a duration of
up to ~ 2.0 kyr, serves as the primary guide to theboundary. The two other candidate sections, the Idealesection at Montalbano Jonico and the Valle di Manche,both in Italy, were deemed to have equivocal (Valle diManche) or imprecise (Montalbano Jonico) reversal records (Head 2019). This finalizes a process initiated by
INQUA in 1973 to define the base of the Middle Pleistocene, a term in use since at least 1869. Although the
Chibanian Stage is presently concurrent with the Middle
Pleistocene Subseries, the introduction of a second
stage for the Middle Pleistocene Subseries with its basenear the onset of the mid-Brunhes event (MIS 12–11
transition, ~ 424 ka; Fig. 2) should be considered.
The M–B reversal facilitates global recognition in marine, terrestrial, and ice-core records, and places the
GSSP appropriately in the middle of the Early–Middle
Pleistocene Transition, an interval of profound and lasting climatic change (Head and Gibbard 2015b). MIS 19
was first labelled by Shackleton and Opdyke (1973; Fig. 6)
who confirmed an earlier association of this interglacial
stage with the M–B reversal (Hays et al. 1969; Fig. 5).
Bassinot et al. (1994) were the first to subdivide MIS 19
formally, labelling events MIS 19.1, 19.2, and 19.3
(Fig. 8b). Tzedakis et al. (2012a, 2012b) were apparently
the first to subdivide MIS 19 into lettered substages,
MIS 19a, 19b, and 19c (Fig. 8c). Most subsequent authors have followed this three-lettered scheme but theboundary between MIS 19b and 19a has been applied inconsistently. The approach here is to restrict MIS 19b tothe first interval of high foraminiferal isotopic values following MIS 19c (Fig. 7) following Nomade et al. (2019).
The fine-scale subdivision of MIS 19 has been treated inHead Progress in Earth and Planetary Science(2021) 8:50

various ways. However, climatostratigraphic units based
potentially on multiple paleoenvironmental criteria, forwhich stadial-interstadial terminology is appropriate, areconceptually different from the benthic isotope signal
which includes a global ice-volume component. For thelatter part of MIS 19, it is proposed that stadial-interstadial
labelling MIS 19-s1 to -s4 and MIS 19-i1 to -i4 be used
independently of substages, with the three or four peaks inthe benthic isotope record of MIS 19a separately labelled asbenthic isotope oscillations MIS 19a-o1 to -o4 (Fig. 7).
MIS 19 is characterized by a reduced-amplitude 400
ky eccentricity cycle similar to our present interglacial(Fig. 2) but obliquity increased less rapidly and to alower amplitude, and peak temperatures seem to havebeen generally lower than for the pre-industrial Holocene. CO2 levels seem to have been similar until about
8000 years ago when they began to rise in the Middle
Holocene (Fig. 13). A Younger Dryas-like oscillation interrupts the deglaciation of Termination IX at several
sites, possibly triggered by a brief AMOC disruption
under these unusual orbital conditions (Haneda et al.
2020b; Marino et al. 2020). The onset of MIS 19 was
driven by a steep rise in June insolation at 65° N withmaximum obliquity in phase with minimum precession.
MIS 19c extends from 790 to 785 ka to the expansion ofice sheets (glacial inception) at 774–777 ka, the timing
influenced by the time scales used, and spans full interglacial conditions which lasted for around 10 to 12.5 kyr.
Records (Table 2) confirm the brevity of full interglacialconditions during MIS 19 compared with most later interglacials, including MIS 11 which has a similar orbital
configuration, and results from an unusually early glacial
inception relative to the obliquity cycle (Tzedakis et al.
2012a, their fig. 6). Any comparisons with later interglacials should consider not just orbital configuration but
also the causes and effects of increased quasi-100 kyr
periodicity during and after the Early–Middle Pleistocene transition (Head and Gibbard 2015b).
During MIS 19c, both Pacific and North Atlantic–Mediterranean planktonic records show variability in the halfor quarter-precession bandwidth, indicating the influenceof equatorial insolation variation at low and mid-latitudes
at a time when AMOC disruption was minimal. The interruption of westerlies carrying moisture to the western
Mediterranean may have led to the northward transportof moisture, feeding ice sheets which were progressively
expanding through much of MIS 19c as boreal summer
insolation decreased (Sánchez-Goñi et al. 2016). The inception of glaciation (774–777 ka) at the end of MIS 19c
presents a cluster of climatostratigraphic signals that can
assist in identifying the Early–Middle Pleistocene boundary (774.1 ka) globally.
MIS 19b–a corresponds to a second precessional minimum, in antiphase with obliquity, that results in a

Page 33 of 38

suppressed insolation peak (Fig. 13) transposed onto alonger-term trend of increasing global ice volume. Global ice accumulation during the latter part of MIS 19c
appears to have crossed a climate threshold, with MIS
19b marking the first of three or four AMOC disruptions triggered by ice-calving and freshwater release into
the northern North Atlantic, and indicated by ice-rafted
debris and sortable silt records at ODP Site 983 andelsewhere. These AMOC disruptions led to activation of
the thermal bipolar seesaw, explaining a slightly lagged
phase relationship with three AIMs (warming events) in
the Antarctic ice-core record (Tzedakis et al. 2012b;
Fig. 8c). As a result of these oscillations, three or four interstadials with characteristically abrupt transitions are
represented during MIS 19b–a, both in the Asian–Pacific and North Atlantic–Mediterranean realms. The coherence of these oscillations on a global scale is best
explained by shifts in the ITCZ (Fig. 4) as a result of
thermal contrast between northern and southern hemispheres created by the bipolar seesaw (Chiang and Friedman 2012; Schneider et al. 2014; Nomade et al. 2019;
Haneda et al. 2020b). For example, a warming of thesouthern hemisphere would displace the ITCZ southwards along with the mid-latitude westerlies. This inturn would have shifted the Subarctic Front southwards,
initiating stadial conditions in the northern hemisphere.
Stadial–interstadial oscillations occur at or close to theharmonics of precession, and their transition from one
state to the other may take less than 200 years. These
AMOC-triggered oscillations may therefore have been
paced by equatorial insolation forcing, even at a time
when the effects of precession were subdued (Ferretti
et al. 2015), and in some regions amplified by the monsoon system (Nomade et al. 2019; Haneda et al. 2020b).
Although MIS 19c represents a close orbital analogue tothe present interglacial, the dominant ~ 5 kyr cyclicity in global records of MIS 19 contrasts with the dominant 2.5 kyr
cyclicity of MIS 1, calling into question the assumed close
similarity of these two interglacial stages (Sánchez-Goñi et al.
2016). The precise mechanism driving AMOC events in thenorthern North Atlantic particularly during MIS 19b–a, andthe teleconnections linking these events with stadial–interstadial oscillations in the eastern Siberian El-gygytgyn record(Fig. 12c), is not known. The Siberian High, North PacificOscillation, Arctic Oscillation, and North Atlantic Oscillation, representing the northern hemisphere’s major atmospheric pressure systems, appear to be interlinked overhistorical time scales and may have provided high-latitude
teleconnections during MIS 19. Additional high-resolution
studies are needed to explore these potential connections
during MIS 19. Detailed marine records of MIS 19 are conspicuously missing from the southern hemisphere (Fig. 4) yet
such information is needed to test hypotheses involving interhemispheric processes.

Head Progress in Earth and Planetary Science(2021) 8:50

Abbreviations
AIM: Antarctic Isotope Maximum; AMOC: Atlantic meridional overturning
circulation; CbCS: Chiba composite section; EPICA: European Project for Ice
Coring in Antarctica; GSSP: Global Boundary Stratotype Section and Point;
ICS: International Commission on Stratigraphy; IODP: Integrated Ocean
Drilling Program; INQUA: International Union for Quaternary Research;
IRD: Ice-rafted debris; ITCZ: Intertropical Convergence Zone; IUGS
EC: Executive Committee of the International Union of Geological Sciences;
M–B: Matuyama–Brunhes; MIS: Marine Isotope Stage; ODP: Ocean Drilling
Program; SQS: ICS Subcommission on Quaternary Stratigraphy
Acknowledgements
This contribution is supported by a Natural Sciences and Engineering
Research Council of Canada Discovery Grant. The library staff at Brock
University and Western University provided helpful assistance. I am mostgrateful to Yusuke Suganuma for encouragement in writing this review andfor his helpful comments at all stages of manuscript preparation. Sincere
appreciation is extended to the journal reviewers, Maria Fernanda SánchezGoñi and Yuki Haneda, whose suggestions have considerably improved thispaper.
Author’s contributions
This review is the sole work of the author, with contributions as noted in the
Acknowledgements. The author read and approved the final manuscript.
Author’s information
MJH is a professor of Earth Sciences at Brock University, Canada, with aninterest in Cenozoic dinoflagellate cysts and acritarchs, stratigraphic
classification, and the formal subdivision of the Quaternary System. He is theprevious Chair and present Vice-Chair of the International Subcommission on
Quaternary Stratigraphy (SQS), Co-Convener of the now-dissolved SQS Working Group on the Lower–Middle Pleistocene Boundary, and a voting member of the International Subcommission on Stratigraphic Classification. He
has a Ph.D. from the University of Aberdeen.
Funding
This work was supported by a Natural Sciences and Engineering ResearchCouncil of Canada Discovery Grant to MJH.
Availability of data and materials
Not applicable

Declarations
Competing interests
The author declares no competing interest.
Received: 18 June 2020 Accepted: 4 June 2021

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