© PLOSI o -
OPEN 3 ACCESS Freely available online
Northern Hemisphere Glaciation during the Globally
Warm Early Late Pliocene
Stijn De Schepper1'2*, Jeroen G roeneveld3, B. David A Naafs40, Cédéric Van Renterghem5,
Jan Hennissen6, Martin J. Head6'7, Stephen Louwye5, Karl Fabian8
1 Department of Earth Science, University of Bergen, Bergen, Norway, 2 Geosciences Department, University of Bremen, Bremen, Germany, 3 MARUM - Center for Marine
Environmental Sciences, University of Bremen, Bremen, Germany, 4 Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany, 5 Research Unit
Palaeontology, Ghent University, Ghent, Belgium, 6 Department of Earth Sciences, University of Toronto, Toronto, Ontario, Canada, 7 Department of Earth Sciences, Brock
University, St. Catharines, Ontario, Canada, 8 Norwegian Geological Survey, Trondheim, Norway
Abstract
The early Late Pliocene (3.6 to ~ 3 .0 m illion years ago) Is the last extended Interval In Earth's history when atm ospheric C 02
concentrations were com parable to today's and global clim ate was warmer. Yet a severe global glaciation during marine
Isotope stage (MIS) M2 Interrupted this phase o f global w arm th ~ 3.30 m illion years ago, and Is seen as a prem ature atte m p t
o f the clim ate system to establish an Ice-age w orld. Here we propose a conceptual model fo r the glaciation and déglaciation
o f MIS M2 based on geochemical and palynologlcal records from five marine sedim ent cores along a Caribbean to eastern
North A tlantic transect. O ur records show th a t Increased Paclfic-to-Atlantlc flo w via the Central American Seaway weakened
the North A tlantic Current and atten dan t northw ard heat transport prior to MIS M2. The consequent cooling o f the northern
high latitude oceans perm itted expansion o f the continental lee sheets during MIS M2, despite near-modern atm ospheric
C 02 concentrations. Sea level drop during this glaciation halted the Inflow o f Pacific w ater to the A tlantic via the Central
American Seaway, allow ing the build-up o f a Caribbean Warm Pool. Once this warm pool was large enough, the G ulf
S tream -N orth A tlantic Current system was relnvlgorated, leading to significant northw ard heat transport th a t term inated
the glaciation. Before and after MIS M2, heat transport via the North A tlantic Current was crucial In m aintaining warm
climates com parable to those predicted for the end o f this century.
C itation : De Schepper S, Groeneveld J, Naafs BDA, Van Renterghem C, Hennissen J, et al. (2013) Northern Hemisphere Glaciation during the Globally Warm Early
Late Pliocene. PLoS ONE 8(12): e81508. doi:10.1371/journal.pone.0081508
Editor: Victoria C Smith, University of Oxford, United Kingdom
Received May 7, 2013; A ccepted October 14, 2013; Published December 12, 2013
C opyrigh t: © 2013 De Schepper et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
F unding: This research was supported by Deutsche Forschungsgemeinschaft grants SCHE 1665/2-1 and SCHE 1665/2-2 (SDS) and NA973/1-1 (BDAN), the
University of Bergen (SDS), a MARUM Student Summer Fellowship (CVR), and a NSERC Canada Discovery Grant (MJH). The funders had no role in study design,
data collection and analysis, decision to publish, or preparation of the manuscript.
C om peting Interests: The authors have declared that no competing interests exist.
* E-mail: smad2@cantab.net
a Current address: Organic Geochemistry Unit, School of Chemistry, University of Bristol, Bristol, United Kingdom
Introduction
Q u a te rn a ry glaciations. W e distinguish this b rie f interval o f intense
glaciation (3.305-3.285 M a) w ithin the longer interval o f M IS M 2
(3.312-3.264 M a) as defined in L R 0 4 [10], T h e associated glacioeustatic sea level d rop is reflected in a m ajor depositional sequence
b o u n d a ry [12] w ith sea level estim ated at 10 m ± 10-15 m,
40 m ± 1 0 m , o r indeed u p to 65 m ± 15-25 m below present
[13-15] (Figure IB). G iven this large u n certain ty in reconstructed
sea level for M IS M 2, it is difficult to quantify how the volum e o f
the n o rth e rn a n d southern hem isphere ice sheets changed. LTsing
the H olocene-like, relatively cool a n d dry A rctic clim ate at Lake
E l’gygytgyn (northeast A rctic Russia) as a n ap p ro x im atio n o f the
b ro a d e r A rctic clim ate, ice advance d u rin g M IS M 2 is th o u g h t to
have o c cu rre d in Alaska, G reen lan d , S valbard a n d A ntarctica,
w hereas substantial expansion in N o rth A m erica was less likely
[16], E stim ates for ice volum e increase in A ntarctica c orrespond to
a sea level d rop o f ~ 8 m [17] or even ~ 1 8 m [13], b u t c an n o t not
fully explain the ~0.5% o b enthic foram inifera! S 180 shift at this
tim e [10], D irect a n d indirect evidence o f glaciation support
expansion o f the A ntarctic ice sheet [18,19], a considerable ice
advance o f the G re en la n d a n d S v a lb ard /B a re n ts Sea ice sheets
T h e early L ate Pliocene (early Piacenzian) from 3.6 to
~ 3 .0 m illion years ago (Ma) is the last sustained interval in
E a rth ’s history w hen global clim ate was w a rm er th a n today. T h e
~ 3 . 3—3.0 M a tim e slab know n as the m id-P iacenzian W arm
P eriod (m PW P, Figure 1) has b e en studied intensively as a
potential analogue for o u r future global clim ate [1]. T h e m P W P is
c haracterised by ~ 3 " C w a rm er global tem p eratu res [2], 10M 0 m
h igher sea-level [3], red u ced continental ice sheets [4], a n d an
A tlantic m eridional ov ertu rn in g circulation (A M O C ) co m parable
to [5] or stronger th a n [6] p reindustrial levels. A tm ospheric C 0 2
concentrations w ere h igher th a n p reindustrial values, a n d likely as
high as the m o d ern a nthropogenic values o f ~ 4 0 0 p p m [7-9]
(Figure 1C). T h e m P W P clim ate is a good ap p ro x im atio n for the
w arm clim atic conditions o f the entire early L ate Pliocene. T his
w arm stable clim ate was nonetheless in te rru p ted b y a short-lived,
intense global glaciation (3.305-3.285 M a) d u rin g m arin e isotope
stage (MIS) M 2 [10,11] (Figure 1). In the L R 0 4 Plio-Pleistocene
benthic Sl80 stack [10], M IS M 2 starts as a low -am plitude
glaciation typical o f the Pliocene, b u t deepens steeply betw een
3.305 a n d 3.285 M a to reach values characteristic o f early
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N orthern Hemisphere G laciation in a W arm Climate
2.40
Series/Stage
PMAG
2.50
2.60
2.70
2.80
2.90
3.00
3.10
Age (Ma)
3.20
3.30
Pleistocene
Matuyama
3.40
SjemmothJ '
3.50
3.60
3.70
3.80
3.90
4.00
Early Pliocene (Zanclean)
Gilbert
’■
MG6
G ¡2
modern value
M2
/ LR04
\
refs. [13], [14], [15], [79]
500
pCO^anno 2012]
400
eg /—
o £ 300
O ra
o. n 200
Preindustrial pCO
refs. [8] alk e n o n e , [80] s to m a ta , [8] a n d [9] boron
100
40
AMOC+
30
20
10
0
2.40
2.50
T
2.60
T
2.70
T
2.80
T
2.90
T~
3.00
3.10
3.20
3.30
Age (Ma)
T
3.40
T
3.50
T"
3.60
—
I—
3.70
AMOC~r
T
3.80
3.90
4.00
Figure 1. M arin e isotope stage M 2 in th e long-term clim ate evolu tio n o f the Pliocene. (A) Time scale, including p alaeo m ag n etic reversals
(PMAG) and th e LR04 b en th ic iso to p e stack [10], o ran g e shading show s m id-Piacenzian Warm Period (= m id-Pliocene Warm Period), grey shading
show s m arine iso to p e sta g e MIS M2; (B) sea level estim ates for th e Pliocene to Pleistocene [13-15,79]; (C) Late Pliocene atm o sp h eric carb o n dioxide
co n cen tratio n s b ased on boron, alkenones and leaf sto m a ta [8,9,80]; (D) long-term carb o n ate-san d record a t ODP Site 999 as an indicator for Pacific
w ater flow th ro u g h th e Central American Seaw ay into th e Atlantic and AMOC [27],
doi:10.1371/journal.pone.0081508.g001
(SST), relative salinity reconstructions, a n d c arb o n ate-san d rec­
ords provide direct evidence th a t the uniq u e conditions responsible
for glaciation du rin g M IS M 2 relate to a n increased Pacific-toA tlantic flow via the C en tral A m erican Seaw ay (CAS) p rio r to
M IS M 2. T his w eakened n o rth w a rd heat tran sp o rt due to a shift
o f the NACI. T h e conceptual m odel p ro p o sed here links a n open
CIAS w ith glaciation in the N o rth e rn H em isphere a n d contrasts
w ith hypotheses th a t propose the closure o f the CIAS as a cause for
the intensification o f N o rth e rn H em isphere glaciation a ro u n d 2.6
M a [33], o r as a delaying factor [34] or a p reco n d itio n for ice sheet
expansion in the N o rth e rn H em isphere [27].
[20-23], ice cap expansion in Iceland [24], a n d possibly in A laska
a n d the C a n a d ia n R ocky M o untains [25] (Figure 2).
In te rru p tin g a n interval o f global w arm th , M IS M 2 has been
p roposed as a n early, failed a ttem p t by the E a rth ’s clim ate to
establish a p a tte rn o f intense a n d frequent N o rth e rn H em isphere
glaciations [26,27]. It was n o t until ~ 5 0 0 ,0 0 0 years later th a t this
p a tte rn em erged, likely due to decreasing atm ospheric carb o n
dioxide concentrations d u rin g the L ate Pliocene [8,28], T h e
decline in atm ospheric c arb o n dioxide c oncentrations [7-9],
increasing global ice volum e [10,11], cooling o f ocean surface
w aters [29-31], a n d tectonic closure o f ocean gatew ays [27,32]
since the L ate M iocene m ay well have ultim ately facilitated
glaciation in the late L ate Pliocene, b u t these long-term processes
are a n unlikely cause o f the short-lived M IS M 2 glaciation.
Sim ilarly, variations in astronom ical forcing alone c an n o t explain
the intense glaciation o f M IS M 2 because intervals w ith sim ilar
astronom ical forcing occu rred th ro u g h o u t the L ate Pliocene
w ithout leading to intense glaciation. T h e isolated n a tu re o f the
M IS M 2 glaciation in the otherw ise w a rm clim ate o f the early L ate
Pliocene m ust be the result o f a specific forcing, unique w ithin this
tim e period.
W e established high-resolution palynological a n d geochem ical
records from five ocean drilling sites along a south w est-n o rth east
transect in the N o rth A tlantic covering the C a rib b e an W arm Pool,
G u lf S tream , subtropical gyre a n d N o rth A tlantic C u rre n t (NACI)
over the interval 3 .4 0 0 -3 .1 8 0 M a to determ in e the role o f ocean
circulation in causing the extensive glaciation o f M IS M 2
(Figure 2). O u r surface w ater m ass, sea surface tem p e ra tu re
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Materials and M ethods
Sam ples w ere collected a t the IO D P B rem en Clore R epository
(G erm any) a n d G u lf Cloast R epository (College Station, T exas,
LTSA) from five sites constituting a transect b etw een the C a rib b e an
Sea (O D P Site 999), w estern N o rth A tlantic (D SD P Site 603), a n d
the eastern N o rth A tlantic (D S D P Site 610, IO D P Sites FT1308
a n d U 1313). T h e foram inifera! geochem istry d a ta a n d palynom o rp h assem blages w ere a cq u ired from the sam e sam ples at each
o f the five ocean drilling sites, a n d sam ples for bio m ark er
(alkenone) analysis w ere taken from the sam e sam ple depths at
th ree sites. All g e n era te d dinoflagellate cyst a n d geochem ical p roxy
d a ta are accessible th ro u g h the datab ase P A N G A E A at h t t p : / /
d o i.p a n g ae a.d e /1 0 .1 5 9 4 /P A N G A E A .8 0 4 6 7 7 . Previously p u b ­
lished M g /C ia a n d dinoflagellate cyst d a ta [26,35] are also
available a t h ttp ://d o i.p a n g a e a .d e /1 0 .1 5 9 4 /P A N G A E A .7 5 8 7 1 0
a n d h ttp ://d o i.p a n g a e a .d e /1 0 .1 5 9 4 /P A N G A E A .7 5 8 7 1 1 .
2
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N orthern Hemisphere G laciation in a W arm Climate
90°W
60°W
30°W
0“
8CTN
60°N
NAD
U1307
subpolar gyre
yrU1308
40°N
subtropical gyre
sed im e n to lo g ic a l e v id e n c e for
g la ciatio n a n d IRD d uring MIS M2
20°N
te rre stria l e v id e n c e for g laciation
Coordinates
Water
depth
ODP 999
12 4 4 ’N, 7 8 4 4 W
2828 m
DSDP 603
3 5 a3 0 ’N, 7 0 °2 ’W
4633 m
IO D P U 13 1 3
4 1 ‘0 ’N, 3 2 '5 7 ’W
IO D P U 13 0 8
4 9 °5 3 ’N, 2 4 °1 4 ’W
3872 m
DSDP 610
5 3 '1 3 ’N, 1 8°53’W
2417 m
3413 m
G S = G ulf S tre a m
NAC = N orth A tlantic C u rre n t
NAD = N orth A tlantic Drift
S S T ('C )
0
5
10
15
20
25
30
Figure 2. M o d e m N orth Atlantic surface circulation w ith m odern sea surface tem peratures (W orld Ocean Atlas 2 005 [81]). Each
studied site is indicated by th e same colour in subsequent figures, and o th e r sites discussed in the te x t are shown in w h ite , lee caps
on G reenland are schematic representations o f Pliocene reconstructions [4].
doi:10.1371/journal.pone.0081508.g002
relative a b u n d an c e variations th a t are statistically significant
according to the p ro c ed u re described in ref. [38] have b e en used
for interp retatio n . D a ta p re sen te d in refs. [26] a n d [35] w ere used
alongside o u r newly gen erated d a ta (Figure 3).
T h e assem blage com position o f dinoflagellate cysts in core-top
sam ples is largely related to the p resent-day overlying w ater
masses [3 9 ^-2 ], a n d reflects the interplay betw een tem p eratu re,
salinity, nutrients, sea ice cover a n d light availability. Presentday, last interglacial [43] a n d Pliocene [26] dinoflagellate cyst
assem blages recovered from the eastern N o rth A tlantic consisting
o f high a b u n d an ces o f Operculodinium centrocarpum sensu W all &
D ale (1966) (herein 0. centrocarpum) all reflect the presence o f the
N o rth A tlantic C u rre n t (NAC). A t D S D P Site 610 a n d IO D P
Site U l 308, 0. centrocarpum concentrations are highest also w hen
the relative abu n d an ces are high, in d ependently c o rro b o ra tin g
the value o f 0 . centrocarpum as a N A C in dicator species.
Full a u th o rial citations o f species discussed in the text are given
in T ab le 1.
Dinoflagellate cyst preparation technique and
assemblage interpretation
O u r lab o ra to ry technique allows dinoflagellate cysts a n d
foram inifera to be extracted from the sam e sam ples (full details
in [26]). E ac h sam ple was first w et sieved at 125 p m to con cen trate
the foram inifera a n d ensure th a t the p alynom orphs pass th ro u g h
the sieve for furth er processing. T h e fraction re ta in e d o n the sieve
(> 1 2 5 pm) was d ried a n d w eighed before bein g picked for
foram inifera. T h e sedim ent filtrate (< 1 2 5 pm) was d ried a n d
w eighed, a n d Lycopodium clavatum tablets w ere a d d ed before
applying sta n d ard palynological p re p a ra tio n techniques involving
cold HC1 a n d H F acids [36], N o oxidation, alkali or ultrasonic
treatm en ts w ere used. O rg an ic residues w ere sieved th ro u g h a 10p m nylon m esh a n d strew m o u n te d o nto m icroscope slides using
glycerine jelly. D inoflagellate cysts w ere co u n ted u n d e r 400x
m agnification w ith counts varying b etw een 44 a n d 527 (average
267) specim ens p e r sam ple. In addition, acritarchs a n d terrestrial
p alynom orphs w ere also e n u m e rate d du rin g the dinoflagellate cyst
counts. P aly n o m o rp h concentrations a n d e rro r estim ates w ere
th en calculated based o n the p a ly n o m o rp h a n d Lycopodium clavatum
counts a n d the dry w eight o f the < 1 2 5 p m fraction [37]. O nly
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N orthern Hemisphere G laciation in a W arm Climate
Age (Ma)
3. 180
PMAG
3.340
3.360
3.380
3.400
LR04
I 603 IIU1308I
I 610 IIU1313]
MG4
22-
®
O
full g la c ia l,
c o n d itio n s
o
18-
bCi) ^ë 1 4 w 7c
P
“„
e
«
I 22:
y o -2 -io
H
I a?% 18
o ^
w
cn
o
i4
d 14
£
18-
ï p
14-
16-
§ g « 1210-
3.0-.
.
2 .0 -
'
1.0 -
0NAC
Warm
water
taxa
|U1308|
water
]
I
]
I
I
]
]
I
]
I
]
I
]
I
I
Operculodinium centrocarpum
Invertocysta lacrymosa + tabulata
Impagidinium solidum
Spiniferites mirabilis
Impagidinium patulum
Impagidinium paradoxum
Impagidinium aculeatum
Polysphaeridium zoharyi
Pentapharsodinium dalei
Impagidinium pallidum
Filisphaera filifera
Bitectatodinium tepikiense
Nematosphaeropsis labyrinthus
Spiniferites spp.
Others
coring gap
180
3.200
3.220
3.240
3.260
3.320
3.340
3.360
3.380
3.400
Age (Ma)
Figure 3. North A tlantic palaeoceanographic p ro xy records from DSDP Sites 6 03 and 61 0 , and IODP Sites U 13 0 8 and U 1313
b etw een 3 .4 0 0 and 3 .1 8 0 M a. Circles with w hite fill are d ata po in ts from this study, circles w ith colour fill are from [30,55] (C) and [26,35] (D).
Vertical grey bars rep re sen t glacials, w hite bars are interglacials. (A) p alaeo m ag n etic reversals; (B) b en th ic isotope ag e m odels for each site tu n e d to
th e LR04 stack [10] (black line), thin coloured lines are d ata, thick coloured line is 4-point running m ean; (C) alkenone SSTs (0-10 m w ater d ep th )
including calibration related error (shading), horizontal lines rep re sen t m odern average annual tem p e ra tu re at 0 -5 0 m w ater d e p th for each site; (D)
SSTs a t 0 -6 0 m w ater d e p th based on Mg/Ca of G. bulloides, including a 1 C error bar (shading), horizontal lines rep re sen t m odern averag e spring
(March-April-May) te m p e ra tu re a t 0 -5 0 m w ater d e p th for each site; (E) SWTMg/ca of G. inflata at 3 00-400 m w ater d ep th ; (F) calculated S18Osw.¡ce as
estim ate of salinity; thick coloured lines are a 4-point running m ean; (G-J) dinoflagellate cyst assem blage com position. High a b u n d a n c e s of 0.
centrocarpum (yellow) indicate an active NAC. Bluish colours = cool-w ater species, reddish colours = w arm -w ater species; (G) and (H) contain d ata
p resen ted in [26] and [35], (J) B. tepikiense and F. filifera are g ro u p e d to g e th e r a t DSDP Site 603 an d are rep re sen ted by th e colour for B. tepikiense
(purple); Impagidinium cf. pallidum is rep re sen ted as Impagidinium pallidum.
doi:10.1371/journal.pone.0081508.g003
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N orthern Hem isphere Glaciation in a W arm Clim ate
T a b le 1 . Dinoflagellate cyst species m entioned in the text and figures: abbreviation, full authorial citation and grouping.
A b b re via tio n
Full species nam e
B. tepikiense
Bitectatodinium tepikiense Wilson 1973
F. filifera
Filisphaera filifera Bujak 1984 emend. Head 1994
1. aculeatum
Impagidinium aculeatum (Wall 1967) Lentin & Williams 1981
1. paradoxum
Impagidinium paradoxum (Wall 1967) Stover & Evitt 1978
1. pallidum
Impagidinium pallidum Bujak 1984
1. patulum
Impagidinium patulum (Wall 1967) Stover & Evitt 1978
1. solidum
Impagidinium solidum Versteegh & Zevenboom in Versteegh 1995
1. lacrymosa
Invertocysta lacrymosa Edwards 1984
1. tabulata
Invertocysta tabulata Edwards 1984
N. labyrinthus
Nematosphaeropsis labyrinthus (Ostenfeld 1903) Reid 1974
0. centrocarpum
Operculodinium centrocarpum sensu Wall & Dale 1966
0. israelianum
Operculodinium israelianum (Rossignol 1962) Wall 1967
P. dalei
Cyst of Pentapharsodinium dalei Indelicato & Loeblich III 1986
P. zoharyi
Polysphaeridium zoharyi (Rossignol 1962) Bujak et al. 1980
RBC
Round brown cysts
S. mirabilis
Spiniferites mirabilis (Rossignol 1964) Sarjeant 1970, and
Spiniferites hyperacanthus (Deflandre & Cookson 1955) Cookson & Eisenack 1974
Spiniferites/ Achomosphaera sp p.
Spiniferites spp. Mantel I 1850, and
Achomosphaera spp. Evitt 1963
Others
Contains all other dinoflagellate cyst taxa counted.
doi:10.1371 /journal.pone.0081508.t001
five specim ens p e r sam ple o f Globigerina bulloides (250-315 pm ) for
Sites 603, 610, FT1308 a n d LU 313 a n d five specim ens p e r sam ple
o f Globigerinoides sacculifer (250-355 pm ) for Site 999. B enthic
foram inifera! isotope d a ta a re based on a t least one > 2 5 0 pm
specim en o f Cibicidoides wuellerstorfi or Uvigerina perigrina p e r sam ple.
Cibicidoides wuellerstorfi S180 values have b e en corrected by ad d in g
0.64 %o [44]. T h e sta n d ard deviation o f the analyses is based o n an
Geochemistry: 81S0 and Mg/Ca o f foraminifera,
calculating and interpreting sea surface tem perature and
relative salinity (818Osw_ice)
F oram inifera w ere picked from the > 1 2 5 p m dry fraction o f
each sam ple. Planktonic foram inifera isotope d a ta w ere m easured
using a F innigan M A T 251 mass spectrom eter at the Isotope
L aboratory, G eosciences D e p artm en t, LTniversity o f B rem en using
T a b le 2 . Palaeomagnetic data for the reversal at the base o f the M am m oth Subchron in DSDP Hole 603C (depth Indicated In bold).
Site
H ole
Core,
ty p e
603
C
17
603
C
17
603
C
603
C
603
Inclination
Section
H alf
Top
(cm)
B ottom
(cm)
Depth
(mbsf)
S am ple
nam e
n
Inc
(low)
Inc
(high)
X
1
W
91
93
136.51
C17S1091
-8 .4
-20.8
4.8
Questionable
reliability
X
2
W
47
49
137.57
C17S2047
-19.3
-43.1
13.2
Questionable
reliability
17
X
3
W
11
13
138.71
C17S3011
-32.9
-35.0
-30.7
17
X
3
W
42
44
139.02
C17S3042
-15.4
-18.1
-12.6
C
17
X
3
W
102
104
139.62
C17S3102
-50.0
-51.7
-48.3
603
C
17
X
3
w
107
109
139.67
C17S3107
-24.7
-29.3
-19.6
603
C
17
X
4
w
119
121
141.29
C17S4119
-19.7
-34.4
-1 .7
603
C
17
X
5
w
41
43
142.01
C17S5041
16.0
12.0
19.9
18.3
603
C
17
X
6
w
42
44
143.52
C17S6042
14.8
11.1
603
C
18
X
1
w
11
13
145.31
C18S1011
-22.9
-33.0
-11.0
603
C
18
X
1
w
99
101
146.19
C18S1099
28.6
27.1
30.1
603
C
18
X
1
w
117
119
146.37
C18S1117
39.6
36.5
42.5
C om m ents
Questionable
reliability
The reversal at the top of the Mammoth Subchron could not be assigned.
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N orthern H em isphere G laciation in a W arm Climate
in-house Solnhofen c arb o n a te sta n d ard w ith a value o f 0.07%o.
V alues are re p o rte d relative to th a t o f the V ie n n a Pee D ee
Belem nite (VPDB) calib rated using N atio n al B u reau o f S tandards
(NBS) 18, 19, a n d 20 standards.
F o r M g /C a m easurem ents, we used 2 0 -2 5 specim ens p e r
sam ple o f G. bulloides (250-315 pm ) o r G. sacculifer (250-355 pm)
a n d 20 specim ens p e r sam ple o f Globorotalia inflata (250-400 pm)
from Sites 610, U 1308 a n d U 1313. T h e cleaning p ro c ed u re for
M g /C a m easurem ents is described elsew here [45]. A fter
dissolution in 0.5 m L 0.075 M Q D H N 0 3, the sam ples w ere
centrifuged a n d diluted for analysis on a n IC P -O E S (Perkin
E lm er O p tim a 3300R) a t the G eosciences D e p artm en t, U n iv er­
sity o f B rem en. T h e analytical precision o f the M g /C a analyses
for G. bulloides, G. sacculifer, a n d G. inflata co m bined was 0.17%
(n = 459). R eproducibility based on replicate sam ples (n = 32) o f
b o th G. bulloides a n d G. sacculifer was ± 0 .1 1 m m o l/m o l (~ 3.3% ).
T h e validity o f analyses was checked by analysing a n artificial inhouse sta n d ard to m o n ito r drift o f the IC P -O E S (M g /C a =
2.93 m m o l/m o l) a n d the lim estone sta n d ard E C R M 752-1 (M g /
C a = 3.75 m m o l/m o l) to allow inter-lab o rato ry com parison [46].
A l / C a, F e /C a , a n d M n / C a w ere sim ultaneously analysed w ith
M g /C a to p re v en t c o n ta m in a te d sam ples from bein g included in
the interp retatio n . W e used the follow ing calibration, established
from core-top sedim ent sam ples in the N o rth A dantic, to
transform the foram inifera! M g /C a ratios o f G. bulloides into
S S T Mg/ Ca: M g /C a = 0 .5 2 exp 0.10 T [47]. W e in te rp re t the
SSTMg/Ca value o f G. bulloides as spring to sum m er SSTs o f the
u p p e r 60 m o f the w ater colum n [26,48,49] because the oxygen
isotope com position o f G. bulloides reflects the n orthw ardm igrating p h y toplankton spring b loom in the N o rth A tlantic
[47,50]. T h e SSTMg/Ca value o f G. sacculifer represents the an n u al
m ixed-layer te m p e ra tu re o f the u p p e r 75 m o f the w ater colum n
for C a rib b e an Site 999 [51]. M g /C a values w ere transform ed
into palaeo-seaw ater tem p eratu res using the follow ing equation:
M g /C a = 0 .4 9 1 exp 0.033 T [52]. A lthough G. inflata calcifies
th ro u g h o u t the w ater colum n, the S W T Mg/ 0a based o n m ostly
no n -en cru sted G. inflata represents the tem p e ra tu re o f the
p e rm a n e n t th erm ocline [53]. W e used the follow ing calibration
to calculate tem peratures: M g /C a
= 0 .7 2 exp 0.076 T .
C om b in in g analytical a n d calibration errors, we estim ate the
e rro r on M g /C a p ala eo te m p e ratu re reconstruction for shallowdw elling foram inifera as ± 1 .0 -1 .5 ° C [47], w hereas for the
deeper-dw elling G. inflata the e rro r is estim ated to b e ± 2 - 2 . 5°C
the Pliocene is detailed elsew here [30,55]. T h e calibration e rro r on
the alkenone SSTs is ~ 1 .5 ° C [59].
T h e global core-top calibration gives the highest correlation
w ith a n n u al m ean SSTs, b u t locally alkenone-based SST s could
reflect the tem p e ra tu re o f the grow ing season (spring in the N o rth
Atlantic) [60]. A lthough this affects the absolute S S T estim ates, it
does n o t influence the relative trends in o u r records. T h e
exception w ould be if the alkenone p ro d u cers shifted their
p ro d u c tio n season o n a glacial/interglacial basis, b u t th ere is no
evidence for such behaviour. H ow ever, if such shifts did occur
d u rin g glacials the alkenone p ro d u cers w ould have delayed their
p ro d u c tio n tow ards sum m er to avoid the colder spring surface
conditions. T his implies th a t the cooling observed in the alkenone
records d u rin g M IS M 2 w ould actually underestim ate the true
cooling.
Carbonate sand fraction
W e gen erated high-resolution c arb o n a te sand fraction d a ta
from O D P H ole 999A over the study interval. In addition, we used
the available low resolution, long-term c arb o n a te sand fraction
re co rd o f the sam e site [27]. T h e sand c o n te n t (> 6 3 pm) o f deepsea c arbonates is considered [27] a sensitive in dicator o f changes in
c arb o n a te dissolution: sand c o n te n t (foram inifer tests) decreases as
dissolution progresses. A low -carbonate sand fraction was in te r­
p re te d to reflect a poorly-ventilated deep C a rib b e an w ater mass.
C a rb o n ate dissolution a t Site 999, caused by entry o f A ntarctic
In term ed iate W ater (AAIW) into the C a rib b e an Basin in place o f
N o rth A tlantic D eep W ater (NADW ), implies a n open C entral
A m erican Seaw ay a n d a w eak overtu rn in g circulation [27].
Palaeomagnetic measurements
T h e positions o f m agnetic reversals for the M a m m o th S ubchron
in D S D P H oles 603C a n d 610A [61], a n d IO D P H ole 1308C [62]
w ere re-m easured in this study to increase precision by analysing
discrete, orien ted sam ples a t 4 -2 3 cm resolution (Tables 2—41).
O rie n te d cubic polystyrene boxes (7.2 c m 3) w ere taken, avoiding
visible m ineral concretions a n d areas influenced by the coring
process, from the w orking halves o f Sections 603C -17X 1 to 603C18x1 (12 sam ples b etw een 136.51 a n d 146.37 mbsf), Sections
610A -17H 3 to 610A -17H 4 (17 sam ples b etw een 156.74 a n d
158.75 mbsf), Sections 610A -17H 6 to 610A -18H 1 (19 sam ples
betw een 161.20 a n d 163.85 mbsf), a n d Sections U 1 3 0 8 C -2 6 H 2 to
U 13 0 8 C -2 6 H 5 (20 sam ples b etw een 230.48 a n d 239.45 mbsf).
T h e discrete sam ples w ere m easu red a t the G eosciences D e p a rt­
m ent, U niversity o f B rem en o n a cryogenic m ag n eto m eter (model
2 G E nterprises 755 H R ). T h e n a tu ra l re m a n e n t m agnetization
(NRM ) was dem agnetized in nine steps (10-100 m T), a n d
inclination a n d relative declination, a n d th eir confidence intervals
w ere d eterm in ed from line fits o f straight-line segm ents in a
Z ijderveld diagram . N ote th a t absolute declination depends on
section a n d core orientations, w hereas inclination values rely on
the fact th a t the drill hole is to a very good appro x im atio n
perp en d icu lar. All ages o f the m agnetic reversals are acco rd in g to
the A T N T S 2004 [63],
In D S D P H ole 610A, the u p p e r b o u n d a ry o f the M a m m o th
S u b ch ro n was found betw een 158.35 m b sf (positive inclination)
a n d 158.75 m b sf (negative inclination). T h e reversal a t the base o f
the M a m m o th S u b ch ro n is m o re difficult to identify due to a
coring gap betw een C ores 610A -17H a n d 610A -18H , a n d
disturbed sedim ent in the u p p e r 25 cm o f Section 610A -18H 1
[64], b u t m ust b e located b etw een 161.85 m b sf (negative
inclination) a n d 163.11 m b sf (positive inclination). T h e reversals
b o u n d in g the M a m m o th S u b c h ro n in IO D P Site U l 308 are
betw een 254.56 a n d 255.46 m cd a t the top, a n d b etw een 262.91
[53].
T h e oxygen isotope com position o f seaw ater (8 O sw) was
calculated via a sta n d ard form ula [54], Since M g /C a a n d S180
w ere m easured o n the sam e planktonic foram iniferal species, the
possible effects o f seasonality a n d h a b ita t differences are
m inim ised. W e used the L R 0 4 global b enthic foram iniferal S180
stack [10] as a n ap p ro x im atio n for changes in ice volum e over the
studied interval. A fter n orm alizing the L R 0 4 record, we subtracted
it from S18O sw, resulting in a 8 18O sw_;ce re co rd th a t approxim ates
local variations in salinity.
Alkenones
All alkenone d a ta from Sites 610 a n d 1308 are new , w hereas
d a ta from IO D P Site U 1313 have b e en published earlier [30,55]
(Figure 3C). T h e m odified alkenone u n sa tu ratio n index U \-¡
[56,57] was m easured using a G C /T O F - M S system [58] on
separate sam ples, taken from the sam e depths as those used for
foram iniferal M g /C a a n d dinoflagellate cyst analyses. U \-¡ in
com bination w ith a global core-top calibration was used to
calculate a n n u al m ea n S S T (top 10 m) [59]. T h e analytical
technique, calibration a n d reliability o f alkenone-based SSTs for
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Decem ber 2013 | V olum e 8 | Issue 12 | e81508
N orthern H em isphere G laciation in a W arm Climate
Table 3. Palaeomagnetic data fo r the reversals at the base and to p o f the M am m oth Subchron in DSDP Hole 61OA.
Hole
Core,
type
610
A
17
610
A
17
610
A
610
Top
(cm)
Inclination
Bottom
(cm)
D epth
(mbsf)
Sam ple
nam e
n
73
75
156.74
Mam-N1821
88
90
156.89
Mam-N1822
98
100
156.99
w
108
110
3
w
115
3
w
127
H
3
w
H
4
w
17
H
4
w
A
17
H
4
610
A
17
H
610
A
17
610
A
610
Inc
(low)
Inc
(high)
41.2
10.6
57.4
48.9
36.4
57.2
Mam-N1823
69.6
44.4
77.2
157.09
Mam-N1824
62.9
53.9
68.5
117
157.16
Mam-N1825
53
45.9
58.3
129
157.28
Mam-N1826
56.6
50.7
61.1
139
141
157.40
Mam-N1827
55.4
30.4
66.6
6
8
157.57
Mam-N1828
51.9
39.2
60
21
23
157.72
Mam-N1829
42.1
-42.5
69.8
w
26
28
157.77
Mam-N1830
45.4
29
55.8
4
w
36
38
157.87
Mam-N1831
49.8
34.3
59.4
H
4
w
52
54
158.03
Mam-N1832
78.2
71.2
81.5
17
H
4
w
69
71
158.20
Mam-N1833
48
40.1
54.1
A
17
H
4
w
84
86
158.35
Mam-N1834
45.3
25
57.3
610
A
17
H
4
w
92
94
1 58.43
Mam-N1835
22.7
2.5
38.4
610
A
17
H
4
w
114
116
1 58.65
Mam-N1836
-17.2
-4 0
12.4
610
A
17
H
4
w
124
126
158.75
Mam-N1837
-30.7
-41.8
-16.5
610
A
17
H
6
w
69
71
161.20
Mam-N1838
-66.5
-7 0
-61.6
610
A
17
H
6
w
88
90
161.39
Mam-N1839
-58.7
-61.6
-55.3
610
A
17
H
6
w
93
95
161.44
Mam-N1840
-50.2
-53.7
-46.1
610
A
17
H
6
w
101
103
161.52
Mam-N1841
-56.8
-58.9
-54.5
610
A
17
H
6
w
117
119
1 61.68
Mam-N1842
64.1
62.3
65.7
610
A
17
H
6
w
134
136
1 61.85
Mam-N1843
-39.2
-41
-37.3
Mam-N1844
Site
Section
H alf
H
3
W
H
3
W
17
H
3
w
A
17
H
3
610
A
17
H
610
A
17
H
610
A
17
610
A
17
610
A
610
C om m ents
610
A
17
H
CC
w
11
13
1 61.97
20.8
10.9
29.6
610
A
18
H
1
w
2
4
1 62.63
Mam-N1845
44.1
43
45.3
610
A
18
H
1
w
8
10
1 62.69
Mam-N1846
-3 .7
-6.5
-1
Disturbed?
610
A
18
H
1
w
14
16
162.75
Mam-N1847
43.4
41.8
45
Disturbed?
610
A
18
H
1
w
23
25
162.84
Mam-N1848
7.1
6.1
8.1
Disturbed?
610
A
18
H
1
w
27
29
1 62.88
Mam-N1849
-20.6
-23.9
-17.1
Disturbed?
610
A
18
H
1
w
31
33
1 62.92
Mam-N1850
-71.3
-74.9
-65.6
610
A
18
H
1
w
41
43
1 63.02
Mam-N1851
-57.1
-59.4
-54.4
610
A
18
H
1
w
50
52
163.11
Mam-N1852
34.2
31
37.3
610
A
18
H
1
w
68
70
163.29
Mam-N1853
9.7
-8 .9
26.6
610
A
18
H
1
w
86
88
163.47
Mam-N1854
57.3
34.5
67.6
610
A
18
H
1
w
101
103
163.62
Mam-N1855
37.4
-12.4
60.2
610
A
18
H
1
w
124
126
163.85
Mam-N1856
17.5
4.7
28.7
Disturbed?
Depths in bold demonstrate the position of the reversals.
Note: The reversal at the base of the Mammoth Subchron is difficult to identify due to a core gap and potentially disturbed sediments in the upper 0-30 cm of Section
610A-18H1.
a n d 264.41 m cd a t the b o tto m [62]. O u r re-assessm ent places the
top o f the subchron betw een 254.59 a n d 254.67 m cd, a n d the
b o tto m betw een 263.21 a n d 263.46 m cd. T h e offset betw een
pred icted a n d m easured position in IO D P Site U l 308 is small a n d
considered to be w ithin the m arg in o f accuracy o f the m ethods. At
IO D P Site U l 313, the re p o rte d p alaeom agnetic reversal o f the
base o f the M a m m o th S u b ch ro n lies a t 153.68 m cd ± 0 .1 m [65],
w ithin the glacial m ax im u m o f M IS M 2. T h e offset m ay b e the
result o f the field geom etry a n d distance b etw een the sites,
v a riation in the m agnetic lock-in tim e a n d d e p th in the sedim ent,
PLOS ONE I w w w .plosone.org
a n d the sites used to determ in e the M a m m o th S u b ch ro n in the
G eom agnetic Polarity T im e Scale.
Age models
A n age m odel was established for each hole (Figures S1-S 4) by
tu n in g its benthic foram iniferal stable oxygen isotope reco rd to the
L R 0 4 b enthic foram iniferal isotope stack [10], w ith the palaeom agnetic reversals as guidelines only, using the software p ro g ram
A nalySeries 2.0.4.2 [66]. T h e accuracy o f each age m odel depends
on the accuracy o f the L R 0 4 benthic stack w hich is estim ated a t 15
7
Decem ber 2013 | V olum e 8 | Issue 12 | e81508
N orthern H em isphere G laciation in a W arm Climate
Table 4. Palaeomagnetic data fo r the reversals at the base and to p o f the M am m oth Subchron in IODP Hole U1308C.
C ore,
ty p e
H a lf
Top
(cm)
B ottom
(cm)
D ep th
(m bsf)
S am p le nam e
Inclination
(°)
Inc (low)
Inc (high)
6
W
7
6
W
15
9
230 .48
Mam-N1857
44.7
35.1
51.9
17
2 3 0 .5 6
Mam-N1858
- 38.1
-42.7
H
6
W
- 32.8
25
27
230.66
Mam-N1859
-76.6
-79.9
25
H
6
-70.5
W
35
37
230.76
Mam-N1860
-6 2
-64.5
C
25
H
-59.1
6
W
42
44
230.83
Mam-N1861
-5 7
-58.6
-55.2
1308
C
25
H
6
W
48
50
230.89
Mam-N1862
-51.8
-53.6
-49.8
1308
C
1308
C
25
H
6
W
56
58
230.97
Mam-N1863
-60.6
-61.5
-59.6
25
H
6
W
65
67
231.06
Mam-N1864
-6 9
-70.5
1308
C
-67.3
25
H
6
W
77
79
231.18
Mam-N1865
-54.8
-56.4
-53.1
S ite
H o le
Section
1308
C
25
H
1308
C
25
H
1308
C
25
1308
C
1308
1308
C
25
H
6
W
87
89
231.28
Mam-N1866
-72.4
-73.1
-71.6
1308
C
25
H
6
W
94
96
231.35
Mam-N1867
-6 7
-67.7
-66.3
1308
C
26
H
5
W
33
35
238.74
Mam-N1868
-51.7
-52.4
-51
1308
C
26
H
5
W
40
42
238.81
Mam-N1869
-75.4
-76.1
-74.6
1308
C
26
H
5
W
48
50
238.89
Mam-N1870
-54.8
-56.7
-52.8
1308
C
26
H
5
W
56
58
238.97
Mam-N1871
-57.9
-61
-54.2
1308
C
26
H
5
W
63
65
239.04
Mam-N1872
-66.5
-6 9
-63.5
1308
C
26
H
5
W
72
74
239.13
Mam-N1873
13
-31.1
46.8
1308
C
26
H
5
W
83
85
2 3 9 .2 4
Mam-N1874
-42.9
-54.4
-24.8
1308
C
26
H
5
W
94
96
239 .35
Mam-N1875
61
0.6
74.4
1308
C
26
H
5
W
104
106
239.45
Mam-N1876
65.6
62.9
67.9
C om m ents
Depths in bold demonstrate the position of the reversals.
kyr b etw een 3 a n d 4 M a [10] a n d the accuracy o f the graphic
correlation o f the benthic 8 1 8 0 records w ith the L R 0 4 tu n in g
target. T h e tie-points used for the age m odels o f each hole a n d
th eir correlation coefficients are p re sen te d in T ables 5 -9 a n d
Figures S 1 -S 5 . T h e ages in the R esults a n d D iscussion section are
re p o rte d w ith high precision (3 decim als) to d em o n strate (1) the
relative age difference b etw een sam ples w ithin one site, a n d (2) the
relative age o f events in relation to the onset (~ 3 .3 1 5 M a), full
glaciation ( ~ 3 .3 0 5 -3 .2 8 5 M a), a n d term in atio n o f M IS M 2
(~ 3 .2 8 5 M a). T hese ages should neith er be considered as absolute
ages, n o r as evidence for suborbital age control.
T h e original age m odel o f O D P Site 999 [27] is based on
correlating the benthic S 180 re co rd to the astronom ically dated
b enthic S180 records from equ ato rial E ast Pacific O D P Site 846
[67] a n d equ ato rial E ast A tlantic O D P Site 659 [68] for the tim e
interval 5 -2 M a. T h e existing age m odel was u p d a te d [51] to the
newly generated, orbitally-tuned age m odel o f E ast Pacific IO D P
Site 1241 [69] (Figure S5). F o r this study, we used the L R 0 4
b enthic S 180 stack [10] to fine-tune the glacial-interglacial
transitions a ro u n d M IS M 2.
T a b le 5 . Tie-points for the age model o f DSDP Hole 610A.
D ep th (m bsf)
A ge (M a)
158.60
3.207
159.00
3.233
159.32
3.237
159.90
3.253
160.20
3.263
D ep th (mcd)
A ge (M a)
161.27
3.285
254.66
3.207
161.56
3.290
259.27
3.265
161.75
3.301
261.11
3.284
163.05
3.315
262.35
3.302
163.89
3.326
263.25
3.320
164.09
3.332
263.38
3.327
167.08
3.596
264.70
3.340
T a b le 6 . Tie-points for the age model of IODP Hole U1308C
Note: mbsf = metres below sea floor.
doi:10.1371 /journal.pone.0081508.t005
PLOS ONE I w w w .plosone.org
Note: mcd = metres composite depth.
doi:10.1371 /journal.pone.0081508.t006
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Decem ber 2013 | V olum e 8 | Issue 12 | e81508
N orthern H em isphere G laciation in a W arm Climate
T a b le 7 . Tie-points for the age model o f IODP Site U1313
T a b le 8 . Tie-points for the age model o f DSDP Hole 603C.
(prim ary splice).
D ep th (m bsf)
A ge (M a)
136.70
3.194
138.25
3.214
3.237
139.79
3.237
153.07
3.285
140.27
3.252
154.39
3.311
142.85
3.286
157.23
3.372
145.71
3.330
159.72
3.419
D ep th (mcd)
A ge (Ma)
149.92
3.224
150.83
Note: mbsf = metres below sea floor.
doi:10.1371 /journal.pone.0081508.t008
Note: mcd = metres composite depth.
doi:10.1371 /journal.pone.0081508.t007
sea levels at th a t tim e [12,15] a n d a longer-term gradual
w eakening o f the th erm o h alin e circulation since 3.6 M a [27] th at
b ro u g h t the clim ate system closer to a tipping point. P rio r to M IS
M G 1 (Figure ID , 5C) high sea levels also occurred, b u t Pacific-toA tlantic through-flow appears not to have w eakened the N A C ,
a n d glaciation in the N o rth e rn H em isphere rem ain ed restricted.
D u rin g m axim al Pacific inflow via the o p en CA S du rin g
interglacial M IS M G 1 ( ~ 3 .3 1 5 -3 .3 2 0 M a), c o n tem poraneous
changes o ccurred in the N o rth A tlantic surface circulation. At
~ 3.315 M a, a m ajor reduction in n o rth w a rd flow o f w a rm N A C
w aters is reflected at Sites 610 a n d LU 308 by a m ajo r tu rn o v er o f
the dinoflagellate cyst assem blages w ithin 1-2 kyrs, a n d a n initial
cooling o f the surface w aters is registered (Figure 3C, 3D , 3G , 3H).
T his c orroborates m odelling studies show ing th a t a n open CAS
results in a w eakened A M O C a n d hence n o rth w a rd h e at tran sp o rt
[71,72]. Betw een ~ 3 .3 1 5 a n d 3.305 M a, a persistent N A C
influence is reco rd ed a t Site LU 308 w hen SSTs decreased further
a n d 0. centrocarpum (our N A C tracer) rem a in e d presen t in low
a b u n d an c e. A t the sam e tim e w e find a p e ak a b u n d an c e o f 0.
centrocarpum (Figure 31) a n d surface-w ater cooling (Figure 3D) at
subtropical gyre Site LU 313. W e in te rp ret this sequence o f events
as a n initial reduction in n o rth w a rd flowing w a rm w ater o f the
N A C at ~ 3.315 M a, follow ed by a grad u al southw ard deflection
o f the N A C betw een ~ 3.315 a n d 3.305 M a. It is im p o rta n t to note
th a t the initial red u ctio n in n o rth w a rd tran sp o rt o f w arm N A C
w ater o ccurred at ~ 3.315 M a w ithin the interglacial M IS M G 1,
well before the M IS M 2 glacial m axim um a t 3.295 M a.
T h e southw ard shift o f the N A C p rio r to M IS M 2 led to the
cessation o f n o rth w a rd h e at tran sp o rt du rin g the full glacial
conditions o f M IS M 2 (3.305-3.285 M a). D u rin g the glacial
conditions, subtropical gyre circulation persisted as attested by a
Results and Discussion
Events during interglacial MIS MG1 leading to glaciation
T h e early L ate Pliocene was w arm er th a n today, a n d p rio r to
~ 3.315 M a o u r geochem ical proxies a n d dinoflagellate cyst
assem blages d em o n strate a surface circulation com p arab le to
to d ay ’s b u t w ith elevated tem p eratu res in the high-latitude N o rth
A tlantic. W e re co rd a n active G u lf S tream over Site 603 as
illustrated by the high SST s (ca. 19.5"C) a n d the presence o f 0.
centrocarpum a n d such w arm w ater dinoflagellate cyst tax a as
Impagidinium aculeatum, I. paradoxum, I. patulum, I. solidum, a n d
Polysphaeridium, zoharyi w hich are also presen t there today [41],
W arm (ca. 20"C) a n d oligotrophic surface w aters a t the subtropical
gyre Site U 1313 are reflected in the dom inance o f I. aculeatum, I.
paradoxum, I. patulum, a n d Invertocysta spp. A n active N A C b ro u g h t
w arm w aters (15.2—18.6"C in the up p erm o st 60 m) n o rth w a rd
over Sites L T 308 a n d 610 (Figure 3C , 3D , 3G , 3H), expressed in
the dinoflagellate cyst assem blages by the dom in an ce o f 0.
centrocarpum a n d the persistent presence o f the w a rm w ater species
Spiniferites mirabilis. T h e less steep m eridional SS T gradient
c o m p a red to present, especially visible in the S S T aik a n d to lesser
extent in the S S T M?/Ca (Figure 3C, 3D , 4), indicates generally
w a rm er conditions in the h igher latitudes c o m p a red to today.
A lthough deeper-w ater exchange via the CA S h a d been
restricted since ~ 4 .6 M a [27], shallow Pacific-to-A tlantic
exchange o ccu rred well into the L ate Pliocene [51]. T his implies
th a t A tlantic m eridional o v ertu rn in g circulation (A M O C) [6] was
able to function even w h en the CA S was partially open. Follow ing
a m ax im u m in A M O C due to m inim al Pacific-to-A tlantic
through-flow a ro u n d 3.6 M a, a grad u al increase in through-flow
via a n o p en CA S c ulm inated im m ediately p rio r to M IS M 2
[27,51]. A t C a rib b e an Site 999, we re co rd b etw een ~ 3 .3 2 0 a n d
~ 3.315 M a a d ro p in S S T a n d salinity (818O sw_;cc) (Figure 5D , 5E),
a low c arb o n a te sand-fraction (Figure 5F), a n d high productivity
evidenced by high dinoflagellate cyst concentrations d o m in a ted by
h e terotrophic species (round b ro w n cysts; Figure 5G , 5F1). T h e low
c arb o n a te sand-fraction indicates a poorly ventilated deep
C a rib b e an w ater m ass a n d c arb o n a te dissolution caused b y entry
o f A ntarctic In term ed iate W ater (AAIW) into the C a rib b e an Basin
in favour o f N o rth A tlantic D eep W ater (NADW ) - in te rp rete d as
evidence o f a w eak overtu rn in g circulation [2 7]. T h e d ro p in SST
a n d salinity p o in t to a n increased inflow o f cooler, less saline
Pacific w aters to the C a rib b e an . F u rth e rm o re , the inferred high
productivity is fully consistent w ith nu trien t-rich w aters from the
Pacific en terin g the C a rib b e an [70], C onsidered altogether, this
evidence shows th a t Pacific-to-A tlantic through-flow via the CAS
d u rin g interglacial M IS M G 1 exceeded a critical threshold,
thereb y reducing the A M O C . T his was likely aided b y the high
PLOS ONE I w w w .plosone.org
T a b le 9 . Tie-points for the age model o f ODP Site 999.
Old age (M a)
N ew age (M a)
3.205
3.205
3.239
3.245
3.276
3.280
3.296
3.295
3.319
3.320
3.342
3.340
3.355
3.365
3.371
3.375
Note: old age from ref. [78], new age from this study.
doi:10.1371 /journal.pone.0081508.t009
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N orthern Hemisphere G laciation in a W arm Climate
610
U1308
U1313
U1308
U1313
13.2 16.3
Full glacial conditions
3.305-3.285 Ma
610
U1308
J 18.6 ,^ ~ '1 î4 l
U1313
' ...... ]16||20.1|2ÖÖ
08
3.332-3.305 Ma
Temperature C
10
12
14
16
18
20
Figure 4. Reconstructed sea-surface tem peratures fo r the studied intervals prior to , d uring and a fte r the full glacial conditions of
MIS M 2 along an eastern N orth Atlantic transect throu gh DSDP Site 6 1 0 , and IO DP Sites U 1 3 0 8 and U 1313 from 6 0 N , 10 W to
3 0 N , 3 0 W . Background rep re sen ts p resen t day sea-w ater tem p e ra tu re s o f th e u p p e r 500 m (from WOA2005, [81]). The insets belo w each site
rep re sen t th e surface w ater tem p e ra tu re based on alkenones (alk; surface w ater 0 -1 0 m), Mg/Ca ratios o f Globigerina bulloides (mgca; mixed layer 0 60 m) and sea-w ater te m p e ra tu re a t 3 0 0 ^ 0 0 m based on Mg/Ca ratios o f Globorotalia inflata (inf). The surface w aters (SSTa!k, SSTMg/Ca G. bulloides)
sh o w im p o rtan t cooling during MIS M2 w ith a ste e p N-S te m p e ra tu re g rad ien t established. Low resolution sea-w ater tem p e ra tu re (SWTMg/Ca)
reco n stru ctio n s o f Globorotalia inflata show th a t w ater a t th e p e rm an en t therm ocline (300-400 m d epth) rem ained stab le and w arm th ro u g h o u t th e
entire stu d ied period. The glacial had no m ajor effect on th e d e e p e r surface w aters, ex cep t possibly at Site U1308 w here o n e sam ple recorded
tem p e ra tu re s as low as 11.8 C during MIS M2. The average values o f 13.5, 14.2 and 15.1 C at Site 610, U1308 and U1313 respectively over th e entire
period illustrate th a t th e entire u p p er w ater colum n during th e Pliocene w as w arm er th an to d ay in th e North Atlantic.
doi:10.1371/journal.pone.0081508.g004
S candinavia [74], T h e increased m eridional S S T grad ien t will
have red u ced air tem p e ra tu re a n d increased snowfall over m ost o f
N o rth A m erica, b o th factors favourable to ice sheet inception [29].
W e dem onstrate th a t sufficiently cool surface w aters w ere present
in the n o rth e rn high latitude oceans, a n d propose th a t these w ere
crucial for the glaciation in the N o rth e rn H em isphere d u rin g M IS
M 2. T h e m oisture req u ire d to b u ild a large ice sheet in the
N o rth e rn H em isphere was p resum ably already presen t in the
atm osphere, because Pliocene clim ates w ere generally w etter th an
today [75]. It is nevertheless likely th a t after the southw ard shift o f
the N A C a n d cooling o f the n o rth e rn high-latitude surface w aters,
carb o n cycle (vegetation, C 0 2) [28,71] a n d p erh ap s sea ice
(albedo) feedbacks also c o n trib u ted to the m ajo r glaciation du rin g
M IS M 2. N evertheless, the extent o f N o rth e rn H em isphere
glaciation du rin g M IS M 2 rem ain ed sm aller th a n a typical
Q u a te rn a ry glaciation, b u t m ay have b e en larger th a n at present.
T h e h igher 8 18O bcnthic values d u rin g M IS M 2 c o m p a red to today
(Figure 1A; 3.74%o vs. 3.23%o, [10]) in d eed im ply th a t M IS M 2 ice
sheets w ere larger th a n today. In d irect evidence o f ex p an d ed ice
sheets in the N o rth e rn H em isphere is found in several sedim ent
a n d ice-rafted debris records from the A rctic O c ea n , N ordic Seas
a n d n o rth e rn N o rth A tlantic [20-23] w hich indicate th a t the
G re en la n d a n d S v a lb ard /B a re n ts Sea ice sheets re ac h ed the
coastline. G lacial deposits o n Iceland [24] a n d possibly also in the
C a n a d ia n R ocky M ountains a n d A laska [25] dem onstrate the
presence o f ice caps there. W ith SSTs a p p ro ac h in g presen t day
values (Figure 3C, 3D) a n d a H olocene-like A rctic clim ate
prevailing du rin g M IS M 2 [16], the developm ent o f a significant
c ontinuance o f the G u lf S tream at Site 603 w here no m ajor
changes in the dinoflagellate cyst assem blages w ere recorded
(Figure 3J). T h e contrasting p roxy evidence o f cool surface S S T alk
[30], w a rm er m ixed-layer S S T M?/Ca a n d largely u n c h an g e d
dinoflagellate cyst assem blages at subtropical gyre Site U l 313 is
difficult to in te rp ret (Figure 3G, 3D , 31). D u rin g the earlier glacials
M IS M G 4 a n d M G 2, b o th geochem ical proxies re co rd a cooling,
suggesting a fundam entally different ocean o g rap h y for M IS M 2. It
is n o t know n w h eth er the divergence in S S T M?/Ca values du rin g
M IS M 2 was caused by different a genotype o f G. bulloides
b ecom ing d o m in a n t in a ch an g ed oceanographic setting [73].
Irrespective o f the ultim ate cause, we consider the co ntrasting SST
proxy records in com b in atio n w ith the palynological d a ta as
evidence o f a southw ard shift o f the N A C th a t affected Site U 1313
d u rin g M IS M 2 b u t n o t d u rin g p rio r glacials.
Glaciation in the Northern Hemisphere during MIS M2
A t the two n o rth e rn sites, dinoflagellate cyst assem blages
indicate subpolar conditions (Bitectatodinium tepikiense, Filisphaera
filifera, I. pallidum, Nematosphaeropsis labyrinthus, Pentapharsodinium dalei)
at Site 610 a n d oligotrophic conditions (I. aculeatum, I. paradoxum) at
Site U l 308 (Figure 3G , 3H), while surface w aters a t b o th sites
cooled b y 3M "C to tem p eratu res only ju st h igher th a n today
(Figure 3C , 3D). T his cooling a t the n o rth e rn sites established a
steep latitudinal SS T g radient in the N o rth A tlantic (Figure 4),
causing the th erm al isolation o f G re en la n d from n o rth w a rd heat
tran sp o rt. As a com parison, a 3 -4 "C cooling o f the N ordic Seas
was necessary for the last glacial inception (~ 115,000 years ago) in
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Decem ber 2013 | V olum e 8 | Issue 12 | e81508
N orthern H em isphere G laciation in a W arm Climate
Age (Ma)
3.
PMAG
M am m o th
3.285
O
c
a>
fu ll glacial
conditions
Site 999
1
£o
CO
CO
%)
o
~
3.03.23.43 .8 4 .0 -
20— E -2 0 -
Wanganui Basin [12]
B ased on benthic isotope stack [13]
Mg/Ca of ostracoda ¡14]
Cooling starts
to MIS M2
I-
«
23
Warming starts
early in MIS M2
999 8180
modern
NADW
8 -5
i-
£
03
O
1
20
15
13
10-
NADW
q
Round brown cysts
[ Impagidinium aculeatum
Impagidinium paradoxum
Impagidinium patulum
Polysphaeridium zoharyi
Operculodinium israelianum
Invertocysta lacrymosa
Operculodinium centrocarpum
Nematosphaeropsis labyrinthus
Spiniferites spp.
Others
-Q oo
k” w
k
i
Productivity
800
£400
i
3.180
r
3.200
n
3.220
r-
1
3.240
r-
1
3.260
1
r
3.280
3.300
3.320
r-
1
r-
3.340
1
3.360
r-
n
3.380
1------ 1
3.400
Age (Ma)
Figure 5. Caribbean Sea palaeoceanographic pro xy records fro m O DP Site 9 9 9 b etw een 3 .4 0 0 and 3 .1 8 0 M a. Vertical grey bars
rep re sen t glacials, vertical w hite bars are interglacials. All circles with w hite fill are d a ta from this study. (A) P alaeom agnetic reversals; (B) b en th ic
iso to p e ag e m odel for Site 999 tu n e d to th e LR04 stack [10] (black line); (C) sea level estim ates; (D) SST-Mg/ca of G. sacculifer, thick black line rep resen ts
4-p o in t running m ean, shading rep resen ts calibration error; (E) 8 18Osw.ice estim ate of salinity; (F) carb o n ate-san d fraction, filled black circles are d ata
from [27], thick line rep re sen ts 4-point running m ean. Lower (higher) values reflect decreased (in-creased) North Atlantic D eep W ater (NADW)
influence a t th e site. (G) dinoflagellate cyst assem b lag e com position; presence o f round brow n cysts indicate high productivity and inflow of Pacific
w ater; (H) dinoflagellate cyst concentration, including error bar (light blue shading).
doi:10.1371/journal.pone.0081508.g005
N o rth A m erican ice sheet is unlikely. T herefore, to explain the
observed ~0.5% o benthic isotope shift [10], a considerable
expansion o f the A ntarctic ice sheet m ust have occu rred also
[16], N evertheless, the possibility o f a n ice cap in N o rth A m erica
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d u rin g M IS M 2 should not be excluded given the evidence o f an
ice cap in the N o rth A m erican interior th a t did n o t reach the
N o rth A tlantic coastline at ~ 3 .5 M a [76], w hen glacials (e.g. M IS
M G 6) w ere less severe th a n d u rin g M IS M 2 (Figure 1).
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N orthern Hemisphere G laciation in a W arm Climate
(1 ) Northward heat transport via NAC
Interglacial high se a level
Northward heat transport via active NAC.
SSTs in mid- to high-latitude N. Atlantic
are 2 -3 'C higher than today.
Mountain glaciation on Greenland.
Central American Seaw ay is open.
Pacific
C aribbean
cool, lower salinity water
W
*
1
P an am a
high
productivity
\
/
!
AMOC AND NORTHWARD HEAT
TRANSPORT VIA THE NAC
re-established after expansion
and warming of CWP
Î
PLIOCENE
WARM CLIMATE
MAXIMAL PACIFIC-TOATLANTIC TROUGH-FLOW
via open P anam a Seaw ay
during interglacial MG1
- 3 .2 8 5 Ma
GLACIALLY CLOSED PANAMA
Sea-level drop halted inflow of cool
and fresh Pacific water into the
Caribbean. From glacial maximum,
the Caribbean Warm Pool builds up.
Pacific
- 3 .3 1 5 Ma
1
REDUCTION IN NORTHWARD
FLOWING WARM WATER VIA NAC
Southward shift of the NAC
MIS M2
GLACIATION
C aribbean
A
hig h e r S ST
P an am a
Isthm us
No NAC, no northward heat transport.
Mid- to high latitude Atlantic SSTs 3 -4 "C cooler.
Glaciation of Greenland, Iceland, SvalbardBarents S ea and questionably N. America.
Central American S eaw ay is closed.
Glacial lower se a level
Cool high-latitude o ceans
Figure 6. Conceptual m odel o f glaciation and deglaciation o f the N o rthern Hem isphere d uring MIS M 2 in th e otherw ise g lo bally
w arm ea rly Late Pliocene. N um bers show se q u en ce o f events.
doi:10.1371/journal.pone.0081508.g006
sea-surface te m p e ra tu re grad ien t (Figure 4), a n d a G re en la n d
ice sheet th a t was red u ced to isolated m o u n ta in glaciers [4], As
such, the glaciation d u rin g M IS M 2 appears responsible for its
ow n dem ise.
Glacial closure o f the Central American Seaway led to
deglaciation
O u r results furth er d em o n strate th a t the sea level d rop a t the
full glaciation o f M IS M 2 [12,15] closed the CA S a n d effectively
halted the inflow o f Pacific w ater into the A tlantic realm . T h e
oligotrophic conditions at Site 999 show n in the absence o f
h e terotrophic dinoflagellate species a n d low cyst concentrations
(Figure 5G , 5H) d u rin g M IS M 2 suggest no inflow o f nutrientrich Pacific w aters [70], T h e increasing SS T a n d salinity
(S18O sw_;cc) at Site 999 from the glacial m axim um at ~ 3 .2 9 5
M a onw ards show th a t C a rib b e an surface w aters becam e
w a rm er a n d m ore saline w hile rem ain in g oligotrophic
(Figure 5D , 5E, 5G , 5H ). T h is is a reflection o f the build-up o f
the C a rib b e an W a rm Pool alread y from the glacial m axim um
onw ards. T h e expansion a n d w a rm ing o f the C a rib b e an W arm
Pool are essential for re-establishing A M O C a n d n o rth w a rd heat
tran sp o rt, as observed for the last déglaciation [77]. A t a ro u n d
3.285 M a, Site 999 is characterised by high SSTs a n d salinity, a
re tu rn to biologically productive conditions, a n d increased
c arb o n a te
preservation
(high
c arb o n a te
sand-fraction)
(Figure 5F). T his indicates a C a rib b e an W a rm Pool sufficiently
large a n d w arm to re-invigorate the A M O C . T h e re-established
n o rth w a rd h e at tran sp o rt a n d active N A C flowing along its
m o d ern p a th w ay a ro u n d 3.285 M a is reflected b y the rap id
tu rn o v er w ithin 1-2 kyrs o f the dinoflagellate cyst assem blages in
the eastern N o rth A tlantic Sites 610 a n d U l 308 w here 0.
centrocarpum becom es d o m in a n t again (Figure 3G , 3F1). By then,
the w a rm clim ates o f the m P W P [1] w ere established, w ith N o rth
A tlantic SSTs ~ 3 " C above presen t values (Figure 3C!, 3D), a
m odern-like A M O C [5] b u t w ith a red u ced m eridional
PLOS ONE I w w w .plosone.org
Conclusions
O u r study identifies links b etw een CA S through-flow , N A C
variability, high latitude sea-surface tem peratures, a n d N o rth e rn
Flem isphere glaciation d u rin g L ate Pliocene M IS M 2 (~ 3 .3 0 M a).
W e provide a conceptual m odel based o n palynological a n d
geochem ical records in the N o rth A tlantic a n d C a rib b e an for
glacial expansion a n d consequent déglaciation d u rin g a n otherw ise
globally w arm er w orld (Figure 6).
A long-term global cooling tre n d (reflected in SST, C 0 2, a n d
ice volum e records; Figure 1) p reco n d itio n ed the N o rth e rn
Flem isphere for glaciation d u rin g the early L ate Pliocene.
Flowever, the ultim ate tipping p o in t for intense glaciation du rin g
M IS M 2 was the through-flow o f Pacific w ater via a n open CAS
into the A tlantic, ultim ately resulting in a steep S S T grad ien t in
the N o rth A tlantic a n d th erm al isolation o f the high latitudes. A n
open C A S as the trigger for N o rth e rn F lem isphere glaciation
contrasts w ith the usually invoked CA S closure as either the
cause, p recondition, o r delaying factor for the intensification o f
N o rth e rn F lem isphere glaciation w hich occu rred 500,000 years
late r [27,33,34], R ecen t m odelling experim ents indicate th a t the
closure o f the CA S actually h a d no effect o n the L ate Pliocene
G re en la n d ice sheet, a n d d em o n strate th a t declining atm ospheric
c arb o n dioxide concentrations w ere the driving factor b e h in d the
intensification o f N o rth e rn F lem isphere glaciation a t ~ 2 .7 5 M a
[28,71],
12
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N orthern H em isphere G laciation in a W arm Climate
O u r records in fact dem onstrate th a t the glacio-eustatic closure
o f the CA S d u rin g M IS M 2 eventually re-established n o rth w a rd
h e a t tran sp o rt in the N o rth A dantic. Follow ing the expansion o f
the A ntarctic a n d N o rth e rn H em isphere ice sheets (including
G reenland, Iceland, S v a lb ard /B a re n ts region a n d questionably
the interior o f N o rth A m erica) to a volum e seem ingly larger th an
present, sea level fell to m ore th a n 10 m a n d possibly as m u ch as
65 m below p re sen t (Figure IB). T h is closed the CA S a n d halted
the flow o f Pacific w ater into the N o rth A tlantic, allow ing the
C a rib b e an W arm Pool to accum ulate. In tim e, this re-invigorated
the G u lf S tre a m /N o rth A tlantic C u rre n t system a n d provided
n o rth w a rd h e a t tran sp o rt, leading to high-latitude N o rth A tlantic
surface w aters th a t w ere 3°C w a rm er th a n p re sen t a n d consequent
re tre a t o f the G re en la n d ice sheet to m ountainous areas in the east
a n d southeast d u rin g the m PW P.
T h e transition from M IS M 2 to the m P W P can be seen as the
evolution o f a w orld w ith com p arab le global tem p eratu res to
p re sen t a n d slightly larger ice sheets, to a w orld w ith global
tem p eratu res ~ 3 ° C higher th a n today a n d glaciation strongly
dim inished a n d localised in the N o rth e rn H em isphere. A lthough
o p eratin g o n a longer tim e scale, this clim ate transition can
provide valuable insights into the p re sen t anthropogenically-forced
clim ate transition tow ards a globally w arm er planet, being
com p arab le to projections for the e n d o f this century. In view o f
this p rojected clim ate w arm ing, o u r results from the L ate Pliocene
show th a t high-latitude N o rth A tlantic surface circulation a n d
SSTs are a crucial factor in the expansion a n d con tractio n o f
N o rth e rn H em isphere ice sheets.
points used, a n d correlation values o f the b enthic reco rd ru n n in g
m ea n a n d raw d a ta w ith the L R 0 4 global stack. N ote: H ole 610A
shows a coring gap b etw een C ores 610A -17H a n d 610A -18H , a n d
sedim ent d isturbance in the u p p e r 25 cm o f Section 610A -18H 1.
(TIF)
Figure S2 A ge m o d e l fo r IO D P S ite U 1308 b a s e d o n th e
c o r r e la tio n o f o x y g en is o to p e r e c o r d s fr o m th e stu d ie d
in te r v a ls w ith th e LR 04 b e n th ic o x y g e n is o to p e g lo b a l
s ta c k [10]. Left, middle and right panel and inset as for Figure S I.
(TIF)
Figure S3 A ge m o d e l fo r IO D P S ite U 1313 b a s e d o n th e
c o r r e la tio n o f o x y g en is o to p e r e c o r d s fr o m th e stu d ie d
in te r v a ls w ith th e LR 04 b e n th ic o x y g e n is o to p e g lo b a l
s ta c k [10]. Left, middle and right panel and inset as for Figure S I.
(TIF)
Figure S4 A ge m o d e l fo r D S D P S ite 603 b a s e d o n th e
c o r r e la tio n o f o x y g en is o to p e r e c o r d s fr o m th e stu d ie d
in te r v a ls a n d p a la e o m a g n e tic r e v e r s a ls w ith th e LR04
b e n th ic o x y g e n is o to p e g lo b a l sta c k [10]. Left, middle and right
panel and inset as for Figure S 1.
(TIF)
Figure S5 S h ow n o n th e le f t a re th e b e n th ic 5 ls O g lo b a l
LR 04 sta c k [10] c o m p a r e d to th e b e n th ic 5 ls O r e c o r d o f
IO D P S ite 1241 [69]. O n the right, the L R 0 4 global stack is
c o m p a red to the old [51] a n d new (this study) b en th ic S180 curve
o f O D P Site 999. T h e latter is a fine-tuning o f the [51] reco rd to
the L R 0 4 stack.
(TIF)
Supporting Information
Figure SI A ge m o d e l fo r D S D P H o le 610A b a s e d o n th e
c o r r e la tio n o f o x y g en is o to p e r e c o r d s fr o m th e stu d ie d
in te r v a ls w ith th e LR 04 b e n th ic o x y g en is o to p e g lo b a l
sta c k [10]. Left panel: core-sections, polarity subchrons, including
Acknowledgm ents
M. Segl is thanked for isotope m easurem ents, S. Pape for assistance with
the IC P-O ES, and M . Hoins, S. Forke and J. Engelke for assistance in the
lab (M ARUM and University of Bremen). Samples were supplied by the
Integrated O cean Drilling Program . W e thank Alan M. H ayw ood and one
anonym ous reviewer for the constructive comments.
uncertain ty interval for the exact position o f each reversal o f the
M a m m o th Subchron, a n d b en th ic isotope re co rd against d ep th
(mbsf). Middle panel: correlation o f the b e n th ic reco rd (thin re d line,
raw data; thick re d line, 4 -point ru n n in g m ean) to the L R 04 global
stack o f b en th ic isotope records [10] plo tted against tim e. G rey
shading represents the m arin e isotope stage bo u n d aries from [10]:
m arin e isotope stage M 2 was defined b etw een 3.264 a n d 3.312
M a. W e consider the full glaciation to occur b etw een 3.305 a n d
3.385 M a (light grey). T h in black lines b etw een left a n d m iddle
pan el show the tie points used (listed in inset). Right panel:
sedim entation rate based on o u r age m odel. Inset gives the tie
Author Contributions
Conceived and designed the experiments: SDS J G BDAN. Perform ed the
experiments: SDS J G BDAN C V R J H KF. Analyzed the data: SDS J G
BDAN C V R J H M JH SL KF. C ontributed reagents/m aterials/analysis
tools: SDS JG BDAN M JH SL KF. W rote the paper: SDS JG BDAN C V R
J H M JH SL KF.
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