Pliocene Warmth, Polar Amplification, and Stepped Pleistocene
Cooling Recorded in NE Arctic Russia
Julie Brigham-Grette et al.
Science 340, 1421 (2013);
DOI: 10.1126/science.1233137

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RESEARCH ARTICLE
Pliocene Warmth, Polar Amplification,and Stepped Pleistocene Cooling
Recorded in NE Arctic Russia
Julie Brigham-Grette,1* Martin Melles,2 Pavel Minyuk,3 Andrei Andreev,2 Pavel Tarasov,4Robert DeConto,1 Sebastian Koenig,1 Norbert Nowaczyk,5 Volker Wennrich,2Peter Rosén,6 Eeva Haltia,5† Tim Cook,7 Catalina Gebhardt,8 Carsten Meyer-Jacob,6Jeff Snyder,9 Ulrike Herzschuh10
Understanding the evolution of Arctic polar climate from the protracted warmth of the middle
Pliocene into the earliest glacial cycles in the Northern Hemisphere has been hindered by the lackof continuous, highly resolved Arctic time series. Evidence from Lake El’gygytgyn, in northeast(NE) Arctic Russia, shows that 3.6 to 3.4 million years ago, summer temperatures were ~8°C
warmer than today, when the partial pressure of CO2 was ~400 parts per million. Multiproxyevidence suggests extreme warmth and polar amplification during the middle Pliocene, sudden
stepped cooling events during the Pliocene-Pleistocene transition, and warmer than present Arctic
summers until ~2.2 million years ago, after the onset of Northern Hemispheric glaciation. Our data
are consistent with sea-level records and other proxies indicating that Arctic cooling wasinsufficient to support large-scale ice sheets until the early Pleistocene.

*Corresponding author. E-mail: juliebg@geo.umass.edu†Present address: Department of Geology, Lund University,
Sölvegatan 12, S-223 62 Lund, Sweden.

www.sciencemag.org

Lacustrine Impact
sediments rocks

1
Department of Geosciences, University of Massachusetts, 611
North Pleasant Street, Amherst, MA 01003, USA. 2Institute ofGeology and Mineralogy, University of Cologne, Zuelpicher
Strasse 49a, D-50674 Cologne, Germany. 3North-East Interdisciplinary Scientific Research Institute, Far East Branch of
the Russian Academy of Sciences, Portovaya Street 16, 685000
Magadan, Russia. 4Free University Berlin, Institute of Geological
Sciences, Malteser Strasse 74-100, Haus D, D-12249 Berlin,
Germany. 5Helmholtz Centre Potsdam, GFZ German ResearchCentre for Geosciences, Telegrafenberg, D-14473 Potsdam,
Germany. 6Department of Ecology and Environmental Science,Umeå University, SE-901 87 Umeå, Sweden. 7Department of
Physical and Earth Sciences, Worcester State University,
Worcester, MA 01602, USA. 8Alfred Wegener Institute for
Polar and Marine Research, Am Handelshafen 12, D-27570
Bremerhaven, Germany. 9Department of Geology, BowlingGreen State University, OH 43403, USA. 10Alfred Wegener
Institute for Polar and Marine Research, Research Unit
Potsdam, Telegrafenberg A43, D-14473 Potsdam, Germany.

Fig. 1. Location of Lake
El’gygytgyn and strati- A
graphic cross section. (A)9Lake El´gygytgyn (star) formed
10
3.58 Ma in NE Russia, when
867
1 2
North America remained a5contiguous landmass (and3 4
had a more continental climate), unbroken by tectonics
and glacially eroded marine
troughs (33) (map courtesyof Ron Blakely). Selected Pliocene and early Pleistocene
sites mentioned in the text
and supplemental materials
NW
SE
are shown (1, Nome; 2, Gubik
Formation; 3, Klondike Area;
4, Lost Chicken; 5, Banks Island; 6, Hvitland Beds; 7,5011-3Lake El'gygytgyn
Meighen Island; 8, Strathcona
55
Fjord; 9, Kap København; 10,5011-1141
Lz1024 A B C
Lodin Elv). (B) Schematic cross0
Quaternary
section of the El´gygytgyn
123basin stratigraphy, showing(m)Pliocene
the location of ICDP sites
318
5011-1 and 5011-3. At site
supposed talik/420
permafrost border
central
5011-1, three holes (1A, 1B,
B
uplift0
km 2
517
and 1C) were drilled to replicate the Quaternary and upCore loss/gap
Impact rock
Permafrost sediments
Lacustrine sediments
permost Pliocene sections.
Hole 1C penetrated the remaining lacustrine sequence down to a depth of 318 m and then ~200 m into
the impact rock sequence underneath. Lz1024 is a 16-m-long percussion piston core taken in 2003 thatfills the stratigraphic gap between the lake sediment surface and the top of drill cores 1A and 1B (18).
Permafrost
sedimentsTthe earlier middle Pliocene, millennial-scalevariability of cooler Pliocene climate states, and
the Pliocene/Pleistocene transition [boundary
at 2.588 Ma (7)]. The terrestrial Arctic is richin Pliocene stratigraphic sequences that docu-BB

he geologic record makes it clear that
global temperatures are intimately linked
to changes in atmospheric greenhouse gas
concentrations (1). Today’s CO2 levels are similar
to Pliocene estimates [4.5 to 2.588 million years
ago (Ma)], ranging from 380 to 450 parts per million (ppm), or some 30% higher than preindustrial levels (2, 3). Pliocene warmth, considered
an analog for future climate scenarios, persisted
across repeated warm and cool Milankovitch cycles, raising questions concerning climate sensitivity to CO2. Moreover, it remains unclear whatmechanisms drove the amplification of polar
temperatures and past reductions in sea ice (4, 5)
and the eventual cooling and growth of continental Northern Hemispheric ice sheets. The
PRISM climate mapping program (6) compiled
important global climate information for theinterval from 3.26 to 3.02 Ma but overlooked

ment time slices when conditions were much
warmer than today (8–10). Yet marine sediments on the Lomonosov Ridge suggest that
the Arctic Ocean has maintained perennial sea
ice (11–13) or perhaps periods of seasonal sea
ice since the middle Miocene (14). The discordance between terrestrial and Arctic marine records
has remained unresolved largely because of thelack of continuous land records for comparison(15, 16).
Here we present results from a continuous,
middle Pliocene to early Pleistocene lacustrine
record from the terrestrial Arctic, derived from
Lake El’gygytgyn, located 100 km north of the
Arctic Circle in Chukotka, northeastern Russia(67°30’N, 172°05’E; Fig. 1). The basin was createdby a meteorite impact 3.58 T 0.04 Ma (17) andcontains a continuous sediment sequence recording a complete Late Cenozoic climate history on
centennial to millennial time scales. The data here
demonstrate extreme warmth and polar amplification during the middle Pliocene, stepped cooling
events during the Pliocene-Pleistocene transition,and the recurrence of Arctic summer warmth until
after 2.2 Ma, after the paced onset of Northern
Hemispheric glaciation (3, 18, 19). Recurring
but declining warmth in the Arctic borderlands
has implications for understanding the transition from a forested Arctic in the middle Pliocene
to the tundra-dominated landscape of today.
Moreover, the data raise questions concerning

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the seasonal persistence of Arctic sea ice in thepast, the evolution of Northern Hemisphere ice
sheets, and the onset of major glacial-interglacial
cycles (20, 21).
Lake Setting, Core Acquisition,and Methods
At the time of the El´gygytgyn impact 3.6 Ma(17), the North American Arctic had a more
continental geography (Fig. 1), forests reachedthe Arctic Ocean coast (10), Greenland was mostly ice-free (22, 23), and permafrost was notwidespread (10, 24). Today Lake El’gygytgyn
is 170 m deep and 12 km in diameter, with anarea of 110 km2 within a catchment of only
293 km2 (25). It is cold, monomictic, and oligotrophic, with a summer temperature of ~3°C.
Distinct lake-level changes are recorded by terraces
and wave-cut notches as high as 35 to 40 m above
and 10 m below the modern lake level, with thehighest benches occurring in the early basin
history (26, 27).
The Lake El’gygytgyn region today has amean annual air temperature (MAT) of –10°C,
with mean July temperatures of ~+8°C and average winter lows of ~–35°C. Mean annual
precipitation (PANN) is about 200 mm/year.
Reanalysis data show that the climatology of thebasin is representative of much of the western
Arctic (25). Although the lake area currently lies
within the subzone of southern shrub tundra,
today’s climatology supports only sparse vegetation, dominated by hummock and moss tundra
over deep permafrost with some prostrate willows
and dwarf birch.
Drilling at Lake El’gygytgyn occurred inspring 2009 from a lake-ice platform (28). Lakebasin stratigraphy is based on seismic surveys (29)suggesting continuous sedimentation over thetime period of interest. The 318-m-long composite sequence representing the complete 3.6 Ma
record in International Continental Scientific Drilling Program (ICDP) core 5011-1 (Fig. 1) was investigated using initial nondestructive scanningand logging technologies along with multiproxyinvestigations on subsamples (supplementarymaterials). The core chronology is based on magnetostratigraphy and tuning of proxy data withmarine isotope stratigraphy (21) and summer
insolation at 67.5°N (30) (supplementary materials and figs. S4 and S5).
The sedimentology is composed of five pelagic facies (Fig. 2D; supplementary materials
and figs. S1 and S2). One striking feature of
the sediments is the remarkably cyclic nature ofmany proxies, including biogenic silica content(BSi%) and accumulation rate (BSi acc. rate),
Si/Ti ratio, and the concentration of total organiccarbon (TOC) throughout the record younger
than 2.65 Ma (18) (Fig. 2, E and F, and fig. S4).
BSi percentage and acc. rate and Si/Ti ratios areshown to be robust proxies of the lake’s primary
productivity, with higher values generally associated with high summer insolation and relatively
light marine isotopic values (Fig. 2, A and B).

1422

Similar regular cycles are not seen >2.65 Ma.
In contrast, high productivity is implied by highSi/Ti ratios and BSi acc. rate from 3.3 to 3.1 Ma(Fig. 2, E and F), but in the early lake history
before 3.2 Ma, the productivity signal is diluted
by higher clastic flux rates, as evoked by the BSi
acc. rate, low Si/Ti ratios, and higher sedimentation rate (Fig. 2E and fig. S3).
Mid-Pliocene Extreme
Our Lake El’gygytgyn record provides a continuous terrestrial record showing the extreme
Arctic warmth from 3.6 to 3.4 Ma that other,
shorter records of the middle Pliocene have inferred (31). The earliest sediment history in thelake begins a few thousand years after the impact, as hydrothermal activity ceased and thecrater cooled and filled with water. The first 17 mof post-impact sediment (~the first 20,000 years)
is almost devoid of pollen and other microfossils, but after 3.56 Ma, the pollen stratigraphy
provides the best descriptor of paleoenvironmental conditions (supplementary materials). The
mean temperature of the warmest month (MTWM,
Fig. 2H) and PANN (Fig. 2I) reconstructed fromthe pollen record imply values of +15° to 16°C
and ~600 mm/year, respectively. These estimatesare 7° to 8°C warmer and 400 mm wetter than
today. Similar values were reconstructed for the
Pleistocene super-interglacial marine isotope stage(MIS) 11c (18), but this time interval lacks fir
and hemlock pollen, highlighting the exceptional warmth and wetter conditions of the middle
Pliocene. Cooler intervals of the record occur
at 3.46 and 3.42 Ma during summer insolation
minima and maxima in the marine oxygen isotope stack (e.g., MG6).
Elevated BSi deposition (Fig. 2E) suggests
greater seasonal productivity and is consistent
with generally warmer and wetter conditions between 3.56 and 3.4 Ma. The dominant planktonic
diatom (Pliocaenicus sp.) is significantly larger
there than during any other interval in the entire lake record, suggesting extremes in nutrientdelivery and reduced seasonal ice cover. This isconsistent with higher rates of siliciclastic sedimentation, probably caused by increased precipitation, steeper relief, and reduced or absent
permafrost within the catchment (supplementary materials, fig. S3).
Mid-Pliocene warmth has previously been
documented in other parts of the terrestrial Arctic,
but those records are limited in duration, precluding comparisons with orbital forcing. Nonetheless, they provide a vital spatial network ofsites for characterizing the Arctic borderlands.
The Strathcona Fjord Beaver Pond dated to
3.4 Ma (32) (Fig. 1, site 7) spans ~40,000 years.
Reconstructed MATs were continuously +19°C
warmer than today (–1.6°C), and summer growing season temperatures were in the range of
14°C (8, 9, 32). The Canadian Archipelago remained a contiguous landmass at this time (33)(Fig. 1), with forests likely extending to the coastof the Arctic Ocean (10). At the same time, sea

21 JUNE 2013

VOL 340SCIENCEsurface temperatures (SSTs) in the North Atlantic
were 18°C [~7°C warmer than today (Fig. 3D)(34)], and latitudinal temperature gradients werelower (35, 36) (Fig. 3H). In Antarctica, persistentopen-water conditions in the Ross Embayment
and a reduced West Antarctic Ice Sheet from
4.5 to 3.4 Ma imply 1.2 million years of continuous warmth (37) (Fig. 3J). Hence, in the midPliocene, both polar regions were substantiallywarmer than at present. At Lake El’gygytgyn,
warm conditions continued even during orbits
with reduced boreal summer insolation (Fig. 2,
B versus H).
Pliocene-Pleistocene Warmth
Reconstructed MTWM (Fig. 2H) and PANN
(Fig. 2I) from 3.26 to 2.2 Ma also indicate a
protracted period of warm and moist conditions~3° to 6°C warmer and <100 to >200 mm wetter than in modern times and warmer than thelast interglacial MIS 5e (18). Beyond the exceptional vegetation response (Fig. 2, G to I),
BSi (Fig. 2E) and TOC (fig. S4) are also elevated during most of this time. Important exceptions to these warm conditions include
marine isotopic intervals labeled MIS 100, 96,
and 86 (Fig. 2A), when MTWM estimates dip
near or below modern values (Fig. 2H) andapproach cold temperatures similar to thoseestimated by the same proxies for late Pleistocene glacial conditions [the Last Glacial Maximum (LGM), Fig. 3A]. Summers cooler than
today are largely younger than 2.5 Ma, with only
one exception, the M2 cooling event at ~3.3 Ma.
Even most early Pleistocene temperatures remained higher than those of the middle and late
Pleistocene.
Changes in boreal forest composition occur
with the loss of conifers at 2.71 to 2.69 Ma andthen the transition from forested habitats totreeless and shrubby environments in steps at
2.6, 2.56, and 2.53 Ma (Fig. 3, G and I). Thesechanges provide estimates of the mean temperature of the coldest month (MTCM; fig. S8), with
important implications for Beringian vegetationclimate feedbacks (38) as expanding tundra
increased land surface albedo, especially during snow-covered months. These cooler vegetation types become dominant < 2.53 Ma, evenduring high summer insolation orbits, except during super-interglacials 93, 91, and 87 (18) (supplementary materials). Climate-vegetation model
simulations using 300 and 400 ppm CO2 (Fig. 4
and supplementary materials) are consistent
with forest cover at Lake El’gygytgyn before~2.53 Ma and restricted glacial ice over Greenland in both cold and warm boreal summer
orbits (39), especially for the PRISM intervalfrom 3.26 to 3.02 Ma (6).
Cross-correlation of MTWM (Fig. 3A) with
the LR04 stack (Fig. 3B) provides a rather goodfit [correlation coefficient (R2) = 0.41, fig. S8],
illustrating the expected link between summer
temperatures at Lake El’gygytgyn and obliquity
via the isotopic stack (Fig. 3C). Several intervals

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RESEARCH ARTICLE
of coniferous-dominated forests occurred during
MIS 101, 93, 91, and 87 interglacials (40). Summers at Lake El’gygytgyn remain 1° to 3°C
warmer than at present for ~200,000 years after

stratification of the North Pacific at 2.73 Ma(41) (Fig. 3F), even during the coldest boreal summer orbits (MIS G6, Fig. 2H). These data, alongwith eustatic sea-level reconstructions (Fig. 3E),

support the notion that Pleistocene coolingand the intensification of Northern Hemisphere
glaciation (NHG) were gradual, occurring inseveral steps (5, 34–36), but with warm Arctic

Age (ka)
2200

2400

2600

2800

3000

G10

G20

3200

3400

3600

2.5

MIS 86 88
LR04 δ18O

96

100

104

G6

KM2

MG6

M2

520
4.5

B
Polarity

C

480
440

Matuyama

Gauss

Kaena

Gauss

Mammoth

E
B, C
A, D

D

E

BSi acc. rate
(g m-2 a-1)

102

first occurrence

30
20

101

Facies4.0

July insolation (W m-2)

3.5

BSi (%)

A

92

3.0

10
0

100

2.0

1.0

Si/Ti

1.5

F

H

MTWM (°C)

100

100

80

80

60

60

40

40

20

20

0
20

0

Spruce (%)

G

Trees & Shrubs (%)

0.5

15

10

5

modernI

PANN (mm a-1)

800

600

400

200
2200

2400

2600

2800

Fig. 2. Chronology, sediment facies, and proxy data from 3.58 to
2.2 Ma from Lake El’gygytgyn. (A) Global marine isotope stack (21)(ka, thousand years ago) and (B) mean July insolation for 67.5°N (30),next to (C) the magnetostratigraphy, (D) sediment facies defined in the
text and supplementary materials, (E) FTIRS-inferred biogenic silica plotted
as BSi acc. rate (note logarithmic scale) and % BSi, and (F) Si/Ti ratio. These
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3000

3200

3400

3600

data are compared to (G) percent of trees and shrubs (green line) and spruce(black bars) pollen. (H) Reconstructed MTWM and (I) annual precipitation(PANN), based on the pollen spectra and the best modern analog approach
applied in (18). The darker colored line in both (H) and (I) represents themean values, whereas colored shading is the uncertainty range [modern
values from (63)].

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land (Fig. 1, site 6), fossil plant and beetle assemblages suggest July temperatures +10° to
11°C warmer than today (42). Data from several sites show that boreal forests stretchingfrom 60° to 80°N included pine, not found inmodern forests of Alaska or the Yukon (31).

summers persisting much later than previously
documented.
At Lake El’gygytgyn, the PRISM interval(Fig. 3) was nearly as warm as it was between
3.56 and 3.4 Ma, and boreal forests changed
little in composition (Fig. 3G). At Meighen Is-

Age (ka)
2400

2600

2800

3000

3200

3400

Holocene avg.

10

Benthic δ O (‰)

3.6

20
18

4.0
16

20
80

0
-20

PRISM M2

60

-40
-60
-80

40

20

250

C

E

warm
cool mixed forest

cold deciduous forest
cold steppe
cold

10

60

8

40
20

6

0
4
2

Northern Hemispheric Glaciation

2400

2600

2800 Age (ka)3000

3200

3400

3600

Mud

Sand Grav.

2200

N3

21 JUNE 2013

Mudstone/sandstone

VOL 340SCIENCE

Siltstone

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R4 N5

3.596
<3.56
>4.29
4.493
<4.78

3.330

>2.21

Diatomite

N4

R3

3.03

R2

>1.945
>2.01

Age
(Ma)

Polarity

2.581
<2.79
<2.87
>2.87

(°C)
ODP 846 - ODP 1012

300

-100

cool conifer forest

Lithologic log
AND-1B

Eustatic sea level(m)

40

12

taiga

J

350

14

0

H

B

Lake El‘gygytgyn Lake El‘gygytgyn
spruce pollen (%) dominant biomeF

ODP 982AUk'37 SST (°C)

0

400

18

3.2

D

450

2.8
5

Atm. p CO2 (ppm)

15

tundra

1424

3600

20

LGM avg. MTWM (°C)

A

Lake El‘gygytgyn

2200

ODP 882 BSi acc. rate
(g m-2 a-1)

Fig. 3. Global proxies of change
compared to Lake El’gygytgyn.
(A) MTWM from Fig. 2, with solid
horizontal lines shown as meanvalues over the selected intervals;
the long dashed line is the Holocene average temperature, andthe short dashed line is the LGM
average temperature from (18).
(B) Benthic marine LR04 stack(21); (C) changes in pCO2 shown
as a range window in yellow, estimated from minimum and maximum of (2), with low-resolution
estimates using boron methods(orange and red solid circles) in(3); (D) alkenone reconstructed
SSTs for the North Atlantic (34)
(ODP, Ocean Drilling Program);(E) eustatic sea-level record fromNew Zealand, representative of
the global stack (55); (F) massaccumulation rate of biogenic silica
from the North Pacific (41); (G)dominant reconstructed biomes
from Lake El’gygytgyn; (H) change
in SST gradient between the North
Pacific tropics and subtropics from(59); (I) percent of spruce pollen;compared with (J) lithology of
ANDRILL core ANT-1B from a depthof 438 to 190 m in (37). The blue
vertical shading is the M2 isotopic event, and the gray verticalbar is the PRISM research interval. The dashed vertical line marks
2.72 Ma.

Yet the average MTWM reconstructed over the
PRISM interval is slightly warmer (~1.5°C) thanin multimodel climate simulations of the Pliocene
Model Intercomparison Project (PlioMIP) (43).
In the PlioMIP simulations using eight different coupled atmosphere-ocean general circula-Diamictite

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RESEARCH ARTICLE
tion models (43), the multimodel MAT at LakeEl´gygytgyn is +7.05° T 2.7°C warmer than in
preindustrial times and the MTWM is 5.5° T
2°C warmer. The simulated MTWM is close to
the PRISM-average reconstructed warming of~7°C. However, at times during PRISM, MTWM
temperatures at the lake were >8°C (Fig. 3A)
warmer than present. In sum, model sensitivity
to Pliocene boundary conditions, including atmospheric CO2, appears to be slightly lower thanthe observed sensitivity at Lake El’gygytgyn.
M2, Pliocene Transition, and Paceof Pleistocene Cooling
The pollen-based reconstructions from Lake
El’gygytgyn show that the largest cooling eventof the mid-Pliocene took place from 3.31 to
3.28 Ma (MIS M2) (21). Conditions were cooler
and dryer, as documented by a dramatic decreasein the relative abundance of arboreal pollen
and increases in Artemisia, Poaceae, and otherherbaceous pollen (fig. S6). Although larch, pine,
birch, alder, and possibly spruce stands remained
around the lake, steppelike habitats dominatedA(Fig. 3G). Coprophilous fungi indirectly point
to the presence of large grazing animals, suchas mammoth, horse, and bison (40). Climaticconditions were not “glacial”; rather, it was aswarm as the Holocene average or slightly warmer(44) (Fig. 3A). This has implications for interpreting the M2 isotopic shift of ~ 0.5 per mil in
the North Atlantic (21). Modeling sensitivity studies(39) do not preclude the existence of a Greenland
Ice Sheet during M2 as a major source of icerafted debris (IRD) (45). However, if Holocenelike conditions at Lake El’gygytgyn are indicativeof broader circum-Arctic climate, conditions forthe initiation of ice over North America would
have been unfavorable until long after 3 Ma (46).
Our results are consistent with gradual alkenonebased evidence of SST cooling in the eastern
North Atlantic in the range of 2° to 3°C (47) and6°C (34) during M2, but with ocean temperatures remaining warmer than modern ones both
before and after (Fig. 3D). Our findings are also
consistent with drilling results from the Ross
Sea (ANDRILL), suggesting that most M2 ice
advance outside Alaska, Greenland, and Svalbard

B

200 ppm CO2, cold summer orbit

o

80 N

80 N

o

70 N

70oN

60oN

60oN

50oN o
0

60oE

C

120oE

180oW

120oW

60oW

0o

50oN o060oE

D300 ppm CO2, cold summer orbit

80 N

80 N

70oN

70oN

60oN

60oN

120oE

180oW

120oW

60oW

0o

300 ppm CO2, warm summer orbit

o

60oE

E

120oE

180oW

120oW

60oW

0o

50oN o
0

80oN

70oN

70oN

60oN

60oN
60oE

120oE

TundraCushion-forb tundra
Prostrate dwarf-shrub tundra
Erect dwarf-shrub tundra
Low and high shrub tundra
Graminoid and forb tundra

180oW

120oW

60oW

60oE

F

400 ppm CO2, cold summer orbit

80oN

50oN o0200 ppm CO2, warm summer orbitoo50oN o0

took place in Antarctica rather than North America(Fig. 3J) (4, 37, 48).
The gradual but paced onset of NHG took
place as step changes starting after 3.6 Ma (49),but temperatures in the Arctic remained elevateduntil ~2.2 Ma. Although records of IRD fromthe North Atlantic/Barents Sea (50) and the North
Pacific (41) indicate the presence of glacial margins reaching tidewater, IRD records do notprovide a measure of ice-sheet size. Eustaticequivalent changes in sea level, in contrast, offer
an index of global ice volume (21) but not where
the ice was located (51).
Direct evidence of North American continentalscale ice sheets extending southward from Canada
does not appear until after 2.4 Ma (52, 53) andmay have had a different geography with warmbased, subglacial conditions (54). This may explain why sea-level variability during initial cold
cycles after 2.6 Ma (MIS 100) was in the range of~70 m, which is much less than ~125 m for the
LGM (Fig. 3E) (55). In the Alaskan Brooks Range,
the earliest glacial cycles were after 2.7 Ma andwere most extensive toward the Arctic Ocean,

0o

60oE

120oE

Grassland and Dry Shrubland
Temperate grassland
Tropical grassland
Temperate xerophytic shrubland
Tropical xerophytic shrubland
BarrenDesertFig. 4. Coupled climate-vegetation model simulations spanning the
Pliocene-Pleistocene transition. All simulations assume an ice-free Northern Hemisphere, including Greenland. CO2 atmospheric mixing ratios span therange assumed to have existed at the onset of major Pleistocene glacials [A and
B; 200 ppm by volume (ppmv)], during the Pliocene-Pleistocene transition (C and
D; 300 ppmv), and in the warm Pliocene (E and F; 400 ppmv). (A), (C), and (E)assume an orbital configuration like that at 116 ka, producing relatively coldboreal summers, whereas (B), (D), and (F) use a warm boreal summer orbit likewww.sciencemag.org

180oW

120oW

60oW

0o

400 ppm CO2, warm summer orbit

50oN o0

Boreal Forest
Cold deciduous forest
Cold evergreen needleaf forest

120oE

180oW

120oW

60oW

0o

Temperate Forest
Temperate deciduous forest
Cold mixed forest
Cool conifer forest
Cool mixed forest
Warm mixed forest
Temperate conifer forestthat at 125 ka. The simulations place the pollen-based vegetation reconstruction(Fig. 3G) into a spatial and temporal context, relative to proxy-based CO2 reconstructions. Although the persistent dominance of forest biomes at Lake El’gygytgyn
until 2.6 Ma is in broad agreement with the model, the lack of temperate forest at
400 ppm of CO2 demonstrates either the undersensitivity of the model to CO2
forcing or the possibility that Pliocene greenhouse gas levels were higher than inmost proxy reconstructions. Descriptions of the model and boundary conditions
are provided in the supplementary materials.

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RESEARCH ARTICLE
suggesting that a more sea-ice–free Arctic Ocean
may have provided an important precipitation
source (56). A major drop in the partial pressure of CO2 (pCO2) at ~2.8 Ma that could have
driven additional cooling is indicated in someproxy records (3) (Fig. 3C). Increases in ocean
stratification and IRD in the North Pacific fromthe mountainous Pacific Rim at 2.73 Ma (57),when inferred pCO2 is in the range of 280 to
370 ppm (2, 3) (Fig. 3C), coincide with a significant drop in precipitation at Lake El’gygytgyn(Fig. 2I) and an abrupt change in boreal forest
composition, characterized by vegetation still requiring summer temperatures warmer than the
Holocene average (Fig. 3A). Thus, our vegetation record is consistent with estimates for pCO2
(2, 3), but Arctic temperatures consistent withthat vegetation make it difficult to reconcile thebuildup of major continental ice sheets with summers warmer than today, even during cold Arctic
summer orbits. The substantial PANN drop at
2.73 Ma (Fig. 2I) and onset of cold winter temperatures, at times colder than today (fig. S8), areconsistent with preliminary model simulations
showing the climatic impact of large Northern
Hemispheric ice sheets on the Arctic basin andBeringia (fig. S9).
At Lake El’gygytgyn, the first cold “glacial”
sediment, Facies A (Fig. 2D), occurs at ~2.6 Ma(18); this facies, indicative of perennial summer lake-ice cover and MAT at least 4° T 0.5°C
colder than today (58), becomes common after
2.3 Ma during cooler summer orbits, after anincrease in low latitudinal temperature gradientsin the Pacific (Fig. 3H) (59). Perennial lake-ice
cover implied by Facies A does not appear during
all cold orbits until after 1.8 Ma (18). Likewise,
the first cold interval in Lake Baikal occurs at
2.7 Ma, but cyclic patterns of glacial/interglacial
BSi do not take hold until after about 1.8 Ma(36, 60).
Warm temperatures and changing boreal forest composition at Lake El’gygytgyn, together
with other proxies, contribute to a growing bodyof marine and terrestrial evidence that climate
system change from the warm Pliocene to the late
Pleistocene glacial world was a punctuated sequence of stepped Northern Hemispheric cooling
events and ice buildup (34, 36, 49, 59). Althoughlatitudinal temperature gradients steepened after
2.65 Ma (Fig. 3H), swings in global sea level
increase only after 2.6 Ma (Fig. 3E), despite
warmer Arctic summers, which is consistent
with the waxing and waning of the West Antarctic Ice Sheet (Fig. 3J) (37). Polar amplification
throughout the interval from 3.56 to 2.2 Ma
makes it likely that seasonal rather than perennial Arctic sea ice (5) was the norm, particularly
during warm summer orbits [(61) and supplementary materials].
Conclusions
The sediments recovered at Lake El’gygytgyn
provide an exceptional window into environmental dynamics from a terrestrial high-latitude1426

site against which other Arctic and lower-latitude
paleoenvironmental records can be compared.
Although the geologic record has already informed us of extreme warmth at intervals in the
middle Pliocene [(31) and references therein],
Lake El’gygytgyn provides a continuous timeseries of the timing of changes responsible fortransforming a virtually ice-free forested Arctic
into one capable of supporting major glacial cycles.
Elevated warmth across cold and warm orbital cycles during the “41,000-year obliquity-dominatedworld” of the Pliocene and early Pleistocene isconsistent with pCO2 estimates of 280 to 400 ppm(2, 3, 62), but evidence of extreme polar amplification exceeds that simulated by many climate
models. Mechanistic explanations for observed
trends in temperature and precipitation have yet
to be determined, but imply high sensitivity to
CO2 forcing. The delay of summer cooling in
the terrestrial Arctic provides new constraints on
Arctic climate and glacial evolution during theonset of NHG.
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Acknowledgments: Drilling operations were funded by the
ICDP, the U.S. National Science Foundation (NSF), the German
Federal Ministry of Education and Research (BMBF), theAlfred Wegener Institute and Helmholtz Centre Potsdam, theFar East Branch of the Russian Academy of Sciences, theRussian Foundation for Basic Research, and the AustrianFederal Ministry of Science and Research (BMWF). The Russian
GLAD 800 drilling system was developed and operated by
DOSECC Inc. We thank all participants of the expedition for
their engagement during the recovery of the ICDP 5011-1
cores. Funding of core analyses was provided by BMBF (grant
no. 03G0642), the German Research Foundation (DFG, grantnos. ME 1169/21 and ME 1169/24), NSF (grant no. 0602471),
the Global Civilian Research and Development Foundation(CGP-RFBR IV, grant no. RUG1-2987-MA-10), Vetenskapsrådet,the Swedish Research Council Formas, and the Kempe
Foundation. P.T. acknowledges DFG Heisenberg Program(grant no. TA 540/5). We are grateful to N. Mantke,

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A. Shahnazarian, and various students (Univ. of Cologne)
for their help in core processing and to T. Matrosova forcontributing modern surface pollen data. Thanks to A. Dolanfor supplying PlioMIP model output and I. Castañeda for
discussions and help with figures. The data reported in thispaper are archived in the databases of PANGAEA (doi:10.1594/
PANGAEA.808834) and the U.S. National Geophysical Data

Center (ftp://ftp.ncdc.noaa.gov/pub/data/paleo/paleolimnology/
asia/russia/elgygytgyn2013.xls).

Figs. S1 to S9Tables S1 to S5
References (64–105)

Supplementary Materials
www.sciencemag.org/cgi/content/full/science.1233137/DC1
Materials and Methods
Supplementary Text

23 November 2012; accepted 24 April 2013
Published online 9 May 2013;10.1126/science.1233137

REPORTS
Massive Dirac Fermions andHofstadter Butterfly in avan der Waals Heterostructure
B. Hunt,1* J. D. Sanchez-Yamagishi,1* A. F. Young,1* M. Yankowitz,2 B. J. LeRoy,2 K. Watanabe,3T. Taniguchi,3 P. Moon,4† M. Koshino,4 P. Jarillo-Herrero,1‡ R. C. Ashoori1‡
van der Waals heterostructures constitute a new class of artificial materials formed by stacking
atomically thin planar crystals. We demonstrated band structure engineering in a van der Waals
heterostructure composed of a monolayer graphene flake coupled to a rotationally aligned hexagonalboron nitride substrate. The spatially varying interlayer atomic registry results in both a local breakingof the carbon sublattice symmetry and a long-range moiré superlattice potential in the graphene. In our
samples, this interplay between short- and long-wavelength effects resulted in a band structure
described by isolated superlattice minibands and an unexpectedly large band gap at charge neutrality.
This picture is confirmed by our observation of fractional quantum Hall states at T5/3 filling and featuresassociated with the Hofstadter butterfly at ultrahigh magnetic fields.

1
Department of Physics, Massachusetts Institute of Technology,
Cambridge, MA 02139, USA. 2Department of Physics, University of Arizona, Tucson, AZ 85721, USA. 3Advanced Materials
Laboratory, National Institute for Materials Science, Tsukuba,
Ibaraki 305-0044, Japan. 4Department of Physics, Tohoku University, Sendai 980-8578, Japan.

*These authors contributed equally to this work.
†Present address: Korea Institute for Advanced Study, Seoul
130-722, Republic of Korea.
‡Corresponding author. E-mail: pjarillo@mit.edu (P.J.-H.);ashoori@mit.edu (R.C.A.)

A

B 25

m( r ) > 0

graphene
hBN

A2

20

Conductivity (e2/h)Tvices based on band structure engineering, however,
have been hindered by growth and nanofabrication limitations (5).

VAgraphite

15

B2

B1

Vg

A1

10

5

0

−20

−10

0

Vg

0

10

20

(mV)

C
3

R( )

he ability to tailor the properties of electronic
devices is one of the landmark achievements of modern technology and motivates a sizable fraction of modern research incondensed matter physics. Just as crystal structure can determine the electronic properties of amaterial, artificial periodic superstructures can beused to modify the electronic band structure ofexisting materials (1). The band structure of pristine graphene consists of linearly dispersing energy bands, which touch at two degenerate “Diracpoints.” This degeneracy is protected by the equivalence of the A and B triangular sublattices thatmake up the graphene honeycomb (2) and is responsible for graphene’s semimetallic behavior.
Theory suggests that the electronic properties ofgraphene can be tuned via external periodic potentials: Long-wavelength superlattices have been
predicted to lead to the formation of additional
gapless Dirac points at finite energy (3), whereas
atomic-scale modulations, by breaking the A-B
sublattice symmetry, may turn graphene from asemimetal into a semiconductor (4). Experimental efforts to make high-mobility functional de-

Recently, a new approach has become available through the use of hexagonal boron nitride(hBN) as a planar crystalline substrate. hBN is
isostructural to graphene but has boron and nitrogen atoms on the A and B sublattices, leading to alarge band gap (6). The weak interlayer van der
Waals forces in both graphene and hBN permit
the fabrication of multilayer heterostructures by
sequential transfer of individual layers (7). During
the transfer process, the angular alignment of theconstituent crystals (q) can in principle be controlled, but the graphene and hBN lattices retain
their natural 1.8% mismatch (4). The frequency
difference of the mismatched lattices leads tothe formation of a moiré pattern with wavelengthl(q) that can be much larger than the lattice constant (8, 9) (Fig. 1A).
The effect of the moiré on the graphene electronic structure can be decomposed into two parts(10). The moiré pattern results in a l-scale modulation of the graphene-hBN coupling, forming
a smooth superlattice potential. More subtly,
the moiré also modulates the local asymmetry

10

2

10

m( r ) < 0

−3

−2

−1

0

1

2

3

Vg (V)

Fig. 1. Insulating states and superlattice minibands in a graphene-hBN heterostructure. (A)Schematic of the moiré pattern for graphene (gray) on hBN (red and blue), for zero misalignment angle and an
exaggerated lattice mismatch of ~10%. The moiré unit cell is outlined in green. Regions of local quasi-epitaxialalignment lead to opposite signs of the sublattice asymmetry, m(r), in different regions (gray, carbon; red,boron; blue, nitrogen). (B) Low-temperature (T = 150 mK) conductivity near charge neutrality of four
heterostructure devices (A1, A2, B1, and B2). The CNP offset V0 = 37, 37, 46, and 42 mV, respectively. Left
inset: Measurement schematic. Right inset: False-color atomic force microscopy image. Scale bar, 3 mm. (C)Resistance over a larger gate range. Finite-density resistance peaks indicate full filling of the lowest superlatticeminiband in two of the four measured devices (A1 and A2) within the experimentally accessible density range.

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