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Natural Variability of Greenland Climate, Vegetation, and Ice Volume During the Past Million Years


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The response of the Greenland ice sheet to global warming is a source of concern notably because of its potential contribution to changes in the sea level. We demonstrated the natural vulnerability of the ice sheet by using pollen records from marine sediment off southwest Greenland that indicate important changes of the vegetation in Greenland over the past million years. The vegetation that developed over southern Greenland during the last interglacial period is consistent with model experiments, suggesting a reduced volume of the Greenland ice sheet. Abundant spruce pollen indicates that boreal coniferous forest developed some 400,000 years ago during the "warm" interval of marine isotope stage 11, providing a time frame for the development and decline of boreal ecosystems over a nearly ice-free Greenland.
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DOI: 10.1126/science.1153929
, 1622 (2008); 320Science
et al.Anne de Vernal,
and Ice Volume During the Past Million Years
Natural Variability of Greenland Climate, Vegetation, (this information is current as of June 20, 2008 ):
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olivines in the light isotopes of Fe (and the heavy
isotopes of Mg) (16, 20).
The extent of equilibrium isotope fractionation
is mainly controlled by the relative mass dif-
ference between the isotopes, and more fraction-
ation happens in isotopes with a larger relative
mass difference (14, 24).IftheFeisotopicvaria-
tion in the lava lake was produced by equilibrium
isot o pe fract io na ti on , Mg isotopes should show
more significant fractionation than Fe isotopes
because of their larger relative mass difference.
Furthermore, kinetic isotope fractionation driven
by th ermal and chemical diffusion should also
result in larger fractionation in Mg isotopes as
compared with that in Fe isotopes (16, 17, 20).
The absence of Mg isotope fractionation in Kilauea
Iki lavas may result from the low-precision iso-
topic analysis of Mg relative to Fe (e.g., 0.1
versus 0.04), which prevents the detection of
Mg isotopic variation. More likely, the presence
of Fe isotope fractionation and the absence of Mg
isotope fractionation may reflect the influence of
Fe oxidation states on kinetic or equilibrium iso-
tope fractionation (as compared with those of Mg,
two oxidation states of Fe exist in terrestrial mag-
matic systems) (5, 25).
Our study suggests that, unlike Li and Mg
isotopes (2, 3), Fe isotopes fractionate during ba-
saltic differentiation at both whole-rock and crys-
tal scales. Mineral compositions should therefore
be used to help interpret whole-rock basalt Fe
isotopic data. The elevated d
Fe of crustal igne-
ous rocks, which is more evolved than that in
basalts, could be explained by fractional crystal-
lization (10).
References and Notes
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F. von Blanckenburg, Geochim. Cosmochim. Acta 71,
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Lett. 268, 330 (2008).
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Lett. 252, 342 (2006).
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Principles, B. O. Mysen, Ed. (Geochemical Society,
University Park, PA, 1987), vol. 1, pp. 241258.
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available as supporting material on Science Online.
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Geochim. Cosmochim. Acta 71, 3833 (2007).
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Isotopes, C. M. Johnson, B. L. Beard, F. Albarede, Eds.
(Mineralogical Society of America, Washington, DC,
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24. H. C. Urey, J. Chem. Soc. (London) 1947, 562 (1947).
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H. L. Krivoy, U.S. Geol. Surv. Prof. Pap. 537-E, 1 (1970).
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U.S. Geol. Surv. Open-File Rep. 94-684, 1 (1994).
28. Discussions with S. Huang, A. T. Anderson Jr., F. M. Richter,
are appreciated. We thank three anonymous reviewers for
constructive comments. This work was supported by a
Packard fellowship, the France Chicago Center, and NASA
through grant NNG06GG75G to N.D.
Supporting Online Material
SOM Text S1 to S5
Fig. S1
Tables S1 to S4
29 February 2008; accepted 12 May 2008
Natural Variability of Greenland Climate,
Vegetation, and Ice Volume During
the Past Million Years
Anne de Vernal* and Claude Hillaire-Marcel
The response of the Greenland ice sheet to global warming is a source of concern notably because of its
potential contribution to changes in the sea level. We demonstrated the natural vulnerability of the ice
sheet by using pollen records from marine sediment off southwest Greenland that indicate important
changes of the vegetation in Greenland over the past million years. The vegetation that developed over
southern Greenland during the last interglacial period is consistent with model experiments, suggesting a
reduced volume of the Greenland ice sheet. Abundant spruce pollen indicates that boreal coniferous forest
developed some 400,000 years ago during the warm interval of marine isotope stage 11, providing a
time frame for the development and decline of boreal ecosystems over a nearly ice-free Greenland.
he potential for sea-level rise, caused by
melting of the Greenland ice-sheet as sur-
face air temperature increases, is consid-
erable (1). Although there is evidence that the
velocity of ice streams flowing into the ocean and
cently (2, 3), large uncertainties remain about the
long-term stability of the ice sheet. The climate
Fig. 4. Modeling of Fe
isotopic variations dur-
ing magmatic differen-
tiation in Kilauea Iki
lava lake (12). Solid
lines represent calcu-
lated Fe isotopic com-
positions of residual
melts during fractional
crystallization by assum-
ing a Rayleigh distilla-
tion process with average
crystal-melt fractionation
factors (Dd
0.1, 0.2, and 0.3.
Dashed horizontal lines
represent calculated mix-
ing lines between the
most magnesian melt
from the 1959 eruption
Fe = 0, 0.1, and 0.2 (black squares)]. The blue star
represents the most magnesian melt (MgO = 10.7 wt %; assumed d
Fe = 0.11). The green bars
represent the ranges of measured d
Fe and estimated MgO in olivine grains from two drill core samples
(MgO = 33.6 to 39.8 wt % and 41.9 to 42.7 wt %; table S3). Sample crystallization sequences are the same
as those in Fig. 2. Error bars indicate 95% CI of the mean.
Melt + olivines
MgO (wt%)
Fe = –0.41 to + 0.01
Fe = –1.10 to + 0.09
Fe (‰)
20 JUNE 2008 VOL 320 SCIENCE www.sciencemag.org1622
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and ice volume of Greenland seem to have varied
considerably in the recent geological past, as shown
by paleoecological data indicating a warmer re-
gional climate and reduced ice volume during the
last interglacial period (4) and by biogenic remains
of coniferous trees from forests that grew on Green-
land during Pliocene and early-to-mid-Pleistocene
times some hundred thousands to million years ago
(5, 6). However, although the climate and ice sheet
history of Greenland during the last climatic cycle
are well documented by isotope and geochemical
records from ice cores, which reveal high sensitiv-
ity to sea-surface conditions over the northern
North Atlantic Ocean (7), very little is known
about conditions preceding the onset of the last
glaciation because of the lack of continuous direct
records. On one hand, glacial activity on Green-
land over millions of years is evidenced by ice-
rafted debris in marine cores from continental
margins (8), but the precise size of the Greenland
ice sheet and its relative stability over time remain
unknown. On the other hand, sedimentary out-
crops from the Greenland coasts and near-shore
marine sediment cores suggest the recurrence of
relatively warm climatic conditions during the past
(5, 6, 9), but the duration and timing of these
phases remain uncertain. We used the pollen con-
tent of sediment cores from the Ocean Drilling
Program (ODP) site 646 on the continental rise,
off southern Greenland (Figs. 1 and 2), as an in-
dependent proxy for assessing the dominant type
of vegetation and the timing of the last forested
episodes. The stratigraphy of the cores was estab-
lished from d
O in foraminifer shells (10), which
permits correlation with the stack marine isotope
stratigraphy of Lisiecki and Raymo (11)andthe
setting of a time scale (12) (fig. S1).
One difficulty in interpreting pollen assem-
blages from marine sediments is the identification
necessarily exotic and derives from more or less
long-distance transport. T wo main transport mech-
anisms have to be taken into account: atmospheric
transport by winds, and hydrodynamic transport
through runof f, rivers, and marine currents. The
long-distan ce atmospher ic tran sport of pollen re-
sults in low concentrations with distorted assem-
blages characterized by an overrepresentation of
Pinus pollen grains that show exceptional aerial
dispersion properties (13). Along continental mar -
gins, detailed studies have shown that most of the
pollen in marine sediments is due to fluvial inputs
from adjacent lands, therefore allowing direct com-
parison with terrestrial palynostratigraphy (14). In
the Labrador Sea, pollen analyses along a near-
shore to offshore transect showed that atmospheric
transport is accompanied with an asymptotic de-
crease in the concentration of pollen from the coast-
line and an increase in the relative proportion of
GEOTOP Geochemistry and Geodynamics Research Center
Université du Québec à Montal, Case Postale 8888,
succursale Centre-Ville, Montréal, Québec H3C 3P8, Canada.
*To whom correspondence should be addressed. E-mail:
Fig. 1. Location of ODP
site 646 (58°12.56 N,
48°22.15 W; water
depth 3460 m) in the
northern North Atlantic
and of other coring sites
referred to in the text:
HU-90-013-013 (58°12.59
N, 48°22.40 W; water
depth 3379 m); ODP
site 647 (53.19.9 N,
45°14.7 W, water depth
3862 m); and HU-84-
030-003 (53.19.8 N,
45°14.7 W, water depth
3771 m). The Dye 3 cor-
ing site, where spruce
DNA was found, is indi-
cated by a blue square
(6). The white arrows
correspond to the mean
surface vector wind from
June to September based
on the 1968-to-1996 clima-
tology [see the National
Centers for Environmen-
tal Prediction/National
Center for Atmospheric
Research reanalysis (www.], available from the Earth System Research Laboratory, Physical
Science Division, of the National Oceanic and Atmospheric Administration ( The thin
, respectively. The blue
arrows schematically illustrate the surface ocean circulation pattern along the Greenland coast, in the
Labrador Sea, and in Baffin Bay. The dashed green line corresponds to the present-day northern limit of the
potential natural tree line or cold evergreen needle-leaf forest in Biome models (27).
Fig. 2. Stratigraphy and chronology of the upper 76 m at ODP site 646 (58°12.56N, 48°22.15 W; water
depth 3460 m) based on
OmeasurementsinNeogloboquadrina pachyderma (10) and correlation with the
stack curve LR04 of Lisiecki and Raymo (11). The abundance of pollen grains and spores of pteridophytes is
expressed in concentration per cm
of sediments. Sedimentation rates are uniform and average 7.8 cm per ka,
which permits the assumption that pollen concentrations are approximately proportional to fluxes (fig. S1)
(12). The vertical gray bands correspond to modern values of concentrations, and the horizontal green bands
correspond to phases with concentrations at least twice that of those recorded during the late Holocene. SCIENCE VOL 320 20 JUNE 2008
on June 20, 2008 www.sciencemag.orgDownloaded from
Pinus (15). Pliocene to recent pollen contents at
offshore sites from the northwest North Atlantic
(ODP site 647, core sample HU84-030-003) (Fig.
1), where mostly wind inputs can be recorded, are
characterized by pollen fluxes lower than 0.5 grain
and largely dominated by Pinus (16).
The analyses of Arctic snow, Greenland ice, and
pollen traps along the southern Greenland edge
show an exotic component from boreal forests of
North America, but illustrate that long-distance
atmospheric transport is responsible for low inputs
(17, 18). Therefore, the large-amplitude variations
in pollen content from the southern Greenland
margin records at ODP site 646, with fluxes
well above modern or Holocene (that is, the past
1 1, 500 years) values, can be interpreted as reflecting
changes in hydrodynamic inputs from a relatively
proximal source-vegeta tion located on southern
Greenland (12). Furthe r evidence for the promi-
nence of proximal sources during interglacials is
provided by the comparison of total pollen content
to long-distance transported grains of Pinus (12).
The pollen record of the last million years at
ODP site 646 shows important variations both in
concentrations [thus fluxes, because sedimentation
rates remained fairly constant in the study se-
quence (12)] and dominance of the main taxa
(Figs.2and3andfigs.S2andS3)(12). Pollen
concentrations vary by orders of magnitude, from
less than 10 grains cm
to more tha n 10
. In general, low concentrations are recorded
during glacial stages. Minimum values (close to
zero) characterize the marine isotope stage (MIS)
6, indicating very low fluxes from both long-
distance and proximal sources, which is consistent
with extensive development of the Laur entide and
Greenland ice sheets (9). Higher concentrations
are seen in interglacial sediments. The Holocene
is characterized by concentrations of about 100
grains cm
. The assemblages include inputs from
the boreal forest of southeastern Canada linked to
predominant southwest-northeast summer winds,
but show components (12) that are from more-
proximal shrub-tundra vegetation.
Earlier interglacial stages record much higher
pollen concentrations than the Holocene. Those of
MIS 5e are five times higher, and the concen-
trations of pteridophytes spores are also higher .
The assemblages are characterized by dominant
Alnus and abundant spores of Osmunda (Fig. 3B
and fig. S2). In core sample HU-90-013-013 col-
lected near ODP site 646 (19), more detailed anal-
yses of MIS 5e document the pollen succession
(Fig. 4). A rapid increase of Alnus occurred during
an early phase of MIS 5e characterized by high
summer sea-surface temperatures, which suggests
rapid development of shrub tundra after the ice
retreat (9). The subsequent increase of Osmunda
represents a unique event in the last million years.
It coincides with maximum sea-surface temper-
atures in winter and suggests the development of
dense fern vegetation over southern Greenland un-
der climatic conditions not unlike those of the
modern boreal forest, given the present distri-
bution of the genus. Osmunda expanded possibly
in a la r g e geog r ap h i c a l do m a in, because spores
were identified at the base of ice cores drilled in
the Agassiz ice cap (20). Toward t h e end of MIS
5e, pollen and spore influxes decreased concomi-
tantly with the augmentation of herb percentages.
This event corresponds to the fi r st ste p to w a r d
higher d
OvaluesinGlobigerina bulloides and
Neogloboquadrina pachyderma. It suggests a
change to the herb tundra resulting from regional
cooling at the onset of ice growth.
MIS 7, the penultimate interglacial period, dif-
fered from MIS 5e in many respects (Fig. 3C).
Sea-surface temperatures never reached those of
MIS 5e, and the pollen and spore content of sed-
iment remained lower . Its pollen assemblages are
characterized by dominant herb taxa (notably
Poaceae and Cyperaceae), suggesting the devel-
opment of tundra along southern Greenland coasts.
The MIS 1 1 interglacial is different than others
because of its near 50,000-year duration [374 to
424 thousand years ago (ka) (11)]. At site 646,
MIS 11 is also unique because of pollen concen-
trations one order of magnitude higher than those
of the Holocene, the dominance of Picea spp., and
the occurrence of Abies pollen grains (Figs. 2 and
3D and fig. S2). The dominance of Picea from the
beginning to the end of the interglacial period sug-
gests the presence of forest vegetation throughout
the entire interval, at least over southern Greenland.
The base of MIS 1 1 is marked by higher pro-
Fig. 3. (A to E)Stratig-
raphy, sea-surface temper-
atures, and concentration
of dominant pollen and
spore taxa during the
present interglacial pe-
riod [MIS 1 (A)], the last
interglacial period [MIS
5e (B)], MIS 7 (C), MIS
11 (D), and MIS 13 (E).
Sea-surface temperatures
are estimated from dino-
cysts [the thin lines are
best estimates from five
modern analogs and the
thick lines correspond to
three-point running av-
erag e s (28)]. Dinocyst as-
semblages were reported
by de Vernal and Mudie
(29). The modern sea-
surface temperatures at
0.7°C and 7.3 ± 1.1°C
in winter and summer, re-
spectively [data from the
World Ocean Atlas, 2001
(30)]. Among pollen assem-
blages, herb taxa include
mostly Pocacea, Cyperace-
ae, and Asteraceae. Shrub
pollen is dominated by
Alnus and Betula with the
occasional occurrence of
Salix and Ericaceae (fig. S2).
Pinus has been excluded
because of its overrepresen-
tation due to long-distance
atmospheric transport.
20 JUNE 2008 VOL 320 SCIENCE www.sciencemag.org1624
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portions of shrub and herb pollen, indicating more
open vegetation and a cooler climate, but Picea
was probably already present regionally , taking
into account the fact that its concentrations reached
hundreds of grains per cm
. The covariance of d
in planktic foraminifers and of Picea concentra-
tio ns suggests a synchronous ice retreat and early
forest development, a maximum of Picea concen-
tration when maximum sea-surface temperatures
occurred, and a concomitant glacial onset and
forest decline at the end of MIS 11. The devel-
opment of spruce forest over Greenland probably
indicates relatively mild conditions (6)anda
substantial reduction of the Greenland ice sheet
during the long MIS 11 interglacial period. How-
ever , precise paleoclimatic and paleoecological in-
ferences from pollen or DNA are uncertain without
knowing the species of conifer trees. The identifi-
cation of Picea pollen grains down to the species
level is difficult because of uniform morphologica l
characteristics of the genus. Nevertheless, detailed
microscopic examination suggests the occurrence
of several species, among which Picea abies
dominated (fig. S4). In northern Europe and
Fennoscandia, Picea abies is a common conifer
tree of montane and boreal environments that
often occurs at the tree limit and acted as a pi-
oneer along emerging postglacial coasts (21).
Growth of Picea abies is fostered by high July
temperatures and cool and snowy winters, but
has a low temperature threshold (2.6°C) for the
initiation of bud and stem growth. Picea abies
has adapted to survive severe climate; it can
persist for hundreds of years by vegetative prop-
agation. Therefore, its development, at least over
southern Greenland during MIS 11, does not nec-
essarily imply a zonal climate that was warmer
than at present, because the northern tree limit
and the Picea abies timberline occur now near
the polar circle in Europe. However, it certainly
indicates ice-free conditions over a large area of
Greenland, and thus a much-reduced ice-sheet
volume, otherwise katabatic winds (22)would
have restricted any forest development.
Before MIS 11, the pollen content was rarely
less abundant than it was during the Holocene,
thus suggesting vegetation that was generally as
extensive as it is at present. Pollen was particu-
larly abundant during MIS 13 (Figs. 2 and 3E),
but Picea concentrations did not reach values as
high as those during MIS 11, thus suggesting
shrub-tundratype vegetation.
In conclusion, although the pollen record
from site 646 does not provide a direct picture of
climate changes over Greenland, it yields impor-
tant information that helps link fragmentary
terrestrial records into a continuous sequence.
Furthermore, the pollen record is as a proxy for
the ice volume of Greenland in two ways. First, it
provides information on pollen production, and
thus on the vegetation density on adjacent land,
which implies ice-free conditions. Second, it
depends on the distance to site 646 from the
source vegetation, which has been shorter during
ice-free episodes in southern Greenland because
of low relative sea levels that are a result of iso-
static adjustment. A substantially reduced Green-
land ice volume seems to have characterized MIS
5e, 1 1, and 13, as well as the Pliocene (23), indi-
cating a long-term sensitivity of the Greenland
ice sheet to warm temperatures. Among warm
climate intervals of the last million years, MIS 11
stands out in terms of forest vegetation spreading
over southern Greenland. Thus, if the melting of
Greenland and other Arctic ice caps are assumed
to have contributed to the equivalent of a 2.2- to
3.4-m-higher sea level during MIS 5e (24), we
may assume that they contributed some more
during MIS 11. The actual volume of the ice-
sheet decline during these episodes is difficult to
estimate, but it did occur under natural forcing
with an atmospheric partial pressure of CO
parts per million by volume (25). During MIS 5e,
particularly high summer insolation probably con-
tributed to the Greenland ice melt (4), whereas
the long duration of MIS 1 1 might explain the
retreat of the ice sheet under an insolation pattern
that is similar to that of the Holocene (26). The
data presented here provide evidence of the
vulnerability of the Greenland ice sheet to natural
forcing and should increase concerns about its
fate during the anticipated global warming.
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financial support from the Natural Sciences and
Engineering Resear ch Council of Canada and the Fonds
Quécois de Recherche sur les Sciences de la Nature et
les Technologies.
Supporting Online Material
Materials and Methods
Figs. S1 to S4
10 December 2007; accepted 9 May 2008
Fig. 4. Close-up on the stratigraphy of the last
interglacial period (MIS 5e) from core sample HU-90-
013-013, collected near ODP site 646. Shown are the
isotope stratigraphy based on Globigerina bulloides
and Neogloboquadrina pachyderma (19), the sea-
surface temperatures estimated from dinocysts (28),
and the concentration and percentages of the
dominant pollen and spore taxa. The percentages
of Osmunda were calculated from the pollen sum,
excluding spores. SCIENCE VOL 320 20 JUNE 2008 1625
on June 20, 2008 www.sciencemag.orgDownloaded from
... The interglacial known as Marine Isotope Stage (MIS) 11c has long been of interest to the palaeoclimate community (see Candy et al. 2014 for review). Not only is it often cited as being a good analogue for the current interglacial (Loutre and Berger, 2003), it was also characterised by major shifts in the Earth system that may be relevant to understanding future warming scenarios, i.e. largescale ice-sheet wastage (de Vernal and Hillaire-Marcel, 2008;Reyes et al., 2014) and global sea levels 10-14 m above the present (Roberts et al., 2012;Raymo and Mitrovica, 2012). Furthermore, terrestrial records of MIS 11c from western and central Europe, along with marine archives from the North Atlantic, contain the clearest evidence for abrupt cold events in any pre-Holocene interglacial (Koutsodendris et al., 2012;Candy et al., 2014). ...
... The discrepancy between the timing of these events is within the ±4 a uncertainty that is cited as the predicted uncertainty of records tuned to the LR04 stack at this time interval (Lisiecki and Raymo, 2005). The location of ODP 646 is shown as it contains the key evidence for long-term changes in ice coverage over southern Greenland which may be a driver of abrupt change during MIS 11 (de Vernal and Hillaire-Marcel, 2008;Reyes et al., 2014). The base map for this figure was created using Ocean Data View ( ...
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Resolving marine and terrestrial records of glacial and interglacial stages has long been a challenge for Quaternary studies. We present a tephra-based correlation for MIS 11c of a North Atlantic marine core and a British terrestrial record (Marks Tey). A varved chronology is presented for the annually laminated lake sequence at Marks Tey and the proxy data from this site are plotted on an annual timescale for the first time. This record shows clear evidence for an abrupt cold event and corresponding ecological and landscape response. Varve counting shows that the abrupt event lasted for ca 185 a but the impact on the landscape persisted for ca 560 a. The co-occurrence of a single tephra layer in Marks Tey and ODP 980 allows these two records to be correlated. Furthermore, this synchronisation shows that the abrupt cold event in Marks Tey was coincident with a centennial-scale cold event in ODP 980, as evidenced by an increase in Neogloboquadrina pachyderma (s) percentage. In both ODP 980 and Marks Tey the age of this abrupt event was ca 414 500 a indicating that, at this point in MIS 11c, a widespread cold event occurred in the northeastern Atlantic and the British Isles.
... During MIS 11c, these records show a period of ice melting, followed by the near cessation of ice-eroded sediment discharge, suggesting the near complete deglaciation of southern Greenland. This is supported by a marine pollen record off southern Greenland, indicating the presence of spruce forest in southern Greenland, with the maximum in spruce pollen values occurring near 400 ka (de Vernal and Hillaire-Marcel, 2008). Exposure-burial scenarios for the GISP2 bedrock core are consistent with GrIS reductions during MIS 11c (Schaefer et al., 2016). ...
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Notoriety of the Marine Isotope Stage (MIS) 11c interglacial arises from its long duration, extending over two precessional cycles, high sea level, and persistence of high atmospheric CO2 concentrations. The strong climatic response is often considered paradoxical because it was attained under weak boreal summer insolation forcing, a function of an extended eccentricity minimum and of precession and obliquity being almost opposite in phase. Here, we trace the characteristics of MIS 11c and explore their most likely causes. MIS 11c was preceded by the largest Quaternary ice volume expansion of MIS 12, which ended with a long period of ice rafting and interhemispheric heat transfer. We suggest that the duration of MIS 12 and the size of ice sheets exceeded a critical threshold that triggered a deglaciation despite the weak insolation forcing. The weak forcing led to a slow but steady loss of ice volume, that was sufficient to allow ocean outgassing of CO2, but insufficient to raise sea level within a single precessional cycle. This gave rise to a prolonged interval with large residual ice sheets and high CO2 concentrations that is unique in the last 800,000 years. The obliquity-precession antiphasing produced a weak boreal summer insolation minimum, skipping a glacial inception and leading to continued sea-level rise and high CO2 concentrations, sustained by carbonate compensation. Full interglacial conditions were achieved in the second precessional cycle, and the combined strength and length of the interglacial probably led to loss of some Greenland and Antarctic ice compared to other interglacials. While MIS 11c is highly unusual in many respects, these appear to be linked to each other through the very weak insolation forcing, which led to its extended duration, slow sea-level rise and stable CO2 concentrations through a cocktail of counteracting carbon cycle processes. Although some of these features are also encountered in other interglacials, their combination with strong interglacial intensity is unique to MIS 11c and this appears to be a function of the large MIS 12 ice sheets and the high CO2 concentrations from the beginning of the interglacial.
... This distinct timing suggests that the rapid increase in atmospheric pCO 2 during Termination V was in part triggered by the reinvigoration of the SO upwelling early in the deglacial progression. Concomitant with the reinvigoration of SO upwelling, the increase in global biosphere productivity (Brandon et al., 2020) that reflects in part, an increase in vegetation cover and terrestrial carbon storage (Tzedakis et al., 2006;de Vernal and Hillaire-Marcel, 2008;Prokopenko et al., 2010;Melles et al., 2012;Kousis et al., 2018) has probably acted as a sink of atmospheric CO 2 , thereby transiently buffering the impact of oceanic CO 2 degassing (Fig. 8). Atmospheric pCO 2 reached maximum values (up to~282 ppmv) during the second part of MIS 11, as the CCP functioned at maximum strength. ...
Article can be downloaded until 15th of July using this link : While numerous studies have highlighted the central role of Southern Ocean (SO) dynamics in modulating rapid increases in atmospheric CO2 concentrations during deglaciations, fewer studies have yet focused on the impact of the Biological Carbon Pump - and more specifically the Carbonate Counter Pump (CCP) - in contributing to increase the CO2 concentration in oceanic surface waters and thus, in the atmosphere. Here, we present micropaleontological (coccolith, planktonic foraminifera) and geochemical (CaCO3, CaXRF, δ13CN. pachyderma) constraints from sediment core MD04-2718 retrieved in the Polar Front Zone of the Indian Ocean covering the time interval spanning Marine Isotope Stage (MIS) 12 to MIS 10 (440,000–360,000 years). We compare our results with published records from the SO to reconstruct past changes in CCP and upwelling dynamics and understand their leverage on the ocean-atmosphere portioning of CO2. We demonstrate that the sharp increase in atmospheric pCO2 during Termination V was likely associated with enhanced deep-water ventilation in the SO, that promoted the release of previously sequestered CO2 to the ocean surface as the westerly wind belt and the frontal system migrated southwards. Enhanced CCP is observed later, during MIS 11, and is likely the consequence of higher sea surface temperature and higher nutrient availability due to the reinvigoration of SO upwelling leading to increased coccolith (and to a lesser degree, planktonic foraminifera) production and export. The low eccentricity signal recorded during MIS 11 might have additionally strengthened the CCP, exerting a specific control on Gephyrocapsa morphotypes. In addition to the strong global biological productivity and higher carbon storage on land, these synergistic mechanisms may have permitted to shape the distinctive 30 ka-long pCO2 plateau characteristic of MIS 11.
Among the 100 kyr climatic cycles of the Late Pleistocene, Termination V (TV, ∼ 433–404 kyr BP), the fifth last deglaciation, stands out for its minimum in astronomical forcing associated paradoxically with maxima in sea level, Antarctic temperature and atmospheric CO2 concentration. However, the driving mechanisms explaining TV remain only partially understood. For instance, climate models cannot fully represent the atmospheric CO2 variation observed in paleoclimate data. Aside from essential oceanic circulation processes, there is increasing evidence that terrestrial biosphere may have played a key role in the global carbon cycle. This study proposes a three-step integrated approach, combining regional and global vegetation records with modelling results, to unveil the evolution of terrestrial biosphere and its contribution to the carbon cycle during TV. First, we provide a new high-resolution (∼ 700 years) deep-sea pollen record from the Gulf of Cádiz (site U1386, 36∘49.680′ N; 7∘45.320′ W) for TV, which shows a moderate expansion of the Mediterranean forest. We then construct the first global forest pollen database for this period. Our compilation features distinct evolutions for different types of forest, highlighting a strong development of temperate and boreal forest which might have delayed the atmospheric CO2 increase during TV. Finally, the direct comparison of global simulated forests (iLOVECLIM model) to our pollen database reveals overall consistent temperate and boreal forest evolutions despite model biases, thereby supporting the hypothesis of a significant CO2 sequestration by middle and high-latitude forests of the Northern Hemisphere shortly after the onset of TV.
Sediment cores from 13 lakes in a 1500 km transect along the eastern North American Arctic contain up to four superposed stratified interglacial units. All 13 lakes contain one unit with sediment similar in character and mass to Holocene gyttja, with 14C ages >40 ka, luminescence ages 90 to 120 ka, and pollen assemblages that require nearly complete Laurentide deglaciation, supporting a Last Interglacial (LIG; MIS 5e) age. Two lakes preserve an older interglacial, with luminescence ages suggesting an MIS 7 age. Four adjacent lakes record a thin, stratified organic unit between the LIG and Holocene units with 14C ages >50 ka, that is probably from late in MIS 5. Temperature estimates from biotic proxies suggest LIG summer temperatures 4–6°C above mid‐20th century values; pollen, chironomids and DNA document a poleward expansion of woody plants and invertebrate species during the LIG, supporting arguments that positive feedbacks native to the Arctic amplified insolation‐driven summer temperature increases. The stratigraphic succession implies the Laurentide Ice Sheet remained intact with sea level below ‐40 m from ~115 ka to ~11 ka, and places new constraints on the interpretation of cosmogenic radionuclide inventories in erratic boulders older than the Holocene throughout this region.
Sediments deposited during glacial-interglacial cycles through the Early to Mid-Pleistocene in the North Sea are chronologically poorly constrained. To contribute to the chronology of these units, amino acid racemization (AAR) and strontium (Sr) isotope analyses have been performed on samples from four shallow borings and one oil well along a transect in the northern North Sea. D/L Asp (aspartic acid) values obtained through reverse-phase liquid chromatography in the benthic foraminiferal species Elphidium excavatum is focused on because of consistent results and a good stratigraphic distribution of this benthic species. For the Early Pleistocene, an age model for the well 16/1–8, from the central part of the northern North Sea based on Sr ages allows for dating of the prograding wedges filling the pre-Quaternary central basin. A regional calibration curve for the racemization of Asp in Elphidium excavatum is developed using published ages of radiocarbon-dated samples and samples associated with the previously identified Bruhnes/Matuyama (B/M) paleomagnetic boundary and a Sr age from this study. Based on all the available geochronological evidence, samples were assigned to marine oxygen isotope stages (MIS) with uncertainties on the order of 10–70 ka. Sr ages suggest a hiatus of <2 million years (Ma) possibly due to non-deposition or low sedimentation between the Utsira Formation (Pliocene) and the Early Pleistocene. An increase in sedimentation rates around 1.5 ± 0.07 Ma (∼MIS 51) may partly be due to sediment supply from rivers from the south-east and partly due to the extension of ice sheet around 1.36 ± 0.07 Ma from the Norwegian coast to the central North Sea. A possible basin-wide glaciation occurred around 1.1 Ma (∼ MIS 32) (upper regional unconformity/top of unit Q4 in this study), resulting in erosion and regional unconformity. Two interglacials in the Norwegian Channel have been dated: the Radøy Interglacial to 1.07 ± 0.01 Ma (possibly MIS 31, the ‘super interglacial’), and the Norwegian Trench Interglacial to 0.50 ± 0.02 Ma (possibly MIS 13). A massive till unit identified at the same stratigraphic level in all shallow borings may partly represent an extensive MIS 12 glaciation. This study shows that the combined use of amino acid racemization data and Sr isotope chronology can refine the chronological ambiguities of Quaternary North Sea sediments related partly to the impact of glacial processes.
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While a large cryosphere may be a necessary boundary condition for millennial-scale events to persist, a growing body of evidence from previous interglacial periods suggests that high-magnitude climate events are possible during low-cryosphere climate states. However, the full spectrum of variability, and the antecedent conditions under which such variability can occur, have not been fully described. As a result, the mechanisms generating high-magnitude climate variability during low-cryosphere boundary conditions remain unclear. In this study, high-resolution climate records from Deep Sea Drilling Project (DSDP) site 610 are used to portray the North Atlantic climate's progression through low ice, boundary conditions of Marine Isotope Stage (MIS) 11c into the glacial inception. We show that this period is marked by two climate events displaying rapid shifts in both deep overflow and surface climate. The reorganization between Polar Water and Atlantic Water at subpolar latitudes appears to accompany changes in the flow of deep water emanating from the Nordic Seas, regardless of magnitude or boundary conditions. Further, during both intermediate and low ice boundary conditions, we find that a reduction in deep water precedes surface hydrographic change. The existence of surface and deep-ocean events, with similar magnitudes, abruptness, and surface–deep phasing, advances our mechanistic understanding of, and elucidates antecedent conditions that can lead to, high-magnitude climate instability.
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The unique alignment of orbital precession and obliquity during the Marine Isotope Stage 11 (MIS-11) interglacial produced perhaps the longest period of planetary warmth above preindustrial conditions in the past 800 kyr. Reconstructions point to a significantly reduced Greenland ice sheet volume during this period as a result, although the remaining extent and volume of the ice sheet are poorly constrained. A series of time slice simulations across MIS-11 using a coupled climate model indicates that boreal summer was particularly warm around Greenland and the high latitudes of the Atlantic sector for a period of at least 20 kyr. This state of reduced atmospheric baroclinicity, coupled with an enhanced and poleward-shifted intertropical convergence zone and North African monsoon, favored weakened high-latitude winds and the emergence of a single, unified midlatitude jet stream across the North Atlantic sector during boreal summer. Consequent reductions in the lower-tropospheric meridional eddy heat flux over the North Atlantic therefore emerge as negative feedback to additional warming over Greenland. The relationship between Greenland precipitation and the state of the North Atlantic jet is less apparent, but slight changes in summer precipitation appear to be dominated by increases during the remainder of the year. Such a dynamic state is surprising, as it bears stronger resemblance to the unified-jet state postulated as typical for glacial states than to the modern-day interglacial state.
A l’échelle des cycles glaciaires-interglaciaires (G/IG) du Quaternaire, la concentration atmosphérique en CO₂ (p CO₂) est associée à d’importantes variations, notamment pendant les transitions entre périodes glaciaires et interglaciaires, aussi appelées terminaisons, qui enregistrent des augmentations de pCO₂ de l’ordre de 100 ppm en quelques milliers d’années. Alors que les rôles de la circulation océanique et de l’étendue de la couverture de glace sur le pCO₂ sont étudiés depuis plusieurs années, peu de travaux s’intéressent à l’impact de la productivité biologique. L’objectif de cette thèse est de reconstruire les changements de productivité biologique et d’estimer leur contribution sur les variations de pCO₂ atmosphérique au cours des derniers 800 000 ans (ka), en portant une attention particulière à la Terminaison V (~425 ka) et au stade isotopique (MIS) 11, l’interglaciaire enregistré vers 400 ka. Pour cela, deux approches ont été combinées pour remonter aux variations globales et locales de la productivité biologique. Des mesures de Δ¹⁷O de O₂ effectuées dans les bulles d’air piégé dans la carotte de glace EPICA Dome C (Antarctique) entre 400 et 800 ka, ont permis de compléter les mesures préexistantes et remonter aux variations de la productivité biosphérique globale exprimée en flux d’oxygène au cours des derniers 800 ka. Des analyses micropaléontologiques (coccolithes, foraminifères) et géochimiques (COT, CaCO₃, XRF) ont été effectuées sur une carotte sédimentaire du secteur Indien de l’Océan Austral (MD04-2718) pour remonter aux variations d’efficacité de la pompe biologique au cours des derniers 800 ka. Les résultats obtenus durant cette thèse ont permis de montrer qu’à l’échelle des cycles G/IG, la productivité biosphérique globale et la productivité carbonatée dans l’Océan Austral sont plus importantes pendant les interglaciaires comparées aux glaciaires, ce qui minimise l’impact de la productivité organique marine sur la pCO₂ atmosphérique. La Terminaison V et MIS 11 enregistrent la plus forte productivité biosphérique des derniers 800 ka et la plus importante production carbonatée marine des 9 derniers interglaciaires dans l’Océan Austral. Alors que la forte productivité carbonatée est une source de CO₂ pour l’atmosphère, l’augmentation de la productivité de la biosphère permettrait de contrebalancer l’augmentation de pCO₂ via la photosynthèse.
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During the Last Interglacial, approximately 129 to 116 ka (thousand years ago), the Arctic summer climate was warmer than the present, and the Greenland Ice Sheet retreated to a smaller extent than its current state. Previous model‐derived and geological reconstruction estimates of the sea‐level contribution of the Greenland Ice Sheet during the Last Interglacial vary widely. Here, we conduct a transient climate simulation from 127 to 119 ka using the Community Earth System Model (CESM2), which includes a dynamic ice sheet component (the Community Ice Sheet Model, CISM2) that is interactively coupled to the atmosphere, land, ocean, and sea ice components. Vegetation distribution is updated every 500 years based on biomes simulated using a monthly climatology to force the BIOME4 equilibrium vegetation model. Results show a substantial retreat of the Greenland Ice Sheet, reaching a minimum extent at 121.9 ka, equivalent to a 3.0 m rise in sea level relative to the present day, followed by gradual regrowth. In contrast, a companion simulation employing static vegetation based on pre‐industrial conditions shows a much smaller ice‐sheet retreat, highlighting the importance of the changes in high‐latitude vegetation distribution for amplifying the ice‐sheet response.
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Previous pollen analyses of ice cores from Devon and Ellesmere islands have contributed considerably to our knowledge of past climate in the Canadian High Arctic. In this case, in 1979, bulk (35–83 litres) water samples were melted down a hole 139 m deep, drilled to bedrock, 1.2 km from the top of the flow line in Agassiz Ice Cap in northern Ellesmere Island. Analysis of ten of these samples, plus some taken in very dirty ice from the melt tank during drilling 7 years ago, has yielded pollen concentrations that, together with the oygen-isotope (6) signatures, suggest the Agassiz Ice Cap began its growth during the last interglacial period. A discrepancy between melt-tank and bulk-sample pollen concentrations is believed to be due to a loss of pollen from the melt-tank samples during the drilling process.
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Correlations of isotopic and palynological records from deep sea cores in Baffin Bay and Labrador Sea with terrestrial palynological sequences, supported by a few Th/U chronological controls, allow the establishment of a regional climatostratigraphic scheme for the Late Pleistocene climatic fluctuations in eastern Canada. During the climatic optimum of isotopic substage 5e, warmer conditions than present prevailed both on land and in oceanic surface water masses. The 5e/5d transition is marked by an abrupt shift in 818O values in Baffin Bay and Labrador Sea as a consequence of ice growth over circumpolar areas of northeastern Canada. From substage 5d to substage 5a, the Baffin Bay border lands experienced glacial conditions while subarctic to cool-temperate and humid climates persisted over Labrador Sea and Atlantic Canada. A short (<104yrs) stage 4 is recorded in deep sea cores with high 8'8O values. It corresponds to the Early Wisconsinan southward extension of the Laurentide Ice Sheet dated at ca. 80,000 yrs in the central St. Lawrence Lowland. There is no clear evidence of full glacial conditions in the Atlantic Provinces during this episode. Stage 3 (Middle Wisconsinan) corresponds in the isotopic records to large oscillations in 818O values suggesting meltwater transits in both Baffin Bay and Labrador Sea. The ice cover remained relatively extensive over eastern Canada, although some areas experienced ice-marginal conditions : in the Atlantic Provinces, notably on Cape Breton Island, hemiarctic to subarctic climate is inferred from palynological records; in the Appalachian foothills of Quebec glacial Lake Gayhurst developed some 46,000 yrs ago. During isotopic stage 2 (Late Wisconsinan), the Laurentide Ice Sheet reached its maximum extent while satellitic ice-caps developed over the Atlantic Provinces. In deep sea cores, high 818O values mark the full glacial conditions of isotopic stage 2, although slightly lower values in western Labrador Sea indicate discrete but continuous meltwater influxes. An early melting phase of the northeastern margin of the Laurentide Ice Sheet is recorded shortly after 16,700BP. The full ice-retreat is observed after ca. 11,000BP. At that time, southeastern Canada was already largely ice-free. Finally, the optimum climatic conditions of isotopic stage 1 settled diachronously in the adjacent basins of eastern Canada.
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Glacial till, glaciomarine diamictites, and ice-rafted detritus found in marine cores collected off the shore of southeast Greenland record multiple Late Cenozoic glaciations beginning in the Late Miocene. Distinct rock assemblages and seismic stratigraphic control correlate the diamictites with glaciation of the southeast Greenland margin. Glaciers advanced to the sea during several intervals in the Pliocene and Pleistocene. North Atlantic glaciation may have nucleated in southern Greenland rather than further north because of the high mountains and the high levels of precipitation in this region.
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We present a 5.3-Myr stack (the "LR04" stack) of benthic δ 18 O records from 57 globally distributed sites aligned by an automated graphic correlation algorithm. This is the first benthic δ 18 O stack comprised of more than three records to extend beyond 850 ka, and we use its improved signal quality to identify 24 new marine isotope stages in the early Pliocene. We also present a new LR04 age model for the Plio-Pleistocene derived from tuning the δ 18 O stack to a simple ice model based on June 21 insolation at 65 • N. Stacked sedimentation rates provide additional age model constraints to prevent overtuning. Despite a conservative tuning strategy, the LR04 benthic stack exhibits significant coherency with insolation in the obliquity band throughout the entire 5.3 Myr and in the precession band for more than half of the record. The LR04 stack contains significantly more variance in benthic δ 18 O than previously published stacks of the late Pleistocene as the result of higher resolution records, a better alignment technique, and a greater percentage of records from the Atlantic. Finally, the relative phases of the stack's 41-and 23-kyr components suggest that the precession component of δ 18 O from 2.7–1.6 Ma is primarily a deep-water temperature signal and that the phase of δ 18 O precession response changed suddenly at 1.6 Ma.
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Previous pollen analyses of ice cores from Devon and Ellesmere islands have contributed considerably to our knowledge of past climate in the Canadian High Arctic. In this case, in 1979, bulk (35–83 litres) water samples were melted down a hole 139 m deep, drilled to bedrock, 1.2 km from the top of the flow line in Agassiz Ice Cap in northern Ellesmere Island. Analysis of ten of these samples, plus some taken in very dirty ice from the melt tank during drilling 7 years ago, has yielded pollen concentrations that, together with the oygen-isotope (6) signatures, suggest the Agassiz Ice Cap began its growth during the last interglacial period. A discrepancy between melt-tank and bulk-sample pollen concentrations is believed to be due to a loss of pollen from the melt-tank samples during the drilling process.
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New observations of long-distance pollen transport to southern Greenland are recorded during the last 2 weeks of May, 2003. The results indicate northeastern North America as the source area of the transported pollen grains as shown in earlier investigations. Backward trajectories indicate that transport occurred twice during the first week corresponding to a time of maximum pollen flux emitted to the atmosphere in the source area. A large percentage of exotic pollen grains were identified, about 11% of the total counted. However, transport during the second week appears to have occurred during a single day at a time of reduced pollen emission into the atmosphere, which was subjected later to severe washout. As a result, only 1% of the total pollen spectra was identified as exotic grains. The back trajectories modeled by the HYSPLIT application differ somewhat from those previously identified in 2002. Although in both years air passing over southern Greenland at 3000 m carried out the main transport, additional transport could have occurred at a much lower altitude in 2003.
A multi-technique approach has been used to study a Pliocene shallow water marine deposit, designated the I OE le de France Formation, in North-East Greenland. The sequence is correlated on the basis of 87 Sr^ 86 Sr ratios in shells and palaeomagnetic studies with the Gauss normal polarity chron, which is dated to between 2.60 and 3.58 Ma years BP. This dating is in accordance with amino acid epimerisation and evidence from dinoflagellates, foraminifers and molluscs. Sediments, marine molluscs and foraminifers show that the sequence was deposited on the inner shelf, below storm wave base. Seawater temperatures were much higher than today, as demonstrated by the occurrence of a number of southern extra-limital species. The same applies to air temperature, and the few remains of land plants may indicate a forested upland with Picea and Thuja. A number of extinct taxa are present, including Nucula jensenii that is erected as a new species.
The pollen content of 77 snow samples, collected at 41 sites in the Canadian Arctic, the adjacent Arctic Ocean and Greenland can be used to identify source regions that produced the assemblages. The major vegetation zones of northern Canada produce distinctive pollen assemblages, and principal components analysis (PCA) indicate that these assemblages are retained even in snow on the sea ice surface. It is shown that pollen percentages and concentrations are related to the density of the regional vegetation and to the distance to the source of more productive regions. Because the pollen grains may be transported for great distances (even to the central regions of the Arctic Ocean), they may be used to indicate the source of that pollen and the trajectory of the air masses that carried and deposited them. Pine is particularly valuable in this sense because it has longer trajectories than other tree pollen. For example, there are indications of "over-the-pole" transport of pollen from higher pine pollen concentrations at the North Pole than on northern Ellesmere Island. Pollen concentrations of certain taxa change significantly at ˜75°N, north of which the concentrations become lower, thereby suggesting that there is a climatic boundary at that latitude. Therefore it would appear that studies of the concentration of pollen in snow have the potential for determining past and present characteristics of atmospheric circulation and also for helping in the development and interpretation of paleoenvironmental records in regions without vegetation, such as ice caps.
Aim The Holocene spread of Picea abies in Fennoscandia is well established from many sites and thus provides an opportunity for detailed study of the dynamics of tree spread and population expansion. Early- and mid-Holocene macrofossil evidence for presence of P. abies in Fennoscandia has questioned traditional interpretations of the timing and direction of its spread. This paper aims to determine when, from where and by which pathways P. abies spread into Fennoscandia. Understanding the character and dynamics of this spread may give insight into the general understanding of Holocene tree spread.
A new calibration database of census counts of organic-walled dinoflagellate cyst (dinocyst) assemblages has been developed from the analyses of surface sediment samples collected at middle to high latitudes of the Northern Hemisphere after standardisation of taxonomy and laboratory procedures. The database comprises 940 reference data points from the North Atlantic, Arctic and North Pacific oceans and their adjacent seas, including the Mediterranean Sea, as well as epicontinental environments such as the Estuary and Gulf of St. Lawrence, the Bering Sea and the Hudson Bay. The relative abundance of taxa was analysed to describe the distribution of assemblages. The best analogue technique was used for the reconstruction of Last Glacial Maximum (LGM) sea-surface temperature and salinity during summer and winter, in addition to sea-ice cover extent, at sites from the North Atlantic (n=63), Mediterranean Sea (n=1) and eastern North Pacific (n=1). Three of the North Atlantic cores, from the continental margin of eastern Canada, revealed a barren LGM interval, probably because of quasi-permanent sea ice. Six other cores from the Greenland and Norwegian seas were excluded from the compilation because of too sparse assemblages and poor analogue situation. At the remaining sites (n= 54), relatively close modern analogues were found for most LGM samples, which allowed reconstructions. The new LGM results are consistent with previous reconstructions based on dinocyst data, which show much cooler conditions than at present along the continental margins of Canada and Europe, but sharp gradients of increasing temperature offshore. The results also suggest low salinity and larger than present contrasts in seasonal temperatures with colder winters and more extensive sea-ice cover, whereas relatively warm conditions may have prevailed offshore in summer. From these data, we hypothesise low thermal inertia in a shallow and low-density surface water layer.