PII: S0277-3791(97)00047-4
Quaternary Science Reviews, Vol. 17, pp. 411—426, 1998
( 1998 Elsevier Science Ltd.
Printed in Great Britain. All rights reserved.
0277—3791/98, $19.00
THE CONTRIBUTION OF ORBITAL FORCING TO THE PROGRESSIVE
INTENSIFICATION OF NORTHERN HEMISPHERE GLACIATION
M.A. MASLIN-‡*, X.S. LI°, M.-F. LOUTRE° and A. BERGER°
-Environmental Change Research Centre, Department of Geography, University College London,
26 Bedford Way, London WC1H 0AP, United Kingdom (e-mail: MMaslin@geog.ucl.ac.uk)
‡Geologisch-Paläontologisches Institut, Universität Kiel, Olshausenstrasse 40, 24098 Kiel, Germany
° Institut d+Astronomie et de Géophysique G. Lemaı̂tre, Université catholique de Louvain,
2 Chemin du Cyclotron, B-1348, Louvain-la-Neuve, Belgium
Abstract—In this study, we reconstruct the timing of the onset of Northern Hemisphere
glaciation. This began in the late Miocene with a significant build-up of ice on Southern
Greenland. However, progressive intensification of glaciation did not begin until 3.5—3 Ma,
when the Greenland ice sheet expanded to include Northern Greenland. Following this stage
we suggest that the Eurasian Arctic and Northeast Asia were glaciated at approximately
2.74 Ma, 40 ka before the glaciation of Alaska (2.70 Ma) and about 200 ka before significant
glaciation of the North East American continent (2.54 Ma). We also review the suggested causes
of Northern Hemisphere glaciation. Tectonic changes, such as the uplift of the Himalayan and
Tibetan Plateau, the deepening of the Bering Strait and the emergence of the Panama Isthmus,
are too gradual to account entirely for the speed of Northern Hemisphere glaciation. We,
therefore, postulate that tectonic changes may have brought global climate to a critical
threshold, but the relatively rapid variations in the Earth’s orbital parameters and thus
insolation, triggered the intensification of Northern Hemisphere glaciation. This theory is
supported by computer simulations, which despite the relative simplicity of the model and the
approximation of some factors (e.g. using a linear carbon dioxide scenario, neglecting the
geographical difference between the Pliocene and the present) suggest that it is possible to
build-up Northern Hemisphere ice sheets, between 2.75 and 2.55 Ma, by varying only the
insolation controlled by the orbital parameters. ( 1998 Elsevier Science Ltd. All rights reserved
the Panama Isthmus (Keigwin, 1978, 1982; Keller et al.,
1989; Mann and Corrigan, 1990) and the deepening of
the Bering Straits (Einarsson et al., 1967) and/or the
Greenland—Scotland ridge (Wright and Miller, 1996)
(Fig. 1). A recent dating of the closure of the Pacific—Caribbean gateway (Keller et al., 1989) suggests
that the Panama Isthmus began to emerge gradually at
6.2 Ma and finally closed at 1.8 Ma. Keller et al. (1989)
also documented four major events in the progressive
closure of the Pacific—Caribbean gateway dated at
6.2 Ma, 4.2 Ma, 2.4 Ma (2.55 Ma with new time scale of
Shackleton et al., 1995) and 1.8 Ma respectively. Keller
et al. (1989) showed that there was an increasing
abundance of salinity-tolerant planktonic foraminifera
in the Caribbean from 2.55 Ma onwards, suggesting
that the restriction of water flow between the Pacific
and Caribbean started at 2.55 Ma and finally ceased at
1.8 Ma. Subsequent work by McDougall (1996), Collins (1996) and Geary et al. (1996) seem however to
disagree with Keller et al. (1989) timing of the emergence of the Panama Isthmus. For example, McDougall
(1996) study of the relative abundances of benthic
foraminifera suggests the major changes due to the
emergence of the Panama Isthmus occurred at
6.7—6.2 Ma, 3.4 Ma, 2.0 Ma and 1.1 Ma in the Caribbean and 6.7—6.4 Ma, 4.0—3.2 Ma, 2.1 Ma, 1.4 Ma
and 0.7 Ma in Pacific. Detailed results from recent
ODP Legs, particularly Leg 165, are eagerly awaited,
THEORIES ON THE CAUSES OF NORTHERN
HEMISPHERE GLACIATION
Many explanations have been put forward to explain the initiation of Northern Hemisphere glaciation.
One group of theories suggests changes in atmospheric
composition or a change in total solar radiation. Theories involving changes in solar radiation are not testable (Opik, 1959), whereas changes in atmospheric
CO content could be detected in the geological record
2
(Sarnthein and Fenner, 1988). Increased volcanism
during the latest Cenozoic (Kennett and Thunell, 1975)
has also been suggested as a possible cause of glaciation. It is now believed by some that the onset of
glaciation may have caused the observed increase in
Northern Hemisphere volcanism (e.g. Rea et al., 1995).
Other theories include virtual polar wandering (Ewing
and Donn, 1956; Schneider and Kent, 1986); uplift of
the high-lands of northern Canada (Flint, 1957;
Emiliani and Geiss, 1958; Birchfield et al., 1982); and
changes in land—sea distribution by sea floor spreading
(North et al., 1983). These theories are either too negligible in effect or too long-term to have caused the
sudden initiation of Northern Hemisphere glaciation.
Tectonic explanations have also been suggested
(Hay, 1992; Raymo, 1994a), such as the emergence of
* To whom correspondence should be addressed.
411
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Quaternary Science Reviews: Volume 17
as they will provide new evidence for the timing of the
closing of the Panama gateway and its affect on the
palaeoceanography of the Caribbean, Pacific and the
Atlantic. The very latest work by Hang and
Tredemann (in press) suggest the closure began at
4.6 Ma and continued until 2 Ma.
None of the present dating suggests that any key
event in the closure of the Panama gateway was coincident with the timing of the intensification of North
Hemisphere glaciation. Keller et al. (1989) suggested
progressive and gradual closure from 2.55 Ma onwards
is too late to have been an initiating cause. There is also
a debate whether the closure of the Panama gateway
would have helped or hindered the intensification of
North Hemisphere glaciation. The reduced inflow of
Pacific surface water to the Caribbean increased the
salinity of the Caribbean. This would have both increased the salinity and strength of the Gulf Stream,
thus enhancing deep water formation (Mikolajewicz et
al., 1993). Increased deep-water formation could have
worked against the initiation of Northern Hemisphere
glaciation as it enhances the heat transport of heat to
the high latitudes and would have tended to prevent ice
sheet formation. A contrasting argument is that the
enhanced Gulf Stream could have pumped moisture
north, stimulating the formation of ice sheets
(Mikolajewicz et al., 1993). If the closure of the Panama
gateway did increase the strength of the Gulf Stream,
this in turn should have increased deep water ventilation. However, the benthic foraminifera d13C records
from Site 552, 607, 610, 659, 704, 846 (Tiedemann, 1991;
Raymo et al., 1992, 1996; Dwyer et al., 1995; Shackleton
et al., 1995), all show a long-term decrease in d13C
between 3.5 Ma and 2.0 Ma which is attributed to increased suppression of NADW formation. The records
also do not contain a step-like increase at 2.55 Ma,
which might have been expected to occur with an abrupt
closure of the Panama gateway. Therefore, at present, the
evidence seems ambiguous as to whether or not the
formation of the Panama Isthmus caused or even enhanced the glaciation of the Northern Hemisphere.
The timing of tectonic changes in the Bering Sea was
initially dated at between 3.5 Ma and 3 Ma (Einarsson
et al., 1967), and, more recently, the submergence of the
Bering Strait has been dated at 3.2 Ma (Fyles et al.,
1991), which is too early to have caused the dramatic
changes near 2.7 Ma, but they may indeed have contributed to the long-term global cooling that started at
about 3.2 Ma. More recently it has been suggested that
changes in the Greenland—Scotland Ridge bathymetry
in the Neogene may have affected the production of
Northern Component Water (NCW) (Wright and Miller, 1996). Wright and Miller (1996) suggest that reduction of the NCW on its own is unlikely to have caused
the long-term Cenozoic climatic cooling. However, it
may have triggered the development of a permanent ice
sheet on Antarctica and the onset of Northern Hemisphere glaciation.
Ruddiman and Raymo (1988), Ruddiman et al.
(1989a,b), and Ruddiman and Kutzbach (1991) dis-
cussed how the initiation of Northern Hemisphere
glaciation could have been caused by progressive uplift
of the Tibetan—Himalayan and Sierran—Coloradan
regions. They suggested that this uplift altered the
circulation of atmospheric planetary waves such that
summer ablation was decreased, which allowed snow
and ice to build-up in the Northern Hemisphere. Discussion is on-going as to whether orography (Charney
and Eliassen, 1949; Bolin, 1950), differential heating of
land and sea surfaces (Sutcliffe, 1951; Smagorinsky,
1953), or a combination of both (Trenberth, 1983),
control the structure and direction of the planetary
waves. In contrast, Copeland et al. (1987) and Molnar
and England (1990) have suggested that the majority of
the Himalayan uplift occurred much earlier between
20 Ma and 17 Ma; while Harrison et al. (1992), from
thermochronologic results, have suggested the Tibetan
Plateau reached its maximum elevation during the late
Miocene at about 7—8 Ma and not during the midPliocene as suggested by palaeobotanical studies (Mercier et al., 1987). Quade et al. (1987) also showed, from
carbon and oxygen isotopes of a pedogenic sediment
profile in Pakistan, that the Asian monsoon underwent
a strong intensification at 7.4 Ma, which they present
as further evidence of a late Miocene uplift, thus suggesting the Ruddiman and Raymo (1988) hypothesis
may be invalid.
Raymo et al. (1988), Raymo (1991, 1994b), and
Raymo and Ruddiman (1992) have taken the
Himalayan debate one stage further suggesting that the
uplift caused a massive increase in tectonically driven
chemical weathering in the late Cenozoic. They argue
that carbonation of rainwater removes CO from the
2
atmosphere and forms a weak solution of carbonic
acid. Dissociated H` ions in the acidified rainwater by
hydrolysis chemically weathers the rocks. However,
only weathering of silicate minerals makes a difference
to atmospheric CO levels, as weathering of carbonate
2
rocks by carbonic acid returns CO to the atmosphere.
2
Bi-products of the hydrolysis reaction affecting silicate
minerals are bicarbonate (HCO~) anions and calcium
3
cations. These, when washed into the oceans, become
metabolised by marine plankton and are converted to
calcium carbonate. The calcite skeletal remains of the
marine plants and animals are ultimately deposited as
deep-sea sediments and hence lost from the global biogeochemical carbon cycle for the duration of the life
cycle of the oceanic crust on which they were deposited
(i.e. at least 30—40 Ma). Consequently, atmospheric
CO is depleted causing a cooling of the global climate
2
and thus the onset of Northern Hemisphere Glaciation.
This theory suffers from one major problem, there is
no negative feedback mechanism to prevent a complete
depletion of the relatively small atmospheric CO res2
ervoir (e.g. Berner, 1994; Compton and Mallinson,
1996; Raymo, in press). There have been a number of
proposals which have been suggested to deal with this
problem. There are three main possibilities: the geological evidence for increased chemical weathering
over the last 40 Ma may have been misinterpreted
M.A. Maslin et al.: The Contribution of Northern Hemisphere Glaciation
413
Fig. 1. Summary cartoon of the suggested causes of Northern Hemisphere Glaciation and map of the location of the major tectonic changes
associated with the onset of Northern Hemisphere Glaciation.
414
Quaternary Science Reviews: Volume 17
(Kump and Arthur, in press), the assumption of constant mantle CO input over the Neogene is incorrect,
2
or another significant flux of C into the atmosphere,
which is sensitive to global climate or ocean/atmosphere CO levels must exist (Raymo, in press). Unfor2
tunately, as Raymo (in press) very succinctly suggests,
we do not at present have the geological data to fully
understand global carbon cycles in the Cenozoic, thus
our present geochemical mass balance models are only
reflecting our ignorance.
In general it has been accepted that the Raymo
model is a good candidate for the cause of long-term
cooling of the late Cenozoic and therefore a precursor
to the onset of Northern Hemisphere Glaciation. As
the dates for the Himalayan uplift lie between 21 and
8 Ma, the model cannot explain the sudden step-like
nature of the intensification of Northern Hemisphere
Glaciation at about 2.7—2.5 Ma, unless we invoke
a long-term nonlinear amplification.
A recent suggestion is that changes in orbital forcing
may have been an important mechanism contributing
to the gradual global cooling and the subsequent rapid
intensification of Northern Hemisphere glaciation
(Lourens and Hilgen, 1994; Maslin et al., 1995). This
theory expands on the ideas of Berger et al. (1993) of
characterising different time periods, in the Pleistocene
and late Pliocene, by the relative strength of the different orbital parameters identified. Maslin et al. (1995)
suggested that the observed increase in the amplitude
of orbital obliquity cycles, from 3.2 Ma onwards, may
have increased the seasonality of the Northern Hemisphere, thus initiating the long-term global cooling
trend. The subsequent sharp rise in the amplitude of
precession and thus insolation between 2.8 Ma and
2.55 Ma may have forced the rapid glaciation of the
Northern Hemisphere. Essentially, however, the causes
of the long-term global cooling trend which started at
3.2 Ma and the significant increase in Northern Hemisphere glaciation near 2.75 Ma, are still unresolved.
REVIEW OF THE NEW EVIDENCE FROM THE
NORTH PACIFIC
Ocean Drilling Program (ODP) Site 882 in the
North West Pacific Ocean is located at the northern
end of the Emperor Seamount Chain, on the western
flank of the Detroit Seamount (50°22@N, 167°36@E) in
a water depth of 3244 m (Rea et al., 1993; ODP
Leg 145, 1993) (Fig. 2). Site 882 provides the first
Fig. 2. Major surface water currents of the North Pacific and the location of Site 882 (50°21.79@N, 167°35.99@E) and the other ODP Leg 145 Sites.
c
Fig. 3. Comparison of age models of ODP Sites 882 and 887 for the time interval 3.2 Ma to 2.4 Ma with the benthic isotopes of Site 659
(Tiedemann et al., 1994). Based on the Matuyama/Gauss magnetic reversal boundary at 2.6 Ma for initial age control, the GRAPE density record
was fine-tuned to precession-related oscillations of the summer (July) insolation record for 65°N, assuming no phase differences. After tuning, the
precession (22-ka filter output) and obliquity (41-ka filter output) components were isolated from the GRAPE-density records and compared with
orbital precession and obliquity records to test the tuning strategy. The magnetic susceptibility records indicate the input of ice-rafted debris (IRD)
see text. Based on the finely-tuned stratigraphy, the major onset of ice rafting occurred at 2.74 Ma in the North West Pacific (Site 882), whereas the
first onset of ice rafting in the North East Pacific (Site 887) occurred at 2.7 Ma, about one obliquity cycle later. Note: At Site 882, the sediment
composition changes abruptly at approximately 2.74 Ma as a consequence of the major onset of IRD deposition. Prior to 2.74 Ma, a time interval
with no significant IRD input, high GRAPE-density values reflect minima in opal and maxima in carbonate accumulation (Tiedemann and Haug,
1995; Haug et al., 1995a and b). Benthic and planktonic isotope records indicate that GRAPE density maxima correlate with warm stages (Maslin
et al., 1995). Between 2.74 and 2.4 Ma, GRAPE density minima are related to IRD input minima and biogenic opal maxima (Haug et al., 1995a and
b). The result is a negative correlation between GRAPE density values and the summer (July) insolation for 65°N before and after 2.74 Ma. This
inverse correlation was not noted in the North East Pacific at Site 887.
M.A. Maslin et al.: The Contribution of Northern Hemisphere Glaciation
415
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Quaternary Science Reviews: Volume 17
Fig. 4. Comparison between the calculated insolation for 65°N (Loutre and Berger, 1993), benthic d18O and d13C isotope records from the
equatorial Pacific at Site 846 (Shackleton et al., 1995), the d18O records from N. pachyderma (r) and G. bulloides and magnetic susceptibility record
from Site 882 for the time interval 3.2 Ma to 2.4 Ma. The shaded regions represent global cold periods and are labelled according to the
nomenclature of Shackleton et al. (1995) and Tiedemann et al. (1994).
high-resolution carbonate, opal, and foraminifera
stable isotope records for the interval between 3.2 and
2.4 Ma in the North Pacific (Haug, 1995; Haug et al.,
1995a; Maslin et al., 1995). The sedimentation rate at
this site varied between 12 to 4 cm/1000 years between
3.2 Ma and 2.4 Ma, with a major drop in the sedimentation rate occurring near 2.75 Ma (Tiedemann
and Haug, 1995). ODP Site 887 is also used in this
study and provides the high resolution records of climate changes in the far North East Pacific. The site is
located on the Patton—Murray Seamount (54°22@N,
148°27@W) in a water depth of 3645 m (Rea et al., 1993;
ODP Leg 145, 1993) (Fig. 2).
The time scale for Sites 882 and 887 were based
initially on magnetostratigraphy. The ages for the magnetic reversal boundaries were derived from the orbitally tuned time scale of Shackleton et al. (1990, 1995)
and Tiedemann et al. (1994) which modified the ages
published by Hilgen (1991). Using the astronomically
dated magnetic reversals for initial age control, we
found that the fluctuations in the gamma ray attenuation porosity evaluator (GRAPE) density and magnetic susceptibility records were linked to variations in
the Earth’s orbit, as expected. GRAPE-density provides an analog data of the sediment wet-bulk density,
while magnetic susceptibility at this location provides
M.A. Maslin et al.: The Contribution of Northern Hemisphere Glaciation
a proxy for ice rafted debris (Haug et al., 1995a; Maslin
et al., 1995).
Tiedemann and Haug (1995) generated an astronomically calibrated stratigraphy for Site 882 for the
last 4 Ma based on fine-tuning the GRAPE-density
oscillations in the precession band to the summer
insolation at 65°N (Berger and Loutre, 1991). This
calibration was based on the assumption that GRAPEdensity maxima are linked to minima in biogenic opal
and maxima in ice-rafted debris, and hence to glacial
stages (insolation minima) during the last 2.75 Ma,
confirmed by benthic and planktonic stable isotopes
for the last 750 ka (Haug, 1995; Haug et al., 1995a,b).
Prior to 2.75 Ma, the relationship between GRAPEdensity and climate was the reverse, which is confirmed
by the benthic d18O results from Site 882 (Maslin et al.,
1995). A detailed description of the astronomically
calibrated age model is given by Tiedemann and Haug
(1995). The age model for Site 887 (Fig. 3) between
2.4 Ma and 3.2 Ma was also based on the assumption
that GRAPE-density maxima are linked to minima in
biogenic opal and maxima in ice-rafted debris, and
hence to glacial stages. There is no evidence of an
inverse correlation which was found at Site 882 by
Tiedemann and Haug (1995).
The d18O and d13C of benthic and planktic
foraminifera species were measured according to the
standard techniques at the University of Kiel (Maslin
et al., 1995). Three planktonic foraminifera species were
analyzed, Globigerina bulloides, Neogloboquadrina
pachyderma (r), and Neogloboquadrina pachyderma (l).
G. bulloides and N. pachyderma (r) occurred with a sufficient time resolution to have near continuous records
between 3.4 and 2.4 Ma, whereas N. pachyderma (l)
occurred sporadically and was used only to confirm
the pattern of the other two species. Using the Site 882
age model, it is possible to compare Site 882 planktonic
d18O with the benthic Cibicidoides wuellerstorfi d18O
and d13C records from the Equatorial Pacific Site 846
(Shackleton et al., 1995). Fig. 4 indicates a high degree
of similarity between the two records, the most significant of which occurs between 2.7 Ma and 2.9 Ma
(Stage G14 to G4, using the nomenclature of Tiedemann et al., 1994). Between 2.85 Ma and 2.75 Ma, the
planktonic foraminifera d18O values show a trend towards lighter values ('1&), overlying the warm—cold
period variations, the lightest values are at 2.76 Ma,
indicating either a very high surface water temperature
or low salinity or a combination of both. Subsequently,
at 2.75 Ma, coeval with the onset of the G6 glaciation,
significant ice rafting occurred in the Northwest Pacific
Ocean and the planktonic foraminiferal d18O drops
dramatically by 2.6&.
If ice volume and sea level effects are taken into
consideration and sea surface temperature (SST) is
assumed to be the only other variable, then, as Maslin
et al. (1996) suggests the SSTs change between cold to
warm periods was at least 5°C before 2.75 Ma with an
overall warming trend of 4°C and a dramatic drop of
no less than 7.5°C at 2.75 Ma (using the SST-d18O
417
equation of Shackleton (1974)). This led Sarnthein et al.
(1995) to suggest that these very high Northwest Pacific SSTs and the possible warming trend prior to
2.75 Ma would have produced enhanced evaporation
and thus a mechanism to transport additional moisture to the growing Northern Hemisphere continental
ice sheets. They stressed, however, that surface water
salinity changes could not be ruled out.
Biomarkers, such as the ºK37* index (Brassell et al.,
1986a,b) and its simplification, ºK37{ (Brassell et al.,
1986b), have been developed as an independent estimate of sea surface temperatures. The ºK37* index is
based on the relative abundances of long-chain alkenones with 37 atoms, which are ubiquitous in the
World’s oceans, both in the water column and in the
sediment (Brassell et al., 1986a; Rosell i Mele, 1994).
Haug et al. (1995b), Haug (1995) and Haug et al. (in
prep.) were able to measure the ºK37 index for the last
6 Ma at ODP Site 882. The preliminary ºK37 index
results indicate that before 2.75 Ma at Site 882, there
was a moderate cooling, indicating that the very light
planktonic d18O was probably not due to warming,
but to a strong freshening of the North Pacific, about
1—2& over 80 ka. The ºK37 index also indicates a
warming of the surface waters at 2.75 Ma (Haug et al.,
1995b and Haug, 1995), though this could be due to
contamination from older material brought in by the
initiation of ice rafting, as indicated by the magnetic
susceptibility record in Fig. 3 (Haug, 1995). Therefore
at present the true cause of the extremely light planktonic foraminifera d18O prior to 2.75 Ma, and the large
increase at 2.75 Ma, is unclear, but the evidence at
present points to strong salinity changes in the Northwest Pacific during the mid to late Pliocene.
DISCUSSION
Timing of the initiation and intensification of Northern
Hemisphere glaciation
The earliest recorded onset of significant global glaciation during the last 100 Ma was the continent-wide
glaciation in Antarctica at about 34 Ma (e.g. Hambrey
et al., 1991; Breza and Wise, 1992; Miller et al., 1991;
Zachos et al., 1992, 1996). In contrast the earliest
recorded glaciation in the Northern Hemisphere is
between 10 and 6 Ma (e.g. ODP Leg 151, 1994; Jansen
et al., 1990; Wolf and Thiede, 1991; Jansen and
Sj+holm, 1991; Wolf-Welling et al., 1995). This Miocene
initiation of the Northern Hemisphere glaciation has
been recorded in the Greenland Sea (Wolf and Thiede,
1991; Wolf-Welling et al., 1996; Fronval and Jansen
(1996); Thiede and Myhre (1996)), the Norwegian Sea
(Jansen et al., 1990; Jansen and Sj+holm, 1991), the
Arctic (ODP Leg 151, 1994) and in the Northwest
Pacific (Haug et al., 1995a; Haug, 1995).
The first occurrence of significant ice sheets in the
Northern Hemisphere was thought not to occur until
about 2.55 Ma (Shackleton et al. (1984), a date which is
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Quaternary Science Reviews: Volume 17
adjusted to the new time scale of Shackleton et al.
(1990) and Hilgen (1991)), although these were preceded by smaller increases in ice volume from 2.7 Ma
onwards (Backman, 1979; Shackleton et al., 1984; Zimmerman et al., 1985). These conclusions were based
primarily on relatively low-resolution records of
%CaCO and d18O from Deep Sea Drilling Project
3
(DSDP) Site 552 in the Atlantic Ocean (Shackleton
et al., 1984). The sudden decrease in %CaCO values
3
was explained by an increase in ice-rafted debris coeval
with a drop in d18O which indicated an increase in
global ice volume (Shackleton et al., 1984).
Recent evidence, though, suggests that the initiation
of major Northern Hemisphere glaciation was the culmination of a longer term, high latitude cooling, which
began in the late Miocene with the glaciation of Greenland and the Arctic (Wolf and Thiede, 1991; ODP Leg
151, 1994; Wolf-Welling et al., 1995; Wolf-Welling
et al., 1996; Fronval and Jansen, 1996; Thiede and
Myhre, 1996). This long-term cooling intensified at
3.2 Ma (Ruddiman et al., 1986b; Tiedemann et al.,
1994) with the progressive and oscillatory enrichment
of benthic foraminiferal d18O and suggests that there
was significant deep water cooling and an increase in
global ice volume after that time (Prell, 1984; Keigwin,
1986; Ruddiman et al., 1986a,b; Sarnthein and
Tiedemann, 1989; Tiedemann et al., 1994).
There is strong evidence for this progressive cooling
of the Northern Hemisphere from the increase in the
percentage of cold water-dwelling planktonic
foraminifera (Loubere and Moss, 1986; Raymo et al.,
1986) and by decreasing abundances of discoaster
(Backman and Pestiaux, 1986). Sea-surface temperatures (SSTs), for this time interval, however, are not
easily reconstructed, as the assemblages of planktonic
foraminifera are not analogous to modern faunas (Ruddiman and Raymo, 1988). Chapman (1992, 1995), however, has demonstrated using evolutionary-equivalent
species that it is possible to reconstruct Pliocene subtropical SSTs. His work on ODP Site 659 shows a major cooling of the tropical Eastern Atlantic at 2.55 Ma.
The most recent benthic foraminiferal d18O results
from ODP Site 659 in the tropical East Atlantic
(Tiedemann et al., 1994), and Site 846 in the equatorial
East Pacific (Shackleton et al., 1995), show a gradual
enrichment of 18O in the ocean between 3.2 Ma and
2.75 Ma. From 2.75 to 2.68 Ma, there are three successive intervals with very heavy d18O values, but with
minimal climatic recovery between them. Subsequently, there were still further increases in the average
d18O value, and a marked increase in the amplitude of
variation between warm and colder periods. The major
controversy at present is how much of these d18O shifts
represent cooling of the deep waters and how much
represent the first build up of ice in the Northern
Hemisphere. It is believed that the three major increases in benthic d18O between 2.75 and 2.68 Ma,
were primarily due to a significant increase in global
ice volume (Shackleton et al., 1995; Tiedemann et al.,
1994); which were associated with an increased sup-
pression of the formation of North Atlantic Deep
Water (NADW) (Sarnthein and Tiedemann, 1989;
Tiedemann, 1991; Raymo et al., 1992).
To provide more detail of the intensification of
Northern Hemisphere glaciation during the Pliocene,
a number of ODP deep-sea sediment proxy climate
records have been tuned to the orbital ‘Milankovitch’
variations, with a resolution of between 3—10 ka. Taking
into account the criteria and the pitfalls suggested by
Shackleton et al. (1995), the Pliocene sections from the
North Pacific have been age modelled (ODP Sites 882
and 887; Tiedemann and Haug (1995)) while other key
ODP records (Sites, 609, 610, 642, 644, 552) have been
evaluated and adjusted to the revised astronomically
calibrated time scale (Shackleton et al., 1990; Hilgen,
1991; Tiedemann et al., 1994; Shackleton et al., 1995).
Evidence from the Norwegian Sea and Iceland
suggests that the first minor increase in Arctic, Iceland
and Scandinavian glaciation occurred at 3 Ma, while
a more pronounced and sustained increase occurred
at about 2.74 Ma (Jansen et al., 1990; Jansen and
Sj+holm, 1991; Einarsson and Albertsson, 1988; Geirsdottir and Eiriksson, 1994; Fronval and Jansen 1996).
This increase is coeval with the increase in the magnetic susceptibility record of Site 882 which demonstrates a minor increase in ice rafting at 3 Ma (Fig. 4),
and a major increase at 2.74 Ma. McKelvey et al.
(1995) has traced the source of late Pliocene ice rafting
at Site 882 to the Kamchatka peninsula and the northern coast of the Sea of Okhotsk. This suggests that the
ice rafted debris observed at Site 882 was predominantly derived from the Eurasian Arctic (via the
Chukchi and Bering Sea) and Northeast Asia. Evidence from the Norwegian Sea and Site 882 confirms
that there was ice cover in the circum-Arctic continents
from about 3 Ma, and that this significantly expanded
at about 2.74 Ma.
Evidence from Northern Alaska (including pollen,
plant macrofossil and marine vertebrate records) suggests that there were three major marine transgressions
in the late Pliocene (Brigham-Greete and Carter, 1992;
Kaufman and Brighamn-Grette, 1993). These were
dated using amino acid geochemistry, palaeomagnetic
studies, vertebrate and invertebrate paleontology and
strontium isotopes to between 2.48 Ma and 2.7 Ma
(2.62 Ma and 2.86 Ma using the new time scale), which
is equivalent to the three warm periods observed in the
deep-sea sediment records between Oxygen Isotope
Stage 104 and G12. They also found that these periods
were too warm for permafrost and even seasonal sea
ice. During the waning stages of these transgressions
the terrestrial conditions were cool enough to support
a herbaceous tundra vegeta tion with scattered larch
trees. They found no evidence for any glaciation of
Northern Alaska between 2.62 Ma and 2.86 Ma. However, the magnetic susceptibility and GRAPE density
records from Site 887 in the Gulf of Alaska (Figs. 1 and
3) indicates that there was a dramatic increase in the
supply of ice-rafted debris at 2.70 Ma. This suggests
that Southern Alaska was glaciated 40 ka later i.e. one
M.A. Maslin et al.: The Contribution of Northern Hemisphere Glaciation
419
Fig. 5. Timing of the intensification of Northern Hemisphere Glaciation. Map shows the extent of the Northern Hemisphere ice sheets during the
Last Glacial Maximum (CLIMAP, 1976, 1981). The separate ice sheets are identified by different colours. The modern climatic fronts are also
shown to represent the possible atmospheric circulation of the pre-glacial Pliocene. The graph shows the estimated time at which each ice sheet was
large enough to reach the edge of the continents and thus release icebergs, compared with its relative effect on the sea-level, estimated from the
change in the global d18O signal (Tiedemann et al., 1994; Shackleton et al., 1995). Iceland seems to have had repeated glacial episodes from the midPliocene onwards, but no clear dates are yet available for when this became sustained (Einarsson and Albertsson, 1988; Geirsdottir and Eiriksson,
1994). The data for the onset of ice rafting of the different ice sheets come from the following sources: Southern Greenland — coarse fraction
analysis (e.g. Wolf and Thiede, 1991; ODP Leg 151, 1994; Wolf-Welling et al., 1995; Wolf-Welling et al., 1996; Fronval and Jansen 1996 and Thiede
and Myhre 1996). Northern Greenland — coarse fraction analysis (e.g. Wolf and Thiede, 1991; ODP Leg 151, 1994; Wolf-Welling et al., 1995;
Wolf-Welling et al., 1996; Fronval and Jansen 1996 and Thiede and Myhre 1996). Eurasian Arctic — lithic fragment counts (Jansen et al., 1990;
Jansen and Sj+holm, 1991) and Northeast Asia — magnetic susceptibility and coarse fraction analysis (Haug, 1995; Haug et al., 1995a,b; Maslin et
al., 1995 and this study), Alaska and the Northwest coast of America — magnetic susceptibility (Rea et al., 1993; Maslin et al., 1995 and this study)
and Northeast America — calcium carbonate, magnetic susceptibility, stable isotopes, ostracods and coarse fraction analysis (e.g. Shackleton et al.,
1984; Ruddiman and Raymo, 1988; Raymo et al., 1989, 1992; Cronin et al., 1996).
420
Quaternary Science Reviews: Volume 17
obliquity cycle after the Eurasian Arctic and Northeast
Asia (Fig. 5).
Evidence from ODP Sites 607, 609 and 610 (Ruddiman and Raymo, 1988; Raymo et al., 1989, 1992;
Cronin et al., 1996) suggests that the first incidence of
ice-rafting in the North Atlantic occurred at about
2.74 Ma coeval with the expansion and increased production of icebergs from the Eurasian ice sheet. However evidence from these sites and DSDP Site 552
(Shackleton et al., 1984) suggests that the North American ice sheets did not expand until 2.54 Ma, when there
was a significant increase in the deposition of ice rafted
debris as recorded in the sediments of the North Atlantic Ocean (Oxygen Isotope Stage 100). There may have
been, therefore, a delay of 200 ka between the intensification of glaciation in the Arctic and Northeast Asia
and the major build-up ice on the North East America
continent (Fig. 5). This suggests that the initiation and
intensification of Northern Hemisphere glaciation was
driven by climate changes in the Arctic and North
Pacific regions, opposed to the Laurentide ice sheet dominated scenario of the Last Glacial Maximum, and much
of the late Pleistocene (Ruddiman and McIntyre, 1981).
Sea-level changes inferred from the benthic
foraminiferal oxygen isotope records of Sites 659
(Tiedemann et al., 1994) and 846 (Shackleton et al.,
1995; see Fig. 4), corroborate the timing described
above and suggest that the two most important stages
in the glaciation of the Northern Hemisphere were the
maturing of the Eurasian—Northeast Asian ice sheets
(Oxygen Isotope Stage 110 or G6) and the protoLaurentide ice sheet on the eastern North American
continent (Oxygen Isotope Stage 100). We stress
though that the step-like nature of the ice rafting records may conceal a more gradual process of ice
build-up, indicated by the progressive 18O enrichment
of benthic isotope records (Tiedemann et al., 1994;
Shackleton et al., 1995). This is because the ice-rafting
records indicate only when the continental ice sheets
were mature enough to impinge on the adjacent
oceans. Dramatic changes, however, are observed in
each ocean basin when ice-rafting first occurs.
Was orbital forcing a contributing factor in the
intensification of Northern Hemisphere glaciation?
According to the astronomical theory of palaeoclimates (Milankovitch, 1949; Berger, 1988), the longterm variations in the geometry of the Earth’s orbit
and rotation are the fundamental causes of the
comings and goings of Pleistocene ice ages (Berger,
1989). It has been suggested (Berger, 1976, 1979) that it
is changes in the latitudinal distribution and seasonal
pattern of insolation which are the key factors driving
the behaviour of the climate system. The causes of the
initiation and intensification of Northern Hemisphere
glaciation essentially remain unresolved. We suggest
that the long-term cooling, and the dramatic intensification of Northern Hemisphere glaciation, can be in
part explained by both tectonics and changes in orbital
forcing between 4 Ma and 2 Ma (Figs. 1 and 6).
Changes in the climate of the Pliocene are dominated
by the 41 ka periodicity of orbital obliquity (Ruddiman
et al., 1986a; Tiedemann et al., 1994; Shackleton et al.,
1995). This climate response to orbital forcing does not
change until approximately 900—800 ka ago when the
41 ka cyclicity is progressively superseded by the
100 ka period of orbital eccentricity (Shackleton and
Opdyke, 1973; Imbrie et al., 1992 and 1993). Berger
et al. (1993) has demonstrated that longer-term climate
fluctuations of the last 1.8 Ma can be subdivided into
three distinct response periods to orbital forcing: (1)
0—610 ka, where the eccentricity-related signal is dominant, (2) 610 ka to 1.22 Ma, where the obliquity signal
is dominant and (3) 1.8 Ma to 1.22 Ma, where both
signals are equally important. DeMenncal (1995)
documented that African climate was dominated by a precession-related signal before 2.8 Ma, by
an obliquity signal between 2.8 and 1 Ma, and by an
eccentricity-related signal after 1 Ma.
Fig. 6 shows the key orbital parameters of the late
Pliocene, obliquity and precession and the resultant
July insolation at 65°N, as calculated by Loutre and
Berger (1993). From 3.5 Ma until 2.5 Ma the amplitude
of obliquity cycles gradually increased until it reach its
peak between 2.5 Ma and 2.1 Ma. Between 2 Ma and
3.5 Ma, the eccentricity-modulated amplitude of the
precessional signal was dominated by a period of
roughly 400 ka, with the largest amplitudes centred
around 2.2 Ma, 2.6 Ma, 3.05 Ma and 3.45 Ma, though
the peak in amplitude at around 2.6 Ma is slightly
smaller than the three other peaks. This pattern of the
orbital elements resulted in the amplitude of the high
latitude summer insolation cycles being dominated by
an approximate 400-ka period. As July 65°N insolation
is dominated by the precessional signal, four stages
with large amplitudes of the signal can be identified,
roughly between 3.45 Ma and 3.3 Ma, between 3.2 Ma
and 2.9 Ma, between 2.75 Ma and 2.45 Ma, and between 2.35 Ma and 2.1 Ma.
For a glaciation to develop, the astronomical theory
requires that summers in the northern high latitudes
must be cool enough to prevent winter snows from
melting. This allows there to be a positive annual
budget of snow and ice, thus initiating a positive
feedback via increased surface albedo. In the late
Pliocene, lower summer insolation during a period of
large amplitude in the insolation signal, should favour
the development of Northern Hemisphere ice sheets
(especially if the tectonically-induced cooling had
brought the hemisphere to a critical threshold), as the
high-latitude winter snow would not be fully removed
by the cooler summers.
The observed rapid intensification of Northern
Hemisphere glaciation between 3.2 Ma and 2.4 Ma
occurred within one of these periods of high-amplitude
in insolation. Tectonic forcing alone cannot explain the
pronounced transition in both the intensity of glacial—interglacial cycles and mean global ice volume.
We suggest that the gradual long-term tectonic forcing
M.A. Maslin et al.: The Contribution of Northern Hemisphere Glaciation
421
Fig. 6. Comparison between the key orbital parameters of the Pliocene, obliquity and precession and the resultant insolation at 65°N, as
calculated by Loutre and Berger (1993) with the benthic d18O record from the equatorial Pacific at Site 846 (Shackleton et al., 1995) and the
magnetic susceptibility record from Site 882. Arrows indicate general tends in the data.
422
Quaternary Science Reviews: Volume 17
role of orbital forcing in triggering the intensification of
Northern Hemisphere glaciation at around 2.75 Ma.
CONCLUSIONS
Fig. 7. July 65°N insolation variation (Loutre and Berger, 1993)
(solid line, upper panel), the tectonically-induced linear CO concen2
trations (Saltzman et al., 1993) (dashed line, upper panel), and the
simulated Northern Hemisphere ice sheet volume with the LLN 2-D
model (solid line, lower panel), from 3.05 Ma to 2 Ma BP (Li et al.,
submitted).
brought the Northern Hemisphere to critical conditions for glaciation after 3 Ma, and then orbital forcing
induced a rapid transition to the glacial—interglacial
climate regime in the Northern Hemisphere between
2.75 Ma and 2.55 Ma. This theory is supported by the
recent simulation of the Northern Hemisphere icesheet volume variation made by Li et al. (in press) with
the LLN 2-D model (Galleé et al., 1991, 1992). In this
experiment, ice volume fluctuations were forced by
insolation variations (Loutre and Berger, 1993) and the
assumption of a linearly decreasing atmospheric CO
2
concentration (Saltzman et al., 1993), and ignoring
other plausible differences in boundary conditions
(e.g. meridional heat flux, geography). Three major
periods of glaciation were simulated between 3 Ma and
2 Ma (Fig. 7): one before 2.9 Ma, one between 2.75 Ma
and 2.45 Ma, and one between 2.35 and 2.1 Ma, each
one corresponding to the periods of large amplitude in
the insolation signal. Among them, the glaciations before 2.9 Ma had the smallest amplitude, coeval with the
minor increase in the magnetic susceptibility (IRD
proxy) record of Site 882 and with geological evidence
from the Norwegian Sea and Iceland which suggests
a minor increase in glaciation in the Arctic, Iceland and
Scandinavia at 3 Ma. A significant increase in ice rafting occurred in the Northwest Pacific Ocean at
2.75 Ma, as revealed by a dramatic drop of the planktonic foraminifera d18O record of Site 882. This is
reinforced by a sudden increase in simulated Northern
Hemisphere ice sheet volume at around that time. As
the ice volume simulation is mainly forced by insolation variations, the coincidence between the geological
observations and the numerical modelling confirms the
(1) Minor increases in ice rafting have been detected
in the Norwegian Sea and the Northwest Pacific from
3 Ma onwards.
(2) A dramatic increase in ice rafting occurred in the
Northwest Pacific and the Norwegian Sea at 2.75 Ma,
suggesting that the Eurasian Arctic and Northeast Asia
was significantly glaciated after 2.75 Ma. In the Northwest Pacific, this major change was accompanied by
a dramatic drop in sea-surface temperatures ('7.5°C),
or an increase in salinity (1—2&). This dramatic change
was preceded by 80 ka of unusually warm or fresh
Northwest Pacific surface waters.
(3) Data from Site 887 in the Northeast Pacific
suggest that Alaska became glaciated at 2.70 Ma, 40 ka
(one obliquity cycle) after the glaciation of Eurasian
Arctic and Northeast Asia. Sites 609, 610 and 552
in the Atlantic Ocean indicate that the North East
American continent was first glaciated at about
2.74 Ma and its ice volume increased dramatically at
2.54 Ma, about 200 ka after the glaciation of Eurasian
Arctic and Northeast Asia. Hence, the initiation and
intensification of Northern Hemisphere Glaciation
was driven by increases in ice volume in the Arctic and
North Pacific, as opposed to the Laurentide ice sheet
dominated scenario of the late Pleistocene.
(4) Tectonic forcing, and the resultant lowering of
atmospheric CO might have gradually cooled the
2
Northern Hemisphere, but it is the orbital forcing that
triggered the intensification of Northern Hemisphere
glaciation between 2.74 Ma and 2.54 Ma.
(5) Simulation of Northern Hemisphere ice sheet
volume with the LLN 2-D model shown in this study,
supports the theory that orbital forcing was the triggering mechanism of the intensification of Northern
Hemisphere glaciation.
ACKNOWLEDGEMENTS
The authors are grateful to N.J. Shackleton for providing the Site 846 isotope data and for M. Sarnthein,
R. Tiedemann, E. Jansen, M. Chapman, W. Berger, T.
Bickert, J. Thiede, and M. Raymo for providing data,
valuable discussions and great insight. We gratefully
acknowledge the cooperation of H. Erlenkeuser and H.
Cordt, who supervised the operation of the mass spectrometer in Kiel. We thank E. Heinrich, J. Hennings,
and C. Hohnemann for technical assistance at Kiel
University and Catherine Pyke and Nick Mann of the
Department of Geography, UCL, Drawing Office.
The Deutsche Forschungsgemeinschaft and the CEE
(grant CEE-EV5V-CT94-0457) for supporting this study.
We would like to thank Ellen Thomas and Simon Robinson whose extensive suggestions greatly improved this
paper.
M.A. Maslin et al.: The Contribution of Northern Hemisphere Glaciation
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