Science, Vol 292,
Issue 5517, 686-693 , 27 April 2001
Trends, Rhythms, and Aberrations in Global Climate
65 Ma to Present
Since 65 million years ago (Ma), Earth's climate has
undergone a significant and complex evolution, the finer details of
which are now coming to light through investigations of
deep-sea sediment cores. This evolution includes gradual
trends of warming and cooling driven by tectonic
processes on time scales of 105 to 107 years,
rhythmic or periodic cycles driven by orbital processes
with 104- to 106-year cyclicity, and
rare rapid aberrant shifts and extreme climate transients
with durations of 103 to 105 years. Here,
recent progress in defining the evolution of global
climate over the Cenozoic Era is reviewed. We focus
primarily on the periodic and anomalous components of
variability over the early portion of this era, as
constrained by the latest generation of deep-sea isotope
records. We also consider how this improved perspective
has led to the recognition of previously unforeseen
mechanisms for altering climate.
1 Earth Sciences
Department, University of California, Santa Cruz, CA
Department of Earth and Environmental Sciences, Wesleyan University,
Middletown, CT 06459, USA.
3 Center for the
Study of Global Change, Yale University, New Haven, CT 06520-8105,
4 College of
Marine Studies, University of Delaware, Lewes, DE 19958, USA.
whom correspondence should be addressed. E-mail: email@example.com
Through study of sedimentary archives, it has become increasingly
apparent that during much of the last 65 million years and
beyond, Earth's climate system has experienced continuous
change, drifting from extremes of expansive warmth with
ice-free poles, to extremes of cold with massive
continental ice-sheets and polar ice caps. Such change is
not unexpected, because the primary forces that drive
long-term climate, Earth's orbital geometry and plate
tectonics, are also in perpetual motion. Much of the higher
frequency change in climate (104 to
105 years) is generated by periodic and quasi-periodic
oscillations in Earth's orbital parameters of
eccentricity, obliquity, and precession that affect the
distribution and amount of incident solar energy (Fig.
Whereas eccentricity affects climate by modulating the
amplitude of precession and thus influencing the total
annual/seasonal solar energy budget, obliquity changes
the latitudinal distribution of insolation. Because the
orbital parameters vary with distinct tempos that remain
stable for tens of millions of years (2),
they provide a steady and, hence, predictable pacing of
Fig. 1. Primary orbital
components are displayed on the left, and Cenozoic paleogeography on
the right. The gravitational forces exerted by other celestial
bodies affect Earth's orbit. As a result, the amount and, more
importantly, the distribution of incoming solar radiation oscillate
with time (123).
There are three orbital perturbations with five periods:
eccentricity (at 400 and 100 ky), obliquity (41 ky),
and precession (23 and 19 ky). (A) Eccentricity
refers to the shape of Earth's orbit around the sun, varying from
near circular to elliptical. This effect on insolation is very
small, however, and by itself should not account for changes in
Earth's climate during the past. (B) Obliquity refers to the
tilt of Earth's axis relative to the plane of the ecliptic varying
between 22.1° and 24.5°. A high angle of tilt increases the seasonal
contrast, most effectively at high latitudes (e.g., winters in both
hemispheres will be colder and summers hotter as obliquity
increases). (C) Precession refers to the wobble of the axis
of rotation describing a circle in space with a period of
26 ky. Modulated by orbital eccentricity, precession determines
where on the orbit around the sun (e.g., with relation to aphelion
or perihelion) seasons occur, thereby increasing the seasonal
contrast in one hemisphere and decreasing it in the other. The
effect is largest at the equator and decreases with increasing
latitude. The periods of the precessional signal modulated by
eccentricity are 23 and 19 ky, the periods observed in
geological records. (D) Continental geography reconstructed
for five intervals of the last 70 My (designed using the
commercial Paleogeographic Information System). [View
Larger Version of this Image (74K GIF file)]
The orbitally related rhythms, in turn, oscillate about a
climatic mean that is constantly drifting in response to
gradual changes in Earth's major boundary conditions.
These include continental geography and topography,
oceanic gateway locations and bathymetry, and the
concentrations of atmospheric greenhouse gases (3).
These boundary conditions are controlled largely by plate
tectonics, and thus tend to change gradually, and for the
most part, unidirectionally, on million-year (My) time
scales. Some of the more consequential changes in
boundary conditions over the last 65 My include: North
Atlantic rift volcanism, opening and widening of the two
Antarctic gateways, Tasmanian and Drake Passages (4);
collision of India with Asia and subsequent uplift of the
Himalayas and Tibetan Plateau (5);
uplift of Panama and closure of the Central American
1 and 2);
and a sharp decline in pCO2 (7).
Each of these tectonically driven events triggered a
major shift in the dynamics of the global climate system
Moreover, in altering the primary boundary conditions and/or
mean climate state, some or all of these events have
altered system sensitivity to orbital forcing (16),
thereby increasing the potential complexity and diversity
of the climate spectrum. This would include the potential
for unusually rapid or extreme changes in climate (17,
Fig. 2. Global deep-sea
oxygen and carbon isotope records based on data compiled from more
than 40 DSDP and ODP sites (36).
The sedimentary sections from which these data were generated are
classified as pelagic (e.g., from depths >1000 m) with
lithologies that are predominantly fine-grained, carbonate-rich
(>50%) oozes or chalks. Most of the data are derived from
analyses of two common and long-lived benthic taxa,
Cibicidoides and Nuttallides. To correct for
genus-specific isotope vital effects, the 18O values were adjusted by +0.64 and
respectively. The absolute ages are relative to the standard GPTS
The raw data were smoothed using a five-point running mean, and
curve-fitted with a locally weighted mean. With the carbon isotope
record, separate curve fits were derived for the Atlantic (blue) and
Pacific above the middle Miocene to illustrate the increase in
basin-to-basin fractionation that exceeds ~1.0 in some intervals. Prior to 15 Ma, interbasin
gradients are insignificant or nonexistent (39).
The 18O temperature scale was computed for an
ice-free ocean [~1.2 Standard Mean Ocean Water (SMOW)], and thus only
applies to the time preceding the onset of large-scale glaciation on
Antarctica (~35 Ma) (43).
From the early Oligocene to present, much of the variability (~70%)
in the 18O record reflects changes in Antarctica
and Northern Hemisphere ice volume (40).
The vertical bars provide a rough qualitative representation of ice
volume in each hemisphere relative to the LGM, with the dashed bar
representing periods of minimal ice coverage (50%), and the full bar representing close to maximum
ice coverage (>50% of present). Some key tectonic and biotic
events are listed as well (4,
Larger Version of this Image (39K GIF file)]
Although Earth's climatic history has been reconstructed with an
array of proxies applied to both marine and terrestrial
sediment archives, much of the progress in resolving the
rates and scales of Cenozoic climate change can be
attributed to the development of high-resolution deep-sea
oxygen (18O) and carbon (13C) isotope records (19).
Since the early 1970s, 18O data have served as the principal means
of reconstructing global and regional climate change on a
variety of geologic time-scales, from millennial to
tectonic. These records are multidimensional in that they
provide both climatic and stratigraphic information, and
can be quickly generated with automated mass spectrometers.
The first marine isotope records were relatively coarse, but
still provided valuable insight into the general
structure of the Pleistocene glacial and interglacial
These were followed by records delineating the long-term
patterns of Cenozoic climate change (21-23)
and, eventually, the first global compilation of records
for the Cenozoic (resolution of 105 to
106 years) (24).
The last decade has witnessed a rapid growth in the inventory of
high-resolution isotope records across the Cenozoic, aided
by the greater availability of high-quality sediment cores
recovered by the Deep Sea Drilling Project (DSDP) and
Ocean Drilling Program (ODP). The improved perspective
provided by these records has led to some of the most
exciting scientific developments of the last decade,
including the discovery of geologically abrupt shifts in
climate, as well as "transient" events, brief but extreme
excursions often associated with profound impacts on
global environments and the biosphere (25-28).
Moreover, these high-fidelity deep-sea records have
facilitated efforts to extend the "astronomically
calibrated" geological time scale back into the early
an achievement previously considered difficult, if not
impossible. Carbon isotope data have proved to be equally
invaluable for stratigraphic correlation, and for
providing insight into the operation of the global carbon
In essence, by detailing both the rate and magnitude of
past environmental perturbations, the latest generation
of Cenozoic deep-sea isotope records has opened windows
into a climatically dynamic period in Earth history. This,
in turn, has proven invaluable for developing and testing
new theories on mechanisms of past climate change (32-34),
and for providing the framework to assess the influence of
climate on the environment (35).
The Deep-Sea Stable Isotope RecordAs a framework for this
review, oxygen and carbon isotope data for bottom-dwelling, deep-sea
foraminifera from over 40 DSDP and ODP sites
representing various intervals of the Cenozoic were
culled from the literature and compiled into a single global
deep-sea isotope record (Fig.
2) [Web table 1 (36)].
The numerical ages are relative to the standard geomagnetic
polarity time scale (GPTS) for the Cenozoic [Web note
To facilitate visualization and minimize biases related
to inconsistencies in sampling density in space and time,
the raw data were smoothed and curve-fitted with a locally
weighted mean. The smoothing results in a loss of detail
that is undetectable in the long-time scale perspective.
The oxygen isotope data provide constraints on the
evolution of deep-sea temperature and continental ice
volume [Web note 2 (36)].
Because deep ocean waters are derived primarily from
cooling and sinking of water in polar regions, the
deep-sea temperature data also double as a time-averaged
record of high-latitude sea-surface temperatures (SST). The
deep-sea carbon isotope data, on the other hand, provide
insight into the nature of global carbon cycle
perturbations [Web note 2 (36)]
and on first-order changes in deep-sea circulation
patterns [Web note 3 (36)]
that might trigger or arise from the climatic
Cenozoic Climate: From Greenhouse to IcehouseOur benthic
compilation shows a total 18O range of 5.4 over the course of the Cenozoic (Fig.
2). Roughly ~3.1 of this reflects deep-sea cooling, the
remainder growth of ice-sheets, first on Antarctica
(~1.2), and then in the Northern Hemisphere
(~1.1). We consider the climate evolution depicted by this
record under three categories: (i) long-term
(~106 to 107 years), (ii) short-term or
orbital-scale (~104 to 105 years), and (iii)
aberrations or event-scale (~103 to 104
Long-term trends. The 18O record exhibits a number of steps and
peaks that reflect on episodes of global warming and cooling,
and ice-sheet growth and decay (Fig.
2). The most pronounced warming trend, as expressed
by a 1.5 decrease in 18O, occurred early in the Cenozoic, from
the mid-Paleocene (59 Ma) to early Eocene (52 Ma),
and peaked with the early Eocene Climatic Optimum (EECO;
52 to 50 Ma). The EECO was followed by a
17-My-long trend toward cooler conditions as expressed by a
3.0 rise in 18O with much of the change occurring over
the early-middle (50 to 48 Ma) and late Eocene
(40 to 36 Ma), and the early Oligocene
(35 to 34 Ma). Of this total, the entire increase in
18O prior to the late Eocene (~1.8) can be attributed to a 7.0°C decline in
deep-sea temperature (from ~12° to ~4.5°C). All subsequent
18O change reflects a combined effect of
ice-volume and temperature (40),
particularly for the rapid >1.0 step in 18O at 34 Ma. On the basis of limits
imposed by bottom-water and tropical temperatures, it has
been estimated that roughly half this signal (~0.6) must reflect increased ice volume (24,
though independent constraints on temperature derived
from benthic foraminiferal Mg/Ca ratios argue for a slightly
greater ice-volume component (~0.8 to 1.0) (42).
This long-term pattern of deep-sea warming and cooling is
consistent with reconstructions of early Cenozoic
subpolar climates based on both marine and terrestrial
geochemical and fossil evidence (43-47).
Following the cooling and rapid expansion of Antarctic
continental ice-sheets in the earliest Oligocene, deep-sea 18O values remained relatively high
(>2.5), indicating a permanent ice sheet(s),
likely temperate in character (48),
with a mass as great as 50% of that of the present-day
ice sheet and bottom temperatures of ~4°C (18).
These ice sheets persisted until the latter part of the
Oligocene (26 to 27 Ma), when a warming trend
reduced the extent of Antarctic ice. From this point until
the middle Miocene (~15 Ma), global ice volume remained low
and bottom water temperatures trended slightly higher (49,
with the exception of several brief periods of glaciation
(e.g., Mi-events) (39).
This warm phase peaked in the late middle Miocene
climatic optimum (17 to 15 Ma), and was
followed by a gradual cooling and reestablishment of a major
ice-sheet on Antarctica by 10 Ma (51,
Mean 18O values then continued to rise gently
through the late Miocene until the early Pliocene
(6 Ma), indicating additional cooling and
small-scale ice-sheet expansion on west-Antarctica (53)
and in the Arctic (54).
The early Pliocene is marked by a subtle warming trend
until ~3.2 Ma, when 18O again increased reflecting the onset of
Northern Hemisphere Glaciation (NHG) (56,
Rhythms. Given this framework for long-term trends, how
has the tempo and amplitude of orbital scale climate variability
evolved through the Cenozoic, particularly during the
transitions between different glacial states (unipolar to
bipolar)? To address this, we turn to high-resolution
time-series spanning four intervals: 0.0 to
4.0, 12.5 to 16.5, 20.5 to 24.5, and
31.0 to 35.0 Ma, each representing an interval
of major continental ice-sheet growth or decay. The
time-series are from DSDP and ODP Sites 659 [0 to
4 Ma (58)],
588 [12.5 to 16.5 Ma (59)],
929 [20.5 to 24.5 (60)],
and 522 [31 to 35 Ma (61,
3). Two of the records, Sites 659 and 929, have
orbitally tuned age models. The mean sampling density
varies from roughly 2 ky for the 0- to 4-Ma time
slice to 9 ky for the 31- to 35-Ma time slice,
thereby limiting resolution of the high-frequency
orbital-scale periodicities in the oldest intervals.
Nevertheless, resolution is high enough to avoid signal
aliasing of lower frequency periods.
Fig. 3. (A through
D) High-resolution 4-My-long 18O time series representing four intervals
of the Cenozoic. The data are from Site 659, eastern equatorial
Site 588, southwest Pacific (59);
Site 929, western equatorial Atlantic (60);
Site 522, south Atlantic (61);
and Site 689, Southern Ocean (68).
Sampling intervals range from 3 to 10 ky. Note that the
18O axes on all plots are set to the same
scale (3.0), though at different ranges to accommodate the change
in mean ocean temperature/ice volume with time. The Plio-Pleistocene
ages for Site 659 are constrained by oxygen isotope records
directly or indirectly calibrated to Northern Hemisphere summer
insolation at 65°N, based on the astronomical solutions of Berger
and Loutre (123).
The Site 929 age model is also calibrated to an orbital curve
derived from the formulations of Laskar (2)
with corrections for tidal dissipation (29).
The upper curves in (A) and (C) represent Gaussian band-pass filters
designed to isolate variance associated with the 400- and
100-ky eccentricity cycles. The 400-ky filter has a central
frequency = 0.0025 and a
bandwidth = 0.0002; the 100-ky central
frequency = 0.01 and
bandwidth = 0.002. Filters were not constructed for
the two records, at sites 588 and 522, which have not been
orbitally tuned. [View
Larger Version of this Image (37K GIF file)]
These and other benthic 18O time-series demonstrate that climate
varies in a quasi-periodic fashion during all intervals
characterized by glaciation, regardless of the location
and extent of ice-sheets. In terms of frequency, much of
the power in the climate spectrum since the early
Oligocene appears to be concentrated in the obliquity
band (~40 ky) (Fig.
4). Additional power resides in the eccentricity
bands, although the signal strength is more variable. For
example, 18O variance in the 100-ky frequency band is
exceptionally pronounced over the last 800 to
900 ky following a mid-Pleistocene shift (63),
but weaker through the early Pleistocene and Pliocene
when the signal was dominated by variance in the 41-ky band
Similar secular shifts in the power of the 100-ky cycle
occurred in the late Oligocene and early Miocene. Power
in the 400-ky band is exceptionally pronounced in the early
Miocene, whereas it is relatively weak in the Pleistocene
and early Oligocene (61,
Fig. 4. Spectral density
as a function of frequency for (A) the Plio-Pleistocene
(0 to 4 Ma) and (B) Oligocene-Miocene (20.5 to
24.5), as based on the benthic 18O time series of Sites 659 and
929. The analyses were performed using the Blackman-Tuckey
method (Arand Software). Both records were detrended and resampled
at 1-ky steps. Both records have been tuned to the orbital spectrum
Atlantic [Web note 1 (36)]
Larger Version of this Image (28K GIF file)]
The variations in the amplitude of the Cenozoic deep sea 18O signal largely reflect on changes in
continental ice-volume and temperature. For example, the
largest oscillations are recorded over the last
800 ky during the period of maximum NHG. The most
recent independent constraints on the isotopic composition
of seawater during the last major ice advance
(20 ka) suggest that <1.0 of the total range of ~2.4 for this period may reflect changes in ice
volume, the remainder temperature (69,
Conversely, the lowest amplitude oscillations (~0.2 to
0.3) were in the late Eocene prior to the appearance of
permanent Antarctic ice-sheets. Slightly higher amplitude
oscillations (~0.5) occurred in the early Oligocene, late
and early Pliocene (72),
when Antarctica was close to fully glaciated. Conversely,
larger amplitude (0.5 to 1.0) oscillations are recorded in the latest
Oligocene and early Miocene, the period when Antarctica
was minimally or only partially glaciated.
Aberrations. Perhaps the most
interesting and unexpected discoveries of the last decade are the
aberrations. These are loosely defined as brief
(~103 to 105 y) anomalies that stand out well
above "normal" background variability in terms of rate
and/or amplitude, and are usually accompanied by a major
perturbation in the global carbon cycle as inferred from
carbon isotope data. The three largest occurred at ~55, 34,
and 23 Ma, all near or at epoch boundaries. This last
distinction is significant in that it implies that each
of these climate events may have also had widespread and
long-lasting impacts on the biosphere.
The most prominent of the climatic aberrations is the Late
Paleocene Thermal Maximum (LPTM), which occurred at 55 Ma
near the Paleocene/Eocene (P/E) boundary. This event is
characterized by a 5° to 6°C rise in deep-sea temperature
(>1.0 negative isotope excursion) in less than
10 ky (Fig.
Sea surface temperatures as constrained by planktonic
isotope records also increased, by as much as 8°C at high
latitudes and lesser amounts toward the equator (47,
Recovery was gradual, taking ~200 ky from the onset of
the event (30).
An associated notable change in climate was globally
higher humidity and precipitation, as evidenced by
changes in the character and patterns of continental
The event is also characterized by a ~3.0 negative carbon isotope excursion of the marine,
atmospheric, and terrestrial carbon reservoirs (Fig.
5) (25, 78-80); widespread
dissolution of seafloor carbonate (75,
mass extinction of benthic foraminifera (82);
widespread proliferation of exotic planktic foraminifera
and the dinoflagellate Apectodinium (84);
and the dispersal and subsequent radiation of Northern
Hemisphere land plants and mammals
(78, 85-88). The recovery interval is
marked by a possible rise in marine and terrestrial
productivity and organic carbon deposition (89,
Fig. 5. The LPTM as
recorded in benthic 13C and 18O records (A and B,
respectively) from Sites 527 and 690 in the south Atlantic
and Site 865 in the western Pacific (26).
The time scale is based on the cycle stratigraphy of Site
with the base of the excursion placed at 54.95 Ma. The other
records have been correlated to Site 690 using the carbon
isotope stratigraphy. Apparent leads and lags are artifacts of
differences in sample spacing. The oxygen isotope values have been
adjusted for species-specific vital effects (118),
and the temperature scale on the right is for an ice-free ocean. The
negative carbon isotope excursion is thought to represent the influx
of up to 2600 Gt of methane from dissociation of seafloor
Larger Version of this Image (29K GIF file)]
In contrast, the next two climatic aberrations are characterized
by positive oxygen isotope excursions that reflect brief
extremes in Antarctic ice-volume and temperature (27,
The first of these lies just above the Eocene/Oligocene
boundary (34.0 Ma) (Fig.
3). It is a 400-ky-long glacial that initiated with
the sudden appearance of large continental ice sheets on
Antarctica. This transition, referred to as Oi-1 (50),
appears to involve reorganization of the climate/ocean
system as evidenced by global wide shifts in the
distribution of marine biogenic sediments and an overall
increase in ocean fertility (62,
and by a major drop in the calcium carbonate compensation
The second aberration coincided with the Oligocene/Miocene
boundary (~23 Ma) (95)
and consists of a brief but deep (~200 ky) glacial
This event, referred to as Mi-1 (50),
was followed by a series of intermittent but smaller
glaciations. Both Oi-1 and Mi-1 were accompanied by
accelerated rates of turnover and speciation in certain
groups of biota, although on a smaller scale than at the
Of particular significance are the rise of modern whales
(i.e., baleen) and shift in continental floral communities
at the E/O boundary (97,
and the extinction of Caribbean corals at the O/M
Furthermore, both transients are characterized by small
but sharp positive carbon isotope excursions (~0.8) suggestive of perturbations to the global
carbon cycle (Fig.
2). Although records indicate a number of lesser
events in the Oligocene and Miocene, none appear to
approach Oi-1 and Mi-1 events in terms of magnitude.
Implications for Climate Forcing MechanismsHas the greater
temporal resolution of Cenozoic climate afforded by the latest
isotope reconstructions altered our understanding of the
nature of long- and short-term climate change? The answer
to this is both yes and no. Perhaps the most important
developments concern the glacial history of Antarctica,
and the scale and timing of climatic aberrations. In the
case of the former, it is evident that ice sheets have
been present on Antarctica for the last 40 My, and over
much of that time have been extremely dynamic, implying a
high degree of instability and/or sensitivity to forcing. As
for the aberrations, their mere existence points toward the
potential for highly nonlinear responses in climate to
forcing, or the possibility of unexpected anomalies in
Gateways or pCO2? With the previous
less-detailed perspective of Cenozoic climate--that is, warm and
ice-free in the beginning to cold and glaciated at
present--there was tendency to attribute the
unidirectional trend, Cenozoic cooling, to a single factor
such as the increased thermal isolation of Antarctica due to
the increased widening of the oceanic passages. However,
as the complex nature of the long-term trend comes into
focus, it is becoming clear that more than one factor was
responsible. A case in point is the transition into and
out of the long-term Oligocene glaciation. Thermal
isolation of Antarctica by widening oceanic passages may
explain the initial appearance of Antarctic ice-sheets, but
fails to explain the subsequent termination. New
reconstructions of Cenozoic pCO2 (Fig.
have added another dimension to this argument, indicating
that this termination occurred at a time when greenhouse
gas levels were declining or already relatively low. This
reinforces the notion that moisture supply was the
critical element in maintaining large polar ice-sheets,
at least during the middle Cenozoic (101,
Although globally averaged precipitation should covary
with pCO2, on regional scales other parameters such
as circulation patterns need to be considered as well.
Future efforts to model the onset of Oligocene glaciation
should investigate the role of the hydrological cycle in
maintaining large ice-sheets on an otherwise warmer than
present Antarctic continent. Similarly, with low
pCO2 over the last 25 My, tectonic events such as
mountain building or oceanic gateway reconfigurations,
which can alter ocean/atmosphere circulation and heat and
vapor transport, may have had a dominant role in
triggering large-scale shifts in climate (10,
Conversely, at these low levels, subtle changes in
pCO2, at least within the error of the proxy
estimates, may be important in triggering ice-volume
changes, again not just through influences on radiative
forcing, but also on atmospheric circulation patterns and
humidity. Clearly, in the case of long-term trends, with
so many variables, some still not well constrained (i.e.,
pCO2, approximate timing of tectonic events),
the task of relating response to forcing is still far
Fig. 6. Estimates of
Cenozoic atmospheric pCO2 based on two independent
proxies as measured in sub-tropical deep-sea sediment cores from the
Pacific. The first curve spanning most of the Cenozoic is estimated
from surface ocean pH as derived from the boron isotope ratios of
planktonic foraminifers (7).
The second pCO2 curve spanning the Miocene is based on
the 13C values of phytoplankton organic
compounds known as alkenones (100).
Both approaches assume chemical equilibrium between the ocean and
atmosphere. In the intervals of overlap, both proxies provide nearly
identical estimates of paleo-pCO2. [View
Larger Version of this Image (16K GIF file)]
Orbital pacing. Efforts to relate periodic climate
variability to forcing through the Cenozoic have proven to been far
more successful. For example, it is now evident that the
primary beat of the glaciated Cenozoic is in the
obliquity band, regardless of the state of other boundary
conditions or the location of ice sheets (e.g., Fig.
4). This is true for the lower frequency 1.25-My
period of obliquity as well (104).
This observation confirms the highly sensitive nature of
ice-sheets to obliquity-generated changes in
high-latitude insolation, particularly when the polar
regions (i.e., Antarctica) are only partially ice-covered,
as in the Miocene. Although the benthic isotope records
currently available for the ice-free Cenozoic lack
adequate resolution to fully characterize obliquity
variance, other proxy records (i.e., physical properties)
suggest that the global climatic response was dominated
by variance in the precession-related bands (30,
This supports the notion that the overall influence of
obliquity on global climate during ice-free periods, without
an ice-sheet amplifier, is weaker or less apparent.
A more definitive finding, however, is the verification of a
strong pre-Pleistocene climate response to eccentricity
oscillations, as exemplified by the concentration of
power at the 100- and 400-ky periods (27,
Analyses of the climate signal over those intervals where
it is pronounced (i.e., the Miocene) reveal a high degree
of coherency with eccentricity in terms of frequency and
amplitude modulation. This finding supports the class of
models that relate amplification of power in the
eccentricity bands to the filtering effects of continental
geography and differences in land-sea heating on
precession, especially in the tropics (16).
Here, power (temperature) can be shifted into the primary
eccentricity bands via truncation of the cooler portion
of precession-related insolation change. What remains
unclear is how these effects are then exported to higher
latitudes. Researchers have considered a variety of
mechanisms for directly and indirectly amplifying the
response to precession forcing (106-109).
This includes processes such as ocean and atmospheric
circulation that directly or indirectly influence
heat-transport, precipitation, and/or the global carbon
cycle and pCO2. Of these, the carbon cycle is
most appealing because of its long time constants, but is
difficult to verify because of the large number of
variables involved. Still, support for a carbon cycle
amplifier is provided by Oligocene-Miocene carbon isotope
records, which exhibit pervasive large-amplitude
100- and 400-ky oscillations that are highly coherent
with the glacial cycles (60).
Furthermore, reanalysis of ice-core data and other
records indicate that the primary response to eccentricity
in the late Pleistocene benthic 18O record is in temperature, not ice volume
as originally believed, and that ice volume lagged
eccentricity forcing, CO2, and deep-sea
temperature by the appropriate phase (70).
Thresholds, methane eruptions, and orbital anomalies.
Characterizing the timing and scale of the three aberrations
discussed here in the context of longer-term background
paleoenvironmental variability has been critical to the
development and testing of hypotheses on their origins.
To begin with, each aberration was superimposed on a
long-term gradual trend in the same direction. In terms
of tempo, the step into the LPTM was much more abrupt
(~103 to 104 years) than that into the
Oi-1 and Mi-1 events (~105 years), and the recovery more
gradual. This, and the fact that the direction of
climatic change is opposite (e.g., a warming instead of
cooling), hints at a different mechanism. For the LPTM,
the abrupt negative ~3.0 global carbon isotope excursion (CIE) (Fig.
5) implicates a rise in greenhouse gas concentrations,
most likely from the dissociation and subsequent oxidation
of 2000 to 2600 Gt of isotopically light
(~-60) methane from marine clathrates as proposed
by Dickens et al. (33,
The carbon mass from this is consistent with a reduction in
ocean pH as inferred from evidence for seafloor carbonate
Although other sources of CO2 have been
considered (i.e., volcanic), the much greater masses
required to generate the CIE would alter ocean chemistry to
an extent unsupported by data. Why would such a large
mass of methane hydrate suddenly dissociate at
55 Ma? Suggested triggering mechanisms range from
the gradual crossing of a thermal threshold via long-term
deep-sea warming (110),
to more abrupt deep-sea warming resulting from sudden
changes in ocean circulation (113,
to massive regional slope failure (115).
In contrast to the LPTM, the much smaller magnitude and positive
carbon isotope excursions for the Oi-1 and Mi-1 events indicate
that greenhouse gas forcing was probably not the primary
causal mechanism, but instead may have served as a
positive or amplifying feedback. In this scenario,
tectonic forcing is viewed at the primary triggering
mechanism that drives the climate system across a
physical threshold (i.e., temperature), which then initiates
rapid growth of ice-sheets along with reorganization of
ocean/atmosphere circulation (17).
The physical changes, in turn, trigger large-scale
biogeochemical feedbacks in the carbon cycle that
initially amplify the climatic changes (18).
Such feedbacks would be short-lived, because other
coupled biogeochemical processes would eventually restore
equilibrium to the system.
Orbital forcing may have had a hand in triggering these events as
well, possibly as a means of providing the climate system
with a final and relatively rapid push across a climatic
threshold, or even as the principal driving force. For
example, the Mi-1 event appears to be in phase with a
long series of regular low-frequency oscillations (i.e.,
~400 ky cycles) (27).
Comparison of the Site 929 isotope records with the
orbital curve (2),
revealed that the low-frequency (400 ky) glacial maxima,
including Mi-1, coincided with, and hence, were being
paced by eccentricity minima (60).
A normal cycle of low eccentricity, however, fails to
explain the unusual amplitude of Mi-1. What is unusual is
the congruence this low in eccentricity with a protracted node
in obliquity (34).
This rare orbital alignment involved four consecutive
cycles of low-amplitude variance in obliquity (a node)
coincident with the low eccentricity that resulted in an
extended period (~200 ky) of cool summer orbits, and possibly,
ice-sheet expansion on Antarctica.
In sum, it now appears that extreme aberrations in global climate
can arise through a number of mechanisms. This would explain
both the random distribution and frequency of such events
over time. Some, such as rare anomalies in Earth's orbit,
are predictable, at least to the extent that the orbital
computations are correct. Others, like catastrophic
methane release, are less so, although closer scrutiny of
earlier time intervals when boundary conditions were
similar to those preceding the LPTM might reveal the existence
of similar aberrations (116).
Correlation does not necessarily prove causation, but in
the case of aberrations, the short time scales involved
significantly reduces the number of potential variables,
thereby rendering the task of identifying and testing
mechanisms a more tractable proposition. Moreover, the
abrupt transitions and transients offer a unique opportunity
to study the dynamics of a rapidly changing climate system,
as well as the response of the biosphere and
biogeochemical cycles on global or regional scales to
significant, sudden changes in greenhouse gas levels. To
this end, future efforts should concentrate on
establishing, in greater temporal detail, the global- and
regional-scale changes associated with these short-lived
events, particularly in climatically and/or
environmentally sensitive regions, both marine and
terrestrial (i.e., high latitudes, tropics, marginal
seas, and continental interiors).
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