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REVIEW
Trends, Rhythms, and Aberrations in
Global Climate 65 Ma to Present
James Zachos,
1
* Mark Pagani,
1
Lisa Sloan,
1
Ellen Thomas,
2,3
Katharina Billups
4
Since 65 million years ago (Ma), Earth’s climate has undergone a signifi-
cant 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 pro-
cesses on time scales of 10
5
to 10
7
years, rhythmic or periodic cycles
driven by orbital processes with 10
4
-to10
6
-year cyclicity, and rare rapid
aberrant shifts and extreme climate transients with durations of 10
3
to
10
5
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.
Through study of sedimentary archives, it
has become increasingly apparent that dur-
ing much of the last 65 million years and
beyond, Earth’s climate system has experi-
enced continuous change, drifting from ex-
tremes 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 tecton-
ics, are also in perpetual motion. Much of
the higher frequency change in climate (10
4
to 10
5
years) is generated by periodic and
quasi-periodic oscillations in Earth’s orbit-
al parameters of eccentricity, obliquity, and
precession that affect the distribution and
amount of incident solar energy (Fig. 1)
(1). 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. Be-
cause the orbital parameters vary with dis-
tinct tempos that remain stable for tens of
millions of years (2), they provide a steady
and, hence, predictable pacing of climate.
The orbitally related rhythms, in turn,
oscillate about a climatic mean that is con-
stantly drifting in response to gradual
changes in Earth’s major boundary condi-
tions. 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, unidirec-
tionally, 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 sub-
sequent uplift of the Himalayas and Tibetan
Plateau (5); uplift of Panama and closure of
the Central American Seaway (6 ) (Figs. 1
and 2); and a sharp decline in pCO
2
(7 ).
Each of these tectonically driven events
triggered a major shift in the dynamics of
the global climate system (8–15). More-
over, in altering the primary boundary con-
ditions and/or mean climate state, some or
all of these events have altered system sen-
sitivity to orbital forcing (16), thereby in-
creasing the potential complexity and di-
versity of the climate spectrum. This would
include the potential for unusually rapid or
extreme changes in climate (17, 18).
Although Earth’s climatic history has
been reconstructed with an array of proxies
applied to both marine and terrestrial sedi-
ment 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 (d
18
O) and carbon (d
13
C) isotope
records (19). Since the early 1970s, d
18
O
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 automat-
ed mass spectrometers. The first marine
isotope records were relatively coarse, but
still provided valuable insight into the gen-
eral structure of the Pleistocene glacial and
interglacial cycles (20). These were fol-
lowed by records delineating the long-term
patterns of Cenozoic climate change (21–
23) and, eventually, the first global compi-
lation of records for the Cenozoic (resolu-
tion of 10
5
to 10
6
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 sedi-
ment cores recovered by the Deep Sea Dril-
ling Project (DSDP) and Ocean Drilling Pro-
gram (ODP). The improved perspective pro-
vided by these records has led to some of the
most exciting scientific developments of the
1
Earth Sciences Department, University of California,
Santa Cruz, CA 95064, USA.
2
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, USA.
4
College of Marine Studies, Uni-
versity of Delaware, Lewes, DE 19958, USA.
*To whom correspondence should be addressed. E-
mail: jzachos@es.ucsc.edu
27 APRIL 2001 VOL 292 SCIENCE www.sciencemag.org686
P ALEOCLIMATE

last decade, including the discovery of geo-
logically abrupt shifts in climate, as well as
“transient” events, brief but extreme excur-
sions 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 Cenozoic (29,
30), 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
cycle (31). In essence, by detailing both the
rate and magnitude of past environmental
perturbations, the latest generation of Ceno-
zoic 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 frame-
work to assess the influence of climate on the
environment (35).
The Deep-Sea Stable Isotope Record
As 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 inter-
vals 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 1
(36)] (37). 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 smooth-
ing results in a loss of detail that is unde-
tectable in the long-time scale perspective.
The oxygen isotope data provide con-
straints on the evolution of deep-sea tem-
perature 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-sur-
face temperatures (SST). The deep-sea car-
bon isotope data, on the other hand, provide
insight into the nature of global carbon
cycle perturbations [Web note 2 (36)] (38),
and on first-order changes in deep-sea cir-
culation patterns [Web note 3 (36)] (39)
that might trigger or arise from the climatic
changes.
Cenozoic Climate: From Greenhouse
to Icehouse
Our benthic compilation shows a total d
18
O
range of 5.4‰ over the course of the Ceno-
zoic (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 (;10
6
to 107 years), (ii) short-term or
orbital-scale (;10
4
to 10
5
years), and (iii)
aberrations or event-scale (;10
3
to 10
4
years).
Long-term trends. The d
18
O record exhib-
its 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 d
18
O, 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
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 recon-
structed for five intervals of the last 70 My (designed using the commercial Paleogeographic
Information System).
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