cenozoic antarctic cryosphere evolution tales from deep sea sedimentary records 2007 deep sea...
TRANSCRIPT
-
8/16/2019 Cenozoic Antarctic Cryosphere Evolution Tales From Deep Sea Sedimentary Records 2007 Deep Sea Research Par…
1/17
Deep-Sea Research II 54 (2007) 2308–2324
Cenozoic Antarctic cryosphere evolution: Tales from deep-sea
sedimentary records
Amelia E. Shevenella,, James P. Kennettb
aSchool of Oceanography, University of Washington, Seattle, WA 98195, USAbDepartment of Earth Science, University of California Santa Barbara, Santa Barbara, CA 93106, USA
Accepted 24 July 2007
Available online 29 October 2007
Abstract
Antarctica and the Southern Ocean system evolved in the Cenozoic, but the details of this complex evolution are just
beginning to emerge via high-resolution investigations of globally distributed marine sedimentary sequences. Here we
review the recent progress in defining the orbital-scale evolution of the Antarctic/Southern Ocean system, with particular
attention paid to new high-resolution multi-proxy records generated across intervals of abrupt Antarctic ice growth in the
Paleogene and early Neogene. This more detailed perspective has allowed researchers to assess the processes and feedbacks
involved in the Cenozoic evolution of the Antarctic cryosphere, absent potential complication of the paleoceanographic
record by a substantial Northern Hemisphere ice volume signal. In this paper, we review the new tools being used to
examine these high-resolution records, assess lead–lag relationships between ice volume, temperature, and carbon cycling
during intervals of abrupt Antarctic ice growth, and consider the resulting implications for the global climate system.
r 2007 Elsevier Ltd. All rights reserved.
Keywords: Antarctica; Cenozoic; Paleoceanography
1. Introduction
Antarctica and the surrounding Southern Ocean
are presently integral components of Earth’s climate
system, exerting influence on the global climate:
Antarctic ice sheets regulate global sea level, large-
scale physical processes occurring in the SouthernOcean catalyze global thermohaline circulation and
carbon cycle dynamics, and as the main global heat
sink, Antarctica drives the southward flux of heat
and hence atmospheric circulation. The Cenozoic
evolution of these conditions that define the
contemporary Antarctic cryosphere was one of the
fundamental reorganizations of the global climate
system. Over the past several decades, substantial
research efforts have been undertaken to improve
our understanding of the Cenozoic evolution of the
Antarctic/Southern Ocean system and the processes
and feedbacks involved in this evolution.The Antarctic continent has been situated in
approximately its current location over the South
Pole since the mid-Cretaceous (Lawver et al., 1992),
but remained predominantly ice-free until the
middle to late Eocene (35 Ma; see Zachos et al.,
2001a for a review). Thus, Antarctica’s geographic
location has never been a sufficient explanation for
widely recognized Cenozoic global cooling and ice
growth trends (Fig. 1). Considerable progress has
ARTICLE IN PRESS
www.elsevier.com/locate/dsr2
0967-0645/$- see front matterr 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dsr2.2007.07.018
Corresponding author.
E-mail address: [email protected]
(A.E. Shevenell).
http://www.elsevier.com/locate/dsr2http://localhost/var/www/apps/conversion/tmp/scratch_4/dx.doi.org/10.1016/j.dsr2.2007.07.018mailto:[email protected]:[email protected]:[email protected]:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_4/dx.doi.org/10.1016/j.dsr2.2007.07.018http://www.elsevier.com/locate/dsr2
-
8/16/2019 Cenozoic Antarctic Cryosphere Evolution Tales From Deep Sea Sedimentary Records 2007 Deep Sea Research Par…
2/17
been made toward developing a detailed record of
Cenozoic Antarctic ice-sheet evolution, much of
which is owed to technological advances, including
the improved quality of acquisition and recovery of
marine sedimentary sequences and the development
of powerful geochemical techniques and paleocli-mate proxies (e.g., Mg/Ca, alkenone, and TEX86paleothermometry).
Over the past several decades, two general groups
of hypotheses have emerged to explain the Cenozoic
evolution of global temperatures and ice volume: (1)
those positing changes in global carbon cycling and
atmospheric pCO2 (Raymo, 1994; Vincent and
Berger, 1985), and (2) those requiring a redistribu-
tion of heat over the Earth’s surface (Kennett, 1975;
Schnitker, 1980; Woodruff and Savin, 1989; De-
Conto and Pollard, 2003). Ultimately, long-term
Cenozoic cooling is likely a function of Earth’sboundary conditions (e.g., plate tectonics and/or
atmospheric greenhouse gas concentrations), while
short-term climate oscillations appear to be paced
by the sensitivity of the Earth’s climate system to
orbital forcing (Zachos et al., 2001a; Pa ¨ like et al.,
2006).
This review provides a summary of the current
understanding of both the long- and short-term
Cenozoic evolution of the Antarctic/Southern
Ocean system, as gleaned from geochemical records
of deep-sea sediments. Emphasis is placed on the
late Paleogene and early Neogene (late Eocene to
late Miocene) record, during which period geo-
chemical signals are not substantially complicated
by Northern Hemisphere ice volume (Zachos et al.,2001a). The processes and feedbacks involved in
early Antarctic ice-sheet development and later ice-
sheet expansions are investigated, with attention to
the role of atmospheric pCO2 in Antarctic glacial
advances.
1.1. New paleoclimate proxies
1.1.1. Foraminifer Mg/Ca
The benthic foraminifer d18O proxy, though it
reliably documents shifts in climate, is complicated
because it includes the influence of both global icevolume and deep-ocean temperatures. These two
variables may have distinct and autonomous effects
on climate; extracting ice volume from the aggregate
d18O record provides a clearer picture of continental
ice-volume variability on both long and short
timescales and also has the potential to constrain
the phasing of ice growth and temperature change.
Recent research efforts have focused on developing
a temperature proxy, independent of salinity that
ARTICLE IN PRESS
Fig. 1. Composite benthic foraminifer d18O and d13C records compiled by Zachos et al. (2001). The three abrupt expansions of the
Antarctic cryosphere discussed in the text are indicated by arrows (modified from Zachos et al., 2001).
A.E. Shevenell, J.P. Kennett / Deep-Sea Research II 54 (2007) 2308–2324 2309
-
8/16/2019 Cenozoic Antarctic Cryosphere Evolution Tales From Deep Sea Sedimentary Records 2007 Deep Sea Research Par…
3/17
may be paired with benthic foraminifer d18O records
to isolate the global ice-volume signal. Of the
recently developed paleotemperature proxies, in-
cluding alkenone unsaturation, TEX86, and benthic
foraminifer Mg/Ca, the benthic foraminifer Mg/Ca
proxy is the most appealing because it can bemeasured on the same foraminiferal calcite as d18O.
Mg/Ca paleothermometry relies on laboratory
observations, indicating that the partition coeffi-
cient of Mg2þ into inorganic calcite is temperature
dependent. Culture and in situ studies of marine
biogenic calcites reveal a similar temperature
dependency, albeit with species-specific vital effects
(Hastings et al., 1998; Rosenthal et al., 1997;
Mashiotta et al., 1999 and others). Thus, the most
reliable foraminifer Mg/Ca records derive from
species for which an empirical Mg/Ca-temperature
calibration exists (Martin et al., 2002; Lear et al.,2000, 2002; Marchitto et al., 2007). However, these
calibrations are limited by our present inability to
successfully culture benthic foraminifers and the
lack of a robust temperature calibration at lower
temperatures ðo5 CÞ. Many researchers are pre-
sently working to improve existing calibrations and
culture benthic foraminifers in a variety of condi-
tions.
To ensure accurate paleotemperature estimates,
the Mg/Ca signal preserved in the foraminifers must
be primary and not altered. As with all paleoclimateproxies, the benthic foraminifer Mg/Ca proxy is not
foolproof. Caveats include the influence of diagen-
esis/dissolution and carbonate saturation (Lear
et al., 2000; Lea et al., 2000). Perhaps the most
difficult issue to overcome when applying the
Mg/Ca paleotemperature proxy on longer Cenozoic
timescales relates to temporal variations in seawater
Mg/Ca (Lear et al., 2000; Billups and Schrag,
2002; Shevenell et al., 2004). Several geochemical
models of seawater predict significant background
variability in Mg/Ca during the Cenozoic (Wilk-
inson and Algeo, 1989; Stanley and Hardie, 1998;
Berner et al., 1983). However, it is important to note
that because the residence times of Mg and Ca in
the ocean are relatively long (13 and 1 My,
respectively), downcore variations in foraminifer
Mg/Ca that occur in o1 My should not be impacted
by a dynamic seawater Mg/Ca ratio (Lear et al.,
2000; Shevenell et al., 2004). To alleviate this
uncertainty, Lear et al. (2000) proposed that
existing foraminifer calibration equations may be
modified to reflect past seawater Mg/Ca (as
estimated by existing models) using the equation
of Lear et al. (2002):
BWT
¼ lnðMg/CaM=ð0:9nðMg/CaSWM=Mg/CaSWPÞÞÞ=0:11,
ð1Þ
where Mg/CaM refers to measured Mg/Ca,Mg/CaSWM to the modeled Mg/Ca of seawater,
and Mg/CaSWP to the Mg/Ca of present day
seawater. Researchers are actively attempting to
reconstruct the evolution of seawater Mg/Ca. Thus,
in the future, we may be able to reconstruct more
accurately the absolute temperatures of seawater
through the Cenozoic. It should be emphasized that
this weakness is essentially limited to the absolute
temperatures. Relative changes in temperatures are
robust and instructive. While there are many
caveats to the Mg/Ca paleotemperature proxy, itis important to recognize that this proxy is the best
available at present and significant advances in our
understanding of the global climate system have
been made using this approach (Lear et al., 2000;
Lea et al., 2000; Billups and Schrag, 2002; Shevenell
et al., 2004 and others).
1.1.2. pCO2 proxies
Several proxies, including boron isotopes ðd11BÞ
and the d13C of alkenones, have been developed so
that researchers may obtain an accurate estimate of atmospheric pCO2 through the Cenozoic. Although
these proxies must be interpreted with caution, the
new techniques have provided researchers with a
means by which to assess the influence of atmo-
spheric pCO2 changes on the evolution of the
Antarctic cryosphere and Earth’s climate system
(Pagani et al., 1999, 2005; Pearson and Palmer,
2000) The d11B proxy is based on the premise that
the pH of the surface ocean will change as atmo-
spheric pCO2 changes (Pearson and Palmer, 2000).
The d11B value of foraminiferal calcite is correlated
with surface ocean pH (Sanyal et al., 1995, 1996);
thus, the d11B of fossil foraminifers should indicate
changes in surface seawater pH through time. These
pH estimates can be used to calculate the aqueous
CO2 concentration and then estimate the atmo-
spheric pCO2 (see Pearson and Palmer, 2000, for
details of the method). The proxy may be compli-
cated by the fact that the pH and aqueous CO2concentrations of the surface ocean vary spatially as
a result of regional productivity, freshwater influx,
and upwelling (Sanyal et al., 1995; Pearson and
Palmer, 2000). Therefore, the best estimates of
ARTICLE IN PRESS
A.E. Shevenell, J.P. Kennett / Deep-Sea Research II 54 (2007) 2308–23242310
-
8/16/2019 Cenozoic Antarctic Cryosphere Evolution Tales From Deep Sea Sedimentary Records 2007 Deep Sea Research Par…
4/17
atmospheric pCO2 derived from foraminiferal d11B
come from sites in the low-latitude gyres (Pearson
and Palmer, 2000).
The d13C of sedimentary alkenones is another
proxy used to estimate past changes in atmospheric
pCO2 (Pagani et al., 1999, 2005). The proxy is basedon the observation that changes in atmospheric
pCO2 alter the relationship between d13CDIC and the
d13C of phytoplankton ðd13CorgÞ (see Pagani et al.,
1999, for a detailed discussion of the method). This
proxy is also not without complication, as there is
some indication that changes in cellular growth rate
and geometry may influence the intercellular CO2available for carbon fixation (see Pagani et al., 1999,
for a review). However, by analyzing the d13Corg of
one particular biomarker (in this case alkenones
from haptophyte algae), it is possible to limit
complications arising from different classes of organisms with different cell geometries (Pagani et
al., 1999, 2005). The alkenone d13C is then
converted to paleo [CO2ðaqÞ] and then to pCO2 via
Henry’s Law (see Pagani et al., 1999). Alkenones are
thought to be relatively stable within the sediments
and thus may be used to generate pCO2 records on
long geologic timescales.
1.2. Long-term Cenozoic climate trends
1.2.1. The benthic foraminifer d18O record
Geochemical records from marine sediments
reveal both the long- and short-term evolution of
high-latitude temperatures and global ice volume
during the Cenozoic and provide the framework for
our understanding of Antarctic cryospheric evolu-
tion. Traditionally, the oxygen isotopic ðd18OÞ
composition of benthic foraminifer calcite
ðCaCO3Þ has been used to reconstruct past changes
in ice volume and temperature (Shackleton and
Kennett, 1975; Miller et al., 1987; Zachos et al.,
2001a). The most recent global compilation of Cenozoic benthic foraminifer d18O records from
marine sediments (Zachos et al., 2001a) exhibits a
long-term 4% increase in d18O between 50 and 0 Ma
that reflects both high-latitude cooling and ice
growth (Fig. 1). Significantly, d18O values do not
increase steadily over the 50 Ma interval; abrupt
step-like increases ð1%Þ occur at the Eocene/
Oligocene boundary ð35MaÞ, in the middle
Miocene ð14MaÞ, and in the middle Pliocene
ð3 MaÞ (Fig. 1; Zachos et al., 2001a). These abrupt
d18
O increases are typically inferred to reflect rapidaccumulations of ice on Antarctica in the Eocene/
Oligocene (Zachos et al., 1993; Exon et al., 2001)
and middle Miocene (Shackleton and Kennett,
1975; Flower and Kennett, 1994; Shevenell and
Kennett, 2004) and the onset of Northern Hemi-
sphere glaciation in the Pliocene (Zachos et al.,
2001a).
Early lower-resolution d18O records revealed a
unidirectional d18O increase, suggesting the possi-
bility that one mechanism may account for Cen-
ozoic cooling and ice growth (e.g., opening and or
closing of tectonic gateways) and that positivefeedbacks reinforced the initial forcing (Shackleton
and Kennett, 1975; Miller et al., 1987). However,
the most recent global Cenozoic d18O compilation
does not exhibit a unidirectional increase since the
late Eocene (Fig. 1; Zachos et al., 2001a). Global
benthic foraminifer d18O values decrease abruptly
ð0:521%Þ at the end of the Oligocene (26–27 Ma)and remain relatively low until the middle Miocene
ð14MaÞ, suggesting an interval of reduced Ant-
arctic ice volume and/or warmer temperatures
following the rapid warming and/or expansion of Antarctic ice at 3 4 M a (Zachos et al., 2001a).
There is some debate as to the global relevance of
this warming, as composite records may be biased
toward particular regions or ocean basins (Lear
et al., 2004). Nonetheless, researchers have begun to
focus on identifying alternate and multiple mechan-
isms and/or feedbacks to explain the abrupt climate
events of the Cenozoic.
1.2.2. The benthic foraminifer Mg/Ca record
In 2000, Lear et al. published the first low-resolution Cenozoic Mg/Ca paleotemperature re-
cord generated using the benthic species Cibicidoides
floridanus and the Mg–temperature relationship of
Hastings et al. (1998) (Fig. 2A). This record reveals
a 12 C cooling of deep-ocean temperatures
ARTICLE IN PRESS
Fig. 2. (A) Composite benthic foraminifer d18O record complied by Miller et al. (1987) (center). A benthic foraminifer Mg/Ca temperature
record generated by Lear et al. (2000) indicating a 12 C cooling over the Cenozoic (left). Estimates of Cenozoic seawater d18O change, a
proxy for global ice volume, extracted from the d18O and Mg-temperature records using the d18O-based paleotemperature equation of
Shackleton (1974) (right) (modified from Lear et al., 2000). (B) A more detailed benthic foraminifer d18O and seawater d18O record
(calculated from Mg/Ca paleotemperatures) from Southern Ocean Site 747 (Kerguelen Plateau) spanning the late Oligocene to Present
with the Haq et al. (1987) sea-level curve plotted for reference (modified from Billups and Schrag, 2002).
A.E. Shevenell, J.P. Kennett / Deep-Sea Research II 54 (2007) 2308–2324 2311
-
8/16/2019 Cenozoic Antarctic Cryosphere Evolution Tales From Deep Sea Sedimentary Records 2007 Deep Sea Research Par…
5/17
ARTICLE IN PRESS
A.E. Shevenell, J.P. Kennett / Deep-Sea Research II 54 (2007) 2308–23242312
-
8/16/2019 Cenozoic Antarctic Cryosphere Evolution Tales From Deep Sea Sedimentary Records 2007 Deep Sea Research Par…
6/17
between 50 and 0 Ma that followed a similar pattern
to the global Cenozoic d18O curve (Fig. 2A; Lear
et al., 2000). However, abrupt cooling steps similar
to the abrupt d18O steps are not recognized in the
record, suggesting that the d18O steps predomi-
nantly reflect changes in global ice volume(Fig. 2A).
This low-resolution Mg/Ca paleotemperature
record was paired with the benthic d18O a n d a
record of seawater d18O (d18Osw, a proxy for ice
volume) was extracted using the d18O paleotem-
perature equation of Shackleton (1974). The results
revealed major increases in global ice volume
associated with each of the previously recognized
abrupt d18O increases at the Eocene/Oligocene
boundary and in the middle Miocene and confirmed
that the first significant glaciation of Antarctica
occurred at the Eocene/Oligocene boundary(Fig. 2A). The record also identified decreases in
Antarctic ice volume during the Oligocene and early
to middle Miocene. Billups and Schrag (2002)
produced a higher-resolution benthic Mg/Ca record
between 27 and 0 Ma that was consistent with Lear
et al.’s (2000) findings regarding the middle Miocene
d18O increase (Fig. 2B).
A difficulty with the paired d18O and Mg/Ca
approach is that the isotopic composition of past
Antarctic ice sheets remains unknown. Juxtaposing
calculated d18
Osw records with independent recordsof sea-level change of similar resolution may
buttress Mg/Ca-derived d18Osw records and illumi-
nate the past d18O composition of Antarctic ice
sheets (Billups and Schrag, 2002; Lear et al., 2004;
Fig. 2B). Efforts are underway to increase the
resolution of the global sea-level curve, particularly
in the middle Miocene (K. Miller, pers. comm.).
1.2.3. The Cenozoic pCO2 record
Recently, million-year to orbital scale records of
Cenozoic atmospheric pCO2
have been generated
using the d11B and alkenone d13C techniques. In
general, these records (Fig. 3) exhibit elevated
atmospheric pCO2 levels in the Paleocene and
Eocene (d11B: 1000–4000 ppmv; Pearson and Pal-
mer, 2000; alkenone d13C: 500–2000 ppmv) and
atmospheric pCO2 levels approaching present day
values after the Oligocene (150–300 ppmv for both
d11B and alkenone d13C, Pearson and Palmer, 2000;
Pagani et al., 1999, 2005). Atmospheric pCO2estimates from alkenone d13C reveal a trend toward
lower concentrations during the Eocene (as does the
d11B record) and an abrupt decline to levels
conducive to ice-sheet growth at the Eocene/
Oligocene boundary (Fig. 3A; Pagani et al., 2005).
This relationship suggests linkages between the
global carbon cycle and climate existed at least
through the Oligocene. Both the d11B and alkenone
d13
C records indicate a shift to modern pCO2 levelsoccurred in the Neogene, just following the Oligo-
cene/Miocene boundary (Fig. 3A). High-resolution
pCO2 proxy records from the early to middle
Miocene suggest orbitally forced (400 kyr) changes
in pCO2 that coincide with episodes of Antarctic
glaciation inferred from the Cenozoic d18O record
(Miller et al., 1991; Zachos et al., 2001a; Figs. 1 and
4). However, these records are presently of too low a
resolution to determine the phasing of pCO2changes and ice growth (Pagani et al., 1999; Pearson
and Palmer, 2000).
2. Major ice expansion events in the Cenozoic
evolution of the Antarctic cryosphere
The pairing of benthic foraminifer d18O and
foraminiferal trace-metal records has revolutionized
understanding of the long-term evolution of the
Antarctic cryosphere, particularly in the Paleogene
and early Neogene. Armed with this knowledge,
recent research efforts have focused on reconstruct-
ing high-resolution paleoclimate records across the
intervals of rapid cryosphere expansion to deter-mine both the structure of the transitions and the
lead–lag relationships between orbital forcing,
temperature, ice volume, and carbon cycling.
Further high-resolution work is required at globally
distributed sites
2.1. The Oi-1 glaciation (33.7 Ma)
After much debate, the paleoclimate community
has reached a consensus that the prominent abrupt
1% d18O increase at the Eocene/Oligocene (Oi-1,
Miller et al., 1991; 33.7 Ma) boundary reflects the
initiation of substantial and permanent ice sheets on
East Antarctica. Lear et al.’s (2000) pioneering work
with benthic foraminifer Mg/Ca across Oi-1 sug-
gests that the majority of the d18O signal relates to
Antarctic ice growth occurred with little or no
change in deep-ocean temperatures, and by exten-
sion, polar surface temperatures (Fig. 5A). In
addition to the geochemical evidence, direct evi-
dence of Antarctic ice growth comes from Southern
Ocean marine sediments, which reveal a shift in
the clay mineral assemblage toward assemblages
ARTICLE IN PRESS
A.E. Shevenell, J.P. Kennett / Deep-Sea Research II 54 (2007) 2308–2324 2313
-
8/16/2019 Cenozoic Antarctic Cryosphere Evolution Tales From Deep Sea Sedimentary Records 2007 Deep Sea Research Par…
7/17
associated with more physical weathering of the
Antarctic continent (Robert and Kennett, 1997) and
an increase in ice-rafted debris (Zachos et al., 1993)
coincident with the global d18O increase, as well as
an estimated 80 15m global sea-level drop
(Kominz and Pekar, 2001; Miller et al., 2005).
Thus, a preponderance of the geologic evidence
supports rapid Antarctic ice growth during Oi-1.
Recent research efforts have focused on develop-
ing globally distributed high-resolution geochemical
records and climate models necessary to identify the
elusive triggers and feedbacks involved in the Oi-1
climate reorganization. Presently, the highest-reso-
lution sedimentary record spanning Oi-1 comes
from the equatorial Pacific (ODP Leg 199, Site
1218; 3800 m paleodepth) (Lyle et al., 2002; Lear
et al., 2004; Coxall et al., 2005). Such records are
providing researchers with the ability to assess leads
and lags between temperature, ice volume, and
carbon cycle proxies at orbital time scales. The
benthic foraminifer d18O record from Site 1218
reveals that the Oi-1 glaciation occurred in two
distinct 40-kyr-long steps separated by 200 kyr at
a node of low eccentricity and obliquity (Coxall
et al., 2005). The 1:5% increase in d18O is coinci-
dent with a 1-km increase in the Pacific calcite
ARTICLE IN PRESS
Fig. 3. (A) Cenozoic pCO2 estimates derived from the carbon isotopic composition of sedimentary alkenones ðd13C37:2Þ (see Pagani et al.,
2005, for method details). The shaded region denotes the error estimates. Antarctic cryosphere expansions discussed in the text are
indicated by arrows (modified from Pagani et al., 2005). (B.I) Cenozoic pCO2 estimates derived from boron isotopes ðd11BÞ (see Pearson
and Palmer, 2000 for method details). (B.II) Close up of atmospheric pCO2 derived from d11B across the middle Miocene d 18O increase
(modified from Pearson and Palmer, 2000).
A.E. Shevenell, J.P. Kennett / Deep-Sea Research II 54 (2007) 2308–23242314
-
8/16/2019 Cenozoic Antarctic Cryosphere Evolution Tales From Deep Sea Sedimentary Records 2007 Deep Sea Research Par…
8/17
compensation depth (CCD) inferred from the
CaCO3 mass accumulation rate (MAR) (Rea and
Lyle, 2005; Coxall et al., 2005).A paired benthic foraminifer d18O and Mg/Ca
record from Site 1218 (Lear et al., 2004) exhibits
similarities to initial Mg/Ca studies at DSDP Site
522 (Fig. 5B; Lear et al., 2000), which suggests no
permanent cooling of global deep waters across
Oi-1. At Site 1218, average deep-water temperatures
across Oi-1 are 3:7 1:5 C, with a 2 C coolingacross the first d18O step and a 2C warming
associated with the second phase of ice growth
(Lear et al., 2004). Seawater Mg/Ca changes would
affect the absolute value of the Mg/Ca by þ2 C
(Wilkinson and Algeo, 1989) but not any Mg/Ca
changes that occur in o1 M y (Lear et al., 2004;
Fig. 5B). Lear et al. (2004) cautiously suggest that
this pattern of temperature change may be asso-
ciated with the cause of glaciation as well as the
response of the global carbon cycle (e.g., a reduction
in silicate weathering on Antarctica) to glaciation,
respectively. Alternatively, Lear et al. (2004) suggest
that the warming observed in the benthic Mg/Ca
record at Site 1218 may reflect a carbonate
saturation effect on Mg partitioning in CaCO3
related to the deepening of the CCD in the Pacific
and a global sea-level fall. Further evidence to
suggest that the Mg/Ca temperature response across
Oi-1 may be damped by the carbonate saturationeffect comes from the Li/Ca record at Site 1218
(Lear and Rosenthal, 2006).
A record of seawater d18O (d18OswÞ was generated
using the equation of Bemis et al. (1998)) and
documents a 1:5% d18Osw increase across Oi-1 (Learet al., 2004). This d18Osw increase suggests an
apparent sea-level lowering of 90 m across Oi-1 that
is comparable with backstripping estimates from the
New Jersey Margin (Kominz and Pekar, 2001).
Following Oi-1, sea level rises between 33 and
31 Ma, suggesting a 50-m decline in Antarctic ice
volume. However, this decline might be too large if
Mg/Ca is influenced by a carbonate saturation effect
(Lear et al., 2004, 2006).
Several lines of evidence suggest changes in global
carbon cycling associated with Oi-1. It has long
been established that a dramatic increase in depth of
the CCD in the Pacific Ocean occurred at the
Eocene/Oligocene boundary (Van Andel, 1975;
Exon et al., 2000; Coxall et al., 2005). Site 1218 in
the Equatorial Pacific contains the most detailed
record of this increase, and the %CaCO3 at the site
increases abruptly at 34 Ma in two 40-kyr steps
ARTICLE IN PRESS
Fig. 4. Atmospheric pCO2 estimates from Southwest Pacific Site 588 based upon the d13C of alkenones (Pagani et al., 1999). CM events
represent carbon maxima events of Woodruff and Savin (1991) and Mi events are the inferred orbitally paced glacial maxima of Miller et
al. (1991) (modified from Pagani et al., 1999).
A.E. Shevenell, J.P. Kennett / Deep-Sea Research II 54 (2007) 2308–2324 2315
-
8/16/2019 Cenozoic Antarctic Cryosphere Evolution Tales From Deep Sea Sedimentary Records 2007 Deep Sea Research Par…
9/17
separated by a 200-kyr plateau (Coxall et al.,
2005). This pattern of change is similar to that of the
benthic foraminifer d18O record. Although the Site
1218 benthic foraminifer d13C record reveals a
similar pattern of change exhibited in the d18O
and %CaCO3 records, a slight (o10 kyr) lag of
d18O in the 40-kyr band with respect to d13C
suggests that changes in the global carbon cycle
related to Earth’s obliquity (damped seasonality)
may have forced the Oi-1 event (Coxall et al., 2005).
Furthermore, the lag of the d18O and CCD records
with respect to the d13C record indicates that the
ARTICLE IN PRESS
Fig. 5. (A) Mg/Ca temperatures based upon three benthic foraminifer species (Lear et al., 2000) and d18O across the Oi-1 glaciation at
DSDP Site 522 (Zachos et al., 1993). There is no decrease in Mg-derived temperature across Oi-1 indicating that the majority of the d18O
increase must reflect an increase in global ice volume (modified from Lear et al., 2000). (B) Benthic foraminifer stable isotope and trace
metal data versus age across Oi-1 from DSDP Site 522 (triangles; Zachos et al., 1993; Lear et al., 2000) and Site 1218 (circles; Lear et al.,
2004) (modified from Lear et al., 2004).
A.E. Shevenell, J.P. Kennett / Deep-Sea Research II 54 (2007) 2308–23242316
-
8/16/2019 Cenozoic Antarctic Cryosphere Evolution Tales From Deep Sea Sedimentary Records 2007 Deep Sea Research Par…
10/17
CCD increase was likely a result of Antarctic
cryosphere expansion and related to a shift in
global CaCO3 sedimentation from the continental
shelves to the deep ocean (Coxall et al., 2005).
2.2. The Mi-1 glaciation (23 Ma)
A transient (200 kyr) 1% benthic foraminifer
d18O increase is observed in globally distributed
deep-sea records at the Oligocene/Miocene bound-
ary (23.7 Ma; Mi-1; Fig. 6A). Although Mi-1 is one
in a series of orbital scale Antarctic glaciations that
occurred during the Oligocene and Miocene
(Zachos et al., 2001b; Pa ¨ like et al., 2006), the
amplitude of the isotopic signal associated with
Mi-1 suggests that this event represents a brief but
extensive expansion of the East Antarctic ice sheet
during a period of relative global warmth and
reduced global ice volumes (Zachos et al., 1997).
Support for this interpretation comes from a glacialmarine sediment sequence collected from the
Antarctic Margin, which suggests a period of ice
sheet expansion and contraction in the eastern Ross
Sea region associated with Mi-1 (Naish et al., 2001).
This observation indicates that the Antarctic ice
sheet reached the continental shelf in the region
during Mi-1 and is thought to have acted similarly
to the Northern Hemisphere ice sheets of the
ARTICLE IN PRESS
Fig. 6. (A) The d18O record of the Mi-1 event (bounded by the gray box) and orbital eccentricity and obliquity curves from 22 to 24 Ma.
Mi-1 corresponds with an interval of low eccentricity related to the 400-kyr cycle and an extended low-obliquity node (modified from
Zachos et al., 2001b). (B) Benthic foraminifer Cibicidoides spp. stable isotope and trace metal data versus age from Site 1218. Mg-derived
temperatures were obtained using the Lear et al. (2002) equation. Vertical shaded bars reflect intervals of bottom water cooling (modified
from Lear et al., 2004).
A.E. Shevenell, J.P. Kennett / Deep-Sea Research II 54 (2007) 2308–2324 2317
-
8/16/2019 Cenozoic Antarctic Cryosphere Evolution Tales From Deep Sea Sedimentary Records 2007 Deep Sea Research Par…
11/17
Plio-Pleistocene (Naish et al., 2001). Thus, substan-
tial research effort has gone into trying to under-
stand the origin of this glacial expansion.
Detailed benthic foraminifer stable isotope re-
cords across Mi-1 from at Ceara Rise ODP Site 929
and at Equatorial Pacific Site 1218 exhibit a positive1:2% d18O excursion (23.3–23.0 Ma) coeval with a0:8% d13C increase centered at 23 Ma (Zachos et al.,2001b; Fig. 6A). Oxygen isotope records from both
sites exhibit strong obliquity (41 kyr) pacing be-
tween 23.3 and 23 Ma, suggesting a high latitude
climate control on the d18O signal prior to the Mi-1
event (Zachos et al., 2001b). A 3-kyr lag in the d18O
record from Ceara Rise with respect to obliquity
indicates that Antarctic ice growth and/or cooling
was paced by changes in Earth’s orbital parameters
during a period of reduced seasonality and cool
summers (Zachos et al., 2001b). d18O records acrossMi-1 also exhibit sensitivity to Earth’s eccentricity
pacing; power is observed in the 400-kyr band
between 24 and 23 Ma and then shifts to the 100-kyr
band at 23.0 Ma (Zachos et al., 2001b). A similar
shift in orbital sensitivity from the obliquity (41 kyr)
to the eccentricity (100 kyr) band is observed in the
Antarctic margin glacial marine sedimentary se-
quence (Naish et al., 2001).
The 400-kyr eccentricity period is rare in the
geologic record. However, power at this period is
enhanced between 24 and 23 Ma and most pro-nounced in the d13C record, which exhibits a mean
increase and enhanced 400-kyr variability 1 My
before the d18O increase of Mi-1 (Zachos et al.,
2001b). This pattern of change has been observed
elsewhere in the geologic record and has been
attributed to enhanced burial of organic carbon on
the margins and a drawdown of atmospheric pCO2(Vincent and Berger, 1985). However, there is little
evidence in the geologic record for organic-rich
sediments deposited at this time. Some researchers
have proposed that lower pCO2
levels may increase
the climate system’s sensitivity to eccentricity
forcing (Zachos et al., 2001b). This interpretation
is further supported by an increase in the sensitivity
of d18O to the 100-kyr eccentricity forcing at a time
when d13C values reach a maximum (Zachos et al.,
2001b). Interestingly, the d18O signal leads d13C in
this interval, suggesting that climate and ice volume
changes are feeding back into the carbon cycle.
Benthic foraminifer stable isotope records have
yielded important information regarding the phas-
ing of carbon cycling and ice-volume/temperature
changes on orbital timescales. However, little was
known about how deep-sea temperatures changed
across Mi-1 (Billups and Schrag, 2002). Lear et al.
(2004) generated a benthic Mg/Ca record across
Mi-1 at Site 1218 that exhibits a 2 C cooling
between 23.8 and 23.7 Ma, a 2 C warming between
23.7 and 23.3 Ma, and additional cooling from 23.3to 23.1Ma (Fig. 6B). This temperature record
suggests that cooling of deep-ocean waters may
have played a role in triggering Mi-1. Interestingly,
deep waters (and by inference polar surface waters)
appear to have warmed shortly after the advances of
the Antarctic ice sheet. Lear et al. (2004) propose a
negative feedback toward ice growth at this time
caused by the blanketing of Antarctica by ice and a
subsequent reduction in global chemical weathering.
This hypothesized increase in atmospheric pCO2may have led to the partial melting of the newly
formed Antarctic ice sheet, and deep-sea warming(Lear et al., 2004); an observation supported by the
alkenone d13C record of Pagani et al. (1999) (Fig. 4).
Finally, the Mi-1 event is unusual in that it
coincides with an interval of low eccentricity
associated with the 400-kyr cycle and an extended
period of low obliquity associated with the 1.25 My
cycle (Fig. 6A; Zachos et al., 2001b). It has been
proposed that the accumulation of ice on Antarctica
is limited by high seasonality (warm summers).
A protracted period of low amplitude obliquity
variations coupled with low eccentricity suggeststhat Mi-1 occurred at a time of low seasonality
when Antarctic summer temperatures were rela-
tively cool (Zachos et al., 2001b). The Mi-1
glaciation reversed itself as eccentricity increased.
Similar nodes exist in the climate record though not
all are associated with transient glaciations of the
magnitude of the Mi-1 event (Zachos et al., 2001b).
It appears that changes in global carbon cycling that
began 1 My prior to Mi-1 may have primed to
system to react sensitively to this astronomical event
(Zachos et al., 2001b).
2.3. The middle Miocene climate transition
(16– 13 Ma)
A significant reorganization of Earth’s climate
system occurred in the middle Miocene, as evi-
denced by the abrupt 1% increase in global
benthic foraminifer d18O at 14Ma (Fig. 1;
Shackleton and Kennett, 1975; Miller et al., 1987;
Flower and Kennett, 1994; Zachos et al., 2001).
This step-like d18O increase is one of three major
events that punctuate the long-term Cenozoic d18O
ARTICLE IN PRESS
A.E. Shevenell, J.P. Kennett / Deep-Sea Research II 54 (2007) 2308–23242318
-
8/16/2019 Cenozoic Antarctic Cryosphere Evolution Tales From Deep Sea Sedimentary Records 2007 Deep Sea Research Par…
12/17
record and reflects some combination of Antarctic
ice growth and global cooling (Shackleton and
Kennett, 1975; Matthews and Poore, 1980; Miller
et al., 1987; Prentice and Matthews, 1991; Flower
and Kennett, 1994). Evidences for both ice growth
and global cooling are found throughout thegeologic record of the middle Miocene: Southern
Ocean ice-rafted debris is more abundant after
14 M a (Margolis, 1975; Kennett and Barker,
1990), large fluctuations in global sea level are
inferred (Haq et al., 1987; John et al., 2004),
paleobotanical and faunal change occurred (Flower
and Kennett, 1994 and references therein), and the
East Antarctic Ice Sheet expanded across the
Antarctic continental margin (Ross Sea sector;
Anderson, 1999; Cape Roberts Science Team,
2000).
The magnitudes of middle Miocene Antarctic icegrowth and temperature change (17–13 Ma) have
been estimated using indirect methods, including
meridional stable isotope gradients (Shackleton and
Opdyke, 1973; Miller et al., 1991; Wright et al.,
1992) and sequence stratigraphy (Haq et al., 1987;
Miller et al., 1987; John et al., 2004). Results from
these studies suggest that 70% of the global 1%
benthic d18O increase at 14 Ma relates to Antarctic
ice volume. Thus, global deep waters are inferred to
have cooled 1.8–2:5 C between 14.2 and 13.8 Ma
(Miller et al., 1991; Wright et al., 1992; Flower andKennett, 1994; John et al., 2004). While indirect
methods provide useful approximations of the
relative contributions of ice volume and tempera-
ture to the middle Miocene d18O signal, none
involves a truly independent measure of either
deep-water temperature or ice volume.
Benthic foraminifer (Cibicidoides mundulus)
Mg/Ca data from the Southern Ocean reveal a 2
1:5 C cooling of regional bottom waters during themiddle Miocene climate transition (14.2–13:8 Ma),which indicates that the globally recognized d18O
increase (14 Ma) describes a major expansion of
the Antarctic cryosphere and that 80% of the
d18O signal relates to ice volume (Shevenell et al., in
review; Fig. 6). Interestingly, this cooling is not
significant or permanent, and temperatures warm
following the d18O increase, much like the Mg/Ca
records across Oi-1 and Mi-1 (Lear et al., 2004;
Figs. 4 – 6). A record of seawater d18O, calculated
using paired C. mundulus Mg/Ca and d18O records
and the paleotemperature relationship of Lynch-
Stieglitz et al. (1999), suggests that Antarctic ice
sheets entered an interval of eccentricity-modulated
glacial advance and retreat at 15Ma, 1 My
before the 1% middle Miocene d18O increase at
a time when Mg/Ca-derived surface and deep-water
temperatures were relatively warm (Fig. 7; Shevenell
et al., 2004, in review). Glacial episodes increased in
intensity between 15 and 13.8 Ma, revealing acentral role for internal climate feedbacks (e.g., ice
albedo feedbacks) in this major Cenozoic climate
transition (Shevenell et al., in review).
The positive d13C excursion of the middle
Miocene suggests a major reorganization of the
global carbon cycle and has been studied extensively
as it is the largest and longest d13C increase of the
Cenozoic (Fig. 1; 16:5–13.5 Ma). Several hypoth-eses have been put forth to account for this d13C
increase including the silicate weathering hypothesis
of Raymo (1994) and the Monterey hypothesis of
Vincent and Berger (1985). Substantial organic-richdeposits surrounding the Pacific Rim have been
dated to this time and suggest that large amounts of
organic carbon were sequestered in continental
margin basins as a result of invigorated ocean
circulation, upwelling of nutrients, and an expan-
sion of oxygen minimum zones (Vincent and Berger,
1985). One problem with hypotheses that relates
changes in carbon cycling with Antarctic ice growth
during the middle Miocene is the 2:5-My offsetbetween the changes in global carbon cycling (the
initial increase in d13
C at 16.5 Ma) and the d18
O stepat 14 Ma. However, the Mg/Ca-derived d18Oswrecord of Shevenell et al. (in review) seems to
indicate that ice growth began 1Ma before the
major d18 O step, suggesting only a 1.5 My lead of
the carbon cycle. This lead, coupled with the strong
400-kyr cycle observed in both the d13C and d18O
records prior to the middle Miocene d18O increase,
is similar to that observed before Mi-1 (Zachos
et al., 2001b). This similarity and the initiation of ice
growth at 15 Ma during the Miocene Climatic
Optimum provides new support for a substantial
role of the global carbon cycle in the middle
Miocene climate transition (Shevenell et al., in
review). Furthermore, comparison of ice volume,
paleotemperature, and paleo- pCO2 records indicate
that middle Miocene expansion of the Antarctic
cryosphere coincided with an interval of relatively
warm Southern Ocean surface and deep-ocean
waters and inferred low atmospheric pCO2 (Fig. 7;
Shevenell et al., in review). This relationship
suggests that changes in heat and moisture trans-
port were important in the development of the
Antarctic cryosphere and that atmospheric pCO2
ARTICLE IN PRESS
A.E. Shevenell, J.P. Kennett / Deep-Sea Research II 54 (2007) 2308–2324 2319
-
8/16/2019 Cenozoic Antarctic Cryosphere Evolution Tales From Deep Sea Sedimentary Records 2007 Deep Sea Research Par…
13/17
concentrations may dictate the sensitivity of the
Antarctic system to low-latitude-derived heat and
moisture.
3. The role of the global carbon cycle in Cenozoic
Antarctic cryosphere evolution
The Cenozoic evolution of the Antarctic Cryo-
sphere and Southern Ocean system has been linked
to the progressive tectonic, oceanographic, and
thermal isolation of Antarctica. Substantial geologic
evidence exists for the opening of the Tasman
Gateway between Australia and Antarctica at the
Eocene/Oligocene boundary (Kennett, 1977; Exon
et al., 2000), but the precise timing of the opening of
the Drake Passage and the initiation of the
Antarctic Circumpolar Current remains unresolved.
Other gateways, such as the Indonesian gateway,
may have played a major role in the middle Miocene
expansion of the East Antarctic Ice Sheet; however,
a definitive chronology of that closure continues to
be elusive.
Recent high-resolution records across the major
Antarctic ice advances of the Paleogene and early
Neogene (including Oi-1, Mi-1, and the middle
Miocene d18O step) suggest that climate feedbacks
beyond the opening of tectonic gateways and
thermal isolation of Antarctica must be considered
to explain the timing and evolution of the Antarctic
cryosphere. For example, benthic foraminifer
Mg/Ca records indicate little or no permanent
deep-ocean cooling after the major ice expansion
at Oi-1 and after the middle Miocene d18O step.
This observation suggests that meridional heat
transport related to the opening and closing of
tectonic gateways cannot have been the sole arbiter
of Antarctic cryosphere expansion through the
Cenozoic (Huber et al., 2004). Additional mechan-
isms must have been involved.
A model simulating ice growth during Oi-1
(DeConto and Pollard, 2003) gradually reduced
ARTICLE IN PRESS
Fig. 7. Southern Ocean paleoclimate records from South Tasman Rise (STR) ODP Site 1171 (48300S, 14906:690E; 2150 m) based on thebenthic foraminifer C. mundulus and planktonic foraminifer G. bulloides. Gaps in the record are due to coring gaps. The age scale is based
on magnetostratigraphic and stable isotopic datums (Exon et al., 2000; Shevenell et al., 2004). Bottom panel: seawater d18O (d18Ow;
SMOW; Standard Mean Ocean Water scale) calculated from C. mundulus d18O and BWT estimates using the paleotemperature equation
of Lynch-Stieglitz et al. (1999). More positive d18Ow intervals (marked by arrows) are interpreted as Antarctic glaciations. Middle panel:
Mg/Ca-derived bottom water temperature (BWT). The temperature scale is exponential and based on conversion of Mg/Ca using the
relationship of Lear et al. (2003): SST ¼ ln(Mg/Ca/0.9)/0.1. Top panel: Mg/Ca-based SST (Shevenell et al., 2004) record derived fromplanktonic foraminifer G. bulloides (modified from Shevenell et al., in review).
A.E. Shevenell, J.P. Kennett / Deep-Sea Research II 54 (2007) 2308–23242320
-
8/16/2019 Cenozoic Antarctic Cryosphere Evolution Tales From Deep Sea Sedimentary Records 2007 Deep Sea Research Par…
14/17
atmospheric pCO2 concentrations to observe the
response of the system. The results suggested that a
gradual lowering of snowline elevations with lower
atmospheric pCO2 concentrations initiated the
formation and gradual coalescence of small but
dynamic ice sheets under favorable orbital condi-tions (cool summers) (DeConto and Pollard, 2003).
These ice sheets continued to increase in size due to
ice albedo feedbacks. This scenario is further
supported by additional iterations of the model in
which pCO2 is lowered at different times prior to
Oi-1 (Pa ¨ like et al., 2006). Results suggest that the
timing of pCO2 change does not influence the timing
of ice growth; rather, ice growth is triggered by
astronomical forcing if atmospheric pCO2 levels are
near a threshold. To determine the influence of
tectonic gateways on the initiation of Antarctic ice
growth at Oi-1, DeConto and Pollard (2003)simulated the opening of the Drake Passage. They
found that this produced only an 20% change in
oceanic heat transport if the gateway was opened
within a narrow range of atmospheric pCO2. They
concluded that atmospheric pCO2 is likely a more
important boundary condition for Cenozoic climate
change than tectonic configuration of the Southern
Ocean (DeConto and Pollard, 2003). Because the
threshold remains unknown, we argue that the
opening of Southern Ocean gateways during a time
of low atmospheric pCO2 and favorable orbitalconditions may have been the trigger for the
initiation of the rapid expansion of the East
Antarctic Ice Sheet at Oi-1.
It is likely that atmospheric pCO2 is a funda-
mental boundary condition for other glacial ad-
vances such as Mi-1 and the middle Miocene d18O
increase at 14 Ma. Atmospheric pCO2 also may
play an important role as a negative feedback
toward continued ice expansion after a glaciation.
Both the Mi-1 and the middle Miocene glacial
events occurred 1–1.5 Ma after a major change in
the state of the global carbon cycle, as inferred from
foraminiferal d13C records. Models suggest that
during intervals of low pCO2, Antarctic glaciations
may be triggered by favorable astronomical condi-
tions; such a relationship has been observed for
Mi-1 but not during the middle Miocene. After Mi-
1, deep-ocean temperatures warm and atmospheric
pCO2 increases (Lear et al., 2004; Pagani et al.,
1999). Lear et al. (2004) propose a negative
feedback toward runaway ice growth driven by
global carbon cycling: that the expanding ice sheet
removed a substantial source of silicate rock
available for weathering from the global carbon
cycle. The removal of this sink for pCO2 enabled
atmospheric pCO2 to increase enough to halt
Antarctic ice growth.
A similar pattern of change is seen after the
middle Miocene d
18
O increase at the end of theMonterey d13C excursion (Shevenell et al., in
review). Alternatively, meridional heat and moisture
transport may play a significant role in the advance
and retreat of Antarctic ice sheets under low pCO2conditions of the Neogene. The advance of the East
Antarctic Ice Sheet in the middle Miocene (15 Ma)
appears to have taken place during an interval of
relatively warm climate conditions when atmo-
spheric pCO2 was paradoxically low (Figs. 4 and
7; Pagani et al., 1999; Shevenell et al., 2004). At this
time, moisture transport regimes were altered such
that moisture was being supplied to the AntarcticDry Valleys (Sugden and Denton, 2004). Shevenell
et al. (2004, in review) use the relationships between
warm planktonic and benthic foraminifer Mg/Ca-
derived ocean temperatures (Fig. 7) to suggest that
middle Miocene ice growth occurred at a time of
reduced latitudinal thermal gradients and increased
moisture supply to Antarctica.
An orbitally paced cooling of Southern Ocean
surface temperatures south of Tasmania suggests an
expansion of the Antarctic Circumpolar Current
related to ice albedo feedbacks isolated Antarcticafrom receiving additional low-latitude-derived
moisture and heat. This isolation coupled with the
silicate weathering feedback discussed below acted
in concert to halt continued expansion of the East
Antarctic Ice Sheet and the Monterey d13C excur-
sion. Further modeling studies are required to
determine if the low atmospheric pCO2 of the
Monterey interval was the primary boundary
condition that allowed for the growth of the East
Antarctic Ice Sheet during the middle Miocene. It is
likely that the expansion of the East Antarctic Ice
Sheet in the middle Miocene led to a reorganization
of the global carbon cycle and that this reorganiza-
tion resulted in the stabilization of the East
Antarctic Ice Sheet (Shevenell et al., in review).
4. Continuing challenges
In the past decade, technological advances have
improved our ability to recover and generate high-
resolution paleoclimate/paleoceanographic records.
Consequently a more detailed understanding of
Cenozoic climate change and Antarctic ice sheet
ARTICLE IN PRESS
A.E. Shevenell, J.P. Kennett / Deep-Sea Research II 54 (2007) 2308–2324 2321
-
8/16/2019 Cenozoic Antarctic Cryosphere Evolution Tales From Deep Sea Sedimentary Records 2007 Deep Sea Research Par…
15/17
evolution is emerging. For example, there is consensus
within the paleoclimate community regarding the
timing of major ice expansions during the early
Oligocene (Oi-1) and the middle Miocene as well as
the existence of a transient glaciation at the Oligocene/
Miocene boundary (Mi-1). Furthermore, a handful of detailed studies have provided important insights into
the orbital scale structure of ice growth events as well
as preliminary insights into lead–lag relationships
between ice growth, temperature change, and global
carbon cycling. These results highlight the importance
of generating additional globally distributed high-
resolution multi-proxy records across major Cenozoic
climate reorganizations, including those during which
Antarctic ice sheets were thought to have retreated
(e.g., the early Miocene).
Despite recent advances, understanding of the
Cenozoic evolution of the Antarctic/Southern oceansystem and its influence on global climate remains
rudimentary at best. To gain an improved under-
standing of this evolution, high-resolution multi-
proxy geochemical studies must be generated at a
network of high-resolution deep-sea sites (with an
emphasis on the high latitudes and the tropics) in
which the lead–lag relationships between orbital
forcing, ice volume, carbon cycling, and tempera-
ture may be assessed across known intervals of
Antarctic ice sheet expansion (e.g., orbital scale
glaciations in the Oligocene and Miocene). Inaddition to deep-sea sedimentary sequences, we
must focus on obtaining dateable high-resolution
sedimentary sequences from the Antarctic continen-
tal margin that may be integrated with deep-sea
records to provide a complete and more direct
perspective on Cenozoic Antarctic cryosphere ex-
pansion. As high-resolution proxy records are only
as useful as the current proxies are reliable, future
research is required to ensure the utility of the Mg/
Ca and pCO2 proxies on Paleogene and Neogene
timescales. Finally, geochemists and modelers must
work closely with one another to develop detailed
simulations of both Antarctic ice sheet advances
and periods of ice retreat through the Cenozoic.
Such studies will assist in improving understanding
of our present climate system.
Acknowledgments
We thank G. Filippelli, D. Warnke, J.A. Flores,
and T. Marchitto for organizing the JOI USSSP
sponsored ‘‘Paleoceanography and Paleoclimatol-
ogy of the Southern Ocean: A Synthesis of Three
Decades of Scientific Ocean Drilling’’ workshop in
Boulder, CO in 2005. This research used samples
provided by the Ocean Drilling Program (ODP).
ODP is sponsored by the US National Science
Foundation and participating countries under the
management of JOI. This research was supportedby NSF Grant OPP0229898 to J.P. Kennett and
JOI/USSSP postcruise funds to A.E. Shevenell, and
a University of Washington Program on Climate
Change Postdoctoral fellowship to A.E. Shevenell.
We thank Carrie Lear, Jim Zachos, and Katarina
Billups for discussions and reviewers for suggestions
and improvements.
References
Anderson, J.B., 1999. Antarctic Marine Geology. CambridgeUniversity Press, Cambridge, 289 pp.
Bemis, B.E., Spero, H.J., Bijma, J., Lea, D.W., 1998. Reevalua-
tion of the oxygen isotopic composition of planktonic
foraminifera: experimental results and revised paleotempera-
ture equations. Paleoceanography 13 (2), 150–160.
Berner, R.A., Lasaga, A.C., Garrels, R.M., 1983. The carbona-
te–silicate geochemical cycle and its effect on atmospheric
carbon dioxide over the past 100 million years. American
Journal of Science 283, 641–683.
Billups, K., Schrag, D.P., 2002. Paleotemperatures and ice
volume of the past 27 Myr revisited with paired Mg/Ca and18O/16O measurements on benthic foraminifera. Paleoceano-
graphy, 17 doi:10.1029/2000PA000567.
Cape Roberts Science Team, 2000, Summary of results. In:
Barrett, P.J., Sarti, M., Wise, S. (Eds.), Studies from Cape
Roberts Project: Initial Report on CRP-3, Ross Sea,
Antarctica. Terra Antarctica, 8, 185–203.
Coxall, H.K., Wilson, P.A., Pa ¨ like, H., Lear, C.H., Backman, J.,
2005. Rapid stepwise onset of Antarctic glaciation and deeper
calcite compensation in the Pacific Ocean. Nature 433, 53–57.
DeConto, R.M., Pollard, D., 2003. Rapid Cenozoic glaciation of
Antarctica induced by declining atmospheric CO2. Nature
421, 245–249.
Exon, N.F., Kennett, J.P., Malone, M.L., the Leg 189 Shipboard
Scientific Party, 2001. Proc. ODP, Initial Reports 189 (CD-
ROM): Ocean Drilling Program, College Station, TX 77845-
9547, USA.Flower, B.P., Kennett, J.P., 1994. The middle Miocene climate
transition: east Antarctic ice sheet development deep ocean
circulation and global carbon cycling. Palaeogeography
Palaeoclimatology Palaeoecology 108, 537–555.
Haq, B.U., Hardenbol, J., Vail, P., 1987. Chronology of fluctuating
sea levels since the Triassic. Science 235, 1156–1167.
Hastings, D.H., Russell, A.D., Emerson, S.R., 1998. Foraminif-
eral magnesium in Globigerinoides sacculifer as a paleotem-
perature proxy. Paleoceanography 13, 161–169.
Huber, M., Brinkhuis, H., Stickley, C.E., Doos, K., Sluijs, A.,
Warnaar, J., Williams, G.L., Schellenberg, S.A., 2004. Eocene
circulation of the Southern Ocean: was Antarctica kept warm
by subtropical waters? Paleoceanography, PA4026 doi:10.
1029/2004PA001014.
ARTICLE IN PRESS
A.E. Shevenell, J.P. Kennett / Deep-Sea Research II 54 (2007) 2308–23242322
http://dx.doi.org/10.1029/2000PA000567http://dx.doi.org/10.1029/2004PA001014http://dx.doi.org/10.1029/2004PA001014http://dx.doi.org/10.1029/2004PA001014http://dx.doi.org/10.1029/2004PA001014http://dx.doi.org/10.1029/2000PA000567
-
8/16/2019 Cenozoic Antarctic Cryosphere Evolution Tales From Deep Sea Sedimentary Records 2007 Deep Sea Research Par…
16/17
John, C.M., Karner, G.D., Muiti, M., 2004. d18O and Marion
Plateau backstripping: combining two approaches to con-
strain late middle Miocene eustatic amplitude. Geology 32,
829–832.
Kennett, J.P., 1975. Cenozoic evolution of Antarctic glaciation,
the Circum-Antarctic Ocean and their impact on global
paleoceanography. Journal of Geophysical Research 82,3843–3860.
Kennett, J.P., 1977. Cenozoic evolution of Antarctic glaciation,
the circum-Antarctic Ocean, and their impact on global
paleoceanography. Journal of Geophysical Research 82,
3843–3860.
Kennett, J.P., Barker, P.F., 1990. Latest Cretaceous to Cenozoic
climate and oceanographic developments in the Weddell Sea,
Antarctica: an ocean-drilling perspective. In: Kennett, J.P.,
Warnke, D.A. (Eds.), The Antarctic Paleoenvironment: a
Perspective on Global Change, Part 1. Antartica Research
Series, vol. 56. AGU, Washington, DC, pp. 7–30.
Kominz, M.A., Pekar, S.F., 2001. Oligocene eustasy from two-
dimensional sequence stratigraphic backstripping. Geological
Society of America Bulletin 113, 291–304.Lawver, L.A., Gahagan, L.M., Coffin, M.F., 1992. The devel-
opment of paleoseaways around Antarctica. In: Kennett, J.P.,
Warnke, D.A. (Eds.), The Antarctic Paleoenvironment: a
perspective on Global Change, Part 1. Antartica Research
Series, vol. 56. AGU, Washington, DC, pp. 7–30.
Lea, D.W., Pak, D.K., Spero, H.J., 2000. Climate impact of Late
Quaternary equatorial Pacific sea surface temperature varia-
tions. Science 289, 1719–1724.
Lear, C.H., Elderfield, H., Wilson, P.A., 2000. Cenozoic deep-sea
temperatures and global ice volumes from Mg/Ca in benthic
foraminiferal calcite. Science 287, 269–272.
Lear, C.H., Rosenthal, Y., Slowey, N., 2002. Benthic forami-
niferal Mg/Ca-paleothermometry: a revised core-top calibra-
tion. Geochimica, et Cosmochimica Acta 66, 3375–3387.
Lear, C.H., Rosenthal, Y., Wright, J.D., 2003. The closing of a
seaway: ocean water masses and global climate change. Earth
and Planetary Science Letters 210, 425–436.
Lear, C.H., Rosenthal, Y., Coxall, H.K., Wilson, P.A., 2004.
Late Eocene to early Miocene ice sheet dynamics and the
global carbon cycle. Paleoceanography 19.
Lear, C.H., Rosenthal, Y., 2006. Benthic foraminiferal Li/Ca:
insights into Cenozoic seawater carbonate saturation state.
Geology 34, 985–988.
Lyle, M., Wilson, P., Janecek, T.R., Backman, J., Busch, W.H.,
Coxall, H.K., Faul, K., Gaillot, P., Hovan, S.A., Knoop, P.,
Kruse, S., Lanci, L., Lear, C., Moore Jr., T.C., Nigrini,
C.A., Nishi, H., Nomura, R., Norris, R.D., Pa ¨ like, H., Pare ´ s,J.M., Quinton, L., Raffi, I., Rea, B.R., Rea, D.K., Steiger,
T.H., Tripati, A., Vanden Berg, M.D., Wade, B., 2002.
Proceedings of the Ocean Drilling Program, Initial Reports,
199.
Lynch-Stieglitz, J., Curry, W.B., Slowey, N., 1999. A geostrophic
transport estimate for the Florida Current from the oxygen
isotope composition of benthic foraminifera. Paleoceanogra-
phy 14, 360–373.
Marchitto, T.M., Bryan, S.P., Curry, W.B., McCorkle, D.C.,
2007. Mg/Ca temperature calibration for the benthic for-
aminifer Cibicidoides pachyderma, Paleoceanography 22 (1),
doi:PA1203, 10.1029/2006PA001287.
Margolis, S.V., 1975. Paleoglacial history of Antarctica inferred
from analysis of Leg 29 sediments by scanning electron
microscopy. In: Kennett, J.P., Houtz, R.E., et al. (Eds.),
Initial Reports DSDP 29, 1039–1048.
Martin, P.A., Lea, D.W., Rosenthal, Y., Shackleton, N.J.,
Papenfuss, T.P., Sarnthein, M., 2002. Quaternary deep sea
temperature histories derived from benthic foraminiferal Mg/
Ca. Earth and Planetary Science Letters 198 (1–2), 193–209.
Matthews, R.K., Poore, R.Z., 1980. Tertiary d
18
O record andglacio-eustatic sea-level fluctuation. Geology 8, 501–504.
Miller, K.G., Fairbanks, R.G., Mountain, G.S., 1987. Tertiary
oxygen isotope synthesis, sea-level history, and continental
margin erosion. Paleoceanography 2, 1–19.
Miller, K.G., Wright, J.D., Fairbanks, R.G., 1991. Unlocking the
icehouse: Oligocene–Miocene oxygen isotope, eustasy, and
margin erosion. Journal of Geophysical Research 96,
6829–6848.
Miller, K.G., Kominz, M.A., Browning, J.V., Wright, J.D.,
Mountain, G.S., Katz, M.E., Sugarman, P.J., Cramer, B.S.,
Christie-Click, N., Pekar, S.F., 2005. The Phanerozoic record
of global sea-level change. Science 310, 1293–1298.
Naish, T.R., Woolfe, K.J., Barrett, P.J., et al., 2001. Orbitally
induced oscillations in the East Antarctic ice sheet at theOligocene/Miocene boundary. Nature 413, 719–723.
Pa ¨ like, H., Norris, R.D., Herrle, J.O., Wilson, P.A., Coxall,
H.K., Lear, C.H., Shackleton, N.J., Tripati, A.K., Wade,
B.S., 2006. The heartbeat of the Oligocene climate system.
Science 314(5807), 1894–1898.
Pagani, M., Arthur, M.A., Freeman, K.H., 1999. Miocene
evolution of atmospheric carbon dioxide. Paleoceanography
14, 273–292.
Pagani, M., Zachos, J.C., Freeman, K.H., Tipple, B., Bohaty, S.,
2005. Marked decline in atmospheric carbon dioxide con-
centrations during the Paleogene. Science 309, 600–603.
Pearson, P.N., Palmer, M.R., 2000. Atmospheric carbon dioxide
concentrations over the past 60 million years. Nature 406,
695–699.
Prentice, M.P., Matthews, R.K., 1991. Tertiary ice sheet
dynamics: the snow gun hypotheses. Journal of Geophysical
Research 96, 6811–6827.
Raymo, M.E., 1994. The Himalayas, organic carbon burial, and
climate in the Miocene. Paleoceanography 9, 399–404.
Rea, D.K., Lyle, M.W., 2005. Paleogene calcite compensation
depth in the eastern subtropical Pacific: answers and
questions. Paleoceanography 20.
Robert, C., Kennett, J.P., 1997. Antarctic continental weathering
changes during Eocene–Oligocene cryosphere expansion: clay
mineral and oxygen isotope evidence. Geology 25, 587–590.
Rosenthal, Y., Boyle, E.A., Slowey, N., 1997. Temperature
control on the incorporation of Mg, Sr, F and Cd into benthicforaminiferal shells from Little Bahama Bank: prospects for
thermocline paleoceanography. Geochimica et Cosmochimica
Acta 61, 3633–3643.
Sanyal, A., Hemming, N.G., Hanson, G.N., Broecker, W.S.,
1995. The pH of the glacial ocean as reconstructed from
boron isotope measurements on foraminifera. Nature 373,
234–236.
Sanyal, A., Hemming, N.G., Broecker, W.S., Lea, D.W., Spero,
H.J., Hanson, G.N., 1996. Oceanic pH control on the boron
isotopic composition of foraminifera: evidence from culture
experiments. Paleoceanography 11, 513–517.
Schnitker, D., 1980. North Atlantic oceanography as possible
cause of Antarctic glaciation and eutrophication. Nature 284,
615–616.
ARTICLE IN PRESS
A.E. Shevenell, J.P. Kennett / Deep-Sea Research II 54 (2007) 2308–2324 2323
http://dx.doi.org/10.1029/2006PA001287http://dx.doi.org/10.1029/2006PA001287
-
8/16/2019 Cenozoic Antarctic Cryosphere Evolution Tales From Deep Sea Sedimentary Records 2007 Deep Sea Research Par…
17/17
Shackleton, N.J., 1974. Attainment of isotopic equilibrium
between ocean water and benthonic foraminifera genus
Uvigerina: isotopic changes in the ocean during the last
glacial Colloq. Int. CNRS No. 219, Les Mthodes Quanti-
tative d’Etudes des Variations du Climat au cours du
Plkistoc6ne, pp. 203–209.
Shackleton, N.J., Kennett, J.P., 1975. Paleotemperature historyof the Cenozoic and the initiation of Antarctic glaciation:
oxygen and carbon isotopic analyses in DSDP Sites 277, 279,
and 281. In: Kennett, J.P., Houtz, R.E., et al. (Eds.), Initial
Reports DSDP, vol. 29. U.S. Government Printing Office,
Washington, DC, pp. 143–156.
Shackleton, N.J., Opdyke, N.D., 1973. Oxygen isotope and
palaeomagnetic stratigraphy of equatorial Pacific core V28-
238: oxygen isotope temperatures and ice volumes on a 105
and 106 year scale. Quaternary Research 3, 39–55.
Shevenell, A.E., Kennett, J.P., Lea, D.W., 2004. Middle Miocene
Southern Ocean cooling and Antarctic cryosphere expansion.
Science 305, 1766–1770.
Shevenell, A.E., Kennett, J.P., Lea, D.W. in review. Middle
Miocene ice sheet dynamics, deep-sea temperatures, andcarbon cycling: a Southern Ocean perspective (G3, 2007).
Stanley, S.M., Hardie, L.M., 1998. Secular oscillations in the
carbonate mineralogy of reef-building and sediment produ-
cing organisms driven by tectonically forced shifts in seawater
chemistry. Palaeogeography Palaeoclimatology Palaeoecol-
ogy 144, 3–19.
Sugden, D.E., Denton, G.H., 2004. Cenezoic landscape evolution
of the Convoy Range to Mckay Glacier area, Transantarctic
Mountains: onshore to offshore synthesis. Geological Society
of America Bulletin 116, 840–857.
Van Andel, T.H., 1975. Mesozoic/Cenozoic calcite compensation
depths and he global distribution of calcareous sediments.
Earth and Planetary Science Letters 26, 187–194.
Vincent, E., Berger, W.H., 1985. Carbon dioxide and polar
cooling in the Miocene. In: Sundquist, E.T., Broecker, W.S.
(Eds.), The carbon cycle and atmospheric CO2: natural
variations Archean to Present, Geophysical Monographyvol. 32. AGU, Washington, DC, pp. 455–468.
Wilkinson, B.H., Algeo, T.J., 1989. Sedimentary carbonate
record of calcium–magnesium cycling. American Journal of
Science 289, 1158–1194.
Woodruff, F., Savin, S.M., 1989. Miocene deepwater oceano-
graphy. Paleoceanography 4, 87–140.
Woodruff, F., Savin, S.M., 1991. Mid-Miocene isotope stratigraphy
in the deep sea: high-resolution correlations, paleoclimatic cycles,
and sediment preservation. Paleoceanography 6, 755–806.
Wright, J.D., Miller, K.G., Fairbanks, R.G., 1992. Evolution of
modern deepwater circulation: evidence from the late
Miocene Southern Ocean. Paleoceanography 6, 275–290.
Zachos, J.C., Lohmann, K.C., Walker, J.C.G., Wise, S.W., 1993.
Abrupt climate change and transient climates in the Paleogene: amarine perspective. Journal of Geology 100, 191–213.
Zachos, J.C., Flower, B.P., Paul, H., 1997. Orbitally paced
climate oscillations across the Oligocene/Miocene boundary.
Nature 388, 567–570.
Zachos, J.C., Pagani, M., Sloan, L.C., Thomas, E., Billups, K.,
2001a. Trends, rhythms, and aberrations in global climate
65 Ma to present. Science 292, 686–693.
Zachos, J.C., Shackleton, N.J., Palike, H., Flower, B.P., 2001b.
Climate response to orbital forcing across the Oligocene–-
Miocene boundary. Science 292, 274–278.
ARTICLE IN PRESS
A.E. Shevenell, J.P. Kennett / Deep-Sea Research II 54 (2007) 2308–23242324