the Eocene/Oligocene boundary. For much of the
Oligocene (B33–25 Ma), planktonic foraminiferal
d
18 O values remained unusually high, averaging
À 0.5%. Beginning around the Oligocene/Miocene
boundary (B25 Ma), planktonic foraminiferal d
18 O
values began a long-term decrease, culminating in
the Pleistocene with average values of À 1.5%. In
contrast, the benthic d
18 O record permanently
changed during the middle Miocene d
18 O shift and
late Pliocene increase.
Apportioning the d
18 O changes recorded by the
benthic and planktonic foraminifera between temperature and ice volume changes requires knowledge
of, or reasonable estimates for, one of these parameters. One promising tool that may help discriminate between each effect is the Mg/Ca ratio
measured in benthic foraminifera. Initial studies
using Mg/Ca ratios confirmed the long-term temperature changes during the Cenozoic calculated
using the d
18
O record and other climate proxies. If
verified, this record implies that small ice sheets grew
during the middle and late Eocene and fluctuated in
size throughout the Oligocene to Miocene. At present, the Mg/Ca record lacks the resolution for key
intervals and still requires verification of interspecies
offsets before it can be applied unequivocally to
isolate the ice volume-induced d
18 O water component
in the foraminiferal d
18 O records. For the discussion
that follows, glacialogical evidence is used to estimate the ice volume/d
18
O water variations.
The Greenhouse World
The oldest unequivocal evidence for ice sheets on
Antarctica, ice-rafted detritus (IRD) deposited by
icebergs in the ocean, places the first large ice sheet in
the earliest Oligocene. Thus, it is reasonable to assume that ice sheets were small to absent and that
surface and deep-water temperature changes controlled much if not all the d
18
O change prior to 34
Ma. The modern Antarctic and Greenland ice sheets
lock up B2% of the total water in the world’s ocean.
If melted, these ice sheets would raise global sea level
by B70–75 m and mean ocean d
18
O water value
would decrease to À 0.9% PDB (see above). One can
then apply eqn [2] to the benthic and planktonic
foraminiferal d
18 O records to estimate deep- and
surface-ocean temperatures for the first half of the
Cenozoic (c. 65–34 Ma).
During the early to middle Paleocene, deep-water
temperatures remained close to 101C (Figure 7B).
The 1% decrease between 58 and 52 Ma translates
into a deep-water warming of 41C, reaching a high
of 141C. This is in sharp contrast to the modern
deep-water temperatures, which range between
0 and 31C. Following the peak warmth at 52 Ma, the
2% increase in benthic foraminiferal values indicates
that the deep waters cooled by 71C and were 71C by
the end of the Eocene. If small ice sheets existed
during the Paleocene and Eocene, then temperature
estimates would be on the order of 11C warmer than
those calculated for the ice-free assumption. (Some
data indicate that smaller ice sheets may have existed
on the inland parts of Antarctica during the late
Eocene. However, these were not large enough to
deposit IRD in the ocean. Therefore, their effect on
the d
18
O values of the ocean was probably less than
0.3%.)
Tropical surface water temperatures warmed from
22 to 241C, based on eqn [2], at the beginning of the
Cenozoic to 281C during the early Eocene (52 Ma;
Figure 7B). The higher estimate is similar to temperatures in the equatorial regions of the modern
oceans. Planktonic foraminiferal d
18 O values recorded a long-term increase of by 2% ( À 2.5 to
À 0.5%) through the remainder of the Eocene. Just
prior to the Eocene/Oligocene boundary, tropical
surface water temperatures were B211C, ending the
long-term tropical cooling of 71C from 52 to 34 Ma.
The Ice House World of the Last 33
Million Years
As mentioned above, southern ocean cores contain
IRD at and above the Eocene/Oligocene boundary.
Widely distributed IRD and glacial tills on parts of
the Antarctic continental margin representing the
Oligocene to Recent mark the onset of large ice
sheets. Whether these sediments represent persistent
or periodic ice cover is uncertain. At least some ice
was present on Antarctica during the Oligocene to
early Miocene. The Antarctic ice sheet has been a
fixture since the middle Miocene (B15 Ma). Our
record of Northern Hemisphere ice sheets suggests
that they were small or nonexistent prior to the late
Pliocene. For the purpose of estimating surface and
deep temperatures, an ice volume estimate slightly
lower than the modern will be applied for the interval that spans from the Oligocene into the middle
Miocene (33–15 Ma). For the interval between 15
and 3 Ma, ice volumes were probably similar to
those of today. From 3 Ma, ice volumes ranged between the modern and LGM. Using these broad estimates for ice volumes, mean ocean d
18 O water values
for those three intervals were À 0.5, À 0.22, and
0.4% PDB, respectively. The 0.4% estimate reflects
the average between the maximum and minimum
conditions during the Plio-Pleistocene. As noted
418 CENOZOIC CLIMATE – OXYGEN ISOTOPE EVIDENCE
Oligocene (B33–25 Ma), planktonic foraminiferal
d
18 O values remained unusually high, averaging
À 0.5%. Beginning around the Oligocene/Miocene
boundary (B25 Ma), planktonic foraminiferal d
18 O
values began a long-term decrease, culminating in
the Pleistocene with average values of À 1.5%. In
contrast, the benthic d
18 O record permanently
changed during the middle Miocene d
18 O shift and
late Pliocene increase.
Apportioning the d
18 O changes recorded by the
benthic and planktonic foraminifera between temperature and ice volume changes requires knowledge
of, or reasonable estimates for, one of these parameters. One promising tool that may help discriminate between each effect is the Mg/Ca ratio
measured in benthic foraminifera. Initial studies
using Mg/Ca ratios confirmed the long-term temperature changes during the Cenozoic calculated
using the d
18
O record and other climate proxies. If
verified, this record implies that small ice sheets grew
during the middle and late Eocene and fluctuated in
size throughout the Oligocene to Miocene. At present, the Mg/Ca record lacks the resolution for key
intervals and still requires verification of interspecies
offsets before it can be applied unequivocally to
isolate the ice volume-induced d
18 O water component
in the foraminiferal d
18 O records. For the discussion
that follows, glacialogical evidence is used to estimate the ice volume/d
18
O water variations.
The Greenhouse World
The oldest unequivocal evidence for ice sheets on
Antarctica, ice-rafted detritus (IRD) deposited by
icebergs in the ocean, places the first large ice sheet in
the earliest Oligocene. Thus, it is reasonable to assume that ice sheets were small to absent and that
surface and deep-water temperature changes controlled much if not all the d
18
O change prior to 34
Ma. The modern Antarctic and Greenland ice sheets
lock up B2% of the total water in the world’s ocean.
If melted, these ice sheets would raise global sea level
by B70–75 m and mean ocean d
18
O water value
would decrease to À 0.9% PDB (see above). One can
then apply eqn [2] to the benthic and planktonic
foraminiferal d
18 O records to estimate deep- and
surface-ocean temperatures for the first half of the
Cenozoic (c. 65–34 Ma).
During the early to middle Paleocene, deep-water
temperatures remained close to 101C (Figure 7B).
The 1% decrease between 58 and 52 Ma translates
into a deep-water warming of 41C, reaching a high
of 141C. This is in sharp contrast to the modern
deep-water temperatures, which range between
0 and 31C. Following the peak warmth at 52 Ma, the
2% increase in benthic foraminiferal values indicates
that the deep waters cooled by 71C and were 71C by
the end of the Eocene. If small ice sheets existed
during the Paleocene and Eocene, then temperature
estimates would be on the order of 11C warmer than
those calculated for the ice-free assumption. (Some
data indicate that smaller ice sheets may have existed
on the inland parts of Antarctica during the late
Eocene. However, these were not large enough to
deposit IRD in the ocean. Therefore, their effect on
the d
18
O values of the ocean was probably less than
0.3%.)
Tropical surface water temperatures warmed from
22 to 241C, based on eqn [2], at the beginning of the
Cenozoic to 281C during the early Eocene (52 Ma;
Figure 7B). The higher estimate is similar to temperatures in the equatorial regions of the modern
oceans. Planktonic foraminiferal d
18 O values recorded a long-term increase of by 2% ( À 2.5 to
À 0.5%) through the remainder of the Eocene. Just
prior to the Eocene/Oligocene boundary, tropical
surface water temperatures were B211C, ending the
long-term tropical cooling of 71C from 52 to 34 Ma.
The Ice House World of the Last 33
Million Years
As mentioned above, southern ocean cores contain
IRD at and above the Eocene/Oligocene boundary.
Widely distributed IRD and glacial tills on parts of
the Antarctic continental margin representing the
Oligocene to Recent mark the onset of large ice
sheets. Whether these sediments represent persistent
or periodic ice cover is uncertain. At least some ice
was present on Antarctica during the Oligocene to
early Miocene. The Antarctic ice sheet has been a
fixture since the middle Miocene (B15 Ma). Our
record of Northern Hemisphere ice sheets suggests
that they were small or nonexistent prior to the late
Pliocene. For the purpose of estimating surface and
deep temperatures, an ice volume estimate slightly
lower than the modern will be applied for the interval that spans from the Oligocene into the middle
Miocene (33–15 Ma). For the interval between 15
and 3 Ma, ice volumes were probably similar to
those of today. From 3 Ma, ice volumes ranged between the modern and LGM. Using these broad estimates for ice volumes, mean ocean d
18 O water values
for those three intervals were À 0.5, À 0.22, and
0.4% PDB, respectively. The 0.4% estimate reflects
the average between the maximum and minimum
conditions during the Plio-Pleistocene. As noted
418 CENOZOIC CLIMATE – OXYGEN ISOTOPE EVIDENCE