an effective barrier to southward-flowing warm surface waters. The development of this current during
the Cenozoic hinged on the deepening for the Tasman Rise and opening of the Drake Passage. Recent
drilling indicates that marine connections developed
across the Tasman Rise at or near the Eocene/
Oligocene boundary (33.5 Ma). Tectonic constraints
on the separation of the Drake Passage are less precise. Estimates range from 35 to 22 Ma for the
opening of this gateway. The uncertainty lies in the
tectonic complexity of the region and what constitutes an effective opening for water to flow through.
The climatic consequence of creating a circumpolar
flow was to thermally isolate Antarctica and promote the growth of the Antarctic ice sheet. As noted
above, the first large ice sheet grew at the beginning
at the Eocene/Oligocene boundary.
The most notable divergence in the d
18 O records
occurred during the middle Miocene (B15 Ma). For
the first time during the Cenozoic, the tropical surface and deep waters recorded a clear divergence in
d
18 O values, a trend that increased in magnitude and
reached a maximum in the modern ocean. Any
poleward transport of heat appears to have been
effectively severed from Antarctica by 15 Ma, promoting further cooling. On the other hand, the tropics have been warming over the past 15 My.
A combination of different factors fueled this
warming. First, less heat was being transported out
of the low- and mid-latitude regions to the high
southern latitudes. Second, the opening of the
Southern Hemisphere gateways that promoted
the formation of the circumpolar circulation led to
the destruction of the circumequatorial circulation.
The effects of the closure of the Tethys Ocean (predecessor to the Mediterranean), shoaling of the
Panamanian Isthmus (4.5–2.6 Ma), and constriction
in the Indonesian Passage (B3 Ma to present)
allowed the east-to-west flowing surface waters in
the tropics to ‘pile’ up and absorb more solar radiation. A consequence of the equatorial warming and
high-latitude cooling was an increase in the equatorto-pole temperature gradient. As the gradient
increased, winds increased, promoting the organization of the surface ocean circulation patterns that
persist today.
Some Caveats
A concern in generating marine isotope records is
that the isotopic analyses should be made on the
same species. This is important because d
18 O calcite
values can vary among the different species of organisms. Coexisting taxa of benthic foraminifera
record d
18 O values that can differ by as much as 1%.
In planktonic foraminifera, variations between species can be as great as 1.5%. For both the planktonic
and benthic foraminifera, interspecific differences are
as large as the glacial–interglacial signal. These
interspecific d
18
O calcite variations are often ascribed
to a vital effect or kinetic fractionation of the oxygen
isotopes within the organism. However, some of the
differences in the planktonic taxa results from different seasonal or depth habitats and therefore provides important information about properties in the
upper part of the water column. It is noteworthy that
the first d
18 O syntheses were based on mixed species
analyses and yet basic features captured in these
curves still persist today. This attests to the robustness of these records and method for reconstructing
climate changes in the ocean.
The high-frequency signal that dominates the late
Pliocene to Pleistocene records is also present in the
Miocene and Oligocene intervals. The cloud of
points about the mean shown in Figure 7 reflects
records that were sampled at a resolution sufficient
to document the high frequency signal. For the
interval between 35 and 1 Ma, the benthic foraminiferal d
18 O record has a 40 000 year frequency
superimposed on the long-term means that are
represented by the smoothed line. The origin of the
40 000 year cycles lies in variations in the tilt of the
earth’s axis that influences the amount of solar radiation received in the high latitudes. This insolation
signal is transmitted to the deep ocean because the
high latitudes were the source regions for deep
waters during much, if not all, of the Cenozoic. The
record prior to 35 Ma is unclear with regard to the
presence or absence of 40 000 year cycles.
See also
Oxygen Isotopes in the Ocean.
Further Reading
Craig H (1957) Isotopic standards for carbon and oxygen
and correction factors for mass spectrometric analysis
of carbon dioxide. Geochemica et Cosmochemica Acta
12: 133--149.
Craig H (1965) The measurement of oxygen isotope
paleotemperatures. In: Tongiorgi E (ed.) Stable Isotopes
in Oceanographic Studies and Paleotemperatures,
pp. 161--182. Spoleto: Consiglio Nazionale delle
Ricerche, Laboratorio di Geologica Nucleare, Pisa.
Craig H and Gordon LI (1965) Deuterium and oxygen-18
variations in the oceans and marine atmosphere.
In: Tongiorgi E (ed.) Stable Isotopes in Oceanographic
CENOZOIC CLIMATE – OXYGEN ISOTOPE EVIDENCE 421
the Cenozoic hinged on the deepening for the Tasman Rise and opening of the Drake Passage. Recent
drilling indicates that marine connections developed
across the Tasman Rise at or near the Eocene/
Oligocene boundary (33.5 Ma). Tectonic constraints
on the separation of the Drake Passage are less precise. Estimates range from 35 to 22 Ma for the
opening of this gateway. The uncertainty lies in the
tectonic complexity of the region and what constitutes an effective opening for water to flow through.
The climatic consequence of creating a circumpolar
flow was to thermally isolate Antarctica and promote the growth of the Antarctic ice sheet. As noted
above, the first large ice sheet grew at the beginning
at the Eocene/Oligocene boundary.
The most notable divergence in the d
18 O records
occurred during the middle Miocene (B15 Ma). For
the first time during the Cenozoic, the tropical surface and deep waters recorded a clear divergence in
d
18 O values, a trend that increased in magnitude and
reached a maximum in the modern ocean. Any
poleward transport of heat appears to have been
effectively severed from Antarctica by 15 Ma, promoting further cooling. On the other hand, the tropics have been warming over the past 15 My.
A combination of different factors fueled this
warming. First, less heat was being transported out
of the low- and mid-latitude regions to the high
southern latitudes. Second, the opening of the
Southern Hemisphere gateways that promoted
the formation of the circumpolar circulation led to
the destruction of the circumequatorial circulation.
The effects of the closure of the Tethys Ocean (predecessor to the Mediterranean), shoaling of the
Panamanian Isthmus (4.5–2.6 Ma), and constriction
in the Indonesian Passage (B3 Ma to present)
allowed the east-to-west flowing surface waters in
the tropics to ‘pile’ up and absorb more solar radiation. A consequence of the equatorial warming and
high-latitude cooling was an increase in the equatorto-pole temperature gradient. As the gradient
increased, winds increased, promoting the organization of the surface ocean circulation patterns that
persist today.
Some Caveats
A concern in generating marine isotope records is
that the isotopic analyses should be made on the
same species. This is important because d
18 O calcite
values can vary among the different species of organisms. Coexisting taxa of benthic foraminifera
record d
18 O values that can differ by as much as 1%.
In planktonic foraminifera, variations between species can be as great as 1.5%. For both the planktonic
and benthic foraminifera, interspecific differences are
as large as the glacial–interglacial signal. These
interspecific d
18
O calcite variations are often ascribed
to a vital effect or kinetic fractionation of the oxygen
isotopes within the organism. However, some of the
differences in the planktonic taxa results from different seasonal or depth habitats and therefore provides important information about properties in the
upper part of the water column. It is noteworthy that
the first d
18 O syntheses were based on mixed species
analyses and yet basic features captured in these
curves still persist today. This attests to the robustness of these records and method for reconstructing
climate changes in the ocean.
The high-frequency signal that dominates the late
Pliocene to Pleistocene records is also present in the
Miocene and Oligocene intervals. The cloud of
points about the mean shown in Figure 7 reflects
records that were sampled at a resolution sufficient
to document the high frequency signal. For the
interval between 35 and 1 Ma, the benthic foraminiferal d
18 O record has a 40 000 year frequency
superimposed on the long-term means that are
represented by the smoothed line. The origin of the
40 000 year cycles lies in variations in the tilt of the
earth’s axis that influences the amount of solar radiation received in the high latitudes. This insolation
signal is transmitted to the deep ocean because the
high latitudes were the source regions for deep
waters during much, if not all, of the Cenozoic. The
record prior to 35 Ma is unclear with regard to the
presence or absence of 40 000 year cycles.
See also
Oxygen Isotopes in the Ocean.
Further Reading
Craig H (1957) Isotopic standards for carbon and oxygen
and correction factors for mass spectrometric analysis
of carbon dioxide. Geochemica et Cosmochemica Acta
12: 133--149.
Craig H (1965) The measurement of oxygen isotope
paleotemperatures. In: Tongiorgi E (ed.) Stable Isotopes
in Oceanographic Studies and Paleotemperatures,
pp. 161--182. Spoleto: Consiglio Nazionale delle
Ricerche, Laboratorio di Geologica Nucleare, Pisa.
Craig H and Gordon LI (1965) Deuterium and oxygen-18
variations in the oceans and marine atmosphere.
In: Tongiorgi E (ed.) Stable Isotopes in Oceanographic
CENOZOIC CLIMATE – OXYGEN ISOTOPE EVIDENCE 421