CENOZOIC CLIMATE – OXYGEN ISOTOPE
EVIDENCE
J. D. Wright, Rutgers University, Piscataway, NJ,
USA
Copyright & 2001 Elsevier Ltd.
Discoveries of fossil remains of 50 million year old
alligators on Ellesmere Island and 30–40 million
year-old forests on Antarctica contrast sharply with
our present vision of polar climates. These are not
isolated discoveries or quirks of nature. An evergrowing body of faunal, floral, and geochemical
evidence shows that the first half of the Cenozoic
Era was much warmer than the present time.
What maintained such a warm climate and could
it be an analog for future global warming? To address these and other questions, one needs more
than a qualitative estimate of planetary temperatures. Quantitative temperature estimates (both
magnitudes and rates of change) are required to
depict how the Earth’s climate has changed through
time. One of the most powerful tools used to reconstruct past climates during the Cenozoic (the last
65 million years of Earth’s history) is the analysis of
oxygen isotopes in the fossil shells of marine organisms. The calcium carbonate shells of the protist
foraminifera are the most often analyzed organisms
because the different species are distributed
throughout surface (planktonic) and deep (benthic)
marine environments.
Oxygen Isotope Systematics
The stable isotopes of oxygen used in paleooceanographic reconstructions are
16 O and
18 O. There are
about 500
16 O atoms for every
18
O atom in the
ocean/atmosphere environment. During the 1940s,
Harold Urey at the University of Chicago predicted
that the
18
O/
16
O ratio in calcite (CaCO 3 ) should
vary as a function of the temperature at which the
mineral precipitated. His prediction spurred on experiments by himself and others at the University of
Chicago who measured
18
O/
16 O ratios in CaCO 3
precipitated in a wide range of temperatures, leading
to the use of stable oxygen isotope measurements as
a paleothermometer.
To determine oxygen isotopic ratios, unknown
18 O/
16 O ratios are compared to the known
18
O/
16 O
ratio of a standard. The resulting values are
expressed in delta notation, d
18 O, where:
d
18 O ¼
18 O=
16 O sample À
18 O=
16 O standard
18 O=
16 O standard
 1000 ½1
Carbonate samples are reacted in phosphoric acid to
produce CO 2 . To analyze water samples, CO 2 gas is
equilibrated with water samples at a constant temperature. Given time, the CO 2 will isotopically
equilibriate with the water. For both the carbonate
and water samples, the isotopic composition of CO 2
gas is compared with CO 2 gas of known isotopic
composition. There are two standards for reporting
d
18 O values. For carbonate samples, the reference
standard is PDB, which was a crushed belemnite
shell (Belemnitella americana) from the Peedee formation of Cretaceous age in South Carolina. The
original PDB material has been exhausted, but other
standards have been calibrated to PDB and are used
as an intermediate reference standard through which
a PDB value can be calculated. For measuring the
isotopic composition of water samples, Standard
Mean Ocean Water (SMOW) is used as the reference. The SMOW reference was developed so that its
d
18 O water value is 0.0% (parts per thousand) and
approximates the average oxygen isotopic composition of the whole ocean. Deep ocean d
18 O water
values are close to the SMOW value, ranging from
À 0.2 to 0.2%. In contrast, surface ocean d
18 O water
values exhibit a much greater variability, varying
between À 0.5 and þ 1.5%.
Oxygen Isotope Paleothermometry
Early studies into the natural variations in oxygen
isotopes led to the development of a paleotemperature
equation. The temperature during the precipitation of
calcite can be estimated by measuring the d
18
O value
in calcite-secreting organisms (foraminifera, corals,
and mollusks) and the value of the water in which the
organisms live. The various paleotemperature equations all follow the original proposed by Sam Epstein
and his colleagues (University of Chicago):
T ¼ 16:5 À 4:3 Â d
18 O calcite À d
18 O water
À
Á
þ0:14 Â d
18 O calcite À d
18 O water
À
Á 2
½2
where T and d
18
O water are the temperature (1C) and
oxygen isotope value of the water in which the
411
EVIDENCE
J. D. Wright, Rutgers University, Piscataway, NJ,
USA
Copyright & 2001 Elsevier Ltd.
Discoveries of fossil remains of 50 million year old
alligators on Ellesmere Island and 30–40 million
year-old forests on Antarctica contrast sharply with
our present vision of polar climates. These are not
isolated discoveries or quirks of nature. An evergrowing body of faunal, floral, and geochemical
evidence shows that the first half of the Cenozoic
Era was much warmer than the present time.
What maintained such a warm climate and could
it be an analog for future global warming? To address these and other questions, one needs more
than a qualitative estimate of planetary temperatures. Quantitative temperature estimates (both
magnitudes and rates of change) are required to
depict how the Earth’s climate has changed through
time. One of the most powerful tools used to reconstruct past climates during the Cenozoic (the last
65 million years of Earth’s history) is the analysis of
oxygen isotopes in the fossil shells of marine organisms. The calcium carbonate shells of the protist
foraminifera are the most often analyzed organisms
because the different species are distributed
throughout surface (planktonic) and deep (benthic)
marine environments.
Oxygen Isotope Systematics
The stable isotopes of oxygen used in paleooceanographic reconstructions are
16 O and
18 O. There are
about 500
16 O atoms for every
18
O atom in the
ocean/atmosphere environment. During the 1940s,
Harold Urey at the University of Chicago predicted
that the
18
O/
16
O ratio in calcite (CaCO 3 ) should
vary as a function of the temperature at which the
mineral precipitated. His prediction spurred on experiments by himself and others at the University of
Chicago who measured
18
O/
16 O ratios in CaCO 3
precipitated in a wide range of temperatures, leading
to the use of stable oxygen isotope measurements as
a paleothermometer.
To determine oxygen isotopic ratios, unknown
18 O/
16 O ratios are compared to the known
18
O/
16 O
ratio of a standard. The resulting values are
expressed in delta notation, d
18 O, where:
d
18 O ¼
18 O=
16 O sample À
18 O=
16 O standard
18 O=
16 O standard
 1000 ½1
Carbonate samples are reacted in phosphoric acid to
produce CO 2 . To analyze water samples, CO 2 gas is
equilibrated with water samples at a constant temperature. Given time, the CO 2 will isotopically
equilibriate with the water. For both the carbonate
and water samples, the isotopic composition of CO 2
gas is compared with CO 2 gas of known isotopic
composition. There are two standards for reporting
d
18 O values. For carbonate samples, the reference
standard is PDB, which was a crushed belemnite
shell (Belemnitella americana) from the Peedee formation of Cretaceous age in South Carolina. The
original PDB material has been exhausted, but other
standards have been calibrated to PDB and are used
as an intermediate reference standard through which
a PDB value can be calculated. For measuring the
isotopic composition of water samples, Standard
Mean Ocean Water (SMOW) is used as the reference. The SMOW reference was developed so that its
d
18 O water value is 0.0% (parts per thousand) and
approximates the average oxygen isotopic composition of the whole ocean. Deep ocean d
18 O water
values are close to the SMOW value, ranging from
À 0.2 to 0.2%. In contrast, surface ocean d
18 O water
values exhibit a much greater variability, varying
between À 0.5 and þ 1.5%.
Oxygen Isotope Paleothermometry
Early studies into the natural variations in oxygen
isotopes led to the development of a paleotemperature
equation. The temperature during the precipitation of
calcite can be estimated by measuring the d
18
O value
in calcite-secreting organisms (foraminifera, corals,
and mollusks) and the value of the water in which the
organisms live. The various paleotemperature equations all follow the original proposed by Sam Epstein
and his colleagues (University of Chicago):
T ¼ 16:5 À 4:3 Â d
18 O calcite À d
18 O water
À
Á
þ0:14 Â d
18 O calcite À d
18 O water
À
Á 2
½2
where T and d
18
O water are the temperature (1C) and
oxygen isotope value of the water in which the
411