Download Oxygen Isotope Stratigraphy of the Oceans

1740 PALEOCEANOGRAPHY, PHYSICAL AND CHEMICAL PROXIES/Oxygen Isotope Stratigraphy
Sigman, D. M., and Boyle, E. A. (2000). Glacial/interglacial variations in atmospheric carbon dioxide. Nature 407, 859–869.
Spero, H. J. (1998). Life history and stable isotope geochemistry of
planktonic foraminifera. In Isotope Paleobiology and
Paleoecology (R. D. Norris and R. M. Corfield, Eds.), pp. 7–
36. Paleontological Society, Pittsburg, PA.
Spero, H. J., and Lea, D. W. (2002). The cause of carbon isotope
minimum events on glacial terminations. Science 296, 522–525.
Zahn, R., Winn, K., and Sarnthein, M. (1986). Benthic foraminiferal
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Oxygen Isotope Stratigraphy of
the Oceans
F C Bassinot, LSCE Gif-sur-Yvette, France
ª
2007 Elsevier B.V. All rights reserved.
Introduction
The Pleistocene, also known as the ‘Ice Ages,’ starts
,2.6 Ma. At the beginning of the nineteenth century,
the first geological evidence of major Quaternary
glacial episodes came from observations by Louis
Agassiz of sediment deposits (moraine) and erosion
features attributed to ancient glaciers in the Jura
mountains of Western Europe. Up until about 1960,
however, most of the Ice Age puzzle remained unexplained. One theory was very promising in explaining
the Ice Ages: the astronomical theory of climate
developed by a Serbian professor of mathematics,
Milutin Milankovitch. Based on his calculations of
seasonal and latitudinal distribution of solar energy
at the surface of the Earth, Milankovitch (1930)
proposed an orbital theory of Ice Ages in which
waxing and waning of continental ice sheets are
under the control of long-term, orbitally driven
changes in summer insolation over the high latitudes
of the Northern Hemisphere. One of the most interesting aspects of this theory was that it made it possible to test predictions about climatic records such as
how many ice-age deposits geologists would find,
and when these deposits had been formed during
the past 650 kyr. During a few decades, however,
the astronomical theory of climate was largely disputed because discussions were based on fragmentary
geological records obtained on land and supported
by incomplete, and frequently incorrect, radiometric
age data. The key to unravel the mystery of the Ice
Ages would come from the oceanic seafloor covered
by continuous sedimentary sections that had
recorded hundreds of thousands of years of the
Earth climatic and oceanographic evolution.
A new branch of Earth’s Sciences emerged in the
1950s: Paleoceanography. One of its major tools for
reconstructing the climate evolution of the Earth are
stable oxygen isotopes measured from the remains of
marine calcareous organisms. This tool provides
clues about past changes in seawater temperature
and seawater isotopic composition, which is controlled by changes in waxing and waning of large
continental ice sheets and is, therefore, directly linked
to glacial/interglacial history of the Earth. This article
presents the major steps in our understanding of
oxygen isotope fluctuations in the Ocean and their
use for stratigraphic purposes.
Oxygen Stable Isotopes in Marine
Carbonate Remains, a Brief Overview
Oxygen Stable Isotopes and the d-Notation
There are three stable isotopes of oxygen in nature:
O, 17O, and 18O, with relative natural abundances of 99.76%, 0.04%, and 0.20%, respectively. Because of the higher abundances and the
greater mass difference between 16O and 18O,
research on oxygen isotopic ratios deals normally
with the 18O/16O ratio. The ratio of stable isotopes
of oxygen in carbonates is analyzed by gas mass
spectrometric determination of the mass ratios of
carbon dioxide (CO2) released during reaction of
the sample with a strong acid, and are expressed
with reference to a standard carbon dioxide of
known composition. These differences in isotope
ratios, known as !-values and given in ‰, are
calculated as follows:
16
d18 O ¼ ðð18 O=16 Osample –
18
18
O=16 Ostandard Þ=
O=16 Ostandard Þ:1000
½1%
A sample enriched in 18O relative to the standard
will show a positive !-value (with a corresponding
negative value for a sample enriched in 16O relative
to the standard). The standard commonly used in
carbonates is referred to as Pee Dee belemnite
(PDB) (a cretaceous belemnite from the Pee Dee
formation in North Carolina, USA). This standard
is not available any longer; however, various international standards have been run against PDB for
comparative purposes. Two standards are commonly used and distributed by the National
Institute of Standards and Technology (NIST) in
the USA, and the International Atomic Energy
Agency (IAEA) in Vienna. They are NBS-18 (carbonatite) and NBS-19 (limestone), (Coplen, 1988,
1994).
PALEOCEANOGRAPHY, PHYSICAL AND CHEMICAL PROXIES/Oxygen Isotope Stratigraphy
Carbonate Fractionation: The Pioneering
Work of H. Urey and S. Epstein
Harold Urey, a Nobel laureate at the University of
Chicago, pioneered the analysis of stable isotopes
of oxygen in carbonates in the late 1940s (Urey,
1947). Urey and his colleagues had found that the
carbonate shells of marine organisms from cold
water contained a higher proportion of the heavier
18
O isotope than did organisms living in warmer
water (Urey et al., 1948). In the early 1950s, pushing this promising approach a step forward, Samuel
Epstein and colleagues looked for empirical calibrations of modern mollusk shell oxygen-isotope ratios
relative to seawater temperatures. This calibration
exercise made it possible to derive equations that
could be used to estimate past temperatures from
fossilized biologic carbonate remains (Epstein et al.,
1951, 1953).
T ¼ 16:9 – 4:2&ðd18 Osample – !18 Owater Þ þ 0:13
& ð!18 Osample – !18 Owater Þ2
½2%
As is readily seen from the above formula, the
oxygen-isotope composition of marine carbonate
samples (!18Osample) is not only related to the temperature of precipitation (T, in ( C), but also to the
isotopic composition of seawater (!18Owater). In
1954, Willi Dansgaard had pointed out that evaporation/precipitation processes might act like distilleries
and concentrate particular oxygen isotopes during
the water cycle (Fig. 1). More particularly, when
snowfall built up continental ice sheets, the isotopic
fractionation process related to evaporation/precipitation mechanisms had withdrawn from the oceans
more of the lighter 16O isotope, leaving the ocean
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water enriched in the heavier 18O isotope. As a
consequence, it was hypothesized that during glaciation
periods, when the glaciers expanded, light 16O atoms of
oxygen were preferentially extracted from the sea and
stored in the ice sheets, leaving the seawater enriched in
the heavier 18O isotope. When the glaciers melted, the
stored isotopes flooded back into the ocean, returning it
to its interglacial composition.
Pleistocene Oxygen-Isotope Stratigraphy:
Early Works
Cesare Emiliani, the Father of Marine
Isotopic Stratigraphy
The early work of Urey and his colleagues had
involved studies of the relation between oxygen isotopes and temperature in recent mollusks. In the
1950s, the Italian–American geologist Cesare
Emiliani started the measurements of oxygen isotopes from calcite shells of foraminifera. The foraminifera are unicellular organisms floating in the water
column (planktonic species) or living on the seafloor
(benthic species) whose calcite tests accumulate in
oceanic sediments after their death. Analyzing tests
of planktonic foraminifera sampled down the length
of several marine sediment cores, Emiliani found a
periodic variation in the ratio of 18O/16O, and interpreted these oscillations as related to glacial–interglacial changes (Emiliani, 1955; Fig. 2A). Before
Emiliani’s work, it was thought that there had been
only four major glaciations during the Pleistocene.
Working on sediments sampled with long piston
cores, Emiliani showed that there had been many
more at least eight glaciations during the late
Pleistocene (Emiliani, 1966; Fig. 2B).
Rayleigh distillation
Precipitation
>>>δ18O
Precipitation
>δ18O
Highly depleted δ18O
18
>>δ O
Ice
Evaporation
18
δ O enrichment
δ18O depletion
through precipitation
or ice melting
Ocean
Figure 1 The hydrological cycle and its influences on the oxygen-isotope ratios. Molecules of water containing the lighter 16O isotope have
a higher vapor pressure and these molecules are preferentially enriched in the vapor phase, showing a much smaller !18O (>>!18O) than the
ocean waters from which they derive. During precipitation processes, fractionation acts in the opposite way than during evaporation, leaving
the remaining water vapor even more depleted in 18O in the clouds. This Rayleigh fractionation process in the clouds explains why the water
vapor that ultimately precipitates at low temperature to form the ice caps is extremely depleted in 18O (>>>!18O) relative to the ocean water.
1742 PALEOCEANOGRAPHY, PHYSICAL AND CHEMICAL PROXIES/Oxygen Isotope Stratigraphy
–2.5
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Figure 2 A, One of the first Upper Pleistocene, marine oxygen-isotope records obtained from planktonic foraminifera (Globigerinoides
ruber) picked along Caribbean core 179-4 (Emiliani, 1955). Emiliani converted the !18O values to sea-surface temperatures (SSTs) assuming
that 60% of the oxygen-isotope variance was related to temperature changes and only 40% to glacial/interglacial changes in ice volume. B, A
longer planktonic isotopic record obtained on the foraminifera species G. Sacculifer picked from Caribbean core P6304-9. This record
shows the eight major glaciation stages from the Upper Pleistocene (Emiliani, 1966). Isotopic stages as defined by Emiliani.
Emiliani proposed a way of numbering the marine
oxygen isotopic stages (MISs) observed in his deepsea records. This numbering scheme is still used
today. Starting from the most recent interglacial
(,warm) period – the Holocene (MIS 1) – Emiliani
attributed an odd number to light isotopic stages,
whereas the glacial (,cold) periods, characterized
by heavier !18O values, were given an even number
(Fig. 2). Later on, more subtle isotopic variations,
such as the episodes of the isotopic stage 5, were
broken into a–e substages (Shackleton, 1969).
Continental Ice-Sheet Waxing/Waning
and the Global d 18O Signal
As seen above (Eqn [2]), it was known that the !18O
measured from biologic marine carbonates reflected
two major effects: the temperature of the seawater in
which the organisms developed and the volume of
continental ice sheets (which would change the isotopic composition of seawater). Cooler temperatures
and greater ice volumes both result in more positive
18
O/16O ratios (higher !18O). Emiliani estimated that
60% of the variations he observed in deep-sea
records were due to the temperature effect, 40%
were due to the ice effect. Based on this estimation,
he concluded that equatorial and tropical ocean
surface temperatures had been several degrees cooler
during times of glaciation. However, this conclusion
was soon to be challenged.
Sir Nicolas Shackleton, from Cambridge University
(UK), compared the !18O measured in the tests of
surface-dwelling foraminifera to oxygen isotopic
ratios obtained, in the same core, from benthic foraminifera that had lived on the seafloor. If, as Cesare
Emiliani had estimated, the ratio of oxygen isotopes in
marine fossils is chiefly governed by sea temperatures
at the glacial–interglacial timescale, the benthic foraminifera would have shown much smaller deviations
than the planktonic foraminifera since the temperature of the water at the bottom of the ocean would
have likely remained unchanged, close to freezing. But
Shackleton’s results showed that the two isotopic
curves were nearly identical, indicating that the temperature was not the main effect at play and that most
of the glacial–interglacial isotopic signal was related to
changes of the continental ice-sheet volume, which in
turn controlled the 18O/16O ratio in the ocean. Instead
of having recorded major temperature changes,
Emiliani’s !18O curve had been an isotopic message
from the ancient ice sheets.
The most accurate constraints to estimate the
change of seawater !18O between the Last Glacial
Maximum (LGM) and the Holocene are provided
by the measurements of pore water !18O and by
PALEOCEANOGRAPHY, PHYSICAL AND CHEMICAL PROXIES/Oxygen Isotope Stratigraphy
high-resolution records of benthic foraminifer !18O
in the high latitudes. They show that the !18O of
seawater in the deep ocean during the LGM was
1.05 ) 0.2‰ heavier than today (see Duplessy et al.
(2002) for a review).
Because seawater is mixed rapidly by currents, any
chemical change in one part of the ocean is reflected
everywhere within a thousand years. Thus, rapidly, it
became obvious that isotopic variations could be
used as a powerful stratigraphic correlation tool.
Further efforts were devoted to test the global nature
of the isotopic signal and to date the major oscillations first observed by Emiliani.
Developing a Timescale for the
Oxygen-Isotope Stratigraphy
Absolute Dating (14C and U/Th on Coral
Terraces)
Since massive corals can grow only within a shallow
water depth, past records of coral terraces provide an
accurate record of former sea-level changes. A geochemist at Lamont Doherty Observatory (New York,
USA), Wallace S. Broecker, dated fossil coral reefs in
the Florida Keys and the Bahamas, and showed that
the sea reached a high level 120 and 80 ka ago, presumably during periods of warm climate when
melted ice sheets had released large amounts of
water to the sea. These periods of high sea levels
closely correspond to warm periods predicted by the
astronomical theory of climates of M. Milankovitch.
van Donk and Broecker (1970), working on a !18O
record from a Caribbean core, concluded that the
major cycle in the marine oxygen isotopic records
was 100 kyr in duration. Moreover, they emphasized
on the fact that the isotopic records showed a characteristic saw-tooth pattern in which periods of glacial expansion averaging about 100 kyr in length
were abruptly terminated by rapid deglaciations.
They labeled these ‘terminations’ (episodes of rapid
deglaciation)
starting with
sponding to
Last Glacial
Holocene.
using roman numbers (I, II, III. . .),
the most recent termination (I) correthe deglaciation located between the
Maximum (at about 20 ka) and the
Magnetostratigraphy and the ‘Rosetta
Stone’ for Quaternary Ice Ages
The dating of long marine sedimentary series was
greatly improved by using magnetic stratigraphy, a
technique first established in the early 1960s. This
approach is based on the fact that the Earth’s magnetic field periodically reversed in the past. By looking at the magnetic signal imprinted into rocks or
sediments, the geologist can pinpoint reversals that
had been previously accurately dated (i.e., K/Ar or
Ar/Ar dating applied to volcanic rock samples). The
magnetostratigraphy approach was applied to cores
from the Lamont’s core repository. Core V28-238,
retrieved in the eastern equatorial Pacific, was particularly promising, with the first distinct magnetic
reversal of the core corresponding to the Brunhes/
Matuyama reversal that had been previously dated
on land at 730 ka by K/Ar method.
The ‘Brunhes–Matuyama’ reversal was found to
occur just before MIS 19 (Shackleton and Opdyke,
1973; Fig. 3). Assuming a constant age-depth model
for the core, it was now possible to assign ages to
each isotope stage by interpolation. Stage 5e, the
penultimate interglacial period equivalent to the
Holocene, came out at an age of 123.5 ka. A timescale was now available for the entire period for
which Milankovitch had made his calculations.
During the decades between the 1930s and the
1960s, the astronomical theory of climate had been
largely disputed, with discussion based on fragmentary
geological records supported by incomplete and frequently incorrect radiometric age data. From the
1960s onward, the access to continuous and longer
oceanic records that revealed many more than the
four glacial periods, recognized from the terrestrial
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730 ka
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polarity
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reversed
Brunhes Normal
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400
500
600
700
800
900
Age (ka)
Figure 3 The ‘Rosetta Stone’ of the Ice Ages, oxygen-isotope record from core V28-238 tied into the magnetostratigraphic timescale.
(Modified from Shackleton and Opdyke, 1973.) This work provided the first accurate chronology of late Pleistocene climate.
1744 PALEOCEANOGRAPHY, PHYSICAL AND CHEMICAL PROXIES/Oxygen Isotope Stratigraphy
records in Europe, provided a strong support to the
astronomical theory of climate. In 1976, Hays and coauthors published a landmark paper in which they
convincingly demonstrated that all the orbital frequencies predicted by the astronomical theory of climate
could be found through spectral analysis of climatesensitive indicators from deep-sea records. One important implication of the ‘pacemaker’ discovery is that
the calculation of past insolation cycle variations could
be used as the basis for an astronomical timescale.
Astronomical Tuning: The SPECMAP
Approach
With the convincing validation of the astronomical
theory of climate by Hays et al. (1976), several paleoceanographers proposed the development of a highprecision timescale from deep-sea sedimentary
records by using orbitally related oscillations of
paleoclimatic proxies as internal ‘pacemakers’
(i.e., Morley and Hays, 1981). The so-called
astronomical (or orbital) ‘tuning’ approach for developing timescales rests upon the phase locking of orbitally related oscillations in sedimentary records to
primary orbital oscillations calculated by astronomers, namely, the precession of equinoxes (with periods of 19–23 kyr) and the obliquity of the Earth’s
axis (with a main period of 41 kyr).
To get a global, marine stratigraphic framework
on which to develop an Upper Pleistocene
astronomical timescale, Imbrie et al. (1984) selected
five planktonic foraminifera oxygen isotopic records
from low- and mid-latitudes in which globally persistent isotopic excursions could be graphically correlated. As we saw above, over the Ice Age glacial–
interglacial cycles, global seawater oxygen isotopic
excursions recorded in the foraminifera calcite reflect
the waxing and waning of continental ice sheets,
enriched in the light 16O isotope relative to seawater.
An initial, radiometrically controlled age model
was developed for each of the five isotopic records,
using few stratigraphic levels: core tops (with a
,zero age); two isotopic events in stage 2 with 14C
ages of 18 and 21 ka; the 6.0 isotopic state boundary
(termination II), taken at 127 ka; and the Brunhes–
Matuyama magnetic reversal, taken at 730 ka. The
astronomical tuning procedure applied by Imbrie
et al. (1984) iteratively modifies the initial depth–
age model to increase the coherency and phase lock
between filtered 19–23 kyr and 41 kyr oscillations of
the oxygen isotopic records, on the one hand, and the
precession and obliquity components of Earth’s orbital variations, on the other hand. Such an orbitaltuning approach assumes constant phase lags at the
primary Milankovitch frequencies between the
marine oxygen isotopic variations and Earth’s orbital
changes. Those phase lags had been determined in an
earlier paper (Imbrie and Imbrie, 1980) by adjusting
the parametrization of a linear (exponential) climatic
model predicting ice-volume variations so that the
model output would match an 130-kyr-long isotopic
record, radiometrically dated.
The five, astronomically dated isotopic records were
finally normalized and stacked, and the resulting curve
smoothed with a nine-point Gaussian filter to produce
a reference stratigraphic record (Fig. 4A). The rational
for the stacking approach is that global isotopic
changes related to ice-sheet waxing/waning would be
enhanced in the resulting composite curve, whereas
local or regional isotopic variations would tend to
cancel out. Stacking is also useful because it results in
a set of defined isotopic events that can in principle be
found in all sediment cores. These events are labeled in
Figure 4B (for the benthic series) and in Figure 4A.
Figure 5 shows the comparison of filtered precessionand obliquity-related oscillations from the stacked isotopic record with obliquity (upper panel) and precession (lower panel) orbital oscillations.
The geological timescale obtained by Imbrie and
co-workers for the Upper Pleistocene is accurate to
within 5 kyr (Imbrie et al., 1984). Since its publication, the orbitally aged isotopic stacked record has
been used in hundreds of paleoceanographic studies
as a reference curve for developing age models and
tying marine sedimentary records into a common
stratigraphic framework.
Three years after the landmark paper by Imbrie
et al. (1984), Martinson et al. developed an orbital
timescale for the last 300 ka (Martinson et al., 1987)
that they transferred to the stacked, benthic oxygenisotope stratigraphy from Pisias et al. (1984; Fig. 4B).
Martinson et al. followed four different tuning strategies. This made it possible to test the robustness and
accuracy of astronomical tuning for developing an
Upper Pleistocene timescale. The error measured by
the standard deviation about the mean of their four
chronologies has an average magnitude of 2.5 kyr.
Transferring the final chronology to the stacked isotopic record of Pisias et al. (1984) leads to additional
errors. The final chronology has an average error of
)5 kyr. Comparison of the younger portion of the
Imbrie et al.’s chronology and Martinson et al.’s
300 kyr chronology shows that ages of isotopic events
agree to within the error bars (Fig. 4).
The 300,000 year-long stacked !18O record
produced by Martinson et al. (1987) is the preferred
target record for upper Pleistocene benthic correlations. Having a higher temporal resolution, it also
contains additional, short events of global
PALEOCEANOGRAPHY, PHYSICAL AND CHEMICAL PROXIES/Oxygen Isotope Stratigraphy
1745
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Figure 4 A, The reference oxygen-isotope record established by the SPECMAP group (Spectral Mapping) by stacking five planktonic
isotopic records (Pisias et al., 1984). The age model was developed by phase-locking oxygen-isotope oscillations to orbital forcing
functions (Imbrie et al., 1984). B, The detailed oxygen-isotopic record from the last 300 kyr obtained by stacking high-resolution benthic
records and tied to orbital-forcing functions (Martinson et al., 1987). Isotopic stages are labeled according to the Pisias et al. (1984)
modification of Emiliani’s scheme.
Age (ka)
Figure 5 In solid lines, variations in obliquity (upper panel) and precession (lower panel), and the corresponding frequency components extracted by band filtering the SPECMAP !18O stack record (dashed line) over the past 780 kyr. Modified from Imbrie J, Hays JD,
Martinson DG, et al. (1984). The orbital theory of Pleistocene climate: Support from a revised chronology of the marine delta 18O record.
In: Berger A et al. (Eds.) Milankovitch and Climate, pp. 269–305. Dordrecht: Plenum Reidel.
significance that were not seen in the SPECMAP
planktonic !18O record.
Oxygen-Isotope Stratigraphy and the
Imprint of the Primary Orbital Oscillations
The Deep Sea Drilling Project (DSDP) and the subsequent Ocean Drilling Program (ODP) provided long,
deep-sea sedimentary records that made it possible to
look at climatic oscillations over the entire Pleistocene
and beyond. These long records made it rapidly
obvious that the 100 kyr oscillations only dominate
the climate of the Earth over the last ,600 kyr,
whereas the early Pleistocene variability is
dominated by 41 kyr, obliquity-related oscillations
(i.e., Ruddiman et al. (1986)). The exact timing and
1746 PALEOCEANOGRAPHY, PHYSICAL AND CHEMICAL PROXIES/Oxygen Isotope Stratigraphy
mechanisms of this shift between dominant
periodicities of the climate system are still a matter of
debate. Some studies have concluded that the transition from 41 to 100 kyr dominant oscillations has
more probably been progressive, taking place approximately in the interval 1.0–1.2 to 0.6 Ma (Ruddiman
et al., 1989; Imbrie et al., 1993; Berger et al., 1994).
However, other studies favored a rapid transition at
about 0.8–0.9 Ma (Shackleton and Opdyke, 1976;
Pisias and Moore, 1981; Bassinot et al., 1997).
Statistical detection of the mid-Pleistocene transition
in mean and variance of Pleistocene !18O records indicates that Earth’s climate system may have experienced an abrupt change around 0.9 Ma (the so-called
‘Mid-Pleistocene Revolution’) that was characterized
by an increase in ice mass and a global cooling
(Maasch, 1988; Mudelsee and Stattegger, 1997).
The dominance of the 100 kyr oscillation in
the Upper Pleistocene is a major puzzle of the
astronomical theory of climates since eccentricity variations of the Earth’s orbit lead to only minor changes
in the insolation budget (see review of this problem in
Imbrie et al. (1993)). Numerous authors have proposed that this 100 kyr oscillation may result from
nonlinear response of the climate system that would
transfer power from the envelop of the precession
oscillations (i.e., Imbrie et al. (1993)). Numerical
modeling has suggested that the existence of large
Northern Hemisphere ice sheets is an essential condition for developing the saw-tooth 100 kyr oscillations
of the late Pleistocene (i.e., Saltzman (1987), Le Treut
and Ghil (1983), and Imbrie et al. (1993)).
State-of-the-Art of the Marine
Oxygen-Isotope Stratigraphy
Improvement in the Marine Oxygen-Isotope
Stratigraphy
Orbital tuning provides a global reference standard
for dating long, continuous sediment records.
However, the approach is not without its problems.
Subsequent revisions of the 0–780 ka SPECMAP timescale showed, for instance, that several oscillations
predicted by the astronomical theory of climate (in
isotopic stages 17 and 18) were apparently missing
in the stacked record (Shackleton et al., 1990;
Bassinot et al., 1994). Their absence is related to (1)
the incompleteness of some of the isotopic records that
reached those stages and to (2) an erroneous Brunhes–
Matuyama magnetic reversal age assignment in the
age model developed at the first step of the Imbrie
et al. (1984) tuning approach. 40Ar/39Ar radiometric
dating of the Brunhes–Matuyama magnetic boundary
gives an age of about 780 ka (Baksi et al., 1992),
significantly older than the K/Ar age (730 ka) available
in the early 1980s, and used by Imbrie et al. (1984) to
constrain their initial age model. Although the time
constraint at the B/M boundary was removed midway
in Imbrie et al.’s orbital-tuning approach, it is quite
clear that the final age model suffered from this erroneous tie point.
Applying astronomical tuning strategies with a
higher degree of freedom (i.e., no initial age assignment to the B/M boundary) on carefully selected,
high-resolution isotopic records, Shackleton et al.
(1990) and Bassinot et al. (1994) obtained astronomical ages for the Brunhes–Matuyama magnetic reversal that are in good agreement with 40Ar/39Ar
radiometric ages (Fig. 6). The Shackleton et al. (1990)
record extends back to 2.5 Ma, and all the magnetic
reversal age assignments are in good accordance with
40
Ar/39Ar radiometric ages. These examples illustrate
the fact that theoretical precision of astronomical timescales (usually a few thousand years) may actually
mask potential, larger uncertainties related to the tuning strategy and/or to incompleteness of paleoclimatic
records used to provide the stratigraphic framework on
which the astronomical tuning is conducted. This
implies that a great care should be taken in selecting
isotopic records used as references.
Developing a Global, Reference Curve
In order to build an accurate stratigraphic reference
curve, a time-consuming and robust approach is to use
as many good and coherent oxygen-isotope records as
possible; this is the approach followed by Lisiecki and
Raymo (2005). These authors recently developed a
5.3 Ma stack (the ‘LR04’ stack) of benthic !18O
records from 57 globally distributed sites aligned by
an automated graphic correlation algorithm (Fig. 7).
They developed an age model for the Pliocene–
Pleistocene, obtained from tuning the !18O stack to a
simple ice model based on 21 Jun. insolation at 65( N
(Imbrie and Imbrie, 1980). Including all sources of
errors (i.e., uncertainty in the orbital solution, number
of records covering the oldest interval analyzed,
assumed response time of the ice sheets), the authors
estimated the uncertainty in the LR04 age model to be
)6 kyr from 3–1 Ma (Lower Pleistocene) and )4 kyr
from 1–0 Ma (Upper Pleistocene). The LR04 age
model, designed to minimize changes in global sedimentation rates, generally supports the SPECMAP
timescale back to 625 ka. As to date, the LR04 record
is the most robust reference for benthic oxygen isotopic stratigraphy of the Pliocene–Pleistocene.
See also: Glaciation, Causes: Milankovitch Theory and
Paleoclimate. K/Ar and Ar/Ar Dating. Quaternary
Stratigraphy: Overview.
PALEOCEANOGRAPHY, PHYSICAL AND CHEMICAL PROXIES/Oxygen Isotope Stratigraphy
δ18O (normalized; % )
δ18O (normalized; % )
–3
2 3 4
5
6
7
8
9 10
11
12
13
14
15 16
17
18 19 20
1747
21 22
–2
–1
0
1
2
Low-Latitude Stack (Bassinot et al., 1994)
–2.5
–1.5
–0.5
0.5
1.5
SPECMAP Stack (Imbrie et al., 1984)
2.5
0
100
200
300
400
Age (ka)
600
700
800
900
Magnetic
stratigraphy
Figure 6 Lower panel, the SPECMAP isotopic record (Imbrie et al., 1984). Upper panel, the Low-Latitude Stack (Bassinot et al., 1994).
The yellow and dark blue connections indicate the missing cycles from the SPECMAP Stack that lead to a revision of the orbital solution
in the astronomical timescale of the Low-Latitude Stack. This new tuning solution led to a revised age of the Brunhes–Matuyama reversal
(found at the base of isotopic stage 19), in good agreement with Ar/Ar dates. Upper panel, from Bassinot FC, Labeyrie LD, Vincent E,
Quidelleur X, Shackleton NJ, and Lancelot Y (1994) The astronomical theory of climate and the age of the Brunhes-Matuyama magnetic
reversal. Earth and Planetary Science Letters 126: 91–108.
Brunhes
1
5
Matuyama
Matuyama
11
9
31
7
15
13
17
3.5
47
37
25
21
19
43
35
29
39
33
27
δ18O normalized (% )
Olduvai
Jaramillo
49
53
41
63
55
45
51
57 59 61
23
4.0
56
3
28
4.5
4
14
8
24 26
18
5.0
2
20
10
6
12
30
60
42
32
34
36
38
40
44
48
46
54
50
58
62
52
22
Benthic δ18O stack (LR04; Lisiecki and Raymo, 2005)
16
5.5
0
200
400
600
800
1,000
1,200
1,400
1,600
1,800
Age (ka)
Figure 7 The state-of-the-art in marine oxygen-isotope stratigraphy: isotopic curve LR04 obtained by stacking 57 global benthic
isotopic records. Age model was obtained through orbital tuning (Lisiecki and Raymo, 2005).
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Oxygen Isotopic Composition
of Seawater
E J Rohling, National Oceanography Centre,
Southampton, UK
ª
2007 Elsevier B.V. All rights reserved.
Introduction
This text deals with processes that affect the ratio of
the two most common stable isotopes of oxygen in
seawater, and explores ways in which this has changed during the Quaternary. Both text and figures
draw heavily on the water-based oxygen isotope
part of a previously published essay on stable oxygen
and carbon isotopes in foraminifera (Rohling and
Cooke, 1999). In contrast to the current text, which
offers only the fundamental key references, the essay
of Rohling and Cooke (1999) contains an extensive
set of references, and would therefore be a useful first
step for further and more specialized reading.