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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 !13C and accumulation rates of organic carbon: Uvigerina peregrina group and Cibicidoides wuellerstorfi. Paleoceanography 1, 27–42. 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 1741 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 1 2 3 4 5 6 7 8 9 10 11 12 13 δ18O (% ) –2.0 –1.5 –1.0 –0.5 Core A179-4 0 100 0 200 (A) 300 400 500 600 700 Depth below top (cm) –2.0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 δ18O (% ) –1.5 –1.0 –0.5 0 0.5 1.0 Core P6304-9 0 200 400 (B) 600 800 1000 1200 1400 Depth below top (cm) 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 –2.25 δ18O (% ) 1743 730 ka –1.75 –1.25 Magnetic polarity –0.75 Matuyama reversed Brunhes Normal 0 100 200 300 400 500 600 700 800 900 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 2.5 δ18O normalized (% ) 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 1.5 18 0.5 19 20 –0.5 –1.5 Age (ka) –2.5 600 700 800 8.5 8.4 7.5 8.02 8.0 7.4 7.2 7.1 7.3 6.5 500 7.0 6.6 6.4 6.42 6.2 6.41 400 6.3 5.53 5.51 6.0 5.4 5.33 300 5.31 5.1 5.2 4.0 4.23 5.0 4.22 3.3 200 3.31 3.1 3.13 3.0 2.2 1.1 2.0 (A) 100 5.3 0 δ18O normalized (% ) 1.0 (B) 0.5 0 –0.5 –1.0 0 100 200 300 0.5 0.3 23.5 0.1 22.5 –0.1 –0.3 Precession (rad) –0.5 0.04 0.5 0.02 0.0 0.00 –0.02 –0.5 –0.04 –0.06 0 100 200 300 400 500 600 700 –1.0 800 Filtered δ18O (% ) Obliquity (°) 0.7 24.5 Filtered δ18O (% ) 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). References Baksi, A. K., Hsu, V., McWilliams, M. O., and Farrer, E. (1992). 40 Ar/39Ar dating of the Brunhes–Matuyama geomagnetic field reversal. 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An alternative astronomical calibration of the lower Pleistocene timescale based on ODP Site 677. Transactions of the Royal Society of Edinburgh: Earth Sciences 81, 251–261. Urey, H. C. (1947). The thermodynamic properties of isotopic substances. Journal of the Chemical Society (London)562–581. Urey, H. C., Epstein, S., McKinney, C., and McCrea, J. (1948). Method for measurement of paleotemperatures. Bulletin of the Geological Society of America 59, 1359–1360. 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.