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19
Cenozoic Variability of Oxygen Isotopes in Benthic Foraminifera

THEODORE C.MOORE, JR.

Exxon Production Research Company

NICKLAS G.PISIAS

Oregon State University

L.D.KEIGWIN, JR.

Woods Hole Oceanographic Institution

INTRODUCTION

The long-term (greater than 106 yr) character of the Earth’s climate appears to exhibit distinct shifts from one state to the next. The succession of such states can be thought of as an evolutionary process with the average characteristics of each successive climatic state fundamentally different from any previous state. Each state may differ in terms of mean condition, in the amount of oscillation around the mean condition, and in the distribution of the amplitude of oscillation as a function of frequency. This long-term evolution of climate appears to be associated with telluric changes (i.e., changes in the geography and topography that form the boundaries to the fluid spheres). The rate of climatic change depends on the nature of the telluric effects. For example, the opening of an ocean gateway to deep and surface flow (such as passage between Antarctica and Australia) may have had a sudden and dramatic effect on average oceanic conditions, whereas the gradual opening of the Atlantic or closing of the Tethyan seaway may have caused longer term shifts in climatic conditions.

The geologic record of the deep sea affords us the opportunity to study the character of global oceanographic conditions over the past 100 million years (Ma) to define the steps in the evolution that have lead from the rather equitable climates of the Cretaceous to the ice ages of the last few million years and to relate these evolutionary steps to the changes in the telluric boundary conditions that are likely to have caused them. Furthermore, we should be able to characterize different parts of the climate system during each stage of this evolution. By studying the way the surface ocean, the deep ocean, the atmosphere, and the cryosphere have changed during each evolutionary stage, we will gain insights into the mechanisms that give rise to long-term climatic change. In addition, the investigation of the climate system under a variety of boundary conditions should give us a better fundamental understanding of how the different elements of this system can, and do, interact.

PREVIOUS WORK

Before such research can proceed, quantitative data on each element of the climate system must be acquired and studied. One of the most extensive quantitative data bases that now exists for the Cenozoic is the record of change in oxygen isotopes of benthic foraminifera (see Table 19.1). These data have been compiled by many investigators (Douglas and Savin, 1973,



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Climate in Earth History: Studies in Geophysics 19 Cenozoic Variability of Oxygen Isotopes in Benthic Foraminifera THEODORE C.MOORE, JR. Exxon Production Research Company NICKLAS G.PISIAS Oregon State University L.D.KEIGWIN, JR. Woods Hole Oceanographic Institution INTRODUCTION The long-term (greater than 106 yr) character of the Earth’s climate appears to exhibit distinct shifts from one state to the next. The succession of such states can be thought of as an evolutionary process with the average characteristics of each successive climatic state fundamentally different from any previous state. Each state may differ in terms of mean condition, in the amount of oscillation around the mean condition, and in the distribution of the amplitude of oscillation as a function of frequency. This long-term evolution of climate appears to be associated with telluric changes (i.e., changes in the geography and topography that form the boundaries to the fluid spheres). The rate of climatic change depends on the nature of the telluric effects. For example, the opening of an ocean gateway to deep and surface flow (such as passage between Antarctica and Australia) may have had a sudden and dramatic effect on average oceanic conditions, whereas the gradual opening of the Atlantic or closing of the Tethyan seaway may have caused longer term shifts in climatic conditions. The geologic record of the deep sea affords us the opportunity to study the character of global oceanographic conditions over the past 100 million years (Ma) to define the steps in the evolution that have lead from the rather equitable climates of the Cretaceous to the ice ages of the last few million years and to relate these evolutionary steps to the changes in the telluric boundary conditions that are likely to have caused them. Furthermore, we should be able to characterize different parts of the climate system during each stage of this evolution. By studying the way the surface ocean, the deep ocean, the atmosphere, and the cryosphere have changed during each evolutionary stage, we will gain insights into the mechanisms that give rise to long-term climatic change. In addition, the investigation of the climate system under a variety of boundary conditions should give us a better fundamental understanding of how the different elements of this system can, and do, interact. PREVIOUS WORK Before such research can proceed, quantitative data on each element of the climate system must be acquired and studied. One of the most extensive quantitative data bases that now exists for the Cenozoic is the record of change in oxygen isotopes of benthic foraminifera (see Table 19.1). These data have been compiled by many investigators (Douglas and Savin, 1973,

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Climate in Earth History: Studies in Geophysics 1975; Savin et al., 1975; Shackleton and Kennett, 1975a, 1975b). More recent work by Cenozoic Paleo-Oceanography Research Project (CENOP) workers has greatly added to this data base, particularly in the Miocene. Oxygen isotopes measured on the shells of benthic foraminifers have given us a good representation (Figure 19.1) of the long-term changes that have occurred in the mean conditions of the deep ocean. The deep ocean contains more than 90 percent of the water on the Earth’s surface and represents a part of the climate system that is important to the storage and transport of heat. It is a rather slow-moving part of the system, with a response time on the order of 103 yr, intermediate between the rapidly responding atmosphere and surface ocean and the much more slowly changing cryosphere and lithosphere. Compared with that of the surface waters, the isotopic composition of the modern deep ocean is relatively homogenous (Craig and Gordon, 1965); thus, an isotopic record of change in the deep ocean from almost any location is likely to give a picture of change in a large and important part of the climate system. If it is assumed that the shells of benthic foraminifera are deposited in isotopic equilibrium (or that any vital effects can be taken into account), then changes in the oxygen isotopic ratio in the carbonate tests of the deep benthic fauna indicate changes in either the temperature of the bottom waters (with each 1°C equivalent to roughly 0.26 ‰ change in the 18O/16O ratio) or in the isotopic composition of deep waters. Changes in the isotopic composition of the deep ocean would most likely be caused by the transfer of a large amount of isotopically light water from the oceans to continental glaciers (δ18O change of 0.1 ‰ is roughly equivalent to a 10-m glacial sealevel change); however, changes in the mode of formation of deep water that involved a significant change in their salinity would also affect their isotopic composition [with about a 0.1 ‰ change in δ18O for every 0.2 ‰ change in salinity (Craig and Gordon, 1965)]. Studies of the long-term record of the isotopic record of foraminifera, together with other geologic and geophysical studies, suggest that in the last 100 Ma there were two times when major continental ice caps were formed and extended into the sea: in the Middle Miocene, marking the buildup of the Antarctic ice cap about 14 Ma ago (Shackleton and Kennett, 1975a, 1975b), and in the late Pliocene, marking the formation of continental glaciers in the northern hemisphere about 3 Ma ago (Shackleton and Opdyke, 1977). If the estimates of the effect of ice volume on the isotopic composition of seawater are taken into account, and if it is assumed that changes in salinity have been small, then the record of the long-term changes in the oxygen isotopes can be interpreted as an oceanic temperature record (Figure 19.1). This record suggests that the deep waters have cooled by about 10–13°C in the last 60 Ma. Surface waters followed this trend to the mid-Miocene and then leveled off or warmed slightly {Douglas and Woodruff, 1981). This overall cooling of deep waters is neither monotonic nor gradual. Short reversals in the trend occur, and the major portion of the cooling appears to occur as distinct steps in the record (e.g., in the mid-Eocene, at the Eocene-Oligocene boundary, in the mid-Miocene, and in the Pliocene (Figure 19.1). These sharp drops in the isotopic record are thought to be associated with evolutionary changes in the climate system that give rise to shifts in the mean conditions. It remains to be seen whether other proxy records and oceanic and climatic changes exhibit the same shifts in their records and whether additional evolutionary steps can be identified in these records. The most recent evolutionary stage as defined in Figure 19.1 is the Quaternary. An important characteristic of the Quaternary climate has been the large degree of variability around the mean climatic state. Most of the Quaternary variability in the benthic oxygen isotope signal is thought to be associated with changes in continental ice volume (Shackleton, 1967; Shackleton and Opdyke, 1973). The variability of both the oxygen isotopes (Shackleton and Opdyke, 1976; Pisias and Moore, 1981) and the planktonic fauna (Ruddiman, 1971; Briskin and Berggren, 1975) changed through the Quaternary and these changes appear to involve both the amplitude and frequency of oscillation. Spectral analyses of one 2-Ma-long record of oxygen isotopes (measured on a planktonic species of foraminifera) indicates that this record can be divided into at least three intervals, each having progressively more variance associated with progressively longer periods of oscillation (Pisias and Moore, 1981). These changes in the spectral character of oxygen isotope records may also indicate evolutionary changes in the climate system. In this example, these changes are thought to result from changing mechanisms of ice-cap growth and decay and may indicate the effects of extensive glacial erosion of continental areas (Pisias and Moore, 1981). Similar spectral studies of Tertiary records have yet to be undertaken, primarily because (a) few long, relatively undisturbed marine sections have been recovered, and (b) establishing a sufficiently accurate time scale for a detailed spectral analysis is a difficult task. Although it may not be possible yet to investigate the spectral character (i.e., the distribution of variance as a function of frequency of oscillation) of Tertiary oxygen isotopic records, the total variance of such records and how this variance has changed with time and place can be studied. It is sometimes assumed that the warmer climes of Tertiary and Cretaceous times were more equable than at present, that is, they were less variable. However, little work has been done on the short-term variability of Cenozoic climate. Certainly before the buildup of continental glaciers one might expect to see less variability in the oxygen isotope signal. But was there less oceanographic variability during the Eocene than during the Oligocene or Miocene? Did the variability of the benthic oxygen isotope record increase as the deep-ocean temperature cooled? How did variability in this record change with evolution of the climate system, and what degree of variability is associated with each evolutionary step? Such questions are addressed here in hopes of better defining the true nature of climatic and oceanographic variability through the Cenozoic. METHODS An accurate estimate of the total variance in a data set does not require as long a record as a spectral analysis. It does not require that the time scale be known accurately, nor does it re-

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Climate in Earth History: Studies in Geophysics TABLE 19.1 Estimated Variance (*σ2) with Linear Trend Removeda Age Site Ocean Water Depth (m) N Species Group *σ2 Data Source L. Quaternary V19–28 P 2720 142 Uviger. 0.1682 Shackleton, 1977   V19–29 P 3157 168 Uviger. 0.1812 Shackleton, 1977   Y6910–2 P 2615 203 Uviger. 0.1422 Shackleton, 1977   157 P 2591 9 Uviger. 0.1732 Keigwin, 1979a   E67–135 A 725 31 Uviger. 0.1842 Keigwin, 1979a   397 A 2900 133 Uviger. 0.2822 Shackleton and Cita, 1979 E. Quaternary 284 P 1068 7 Uviger. 0.1192 Kennett et al., 1979   310 P 3516 21 Uviger. 0.0872 Keigwin, 1979a   E67–135 A 725 32 Uviger. 0.1002 Keigwin, 1979b   397 A 2900 15 Uviger. 0.1132 Shackleton and Cita, 1979 L. Pliocene V28–179 P 4490 36 G. subgl. 0.0642 Shackleton and Opdyke, 1977   157 P 2300 16 Uviger. 0.0522 Keigwin, 1979a   206 P 3110 6 Uviger. 0.0122 Bender (CENOP unpubl.)   207 P 1360 9 Uviger. 0.0182 Bender (CENOP unpubl.)   310 P 3510 23 Uviger. 0.0602 Keigwin, 1979a   503 P 3500 7 P. wuell. 0.0552 Keigwin (CENOP unpubl.)   281 S(P) 1570 6 Uviger. 0.0332 Bender (CENOP unpubl.)   E67–135 A 720 25 Uviger. 0.0682 Keigwin, 1979b   397 A 2890 25 Uviger. 0.1222 Shackleton and Cita, 1979   502 A 3040 9 P. wuell. 0.1462 Keigwin (CENOP unpubl.) E. Pliocene V28–179 P 4480 36 G. subgl. 0.018 Shackleton and Opdyke, 1977   62.1 P 2490 10 G. subgl. 0.007 Keigwin et al., 1979   83A P 3410 14 Uviger. 0.008 Keigwin et al., 1979   84 P 2700 6 Uviger. 0.056 Keigwin et al., 1979   158 P 1710 11 Uviger. 0.050 Keigwin, 1979a   206 P 3100 8 Uviger. 0.006 Keigwin et al., 1979   207A P 1340 7 Uviger. 0.010 Keigwin et al., 1979   208 P 1520 19 Uviger.; G. subgl. 0.0281 Bender (CENOP unpubl.); Keigwin et al., 1979   284 P 1020 19 Uviger. 0.015 Kennett et al., 1979   310 P 3500 11 Uviger. 0.017 Keigwin, 1979a   503 P 3450 21 C. kull. 0.011 Keigwin (CENOP unpubl.)   E67–135 A 710 24 Uviger. 0.025 Keigwin, 1979b   297 A 2890 20 Uviger. 0.090 Shackleton and Cita, 1979   502 A 3030 17 P. wuell. 0.035 Keigwin (CENOP unpubl.) L. Miocene (postcarbon shift) 77B P 4250 12 M.B. 0.090 Savin and Weh, 1981 158 P 1570 27 Uviger.; G. subgl. 0.0301 Keigwin, 1979a   207A P 1320 28 Uviger. 0.034 Bender (CENOP unpubl.)   208 P 1480 12 Uviger. 0.058 Bender (CENOP unpubl).   284 P 990 13 Uviger. 0.015 Kennett et al., 1979   289 P 2190 12 P. wuell. 0.025 Woodruff et al., 1981   292 P 2800 27 Oridos. 0.046 The Benedum Lab., Brown U. (CENOP unpubl.)   296* P 2750 12 Oridos. 0.422 The Benedum Lab., Brown U. (CENOP unpubl.)   310 P 3430 12 Uviger. 0.015 Keigwin, 1979a   503 P 3270 7 C. kull. 0.014 Keigwin (CENOP unpubl.)   278 S(P) 3530 6 Cibicid. 0.034 Bender (CENOP unpubl.)   281 S(P) 1500 6 Uviger. 0.014 Bender (CENOP unpubl.)   329 S(A) 1430 33 M.B. 0.114 Savin et al. (CENOP unpubl.)   238 I 2630 19 Oridos.; P. wuell. 0.0561 Vincent et al., 1980   357 A 2060 8 Oridos. 0.062 The Benedum Lab., Brown U. (CENOP unpubl.)   397 A 2860 18 M.B. 0.055 Shackleton and Cita, 1979

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Climate in Earth History: Studies in Geophysics Age Site Ocean Water Depth (m) N Species Group *σ2 Data Source   408 A 1190 6 Oridos. 0.106 The Benedum Lab., Brown U. (CENOP unpubl.)   502 A 3020 21 P. wuell. 0.035 Keigwin (CENOP unpubl.) L. Miocene (precarbon shift) 77B P 4210 16 M.B. 0.149 Savin et al., 1981 158 P 1410 9 Uviger. 0.057 Keigwin, 1979a   206 P 3100 12 Uviger. 0.042 Bender (CENOP unpubl.)   207 P 1300 13 Uviger. 0.018 Bender (CENOP unpubl.)   208 P 1450 7 Uviger. 0.009 Bender (CENOP unpubl.)   289 P 2180 11 P. wuell. 0.046 Woodruff et al., 1981   296 P 2740 9 Oridos. 0.054 The Benedum Lab., Brown U. (CENOP unpubl.)   310 P 3420 9 Oridos. 0.009 Keigwin, 1979a   503 P 3090 7 P. wuell. 0.006 Keigwin (CENOP unpubl.)   278 S(P) 3510 6 Cibicid. 0.045 Bender (CENOP unpubl.)   281 S(P) 1490 26 G. subglob. 0.097 Loutit (CENOP unpubl.)   329 S(A) 1420 6 M.B. 0.276 Savin et al., (CENOP unpubl.)   238 I 2640 38 P. wuell., Oridos. 0.0291 Vincent et al., 1980   357 A 2050 8 Oridos. 0.095 The Benedum Lab., Brown U. (CENOP unpubl.)   397 A 2850 28 M.B. 0.077 Shackleton and Cita, 1979   397 A 2850 11 P. wuell. 0.011 Bender (CENOP, unpubl.)   408 A 1170 10 Oridos. 0.209 The Benedum Lab., Brown U. (CENOP unpubl.)   502 A 3000 18 P. wuell. 0.020 Keigwin (CENOP unpubl.) L. Mid. Miocene 77B P 4180 27 Cibicid.; G. subgl. 0.0391 Savin et al., 1981; Kennett and Keigwin (CENOP unpubl.)   206 P 3060 12 Oridos. 0.019 Bender (CENOP unpubl.)   206 P 3060 11 P. wuell. 0.065 Bender (CENOP unpubl.)   207A P 1240 7 Uviger. 0.051 Bender (CENOP unpubl.)   208 P 1420 11 P. wuell. 0.013 Bender (CENOP unpubl.)   289 P 2160 36 Cibicid. 0.053 Woodruff et al., 1981   310 P 3380 5 Oridos. 0.036 Keigwin, 1979a   281 S(P) 1400 17 M.B.; Uviger., G. subgl. 0.0041 Shackleton and Kennett, 1975a; Loutit (CENOP unpubl.) Mid. Miocene (trans.) 77B P 4090 5 G. subgl. 0.086 Kennett and Keigwin (CENOP unpubl.)   77B P 4090 23 Cibicid.; C. kull. 0.0241 Kennett and Keigwin (CENOP unpubl.)   289 P 2140 37 Cibicid. 0.099 Woodruff et al., 1981 E. Mid. Miocene 55 P 2750 6 M.B. 0.048 Savin et al., 1975   71 P 4270 6 Cibicid. 0.080 Savin et al., 1981   77B P 4080 17 Cibicid. 0.037 Kennett and Keigwin (CENOP unpubl.)   206 P 3040 18 P. wuell. 0.0241 Bender (CENOP unpubl.)   289 P 2100 26 Cibicid. 0.033 Woodruff et al., 1981   281 S(P) 1380 24 Uviger. 0.041 Loutit (CENOP unpubl.) E. Miocene 71 P 4260 37 Oridos.; Cibicid. 0.1101 Savin et al., 1981   77B P 3980 8 Cibicid. 0.063 Kennett and Keigwin (CENOP unpubl.)   206 P 3000 11 Oridos. 0.056 Bender (CENOP unpubl.)   208 P 1350 20 G. subgl. 0.036 Bender (CENOP unpubl.)   289 P 1900 25 Cibicid. 0.042 Woodruff et al., 1981   296 P 2360 18 Oridos. 0.070 The Benedum Lab., Brown U. (CENOP unpubl.)

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Climate in Earth History: Studies in Geophysics Age Site Ocean Water Depth (m) N Species Group *σ2 Data Source   279 S(P) 2010 35 M.B.; Gyrod. 0.0551 Bender (CENOP unpubl.); Shackleton and Kennett, 1975   281 S(P) 1250 17 Uviger. 0.015 Loutit (CENOP unpubl.)   237 I 1440 25 Oridos. 0.059 Vincent et al. (CENOP unpubl.)   15 A 2550 17 M.B. 0.060 Savin et al., 1975   116 A 200 22 Oridos. 0.026 The Benedum Lab., Brown U. (CENOP unpubl.)   366A A 2700 27 Oridos. 0.050 Vincent et al., (CENOP unpubl.) M.-L. Oligocene 277 S(P) 1222 14 M.B. 0.020 Shackleton and Kennett, 1975a   366A A 2860 11 G. subgl. 0.035 Boersma and Shackleton, 1977 E. Oligocene 277 S(P) 1222 7 Oridos. 0.004 Keigwin, 1980   292 P 2943 7 Oridos. 0.018 Keigwin, 1980   366A A 2860 8 M.B. 0.044 Boersma and Shackleton, 1977 L. Eocene 292 P 2943 14 Oridos. 0.026 Keigwin, 1980   277 S(P) 1222 20 M.B., Oridos. 0.0171 Shackleton and Kennett, 1975a; Keigwin, 1980 M. Eocene 44 P 1478 8 M.B. 0.038 Savin et al., 1975   277 S(P) 1222 9 M.B. 0.083 Shackleton and Kennett, 1975a   398* A 3900 8 M.B. 0.329 Vergnaud-Grazzini, 1979 Paleocene 384 A 3910 13 M.B. 0.065 Boersma et al., 1979 aData sets are arranged according to stratigraphic age and grouped according to ocean basin. Depths given are estimated paleodepths for each stratigraphic age based on standard backtracking techniques. Number of data points (N) in individual data sets, species groups used, and data sources are indicated. M.B. indicates data that are based on the mixed benthic assemblage. An asterisk denotes data sets with very large variances, which appear spurious when compared with other data of similar age and location. These data are not shown in Figure 19.2. A “1” denotes those sites in which data are available from two different species groups and have estimated variances that are not significantly different. Their variances are pooled in this table. A “2” indicates those data sets in which all variance estimates from a given ocean basin and age are not significantly different. These variances are pooled for use in Figure 19.2. Site locations and ocean abbreviations are given in Table 19.2. FIGURE 19.1 Benthic oxygen isotope record for the Cenozoic (after Douglas and Woodruff, 1981).

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Climate in Earth History: Studies in Geophysics quire that the mean values of two data sets be the same before a comparison of the variances can be made. Thus, virtually all of the short-time series of oxygen isotope data (Table 19.1), regardless of the particular species used to obtain the data, can be used in a comparison of the total variance in different data sets. These variances have units of (‰)2; however, this notation is omitted from the text discussion. Most of the samples in the Tertiary data series are rather widely spaced within the recovered sections (usually 100 cm). Accumulation rates of 10–50 meters/million years (m/m.y.) are common in these pelagic sediments and thus would indicate a time spacing of the samples that is often greater than 105 yr. Sample spacing in the Miocene isotopic data collected by the CENOP group (Table 19.1) was designed to be between 50,000– 100,000 yr. Although such sample spacings are much broader than commonly used in Quaternary studies, they do provide an estimate of the total variance in the long-term record. If the Pleistocene record is sampled at a 50,000-yr spacing, the variance estimate is the same as for a 5000-yr sample spacing. Table 19.1 lists data sources to be used in this analysis and Table 19.2 lists site locations. These data have the following characteristics: (1) Oxygen isotope measurements in each data set were carried out on samples of a single species, a species group, or a mixed benthic assemblage. Data derived from different species groups or size fractions were not combined; however, if their variances were not significantly different (F test), they TABLE 19.2 Location, Water Depth, and Stratigraphic Age of Benthic Oxygen Isotope Data Used in this Studya Site Ocean Latitude Longitude Water Depth (m) Stratigraphic Age Studied E67–135 A 29°00′ N 87°00′ W 725 Quaternary, Pliocene V19–28 P 02°22′ S 84°39′ W 2720 L. Quaternary V19–29 P 03°35′ S 83°56′ W 3157 L. Quaternary V28–179 P 04°37′ N 139°36′ W 4502 Pliocene Y6910–2 P 41°16′ N 127°01′ W 2615 L. Quaternary 15 A 30°53′ S 17°59′ W 3927 E. Miocene 44 P 19°18′ N 169°00′ W 1478 M. Eocene 55 P 09°18′ N 142°33′ W 2850 M. Miocene 62 P OJ °52′ N 141°56′ W 2591 Pliocene 71 P 01°26′ S 125°49′ W 4419 M. Miocene, E. Miocene 77 P 01°39′ S 127°52′ W 4290 L. Miocene, M. Miocene, E. Miocene 83 P 04°03′ N 95°44.2′ W 3632 Pliocene 84 P 05°45′ N 82° 53′ W 3096 Pliocene 116 A 57°56′ N 15°56′ W 1161 E. Miocene 157 P 01°46′ S 85°54′ W 2591 L. Quaternary, Pliocene 158 P 06°37′ N 85° 14' W 1953 Pliocene, L. Miocene 206 P 32°01′ S 165°27′ E 3196 Pliocene, L. Miocene, M. Miocene, E. Miocene 207 P 36°58′ S 165°26′ E 1389 Pliocene, L. Miocene, M. Miocene 208 P 26°07′ S 161°13′ E 1545 Pliocene, L. Miocene, E. Miocene 237 I 07°05′ S 58°07′ E 1640 E. Miocene 238 I 11°09′ S 70°32′ E 2844 L. Miocene 277 S(P) 52°13′ S 166°11′ E 1222 L. Oligocene, L. Eocene, M. Eocene 278 S(P) 56°33′ S 160°04′ E 3698 L. Miocene 279 S(P) 51°20′ S 162°38′ E 3371 E. Miocene 281 S(P) 48°00′ S 147°46′ E 1591 Pliocene, L. Miocene, M. Miocene, E. Miocene 284 P 40°30′ S 167°41′ E 1068 E. Quaternary, Pliocene, L. Miocene 289 P 00°30′ S 158°31′ E 2206 L. Miocene, M. Miocene, E. Miocene 292 P 15°49′ N 124°39′ E 2943 L. Miocene, E. Oligocene, L. Eocene 296 P 29°20′ N 133°32′ E 2920 L. Miocene, E. Miocene 310 P 36°52′ N 176°54′ E 3516 E. Quaternary, Pliocene, L. Miocene, M. Miocene 329 S(A) 50°39′ S 46°06′ W 1519 L. Miocene 357 A 30°00′ S 35°34′ W 2109 L. Miocene 366 A 05°41′ N 19°51′ W 2860 E. Miocene, L. Oligocene, E. Oligocene 384 A 40°22′ N 51°40′ W 3910 Paleocene 397 A 26°51′ N 15°11′ W 2900 Quaternary, Pliocene, L. Miocene 398 A 40°58′ N 10°48′ W 3900 M. Eocene 408 A 63°23′ N 28°55′ W 1634 L. Miocene 502 A 11°29′ N 79° 23′ W 3052 Pliocene, L. Miocene 503 A 04°03′ N 95°38′ W 3672 Pliocene, L. Miocene aOcean locations: A, Atlantic; P, Pacific; 1, Indian, S(P), Southern Ocean, Pacific sector; S(A), Southern Ocean, Atlantic sector.

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Climate in Earth History: Studies in Geophysics FIGURE 19.2 Variance in benthic oxygen isotopes as a function of age in the Cenozoic. Data sources are listed in Table 19.1. Horizontal lines indicate stratigraphic range represented by the data sets, vertical lines indicate 80 percent confidence limits of the variance estimates. Closed circles represent Pacific Ocean data sets; open circles, Atlantic Ocean data sets; squares, Southern Ocean data sets; triangles, Indian Ocean data sets. Shaded area encompasses most of the data from the Pacific and Southern Oceans; excluded are the very deep Pacific sites in the Early and Late Miocene and the highly variable sites of the Atlantic and Southern Oceans sites in the Late Miocene. could be pooled. (2) Each data set comes from a single site. (3) Each group of measurements is associated with a known stratigraphic age. Stratigraphic intervals over which the variance was calculated were kept as short as possible to lend more detail to the long-term record of variability. Data were selected to avoid any clear jumps or shifts in the record that might be associated with evolutionary changes in the system. Such shifts in the data would lead to an abnormally large estimate of the variance. To guard further against more gradual shifts in the data, trends were removed from each data set using a simple linear regression. The estimate of variance used in this study is the variance around this regression line. For each stratigraphic age the variances of all data sets were compared using the M test (Thompson and Merrington, 1944) for homogeneity of variances. If the difference in the variance estimates were nonsignificant (p≥0.95), then the variances of all the data sets of the age could be pooled. This procedure allows the combination of several Quaternary time series. If variances were nonhomogeneous, they were subdivided according to ocean basin locations, and again tested for homogeneity. Data sets from the Atlantic, Pacific, and Indian Oceans were compared; when sufficient data were available, their variability as a function of water depth was contrasted. In a few cases the variability in individual data sets is high, greatly exceeding that found in other sites at the same time and depth interval. Such data might result from unrecognized diagenic or stratigraphic problems. They are considered spurious here, and, although included in Table 19.1, they are not plotted in the figures. Sites having extremely low variability might result from severe disturbance and mixing of rotary-drilled sections; however, such sites appear to show some degree of spatial and temporal coherence and are not excluded from the figures. THE LONG-TERM RECORD The record of the variance in benthic oxygen isotopes is shown in Figure 19.2, with vertical lines giving the 80 percent confidence limits of the estimate and horizontal lines indicating the range of the stratigraphic age of the individual data sets. Each symbol is located at the midpoint of the stratigraphic range. Different symbols are used for Pacific, Atlantic, Antarctic, and Indian Ocean data sets. Although the amount of data varies greatly through the Cenozoic (Figure 19.2), there appear to be several important changes in the character of isotopic variance as a function of time: (a) a decrease in the estimated variance from mid-Eocene to Oligocene time, (b) a slight Middle Miocene maximum in variance, (c) an increase in the variance of Atlantic sites relative to Pacific sites in post-Early Miocene times, and (d) a sharp increase in the variance at both Atlantic and Pacific sites beginning in the late Pliocene. The background variability above which the intervals of high variance rise is surprisingly consistent. It averages about 0.04 and is four times greater than the variance associated with laboratory error. In the latest part of the record the high variance in benthic oxygen isotopes increases from values of 0.06–0.1 in the late Pliocene to approximately 0.2 in the late Quaternary. Most of this high degree of variability is related to changes in the isotopic composition of the oceans as continental glaciers waxed and waned (Shackleton, 1967; Shackleton and Opdyke, 1973). There is some indication that the pooled variance in Atlantic sites is slightly higher than in Pacific sites. The difference is not significant in the Quaternary (M test, p≥0.95); however in the Pliocene and Late Miocene the Atlantic and Pacific do appear to show different degrees of variability. Shackleton and Cita (1979) noted the generally higher degree of varability in Atlantic sites during the latest Miocene. Such differences are

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Climate in Earth History: Studies in Geophysics also seen in the Pliocene and Late Miocene data presented here (Figure 19.2), where several {but not all) Atlantic and Southern Ocean sites show a higher variance than most Pacific sites, with the highest variances measured in the earlier part of the Late Miocene. During the Early Miocene and Oligocene, the variance of benthic oxygen isotopes in Atlantic sites were also slightly higher than those of the Pacific; however, the differences are not statistically significant (F test, p≥0.9). In the Pliocene and Miocene, measurements from deep sites (>4000 m) are included in the data set, These Pacific sites (DSDP 71 and DSDP 77 of the Deep Sea Drilling Project) show generally higher variance than others from the Pacific Ocean basin during both the Early and Late Miocene. During the Middle Miocene, however, detailed studies of a mid-depth site (DSDP 289—Woodruff et al., 1981) have an equally high degree of variability. This interval spans the time of glacial buildup in Antarctica (Shackleton and Kennett, 1975a) and appears as a major shift in the benthic isotopic values (Figure 19.1). The maximum in variance associated with this shift in isotopic values is clearly seen in the original data (Woodruff et al., 1981). To remove the effect of this shift in isotopic mean values on estimates of variance, the Middle Miocene data are subdivided into three groups (Table 19.1, Figure 19.1): pretransition, transition, and posttransition. Trends within these data subsets were removed using a linear regression. There are few accurate estimates of oxygen isotopic variance in the Paleogene; however, the data that are available indicate a minimum in variability during Late Eocene through Oligocene times. Discounting the high mid-Eocene variance of DSDP 398 (which may be spurious), the variance in benthic oxygen isotopes of the early Paleogene was near 0.06, whereas those in the Late Eocene through Oligocene were significantly lower (pooled variance=0.027). This apparent decrease in variance parallels a general cooling trend through the mid- to late-Paleogene (Figure 19.1) when bottom-water temperatures are estimated to have dropped from almost 15°C in the mid-Eocene to 6°C in the Oligocene (Figure 19.1). This late Paleogene minimum in variability is also significantly less than that measured in the Miocene, when the variance was usually near 0.04. Many of the changes in the variance of the oxygen isotopic data can be associated with major evolutionary transitions in the Cenozoic climate. The maxima in variance during the Quaternary and mid-Miocene are both correlated with the growth of major ice caps. It is presumed that like those of the Quaternary, the mid-Miocene intervals of high variance are associated with instability in the climate system and that most of the variation is due to changes in the isotopic composition of the oceans. The next older major step in climatic evolution indicated by the oxygen isotopes (Figure 19.1) occurs at the Eocene-Oligocene boundary. Although the data are more sparse around and across this boundary, the existing data (Kennett and Shackleton, 1976; Keigwin, 1980) indicate a rapid, monotonic shift in isotopic values. This shift is observed in both planktonic and benthic species in high latitudes but only in the benthics at low-latitude Pacific sites (Keigwin, 1980). Thus this evolutionary step, which is thought to be associated with the opening of the Australian-Antarctic seaway (Kennett and Shackleton, 1976), appears to be related to a marked cooling of deep waters and high-latitude surface waters. However, there does not appear to have been a change in the variability of deep-ocean waters on either side of this boundary. Nor was there a marked instability associated with the transition from one climatic state to the next, as observed with the growth of continental ice in the Middle Miocene and Quaternary. The increase in benthic isotopic variability that occurred in the mid-Eocene is based on sparse data (Table 19.1); but if further work supports its existence, it is the third maximum in variability associated with a marked shift in the mean 18O content of benthic tests (cf., Figure 19.1). The cause of the mid-Eocene shift in mean isotopic values is not certain. It could have been caused by a telluric change, such as the opening of a passage between South America and Antarctica (Norton and Sclater, 1979), which might have led to a marked cooling of the ocean waters. It might also have been caused by an early buildup of fairly large mountain glaciers in Antarctica (Matthews and Poore, 1980) or by some combination of temperature and ice volume effects. Sufficient data have been gathered from the Miocene and Pliocene (Table 19.1) to allow a view of how benthic isotopic variability is distributed in space as well as time. In the Late Miocene there is an increased oxygen isotopic (temperature) contrast between deep and bottom waters (Douglas and Woodruff, 1981). This is also the time when marked differences between the isotopic variability of the Atlantic and Pacific Oceans are first noted. This divergence of Atlantic and Pacific estimates of isotopic variance in the Late Miocene suggests the development of different source regions for deep waters in the two basins. The general pattern of change with depth seen in the Miocene and Pliocene of the Pacific Ocean is from low variance in shallower sites to higher variance in deeper sites. In both the Early and Late Miocene, the variance in the deepest sites (DSDP 71 and DSDP 77) is higher than in any other depth zone in the Pacific Ocean and is exceeded in magnitude only by the Quaternary data and the Late Miocene data from the Atlantic and Southern Ocean. In the Atlantic, Late Miocene variability of benthic isotopes at shallow depths is high (up to about 0.25). The variance decreases with depth, so that at 3000 m all oceans show approximately the same rather low degree of variability. The marked difference in the variance of Atlantic and Pacific benthic isotopic data has been noted previously (Shackleton and Cita, 1979); however, this difference does not appear to occur prior to the mid-Miocene. DISCUSSION AND CONCLUSIONS The data presented here indicate that the variability of benthic oxygen isotopes have changed with time and that these changes have often been associated with major steps in the evolution of the oceans. Although the data from the Paleogene are sparse, they indicate that the variability in benthic oxygen isotopes of

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Climate in Earth History: Studies in Geophysics the Oligocene and Late Eocene was significantly less than at any other time in the Cenozoic. The Oligocene was a time of relatively cool and equable climate (Kennett, 1978; Fischer and Arthur, 1977) and low eustatic sea levels (Vail et al., 1977). The low isotopic variability of this interval may be attributable to homogeneous deep waters derived from a single source region. The higher variability of the early Paleogene was associated with much warmer high-latitude and bottom-water temperatures. These conditions might have given rise to deep waters with a wider range of isotopic compositions. Different source regions and the initial buildup of sizable glaciers on Antarctica could also have served to introduce variability in the isotopic composition of the deep waters; however, the data are not sufficient to explore these possibilities. The well-documented isotopic shift across the Eocene-Oligocene boundary is interpreted as cooling of the high-latitude ocean and deep waters by about 3°C (Keigwin, 1980). There is no maximum in isotopic variability associated with the buildup of continental ice during the mid-Miocene and Pliocene-Pleistocene. Rather the record indicates a sudden, monotonic shift from one relatively stable oceanic state to another. In the Early Miocene the average variability increased again, but not up to the levels of the early Paleogene. This change is not readily associated with major oceanographic changes (see Figure 19.1). and the data are not sufficient to tell whether this shift to increased variance was relatively sudden (as in the mid-Miocene and Quaternary) or more gradual. The gradual shoaling of the calcite compensation depth and increase in the carbonate dissolution gradients (Heath et al., 1977) through the Oligocene and Early Miocene suggest a slow, long-term change in the character of the deep waters during this time interval. An apparent maximum in variability is, associated with the growth of the Antarctic ice cap during the mid-Miocene and suggests that some degree of instability in this ice cap may have existed during its early growth phase (Woodruff et al., 1981). In the Pacific Ocean. variation in the benthic oxygen isotopes was approximately the same before and after growth of the Antarctic ice cap; however. variability in data from the Atlantic Ocean greatly increased by Late Miocene times. The development of northern hemisphere ice sheets in the Late Pliocene is associated with an increase in oxygen isotopic variability that continues into the Quaternary and reaches a maximum in the late Pleistocene. This increase in variance is readily associated with the fluctuations in the isotopic composition of seawater caused by the growth and decay of continental ice sheets. There are several keys to assessing the changes in the Cenozoic oceans that may have led to changes in the variability of benthic isotopes. Changes in the global ice volume (which caused a 1.6 ‰ change in the oxygen isotopic composition of the oceans during the late Quaternary) is clearly associated with two of the maxima noted in the historical record (Figure 19.2). Such compositional changes may also be associated with the high variance of the early Paleogene (Matthews and Poore, 1980). By itself the range of oxygen isotopic compositions of modern deep waters (0.57 ‰) is close to the range of variation in the Cenozoic record of benthic oxygen isotopes; however, the temperatures of these modern watermasses are such that the isotopically heaviest waters [North Atlantic deep water (NADW) at +0.12 ‰] are also comparatively warm (about 4°C), and the 18O-depleted waters [Antarctic bottom water (AABW) at −0.45 ‰] are cold (about 2°C). Thus, the temperature-fractionation effects of calcite precipitation on oxygen isotopes tend to offset the compositional differences and result in benthic foraminiferal tests with similar isotopic compositions. The temperature and isotopic differences between modern AABW (at about 5°C and −0.15 ‰) and NADW (at 4°C and +0.12 ‰) tend to enhance the differences measured in the benthic foraminifera. If over long periods of time a site were alternately bathed by NADW and AABW, the isotopic record measured on benthic foraminifera would have a variance close to that measured in most of the Cenozoic sites studied. This suggests that the range of isotopic compositions of modern deep waters is at least sufficient to account for most of the long-term Cenozoic variation in benthic oxygen isotopes (excluding the large compositional changes associated with development of the cryosphere). How such a degree of long-term variation actually takes place remains unresolved. There are four mechanisms that seem plausible: (1) changes in the isotopic composition of the deep waters at their area of formation; (2) changes in the temperature of salinity characteristics of the deep water masses [which might also be associated with (1) above]; (3) changes in the number of source areas producing deep waters of dissimilar isotopic composition; and (4) changes in vertical structure of the oceans that would lead to fluctuations in hydrographic boundaries between waters of different physical and/or isotopic character. To evaluate the likelihood of any of these mechanisms operating at a particular time, data are needed from sites that sample a wide depth range in several ocean basins. Such data are not available for the Paleogene but are now being produced for the Neogene. The most common pattern seen in the Miocene data is that of relatively low variability at shallow depths (<1500 m), moderate variability between about 1500 and 4000 m, and increased variability below 4000 m. High variability in the deepest sites is not seen in Pliocene times. The Early Pliocene has relatively low variability (0.01– 0.03) at all depths except between 2000–3000 m. In the Late Pliocene, the data are rather homogeneous within the Atlantic and Pacific Oceans. There is some indication that variability increases slightly with depth and that the Atlantic is more variable than the Pacific:. These tendencies are statistically significant, but the data base is not large. During this time interval, northern hemisphere glaciations are thought to have begun (Shackleton and Opdyke, 1977), Although variance estimates for the Pacific Ocean are only slightly less than those of the mid-Miocene ice buildup in Antarctica, the amount of variability estimated for this interval of northern hemisphere glacial buildup is no greater than that found in the shallower waters of the Atlantic Ocean in the Late Miocene and is nearly the same as that estimated for the Pacific Ocean during the early Paleogene. Thus, relatively high isotopic variability may be closely

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Climate in Earth History: Studies in Geophysics associated with times when major changes occur in the average oxygen isotopic composition of the oceans (such as during glacial buildup), but they may also occur in association with the creation of new, isotopically different types of deep and intermediate water masses. If the later mechanism applies, large differences in the variability of benthic oxygen isotopes may be found between different oceans and between different depths in the same ocean. The oxygen isotopic data presented here are discussed in terms of only one simple statistical characteristic—its variability. Even with this simple tool, major changes in the deep waters of the oceans can be discerned. Clearly, the record of paleo-oceanographic change is dependent not only on its position in time but also on its geographic and depth location. A more thorough understanding of the variability of the oceans awaits an evaluation of the spectral character of oceanic oxygen isotopic variability that could take into account the relative importance of long-term and short-term oscillations. Questions concerning how such variance spectra have changed with time and the likely mechanisms of such change await the gathering of longer time series and the further refinement of the geologic time scale. ACKNOWLEDGMENTS We wish to express our appreciation to the CENOP project scientists for their thoughtful comments and criticisms of this work. Discussions with Sam Savin, Michael Bender, Nick Shackleton, and David Graham were particularly helpful. We also thank Fay Woodruff, Edith Vincent, Robley Matthews, Mike Sommers, Michael Bender, and Sam Savin for making their unpublished data available to us. 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