Carbon isotope records from pacific surface waters and atmospheric carbon dioxide

Quaternary Science Reviews, Vol. 11, pp. 387-400, 1992. 0277-3791/92 $15.00 Printed in Great Britain. All rights reserved. © 1992 Pergamon Press Ltd

CARBON ISOTOPE RECORDS FROM PACIFIC SURFACE WATERS AND ATMOSPHERIC CARBON DIOXIDE

N.J. Shackleton,* J. Le,* A. Mixt and M.A. Hall* * University of Cambridge, Sub Department of Quaternary Research, Godwin Laboratory, Free School Lane,

Cambridge CB2 3RS, U.K. tSchool of Oceanography, Oregon State University, Corvallis, OR 98195, U.S.A.

We have stacked planktonic carbon isotope data from three cores in the western equatorial Pacific in order to generate a new reconstruction of atmospheric carbon dioxide over the past 450,000 years. Our new reconstruction resembles that of Shackleton et al. (1983) based on data from East Pacific core V19-30, which successfully predicted features that were subsequently verified by Barnola et al. (1987) in the record from the Vostock ice core. In addition the new data confirm the discovery of Shackleton and Pisias (1985) that changes in atmospheric CO2 lead changes in ice volume and hence probably contributed to the glacial- interglacial cycles. Our new reconstruction avoids some of the deficiencies of the previous reconstruction: in particular the planktonic species (Neogloboquadrina dutertrei), on which the earlier reconstruction depends, does not calcify in truly nutrient- free surface water as the model assumes, whereas our new reconstruction uses Globigerinoides sacculifer which is expected to be more reliable. In addition, the surface waters in the west Pacific are closer to the nutrient-free ideal on which the model (Broecker, 1982) depends. On the other hand, the amplitude of the new reconstruction is significantly smaller than the amplitude observed by Barnola et al. (1987). It is not clear whether this smaller range is a better estimate of the amplitude of the 'biological pump' effect, or whether the true amplitude is reduced by bioturbation in the west Pacific cores that we studied.

INTRODUCTION

The first reports that the air trapped as bubbles in the Greenland ice from the last ice age contains signifi- cantly less carbon dioxide than Holocene air were by Delmas et al. (1980) and by Neftel et al. (1982). Broecker (1982) reasoned convincingly that the reduc- tion in CO2 concentration to around 200ppm during the last glacial that was reported by these groups could only be explained by postulating Changes in the distribution of dissolved carbon dioxide within the ocean. Specifi- cally, it requires the operation of one or more processes that reduce the average pCO2 of the ocean surface without entailing major changes in global ocean chem- istry. One possibility that Broecker explored was that the changes were driven by sea-level. He suggested that the rapid accumulation of organic-rich muds on the continental shelves as sea-level rose could have pre- ferentially removed nutrients from the ocean, allowing pCO2 at the surface to rise. However, more detailed records of atmospheric pCO2 are not consistent with the timing that this mechanism would require (Shackle- ton and Pisias, 1985). Another group of processes discussed by Broecker (1982) involve an increase in the proportion of dissolved carbon that is removed by photosynthesis from water as it reaches the ocean surface and is exposed to sunlight. Since photosynthesis removes isotopically light carbon in the form of organic matter, it should be possible to monitor this mechanism by means of 813C measurements. Shackleton et al. (1983) tested this approach by measuring bl3C in co- existing specimens of the planktonic Neogloboquadrina dutertrei and the benthonic Uvigerina senticosa from

core V19-30 (Table 1). They showed that the isotopic gradient recorded by these pairs of measurements does indeed increase in glacial times as predicted by this category of Broecker's models, and by using Broecker's calculations they reconstructed an atmospheric CO2 record for the past 150,000 years. The pattern of CO2 change that they reconstructed was to a large extent verified by the longer record of direct CO2 measure- ments published by Barnola et al. (1987). Extending the measurements in the same core Shackleton and Pisias (1985) reconstructed atmospheric CO2 over three glacial-interglacial cycles, and showed that modelled CO2 responds at the periods of the so-called Milan- kovitch cycles. In addition they showed that modelled CO2 leads glacial ice volume at the Milankovitch frequencies, implying that changes in atmospheric CO2 may have played a functional role in the glacial- interglacial climate record.

This application of the model proposed by Broecker (1982) does not require that the process controlling atmospheric CO2 necessarily occurs at the location where the ~)13C measurements are made, because in principle a change in the 'nutrient pump' in any region

TABLE 1. Cores discussed

Core Latitude Longitude Depth s.i.

ERDC93P 2°14.5'S 157°00.5'E 1619 m 2.5 cm V24-109 0°26'N 158°48'E 2367 m 2.5 cm RC17-177 1°45.3'N 159°26.9'E 2600 m 3 cm V19-30 3°23'S 83°21'W 3091 m 3cm V28-238 I°01'N 160°29'E 3120 m

387

388 N.J, Shackleton et aL

that is sufficiently large to change atmospheric pCO2 will be transmitted via changes in the ~3C of atmospheric CO2 to other parts of the surface ocean. However, the measurements should in principle be made in 'nutrient-free warm surface waters, ' and a major weakness of the data sets from core V19-30 is that N. dutertrei does not inhabit nutrient-flee water but rather calcifies low in the thermocline (Fairbanks et al., 1982). In the region from which core V19-30 was collected, calcite dissolution on the sea floor is so intense that surface-dwelling species of foraminifera are almost entirely absent from the sediment. The objective of the present study was to examine ~13C records derived from planktonic species from other parts of the low latitude Pacific with a view to generating a reconstruction of atmospheric CO2 that does not suffer from this weakness.

CORES EXAMINED

For this study we have analyzed G. sacculifer from several cores in the western equatorial Pacific. Here dissolution is much less intense than in the east Pacific, and a substantial area of shallower sea floor, well above the foraminiferal lysocline, is available for coring. The locations of the cores and those discussed in the text are given in Table 1.

The cores have been sampled at closer intervals than has been customary in the past (Table 1). The objective was both to obtain the best resolution consistent with the bioturbation in the sediments, and to maximize the signal-to-noise ratio in the 613C records.

Samples of dry sediment were washed on a 150 micron sieve and dried at about 50°C. In order to minimize noise due to variation in isotopic composition associated with ontogenetic development (Berger et al., 1978), specimens were picked from size fractions extracted by sub-sieving; in this study the size range 300-355 microns was used. G. sacculifer was selected because the study of Fairbanks et al. (1982) shows the species to be abundant in the upper part of the water column and it is the species that most consistently yields isotopically positive 6~3C values consistent with calcifi- cation in the photic zone (Berger et al., 1978). This species also consistently gives i5~80 values indicative of calcification near the sea surface (Emiliani, 1954; Berger et al., 1978).

Isotopic measurements were made using a VG lsogas SIRA triple collector mass spectrometer with an ISOCARB automatic carbonate analyzer of the type developed by R.G. Fairbanks. In this system, a number of consecutive samples (up to 44) react in the same vat of 100% orthophosphoric acid. Our procedure was to fill the carousel with a succession consisting of 5 marble standards; up to 30 unknown foraminiferal samples; 5 additional marble standards, The acid was kept at 90°C and stirred continuously during the reaction. The reaction time was held to 6 min. Cross-contamination between successive samples was about 2%, which is almost invariably negligible except when a marble

standard or a planktonic sample is immediately fol- lowed by a benthonic sample; in this case a blank was inserted. Since this blank (empty space) was processed as a sample, it provided a measure of the residue from the previous sample which would have been present as memory; thus the system memory was in effect moni- tored frequently. Analytical precision was about + 0.06 per mil for ~180 and somewhat better than this for 613C. Real (geological) reproducibility is usually worse than this for planktonic foraminifera, presum- ably because of the substantial inter-specimen variabil- ity and also bioturbation (Schiffelbein, 1986), and is best judged on a core-by-core basis on the basis of sample-to-sample noise (Shackleton and Opdyke, 1973).

RESULTS

The isotopic measurements are given in Tables 2-4. We also obtained measurements for a number of verified core-top samples from the same area. Occasio- nally we obtained what we regard as aberrant measure- ments; possible reasons (in order of decreasing likeli- hood) are: contamination of the specimens (leading to impurities in the gas analyzed); malfunction of a vacuum valve (leading to gas fractionation); and electronic errors. Where possible, suspect measure- ments were repeated twice more and the aberrant measurement discarded; discarded measurements are marked with an asterisk in Tables 2-4 and are not shown on Figs 1 and 2 where measurements of replicate picks from the same sediment sample are averaged,

TIMESCALE DEVELOPMENT

Since the SPECMAP timescale (Imbrie et al., 1984) was developed on the basis of core V28-238 which was recovered from within the geographical area under consideration (Table 1), it proved relatively easy to transfer this timescale to our cores. The most difficult section proved to be in Stage 5 where much of the detail in the composite record of Imbrie et al. (1984) was provided by high-resolution records from other areas. Prell et al. (1986) reasoned that the record of core V28- 238 was physically distorted by the coring process in Stage 5 and it may be that this is so, but other factors such as a reduced accumulation rate and (in V24-109) some re-deposition have conspired to aggravate the difficulties in this interval for cores that we have been able to examine from the western equatorial Pacific.

Table 5 lists the control points that we used to develop the timescales. In some cases controls have been inserted to align features in the dissolution records, and in some cases we have preferred to insert a control at a point of rapid isotopic change (i.e. a stage boundary) rather than at an isotopic extreme. If a core is sampled at very close intervals, the insertion of age controls at stage boundaries provides a more reliable approach to timescale development than the procedure

Carbon Isotope Records 389

T A B L E 2. Isotopic measurements of Globigerinoides sacculifer in core ERDC-93P picked from the sieve range 300-350 microns,

reported as a 6 value %o with reference to the PDB standard

T A B L E 2. continued

Depth Lab rcf. (m) 6180 613C

A 89/1862 1.875 - 1.37 1.52 A 89/1863 1.900 - 1.61 1.52 A 89/1864 1.925 - 1.53 1.50 A 89/1865 1.950 - 1.67 1.58 A 89/1866 1.975 - 1 . 5 6 1.48 A 89/1867 2.000 - 1.57 1.42 A 89/1868 2.025 - 1.60 1.41 A 89/1869 2.050 - 1.66 1.45 A 89/1870 2.075 - 1 . 7 0 1.42 A 89/1881 2.100 - 1 . 7 4 1.32 A 8911882 2.125 - 1.54 1.31 A 89/1883 2.150 - 1.50 1.24 A 89/1884 2.175 - 1.32 1.45 A 89/1885 2.200 - 1.32 1.29 A 89/1886 2.225 - 1 . 3 4 1.34 A 89/1887 2.250 - 1 . 2 3 1.35 A 89/1888 2.275 - 1 . 2 8 1.45 A 89/1889 2.300 - 1.35 1.52 A 89/1890 2.325 - 1.39 1.51 A 89/2001 2.350 - 1 . 4 6 1.51 A 89/2042 2.375 - 1.47 1.54 A 89/2043 2.400 -1 .63 1.29 A 89/2044 2.425 - 1.56 1.25 A 89/2045 2.450 - 1.55 1.49 A 89/2046 2.475 - 1 . 5 4 1.18 A 89/2047 2.500 - 1.63 1.12 A 89/2048 2.525 - 1 . 5 5 1.24 A 89/2049 2.550 - 1.21 1.33 A 89/2050 2.575 - 1.18 1.19 A 89/2061 2.600 - 1.14 1.34 A 89/2062 2.625 - 1.13 1.28 A 89/2063 2.650 - 1.02 1.29 A 89/2064 2.675 - 1 . 1 0 1.32 A 89/2065 2.700 - 1.19 1.23 A 89/2066 2.725 - 1.30 1.27 A 89/2067 2.750 - 1 . 1 4 1.27 A 89/2068 2.775 - 1.03 1.21 A 89/2069 2.800 - 1 . 1 8 1.31 A 89/2070 2.825 - 1.13 1.20 A 89/2081 2.850 - 1.04 1.43 A 89/2082 2.875 - 0 . 9 8 1.34 A 89/2083 2,900 - 0 . 9 3 1.48 A 89/2084 2.925 - 1.02 1.57 A 89/2085 2.950 - 1.19 1.72 A 89/2086 2.975 - 1.38 1.66 A 89/2087 3.000 - 1.34 1.60 A 89/2088 3.025 - 1,48 1,57 A 89/2089 3.050 - 1,24 1.63 A 89/2090 3.075 - 1 . 3 2 1,68 A 89/2101 3.100 - 1 . 2 0 1.76 A 89/2102 3.125 - 1.07 1.66 A 89/2103 3.150 - 1.02 1.61 A 89/2100 3.175 - 1 . 1 6 1.69 A 89/2105 3,200 - 1.09 1.65 A 89/2106 3,225 - 1.35 1.64 A 89/2107 3.250 -1 .33 1.58 A 89/2108 2.275 - 1.31 1.59 A 89/2109 3.300 - 1.47 1.59 A 89/2110 3.325 -1 .31 1.61 A 89/2121 3.350 - 1 . 4 9 1.55 A 89/2122 3.375 - 1.62 1.33 A 89/2123 3.400 - 1 . 5 9 1.51 A 89/2124 3.425 - 1.64 1.66 A 89/2125 3.450 - 1.76 1.45 A 89/2126 3.475 - 1.76 1.46 A 89/2127 3.500 - 1.69 1.26 A 89/2128 3.525 - 1 . 3 2 1.16 A 89/2129 3.550 -1 .51 1.12 A 89/2130 3.575 -1 .31 1.19 A 89/2141 3.600 - 1.34 1.09 A 89/2142 3.625 - 1 . 3 3 1.14 A 89/2143 3.650 - 0 . 8 7 1.25 A 89/2144 3.675 - 0 . 8 7 1.25

continued

Depth Lab ref. (m) 61so 613C

A 89/1651 0.1300 - 2 . 1 3 1.62 A 89/1652 0.025 -2 .01 1.67 A 89/1653 0.050 - 1.90 1.61 A 89/1654 0.075 - 1.54 1.43 A 89/1655 0.100 - 1.30 1.59 A 89/1656 0.125 - 0 . 8 9 1.72 A 89/1657 0.150 - 0 . 7 8 1.66 A 89/1658 0.175 - 1.03 1.77 A 89/1659 0.200 - 1 . 2 0 1.67 A 89/1660 0.225 - l . 2 7 1.73 A 89/2758 0.250 - 1.29 1.78 A 89/2759 0.275 - l . 17 1.73 A 89/2792 0.325 - 1 . 2 6 1.52 A 89/2793 0.350 - 1 . 0 8 1.57 A 89/2794 0.375 - 1 . 3 8 1.30 A 89/2795 0.400 - 1.23 1,46 A 89/2796 0.425 - 1.00 1.69 A 89/2797 0.450 - 1.20 1.49 A 89/2798 0.475 - 1 . 3 0 1.74 A 89/2562 0.500 - 1.57 1.81 A 89/2563 0.525 - 1.55 1.77 A 89/2564 0.550 - 1.61 1.87 A 89/2567 0.625 - 1 . 4 8 1.91 A 89/2568 0.650 - 1 . 6 9 1.84 A 89/2569 0.675 - 1 . 6 9 1.84 A 89/2570 0.700 - 1.62 1.78 A 89/2551 0.725 - 1.61 1,65 A 89/2552 0.750 - 1.53 1.63 A 89/2553 0,775 - 1.61 1.64 A 89/2554 0.800 - 1.62 1.71 A 89/2555 0.825 - 1.34 1.53 A 89/2556 0.850 - 1.69 1.58 A 89/2557 0.875 - 1.14 1.:64 A 89/2558 0.900 - 1.83 1.40 A 89/2559 0.925 - 1,86 1,57 A 89/2560 0.950 - 1,76 1,60 A 89/2571 0.975 - 1.81 1.35 A 89/2572 1.000 - 1 . 7 8 1.40 A 89/2573 1.025 - 1 . 9 2 1.20 A 89/2834 1.050 - 1.88 1.23 A 89/2833 1,075 - 1.92 1.29 A 89/1711 1,100 - 1 . 6 9 1.27 A 89/1712 1,125 - 1 . 8 4 1.23 A 89/1713 1.150 - 1 . 4 6 1.23 A 89/2832 1.175 - 1,14 1,26 A 89/1715 1.200 - 1.08 1.29 A 89/1716 1.225 - 0 . 9 4 1.50 A 89/1717 1.250 - 0 . 9 5 1.43 A 89/1718 1.275 -0 .91 1.39 A 89/1719 1.300 - 0 . 8 4 1.38 A 89/1720 1.325 - 0 . 9 6 1.34 A 89/1731 1.350 -0 .81 1.31 A 89/1732 1.375 -1 .01 1.32 A 89/1733 1.400 - 0 . 9 6 1.24 A 89/1734 1.425 - 0 . 9 8 1.24 A 89/1735 1.450 - 1.06 1.23 A 89/1736 1,475 - 0 . 9 3 1.38 A 89/1737 1.500 - 1 . 2 5 1.13 A 89/1738 1.525 - 1 . 2 0 1.29 A 89/1739 1.550 - 1.19 1.33 A 89/1740 1.575 - 1.23 1.36 A 89/1751 1.600 - 1.25 1.10 A 89/1752 1.625 - 1.26 1.23 A 89/1753 1,650 - 1 . 2 5 1.34 A 89/1754 1.675 - 1 . 2 4 1.27 A 89/1755 1.700 - 1.15 1.32 A 89/1756 1.725 - 1 . 0 5 1.40 A 89/1757 1.750 -1 .11 1.30 A 89/1758 1,775 - 1.32 1.53 A 89/1759 1.800 - 1 . 3 8 1.58 A 89/1760 1.825 - 1.54 1.81

continued

390 N.J. Shackleton et al.

T A B L E 2. continued

Depth Lab ref. (m) 61~0 6h~(7

T A B L E 3, continued

Depth Lab rcl. (m} blsO ~t~(,

A 89/2145 3,700 A 8912146 3,725 A 89/2147 3.75{} A 89/2148 3,775 A 8912149 3.800 A 89/2151} 3.825 A 89/2451 3.850 A 89/2452 3.875 A 89/2453 3.90(} A 8912454 3.925 A 89/2455 3.95{} A 89/2456 3.975 A 89•2457 4.(}(10 A 89•2458 4.025 A 89/2459 4.050 A 89/2460 4.{}75 A 89/2471 4. 100 A 89/2472 4.125 A 8912473 4,150 A 8912474 4. 175 A 89•2475 4.200 A 89/2476 4.225 A 89/2477 4.250 A 89/2478 4.275 A 89/2479 4.300 A 8912491 4.375 A 89•2492 4.400 A 89/2493 4,425 A 89/2494 4,450 A 89/2495 4.475 A 89/2821 4.5{R} A 89/2822 4.525 A 89•2824 4,551/ A 89/2514 4,575 A 89/2515 4.600 A 89/2516 4.625 A 89t2517 4.6511 A 8912518 4,675 A 89/2519 4.7(}0 A 89/2520 4.725 A 89/2534 4.825 A 89/2535 4.850 A 8912536 4,875 A 89/2538 4,925 A 89/2539 4.950 A 89/2540 4.975 A 89/2561 5.(}00

-0 .86 -0 .90 -11.96 -0 .92

l . ( } 5

- 1 . 1 4 - .10 - . 3 1

- . 3 5

- . 1 6

- . 4 7

- . 3 8

- . 2 9

- . 5 6

- . 5 3

- 1 . 9 0

- 1 . 7 8

- 1 . 9 9

- 1.96 -2 .09 - 1 . 8 4

- 1 . 9 4

-1 .71 - 1.72 -1 .55 - ( } . 6 8

- 0 . 5 0

-0.5/} --{}.51 -0 .64 -11.77 - 1 , { } 0

-0 ,89 -{}.90 -0 .98 -1.11 - 1 . ( 1 9

-1 .17 - 1.38 -1.21 - 1.23 -1 .13 - 1.08 -0 .95 - 0 , 9 6 --0.91 - 1 . 0 1

1.39 A 86/1313 1.49 A 86/677 1.59 A 86/1023 1.78 A 86/1843 1.85 A 87/233 1.81 A 86/678 1.59 A 86/1314 1.67 A 87/234 1.48 A 86/679 1.73 A 86/1315 1.57 A 87/235 1.49 A 86/680 1.51 A 86/1351 1,62 A 86/71)1 1,59 A 86/1024 1.73 A 86/1842 1.89 A 87/236 1.63 A 861702 1,60 A 86/1352 1.72 A 86/703 1.65 A 86/1025 1.54 A 86/1841 1.33 A 87/237 1.31} A 86/704 1.(}5 A 86/1353 1.29 A 86/705 1.59 A 86/1026 1.63 A 86/1840 1.49 A 87/238 1.51/ A 86/71}6 1.57 A 86/1354 1.62 A 86/7{}7 1.55 A 86/I(}27 1.26 A 86/1839 1.34 A 87•239 1.29 A 86/708 1.39 A 86/1355 1.58 A 87/240 1.61 A 86/7t}9 1.57 A 86/1356 1.61} A 86/710 1.57 A 86/1028 1.59 A 86/1838 1.62 A 87/241 1.62 A 86/1357 1.46 A 86/1029 1.63 A 86/1837

A 87/242

T A B L E 3, Isotopic measurements of GIobigerinoides sacculqer in core RC17-177 picked from the sieve range 300-3511 microns,

reported as a 6 value %o with reference to the PDB standard

A 86/1358 A 86/103{} A 86/1836 A 87/243 A 87/441 A 87/442 A 87/451

Depth Lab ref. (m) 6t~O 81 ~("

A 86/1359 A 86/1075 A 86/1835 A 87/244

A 86/671 {}.0 - 2 . 0 4 A 861672 {/.05 -2 .11 A 86/1311 0.05 -2 .01 A 86/673 (1.09 - 1,91 A 86/1021 {}.{}9 - 1.94 A 8611861 {}.09 - 1.98 A 871231 0,12 -1 .57 A 86/674 {}. 15 - 1.59 A 86/1312 !}.15 -1.71 A 861675 0.18 - 1.45 A 8611022 {}.18 -1 .31 A 86/1844 0.18 - 1,48 A 871232 0.21 -0 .98 A 861676 {}.25 - 1.20

1.76 A 87/444 1.56 A 87/445 1.63 A 87/454 1.53 A 86/1361} 1.47 A 86/1076 1.68 A 86/1834 1.43 A 87/245 1.47 A 87/447 1.46 A 87/448 1.28 A 87/457 1.42 A 86/1376 1.47 A 86/1077 t.46 A 8611833 1.3 l A 87/246 continued

{},25 1}28 {}28 0.28 O32 035 {}.35 (},41 (}.45 {}. 45 {},52 {},55 I}55 0.58 0.58 0,58 11.62 0.65 {}.65 {},68 (}.68 1}.68 0 7 2 tl.75 0,75 {I 77 {}.77 0,77 1t.8t 0.85 {},85 11.89 {}. 89 0,89 {}.92 {}.95 0.95

.{}2 ,{}5 ,05 {}9

{}9 .09 .13 15 ,18 18 2t

t.25 .28 28 31 .31 31 .31 .35 .40 .4{} .41 .41 .41 .41 45 ,51} .51} .51 .51 .51 .51 .55 ,6() ,60 61

- 1.09 - 1 . 2 9

-1},89 --0.86 - (L64 -0 .76 -0 .88 -1.01 -- l.(}6 .... t.05

1 . { 1 2

-- 1,18 - 1.27 - 1,11 - l . 1 8 -1 .25 - I . ( } 7

- 1.27 - I . 15

- 1.20 - 1 . 2 0

- 1 . 3 0

- 1 . 2 1 /

- 1 . 3 6

- 1 . 2 7

- 1,24 -1 ,19 -1,41 • - t.27 - 1.30 - 1 . 2 8

- 1.22 - I . 1 9 - 1 . 3 0

- 1 . 0 1

- 1.26 -- 1.20 - 1.35 - 1.75 -1 .41 - 1.65 - t . 7 9

- 1.46 -1.61 - 1 . 6 0

-1 .57 - 1 . 5 0

-1 .55 - i,43 -- 1.60

-1 .71 - 1 . 6 7

- 1 . 6 1

- 1 . 6 4

- 1.63 -- 1.70 - 1 . 5 7

- 1 , 6 4

- - 1.58 -- 1 . 0 0 * *

-- 1.42 1.44

- 1.70 -1,71 - 1.79 - 1 . 6 4

- 1 . 6 9

- 1 , 7 7

- 1,69 - 1,65 - 1 . 7 9

- 1 . 7 3

- 1 . 9 1

1.66 1.21 1 . 5 ( 1

1 . 5 0

1 . 6 0

1 . 6 3

1 . 7 0

1 . 5 8

1 . 5 5

1 . 7 4

1 . 4 5

1.75 1 . 6 4

1 . 6 4

1 . 6 2

1.78 1 . 7 2

1 . 6 8

1.83 1.61)

1 . 6 6

t . 7 { )

1 . 4 4

1 . 6 6

1.5tl 1.31 1 . 3 3

1.24 1.3(}

1.16 1.48 1 , 2 4

1.25 1 . 2 7

t . 4 6

1 . 5 8

1.53 1 . 6 5

1.511 1.70 1.50 1 . 4 8

.4O 1.46 1 . 6 3

1 3 8 1.59 1,71 1 , 5 8

1 4 4

1 . 5 0

1.55 t .82 1 . 6 8

.47

.41}

.54 58 .43 .84 67

.51

.51

.53 32 .53 .76 .55 .47 32 27

1.39 1,29

continued

Carbon Isotope Records 391

Lab ref.

A 86/1375 A 86/1078 A 86/1832 A 871661 A 87/662 A 87•663 A 871664 A 871665 A 87/666 A 87/667 A 87/668 A 87/669 A 87/670 A 87/247 A 86/1374 A 86/1079 A 86/1831 A 87/248 A 86/1373 A 86/1080 A 86/1830 A 87/381 A 87/249 A 86/1372 A 86/1171 A 86/1829 A 86/1172 A 86/1828 A 86/1173 A 86/1827 A 86/1371 A 86/2053 A 86/1826 A 86/1825 A 87/252 A 86/2052 A 86/1824 A 86/1823 A 87/253 A 86/2051 A 86/1822 A 86/1821 A 86/1174 A 86/1820 A 86/2010 A 86/1819 A 86/1818 A 87/254 A 86/2009 A 86/1817 A 86/1816 A 87/255 A 86/2008 A 86/1815 A 86/1814 A 87/256 A 86/2007 A 86/1813 A 86/1812 A 87/257 A 86/2006 A 86/1811 A 86/1810 A 87/258 A 86/2005 A 86/1809 A 86/1808 A 87/259 A 86/2004 A 86/1807 A 86/1806 A 87/260 A 86/2003

TABLE 3. continued

Depth (m) 6'~0

1.65 -1 .90 1 . 7 0 - 1 . 9 4 1 . 7 0 - 1 . 8 8 1 . 7 0 - 1 . 9 8 1.70 -1 .60 1.70 -1.91 1 . 7 0 - 1 . 8 2 1 . 7 0 - 1.67 1.70 -2 .03 1 . 7 0 - 1 . 9 5 1 . 7 0 - 1 . 9 5 1.70 - 1.73 1.70 -1 .86 1 . 7 2 - 1 . 6 3 1 . 7 5 - 1 . 4 7 1.80 -1 .51 1.80 -1 .20 1 . 8 1 - 1.04 1.85 -0 .97 1.90 -0 .95 1.90 - 1.01 1 . 9 0 -0.71 1.91 -0 .86 1.95 -0 .88 1.97 -0 .78 1.97 -0 .83 2.00 -0 .75 2.00 -0 .74 2.01 -0 .77 2.01 -0 .94 2.05 -0 .82 2.05 -0 .70 2.07 -0 ,77 2.11 -0 .90 2.12 -0 .94 2.15 -1 .06 2.18 -1 .15 2.21 -1 .35 2.22 -0 .87 2.25 -1 .03 2.28 - 1.04 2.31 -1.11 2.32 - 1.04 2.32 - 1.14 2.35 -1 .04 2.38 -1 .08 2.40 - 1.56 2.41 -1 .37 2.45 -1 .57 2.47 - 1.62 2,50 - 1.72 2,51 -1 .54 2.55 - 1.40 2.58 -1 .53 2.60 - 1.63 2.61 - 1.60 2.65 -1 .58 2.67 - 1.46 2.70 - 1.65 2.71 -1 .54 2.75 - 1.23 2.775 - 1.34 2.80 -1 .36 2.81 -1 .14 2.85 - 1.05 2.87 -1 .34 2.90 -1 .39 2.91 -1.41 2.95 -1 .48 2.97 - 1.57 3,01 - 1.54 3.02 - 1.55 3.05 - 1.32

6t3C Lab ref.

1.35 A 86/1805 1.04 A 86/1804 1.38 A 86/1803 1.45 A 87/261 1.44 A 86/2002 1.45 A 86/1802 1.18 A 86/1862 1.26 A 87/262 1.39 A 86/2001 1.26 A 86/1800 1.24 A 86/1799 1.23 A 87/263 1.36 A 86/1990 1.22 A 86/1798 1.20 A 86/1797 0.99 A 87/264 1.12 A 86/1989 1.14 A 86/1796 1.00 A 86/1795 1.33 A 87/265 1.32 A 86/1988 1.47 A 86/1794 1.23 A 86/1793 1.37 A 87/266 1.26 A 86/1987 1.37 A 86/1792 1.25 A 86/1791 1.13 A 87/267 1.10 A 86/1986 1.07 A 86/1790 1.16 A 86/1789 1.35 A 86/1788 1.27 A 86/1985 1.20 A 86/1787 1,13 A 86/1786 1.21 S 90/1581 1.36 S 90/1582 1.40 A 87/268 1.22 A 86/1984 1.15 A 86/1785 1.10 A 86/1784 0.96 A 86/1983 1.08 A 86/1783 1.13 A 86/1782 1.39 A 86/1982 1.45 A 86/1781 1.33 A 86/1780 1.50 A 86/1981 1.59 A 86/1779 1.63 A 86/1778 1.64 A 86/1980 1.44 A 86/1777 1.53 A 86/1776 1.48 A 86/1979 1.47 A 86/1775 1,31 A 86/1774 1,35 A 86/1978 1.36 A 86/1773 1.18 A 86/1772 1.25 A 86/1977 1.40 A 86/1771 1.16 A 86/1770 1.12 A 86/1976 1.19 A 86/1769 1.46 A 86/1768 1.33 A 86/1975 1.50 A 86/1767 1.38 A 86/1766 1.37 A 86/1974 1.45 A 86/1765 1.37 A 86/1764 1.20 A 86/1970 1.39 A 86/1763

continued

TABLE 3. continued

Depth (m) a180 •13 C

3.07 - 1.32 1.00 3.09 - 1.47 1.16 3.11 -0 .92 1.16 3.13 -0.81 1.12 3.15 -0.85 1.57 3.17 -0 .87 1.30 3.20 - 1.00 1.37 3.21 - 1.09 1.23 3.25 -0 .90 1. I0 3.28 - 1.09 1.30 3.30 -0 .83 1.32 3.31 -0 .82 1.34 3.35 -0 .73 1.35 3.38 -0 .92 1.56 3.40 - 1.25 1.50 3.41 - 1.28 1.61 3.45 - 1.23 1.45 3.48 - 1.24 1.70 3.51 - 1.17 1.66 3.53 -0 .93 1.84 3.55 -0 .87 1.69 3.57 -0 ,88 1.83 3.60 - 1,07 1.77 3.61 -1 .17 1.65 3.65 -1 .08 1.56 3.67 - 1.33 1.72 3.70 - 1.44 1.43 3.71 -1 .18 1.47 3.75 -1 .42 1.36 3.78 - 1.44 1.46 3.80 - 1.41 1.42 3.81 -1 .62 1.17 3.85 - 1.57 1.33 3.87 -1 .37 1.46 3.90 - 1.53 1.43 3.91 - 1.59 1.56 3.91 - 1.66 1.64 3.91 -1 .49 2.52** 3.95 - 1.50 1.42 3.98 - 1.53 1.25 4,00 - 1.48 1.28 4,05 - 1.43 0.98 4.07 -1 .04 1.09 4.10 -0 .70 1.05 4.15 -0 .65 1,50 4.17 -0 .65 1.47 4.20 -0 .68 1.53 4.25 -0 .62 1.60 4.27 -0 .50 1.42 4.30 -0 .69 1.62 4.35 -0 .82 1.60 4.37 -0 .86 1.45 4.40 -1 .20 1.41 4.45 -1 .26 1.40 4.47 - 1.09 1.67 4.50 -1 .19 1.59 4.55 - 1.29 1.45 4.57 -1 .49 1.65 4.60 -1 .59 1.38 4.65 - 1.47 1.49 4.67 - 1.77 1.70 4.70 - 1.79 1.71 4.75 -1 .59 1.38 4.77 -1 .86 1.47 4.79 - 1.72 1.43 4.85 - 1.64 1.44 4.87 - 1.65 1.43 4.90 -1.31 1.45 4.95 -1 .16 1.31 4.97 -0 .64 1.27 5.00 -0 .74 1.15 5.05 -0 .44 1.56 5.07 -0 .37 1.58

continued

392 N.J. Shackleton et al.

Lab ref.

A 86/1762 A 86/1969 A 86/1761 A 86/1760 A 86/1759 A 86/1968 A 86/1758 A 86/1757 A 86/1967 A 86/1756 A 86/1755 A 86/1966 A 86/1754 A 86/1753 A 86/1965 A 86/1752 A 86/1964 A 86/1751 A 86/1750 A 86/1749 A 86/1963 A 86/1748 A 86/1747 A 86/1962 A 86/1746 A 86/1745 A 86/1961 A 86/1744 A 86/1743 A 86/1960 A 86/1742 A 86/1741 A 86/1959 A 86/1740 A 86/1739 A 86/1958 A 86/1738 A 86/1737 A 86/1957 A 86/1736 A 86/1735 A 86/1956 A 86/1734 A 86/1733 A 86/1955 A 86/1732 A 86/1731 A 86/1954 A 86/1730 A 86/1729 A 86/1953 A 86/1728 A 86/1727 A 86/1952 A 86/1726 A 86/1725 A 86/195(I A 86/I724 A 86/1723 A 86/1951 A 86/1722 A 86/1711 A 86/1949 A 86/171(I A 87/151 A 86/1709 A 87/152 A 87/2511 A 87/25 I A 86/1948 A 871153 A 86/1708 A 86/17(/7

T A B L E 3. continued

Depth (m) 61sO

5.1(I -0 .31 5.15 -0 .45 5.17 -0 .71 5.20 - 0 , 8 2 5.21 -0 .71 5.25 - 0 . 7 9 5.27 -0 .81 5.30 -0 .77 5.35 -0 .88 5,37 -0 .85 5.40 -0 .97 5.45 - 1.09 5.47 -1 .24 5.50 - 1 . 1 6 5.55 - 1.20 5.59 - 1.42 5.65 -1 .35 5,67 - 1 , 4 6 5,70 - 1.38 5.71 - 1.22 5.75 - 1.09 5.77 -1 .13 5.80 - 1.20 5.85 - 1,25 5.87 - 1.46 5.9(I - 1.40 5.95 - 1.07 5.97 - 1.20 6.00 -1 .17 6.05 - 1.09 6.07 -(I.82 6.10 - 0 . 9 9 6.15 - 0 . 7 9 6.17 -1.11 6.2(I - l. 14 6.25 - 1.15 6.27 - 1.23 6.3(I - 1.22 6.35 - 1.22 6.37 -1 .25 6,40 - 1.33 6.45 - 1 . 3 4 6.47 - 1.69 6.50 - 1.46 6.55 -1 .55 6.57 - 1.44 6.60 - 1.64 6.65 - 1.59 6.67 - 1.66 6.7l -1 .43 6.75 - 1.32 6.77 - 1.22 6.81 - l . 1 6 6.85 - 1.03 6.87 - 1.14 6.90 -1.31 6.95 -1,41 6,97 - 1.39 7.0(I - 1.36 7.05 - 1.31 7.07 - 1.13 7.10 -1 .18 7.15 -1 .34 7. t7 - l . 3 0 7.17 - 1.37 7.21 - 0 . 9 0 7.21 -1 .39 7.21 -1.41 7.21 -1 .29 7.25 - 1.43 7.25 - 1.31 7.26 -1 .52 7.31 -1 .53

6~3C Lab ref.

1 . 7 2 1.63 A 86/17(_16 1.52 A 86/1705 1.57 A 86/1704 1 . 7 2 1.60 A 86/1700 1.56 A 86/1703 1.54 A 86/1699 1.51 A 86/1702 1.66 A 86/1698 1.61 A 86/1701 1.88 A 86/1697 1.73 A 86/1696 1 . 7 6 1.89 A 86/1695 1.90 A 86/1694 1,83 A 86/1693 1,73 A 8611692 1,72 A 86/1691 1 . 8 3 1.71 A 82/241 1.84 A 82/242 1.73 A 82/247 1.62 A 82/251 1.50 A 86/991 1.66 A 86/992 1.72 A 86/993 1.51 1.66 A 82/244 1.43 A 82/245 1.61 A 82/248 1.49 A 82/249 1.57 A 86/994 1.57 A 86/995 1.52 A 86/996 1 . 4 9 1.44 A 82/243 1.62 A 821246 1.52 A 82/250 1.39 A 82/252 1.66 A 82/253 1.66 A 86/997 1.77 A 86/998 1.64 A 86/999 1.50 1.56 A 8611000 1.41 A 86/t001 1 . 4 0 1.32 A 8611002 1.39 A 86/1003 1.3t A 86/1004 1 . 4 3 1.46 A 86/1005 1,37 A 86/1006 1 . 4 4 1.47 A 86/1007 1.27 A 86/80 1.54 A 86/1(109 1.47 A 86/1010 1.59 A 86/1011 1.58 A 86/1012 1.79 A 8611013 1.58 A 86/1014 1.49 A 86/1015 1.38 A 8611016 1.77 A 86/1017 1.47 A 86/1018 1.45 A 86/1019 1.44 A 86/102(I 1.42 A 86/1031 1.59 A 86/1032 1.07 A 86/1033 1. I l A 86/1034

continued

T A B L E 3. continued

Depth (m) 6 ' s o 813 C

7.35 7,35 7.39 7.40 7.45 7.45 7.45 7.46 7.46 7A7 7.47 7.51) 7,51 7.55 7.55 7.56 7.57 7.60 7.61 7.65 7,65 7.65 7.65 7.65 7,65 7.67 7.70 7.75 7.75 7.75 7.75 7.75 7.75 7.77 7.811 7.85 785 7.85 7.85 7.85 785 7.85 7.87 7.90 795 7 95 8.00 8.06 8.1)6 8.07 8.10 8.15 8.15 8.20 8.25 8,25 8.26 8.3(1 8.35 8.37 8.4(1 8.45 8.47 8.50 855 857 8.60 8.65 8.67 ~.7(1 8.75 8.77 8,80

- 1 . 3 4 - 1 . 6 0 - 1 . 4 3 - 1.24 -1 .18 -0 .95 -(I ,80 -0 .81 -(I.94 - 1 . 1 2 -0 .88 -0 .55 -0 .47 -0 .50 -0 .45 -0 .56 -0 ,49 -0 .43 -0 .67 -(/.61 -0 .70 -0 ,75 - 0 . 6 6 -0 .67 -0 .51 - 0 . 5 6 -0 .69 - 0 . 7 0 - 0 . 7 4 -0 .77 -0 .67 -0 .67 - 0 . 5 9 -(I.69 -0 .73 -0 .70 - 1.09 -0 .79 -0 .77 -0 .74 -(I.66 -0 .66 -0 .79 - 0 . 8 4 -0 .87 -(}.77 -0 .86 -(/.71 .-0.76 - 0 . 8 8 -(t .85 - 1 . 0 8 - 1 .(12 -1 .21 - 1,32 - 1 . 3 9 -1 .43 -1 ,33 - 1 , 4 4 - 1 , 4 0 - 1.511 - 1 . 4 7 - 1 . 4 3 - 1 . 4 8 -1 .41 -1 .23 - 1.23 - 1 . 2 7 - 1 . 0 4 - 1.(12 -0 .54 -(I .50 -(I .76

1.04 1,14 1.15 0.97 1.04 0.89 1 , 2 6 1.16 1.14 0.95 1 .(10 1.11 1.2(1 1.37 1 . 3 7 1 . 2 3 1 , 4 4 1.36 1 . 4 3 1 . 2 9 1 . 2 6 1.18 1 .36 t .42 1.03 1.13 1 , 2 0 1.33 1.32 1 . 2 4 1 . 3 3 1 . 4 7 1.3(I 1.21 1 . 3 3 1.25 1 , 2 4 1 . 2 3 1 . 2 9 1 . 3 0 1.32 1.17 1. I0 0.75 0.92 1 . 5 5 1 .25 1.22 t.30 1.33 1 . 4 6 1.37 1.42 1.36 1 . 5 l 1 . 4 4 1 . 3 2 1 . 5 2 1.61 1 .45 1 . 4 4 1 .49 1 . 3 2 1 . 3 4 1.18 1,14 1.14 1.19 0.98 1 . 0 5 1.14 1 . 5 0 1 . 2 4

continued

Carbon Isotope Records 393

T A B L E 3. continued

Depth Lab ref. (m) 6J~O 813C

A 86/1035 8.85 - 0 . 7 9 1.36 A 86/1036 8.87 - 1 . 0 6 1.47 A 86/1037 8.90 - 1 . 0 3 1.54 A 86/1038 8.95 - 1 . 0 8 1.24 A 86/1039 8.97 - 1 . 1 0 1.35 A 86/1040 9.00 - 1 . 1 7 1.25 A 86/1055 9.05 - 1 . 1 2 1.40 A 86/1056 9.10 - 1 . 1 4 1.29 A 86/1057 9.15 - 1 . 0 6 1.17 A 86/1058 9.17 - 1.00 1.22 A 86/1060 9.25 - 1 . 0 5 1.24 A 86/1059 9.26 - 0 . 9 7 1.19 A 86/1061 9.27 - 0 . 8 2 1.10 A 86/1062 9.30 - 0 . 8 2 1.32 A 86/1063 9.35 - 0 . 7 3 1.30 A 86/1064 9.37 - 0 . 6 8 1.60 A 86/1065 9.40 - 0 . 6 8 1.42 A 86/1066 9.45 - 0 . 7 8 1.30 A 86/1067 9.47 - 1 . 0 0 1.36 A 86/1068 9.50 - 0 . 8 6 1.35 A 86/1069 9.55 - 0 . 9 0 1.26 A 86/1070 9.57 - 0 . 9 9 1.23 A 86/1071 9.60 - 0 . 9 2 1.41 A 86/1072 9.65 - 1 . 1 4 1.27 A 86/1073 9.67 - 1 . 3 7 1.12 A 86/1074 9.70 - 1 . 3 6 1.19 A 86/1141 9.75 - 0 . 9 8 1.25 A 86/1142 9.78 - 1 . 0 8 1.26 A 86/1143 9.85 - 1 . 0 8 1.26 A 86/1144 9.90 -1 .21 1.29 A 86/1145 9.95 - 1.12 1.40 A 86/1146 9.97 - 1 . 2 5 1.30 A 86/1147 10.00 - 1 . 2 5 1.34 A 86/1148 10.05 - 1 . 2 4 1.27 A 86/1149 10.07 - 1 . 5 0 1.19 A 86/1150 10.10 - 1.27 1.33 A 86/1151 10.15 - 1 . 3 6 1.21 A 86/1152 10,17 - l . 2 5 1.26 A 86/1153 10,20 - 1.25 1.22 A 86/1154 10.25 - 1 . 2 6 1.20 A 86/1155 10.27 - 0 . 9 6 1.36 A 86/1156 10.30 - 0 . 9 6 1.29 A 86/1157 10.35 - 1 . 0 0 1.04 A 86/1158 10.37 - 1 . 0 3 0.99 A 86/1159 10.40 - 0 . 7 3 1.11 A 86/1160 10.45 - 0 . 9 0 1.15 A 86/1161 10.47 - 0 . 6 5 1.25 A 86/1162 10.50 -0 .71 1.40 A 86/1163 10.55 - 0 . 7 8 1.41 A 86/1164 10.57 - 0 . 7 6 1.55 A 86/1166 10.60 - 0 . 6 2 1.35 A 86/1165 10.65 - 0 . 7 6 1.54 A 86/1167 10.67 - 0 . 8 6 1.40 A 86/1168 10.70 - 0 . 8 6 1.49 A 86/1169 10.75 - 1 . 0 2 1.24 A 86/1170 10.77 - 0 . 9 4 1.47 A 86/1175 10.80 - 0 . 8 4 1.45 A 86/1176 10.85 - 0 . 9 8 1.36 A 86/1177 10.87 - 1 . 0 7 1.33 A 86/1178 10.90 - 1 . 1 8 1,36 A 86/1179 10.95 -1 ,11 1.44 A 86/1180 10.97 - 1 , 1 6 1,36 A 86/1181 11.05 - 0 , 9 2 1,55

*Mixed sizes.

T A B L E 4. Isotopic measurements of Globigerinoides sacculifer in core V24-109 picked from the sieve range 300-350 microns,

reported as a 8 value %o with reference to the PDB standard

Depth Lab ref. (m) 6J80 8t'~C

A 90/1491 0.100 - 2 . 0 1 1.52 A 90/1492 0.125 - 1 . 9 4 1.59 A 90/1493 0.150 - 1 . 8 8 1.64 A 90/1494 0.175 - 1 . 9 7 1.44 A 90/1495 0.200 - 1 . 7 4 1.59 A 90/1496 0.225 -2 .01 1.63 A 90/1497 0.250 - 1 . 8 2 1.51 A 90/1498 0.275 - 1.69 1.31 A 90/1499 0.300 - 1 . 5 4 1.53 A 90/1500 0.325 - 1 . 7 3 1.51 A 90/1501 0.350 - 1.41 1.49 A 90/1503 0.400 - 1 . 0 6 1.50 A 90/1608 0.400 - 0 . 9 9 1.58 A 90/1504 0.425 - 0 . 3 7 1.45 A 90/1609 0.425 - 0 . 4 3 1.51 A 90/1505 0.450 - 0 . 7 3 1.66 A 90/1506 0,475 - 0 . 7 5 1.69 A 90/1507 0,500 - 0 . 8 0 1.78 A 90/1610 0.500 - 0 . 7 9 1.64 A 90/1508 0.525 -0 .91 1.72 A 90/1509 0.550 -0 .81 1,72 A 90/1510 0.575 - 0 . 8 3 1.73 A 90/1511 0.600 - 0 . 9 4 1,77 A 90/1512 0.625 - 0 . 8 3 1,93 A 90/1513 0.650 - 1 . 0 6 1.72 A 90/1514 0.675 - 1 . 0 9 1.71 A 90/1515 0.700 - 1.07 1.73 A 90/1516 0.725 - 1.09 1.74 A 90/1517 0.750 - 1.19 1.69 A 90/1518 0.775 - 1 . 0 8 1.72 A 90/1519 0.800 - 1.16 1.72 A 90/1520 0.825 - 1 . 0 8 1.81 A 90/1521 0.850* - 1.34 1.73 A 90/1522 0.875 - 1 . 1 8 1.66 A 90/1523 0.900 - 1 . 2 7 1.81 A 90/1524 0.925 - 1 . 4 9 1.64 A 90/1525 0.950 - 1 . 3 2 1.65 A 90/1526 0.975 - 1.37 1.69 A 90/1527 1.000 - 1 . 1 6 1.36 A 90/1528 1.025 - 1 . 2 0 1.47 A 90/1529 1.050 - 1 . 1 6 1.54 A 90/1530 1.075 - 1 . 2 0 1.57 A 90/1531 1.100 - 1 . 0 9 1.59 A 90/1532 1.125 - 1 . 0 7 1.51 A 90/1533 1.150 - 1.14 1.67 A 90/1534 1.175 - 1 . 0 8 1.76 A 90/1535 1.200 - 1.12 1.83 A 90/1536 1.225 - 1 . 4 0 1.76 A 90/1537 1.250 - 1.34 1.77 A 90/1538 1.275 - 1 . 3 9 1.79 A 90/1539 1,300 - 1 . 5 5 1.83 A 90/1540 1.325 - 1 . 5 3 1.83 A 90/1541 1.350 - 1 . 3 9 1.94 A 90/1542 1.375 - 1.53 1.90 A 90/1543 1.400 - 1 . 6 2 1.90 A 90/1544 1.425 - 1 . 6 5 2.07 A 90/1545 1.450 - 1 . 5 4 1.80 A 90/1546 1.475 - 1.41 2.05 A 90/1547 1.500 - 1 . 5 5 1.99 A 90/1548 1.525 - 1.53 1.95 A 90/1611 1.525 - 1 . 4 4 1.97 A 90/1549 1.550 -0 .33** 1.80 A 90/1612 1.550 -1 .51 1.88 A 90/1550 1.575 - 1.49 1.76 A 90/1613 1.575 - 1.54 1.80 A 90/1551 1.600 - 1 . 5 9 1.57 A 90/1552 1.625 - 1 . 5 0 1.57 A 90/1553 1.650 - 1 . 5 7 1.63 A 90/1554 1.675 - 1.55 1.54 A 90/1555 1.700 - 1.58 1.66 A 90/1556 1.725 - 1.48 1.69

continued

394 N.J. Shackleton et al.

T A B L E 4. cont inued

Depth Lab ref. (m) 61sO 6~3C

A 90/1557 1.750 - 1.49 1.67 A 90/1558 1.775 - 1.63 1.61 A 9011559 1.800 - 1.58 1.68 A 9011560 1.825 - 1.65 1.63 A 90/1561 1.8511 - 1.58 1.50 A 90/1562 1.875 - 1.64 1.58 A 9011563 1.900 - 1.40 0.99 A 90/1564 1.925 - 1,49 1.75 A 90/1614 1.925 - 1.35 1.95 A 90/1565 t .950 - 1.72 1.31 A 90/1615 1.950 - 1.61 1.38 A 90/1566 1.975 - 1.87 1.28 A 90/1616 1.975 - 1 . 8 4 1.31 A 90/1567 2.(100 - 1.36 1.39 A 90/1617 2.000 - 1.20 1.63 A 9011568 2.025 - 1.75 1.18 A 90/1618 2.{)25 - 1,62 1/.86 A 90/1569 2.050 - 1.77 1.10 A 90/1619 2.t/51/ - 1.68 1.12 A 90/1570 2.1175 - 1.85 1.19 A 90/1621/ 2.1t75 - 1.70 1.36 A 90/1571 2.1011 - 1.58 1 .(17 S 90/1402 2.100 - 1.53 1.23 A 9011572 2.125 - 1.23 1.25 S 90/1403 2. 125 - 1.27 1.29 A 90/1573 2.150 - 0 . 8 9 1.25 S 90/1404 2.150 - / / . 8 8 1.44 A 9011574 2.175 - 1.1)8 1.29 S 9011405 2.175 -1/ .97 1.41 A 90/1575 2.200 - 0 . 7 5 1.36 S 9011406 2,200 - 0 . 8 8 1.48 A 9011576 2.225 - 0 . 9 0 1,45 A 90/1577 2.2511 - 0 . 7 8 1.42 S 90/1407 2,250 -1/ .98 1.47 A 90/1578 2.275 -11.81 -1.37 A 90/1579 2.3011 -1/ .96 1.51 A 90/1580 2.325 -1/ .86 1.39 S 90/1051 2.3511 -1/.91 1.33 S 90/1052 2.375 -11.61 1.42 S 90/1053 2.400 - 0 . 8 7 1.32 S 90/1054 2.425 - 0 . 8 2 1.34 S 90/1055 2.450 - 0 . 8 5 1.28 S 90/1056 2.475 - 0 . 8 6 1.32 S 90/1057 2.5011 - 0 . 8 1 1.31 S 90/1058 2.525 - 1 .(15 1.27 S 90/1059 2.550 - 1.17 1.25 S 90/1060 2.575 - 1.10 1.24 S 90/1061 2.61X) - 1.26 1.31 S 90/1062 2.625 - 1.(18 1,28 S 90/1063 2.650 - 1.08 1,25 S 90/1064 2,675 - 0 . 7 8 1.18 S 9011065 2,700 - 1.05 1.24 S 90/1066 2.725 - 0 . 9 9 1.32 S 9011067 2.750 - 1.01 1.33 S 90/1068 2.775 - 1.00 1.45 S 90/1069 2.8011 - 1.28 1.44 S 90/1071t 2.825 - 1.44 1.53 S 90/1071 2.850 - 1.39 1.59 S 90/1072 2.875 - 1.53 1.53 S 9011073 2.900 - 1.61 1.68 S 9011074 2.925 - 1.65 1.74 S 9011075 2.950 - 1.74 1.56 S 90/1076 2.975 - 1.56 1.56 S 90/1077 3.0111/ - 1.66 1.61 S 9011078 3,025 - 1.62 1.5 l S 9011079 3.0511 - 1.56 1.56 S 90/1081 3.100 - 1.61 1.64 S 9011082 3.125 - 1.63 t .55 S 90/1083 3.150 - 1.50 1,35 S 90/1084 3.175 - 1.41 1.42 S 90/1085 3.200 - 1.41 1.46 S 90/1086 3.225 - 1,47 1.32

continued

T A B L E 4. continued

Depth Lab ref. (m) ~'¢~O 6 ~ ~C

S 90/1087 3.251t - I. 13 1.49 S 90/1088 3.275 - t .02 1.49 S 911/1089 3.0110 - 1.02 1.43 S 90/1090 3.325 - 0 . 9 0 1.43 8 9011091 3.350 - i. 10 1.48 S 9011092 3.375 - 1.21 1.45 S 90/1093 3.41/5 - 1.49 1.68 S 9011094 3,425 - 1.56 1,77 S 90/1095 3,450 - 1.58 1.bl S 90/1096 4.475 - 1.64 1.51 S 90/1097 3.5011 - 1.52 1.52 S 90/1098 3.525 - 1.44 1.44 S 90/1099 3.550 - 1.39 1.35 S 90/11110 3,575 - 1.36 1.19 S 90/1101 3.60{) - 1.03 1.29 S 90/1102 3.625 - 1.110 1.34 S 90/1103 3.651/ - / I .91 1.44 S 90/1104 3.675 - 1.25 1.27 S 90/1105 3.700 - 0 . 9 2 1.41 S 90/1106 3.725 - 0 . 9 4 1.26 S 90/11117 3.750 - 0 . 9 7 1.22 S 90/1108 3.775 -11.98 1.33 S 90/1109 3,800 - 1.01 1.23 S 90/111{/ 3.825 -1/ .84 1.23 S 90/1111 3.8511 - 0 . 8 3 1.50 S 90/1112 3.875 -(I.82 1.66 S 90/1113 3.9011 - 1.00 1.44 S 9011114 3.925 - 1.10 1.611 S 90/1115 3.9511 - 1.21 1.65 S 90/1116 3,975 -1 .17 1.61 S 90/1117 4.(100 -- 1.33 1.60 S 911/1118 4.025 - 1.32 1.55 S 90/1119 I-.050 - 1.39 1.65 S 9t)/11211 4.075 - 1.43 1.57 S 90/1121 4.1011 - I. 15 1,68 S 90/1122 4.125 - 1 . 3 0 1,71 S 9011123 4.1511 - 1 . 1 5 1,46 S 90/1124 4,175 - 1.32 1.49 S 90/1125 4,200 - 1.46 1.53 S 90/1126 4,225 - 1.54 1.54 S 90/1127 4.250 - 1.50 1.51/ S 90/1128 4,275 - 1.46 1.50 S 90/1129 4.3011 - 1.40 1.51 S 90/1130 4,325 - 1.46 1.53 S 90/1131 4.350 - 1.50 1.46 S 90/1132 4.375 - 1.51 1.52 S 90/1133 4.41111 - 1.67 1.46 S 9011134 4.425 - 1 . 7 l 1.39 S 9011135 4.451t - 1 . 6 0 1.36 S 9(111136 4.475 - 1,63 1.27 S 9011137 4.500 - 1 . 8 2 1.37 S 90/1138 4.525 - 1.87 1.53 S 90/1139 4,550 - 1.80 1.49 S 90/1141 4.601/ - 1 . 7 1 1.59 S 9011142 4.625 - 1.74 1.70 S 90/1143 4.651t - 1.72 1,48 S 90/1144 4.675 - 1.59 1.43 S 90/1145 4.700 - 1,51 1.42 S 90/1146 4.725 - 1.43 1,23 S 90/1147 4 7 5 0 - 1 . 2 2 1.21 S 9011148 4.775 - 1.22 I. 18 S 90/1149 4.800 - 0 . 9 2 1.20 S 90/1150 4.825 - 0 . 6 9 1,39 S 9011151 4,850 - 0 . 6 7 1.47 S 90/1152 4 8 7 5 - 0 . 7 6 1.55 S 90/1153 4 9 0 0 - 0 . 8 3 1.46 S 90/1154 4.925 -1/ ,86 1.5{I S 90/1155 43)50 -1/.81/ 1.52 S 9011156 4,975 - t / .71 1.68 S 90/1157 5.000 - 0 . 5 5 1.61

*Mixed sizes.

Carbon Isotope Records 395

-2.0

-1 .s

> -1.o

-2.0

-1.5

~. -1.0

rr -0.5

J i i : i i

0.1 0.2 013 014

age ka

-2.0 _.e

-1.5 o.

-1.0

-0.5

FIG. 1. Oxygen isotope records of G. sacculifer in cores V24-109, ERDC-93P and RC17-177 plotted versus age (from Tables 2-4 and 5). For clarity the records are offset from each other.

2.0

1.s v - -

1.0 ERDC-93P 2.0

1.5 ~

1.o

1.5 o.5 "7

1.0

i J i =2 i t

0.1 0. 0.3 0.4

age ka

FIG. 2. Carbon isotope records of G. sacculifer in cores V24-109, ERDC-93P, and RC17-177 plotted versus age (from Tables 2- 4 and 5). For clarity the records are offset from one another,

of assigning ages to isotopic extremes that was advo- cated by Prell et al. (1986).

Figure 1 shows the oxygen isotope records of the three cores, and Fig. 2 the carbon isotope records. All three cores display the familiar oxygen isotope record but as expected in cores accumulating at 1.0-1.5 cm/ka, the records are all somewhat 'noisy'.

In order to use the 613C data to estimate changes in atmospheric pCO2 we need to subtract the effect of changes in the 613C of the whole ocean. Instead of assuming that a single high-quality Pacific record is a good estimator of changing ocean ~i13C, we have used a stack that has been generated from benthonic 6~so and 613C records from a range of water depths in the major ocean basins (Mix and Shackleton, in prepara- tion). In this stack each record is weighted according to the proportion of the global ocean carbon inventory that it represents.

MATHEMATICAL TREATMENT

Standard methods of time-series analysis have been used (Jenkins and Watts, 1968). However we have used a gaussian weighting procedure while stacking the measurements in order to generate mean 6tSo and ~13C records using the data from the three cores, which being in the same region, would ideally be essentially identical, both as regards the pattern of variation and the absolute values. Figure 3 shows the stacked 6180 and 613C records together with an indication of the variability expressed as the standard deviation observed at each horizon in the stacking process. Time series were interpolated at 3ka intervals and analyses were performed on a series of 151 data points extending to 450ka. For cross-spectral analysis 60 lags were used; in the coherency plot a line is drawn at the 95% confidence level. In the phase plot, the

396 N.J. Shackleton et al.

TABLE 5. Age control points used to develop the timescale of cores

ERDC-93P, RC17-177 and V24-109

Depth Age ERDC-93P (cm) (Ma)

0.0 0.000 10.0 0.012 15.0 0.019 17.5 0.024 37.5 0.053 40.0 0.059 42.5 0.065 47.5 0.071 72.5 0.105 82.5 0.111

115.0 0.128 122.5 0.146 150.0 O. 166 170.0 0.176 177.5 0.186 197.5 0.212 225.O (̀ ).228 255.0 0.246 272.5 0.257 290.0 0.269 307.5 0.293 315.0 0.299 322.5 0,310 337.5 O,325 347.5 0.334 364.0 0.339 380.0 0,362 390.0 0.368 405.0 0.395 417.5 0.405 442.5 0,434 452.5 0.460 462.5 0.476 470.0 0.482 500.0 I).53(I

TABLE 5. continued

Depth Age RC17-177 (cm) (Ma)

495.0 0.423 510.0 0.434

Depth Age V24-109 (cm) (Ma)

0.0 0.000 35.(/ 0.012 42.0 0.019 65.0 0.024 92.0 0.053

1117.0 0.059 117.(I 0.065 125.0 0.072 140,(I 0,076 150.0 0,095 17(I.(I 0.105 190.(I 0. III 197.0 0. 113 212,(I 0.128 230.0 0.146 255.0 O. 166 265.0 O. 176 282.0 O. 186 312,0 0.212 332.0 (.).228 360.0 0.245 367.0 0.257 385.0 0.269 407.0 0.287 412,0 0.299 425,0 0.310 440.0 0,325 477.0 0.339 500,(I (1.351

Depth Age RC17-177 (cm) (Ma)

0.0 0.000 18.0 0.012 32.0 0.019 52.0 0.024 75.0 0.053 89.0 0.059 92.0 0.065

103.0 0.072 128.0 0.095 140.0 0.104 145.0 0.111 180.0 0.128 190.0 0.146 218,0 0.106 221.0 O. 176 239.0 O. 186 250.0 0,196 255.0 0,206 285.0 0,228 311.0 0.246 321.0 0.257 335.0 0.269 351.0 0.293 355.0 0.299 370.0 0,310 375.0 0.322 381.0 0.325 407.0 0,339 435,0 0.361 465,0 0.395 479.0 (I.407

continued

phase angle is only shown at those f requenc ies at which cohe rency is s ignif icant .

D I S C U S S I O N

Cross - spec t ra l analysis (Fig. 4) conf i rms that our oxygen i so tope da t a have been cor rec t ly c o r r e l a t e d to the ben thon i c s tack o f Mix and Shack le ton (in prepara- tion). The analysis also gives an ind ica t ion o f the l imi ta t ions of r eco rds f rom this reg ion; the p recess iona l signal is p o o r l y r e p r e s e n t e d . Cross - spec t ra l analysis agains t the S P E C M A P s tack ( Imbr i e et al., 1984) gives a very s imi lar resul t .

F igure 5 shows a c ross -spec t ra l analysis of the 6t3C da t a agains t 6 t 8 0 in the same s tack. A s was o b s e r v e d by Shack le ton and Pisias (1985) r ega rd ing ~}~3C in p l ank ton i c spec ies f rom the ea s t e rn Pacif ic , the phase is s ignif icant ly non-ze ro . These au thors r e a s o n e d tha t the 613C of sur face w a t e r is not i tself i nhe ren t ly in te res t ing since it c o m p o u n d s the w h o l e - o c e a n effect (Shack le ton , 1977) with the ver t ica l f r ac t iona t ion tha t ( fo l lowing B r o e c k e r , 1982) we are seeking . In o r d e r to r e m a i n essent ia l ly i n d e p e n d e n t of the da t a pub l i shed by Shack- l e ton and Pisias (1985), we use the s t acked ~)13C record of Mix and Shack le ton (in preparation) to

Carbon Isotope Records 397

0

0

s ~ - - " ~ ~1 ~ ~ ~ . s ~ . ~

• t t t l ~ - ~ . • ~ •

0 i i f i 0 100 200 300 400 500

age ka

FIG. 3. Above: stacked 6~sO record, below: stacked 6t3C record, from Figs 1 and 2, of the three cores from the Ontong Java Plateau. Dotted lines indicate the moving effective standard deviation in the data stacked.

1

bendw~ltfl

i-lL,.o--

90

0

l -90 t t I 0 . 0 2 0 . 0 4 0 .

lOOka 40ka 23ka frequency

FIG. 4. Cross spectral analysis of stacked planktonic 6tsO (Fig. 3) versus 6~aO in the benthonic stack shown in Fig. 6 (Mix and

Shackleton, in preparation).

remove the whole-ocean effect. This stack is con- structed from Cicicidoides records and from some data from other species, only where it is demonstrated for that particular location that the other species gives ~13C data parallelling Cibicidoides values. The record of V19-30 is one of some 20 cores in this stack; high resolution data from Kasten core TR163-31 at the same location as V19-30 show that the Uvigerina ~13C record from this locality is indeed very similar to a Cicicidoides record at least for the past 30 ka. Figure 6

1 g

O.

_E i 1"

,2 ,

l

1o

o

)o o

, " ' ' " , , OJ also plank:ionic

%

t t , ~ ,

T I OOka 40ka 28ka frequency

(cyc~;ka)

FIG. 5. Cross spectral analysis of stacked planktonic 613C versus stacked planktonic 6~80, both from Fig. 3.

shows the difference record generated in this way. For comparison the record generated from core V19-30 is also shown on the same scale.

Following the earlier work, we show in Fig. 7 a cross- spectral analysis of this new stack-generated CO2 record against the oxygen isotope record. Figure 7 confirms the earlier report (Shackleton and Pisias, 1985) that changes in reconstructed atmospheric CO2 lead changes in global ice volume (~180) in such a sense that CO2 should be regarded as part of the forcing of ice volume change. Table 6 compares the

398 N.J. Shackleton et al.

1.5

,,;,

~ 3 C gradient planktonic-benthonic

3.0 benthonic stack ~180

3.5

4.0

4.5

5.0 ~ , , ~ 1 O0 200 300 400 500

age ka

FIG. 6. AbL~C record generated by subtracting the global b~3C stack of Mix and Shackleton (in preparation) from the stacked planktonic 6~3C record of Fig. 3. Dashed: the , ~ 3 C record of Core V19-30 (Shackleton and Pisias, 1985). Below: stacked benth-

onic 6[80 record of Mix and Shackleton (in preparation).

T A B L E 6. Phase estimates and 95% uncertainty ranges (degrees)

Period

Records compared 100ka 40ka 23ka

estimates of phase lead of this n e w A613C record against the stacked planktonic 6180 data from the same cores, this same A613C record against the benthonic b l s o stack of Mix and Shackleton (in preparation), and the phases estimated by Shackleton and Pisias for the shorter but higher-resolution benth- onic bt80 record of core V19-30.

It has been disputed in the past that measured CO2 as found in the Vostok ice core does in fact lead global ice volume. Pending evaluation of the important work of Sowers et al. (in press) which seeks to correlate the Vostok atmospheric record directly with the ocean btSo record this has to be regarded as uncertain. However, in order to demonstrate the likelihood that the record we reconstruct is a valid approximation to the 'true' record and that there is no phase inconsis- tency, we have made a new comparison of the two approaches.

Figure 8 compares the CO2 reconstruction of Shack- leton and Pisias (1985) with a modified version of the Vostok record of Barnola et al. (1987). The modifica- tion consists of simply re-aligning the temperature record of the Vostok ice core (Lorius et al., 1985), which was originally published on a glaciological timescale, with the surface temperature record obtained from sub-antarctic core RCl l -120 as ex-

new 'CO2' to new 6~80 - 3 4 _+ 24 - 4 8 _+ 12 - 6 0 _+ 26 new 'CO2" to Mix bl"O - 3 6 + 25 - 5 2 +_ 14 - 6 3 + 23 V19-30 'COs ' to b]80 - 3 6 +_ 22 - 2 4 _+ 23 - 2 8 +_ 26

/ t 8

bandwidth

,~''', ben~onic ~80stack / / x

E

I ' o.o°' -90 ~ 0.04

lOOka 40ka 23ka frequency (cyck~/ka)

F I G . 7 . C r o s s s p e c t r a l a n a l y s i s o f A 6 1 3 C ( ' C O z ' ) a g a i n s t s t a c k e d b e n t h o n i c b~aO. M o d e l l e d C O 2 l e a d s ( n e g a t i v e ) i ce v o l u m e ( s e e

T a b l e 6 ) .

o=

R

300 290 28O 270 280 250 240 23O 220 210 200 190 180 170

4 3 - 2 - 1 - I

-1 - -2 -

~3 - - 4 - -5 -6 -7 -8 -9

-10

0

i ~ i i r

C02(V19-30) t~ ;, ~ '

,q , s J i

• ~, I I ,,; ;

C 0 2 (

/ temperature (RC11-120)

.~ ii ,I i . t~' ; #x,, , .,. f'tl\ f 1

, , , , , , , , , , ,

20 40 60 80 100 120

aoeka

140 160

FIG. 8. Reconstructed CO2 record from 6~3C data in core V19-30 (modified from Shackleton and Pisias, 1985) compared with CO2 record from Vostok (Barnola et al., 1987) on modified timescale. Below: sea surface temperature reconstruction from sub-antarctic core RCl l -120 (Hays et al., 1976; Martinson et al., 1987) and temperature record from Vostok (Lorius et al. , 1985); triangles show control points used to correlate the Vostok record to RCII*120 and

hence express it on the SPECMAP timescale.

Carbon Isotope Records 399

pressed on the SPECMAP timescale by Martinson et al. (1987). The procedure is analogous to that of Petit et al. (1990), who chose to correlate dust in the Vostok record (de Angelis et al., 1987) with magnetic suscepti- bility in RCll-120 (Kent et al., 1982). We recognize that the comparison of Fig. 8 is not rigorously valid, but we argue that given the uncertainties in the correlations as well as the uncertainties in the two independent sets of CO2 estimates, there is no basis for asserting that the CO2 reconstruction based on 613C data is different in phase from that based on measurements of the CO2 concentration in the bubbles trapped in the Antarctic ice sheet.

Turning back to the longer and less detailed record of Fig. 6, it may be remarked that the reconstruction appears more stable than that given by Shackleton and Pisias (1985); for example the value in Stage 6 resembles that in Stage 2 (as is observed at Vostok; Barnola et al., 1987). This suggests that the correct means for deriving long records of past atmospheric CO2 is to pursue further the approach of stacking good quality planktonic (~13C records from uncomplicated areas in the open ocean. By this method is should be possible to generate much longer records of variability in atmospheric CO:.

These comparisons do not however tell the full story. It will be observed that the glacial-interglacial range in surface 613C in our new stack is generally below 0.5 per mil, whereas in the V19-30 record it approaches twice that figure. The range in global deep water 6a3C estimated in the stack genera tedby Mix and Shackleton (in preparation) is also somewhat less than the range measured in core V19-30. The overall result is that the range in atmospheric CO2 estimated by this measure is only about 50ppm over the last climatic cycle. The earlier estimate from core V19-30 was close to the 100ppm range that is observed in the ice cores (Barnola et al., 1987; Neftel et al., 1988). It is not yet clear whether this reduced range reflects the effect of bioturbation, or whether it provides a more accurate estimate of the contribution of the 'biological pump' in controlling natural variation in atmospheric C02.

CONCLUSIONS

The atmospheric CO2 record generated from mea- surements of planktonic and benthonic b~3C in core V19-30 has substantial validity despite the valid criti- cisms that have been made (especially, that N. dutertrei in that area is not calcifying in nutrient-free surface water). An independent reconstruction generated by stacking several new records from the western Pacific is probably a better representation of the 'biological pump' simulation of the long-term atmospheric CO2 record and this new record supports the notion that changes in atmospheric CO2 have played a role in forcing the glacial-interglacial climate cycles. How- ever, it now appears that the simplified approach that we have taken is probably unable to explain the full

amplitude of observed pCO2 change observed in the ice cores. Boyle (1988) has discussed more realistic model- ling of changes in atmospheric pCOa and this contribu- tion only attempts to improve one of the critical data sets for such modelling.

ACKNOWLEDGEMENTS

This study was supported by EEC grant EV4C-0043-UK and by NERC grant GR3/3606 to NJS, and NSF grant ATM-8812640 to AM. We are grateful to R. Lotti, core curator at Lamont Doherty Geological Observatory, for providing samples curated under NSF grant AMT-8812640. In addition we thank Wolfgang Berger for samples from core ERDC-93P. Additional laboratory assistance by David Pate and Simon Crowhurst is gratefully acknowledged. Claud Lorius generously shared data from the Vostok ice core prior to publication, and discussion with him on more than one occasion has stimulated our attempts to understand the CO2 record. The gaussian weighting method used is an adaptation by Will Howard (Brown University) of the program developed for NJS and J. lmbrie by Angeline Duffy (also Brown University); we thank them both for this and for other help with programs. We are also grateful to Joe Morley (Lamont Doherty Geological Observatory) and Simon Crowhurst for re-arranging the time-series analysis programs for use on MS-DOS machines. We thank Tom Pedersen for a very careful review of an earlier MS.

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