The Astronomical Theory of Climate and the Age of the Brunhes-Matuyama Magnetic Reversal

EPSL ELSEVIER Earth and Planetary Science Letters 126 (1994) 91-108

The astronomical theory of climate and the age of the Brunhes-Matuyama magnetic reversal

Franck C. Bassinot a,1, Laurent D. Labeyrie b, Edith Vincent a, Xavier Quidelleur c Nicholas J. Shackleton d, Yves Lancelot a a Laboratoire de Gdologie du Quaternaire, CNRS-Luminy, Case 907, 13288 Marseille cddex 09, France b Centre des Faibles Radioactivit&, CNRS/CEA, Avenue de la Terrasse, BP 1, 91198 Gif-sur-Yvette, France c Institut de Physique du Globe, Laboratoire de Pal~omagn&isme, 4 Place Jussieu, 75252 Paris c~dex 05, France d Department of Quaternary Research, The Godwin Laboratory, Free School Lane, Cambridge CB2 3RS, UK Received 3 November 1993; revision accepted 30 May 1994

Abstract

Below oxygen isotope stage 16, the orbitally derived time-scale developed by Shackleton et al. [1] from ODP site 677 in the equatorial Pacific differs significantly from previous ones [e.g., 2-5], yielding estimated ages for the last Earth magnetic reversals that are 5-7% older than the K/Ar values [6-8] but are in good agreement with recent Ar/Ar dating [9-11]. These results suggest that in the lower Brunhes and upper Matuyama chronozones most deep-sea climatic records retrieved so far apparently missed or misinterpreted several oscillations predicted by the astronomical theory of climate. To test this hypothesis, we studied a high-resolution oxygen isotope record from giant piston core MD900963 (Maldives area, tropical Indian Ocean) in which precession-related oscillations in t~180 are particularly well expressed, owing to the superimposition of a local salinity signal on the global ice volume signal [12]. Three additional precession-related cycles are observed in oxygen isotope stages 17 and 18 of core MD900963, compared to the SPECUAP composite curves [4,13], and stage 21 clearly presents three precession oscillations, as predicted by Shackleton et al. [1]. The precession peaks found in the 3180 record from core MD900963 are in excellent agreement with climatic oscillations predicted by the astronomical theory of climate. Our ~180 record therefore permits the development of an accurate astronomical time-scale. Based on our age model, the Brunhes- Matuyama reversal is dated at 775 + 10 ka, in good agreement with the age estimate of 780 ka obtained by Shackleton et al. [1] and recent radiochronological Ar/Ar datings on lavas [9-11]. We developed a new low-latitude, Upper Pleistocene ~180 reference record by stacking and tuning the 3180 records from core MD900963 and site 677 to orbital forcing functions.

1. Introduction [PT] Present address: Centre des Faibles Radioactivit&, CNRS/CEA, Avenue de la Terrasse, BP 1, 91198 Gif-sur- Mathematically formulated early in this cen- Yvette, France. tury by Milutin Milankovitch, the astronomical

0012-821X/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0012-821X(94)00127-K 92 F.C. Bassinot et al. /Earth and Planetary Science Letters 126 (1994) 91-108 theory of climate was widely accepted by Earth absolutely certain whether such a composite scientists only about 15 years ago, when Hays et record is complete and permits an accurate astro- al. [14] unambiguously showed that fluctuations nomical calibration. Furthermore, no magne- in paleoclimatic indicators in geologic records do tostratigraphy was available at site 677 and posi- contain the periods associated with the Earth's tions of the magnetic reversals were inferred from orbital components (namely, eccentricity of the biostratigraphy and oxygen isotope stratigraphy. Earth's orbit, tilt and precession of the Earth's We have addressed these uncertainties in the axis). The astronomical theory of climate brought Upper Pleistocene (last 900 kyr) by studying the important insights into the mechanisms that con- oxygen isotope record provided by the ~ 53 m trol global changes and it has also opened the long piston core MD900963 (tropical Indian way to a very powerful means for developing Ocean) in which the Brunhes-Matuyama reversal accurate geological time-scales. By fine tuning has been precisely located. The core MD900963 paleoclimatic indicators to astronomical forcing provides one of the most detailed Late Pleis- functions, Earth scientists have provided detailed tocene climatic records ever retrieved in low lati- Pliocene-Pleistocene chronologies for marine tudes. It allows us to compare in detail paleocli- sediments with a theoretical accuracy of a few matic oscillations with variations in orbital forc- thousand years [e.g., 2-5,15-18]. In the late Pleis- ing, making it possible to test the accuracy of the tocene, with the exception of Johnson's results orbitally derived timescale of Shackleton et al. [15], orbitally derived ages for the last Brunhes- [1]. Matuyama magnetic reversal (e.g., 728 ka [2]; 738 ka [3]; 734 ka [4]) have been in good agreement with the 730 ka age obtained by K/Ar ra- 2. The giant piston core MD900963: location, diochronological dating [6]. This agreement ap- biostratigraphy and magnetostratigraphy parently indicated that all the climatic variations predicted by the astronomical theory of climate Core MD900963 was collected in the In- were observed in the paleoclimatic records avail- dian Ocean, east of the Maldives platform able so far. (05°03.30'N-73°52.60'E) from a water depth of Four years ago, however, Shackleton et al. [1] 2446 m, during the MD65-SEYMAMA expedi- proposed a revised orbitally derived time-scale tion of the French R/V Marion Dufresne in 1990 for the last 2.6 Myr that differs significantly from [19]. The core liner was 52.70 m long and the previous ones below oxygen isotope stage 16 (~ sediment thickness retrieved is 51.70 m. With the 620 ka) and yields estimated ages for the last six exception of minor flow-in structures at the bot- major reversals of the Earth's magnetic field that tom of sections 3 (roughly between about 4.0 and are 5-7% older than the K/Ar radiometric val- 4.5 m) and 4 (between about 5.5 and 6.0 m), the ues [6-8]. These ages have since been confirmed sediment does not present any evidence of coring by precise Ar/Ar dating obtained on carefully disturbance. The sediment is free of turbidite selected single crystals of sanidine from lavas layers but a slump of 60 cm was found between [e.g., 9-11]. At first sight, this independent con- 26.30 and 26.90 m. Measurements performed firmation of Shackleton et al.'s age estimates within this slump interval were discarded and the suggests that several climatic oscillations pre- entire interval was eliminated in the calculation dicted by the astronomical theory of climate were of final sub-bottom depths. missing in paleoclimatic records used by former Biostratigraphy is based on calcareous nanno- investigators for the development of orbitally de- fossil and planktonic foraminifer datums studied rived timescales. However, the 120 m long oxygen at a 10 cm sampling interval. The depositional isotope record used by Shackleton et al. was not history of core MD900963 records a major hiatus retrieved in one single piece but constructed by at about 41 m, as shown by the concomitant carefully splicing together 9.5 m long hydraulic disappearance within a narrow depth range of piston cores using all the available data. It is not Helicosphaera sellii, Calcidiscus macintyrei and F.C Bassinot et aL / Earth and Planetary Science Letters 126 (1994) 91-108 93

Globigerinoides fistulosus, and the appearance of mentation rate (averaging 4.5 cm/kyr) minimized Gephyrocapsa oceanica [for details see 20]. How- smoothing of the climatic signal by bioturbation, ever, the upper 41 m of core MD900963 covers thus permitting the recovery of one of the most the entire Late Pleistocene, down to isotope stage detailed deep-sea records of Late Pleistocene cli- boundary 23.0 (Fig. 1). Isotopic stages defined by matic variability ever retrieved in low latitudes. Emiliani [21] and Shackleton and Opdyke [22] are The sedimentation rate translates into an average particularly well expressed, owing to the superpo- sample spacing of about 2.2 kyr. Unfortunately, sition of the global ice volume signal and a local the uppermost portion of the oxygen isotopic salinity effect, most probably controlled by curve does not extend into the Holocene. This changes in monsoon intensity [12]. The high sedi- suggests that the very top part of the sedimentary

8180 (%0) Paleomagnetic declination (o)

0 -1 -2 -3 -50 50 150 250 0 I I I I I I I I I I I

% 10 ~-- 7

o o o oo © 20 11 © oo E %

1- ~ Y O. % 0 © C3 15 ° o 30 % 17 ©

19 o B/M 21 © 40 Hiatus

Fig. 1. Stable oxygen isotope stratigraphy and magnetic declinations in core MD900963. 94 F.C. Bassinot et al. / Earth and Planetary Science Letters 126 (1994) 91-108 section was not preserved in the core. A loss of (~1 8 0 (%o) this type is not uncommon in piston coring [e.g., 23]. 0 -1 -2 -3 i i I,~ i I I I Paleomagnetic measurements were performed 34 with standard plastic cubes (8 cm 3) taken roughly every 75 cm along the core, from the top of the core and about 40 m. The mean sampling interval was reduced to about 15 cm around the Brun- 34.5 hes-Matuyama reversal. In addition, U-channels were measured in order to get continuous and B/M detailed records within specific intervals, selected 35 A 'ansition on the basis of the results of the single samples. E v We followed the procedure described by Nagy and Valet [24], involving measurements every 2 ,.C 35.5 cm using the high-resolution pick-up coils of a Q. three-axis cryogenic 2G magnetometer. Stepwise alternating field demagnetization of the natural 36 ~ n remanent magnetization (NRM) up to 55 mT for single samples and up to 40 mT for U-channels was performed within a shielded room at the 81 Institut de Physique du Globe de Paris. 36.5 _ 80--11 Results of single samples and U-channels paleomagnetic analyses are plotted versus depth in Fig. 1 (single samples analyses are presented only 37 I I I I for intervals without U-channel data, with the -100 0 100 200 exception of the interval containing the Brun- hes-Matuyama reversal for which both data sets Declination (o) are shown on the figure). The sediments between Fig. 2. Position of the Brunhes-Matuyama reversal in core 24.3 m (middle of section 18) and about 26.3 m MD900963 as inferred from paleomagnetic measurements (end of section 19) present anomalous magnetic performed on discrete samples (squares) and on U-channels (dashed line with circles). See text for details. declinations, with a constant angular deviation of 100 ° compared to values directly measured above and below this interval. However, as is clearly interval, as also suggested by other studies [25]. seen in Fig. 1, the location of the B/M reversal is Therefore, the position of the reversal must be unambiguously identified from both sets of re- defined with an uncertainty linked to the length sults. There is an abrupt change in declination at of this interval. We notice that the only result 35.13 m (observed from the U-channels) between obtained with a single sample within this interval 180 ° and 0 °. Within the same interval the magne- confirms our interpretation. Our best estimate of tization intensity is very low and the angular the position of the Brunhes-Matuyama transition variations between the successive directions iso- is at 35.01 + 0.19 m (Fig. 2). lated after each demagnetization step increase to values exceeding 10 °. This behaviour reveals a zone (between 34.82 and 35.20 m) where it is 3. Late Pleistocene oxygen isotope stratigraphy in impossible to isolate a clear characteristic compo- core MD900963 nent, probably because the field intensity was too low. Among several possibilities, we cannot ex- We measured the oxygen isotope composition clude that transitional directions could not be (~180) of planktonic foraminifers Globigerinoides recorded properly by the sediment within this ruber (white) on samples collected at a 10 cm F.C. Bassinot et aL / Earth and Planetary Science Letters 126 (1994) 91-108 95 interval. The mass spectrometer used was a to stage 11, shows good agreement with the 6180 Finnigan MAT 251, with automated carbonate composite curves produced by the SPECMAP work- preparation system (individual reaction chamber ers, but below stage 11 it shows important differ- device). The data are all reported with respect to ences compared to these records (Fig. 1) [4,13]. In NBS 19 [26]. Mean external reproducibility was stages 12-14, the 6180 signal of core MD900963 0.05%c at ltr. The mean sample size was 3-5 is slightly noisier compared to the rest of the shells per analysis. curve (Fig. 1). We tentatively assume that the The upper part of MD900963 8~So curve, down 6~80 peaks labelled X and Y correspond to iso-

Ice volume 6]SO (%0) (arbitrary scale) 0 - 1 - 2 ~ more ice

0 ~ I ~ I ~ I ~ I

......

300 ...... Z......

---"-"~ 400 i~

< 500

6 0 0 ...... _~15 ......

700

900 Fig. 3. Correlation of the MD900963 &zs0 record with the ice volume curve of [28] used as a target curve for the development of our orbital chronostratigraphy. 96 F.C. Bassinot et al. /Earth and Planetary Science Letters 126 (1994) 91-108 topic events 13.2 and 13.3, respectively, although A band-pass filter was then used to extract the they reach unusually high and low relative values astronomical components of the ~180 stacked compared to values observed either in stacked record and these extracted components were records [4,13] or in other high-resolution 6180 tuned to astronomical functions. However, differ- records [1]. ent initial age models may be developed from In the lower Brunhes chronozone, discrete core MD900963 depending on the radiochrono- substages in the 6180 curve of MD900963 are logical age assigned to the Brunhes/Matuyama much more clearly expressed than in most deep- reversal. Since extraction of astronomical compo- sea isotopic records, and especially those used for nents by band-pass filtering of climatic records is the SPECMAP stack [4,13]. In stage 17, three peaks very sensitive to inaccuracies in the initial are clearly observed in our 6180 record; in the chronology [17], we decided to tune the complete, SPECMAP stack, this interglacial interval is re- unfiltered 6180 curve to orbital functions di- duced to two precession cycles. An additional rectly, without making any preliminary depth-to- peak is also observed in stage 18 of core time conversion based on radiochronologically MD900963 at about 33.50 m (Fig. 1). This peak calibrated control points. In such a tuning ap- can be seen in the 6180 record of ODP site 502, proach, the target curve must obviously have as which is one of the five records used for the many features as possible in common with the construction of the composite record used by oxygen isotope record to allow an unambiguous Imbrie et al. [4]. During the stacking procedure, tuning without preliminary age control points. this extra peak at site 502 has been 'eliminated' The ice model developed by Imbrie and Imbrie (or at least seriously reduced) because it was not [28] offers a good tuning target. The resulting observed in the two other 6180 records that curve has proved to be a powerful tuning target extended into the Lower Pleistocene (V28-238 in the uppermost Pleistocene [17] as well as in the and V22-174; [4]). lower Pleistocene [1]. This model assumes that Finally, oxygen stage 21 of core MD900963 the rate of the climate response (growth or decay clearly shows three peaks that we interpret as of ice sheets) is proportional at any instant to the related to precession cycles (Fig. 1), whereas magnitude of the summer insolation forcing at Ruddiman et al. [16] compressed this stage into a 65°N. Using a slightly longer time constant for single obliquity cycle in ODP site 607, and Hilgen the growth of the ice sheets than for their decay, [27] interpreted it as containing two tilt cycles. the model introduces some non-linearity in the In the following section we examine: (1) response to insolation forcing, which results in a whether these additional oscillations in the 6180 ~ 100 ka oscillation. We constructed this target record are in good agreement with paleoclimatic curve (Fig. 3, right) using the equations and time changes predicted by the astronomical theory of constants for decay and growth of ice sheets given climate; and (2) what the implications of these by Imbrie and ImbHe [28], and the 65°N, July extra peaks are for the astronomical calibration monthly insolation curve of Berger and Loutre of the 6180 record from core MD900963. [29]. The first step of our tuning procedure was to align features in the MD900963 6180 record with 4. Development of an orbitally derived time-scale features in the ice volume curve using the pro- in core MD900963 gram 'LINAGE' developed at the Centre des Faibles Radioactivit6s of Gif-Sur-Yvette [Paillard The first step in the tuning approach used by and Labeyrie, pers. commun., 1993]. The tuned Imbrie et al. [4] to establish the SPECMAP chronol- 6180 curve and the ice volume model are pre- ogy consisted of developing an initial age model sented in Fig. 3. Such correlations are relatively based on a few radiochronologically calibrated easy because precession-related oscillations are control points (including the isotopic stages 5/6 so well expressed in the 6180 record from core transition and the Brunhes/Matuyama reversal). MD900963. In our tuning procedure, the 6180 F.C. Bassinot et al. / Earth and Planetary Science Letters 126 (1994) 91-108 97

Table 1 the 6180 record (Fig. 3). For this interval, we Control points used for our depth-to-age conversion in core adopted the solution of Imbrie et al. [4] and MD900963 assigned two precession cycles to the major 6180 Depth (m) Age (ka) peak of stage 11, found at about 21 m (Figs. 1 and 3). We assumed that 8180 peaks labelled X and Y in Figs. 1 and 3 correspond to isotopic events 0.010 6.0 3.365 70.0 13.2 and 13.3, respectively, and we tuned these 7.735 127.0 peaks to the high and low ice volume peaks found 9.750 160.0 at about 510 and 525 ka. We are confident in the 11.120 187.0 tuning of the entire stage 15 and isotopic event 13.335 243.0 16.2 since the 6180 record and the ice volume 14.525 281.0 17.990 346.0 curve are so similar that mismatch in the tuning 19.320 384.0 procedure seems unlikely. Once the stage 15 is 20.120 396.0 tuned, however, we must shrink the 6180 record 21.020 427.0 in stages 13-14 slightly to keep pace with the ice 22.235 459.0 23.095 480.0 volume model and respect the number of preces- 23.735 500.0 sion-related peaks in this interval. Our age model 24.915 510.0 solution results in two short intervals with very 25.615 525.0 high sedimentation rates (up to 12 cm/kyr). As 26.725 539.0 27.935 577.0 seen above, the sediments between 24.3 m and 29.750 622.0 about 26.3 m present magnetic declinations with 31.910 683.0 a constant angular deviation of 100°, compared to 33.130 721.0 values directly measured above and below this 34.420 759.0 35.515 789.0 interval. We cannot reject the possibility that this 38.410 864.0 2 m long sediment section has rotated during the 39.905 907.0 coring process. However, this interval may also correspond to an allochthonous sediment lens, thus explaining both our difficulties during the stage boundaries were given higher priorities than tuning procedure and the resulting high sedimen- the centres of glacial and interglacial intervals. tation rates. Nevertheless, since there is no clear The sedimentation rates are assumed constant evidence of mass movement in the core descrip- between selected age control points and change tion, this interpretation remains speculative. more or less abruptly at these control points. When we discard the 6180 measurements per- Such an assumption may be realistic when control formed within the anomalous magnetic declina- points correspond to glacial-interglacial transition interval, the tuning procedure does not ap- tions, when environmental changes might be ex- pear to be significantly easier. Another long pis- pected to affect the sedimentation rates. Further- ton core taken in the vicinity of core MD900963 more, locating a rapid change with large ampli- is being processed for 6180 in order to resolve tude is probably more accurate than positioning a uncertainties within this interval. peak or trough with considerable width and small Down to glacial stage 16, our chronology is amplitude in comparison with analytical noise. similar to the SPECMAP chronology [4]. Below that Control points for this initial age model are given stage, however, discrepancies between the two in Table 1. data sets result in different tuning solutions and Down to 8180 stage 10, tuning was a fairly different chronologies. Once the extra peak in straightforward process. For stage 11, low-ampli- stage 17 is associated to a precession oscillation, tude insolation fluctuations at about 400 ka result and if we interpret the small 6180 peak found in in low-amplitude peaks in the ice volume curve stage 18 as containing two precession cycles and that are difficult to correlate unambiguously to the stage 21 as containing three precession cycles, 98 F.C. Bassinot et aL / Earth and Planetary Science Letters 126 (1994) 91-108

a very impressive match can be achieved between filtered using band-pass filters centred at a pe- the t~180 record of core MD900963 and the ice riod of about 22 kyr. The 6180 record was fil- volume curve (Figs. 3 and 4). These additional tered using a slightly larger band-pass filter to peaks are also observed at site 677 [1], suggesting include essentially all of the distorted precession that they are probably true features of the oxygen variance that could have been smeared to neigh- isotope stratigraphy and do not result from dis- bouring frequency bands due to a slight inaccu- tortion of the MD900963 record. racy in our initial age model. This second tuning Further improvements in our time-scale were step lead to minor age adjustments that did not performed by fine tuning the extracted precession exceed 3,500 yr. The depths, final ages and 6180 components of the 6180 record to the precession values of core MD900963 are given in Table 2. components of the ice volume model using the The final tuned 6~80 curve was band-pass inverse approach for signal correlation of Martin- filtered again to extract the precession compo- son [30] with 21 coefficients. Both records were nents and then compared to extracted precession

SPECMAP stack 8180 MD900963 5IsooDP Site 677 (%o) 0;o) (%o) 1 o -1 -2 0 -1 -2 -3 0 -1 -2 -3 600 i i 4 I I I I I I I I I I I == 15.5

650 17.1

17.3 -4 17 700 8.3 -4 -4 19.1 -4 750 ---4

< -t 19 800 1

850

900 I I I I I I I I I I I I I I I

more ice ~--- more ice 4--more ice Ice volume Ice volume Ice volume (arbitrary scale) (arbitrary scale) (arbitrary scale) Fig. 4. Stratigraphy of the lower Brunhes interval. The ice volume model [28] is superimposed on the orbitally tuned SPECMAPstack (left) [4] and the tuned 6180 records from core MD900963 (middle; this paper) and site 677 (right) [1]. An impressive match can be seen between the 'MD900963 record and the ice volume model. On the figures, oxygen isotope events are labelled according to Prell et al. [13]. In the MD900963 record, stage 19 is unambiguously recognized on the basis of the location of the Brunhes- Matuyama boundary, but the details of stages 17-19 cannot be identified by reference to the SPECMAP stack; they do, however, resemble the equivalent part of ODP Site 677. F.C. Bassinot et al. / Earth and Planetary Science Letters 126 (1994) 91-108 99 cycles of ice volume model (Fig. 5). Both records by a direct response of climate to orbitally driven were filtered using the same band-pass filter cen- changes in insolation are present in the 6180 tred at a period of 22 kyr. Precession exhibits a record from core MD900963; and (2) that an strong modulation, which is related to changes in accurate tuning solution has been obtained. eccentricity of the Earth's orbit. In Fig. 5, the Cross-spectral analysis makes it possible to ex- match between the modulation of the two filtered amine coherency across the entire range of statis- records (the ice model and the MD900963 6180 tically visible frequencies at a higher resolution record) is obvious. Especially noteworthy is the than is practical with filters. To determine over good agreement between the two filtered records the entire range of Milankovitch primary fre- in the interval from about 900 ka to 600 ka, the quencies the amount of 6180 variance in core interval within which we assigned additional pre- MD900963 that can be explained by linear re- cession-related peaks. These results suggest: (1) sponse to orbital forcing, we performed cross- that all the climatic oscillations that are predicted spectral analysis between the tuned 6~So record

Ice volume model a180 (%0) 6~80filtered filtered 0 -1 -2 -3 -0.2 0.2 - 4 0 4 0 I t I I I t i i i i i i i i t i i i

200 ...... ~ ......

4oo ......

600

800

1000 Fig. 5. The tuned 8180 curve from core MD900963 (left) is band-pass filtered to extract its precession components (middle). The filtered record is compared with precession cycles of the ice volume curve extracted using the same filter centered at about 22 kyr (right). 100 F.C. Bassinot et al. /Earth and Planetary Science Letters 126 (1994) 91-108

Table 2 Depths, final ages (after improvement of the chronology using inverse approach for signal correlation [30], see text for details) and oxygen isotope values for core MD900963

Depth Age 8180 Depth Age 8180 Depth Age 8180 Depth Age 8180 Depth Age (5180 (m) (ka) (%0) (m) (ka) (%0) (m) (ka) (%0) (m) (ka) (%0 (m) (ka) (%0)

0.010 6.0 -2.58 4.265 81.1 -1.45 8.625 141.9-0.87 13.030 236.2-2.31 17.840 345.0-1.17 0.060 6.9 -2.90 4.375 82.6 -1.88 8.725 143.6 -0.98 13.130 238.8 -2.65 17.910 346.3 -1.13 0.100 7.7 -2.59 4.465 83.8 -1.91 8.825 145.2 -1.32 13.335 244.2 -1.22 17.990 347.8 -1.60 0.155 8.7 -2.07 4.645 86.2 -1.73 8.930 146.9 -1.23 13.435 247.2 -0.62 18.090 350.7 -1.81 0.200 9.5 -1.87 4.745 87.5 -1.61 9.060 149.0 -1.46 13.535 250.5 -1.29 18.260 355.6 -0.94 0.255 10.5 -1.97 4.845 88.9 -1.61 9.150 150.5-0.89 13.635 253.7-1.20 18.340 357.9-0.95 0.300 11.4 -1.48 4.945 90.2 -1.55 9.250 152.1 -1.06 13.735 256,9 -0.94 18.440 360,7 -1.04 0.335 12.0 -t.37 5.045 91.5 -1.34 9.350 153.8 -1.06 13.835 260,1 -1.33 18.540 363.5 -1.12 0,370 12.7 -0.72 5.145 92.9 -1.99 9.450 155.4 -1.29 13.935 263,3 -l.17 18.650 366.6 -1.33 0.400 13.2 -0.72 5.245 94.2 -1.99 9.550 157.0 -0.96 14.035 266,6 -1.02 18.755 369.6 -1.39 0.500 15.1 -0.81 5.345 95.6 -2.30 9.660 158.8-0.93 14.145 270,1 -0.87 18.830 371.7-1.64 0.600 16.9 -0.45 5.445 96.9 -2.34 9.750 160.3-1.50 14,235 273,0-1.29 18.930 374.5 -1.58 0.700 18.8 -0.66 5.545 98.2 -2.18 9.850 162.3 -1.49 14.335 276,2 -1.25 19.030 377.3 -1.44 0.785 20.3 -0.99 5.645 99.6 -2.43 9.950 164.3 -1.68 14.435 279.4 -1.21 19.120 379.8 -1.56 0.900 22.5 -1.06 5.745 100,9-2.15 10.050 166.2-1.38 14.525 282.3-1.48 19.220 382.5-1.96 1.100 26.2 -1.01 5,845 102.2-1.91 10.150 168.2-1.61 14.625 284.2-l.78 19.320 385.2-2.25 1,200 28.1 -1.30 5.945 103.6-2.04 10.210 169.4-1.26 14.845 288.3-1,79 19.420 386.7-2.29 1.300 29.9 -1.30 6.055 105,0 -2.22 10.240 170.0 -0.78 14.945 290.2 -1.71 19.520 388.1 -1.94 1.515 33.9 -1.15 6.170 106,6-1.56 10.270 170.6-1.45 15.045 292.1 -1.49 19.620 389.5 -1.64 1.590 35.4 -1.23 6.270 107,9 -1.81 10.340 172.0 -1.41 15.135 293.8 -1.50 19.730 391.1 -1.44 1.680 37.0 -1.08 6.365 109,2-1.77 10.430 173.7-1.39 15.235 295.7-1.14 19.820 392.4-1.86 1.790 39.1 -1.22 6.465 110,5 -1.82 10.520 175.5 -0.97 15.330 297.5 -1.56 19.920 393.8 -1.81 1.880 40.8 -1.18 6.565 111,8 -1.98 10.620 177.5 -1.33 15.430 299.4 -1.29 20.020 395.2 -1.88 1.980 42.7 -1.18 6.665 113,2 -1.69 10.720 179.5 -1.54 15.535 301.4 -1.51 20.120 396.6 -2.32 2.070 44.4 -1.08 6.765 114,5 -2.39 10.820 181.5 -1.18 15.635 303.2 -1.81 20.230 400.2 -2.42 2.280 48.4 -1.25 6.865 115,8 -2.54 11.020 185.4 -1.77 15.855 307.4 -2.03 20.320 403.1 -2.92 2.375 50.2 -1.49 6.965 117,1 -2.66 11.120 187.4 -1.77 15.955 309.3 -2.08 20.420 406.3 -2.58 2.475 52.1 -1.24 7.065 118.5 -2.64 11.220 190.0 -2.69 16.055 311.2 -2.24 20.520 409.6-2.77 2.675 55.9 -1.37 7.160 119,7 -2.95 11.320 192.5 -2.39 16.155 313.1 -1.98 20.620 412,8 -2.22 2.775 57.8 -1.29 7.260 121,0-3.10 11.420 195.1 -2.34 16.255 315.0-1.73 20.720 416.1 -2.01 2.880 59.8 -1.13 7.355 122,3 -2.96 11.520 197.6 -1.43 16.355 316.8 -1.81 20.820 419.3 -1.94 3.075 63.6 -1.24 7.455 123,6 -2.89 11.670 201.4-1.42 16.455 318.7 -1.51 20.920 422.6 -1.25 3.165 65.3 -1.19 7.555 124,9 -2.91 11.830 205.5 -1.63 16.555 320.6 -1.85 21.020 426.0 -0.49 3.265 67.2 -1.23 7.645 126.1 -2.30 11.950 208.6-2.14 16.655 322.5 -2.06 21.120 428,5 -0.59 3.365 69.2 -1.44 7.735 12713 -1.45 12.030 210.6 -2.16 16.755 324.4 -2.31 21.220 431.1 -0.51 3.475 70.6 -2.00 7.835 128.9 -0.96 12.130 213.2 -2.56 16.855 326.3 -2.65 21.320 433.7 -0.60 3.575 72.0 -1.24 7.930 130.5 -0.87 12.230 215.8 -1.82 16.955 328.2 -2.64 21.410 436.1 -1.23 3.665 73.2 -1.76 8,025 132.1 -0.50 12.430 220.9 -1.61 17.055 330.1 -2.42 21.630 441,9 -1.02 3.765 74.5 -1.80 8.125 133.7 -1,25 12.530 223.4 -1.44 17.260 334.0 -2.00 21.750 445,1 -1.04 3.875 75.9 -1.79 8.230 135.4-0.50 12.630 226.0-1.18 17.300 334.8-0.89 21.835 447.4-2.53 3.985 77.4 -1.94 8.325 137.0-0.98 12.730 228.5 -1.07 17.440 337.4-0.84 21.935 450.1 -1.15 4.065 78.5 -1.88 8.425 138.6-0.82 12.830 231.1 -1.67 17.540 339.3-0.56 22.035 452.8-1.71 4.165 79.8 -1.81 8,525 140.3 -0,91 12.930 233.7 -2.04 17.740 343.1 -0.80 22.135 455.5 -1.11 F.C. Bassinot et al. / Earth and Planetary Science Letters 126 (1994) 91-108 101

Table 2 (continued)

Depth Age 8180 Depth Age 8180 Depth Age 8180 Depth Age 8180 (m) (ka) (%~) (m) (ka) (%o) (m) (ka) (%~) (m) (ka) (%~)

22.235 458.2 -1.59 26.955 544.4 -1.90 31.620 675.6 -1.48 35.915 799.4 -1.00 22.335 460.8 -1.20 27.025 546.5 -1.37 31.670 677.0-1.73 36.015 802.0-0.96 22.435 463.3 -1.37 27.230 553.1 -1.93 31.810 681.0 -1.42 36.170 805.9 -1.28 22.515 465.3 -1.61 27.335 556.2 -1.95 31.910 683.8 -2.71 36.270 808.5 -1.38 22.725 470.6-1.40 27.435 559.3-1.69 32.010 686.9-2.41 36.340 810.3-1.40 22.835 473.4-2.01 27.535 562.4-1.88 32.110 689.9-2.14 36.415 812.2-1.43 22.915 475.4-1.81 27.635 565.6 -1.80 32.210 693.0-1.98 36.515 814.8 -1.74 22.995 477.3 -1.92 27.735 568.7 -2.01 32.310 696.0-1.78 36.615 817.3 -1.72 23.095 479.8-2.10 27.835 571.9 -2.44 32.410 699.0 -1.71 36.715 819.9 -1.86 23.335 487.4 -1.88 27.935 575.0-2.33 32.510 702.0 -1.59 36.815 822.5 -2.06 23.435 490.5 -1.64 28.035 577.5 -1.22 32.610 705.1 -1.88 36.915 825.0-1.80 23.535 493.6-2.13 28.135 580.0-1.57 32.710 708.1 -1.88 37.015 827.6-1.38 23.635 496.7 -2.04 28.235 582.5 -1.26 32.760 709.6 -1.86 37.115 830.2-1.54 23.735 499.8 -2.21 28.335 585.0 -1.38 32.810 711.1 -1.20 37.215 832.8 -1.78 23.845 500.7 -2.15 28.435 587.5 -1.61 32.910 714.1 -1.05 37.315 835.4 -1.69 23.945 501.5 -2.23 28.535 590.0-1.56 33.110 720.2 -0.78 37.435 838.5 -2.41 24.035 502.3 -1.60 28.660 593.1 -2.33 33.130 720.8 -0.73 37.520 840.8 -2.16 24.135 503.1 -1.86 28.750 595.4 -2.05 33.240 724.0-1.43 37.715 845.8 -1.48 24.335 504.7 -1.71 28.850 597.9 -2.23 33.320 726.3 -1.31 37.810 848.3 -1.41 24.435 505.6-1.76 28.940 600.2 -2.14 33.410 728.9-1.66 37.905 850.8 -1.52 24.735 508.0 -1.10 29.035 602.6-1.64 33.520 732.1 -1.51 38.005 853.3 -1.92 24.835 508.8 -0.78 29.140 605.2 -0.97 33.620 735.1 -1.23 38.105 855.9 -2.15 24.915 509.5 -0.55 29.240 607.7 -1.68 33.720 738.0 -1.23 38.205 858.5 -2.29 25.015 511.6 -1.44 29.340 610.3 -2.20 33.820 741.0 -1.22 38.315 861.3 -2.11 25.065 512.6 -1.43 29.440 612.8 -2.10 33.920 744.0 -0.96 38.410 863.7 -1.47 25.115 513.6 -1.89 29.540 615.4 -2.25 34.020 746.9 -1.15 38.515 866.6 -0.73 25.215 515.7 -1.41 29.650 618.2 -2.08 34.120 749.9-1.01 38.605 869.0-0.51 25.315 517.7 -1.27 29.750 620.8 -1.13 34.220 752.9-1.47 38.715 872.0 -0.61 25.415 519.8 -1.73 29.840 623.4 -0.28 34.320 755.9 -1.53 38.815 874.7 -0.78 25.515 521.8 -2.55 29.940 626.4 -0.19 34.420 758.9 -1.81 38.915 877.4 -0.91 25.615 523.9 -2.55 30.035 629.2 -0.23 34.675 766.0 -2.09 39.025 880.3 -0.89 25.715 525.1 -2.08 30.220 634.7 -0.72 34.745 768.0-1.79 39.115 882.6-0.66 25.815 526.3 -1.54 30.320 637.6 -1.10 34.815 769.9 -1.68 39.170 884.0 -0.64 25.915 527.5 -1.84 30.520 643.6-1.17 34.915 772.7 -1.82 39.215 885.2 -0.97 26.015 528.7 -1.54 30.620 646.6 -0.87 35.015 775.5 -1.34 39.265 886.4 -0.92 26.155 530.4 -1.59 30.720 649.5 -0.80 35.115 778.2-1.79 39.315 887.7 -1.38 26.245 531.5 -1.32 30.920 655.4 -1.06 35.215 781.0-2.43 39.415 890.2 -1.35 26.425 533.7 -1.52 31.020 658.3 -1.06 35.315 783.7 -2.03 39,515 892.7 -1.02 26.525 534.9 -1.11 31.120 661.3 -1.54 35.415 786.4 -2.28 39.615 895.1 -1.51 26.625 536.1 -1.41 31.220 664.2 -1.65 35.515 789.2 -1.25 39.715 897.6 -1.50 26.725 537.3 -1.15 31.320 667.1 -1.71 35.615 791.7 -0.96 39.795 899.5 -1.32 26.825 540.4-1.34 31.420 669.9-1.75 35.715 794.3-0.40 39.905 902.2-1.26 26.925 543.5 -1.04 31.520 672.8 -1.57 35.815 796.9 -0.84

and an Eccentricity-Tilt-Precession (ETP) curve, Berger and Loutre [29]. The sign of the preces- constructed by normalizing and stacking the ec- sion index was reversed prior to stacking so that centricity, tilt and precession functions given by positive changes in this signal have the same 102 F.C. Bassinot et aL /Earth and Planetary Science Letters 126 (1994) 91-108 climatic impact in the Northern hemisphere as 5. Age estimate of the Brunhes/Matuyama Earth the positive excursions in eccentricity and obliq- magnetic reversal uity. The resulting coherencies in our cross-spectrum analysis are particularly high; reaching 0.93 The age error associated with our tuning pro- in the obliquity band and 0.99-0.96 for the pre- cedure is estimated to be +5 kyr [17] and our cession bands (23 and 19 kyr, respectively, Fig. 6). tuning solution results in an age estimate of about These values fall significantly above the limit of 775 + 10 ka for the Brunhes/Matuyama mag- the 95% confidence interval, which is at about netic reversal located at 35.01_ 0.19 m in core 0.75 (cross-spectral analysis was performed with MD900963, in good agreement with age estimate 176 lags and 2 kyr sampling intervals). As the of 780 ka given by Shackleton et al. [1]. This age square of coherency is the fraction of the vari- is also in good agreement with recent results from ance in one signal, which is linearly related to the 4°Ar/39Ar incremental heating studies on series variance in the other signal, our results indicate of lavas from Maui, New Zealand and Valles, that, in the 61So record from core MD900963, which give ages of 783 + 11 ka, 780 + 30 ka and about 86% of the variance in the 41 kyr band, 780 + 10 ka, respectively [9-11]. This good agree- 98% in the 23 kyr band and 92% in the 19 kyr ment, and the fact that the tuned 6180 record band is linearly related to orbital variations. from core MD900963 shows very strong co-

...... Ef'P -- ~180

0 : 23 u) 100 E~ 0 m

>, (/) E: ; ";' "? ', 7'" / / L'/ "]"' ";', ,, . ,"":"" ''""',' L ,:""/ '""" " "0

G) (-o

.m > 1

~ 0.8 0 e- 0.6 t- 0.4 O 0 0.2-

0 ' ' ' ' r ' ' ' ' [ ' ' ' ' I .... I ' ' ' ' I ' ' ' ' ' ' ' ' 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 Frequency (cycles/kyr) Fig. 6. Coherency and variance spectra resulting from cross-spectrum analysis of the orbitally tuned 6180 record from core MD900963 and an ETP record (formed by normalizing and stacking variations in eccentricity, obliquity and precession). F. C. Bassinot et aL / Earth and Planetary Science Letters 126 (1994) 91-108 103

Low Latitude Stack (MD900963 + Site 677) SPECMAP Stack (%0) (8180 - %0) 2 1 0 -1 -2 2 1 0 -1 -2 -3 0 I ~ ~ I I .I I -)- 0 2.2 ~ 3.1 3.3 5.1 ~ 5.3 100 100 6.2 ~.5 5.5 6.6 200 7.2 ~ 7.1 200 - 74~~--~--- 7.3 7.5 82 '%'~'------~ 8:4 8.6 ~ 8.5 300 300 9.2 ~ 9.3 10.2 ~10.3 . 10.4 c'------..._.~ 11,1 11.2Z ~ 11.23 400 11.24 400 11. 12.2 ~) 0 < 13.12 ~_~ 13.11 500 13.2 ~13.13 500 " 14.2 ~3.3

600 15.4 ~ 15.5 600

~17,~1 700 17"4~17. 5 700 - 18"2~18.3 18.4 £..______.______....>19. 1 ~19.3 800 8OO

21"4~21. 5 22.2 ~---'--~ 900 I I I I I [ I J I 900 Fig. 7. Left: the SPECMAPstack plotted against age using the time scale developed by Imbrie et al. [4]. Right: the low-latitude stack (this study) tuned to the ice volume prediction model of Imbrie and Imbrie [28]. Based on this alternative stack for the upper Pleistocene, we propose a revision of the numbering of the isotopic events (bold and underlined numbers). 104 F.C. Bassinot et aL /Earth and Planetary Science Letters 126 (1994) 91-108

Table 3 Data of the smoothed, low-latitude oxygen isotope stack as a function of age Age Age Age Age Age Age Age Age Age (ka) (ka) (ka) (ka) (ka) (ka) (ka) (ka) (ka)

6 -2.04 114-1.17 222 0.42 330-1.68 438 1.78 546-0.17 654 1.10 762 -0.47 870 1.86 8 -1.09 116 -1.52 224 0.71 332 -1.09 440 1.59 548 -0.42 656 0.82 764 -0.68 872 1.90 10 -0.15 118-1.87 226 0.65 334-0.40 442 1.45 550-0.74 658 0.47 766 -0.81 874 1.83 12 0.68 120-2.47 228 0.40 336 0.96 444 1.36 552-0.79 660 0.22 768 -0.77 876 1.72 14 1.63 122-2.54 230 0.01 338 1.49 446 1.27 554-0.84 662 0.02 770 -0.65 878 1.60 16 1.85 124 -2.01 232 -0.47 340 1.70 448 1.02 556 -0.92 664 -0.15 772 -0.55 880 1.54 18 1.84 126 -1.07 234 -0.90 342 1.69 450 0.66 558 -1.13 666 -0.20 774 -0.64 882 1.59 20 1.64 128 0.35 236-0.83 344 1.52 452 0,5l 560-1.20 668 -024 776 -0.85 884 1.58 22 1.36 130 1.48 238-0.55 346 0.97 454 0.25 562-1.18 670 -0.26 778 -1.06 886 1.53 24 1.23 132 1.66 240-0.21 348 0.47 456 0.30 564-1.15 672 -0.31 780 -1.20 888 1.25 26 1.11 134 1.72 242 0.12 350 0.45 458 0.45 566 -1.18 674 -0.37 782 -1.16 89O O.7O 28 0.94 136 1.66 244 0.57 352 0.52 460 0.23 568 -1.27 676 -0.45 784 -0.70 30 0,70 138 1.64 246 0.98 354 0.62 462 0.28 570 -1.42 678 -0.56 786 -0.18 32 0.53 140 1.61 248 1.11 356 0.86 464 0.16 572 -1.58 680 -0.73 788 0.45 34 0.50 142 1.55 250 1.12 358 0.99 466 0.12 574-1.64 682 -0.93 790 1.14 36 0.51 144 1.38 252 1.03 360 0.96 468 0.14 576-1.32 684 -1.12 792 1.50 38 0.52 146 1.15 254 1.00 362 0.83 470-0.03 578-0.87 686 -1.27 794 1.59 40 0.55 148 1.02 256 0.97 364 0.63 472-0.23 580-0.33 688 -1.38 796 1.53 42 0.52 150 1.03 258 0.95 366 0.39 474-0.39 582-0.11 690 - 1.41 798 1.35 44 0,5l 152 1.04 260 0.91 368 0.16 476 -0.69 584 -0.13 692 -1.13 800 1.08 46 0.39 154 0.94 262 0.89 370 0.06 478 -0.88 586 -0.23 694 -0.85 802 0.94 48 0.29 156 1.04 264 1.03 372 0.09 480 -0.91 588 -0.50 696 -0.59 804 0.79 50 0.18 158 1.19 266 1.07 374 0.17 482-0.90 590-0.84 698 -0.37 806 0.53 52 0.07 160 1,07 268 1.02 376 0.14 484-0.86 592-1.05 700 -0.24 808 0.36 54 0.14 162 0.78 270 0.94 378 -0.02 486 -0.82 594 -1.31 702 -0.21 810 0.17 56 0.24 164 0.76 272 0.83 380-0.24 488-0.76 596-1.24 704 -0.30 812 -0.06 58 0.35 166 0.80 274 0.68 382 -0.46 490 -0.60 598 -1.08 706 -0.40 814 -0.26 60 0,45 168 0,91 276 0.57 384 -0.60 492 -0.56 600 -0.81 708 -0.42 816 -0.46 62 0.47 170 1.15 278 0.46 386 -0.63 494 -0.61 602 -0.47 710 -0.35 818 -0.72 64 0.49 172 1.02 280 0.26 388 -0.51 496 -0.75 604 -0.36 712 0,47 820 -0.80 66 0.47 174 1.11 282 -0.24 390 -0.37 498 -0,77 606 -0.47 714 0.96 822 -0.76 68 0.35 176 1.07 284-0.57 392-0.40 500-0.85 608-0.75 716 1.19 824 -0.61 70 -0.04 178 0.96 286-0.76 394-0.57 502-0.60 610-1.14 718 1.22 826 -0.38 72 -0,34 180 0.79 288 -0.87 396 -0.91 504 -0.49 612 -1.39 720 1.25 828 -0.32 74 -0.54 182 0.61 290 -0.63 398 -1.27 506 0.08 614 -1.45 722 1.00 830 -0.37 76 -0,78 184 0.49 292 -0.15 400 -1.69 508 0.55 616 -1.29 724 0.69 832 -0.51 78 -0.85 186 -0.12 294 0.06 402 -2.17 510 0.59 618 -0.86 726 0.36 834 -0.79 80 -0.83 188 -0.65 296 0.13 404 -2.49 512 0.24 620 -0.17 728 0.07 836 -0.93 82 -0.82 190 -0.95 298 -0.05 406 -2.60 514 -0.08 622 0.72 730 0.04 838 -1.01 84 -0.53 192 -I,19 300 -0.24 408 -2.64 516 -0.08 624 1.62 732 0.08 840 -0.94 86 -0.08 194-1.27 302-0.55 410-2.50 518-0.13 626 2.15 734 0.12 842 -0.77 88 -0.09 196 -0,96 304 -0.70 412 -2.31 520 -0.43 628 2.37 736 0.21 844 -0.60 90 -0,54 198-0,69 306-0.81 414-2.07 522-0.85 630 2.30 738 0.33 846 -0.41 92 -1.05 200 -0,50 308 -0.95 416 -1.79 524 -1.14 632 2.15 740 0.44 848 -0.19 94 -1.22 202 -0,36 310 -I.00 418 -1.48 526 -0.78 634 2.00 742 0.54 850 -0.29 96 -1.31 204-0,36 312-0.97 420-1.13 528-0.33 636 1.79 744 0.64 852 -0.49 98 -1.30 206 -0.48 314 -0.91 422 -0.75 530 -0.04 638 1.49 746 0.76 854 -0.74 100-1.15 208-0.59 316-0.84 424-0.18 532 0.11 640 1.41 748 0.86 856 -1.01 102-1.00 210-0.67 318-0.93 426 0.45 534 0.18 642 1.35 750 0.88 858 -1.03 104 -0.72 212 -0.82 320 -1.02 428 0.97 536 0.23 644 1.31 752 0.89 860 -0.85 106 -0.53 214 -0.75 322 -1.25 430 1.45 538 0.18 646 1.35 754 0.91 862 -0.47 108 -0.64 216 -0.44 324 -1.55 432 1.82 540 0.01 648 1.29 756 0.92 864 0.24 110 -0.83 218 -0.18 326 -1.95 434 1.96 542 0.04 650 1.21 758 0.57 866 0.94 112 -0.94 220 0,08 328 -2,15 436 1,92 544 -0.01 652 1.16 760 0.11 868 1.52 Ages have been interpolated to a constant sampling interval of 2 kyr. Isotopic variations are expressed in standard deviation units around a zero mean. F.C. Bassinot et al. / Earth and Planetary Science Letters 126 (1994) 91-108 105 herency with orbital curves, clearly argue that our volume model of Imbrie and Imbrie [28] intro- age model is tightly constrained. duces constant phase lags between the oscilla- We shall consider the possibility that the exact tions in the insolation forcing at 65°N in July and stratigraphic position of the B/M reversal rela- the computed ice volume responses to which we tive to the 8180 record may be slightly biased in tuned the oxygen isotopic record from core core MD900963. The Brunhes-Matuyama rever- MD900963. In the precession band, this phase lag sal is located in the uppermost part of the iso- is of the order of 5 kyr [4, 28]. These phase lags topic stage 19 in this core. It has been shown, may be subject to revision. Recent work on U/Th however, that the magnetization which is mea- dating by mass spectrometry [e.g., 35] has shown sured in deep-sea sediments is most probably a that corrections must be performed on the ~4C post-depositional remanent magnetization age of the last glacial maximum, which is one of (PDRM), with the Earth's magnetic field affect- the six control points used by Imbrie and Imbrie ing sediments a few centimetres below the sedi- [28] to constrain their ice volume model. ment-water interface (fine ferrimagnetic grains are free to rotate in high-porosity, unconsolidated surface sediments) [e.g., 31-33]. Recent estimates 6. Alternative low-latitude ~180 stack for the Late of the 'lock-in depth' (depth below the Pleistocene sediment-water interface at which the remanent magnetization was acquired) vary from 7 cm [34] In order to obtain a 8180 record which can be to about 16 cm [32,33]. Thus, the stratigraphic confidently used as a reference curve for strati- position of the B/M reversal in the sedimentary graphic and chronologic purposes in the Late column may not permit a direct and accurate Pleistocene down to about 0.9 Ma, we stacked estimate of its true chronologic occurrence rela- and tuned the 8180 records of core MD900963 tive to the /~180 events. Based on a careful study and site 677 to orbital forcing functions. The of the relative stratigraphic positions of oxygen stacking procedure tends to reduce minor distor- isotopic stage 19 and the B/M reversal for eight tions of individuals ~80 records, thereby en- deep-sea sediment cores, deMenocal et al. [32] hancing the low-latitude climatic signal recorded recently concluded that the B/M reversal oc- at these two sites. We applied the technique of curred about 6 + 2 kyr after the isotopic event graphic correlation in the depth domain [36,13], 19.1. This translates into an age of 776 ka for the using core MD900963 as the reference section B/M reversal in our time-scale (19.1 is dated at and plotting the depth range of isotopic events 782 ka in core MD900963). Although additional that are common in both records. Isotopic records work is necessary to address the 'true' strati- were normalized (zero mean, unit standard devia- graphic position of the B/M reversal in core tion), stacked and slightly smoothed. The result- MD900963 precisely, the good agreement being record was tuned to the Imbrie and Imbrie tween this age estimate of 776 ka and the age of ice volume model [28]. Further improvements in 775 ka we obtained using the 'uncorrected' posi- our time-scale were performed by fine tuning the tion of the B/M reversal in core MD900963 extracted precession components of the /~80 suggests that there might be only a small shift in record to the precession components of the ice the B/M stratigraphic location relative to the volume model, using the inverse approach for 8180 record in this core. Such a small shift could signal correlation of Martinson [30] with 21 coef- result from the combination of relatively high ficients. Data are interpolated at 2 kyr in Table 3. sedimentation rates and/or the small 'lock-in The resulting curve is shown in Fig. 7 and depth' for these sediments. compared with the SPECMAP stack. We have rede- Further improvement in our tuning target may fined events in stages 17-21 in order to take into slightly modify our B/M age estimate and will account the extra details observed in stages 17, 18 probably put it in closer correspondence to 4°Ar/ and 21 that lead to the revised tuning solution 39Ar radiochronological ages. The non-linear ice first proposed by Shackleton et al. [1] in this time 106 F.C. Bassinot et al. / Earth and Planetary Science Letters 126 (1994) 91-108 interval. The revised numbers are shown bold Table 4 and underlined in Fig. 7. For instance, event 19.1, Ages of the isotopic events in the low-latitude oxygen isotope in which the Brunhes/Matuyama reversal takes stack place, becomes 19.3 in our new numbering. Ow- ing to the strong precessional signal recorded in Isotopic Age Isotopic Age Isotopic Age core MD900963 and at site 677, additional 6180 event (ka) event (ka) event (ka) peaks are also clearly observed in stages 10 and 11 compared to the SPECMAP stack and were 2.0 11 8.6 295 15.3 594 2.2 17 9.0 301 15.4 604 numbered according to the procedure presented 3.0 24 9.1 309 15.5 615 by Prell et al. [13] (Fig. 7). The SPECMAP stack 3.1 30 9.2 315 16.0 621 provides a better estimate of stages 6 and 12 than 3.3 52 9.3 928 16.2 628 our low-latitude stack. Disturbance in stage 6 4.0 57 10.0 334 16.3 642 results from the poorly defined structure of this 4.2 62 10.2 340 17.0 659 stage at site 677. Ages of the local minima, max- 5.0 71 10.3 349 17.1 666 ima and stage boundaries of our low-latitude 5.1 79 10.4 357 17.2 ** 6180 stack record are given in Table 4 (with the 5.2 86 11.0 364 17.3 688 exception of stage 6, for which no age determina- 5.3 97 11.1 369 17.4 699 tion was attempted, and isotopic event 17.2, which 5.4 106 11.22 375 17.5 708 does not show distinctly in our low-latitude 3t80 5.5 122 11.23 384 18.0 712 stack). 6.0 127 11.24 390 18.2 718 6.2 133 11.3 406 18.3 729 6.3 ** 12.0 427 18.4 754 7. Conclusions 6.4 ** 12.2 434 19.0 760 6.5 ** 12.3 458 19.1 765 Using the high-resolution t~180 record ob- 6.6 ** 12.4 468 19.2 772 tained in the giant piston core MD900963 (~ 53 7.0 186 13.0 474 19.3 782 m) retrieved east of the Maldives platform, we 7.1 194 13.11 481 20.0 787 show that several oscillations in the lower Brun- 7.2 202 13.12 491 20.2 793 hes-upper Matuyama chronozones that are pre- 7.3 213 13.13 500 21.1 820 dicted by the astronomical theory of climate were 7.4 225 13.2 510 21.2 828 apparently missing in the deep-sea paleoclimatic 7.5 236 13.3 524 21.3 838 records first used for developing astronomically 8.0 242 14.0 528 21.4 847 derived time-scales for the Late Pleistocene. In 8.2 248 14.2 536 21.5 858 the detailed 6180 record of core MD900963, ad- 8.3 258 15.0 568 22.0 865 ditional precession-related peaks are clearly ob- 8.4 266 15.1 573 22.2 871 served in oxygen isotope stages 17 and 18 com- 8.5 287 15.2 582 22.3 879 pared to the composite curves proposed by the Isotopic minima and maxima are shown in Fig. 7; isotopic SPECMAP scientists [4,12] and stage 21 presents events labeled 2.0, 3.0, etc., are stage boundaries not shown in three precession-related oscillations, as recently Fig. 7. suggested by Shackleton et al. [1]. These extra peaks result in an excellent match between the 6180 record and the ice volume model of Imbrie is dated at 775 _+ 10 ka, in good agreement with et al. [13], which is used as a target curve for recent Ar/Ar dating on selected sanidine crystals developing an orbitally derived age model in core [8-10] and the age estimate of 780 ka derived MD900963. from the orbital time-scale of Shackleton et al. Based on our orbitally derived age model, the [1]. Brunhes-Matuyama reversal (which is located in Finally, we developed an alternative, low-lati- the upper part of the stage 19 in core MD900963) tude, Late Pleistocene t~180 reference record by F.C. Bassinot et al. / Earth and Planetary Science Letters 126 (1994) 91-108 107 stacking and tuning to the 6180 records from [8] W.B. Harland, R.L. Armstrong, A.V. Cox, UE. Craig, core MD900963 (Indian Ocean) and ODP site A.G. Smith and D.G. Smith, A Geological Time Scale 677 (Pacific Ocean) orbital forcing functions. 1989, Cambridge Univ. Press, Cambridge, 1989. [9] A. Baksi, B. Hougton, M. McWilliams, H. Tanaka and G. Turner, What is the age of the Brunhes-Matuyama polarity transition? EOS Trans. Am. Gepphys. Union 72(44), 135, 1991. Acknowledgments [10] A.K. Baksi, V. Hsu, M.O. McWilliams and E. Farrer, 4°mr/39Ar dating of the Brunhes-Matuyama geomagnetic We are grateful to L. Beaufort for fruitful field reversal, Science 256, 356-357, 1992. discussions. We thank all the members of the [11] T.U Spell and I. McDougall, Revisions to the age of the Brunhes-Matuyama boundary and the Pleistocene geo- SEYMAMA expedition during which the giant magnetic polarity timescale, Geophys. Res. Lett. 19(12), coring system developed by Y. Balut allowed the 1181-1184, 1992. recovery of exceptionally long cores. Special [12] F. Rostek, G. Ruhland, F.C. Bassinot, P.J. MiJller, L.D. thanks go to B. LeCoat, and J. Antignac for the Labeyrie, Y. Lancelot and E. Bard, Reconstructing sea isotopic analysis and to N. Buchet for help in surface temperature and salinity using 6180 and alkenone records, Nature 364, 319-321, 1993. sample preparation. This work was supported by [13] W.L. Prell, J. Imbrie, D.G. Martinson, J.J. Morley, N.G. funding from INSU/CNRS (under PNEDC, DBT Pisias, N.J. Shackleton and H.F. Streeter, Graphic corre- and IST programs) and TAAF. 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