Relative palaeointensity records from the Ontong-Java Plateau

Transcription

1 Geophys. J. nt. (1994) 117, Relative palaeointensity records from the Ontong-Java Plateau L. Tauxel.* and N. J. Shackleton2 Scripps nstitution of Oceanography, La Jolla, CA , USA University of Cambridge, Sub-Department of Quaternary Research, The Godwin Laboratory, Free School Lane, Cambridge CB2 3RS, UK Accepted 1993 December 3. Received 1993 December 1; in original form 1993 August 26 SUMMARY Rock magnetic, palaeomagnetic and oxygen isotopic results are presented from core RNDB 75p, which was recovered from the Ontong-Java Plateau (OJP). A high degree of uniformity in magnetic properties characterized by relatively small changes in concentration and grain size in the upper 4 m of the core, combined with a lack of coherence between the normalized remanence and rock magnetic data suggests that the natural remanence normalized by saturation remanence reflects variations in relative palaeointensity of the geomagnetic field. The record from RNDB 75p replicates other Ontong-Java records spanning the last 4Ka and extends the record back to some 7Ka. Spectral analysis of the Ontong-Java record suggests periodic behaviour in the relative palaeointensity record with a dominant period of between 3 and 4Ka, which appears not be be an artefact of lithologic variability. This dominant period lies between functions describing climatic precession and obliquity changes in the Earth s orbit. Comparison of the normalized remanence record with astronomical precession (26 Ka period), however, is much more favorable. None the less, tuning the palaeointensity record to that of astronomical precession appears inconsistent with existing isotopic age constraints derived from the SPECMAP time-scale. Based on these data, we must choose between assuming that the Earth s orbit controlled ice volume (inherent in the SPECMAP time-scale) and assuming that the Earth s magnetic field is driven by astronomical precession. The former assumption has a substantial theoretical and observational base and we prefer to interpret the data presented here as suggesting that the Earth s orbit has not played a detectable role in the modulation of the magnetic field. Plots of saturation remanence and magnetic susceptibility are very sensitive to quite subtle changes in magnetic grain size. A slight shift within the pseudo-singledomain grain-size range toward the multidomain (or superparamagnetic) field was detected at about 4m in RNDB 75p. This change in grain size may reflect a diagenetic alteration of the magnetite (such as dissolution) and may be related to the phenomenon responsible for the loss of magnetic remanence at depth detected in other cores from the region. Key words: geodynamo, oxygen isotopes, palaeointensity, rock magnetism. 1 NTRODUCTON t is almost universally accepted that the Earth s magnetic field is produced by flow in the fluid outer core. However, problems in understanding the generation of the Earth s * Now at: Fort Hoofddijk Paleomagnetic Laboratory, The University of Utrecht, Budapestlaan 17, 3584 CD Utrecht, The Netherlands. magnetic field are formidable and include the difficulty in solving the many simultaneous non-linear partial differential equations that govern the system (see Merrill & McElhinny 1983), the fact that several key parameters are only poorly constrained (e.g. temperature, conductivity, etc.), and that there is a dearth of experimental constraints. As Braginsky (1991, p.115) stated, There is no hope of building a realistic theory of such a complicated system purely by mathematical reasoning. Geodynamo theory will inevitably include Some unknown parameters which can only be determined by 769

2 77 L. Tame and N. J. Shackleton fitting the theory to the observations.' ndeed, even the source of energy driving the flow is a matter of discussion. The driving mechanism that is currently most popular is buoyancy-driven convection in the core (e.g. Braginsky 1991). However, excitation by luni-solar precession of the Earth's rotation axis has also been considered (e.g. Malkus 1963, 1968, 1989; Vanyo 1991). Arguments for the latter stem largely from laboratory experiments, scaled to Earth dimensions (see Vanyo 1991, for a recent summary). Enthusiasm for the precessionally driven dynamo was considerably dampened by, for example, Rochester et al. 1975), who pointed out serious difficulties in both experimental design and mathematical formulation. Thus, bouyancy-driven dynamos have been favoured because of mathematical and experimental plausibility. Despite the overwhelming preference for a bouyancydriven dynamo, which would have no direct link to the Earth's orbit, there have been nagging doubts generated by some palaeomagnetic observations. Reports of a link between the variations in the magnetic field and in the Earth's orbit have persisted (e.g. Wollin, Ericson & Ryan 1971; Kent & Opdyke 1977; Napoleone & Ripepe 199; Creer, Thouveny & Blunk 199). Although some of these accounts have been debunked (see Amerigian 1974; Chave & Denham 1979; Kent 1982), the bulk remain unchallenged. What is required is a long, well-documented palaeomagnetic data set that can be compared directly with the orbital forcing functions using a high-resolution time-scale. The data sets of Creer et al. (199) and Napoleone & Ripepe (199) are quite short and the data set of Kent & Opdyke (1977) has poor internal time control. Hence, none of the published records meet minimum requirements needed to provide convincing evidence for precessional control of the geodynamo. t is the purpose of this paper to investigate the possibility of using the relatively long, isotopically dated pelagic carbonate sequences from the Ontong-Java Plateau (OJP) to address the question of the link between the Earth's magnetic field and its orbit. 2 PALAEOMAGNETC AND ROCK MAGNETC BEHAVOUR Core RNDB 75p was recovered during the Roundabout Expedition (1988) from 1.9"N, 16.2"E at a water depth of The core was first split into halves; one half was further split into two quarters. One of these quarters was measured in the through-bore magnetometer housed at Scripps nstitution of Oceanography using the methods described by Tauxe & Wu (199). We then subsampled the remaining half core at 3cm intervals. The natural remanence (NRM), initial susceptibility (x), and saturation remanence (Ms.) were all measured on the subsamples. Finally, splits of the subsamples were measured for 6l8. These data are shown in Fig. 1. The -8m core was cut on the ship into unequal and unoriented pieces up to 1.5 m in length. The abrupt changes in declination shown in Fig. 1 occur at these section boundaries. The upper -2-25cm appears to be substantially disturbed by the coring process, resulting in highly smeared declination and inclinations well off the nearlzero average expected for this latitude. These data are excluded in the following. n general, the continuous measurements made on the archive quarters constitute a somewhat smoothed version of the subsample data set. The approximately 45" offset between the declinations of the archive quarters is due to the placement of the quarters in the measurement boat; no attempt was made to adjust either to zero for the purposes of this figure. However, when the declination for each section (excluding the topmost section) is adjusted to have a mean of zero, and the Fisher average (Fisher 1953) is calculated for all 247 samples, the mean inclination is 1.7" with an ag5 is 1.4", or indistinguishable from the dipole expected inclination at this latitude of.6" calculated assuming a geocentric axial dipole. One large 'excursion' measured in a region of exceptionally low intensity in the through-bore measurements at about 72 cm, was unsubstantiated in the subsamples, highlighting the limitations in resolution of the through-bore measurements. The measurements of x and M,, vary downcore by less than a factor of four suggesting remarkable uniformity of the magnetic concentration and properties of this high carbonate (>9 per cent) core. Because the variability in carbonate is less than the experimental error, we have not corrected the x data for the nearly constant diamagnetic contribution of the carbonate matrix. The rock magnetic properties of RNDB 75p sediments are identical to those measured on other cores from the same area, described in detail by Tauxe & Wu (199) and Constable & Tauxe (1987). n brief, the magnetization is a single component, carried by magnetite. n Fig. 2, we show a summary of additional rock magnetic studies performed on the sediments of RNDB 75p. n Fig. 2(a), we show a plot of saturation remanence M,, versus susceptibility x. The data fall on two well-defined tracks, comprising subsamples from above and below approximately 4 m. The distinction between the two tracks is quite small. A best fit line in the least-squares sense through the data on the 'upper' and 'lower' tracks are shown as dashed and solid lines respectively. The correlation coefficients are.98 and.95 for the upper and lower tracks, reflecting the somewhat larger scatter in the lower track of data. Representative hysteresis loops obtained using an alternating gradient force magnetometer on samples spanning the length of the core are shown in Fig. 2(b). A high field diamagnetic slope has been removed, and the saturation magnetization normalized to one. The curves are quite similar for all samples, but there is a slight tendency for samples from above about 4m (dashed lines) to have more 'upright' hysteresis curves than those from below 4 m. Ratios of the hysteresis parameters of saturation remanence to saturation magnetization (M,r/M5) against the ratio of coercivity of remanence to bulk coercivity (Bcr/Bc) are shown on a Day plot (Day, Fuller & Schmidt 1977) in Fig. 2(c). All the data plot within a small portion of the pseudo-single-domain field, but the data from above and below 4 m fall in distinct clusters. The difference between the two populations can be ascribed to a difference in grain sizes between the two populations. However, the exact nature of the difference is ambiguous without additional information because the lower track could either have larger grains, plotting closer to the multidomain (MD) range, or additional superparamagnetic

3 Relative palaeointensity records r L RNDB 75p.' ,- Y :.L Pi (SP) grains, pulling the data toward the SP range (see Hartl, Tauxe & Herbert 1994 for discussion). n order to examine the character of the change in grain size further, we plot the data of Fig. 2(a) against depth in Fig. 3. We have converted the data to a parameter here called AM,,, which is the observed M,, value minus the M,, predicted from a given x value indicated by the dashed line in Fig. 2(a). Thus, data from the upper track (upper 4m) fluctuate about the expected value and the data from the lower track (below about 4 m) generally plot below this value. Not only do the data from below 4 m have a lower mean value of AM,,, but they are also more scattered, as noted previously, and have a slightly different slope. This change in grain size appears abrupt and is reminiscent of the abrupt loss of magnetization and susceptibility observed at varying depths in a variety of cores

4 772 L. Tame and N. J. Shackleton n c y" 2. r4 E M = 1. L v).5. 3', /o Above 4 m Below 4 m x (lov8 m3 kg-') v) \L.3 v).2.1 SD 1.o n n - 3 \ 3.5 w C a, E. 3 U a, N.- ;-.5 E L z -1.o Applied Field (T) Above 4m Below 4m. SP & MD BcJBc Figure 2. Rock magnetic measurements on discrete samples from RNDB 75p. (a) Saturation remanence (Msr) against low field magnetic susceptibility (x). (b) Representative hysteresis loops from samples taken above 4 m (dashed line) and below 4 m (solid line). A high field diamagnetic slope has been removed from the original data and saturation magnetization was normalized to unity. (c) Hysteresis ratios plotted on a Day diagram (Day et al. 1977) for samples above (open) and below (closed) 4 m., '.4 Y N E c -.2 ti -.4 a ' Metres below core top Figure 3. M,,, data plotted as AM,,, the difference between the measured value and the value predicted from a given x value using the dashed line in Fig. 2(a).

5 Y Relative palaeointensity records Centimetres below core top Figure 4. Abrupt loss of magnetization and magnetic susceptibility with depth as observed at about 54cm (indicated by the arrow) in core ERDC 89p. from the OJP (Musgrave et al. 1993). An example of this almost total loss of magnetization in ERDC 89p is shown in Fig. 4. At about 545cm, both magnetization and susceptibility drop dramatically to some 1 per cent of the average from above that depth. This abrupt and permanent loss of magnetization (occurring at what will be called here the critical depth) has been attributed to diagenetic loss of magnetite due to sulphate reduction (Musgrave et a/. 1993; see also Karlin & Levi 1983, 1985; Canfield & Berner 1987). n Fig. 5 we update the plot of critical depth versus water depth from Musgrave et a/. (1993) by including other cores from the OJP. We suspect that the subtle changes in grain 4 89p/OoN! 3 85/1.2O a3/ /1.2O Critical Depth (m) Figure 5. Depth at which the intensity of remanence or susceptibility drops permanently below 1 per cent of the initial value (critical depth) versus the water depth of the core. Data from Musgrave et al. (1993), Barton & Bloemendahl(1985) and Tauxe & wu (199). size occurring well above the critical depth in RNDB 75p, could be a related phenomenon, signifying diagenetic modification of the magnetic phases. Above -4m, there is superior uniformity of rock magnetic properties of RNDB 75p as compared to recently published paleointensity records (Tauxe & Wu 199; Tric et al. 1992; Meynadier et al. 1992; see Tauxe 1993 for a review). However, the subtle grain size change at -4m suggests that the data from the lower part of the core should be treated with caution. We should note that the data below 4m are not necessarily unsuited for relative palaeointensity as they still pass all of the requirements described by King, Banerjee & Marvin (1983) and Tauxe (1993), but there is sufficient cause for concern for us to distinguish them from the other data sets in every case. 3 CHOCE OF A NORMALZATON PARAMETER The fundamental assumption serving as the basis for sedimentary palaeointensity studies is that the remanent magnetization of the sediment under study is linearly related to the anicent magnetic field in which it was deposited, after suitable normalization for changes in magnetic concentration, grain size and so on. n a recent review of the subject of sedimentary palaeointensity, Tauxe (1993) outlined the pros and cons of various methods of normalization. Based on the experimental evidence (particularly Sugiura 1979 and data published in the review), it is clear that the most popular method for normalization, the use of anhysteretic remanence, has severe drawbacks and may not be the optimum method. These drawbacks are first that anhysteretic remanence is not linear with applied field for fields as low as.6,mt, and secondly that it is not linear with concentration particularly for concentrations as low as in these cores (-1-4v1. per cent). These two factors lead to a large amount of scatter in classic plots of anhysteretic remanence versus x (Banerjee, King & Marvin 1981; King et af. 1982, 1983) obscuring more subtle changes in grain size (see also Hart1 et al. 1994).

6 774 L. Tauxe and N. J. Shackleton For these reasons, we have chosen here to use saturation isothermal remanence M,, because it is linearly related with concentration (unlike anhysteretic remanence at these concentrations). Moreover, the drawbacks of using M,,, such as the sensitivity to multidomain grains, appear not to be serious in these sediments, owing to their uniformly pseudo-single-domain character. Finally, based on hundreds of demagnetization curves of natural, anhysteretic and isothermal remanences for sediments of the OJP (for examples, see Tauxe & Wu 199), the characters of the three parameters are distinctly similar, although anhysteretic is almost always slightly more stable against alternating field demagnetization than isothermal remanence). On balance, although not ideal, isothermal remanence presents several advantages over anhysteretic remanence as a normalizer. We note that the agreement between x (by far the worst.81 NRM/M,, versus M,, NRM/ARM versus ARM.6 - _--- The entire record Above 4 m NRM/x versus x N+.61 N (b)erdc 8 9 ~ NRM/x versus x fc) ERDC Frequency (per cm) Figure 6. Squared coherence ( y2) versus frequency of relative palaeointensity estimate and the normalizer for three OJP cores (RNDB 75p, ERDC 89p and ERDC 113p) and RC1-167 from the North Pacific. choice as a normalizer) and M,, in the upper 4m is encouraging. Finally, the fact that M,, is a saturation parameter is its principle advantage. For this reason, we do not demagnetize it to the same level as the natural remanence (in this case 15mT) prior to normalization. ndeed, as pointed out by Levi & Banerjee (1976), this can be quite an unstable procedure. As stressed by Tauxe & Wu (199), the normalizer of choice should not be coherent in the frequency domain with the normalized remanence. Such a coherence could be an indication that normalization has failed to take completely into account changes in the magnetization potential of the sediments due to, for example, changes in magnetic grain size. n order to test coherence, we plot squared coherence (y ) in Fig. 6 (see Tauxe & Wu 199, for a detailed description of the method). The horizontal dashed line is the level of y that is indistinguishable from zero coherence at the 95 per cent level of confidence (Chave & Filloux 1985). Please note that y can be above this line 5 per cent of the time from chance alone. n Fig. 6(a) we show the coherence between M,, and NRM demagnetized to 15mT and normalized by M,, for both the entire record and for the upper 4m alone. These two time series are quite incoherent and it appears that the normalized remanence record is controlled by factors other than lithologic variability. We will assume in the following that normalized remanence reflects the relative palaeointensity of the Earth s magnetic field at the time of bioturbation. Also shown for purposes of future discussion are the coherence of normalized remanence versus the normalizer for three other published records: ERDC 89p and 113p (Tauxe & Wu 199) and RC1-167 (Kent & Opdyke 1977). The ERDC cores show coherence only at high frequencies corresponding to periods shorter than some 15Ka. The RC1-167 record, however, shows significant coherence in the frequency band from.12 to.25 corresponding to climatically important periods of 4-8 Ka. One final criterion for judging palaeointensity records is whether, when put on a common time-scale, duplicate records from the same region show similar behaviour. There are two other published records from the OJP (ERDC 89p and 113p; Tauxe & Wu 199). These span the latter half of the Brunhes and Chron and will be considered together with the new record presented in this paper. 4 TME FRAMEWORK Oxygen isotopic data from the three OJP cores are shown in Fig. 7 against depth. The data from RNDB 75p were shown in Fig. 1 and those from ERDC 89p and 113p were compiled by Tauxe & Wu (199) and references therein. The reference time-scale and terminology we will use is that of mbrie et af. (1984) for the period -62Ka. Then, assuming that the BrunhedMatuyama boundary falls within the early part of the Stage 19 and that it is 78Ka (Shackleton, Berger & Peltier 199), Stages were dated by interpolation. We interpret the isotropic signature of RNDB 75p (Fig. 7) to mean that the upper -1 m of sediment was not recovered and the top of the core begins in about Stage 6. Support for this interpretation comes from inspection of the foraminiferal content, which revealed the presence of Globigerinoides

7 Relative palaeointensity records 775 EROC 113p.', t n "." nl '. Lo 1.o Metres below core top RNDB 75p Age (ko) Figure 7. Time framework for the Ontong-Java piston cores used in this study. The 6l8 time-scale is that of mbrie er al. (1984) for the period -62 Ka. Between 62 and 78 Ka, data are calibrated by interpolation, using an age of 78 Ka for the Brunhes/Matuyama boundary (Shackleton er al. 199). ruber. This pink-pigmented foraminifera became extinct in the ndo-pacific during Stage 5 (Thompson et al. 1979) but occurs in samples of RNDB 75p up to the top of the core. We plot the relative palaeointensity data from the overlapping portions of the three OJP cores in Fig. 8. n Fig. 8(a), the three cores are adjusted to maximize the correlation of the isotopic data (Fig. 7) alone. Since the sampling density of the ERDC cores was rather poor, there is considerable room for small-scale adjustments without violating any of the isotopic constraints. Therefore, we attempt in Fig. 8(b) to align the peaks in relative palaeointensity within the constraints provided by the isotopic data. n Fig. 8(c), we create an OJP stack based on the data shown in Fig. 8(b). Plots of depth versus age tie points used in creating these plots are shown in Fig. 9. There are only subtle differences between the two time-scales. The three cores have similar sediment accumulation rates of 1.2, 1.3 and 1.5 cm Ka-' for ERDC 89p, 113p and RNDB 75p respectively. 5 COMPARSON OF THE OJP STACK WTH OTHER RECORDS One of the most powerful tests of the reliability of the sedimentary palaeointensity method is reproducibility. There are very few studies with which the data from the OJP can be compared. However, there have been two studies published recently that have the length, replication and the isotopic time control sufficient to allow a meaningful comparison. One was from the Somali Basin (Meynadier et af. 1992) spanning the last 14Ka, and the other was from the Mediterranean Sea (Tric et al. 1992), spanning the last 8Ka. The data from the three regions are plotted in Fig. 1. Each data set has been scaled to have a mean of unity, but has otherwise not been modified. There are similarities among the three data sets, particularly during the last 8 Ka. n particular, there are lows in relative palaeointensity (B*) at 4 and 6-65 Ka, with highs at 5 and 8 Ka. Another record older than 14Ka is that from the North

8 776 L. Tauxe and N. J. Shackleton "'- Using isotopes alone - RNDB 75p - ERDC 113p!+\,, t.1! h u.d 2. C a, 42 [3 'g 1.5 a, m 3 m a a, 1. rz.ri il ri m.5 E (b) Aligning intensity peaks and isotopes.. ', , 2. Ontong-Java Stack 1! Age (ka) Figure 8. Relative palaeointensity versus age for the Ontong-Java cores. (a) Data are aligned using isotopic age constraints; (b) peaks in intensity are aligned as much as possible, without violating the isotropic age constraints; (c) the data in (b) are averaged to give a stacked record for the OJP. Pacific core RC1-167 (Kent 8~ Opdyke 1977). This core had also included those from the lower part of RNDB 75p, no carbonate, hence has no oxygen isotopic time control. indicated by a dashed line. These data come from a single Nevertheless, it is worthwhile comparing the two data sets core only and may have suffered diagenetic modification of seeking broad similarities. n Fig. 11 we plot. the magnetite. However, since they have isotopic age control, NRM/ARM (BA) data of RC1-167 and the OJP stack. n superior rock magnetic uniformity compared with other addition to the data stacked as illustrated in Fig. 8, we have published data (the evidence for diagenetic alteration

9 Using isotopes alone 8oo Relative palaeointensity records 777 Aligning intensity peaks and isotopes ERDC 89p ERDC 113p Depth below core top (crn) Figure 9. The age-depth tie points used to construct Fig. 8. notwithstanding), we feel it worthwhile to include them in Fig. 11, bearing in mind all the potential problems. There are many similarities between the RC1-167 data and those from the OJP. Prominent features have been marked with dashed lines. ndeed, apart from one prominent feaure in the RC1-167 record (indicated with a question mark), the two data sets are remarkably similar. A detailed comparison is impossible, however, because of the lack of isotopic time control for RC SPECTRAL ANALYSS One of the principal findings of Kent & Opdyke (1977) on analysing the palaeointensity record of RC1-167 was that it had substantial spectral power centred at a period of around 43 Ka (.23 Ka-' in terms of frequency)--enticingly close to that of the Earth's obliquity (41 Kal.24 Ka-'; mbrie et al. 1984). (Kent & Opdyke used an age of.7ma for the B/M boundary so this peak would be centred at 48 Ka/.21 Ka-' using the new time-scale employed in this paper.) They noted that a link between obliquity and the generation of the magnetic field was not impossible (citing Malkus 1963, 1968), but evidence from a single core from a single place should be viewed with caution when discussing a global phenomenon such as the geomagnetic field. This record has stood unconfirmed for some 15 years. With the retrieval of RNDB 75p, we are at last in a position to test to some degree whether the results of Kent & Opdyke (1977) are reproducible. Moreover, since the earlier analysis, new spectral techniques have been developed more suited to the short time series commonly available in palaeomagnetism (Thomson 1982). The results of our spectral analysis are shown in Fig. 12. Each of these analyses was performed with six tapers and a time-bandwidth product of 3.5. For the purposes of comparison, we plot the spectrum of the oxygen isotopic time-scale (shown in Fig. 7) as this has been repeatedly shown to exhibit the so-called h43ankovitch periods of Depth below core top (crn) 1 Ka, 41 Ka and about 2 Ka corresponding to eccentricity, obliquity and precession of the Earth's orbit (mbrie et al. 1984). We plot this climatically forced signal as a bold line in Fig. 12(a). The coherence of the record with the isotopic data was tested using 12 tapers and a time-bandwidth product of eight, giving approximately 24 degrees of freedom and a zero coherence criterion of.24 (Chave & Filloux 1985). Results of an analysis of RC1-167 are similar to those reported by Kent & Opdyke (1977) in that the plot of amplitude versus frequency displays a general decline from low frequency to high punctuated by spikes at irregular intervals. However, there are several limitations of the RC1-167 record as already noted. First, it has no internal time control and changes in accumulation rate could distort the power spectrum by shifting the frequencies or obscuring significant peaks completely. Second, there is significant coherence of the record with lithological variability which gives one cause for concern about the adequacy of the normalization procedure. For these reasons, we do not plot the spectrum and turn instead to the more promising OJP stack record. The combined records from the OJP have the advantage that there is an excellent time framework available, and that the relative intensity records display no coherence with the bulk magnetic measurements in frequencies of interest here. Furthermore, there are three overlapping records, giving at least two independent estimates for relative palaeointensity over at least the last 44Ka. Finally, when compared with contemporaneous records from around the globe, they exhibit broad similarities. f there are variations of the Earth's dipole moment at orbital frequencies, then these should be evident in the OJP records. We plot the spectra of the Ontong-Java stacked record in Fig. 12(b) and the coherence with the Brunhes isotopic data in Fig. 12(a). For the entire record, there is a broad peak corresponding approximately to the eccentricity frequency (.1 Ka-'/1 Ka) as well as one centred on about.33 Ka-'/3 Ka. Taking just the -442 Ka portion

10 778 L. Tame and N. J. Shackleton 2. Mediterranean Sea X,i, o.5 < tn \,, %.-* Somali Basin a Figure 1. Relative palaeointensity data from the OJP are compared with two other records with sufficient length and isotopic age constraints. The Mediterranean Sea data are those of Tric et al. (1992) and the Somali Basin data are from Meynadier ei al. (1992). Bi is susceptibility-normalized remanence, and BA is ARM-normalized remanence. of the record the peaks shift slightly toward lower frequencies and the approximately 1 Ka peak becomes poorly expressed. Both time series display slight coherence with the isotopic data in the range of -.2Ka- or near the obliquity band. Based on these analyses, it appears that there may be some power in the palaeointensity variations of the Earth s magnetic field roughly corresponding to the Milankovitch frequencies, at least for the last half of the Brunhes. We note again that none of the individual OJP records displayed significant coherence with its normalizer in the frequencies of interest here. n order to explore the possibility of a link between the Earth s orbit and variations in the magnetic field more directly, we plot the OJP stacked record scaled for comparison with the orbital forcing functions of eccentricity, obliquity and so-called climatic precession as calculated by Berger & Loutre (1991) in Fig. 13. Also shown is the astronomical precession represented as a sine wave with a

11 1.c Relative palaeointensity records 779.e (a) OJP stack B' versus Brunhes 6l8.E N A.4 Zero Coherence.2 ' Ontong-Jova Plateau (stack) Age (ka) Figure 11. Comparison of the OJP stack with the record of RC1-167 from Kent & Opdyke (1977). Possible correlations are indicated by the dashed lines. period of 26 Ka. There is some degree of similarity between the eccentricity function and the OJP stack. However, the record is much too short to establish the reliability of this apparent correspondence as it is based on less than six wavelengths of the 1 Ka period. The OJP stacked record is just long enough, however, to examine the purported link between obliquity and/or precession and magnetic variability. As suggested by the spectral analysis, the dominant period of the magnetic record appears to be at a frequency between those of obliquity and climatic precession (Fig. 13). ndeed there is no coherence between the OJP stack and either obliquity or climatic precession. None the less, there is sufficient variability in frequencies close to the 26 Ka cycle that some input from astronomical precession cannot be excluded. t does not seem to be possible to adjust the palaeointensity peaks to align with the 26 Ka precessional cycles without violating the isotopic time constraints. We have used the SPECMAP time-scale as the basis for comparing our palaeointensity data with the orbital functions. The SPECMAP time-scale was generated by tuning the isotopic data to the orbital variations of the Earth (mbrie et al. 1984). Thus, tuning the palaeointensity data to the 26Ka cycle would imply a choice of believing that the orbit controls the magnetic field over believing that it controls ice volume. The latter assumption has stood the test of time (see e.g. mbrie et al. 1992) and the former has no firm theoretical or experimental foundation (Rochester et al. 1975). On balance, we prefer to interpret the data in Fig. 13 to suggest that the orbit does not play a detectable role in the generation of the Earth's magnetic field. Tauxe & Wu (199) concluded that while there was perhaps a periodicity in the relative palaeointensity data in the -33Ka band, this did not persist throughout the Brunhes but was only evident in the latter half. Their data set consisted of a stack of RC1-167, ERDC 89p and 113p. As noted previously, RC1-167 has no independent age control and the stack prior to about 45Ka was based entirely on this interpolated record. We now see that the W _ E3 a L e,.c (b) Brunhes 6l8O - OJP Stack ka OJP Stack ka n Frequency (per ka) Figure U. Spectral analysis of the OJP stacked record and the Brunhes isotopic data of Fig. 7. (a) Coherence of OJP relative intensity with the isotopic data. Time-bandwidth product = 8, 12 tapers; 95 per cent confidence limit for zero coherence shown as dashed line. (b) Linear amplitude versus frequency of the OJP stack and the Brunhes isotopic data (in bold). -33Ka periodicity may persist into the early Brunhes as well. 7 CONCLUSONS (1) The core RNDB 75p taken from the OJP has excellent rock and palaeomagnetic properties suggesting nearly uniform magnetic properties in the upper 4m of the core. The remanence is carried by apparently pseudo-singledomain magnetite. Subtle changes in magnetic grain size, detected using plots of isothermal remanence versus magnetic susceptibility and ratios of several hysteresis parameters, occur below 4 m indicating the possibility of t

12 78 L. Tauxe and N. J. Shackleton -.8' ,., \ Ko Figure 13. Plot of OJP stack scaled for comparison with orbital forcing (eccentricity, obliquity and climatic precession) calculated by Berger & Loutre (1991). Also shown is the astronomical precession with a 26 Ka period. Bold lines are the portion of the OJP stack based on the most reliable data. Thin lines are data from below 4 m in RNDB 75p and are less reliable. Respective orbital functions are shown by dashed lines. slight diagenetic modification of the magnetite phase below isotopes, the RNDB 75p record was compared with other this depth. OJP records (ERDC 89p and 113p from Tauxe & Wu 199). (2) The normalized remanence varies independently from Coeval portions of the records were combined to create an variations in bulk magnetic properties allowing the OJP stack, for comparison with records from the assumption that it is controlled by variations in the Mediterranean (Tric et al. 1992), the Somali Basin geomagnetic field, rather than lithologic factors. (Meynadier et al. 1992) and the north Pacific (Kent and (3) When placed on a common time-scale using oxygen Opdyke 1977). There are many common features among the

13 various records and the degree of reproducibility is encouraging. (4) Spectral analysis of the OJP stack suggests that there are concentrations of power in the relative palaeointensity record near to, but somewhat offset from, orbital frequencies. t is important to stress that these peaks appear not to be coherent with variations in magnetic properties controlled directly or indirectly by climate, but are likely to represent true variations in the geomagnetic field. However, when compared with the orbital functions of obliquity and precession, no coherence is displayed and the dominant periods of the OJP stack appear to be between the two (at about 33 Ka). Some correspondence with eccentricity can be discerned but the record is too short for a meaningful discussion to be made. 8 ACKNOWLEDGMENTS We would like to thank Steve Didonna for assistance in obtaining the data and Paul Hartl, Cathy Constable, John Tarduno, Fritz Hilgen and Cor Langereis for numerous discussions. Funds for the Scripps palaeomagnetic laboratory were kindly provided by the Keck Foundation. This work was supported in part by NSF grants EAR and OCE Partial support from the University of Utrecht is also gratefully acknowledged. NOTE ADDED N PROOF After submission of the final version of this paper, another record of relative palaeointensity spanning the Brunhes Chron was published: Valet, J.-P., & Meynadier, L., Geomagnetic field intensity and reversals during the last four million years, Nature, 366, REFERENCES Amerigian, C., Sea-floor dynamic processes as the possible cause of correlations between paleoclimatic and paleomagnetic indices in deep-sea sedimentary cores, Earth planet. Sci. Lett., 21, Banerjee, S.K., King, J., & Marvin, J., A rapid method for magnetic granulometry with applications to environmental studies, Geophys. Res. 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