Geomagnetic field variations during the last 400 kyr in the western equatorial Pacific: Paleointensity-inclination correlation revisited

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1 GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L20307, doi: /2008gl035373, 2008 Geomagnetic field variations during the last 400 kyr in the western equatorial Pacific: Paleointensity-inclination correlation revisited Toshitsugu Yamazaki, 1 Toshiya Kanamatsu, 2 Sakiko Mizuno, 3 Natsumi Hokanishi, 1 and Eddy Z. Gaffar 4 Received 22 July 2008; revised 17 September 2008; accepted 19 September 2008; published 24 October [1] A paleomagnetic study was conducted on four piston cores newly obtained from the West Caroline Basin in the western equatorial Pacific in order to investigate variations in paleointensity and inclination during the last 400 kyr. An inclination-intensity correlation was previously reported in this region using giant piston cores, but the quality of the paleomagnetic data of the younger end, the last ca. 300 kyr, was needed to be checked because the upper part of the giant piston cores could suffer from perturbation by oversampling. Age control is based on the oxygen-isotope ratios for one core and inter-core correlation using relative paleointensity for other cores. Stacked curves of paleointensity and inclination were constructed from the four cores. It was confirmed that variations on the order of 10 4 to 10 5 years occur in inclination as well as paleointensity. A cross-correlation analysis showed that significant in-phase correlation occurs between intensity and inclination for periods longer than about 25 kyr, and power spectra of both paleointensity and inclination variations have peaks at 100 kyr periods. The regional paleointensity stack with higher resolution than the Sint-800 stack (Guyodo and Valet, 1999) should be useful for paleointensity-assisted chronostratigraphy. Citation: Yamazaki, T., T. Kanamatsu, S. Mizuno, N. Hokanishi, and E. Z. Gaffar (2008), Geomagnetic field variations during the last 400 kyr in the western equatorial Pacific: Paleointensity-inclination correlation revisited, Geophys. Res. Lett., 35, L20307, doi: /2008gl Introduction [2] Recent progress in sedimentary paleointensity revealed that variations in intensity have significant power in frequencies on the order of 10 4 to 10 5 years [e.g., Tauxe and Yamazaki, 2007]. It has been argued whether the Milankovitch orbital frequencies often found in relative paleointensity records are originated from orbital modulation of the geomagnetic field or lithological changes of sediments induced by paleoclimate changes [Channell et al., 1998; Yamazaki, 1999; Yokoyama and Yamazaki, 2000; Guyodo et al., 2000; Thouveny et al., 2004; Yokoyama et al., 2007]. As for variations in directions, fluctuations on the order of 10 2 to 10 3 years in Holocene have been documented relatively well. However, the number of records of 1 Geological Survey of Japan, AIST, Tsukuba, Japan. 2 Institute for Research on Earth Evolution, Japan Agency for Marine- Earth Science and Technology, Yokosuka, Japan. 3 Center for Advanced Marine Core Research, Kochi University, Kochi, Japan. 4 Research Center for Geotechnology, Indonesian Institute of Science, Bandong, Indonesia. Copyright 2008 by the American Geophysical Union /08/2008GL directional variations on the order of 10 4 to 10 5 years is still scarce, although variations of this timescale are important for understanding the core dynamics. The amplitude of the directional variations is small, which is not much larger than possible errors in sampling and measurements, and thus accumulation of high-quality records is required. [3] Yamazaki and Oda [2002] reported a quasi-periodic signal of 100 kyr frequency in inclination variations from a sediment core in the western equatorial Pacific, although its statistical significance has not yet been established [Roberts et al., 2003]. Yamazaki and Oda [2002, 2004] found significant correlation between paleointensity and inclination, and presented a possible model to explain it; the strength of the geocentric axial dipole (GAD) varies whereas persistent non-dipole components do not. These studies used giant piston cores of the International Marine Past Global Changes Study (IMAGES), MD and MD However, it has become known that significant oversampling often occurs in the upper parts of IMAGES giant piston cores [Skinner and McCave, 2003; Széréméta et al., 2004]. The oversampling seems to have occurred in the particular two cores, which is evidenced by rapid upward increases of apparent sedimentation rates and larger amplitudes of fluctuations in remanent directions in the upper 15 to 20m (ca. the last 300 kyr) [Yamazaki and Oda, 2002, 2004]. Thus the quality of the paleomagnetic records of the younger end needs to be checked using undisturbed sediment cores. [4] We conducted a paleomagnetic study of four piston cores newly obtained from the West Caroline Basin (WCB) in the western equatorial Pacific. We present regional relative paleointensity and inclination stacks during the last ca. 400 kyr, and report large inclination anomalies. We confirm in-phase correlation between paleointensity and inclination during this period of time. 2. Materials and Age Control [5] Four piston cores, KR0515-PC1, PC2, PC3, and PC4 (hereafter KR0515- is omitted from the core ID), of about 12.5 to 19.5m long were taken from WCB during the KR05-15 cruise of R/V Kairei (Table 1). The water depths of the coring sites range from about 3200 to 4300m, constituting a depth transect. The sites are on the outer rise of the Caroline plate subducting at the New Guinea trench, and thus the sites are considered to be free from detritus transported as turbidite. The cores are comprised of greenish gray hemipelagic clay capped with yellowish brown oxic layer of several tens of centimeters. For cores from the shallower sites (PC1 and PC2), a large amount of foraminifera is visible and the color of the sediments is lighter, L of5

2 Table 1. Position, Water Depth, and Inclination Anomaly of Studied Cores Core ID Latitude Longitude Depth (m) Mean ChRM I KR0515-PC S E KR0515-PC S E KR0515-PC S E KR0515-PC N E a DI is the Inclination anomaly. whereas only a minor amount of foraminifera remains and the color is darker at the deepest site (PC4), where the water depth is near the carbonate compensation depth (CCD) in this area, about 4500m at present. Discrete samples for paleomagnetic measurements were taken from half-split core sections sequentially with no gap between them using plastic cubes of 7 cm 3 immediately after the core recovery, and the samples were tightly sealed and kept refrigerated. DI a [6] Oxygen-isotope ratios (d 18 O) were measured on planktonic foraminifera Globigerinoides ruber of >250 mm size picked out from PC1 using a Micromass Optima mass spectrometer at the Geological Survey of Japan (GSJ), AIST. The depths of the core were converted to ages by correlating the measured d 18 O curve to the global d 18 O stack LR04 [Lisiecki and Raymo, 2005] (Figure 1). The ages of other three cores were inferred by inter-core correlation using relative paleointensity as shown later. 3. Paleomagnetic Measurements and Results [7] Paleomagnetic measurements were carried out at GSJ, AIST, for PC1 and PC2, the Japan Agency for Marine-Earth Science and Technology for PC3, and the Center for Advanced Marine Core Research, Kochi University, for PC4. Remanent magnetization was measured using pass-through cryogenic magnetometers (2G Enterprises model 760R) with in-line alternating-field (AF) Figure 1. Age control of the cores. Oxygen-isotope (d 18 O) stratigraphy for KR0515-PC1 (gray lines show tie points), which was transferred to other cores using relative paleointensity. 2of5

3 Figure 2. Stacked curves of relative paleointensity and inclination in the West Caroline Basin during the last 380 kyr. Blue curves in relative paleointensity and inclination are the stacks of Yamazaki and Ioka [1994], orange curve in relative paleointensity is the Sint-800 stack of Guyodo and Valet [1999], and pink curve in inclination is of Yamazaki [2002]. Correspondence between paleointensity and inclination is shown by bold pale green lines. demagnetizers. Stepwise AF demagnetization revealed that remanent magnetization of most samples consists of a single component except for low AF steps up to 15 mt in general. Directions of characteristic remanent magnetization (ChRM) were determined by applying the principal component analysis (PCA) [Kirschvink, 1980]. The maximum angular deviations (MADs) are very small in general; about 90% of the samples show MAD less than 2 for PC1, PC2, and PC4, and less than 4 for PC3. An average ChRM inclination and inclination anomaly DI (observed inclination minus prediction from GAD) of each core are presented in Table 1. After NRM demagnetization, anhysteretic remanent magnetization (ARM) was imparted by superimposing a DC biasing field of 0.1 mt on a smoothly decreasing AF with a peak field of 80 mt, and then demagnetized progressively with AF. Next, isothermal remanent magnetization (IRM) was imparted at 2.5T, which is regarded as saturation IRM (SIRM) in this paper, using pulse magnetizers (2G model 660), and then partially demagnetized with AF. [8] It is known that sediments in WCB are rock-magnetically homogeneous, and suitable for relative paleointensity estimations [Yamazaki and Ioka, 1994; Yamazaki and Oda, 2005]. The homogeneity was verified also for the present four cores. Downcore variations in magnetic concentration are within several times. S-ratios, a magnetic mineralogy proxy, showed little variation between and in general, indicating that remanent magnetization of these cores is mostly carried by magnetite. Occurrence of magnetite is supported by presence of the Verwey transition in lowtemperature magnetometry. Magnetic hysteresis parameters produced a tight cluster in a pseudo-single-domain region on the Day plot [Day et al., 1977] (Mr/Ms 0.15, Hcr/Hc 2.5). Fluctuations of the ratio of ARM to SIRM are also small, within 20% of the mean for each core. The ARM/ SIRM ratio is often interpreted as a magnetic grain-size proxy, but in Pacific deep-sea sediments, it may represents relative abundance of a non-interacting single-domain (SD) component of probably biogenic origin and an interacting SD component of terrigenous origin [Yamazaki, 2008]. In either case, the small variation of the ARM/SIRM ratio indicates rock-magnetic homogeneity. We selected ARM as a normalizer of relative paleointensity estimation mainly because of the consistency with previous works in this area [Yamazaki and Ioka, 1994; Yamazaki and Oda, 2005]. NRM and ARM intensities demagnetized with AF at 30mT were used. Variation patterns of normalized intensity do not depend on the choice of the normalizer between SIRM and ARM, as expected from the small changes of the ARM/ SIRM ratio. Normalized intensity does not depend on the peak field of AF demagnetization either. Relative paleointensity variation patterns of the four cores closely resemble each other (Figures 1 and S1 1 ). [9] Depths of each core were converted to a common depth scale based on inter-core correlation using relative paleointensity, and PC2 was selected as a reference for this purpose because its resolution looks highest among the four cores (Figures 1 and S1). Then the depth scale was changed to age based on the d 18 O stratigraphy of PC1 transferred to PC2. Ages of the bottom of PC2 and PC3 are about 380 ka, which is similar to that of PC1, although the lengths of the cores are different. Core PC4 reached older ages, but the age interval of ka common to other cores is used for further analysis in this paper. The mean sedimentation rates of PC2 and PC3 during the last 380 ka are 46 to 47 m/m.y., a little higher than that of PC1, which is similar to the value (39 m/m.y.) estimated for PC4. Conspicuous paleointensity minima would be associated with geomagnetic excursions; for example the Laschamp excursion at 40 ka and the Iceland Basin excursion at 190 ka [Laj and Channell, 2007]. However, ChRM directions did not show clear excursional directions. This is probably because sedimentation rates of the present cores are not high enough; a sedimentation rate above 10 m/m.y. would be necessary to preserve excursional directions [Roberts and Winklhofer, 2004]. [10] Relative paleointensity and inclination records of each core were resampled at 1 kyr intervals by linear interpolation. Then the mean and standard deviation of the four records were calculated for paleointensity and inclination, respectively, at 1 kyr intervals (Figure 2). Before the calculation, the mean inclination of each core was set to zero by subtracting a constant bias to compensate for latitudinal differences of the sites and non-vertical penetration of the cores. 4. Discussion [11] In this study, we have constructed a regional relative paleointensity stack in WCB as shown in Figure 2. We need to be aware that the ages of the four cores were not 1 Auxiliary materials are available in the HTML. doi: / 2008gl of5

4 Figure 3. (top) Power spectra of relative paleointensity (red) and inclination (blue) and (middle) coherence and (bottom) phase angle between the two. The AnalySeries software [Paillard et al., 1996] was used, and the Blackman-Tukey method with a Bartlett window was applied. independently controlled but tied using paleointensity itself, which resulted in apparently small standard deviations. The paleointensity stack is almost identical to that of Yamazaki and Ioka [1994] during the last 200 kyr, except for about 15 kyr difference in age of the peak at about 50 ka (Figure 2). The discrepancy would be due to the lower quality of the d 18 O curve of Yamazaki and Ioka [1994], which was derived from the site of 4055m in depth, relatively close to the CCD. Comparison with the Sint-800 stack of Guyodo and Valet [1999] shows that long-term variation trends coincide with each other. However, the amplitude of the variations in the Sint-800 stack is considerably smaller, and sharp, short-wavelength features in our stack such as the peaks at about 70, 125, 170, 205, and 320 ka are obscure in the Sint-800. Although the dataset of Yamazaki and Ioka [1994] was used for constructing the Sint-800 stack, inclusion of other records with lower sedimentation rates and disagreement in ages would have resulted in the loss of the short-wavelength features. The high-resolution regional paleointensity stack in WCB established in this study should be useful for global as well as regional inter-core correlation and age assignment, referred to as the paleointensity-assisted chronostratigraphy [e.g., Evans et al., 2007]. Accumulation of reliable paleointensity curves from various regions of the globe is also needed for separating the dipole and non-dipole components; non-dipole components may partly be responsible for the differences between the paleointensity stack in WCB and the Sint-800 stack. [12] The mean inclinations of the four cores show negative DIs, ranging from 5.2 to 11.2 (Table 1). The mean and standard deviation of DIs observed so far in WCB in the Brunhes chron but for various period of time are 6.5 ± 2.8 (N = 13; four cores of the present study, five cores of Yamazaki and Ioka [1994], two cores MD and MD by Yamazaki and Oda [2004], 6.2 for MD (Yamazaki and Oda [2005] for paleomagnetic data but DI was not presented), and 10.4 during the Brunhes chron for MD at N, E [unpublished]). The western equatorial Pacific is documented as a region of a large negative DI, and the observed mean value is comparable to those expected from the timeaveraged field (TAF) models: about 5 for Johnson and Constable [1997], and about 4 for Hatakeyama and Kono [2002]. [13] The inclination stack in Figure 2 demonstrates the occurrence of long-term variations. The fluctuations during the last 200 kyr coincide very well with those of the stacked record of Yamazaki and Ioka [1994] even for minor details. The excellent reproducibility indicates high reliability of the record. On the other hand, there is some inconsistency if compared with the smoothed curve of Yamazaki and Oda [2004] obtained from IMAGES giant piston cores, although long-wavelength patterns of the two roughly coincide. This is probably due to perturbation of sediments suspected for the upper 10 to 15m of IMAGES cores. [14] We can recognize a tendency for a correspondence between positive (negative) inclinations and relative paleointensity highs (lows) in Figure 2, although it is less obvious in the older part (>300 ka). We conducted a crosscorrelation analysis between relative paleointensity and inclination (Figure 3). For periods longer than about 25 kyr (0.04 cycle/kyr), paleointensity and inclination have significant coherence in general, and the phase angle is close to 0. This result confirms that variations in paleointensity and inclination are in-phase in this region in the Brunhes chron, as previously reported [Yamazaki and Oda, 2002, 2004]. A model that can explain the intensity-inclination correlation is that the strength of GAD varies whereas persistent non-dipole components do not [Yamazaki and Oda, 2002]. If the large negative DI in TAF models during the Brunhes chron in the western equatorial Pacific is due to non-dipole components, in particular the quadrupole component, and these strengths do not vary, inclination values smaller (steeper but negative sign) than the prediction of GAD are expected when GAD is weak, that is an in-phase intensity-inclination correlation. We consider lithological changes cannot be a cause of inclination variations. Inclination shallowing might occur in detrital remanent magnetization acquisition and it can depend on lithology, but its effect at equatorial latitudes where geomagnetic inclinations are close to zero would be negligibly small even if it occurs. The mean inclination close to the predictions from TAF models suggests that inclination shallowing is not present in our cores. The agreement of inclination variations among the sites constituting a depth transect where carbonate contents should be different (although not measured) suggests that lithology is not a controlling factor. 4of5

5 [15] Finally, we point out that power density of both paleointensity and inclination has a peak near a 100 kyr period. For paleointensity, the orbital eccentricity frequency has often been reported, and possibility for orbital modulation of the geomagnetic field has been argued. Concerning inclination, the number of reports of a 100 kyr period is a few, which is limited geographically in the western equatorial Pacific [Yamazaki and Oda, 2002; Yamazaki, 2002]. However, the strength of the inclination data is that they should be less susceptible to rock-magnetic changes induced by paleoclimate changes than paleointensity. If the model of the intensity-inclination correlation is correct, such correlation is expected for regions with large DIs in TAF models, for example, the central equatorial Pacific and equatorial Atlantic. [16] Acknowledgments. We thank Hirokuni Oda, Yusuke Suganuma, Mitsuru Yamamura, Dodi R. Galih, Etsuko Usuda, and Emi Kariya for the help of sampling and measurements, Kazuto Kodama for guidance to S.M., Hodaka Kawahata for support, and Yuhji Yamamoto and anonymous two reviewers for the constructive comments to the manuscript. We also thank captain, officers, crew, marine technicians, and others who were concerned with the R/V Kairei KR05-15 cruise. This study was partly supported by the Grant-in-Aid for Scientific Research ((A)(2) ) of the Japan Society for the Promotion of Science. References Channell, J. E. T., D. A. Hodell, J. McManus, and B. Lehman (1998), Orbital modulation of the Earth s magnetic field intensity, Nature, 394, Day, R., M. Fuller, and V. A. Schmidt (1977), Hysteresis properties of titanomagnetites: Grain-size and compositional dependence, Phys. Earth Planet. Inter., 13, Evans, H. F., J. E. T. Channell, J. S. Stoner, C. Hillaire-Marcel, J. D. Wright, L. C. Neitzke, and G. S. Mountain (2007), Paleointensity-assisted chronostratigraphy of detrital layers on the Eirik Drift (North Atlantic) since marine isotope stage 11, Geochem. Geophys. Geosyst., 9, Q02001, doi: /2007gc Guyodo, Y., and J.-P. Valet (1999), Global changes in intensity of the Earth s magnetic field during the past 800 kyr, Nature, 399, Guyodo, Y., P. Gaillot, and J. E. T. Channell (2000), Wavelet analysis of relative geomagnetic paleointensity at ODP Site 983, Earth Planet. Sci. Lett., 184, Hatakeyama, T., and M. Kono (2002), Geomagnetic field model for the last 5 My: Time-averaged field and secular variation, Phys. Earth Planet. Inter., 133, Johnson, C. L., and C. G. Constable (1997), The time-averaged field: Global and regional biases for 0 5 Ma, Geophys. J. Int., 131, Kirschvink, J. L. (1980), The least-squares line and plane and the analysis of paleomagnetic data, Geophys. J. R. Astron. Soc., 62, Laj, C., and J. E. T. Channell (2007), Geomagnetic excursions, in Treatise of Geophysics, vol. 5, Geomagnetism, pp , Elsevier, New York. Lisiecki, L. E., and M. E. Raymo (2005), A Pliocene-Pleistocene stack of 57 globally distributed benthic d 18 O records, Paleoceanography, 20, PA1003, doi: /2004pa Paillard, D., L. Labeyrie, and P. Yiou (1996), Macintosh program performs time-series analysis, Eos Trans. AGU, 77(39), 379. Roberts, A. P., and M. Winklhofer (2004), Why are geomagnetic excursions not always recorded in sediments? Constraints from post-depositional remanent magnetization lock-in modeling, Earth Planet. Sci. Lett., 227, Roberts, A. P., M. Winklhofer, W.-T. Liang, and C.-H. Horng (2003), Testing the hypothesis of orbital (eccentricity) influence on Earth s magnetic field, Earth Planet. Sci. Lett., 216, Skinner, L. C., and I. N. McCave (2003), Analysis and modelling of gravityand piston coring based on soil mechanics, Mar. Geol., 199, Széréméta, N., F. Bassinot, Y. Balut, L. Labeyrie, and M. Pagel (2004), Oversampling of sedimentary series collected by giant piston corer: Evidence and correlations based on 3.5-kHz chirp profiles, Paleoceanography, 19, PA1005, doi: /2002pa Tauxe, L., and T. Yamazaki (2007), Paleointensities, in Treatise of Geophysics, vol. 5, Geomagnetism, pp , Elsevier, New York. Thouveny, N., J. Carcaillet, E. Mareno, G. Leduc, and D. Nérini (2004), Geomagnetic moment variation and paleomagnetic excursions since 400 kyr BP: A stacked record from sedimentary sequences of the Portuguese margin, Earth Planet. Sci. Lett., 219, Yamazaki, T. (1999), Relative paleointensity of the geomagnetic field during Brunhes Chron recorded in North Pacific deep-sea sediment cores: Orbital influence?, Earth Planet. Sci. Lett., 169, Yamazaki, T. (2002), Long-term secular variation in geomagnetic field inclination during Brunhes Chron recorded in sediment cores from Ontong-Java Plateau, Phys. Earth Planet. Inter., 133, Yamazaki, T. (2008), Magnetostatic interactions in deep-sea sediments inferred from first-order reversal curve diagrams: Implications for relative paleointensity normalization, Geochem. Geophys. Geosyst., 6, Q02005, doi: /2007gc Yamazaki, T., and N. Ioka (1994), Long-term secular variation of the geomagnetic field during the last 200 kyr recorded in sediment cores from the western equatorial Pacific, Earth Planet. Sci. Lett., 128, Yamazaki, T., and H. Oda (2002), Orbital influence on Earth s magnetic field: 100,000-year periodicity in inclination, Science, 295, Yamazaki, T., and H. Oda (2004), Intensity-inclination correlation for longterm secular variation of the geomagnetic field and its relevance to persistent non-dipole components, in Timescales of the Paleomagnetic Field, Geophys. Monogr. Ser., vol. 145, edited by J. E. T. Channell, pp , AGU, Washington, D. C. Yamazaki, T., and H. Oda (2005), A geomagnetic paleointensity stack between 0.8 and 3.0 Ma from equatorial Pacific sediment cores, Geochem. Geophys. Geosyst., 6, Q11H20, doi: /2005gc Yokoyama, Y., and T. Yamazaki (2000), Geomagnetic paleointensity variation with a 100 kyr quasi-period, Earth Planet. Sci. Lett., 181, Yokoyama, Y., T. Yamazaki, and H. Oda (2007), Geomagnetic 100-ky variation extracted from paleointensity records of the equatorial and North Pacific sediments, Earth Planets Space, 59, E. Z. Gaffar, Research Center for Geotechnology, Indonesian Institute of Science, Bandong 40135, Indonesia. N. Hokanishi and T. Yamazaki, Geological Survey of Japan, AIST, Tsukuba , Japan. (toshi-yamazaki@aist.go.jp) T. Kanamatsu, Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, Yokosuka , Japan. S. Mizuno, Center for Advanced Marine Core Research, Kochi University, Kochi , Japan. 5of5