Ocean Drilling Program (ODP) Site 984 (Bjorn Drift) since

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. B10, PAGES 22,937-22,951, OCTOBER 10, 1999 Geomagnetic paleointensity and directional secular variation at Ocean Drilling Program (ODP) Site 984 (Bjorn Drift) since 500 ka: Comparisons with ODP Site 983 (Gardar Drift) J. E. T. Channell Department of Geological Sciences, University of Florida, Gainesville Abstract. Normalized remanence (relative geomagnetic paleointensity) records for the last 500 kyr can be matched between two Iceland Basin Ocean Drilling Program (ODP) sites (984 and 983) and correlated with other high-resolution records. Directional secular variation is, however, not easily correlated between sites due to inadequate recording, at these mean sedimentation rates (12-15 cm/kyr), of the characteristic high frequency variability of the directional record. Both sites record the Iceland Basin event at ka in which the characteristic magnetization component rotates through 180 ø and back, coincident with a paleointensity low that lasted about 3 kyr. Other Brunhes Chron geomagnetic excursions appear to be manifest as intervals of higher-amplitude secular variation during lows in paleointensity. There is a tendency for paleointensity lows to correlate with peak interglacials in the oxygen isotope records; however, this does not translate into a correlation between paleointensity and percent carbonate or between paleointensity and magnetic concentration or grain size parameters. 1. Introduction Ocean Drilling Program Leg 162 (July-September 1995) included sites located on sediment drifts south of Iceland [McCave et al., 1980; Manley and Caress, 1994; Wold, 1994]. Site 983 (60.4øN, 23.6øW) and Site 984 (61.4øN, 24.1øW) are located on the Gardar and Bjorn Drifts, respectively (Figure 1) [Shipboard Scientific Party, 1996a,b]. At Sites 983 and 984, mean sedimentation rates within the Brunhes Chron are 10.4 and 12.6 cm/kyr, respectively. Here we report sediment magnetic, paleointensity, and directional secular variation data from Site 984, for the last 500 kyr and compare these data with corresponding data from Site 983. Oxygen isotope age models for the late Brunhes Chron at both sites [see Channell et al., 1997, 1998] were constructed by correlation of the oxygen isotope records to a reference curve. In the case of Site 984, the reference curve was constructed from Imbrie et al. [ 1984], Martinson et al. [ 1987], and Shackleton et al. [1990]. Linear interpolation between age tie points yielded the age models (Figure 2). The shipboard polarity stratigraphies for both Sites 984 and 983, resolved to the base of the Olduvai Subchron, were based on pass-through measurements of the archive halves of core sections after a single alternating field (AF) demagnetization step (25 or 20 mt) [Shipboard Scientific Party, 1996a,b]. This abbreviated treatment was necessitated by the high rate of core recovery and the need to rapidly process the cores. The ground truthing of the shipboard polarity stratigraphy was carried out postcruise using natural remanent magnetization (NRM) and to compare the characteristic magnetization component with the magnetization directions obtained from the shipboard pass-through magnetometer [Channell and Lehman, 1999]. A few of these discrete samples were then used to determine hysteresis parameters and to carry out thermal demagnetization of isothermal remanent magnetization (IRM). The bulk of the magnetic data reported here were derived from u-channel samples [Tauxe et al., 1983], with a 2x2 cm square cross section and up to 1.5 m in length, which were collected postcruise from the "archive" halves of the composite section. A few u channels were also collected from "working" halves in the vicinity of the Iceland Basin event. Composite sections at Sites 983 and 984 were derived by using shipboard measurements of magnetic susceptibility, gamma ray attenuation porosity (GRAPE), and color reflectance to splice the records from multiple holes drilled at the site (four in the case of Site 984) to produce a complete and undisturbed record of the sedimentary sequence [see Hagelberg et al., 1992]. The u-channel record from the composite section, therefore, represents an optimal sedimentary record from the site. Depths in the composite record are measured as meters composite depth (mcd). Cored intervals within the composite splice were neither compressed nor expanded. The u-channel measurements were carried out on a small ac- cess 2G Enterprises pass-through cryogenic magnetometer at Gifsur-Yvette, France [Weeks et al., 1993]. Measurements were discrete samples (7 cm 3) in plastic boxes, collected shipboard, made at 1-cm intervals downcore; however, the response funcfrom each (1.5 m) core section. These samples were stepwise AF tions of the magnetometer pickup coils have an effective width of demagnetized in order to determine the component content of the --4 cm. Thus each fourth or fifth measurement, at best, is independent. As the archive halves of the composite section were demagnetized shipboard at peak fields of 20 or 25 mt, the NRM Copyfight 1999 by American Geophysical Union. of the u channels was measured after AF demagnetization at the Paper number 1999JB following peak fields: 25, 30, 35, 40, 45, 50, and 60 mt. Volume /99/1999JB susceptibility : was then measured at 1-cm intervals using a 45-22,937

2 22,938 CHANNELL: GEOMAGNETIC PALCOINTENSITY AND SECULAR VARIATION 28oW 24oW 20oW 18"W 12øW 62"N 60øN / /! Site "N I 56"N Rockall Plateau 54ON km Figure 1. Location map for Site 983 and Site 984. Bathymetry in meters. Dashed line indicates crest of Gardar Drift, arrows indicate inferred bottom current flows (after Manley and Caress, 1994; McCave et al., 1980). N -oo ::,,', II ] E 20,,' " o 60 ' ' ' ' ' { ' ' ' ' I ' ' ' ' { ' ' [ ' ' l Age (ka) Figure 2. Depth-age plot indicating tie points of the oxygen isotope data to the reference curves with linear interpolation between tie points [see Channell et al., 1997, 1998]. Highs in interval sedimentation rate tend to coincide with peak interglacials during isotopic stages 1, 5, 7, 9 and 11. Open symbols and dashed lines, Site 983; solid symbols and continuous lines, Site 984.

3 CHANNELL: GEOMAGNETIC PALEOINTENSITY AND SECULAR VARIATION 22,939 W N [ JiJo E/Down [ i!! i [ H (nat) mot E/Down! [ [! ] i SO H (nat) mcd F/Down! i i! [ i m H (mt) mcd w N JiJo H (nat) S/Down! [ ]! [! H (mt) mcd S/Down t [ i i i l H (mt) mcd S/Down mcd W - N J/ J/ E/Down [ i i i i!! H (mt) 63.2g mcd ;o ' o ' o ' ' tt (nat) F/Down mcd E/Down,,,! i i i 1o 30 SO H (mt) med Figure 3. Orthogonal projection of alternating field demagnetization data for samples from Hole 984B. Open and solid symbols indicate projection on the vertical and horizontal planes, respectively. The NRM intensity prior to demagnetization (J0) and the position of the sample in meters composite depth (mcd) are indicated. mm-diameter Bartington loop sensor. Anhysteretic remanent magnetization (ARM) was acquired in a 100-mT AF with a mt bias field, then demagnetized at peak fields of 35 and 45 mt. The same demagnetization scheme was used on IRM which was imparted in a dc field of 500 mt. 2. Magnetic Properties at Site 984 Orthogonal projections of AF demagnetization data indicate the presence of well defined magnetization components isolated at peak fields above 20 mt (Figure 3). About 10% of the NRM remains after demagnetization at peak fields of 70 mt. Thermal demagnetization of a three-axis IRM, applied orthogonally and sequentially in dc fields of 1, 0.3, and 0.1 T [see Lowrie, 1990], indicates the dominance of low coercivity remanence carriers with maximum blocking temperatures below 600øC (Figure 4a). The 0.1-T IRM has at least an order of magnitude greater intensity than the 0.3-T and 1-T IRMs (Figure 4a). Blocking temperatures below 600øC are consistent with dominance of magnetite. This is corroborated by IRM acquisition curves (Figure 4b) which indicate that the IRM is carried by low-coercivity minerals. The anhysteretic susceptibility KAR M against susceptibility r plot (Figure 4c) implies that the magnetite remanence carrier has restricted grain size in the -5 gm size range [King et al., 1983]. Hysteresis ratios plot in the pseudo-single domain (PSD) field (Figure 4d) of Day et al. [ 1977]. Sites 983 and 984 are separated by about 106 km (Figure 1) and penetrated different sediment drifts. When the records from the two sites are placed on their respective 5i80 age models [Channell et al., 1997, 1998], the u-channel susceptibility, ARM, and IRM records can be matched in some intervals (Figure 5). For Sites 983 and 984, volume susceptibility, ARM and IRM vary by a factor of 10 or less (Figure 5), within the range generally considered to be suitable for paleointensity determinations [see King et al., 1983; Tauxe, 1993]. 3. Directional Secular Variation Orthogonal projections of AF demagnetization data from Site 984 indicate that the characteristic magnetization component can

4 ß ß ß ß ß,, 22,940 CHANNELL: GEOMAGNETIC PALEOINTENSITY AND SECULAR VARIATION b) -: ' Temperature (øc) o0 Applied Field (mt) c) ß.". $,,...: o. ß l.,,,,i,,,[l[], l,,;i,,,,i z 5 K (* 10 '3) SI Hcr/Hc Figure 4. Site 984. (a) Intensity of remanence during progressive stepwise thermal demagnetization of a composite IRM applied orthogonally and sequentially in dc fields of 1, 0.3, and 0.1 T, (b) acquisition of IRM, (c) anhysteretic susceptibility KAR M plotted against volume susceptibility c, and (d) hysteresis ratio plot. SD, single domain; PSD, pseudo-single domain; MD, multidomain [see Day et al., 1977]. be isolated at peak fields of mt (Figure 3). For both Sites 984 and 983 the characteristic magnetization component was the and ka intervals) can also be attibuted to chronological discrepancies. At Site 983 the Iceland Basin event computed at 1-cm intervals applying the standard least squares has an onset age of 189 ka and a duration of about 3 kyr (Figure technique [Kirschrink, 1980] to the mt demagnetization 7). The directional changes can be replicated between holes at interval. Maximum angular deviation (MAD) values for the remanence components from both Sites 983 and 984 are largely bethe site and coincide with a low in the NRM/IRM paleointensity proxy (see below). Virtual geomagnetic pole (VGP) paths for low 5 ø (Figure 6), indicating that the magnetization components Sites 983 and 984 illustrate that the Iceland Basin event yields are well defined. The mean of virtual geomagnetic poles (VGPs) for the two sites are within 1 o of the geographic pole, and the VGP dispersions are 14.5 ø and 16.8 ø for Sites 983 and 984, respectively. VGP dispersions were determined after applying reverse polarity VGPs (Figure 8). Differences among the VGP paths may be attributed to (unrecognized) core deformation, stochastic pulses in sedimentation, or artifacts produced by the sampling or measurement procedure. VGP cutoffs of 31.5 ø and 35.2 ø for Sites 983 and 984, respectively, using the method of Vandamme [ 1994] to determine cutoff angles. The VGP dispersion values imply that the directional 4. Paleointensity Determinations secular variation is less damped than in most marine sediment drifts [e.g., Lund and Keigwin, 1994], although the values of VGP dispersion are several degrees less than predicted by models of secular variation [e.g., McFadden et al., 1988]. Component inclinations are negative at Sites 983 and 984 in the ka interval (Figure 6). Channell et al. [1997] named this excursion the Iceland Basin event at Site 983. The apparent discrepancy in age of this event at Sites 983 and 984 (Figure 6) is attributed to uncertainty in the isotopic age control, particularly at Site 984 which has a less well defined isotopic record. Certain A number of studies have illustrated that sedimentary paleointensity records can be correlated over large distances [e.g. Meynadier et al., 1992; Tric et al., 1992; Tauxe and Shackleton, 1994; Stoner et al., 1995, 1998; Yamazaki and Ioka, 1994; Guyodo and Valet, 1996; Lehman et al., 1996; Schneider and Mello, 1996; Roberts et al., 1997; Channell et al., 1997; Schwartz et al., 1998], implying that the longer period content of the records is controlled by the global-scale geomagnetic field. The paleointensity proxy of choice is generally either NRM/IRM or mismatches in the two declination records in Figure 6 (such as in NRM/ARM. Both remanences in the ratios are measured after

5 CHANNELL: GEOMAGNETIC PALEOINTENSITY AND SECULAR VARIATION 22,941 a) b) 'site o > ß : o.2 c) 30 - Site 984 I ',ll, I I : o o Age (ka) Figure 5. Comparison of Site 984 and Site 983 magnetic data placed on their individual age models. (a) The u- channel susceptibility, (b) ARM after demagnetization at peak fields of 35 mt, (c) IRM after demagnetization at peak fields of 35 mt. demagnetization at a particular peak AF in order to eliminate viscous or other low-coercivity contributions to remanence. Ideally, the normalizer (IRM or ARM) activates the same grains that carry the NRM, in which case the normalizer compensates for changes in concentration of remanence carrying grains. Volume (low field) susceptibility : is occasionally used as the normalizer; however, large multidomain magnetite grains (and paramagnetic and superparamagnetic grains), which could be important contributors to susceptibility, would not be important contributors to the remanence. Susceptibility is therefore usually not the preferred normalizer. For the composite section at Site 984 the values of NRM/IRM and NRM/ARM were computed after demagnetization at 35 and 45 mt. These peak demagnetizing fields are sufficient to eliminate the steep downward magnetic overprint attributed to the drilling process (Figure 3). The variability of NRM/ARM after demagnetization at peak fields of 35 and 45 mt is similar; however, the value of the ratio is greater after demagnetization at 45 mt, which indicates that the NRM has higher coercivity (in this demagnetization interval) than the ARM. The value of NRM/IRM is also similar after demagnetization at 35 and 45 mt, but in this case, the ratio decreases with increasing demagnetizing field, which indicates that the coercivity of IRM is greater than that of NRM. In Figure 9 the individual records of NRM/ARM and NRM/IRM, their arithmetic means, as well as the slope of NRM versus ARM and NRM versus IRM in the mt inter- val, are shown. In this demagnetization interval the coercivity match between NRM and IRM is closer than for NRM and ARM (Figures 9a and 9b). NRM/IRM is therefore the preferred paleointensity proxy in this case. Comparison of the mean NRM/IRM for Sites 984 and 983, placed on their independentimescales, indicates offsets that are attributable to discrepancies between the age models (Figure 10a). The Site record has higheresolution than that from Site 984 (Figure 10b); hence the Site 984 timescale has been "tuned" by matching the Site 984 NRM/IRM record to that from Site 983 (Figure 10a). The Site 984 planktic record, when placed on the adjusted timescale, shows no discrepancies with the Site curve (Figure 10b), therefore the age adjustments required by the NRM/IRM correlation do not violate the constraints. 5. Discussion 5.1. Secular Variation The normalized remanence (paleointensity) records from Sites 983 and 984 for the last 500 kyr can be matched within the constraints of the data (Figure 10) and can be correlated to other paleointensity records of comparable resolution (Figure 11). Directional secular variation records, on the other hand, are not easily correlated between Sites 983 and 984, a distance of-106

6 22,942 CHANNELL: GEOMAGNETIC PALEOINTENSITY AND SECULAR VARIATION MAD (o) Declination (ø) Inclination (ø) Site 984 o 5 lo Site Site Iceland 200 Event , 500 ' Site Site 983 Site 983 Figure 6. Component declinations and inclinations for Site 983 and Site 984, and corresponding maximum angular deviation (MAD) values, computed at 1-cm intervals downcore for the mt demagnetization interval using the standard three-dimensional least squares method.

7 ,,,, CHANNELL: GEOMAGNETIC PALEOINTENSITY AND SECULAR VARIATION 22,943 Declination (o) Inclination (o) Mean NRM/IRM i!6 ka 20! ka - ' i i : 189 ka -,, 22 ' ' k i II Iii I 1111 Figure 7. Site 983 component declination, component inclination, and NRM/IRM paleointensity proxy in the vicinity of the Iceland Basin event. Open symbols, working halves of core sections; solid symbols, archive halves of core sections; circles, Hole 983C; squares, Hole 983B. Ages are based on oxygen isotope age model. km (57 nautical miles) (Figure 6). When the Site 984 age model is adjusted for optimal fit of the paleointensity records (Figure tion, or artifacts produce by the sampling/measurement procedure. 10), intervals of high amplitude directional secular variation are in somewhat better agreement. The poor correlation between the 5.2. Magnetic Excursions directional secular variation records at Sites 983 and 984 (but the clear correlation of the paleointensity records) is attributed to the difference in characteristic variability of the direction and length (intensity) of the magnetization vector. The global-scale (axial At Sites 983 and 984, lows in paleointensity and higher dispersion in declination are apparent at ka (Laschamp event time) and at ka (Blake event time) (Figures 6 and 10), but dipole) geomagnetic field may dictate the length (intensity) of the the inclination is not anomalous in these intervals. The direcmagnetization vector, whereas its direction is perturbed by the lo- tional secular variation record from Sites 983 and 984 features a cal/regional (nonaxial dipole) components of the geomagnetic single interval where remanence inclinations are negative, in confield. Reproducible directional secular variation records have generally been derived from lake sediments which have order of trast to the numerous geomagnetic excursions recorded farther north in the Norwegian-Greenland Sea and to the south in the magnitude greater sedimentation rates than is characteristic for Blake-Bahama-Bermuda area. Records of the late Brunhes Chron marine sediment drifts. The higher-frequency variability of the directional record (relative to the paleointensity record) is apparently not captured at Site 983/984 mean sedimentation rates (12-15 cm/kyr) due to bioturbation, stochastic pulses in sedimentafrom the Norwegian-Greenland Sea indicate several intervals of negative inclination [Bleil and Gard, 1989], which have also been observed in piston cores from farther north in the Fram Strait [Nowaczyk and Baumann, 1992] and on the Yermak

8 , 22,944 CHANNELL: GEOMAGNETIC PALEOINTENSITY AND SECULAR VARIATION Iceland Basin Event ( ka) Hole 983B (working) Hole 983C (archive) Section break Hole 984A (working) Hole 984A (archive) Hole 984C (working) Hole 984C (archive) Figure 8. Virtual geomagnetic poles (VGPs) in the vicinity of the Iceland Basin event computed from component directions from working and archive sections from drill holes at Sites 983 and 984. Plateau [Nowaczyk et al., 1994]. Lack of well-defined 5180 records and the age control from these early studies are suffirecords from these cores led to uncertainty in the age of the inter- ciently equivocal that the old names should be abandoned in favals of negative inclinations. However, in cores from the vor of names that link the events to a region or specific locality Greenland Basin, north of Jan Mayen Island, Nowaczyk and where the event has been clearly documented. Antonow [ 1997] have documented magnetostratigraphic and 5 80 At Ocean Drilling Program (ODP) Leg 172 sites, from the records which indicate the presence of negative inclinations with Blake-Bahama Outer Ridge and the Bermuda Rise, initial studies ages corresponding to the Mono Lake excursion ( ka), the indicate numerous magnetic field excursions that are character- Laschamp excursion (-40 ka), and the Iceland Basin event (-188 ized by low or negative inclinations within the Brunhes Chron ka). Nowaczyk and Antonow [ 1997] refer to the 188 ka event as [see Lund et al., 1998]. Site 1062 from the Bahama Outer Ridge the Biwa I/Jamaica event after Wollin et al. [1971], Ryan and records the largest number (14) of excursions. The Matuyama- Flood [ 1972], and Kawai et al. [ 1972]. The magnetostratigraphic Brunhes boundary lies in the m below seafloor (rnbsf)

9 CHANNELL: GEOMAGNETIC PALEOINTENSITY AND SECULAR VARIATION 22,945 a) b) o 0.01: C) Z d) 0.01 Z 0 looo i Meters comvosite devth (mcd) o ' Figure 9. Site 984 paleointensity proxies. (a) NRM/ARM after demagnetization at peak fields of 35 and 45 mt. (b) NRM/IRM after demagnetization at peak fields of 35 and 45 mt. (c) Arithmetic mean of the two NRM/ARM estimates, after demagnetization at peak fields of 35 and 45 mt, and slope of the NRM versus ARM plot. (d) Arithmetic mean of the two NRM/IRM estimates, after demagnetization at peak fields of 35 and 45 mt, and slope of the NRM versus IRM plot. range in the seven holes drilled at Site 1062, indicating comparable Brunhes Chron mean sedimentation rates to those at Sites 983 and 984. It is therefore difficult to argue that the recording of magnetic excursions at Site 1062, but not at Sites 983 and 984, is due to the higher mean sedimentation rate at Site The Iceland Basin event appears to be distinct in age from the Pringle Falls event, which has an age close to 220 ka [Herrero- Bervera et al., 1994], and is possibly coeval with a paleointensity low at 218 ka in the Site 983 record (Figure 7). The Iceland Basin event may be coeval with a magnetic excursion that has been recognized elsewhere in the central North Atlantic Ocean [Weeks et al., 1995; Lehman et al., 1996] as well as in the western equatorial Pacific [Yamazaki and Ioka, 1994] and North Pacific [Roberts et al., 1997]. This particular excursion appears therefore to have been documented over a large portion of the globe. Other excursionsuch as the majority of those recorded on the Bahama Outer Ridge [Lund et al., 1998] appear to be locally manifest. As age control for these excursions becomes available, it will be possible to determine whether these excursions are coeval with paleointensity lows recorded at Sites 983 and 984. Synthetic field models have shown that reduction of the axial dipole field to half its present-day value can give very anomalous

10 22,946 CHANNELL: GEOMAGNETIC PALEOINTENSITY AND SECULAR VARIATION a) i,, i i,,, 'Sit 98'4 (c n it o n a'ge rhod l),, q Site 984 (optimally correlated to the Site 983 record[) = Site 983 (on its own age model) = = i i, 0.01:- t o.oo ' 0 b) -- i i i i I i i i i i i i i i i! i i i Age (ka) Figure 10. (a) Site 984 mean NRM/IRM plotted on its own age model, with age adjusted for optimal correlation to the Site 983 mean NRM/IRM record and Site 983 mean NRM/IRM plotted on its own age model. (b) Planktic and benthic 5180 data for Site 983 (solid symbols) and Site 984 (open symbols) [see Channell et al., 1998].

11 CHANNELL: GEOMAGNETIC PALEOINTENSITY AND SECULAR VARIATION 22,947 P-013 (Lab. Sea) Ste 8, I,,,,,,, Si t-20y I I Age (ka) Figure 11. Site 984 paleointensity proxy (mean NRM/IRM) adjusted for optimal fit to Site 983 (see Figure 10) compared with high-resolution (high sedimentation rate) relative paleointensity records from the Sulu Sea [Schneider and Mello, 1996], Mediterranean/Somali Basin [Meynadier et al., 1992; Tric et al., 1992], Labrador Sea [Stoner et al., 1995, 1998] and Sint-200 [Guyodo and Valet, 1996], a global composite based on 18 paleointensity records. excursional directions covering --1/5 of the Earth's surface, with time intervals [Channell et al., 1998]. The key question is apparently normal directions elsewhere [see Quidelleur et al., whether the orbital power in paleointensity data represents the 1999]. influence of climate on lithology, and hence on the paleointensity proxy, or whether it can be attributed to the geomagnetic field Power Spectra For Sites 983 and 984, IRM (the normalizer for the paleointensity proxy) and other magneti concentration parameters (ARM and The visual correlation of paleointensity records to the susceptibility) show significant 100-kyr power (Figures 13a and records indicates that there is a tendency for paleointensity lows 13b) which can be attributed to climatic influences on lithology. to correlate to peak interglacials (low values of 15180) (Figure 10). IRM and the other concentration parameters do not, however, For Sites 983 and 984, Ortiz et al. [ 1999] have used the split-core show 41-kyr power. Therefore the 41-kyr power in NRM/IRM is analysis track (SCAT) reflectance measurements to produce a not obviously attributable to climate/lithology. The magnetite proxy for percent carbonate and have ground truthed these data grain size parameter ARMhc, on the other hand, does show sigusing percent carbonate determined by coulometric titration. nificant power close to a period of 41 kyr, particularly at Site 984 Comparison of paleointensity data with the percent carbonate (Figure 13a). Schwartz et al. [1996] have noted a correlation proxy indicates that the visual correlation between the curve between paleointensity data and ARMhc in cores from the Blake and paleointensity (Figure 10) does not translate into a correla- Outer Ridge. The squared coherency between NRM/IRM and tion between paleointensity and percent carbonate (Figure 12). ARM/ c is, however, not significant in the vicinity of 41 kyr King et al. [1999] have suggested that carbonate content may in- (Figure 13c) implying that the 41-kyr power in NRM/IRM cannot fluence paleointensity records. However, power spectra for be attributed to changes in ARMhc (magnetite grain size). NRM/IRM (paleointensity proxy) and percent carbonate computed over the last 500 kyr for Sites 983 and 984 are very differ Scaling of Paleointensity Data ent (Figures 13a and 13b). The orbital periodicities in NRM/IRM at Sites 983 and 984 were also found in an earlier analysis of Constable and Tauxe [1996] suggested a means of scaling Sites 983 and 984 based on different age models and different sedimentary relative paleointensity records using the assumption

12 22,948 CHANNELL: GEOMAGNETIC PALEOINTENSITY AND SECULAR VARIATION a) Age (ka) ' Site984 b) Site > c) g Age (ka) Figure 12. (a) Percent carbonate proxy for Sites 983 and 984 [Ortiz et al., 1999]. (b) Paleointensity records scaled to gt using the technique advocated by Constable and Tauxe [1996] to give virtual axial dipole moment (VADM). (c) ARM/ c (magnetite grain size proxy). that the axial dipole goes to zero at the time of reversal and that field intensity at that time is due to the non-axial-dipole field (NAD). They use an estimate of 7.5 gt for the strength of the NAD, then multiply the sedimentary paleointensity record by a constant factor which sets the average polarity transition paleointensity to 7.5 gt. Here I set the paleointensity during the Iceland Basin Event to this value and scale the record accordingly (Figure 12). The resulting mean intensities for Sites 983 and 984 are 43.6 and 46.1 gt, respectively. The expected mean dipole field intensity at the site latitudes is 54.2 gt so the scaling process appears to underestimate the field intensity, possibly because the Iceland Basin event paleointensity was not entirely produced by the NAD and/or did not sink as low as 7.5 gt. The mean virtual axial dipole moments (VADMs) for Sites 983 and 984 are 6.2 x1022 A m 2 and 6.6 x1022 A m 2, respectively, with values falling to about 1 x1022 A m 2 at paleointensity low such as the Iceland Basin event (Figure 12). Paleointensity estimates from volcanic rocks give a time averaged VADM that decreases from the present day value (8 x1022 A m 2) to -3 x1022 A m 2 at 40 ka [Merrill et al., 1996]. An obvious inconsistency is that the

13 Illl I 22, O IRM ---- X,,,/. o.xn = = ' ß...,... ".," kyr 41 kyr 23 kyr 19 kyr Frequency (kyr -1) b) 1...t¾,,, I... [... [... [... [... [ Site o 0.6 ß ',,i/ - NRM/IRM O c) p kyr 41 kyr 23 kyr 19 kyr Frequency (kyr -1) 0.6 Site % confidence Sit 100 kyr 41 kyr 23 kyr kyr Frequency (kyr -1) Figure 13. (a) Site 984 power spectra using AnalySerie software [Paillard et al., 1996] and the Blackman-Tukey method with a Bartlett window for NRM/IRM (paleointensity proxy), IRM35mT, percent carbonate and ARM/}c (magnetite grain size proxy). (b) Same as Figure 13a, but for Site 983. (c) Squared coherence between NRM/IRM and ARM/r, indicating insignificant coherence on the kyr -1 frequency interval.

14 22,950 CHANNELL: GEOMAGNETIC PALEOINTENSITY AND SECULAR VARIATION present-day VADM is not reproduced in the Site 983 and 984 records, possibly due to coring related disturbance of the top 2 rn (20 kyr) of the records. Acknowledgments. I am very grateful to C. Laj, C. Kissel, A. Mazaud and B. Lehman for logistical support and scientific advice during and since my sabbatical leave at Gif-sur-Yvette, and to A. Roberts and J.S. Stoner for providing thorough reviews of the manuscript. This research was partly supported by NSF (EAR ) and the U.S. Science Support Program. References Bleil, U., and G. Gard, Chronology and correlation of Quaternary magnetostratigraphy and nannofossil biostratigraphy in Norwegian- Greenland Sea sediments, Geol. Rundsch., 78, , Channell, J.E.T., and B. Lehman, Magnetic stratigraphy of Leg 162 North Atlantic Sites Proc. Ocean Drilling Program, Sci. Results, 162, in press, Channell, J.E.T., D.A. Hodell and B. 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15 CHANNELL: GEOMAGNETIC PALEOINTENSITY AND SECULAR VARIATION 22,951 Tauxe, L., J.L. LaBrecque, R. Dodson, and M. Fuller, U-channels - a Wold, C.N., Cenozoic sediment accumulation on drifts in the northern new technique for paleomagnetic analysis of hydraulic piston cores, North Atlantic, Paleoceanography, 9, , Eos Trans. AGU, 64, 219, Wollin, G., D.B. Ericson, W.B.F. Ryan, and J.H. Foster, Magnetism of Tric, E., J.P. Valet, P. Tucholka, M. Pateme, L. Labeyrie, F. Guichard, L. the Earth and climate changes, Earth Planet. Sci. Lett., 12, , Tauxe, and M. Fontugne, Paleointensity of the geomagnetic field for the last 80,000 years, J. Geophys. Res., 97, , Yamazaki, T., and N. Ioka, Long-term secular variation of the geomag- Vandamme, D., A new method to determine paleosecular variation, netic field during the last 200 kyr recorded in sediment cores from the Phys. Earth. Planet. Inter, 85, , western equatorial Pacific, Earth Planet. Sci. Lett., 128, , Weeks, R., C. Laj, L. Endignoux, M. Fuller, A. Roberts, R. Manganne, E. Blanchard, and W. Goree, Improvements in long-core measurement techniques: applications in palaeomagnetism and palaeoceanography, Geophys. J. Int., 114, , J.E.T. Channell, Department of Geological Sciences, University of Weeks, R., C. Laj, L. Endignoux, A. Mazaud, L. Labeyrie, A. Roberts, C. Florida, P.O. Box , Gainesville, FL Kissel, and E. Blanchard, Normalized NRM intensity during the last (jetc@nersp.nerdc.ufl.edu) 240,000 years in piston cores from central North Atlantic Ocean: Geomagnetic field intensity or environmental signal?, Phys. Earth (Received August 3, 1998; revised June 21, 1999; Planet. Inter., 87, , accepted June 29, 1999.)