RESULTS

Natural Remanent Magnetization

The AF demagnetization diagrams (Fig. 2) show in most cases a characteristic remanent magnetization (ChRM) and a steep downward coring-induced overprint for the u-channel measurements. This overprint appears to be removed with alternating fields between 15 and 20 mT. The CIM was not observed in the discrete samples because they had been already demagnetized at 20 mT during Leg 167. Figure 2 shows typical Zijderveld diagrams of samples from Hole 1020C before, during, and after the upper Jaramillo transition. The discrete sample and the u-channel measurements are from the same depth interval before (Fig. 2A), during (Fig. 2B), and after (Fig. 2C) the transition. The ChRMs were obtained by fitting a line through at least four points of the demagnetization diagrams. The inclination values of discrete samples and u-channels are in good agreement. The differences in declination of ~180º occur because u-channel samples were taken from the archive halves and the discrete samples from working halves. The ChRM can be determined quite reliably for samples outside the transition interval, and the principal component analysis is based on at least four demagnetization steps. A minimum of three points were used to fit a line to the AF-demagnetization data for computation of the transitional directions. The directions during a reversal are not as stable as during constant polarity intervals. The maximum angle of deviation for a line fit to the directional data of single samples during the transition is larger than before or after the transition (Fig. 2).

The inclination, corrected declination of the ChRMs, and the normalized NRM for the upper Jaramillo transition at Site 1020 are plotted vs. depth in Figure 3. For the normalized NRM intensities, the demagnetization level of 20 mT is chosen for ARM and NRM. The NRM intensities of single samples from Site 1020 that were measured in the Munich laboratory were 7%-22% higher than the shipboard intensities, which is most likely caused by differences in magnetometer calibration. The ARM values are normalized to the mean value of the investigated depth interval to make single-sample and u-channel results comparable. Normalized values are used because of the difference in amplitude of the alternating field for the acquisition of ARM between the discrete samples and u-channels. The normalized NRM intensities are probably indicators for paleointensity, as demonstrated in the rock magnetism section below. There is good agreement in inclination, declination, and relative paleointensity between discrete samples and the u-channel measurements (open triangles and solid dots, respectively, in Fig. 3). The dashed lines in the plot of inclination represent the values of ±60º expected for an axial geocentric dipole field at the geographic latitude of 41º for Site 1020. The inclinations of the reversed directions are a little shallower than the expected value. This could be the result of a coring-induced overprint that was not completely removed. The normalized relative paleointensity has a minimum at the upper Jaramillo transition. This minimum in relative paleointensity coincides with the jump in declination from 0° to 180°. From the geomagnetic polarity time scale (Cande and Kent, 1995), this depth can be assigned an age of 990 ka. If one employs a constant sedimentation rate of 8 cm/k.y., based on the duration of the Jaramillo Subchron, the length of the transition can be estimated. The directional change for the upper Jaramillo transition takes ~5 k.y. The labeled points in Figure 3 are the ChRM of the discrete samples and u-channel measurements displayed in Figure 2. The low density of data points in the transition region (Fig. 3) is caused by single samples or u-channel intervals that were not interpretable.

The resulting VGP paths of the upper Jaramillo reversal for the u-channel measurements and the single samples are illustrated in Figure 4. The VGPs from the u-channel measurements (solid dots) and the single samples (open triangles) are different. For the u-channels, the VGP path tracks across the Pacific. The only intermediate VGP from a single sample lies near the Gulf of Mexico. Two intermediate VGPs are marked with "1," and the Zijderveld diagrams of the discrete sample and the corresponding u-channel measurement are shown in Figure 2B. The inclinations of these transitional samples are rather steep, so the declination value is not well defined. After removing a large fraction of the CIM with 20-mT AF, the remaining NRM intensities of transitional samples are low (J < 0.2 mA/m). The determination of ChRMs in these intermediate samples is more difficult than for samples from stable polarity intervals.

Figure 5 shows the inclination and declination of the ChRMs and the normalized remanence record vs. depth for the Brunhes-Matuyama transition at Site 1020 (Gorda Ridge). In this record, a precursor to the reversal occurs in the depth interval from 77.35 to 76.90 mbsf. After a short period of negative inclination, the directional change to normal polarity takes place within 50 cm. With a sedimentation rate of 8 cm/k.y. (Lyle, Koizumi, Richter, et al., 1997), the duration of the directional change corresponds to ~6.2 k.y. An incompletely removed drilling overprint is also seen in this reversal record because the inverse inclinations are considerably shallower than the value expected for a dipole field (dashed lines in Fig. 5).

The VGP path for the B-M transition at Site 1020 is presented in Figure 6A (the results from discrete samples) and in Figure 6B (the u-channel measurements). There is not much similarity between the two VGP paths, but in both cases the VGP jumps several times from normal to reverse polarity and back. For the discrete samples and the u-channel measurements, the VGPs in the Southern Hemisphere do not reach the geographic South Pole. This lack of antipodality can arise from a normal drilling overprint, for example, which leads to shallow negative inclinations.

Magnetic Mineralogy

Several tests have been conducted to identify the main carrier of remanent magnetization at Site 1020. High-coercivity magnetic minerals like hematite are not dominant in this section of the sediments. This is concluded from the S-0.3T parameter (Bloemendal et al., 1992), which is higher than 0.95 for most measurements on u-channels and discrete samples (Fig. 7). In addition, all IRM acquisition curves reach saturation between 200 and 300 mT, which suggests that (titano) magnetite may be present and that high-coercivity minerals are mostly absent. The results of the thermomagnetic curves show no noticeable intensity drop up to 400ºC, and during the heating process, magnetite is formed around this temperature. It was not possible to demonstrate the presence of primary magnetite with the aid of the thermomagnetic curves because secondary magnetite was formed upon heating.

Downcore variations in magnetic concentration can be estimated from ARM and susceptibility . Figure 7 shows the variation of ARM and susceptibility vs. depth. The variations in ARM and are less than a factor 5.2 and 2.6, respectively. The concentration of magnetic minerals, therefore, varies much less than a factor of 10, which is one prerequisite for relative paleointensity determination (Tauxe, 1993).

The uniformity of grain size was determined from the hysteresis parameters Jrs/Js and Bcr/Bc. The plot of Jrs/Js vs. Bcr/Bc (Day et al., 1977) shows that the mean grain size clusters like in many other samples within the pseudo-single-domain range (Fig. 8). The grain sizes of the magnetic particles in the sediments from Hole 1020C (B-M transition and upper Jaramillo) are quite similar, and there are only small variations in magnetic mineralogy across these reversals.

Modeling the Effects of Overprint on the VGP Path

The ChRM from the upper Jaramillo and the B-M transition (Fig. 3, Fig. 5) have negative inclinations that are too shallow compared with the value expected at this location for a dipole field. Evidence for a radial overprint comes from the declinations of NRM after 20-mT AF demagnetization of 13 cores of Hole 1020C. Ideally, the declinations should be randomly distributed between 0º and 360º because of the randomly oriented cores. But there is a concentration of samples with high intensities and with declinations clustering around 180º, 270º, or 330º.

Another example of a reversal with a drilling overprint is given in Figure 9 with a record of the B-M transition at Hole 1014D from u-channel measurements. The negative inclinations are again shallow, and the positive inclinations are steep compared with the expected inclination value of 52º at this latitude (dashed lines in Fig. 9). The record from Site 1014 in the Tanner Basin is a good example for the influence of a coring-induced overprint in the positive z-direction. Drilling overprints in various directions were also observed in other ODP cores (Roberts et al., 1996). During the coring process with the APC, the sediments may get an isothermal remanent magnetization overprint in the vertical direction. Another possibility is a radial overprint, which can be acquired by the sediments during the drilling process (Herr et al., 1998; Fuller et al., 1998). The NRM intensity of the sediments from Site 1014 had decreased by about a factor of 30 between the shipboard measurements and the land-based intensity measurements seven months later (F. Heider, J.M. Bock, J.P. Kennett, I. Hendy, J. Matzka, and J. Schneider, unpubl. data). Reorientation of magnetic particles in the expanding sediment is a possible explanation because susceptibility of the sediments remained unchanged between shipboard and land-based measurements. The record at Site 1014 is further complicated by the presence of two magnetic minerals, which are locked in at different depths below the sediment/water interface.

The effect of a steep vertical overprint on the VGP path of the B-M transition at Site 1014 is illustrated in Figure 10. Because of the high positive inclinations, the northern VGPs do not reach the north rotation pole and remain close to the site. The southern VGPs exhibit the same behavior in the opposite direction because of flattening of negative inclinations.

It is difficult to exactly quantify the direction and intensity of the magnetization overprint in the x-, y-, or z-direction. Instead, we add or subtract a small component of magnetization to the measured NRM20mT intensity and investigate the change in the resulting direction of magnetization. For a modification of the VGP path during the upper Jaramillo transition from Site 1020, the u-channel measurements were taken at a demagnetization step of 20 mT because many transitional VGPs are available from that reversal. If sediments are contaminated by a coring-induced overprint, the size of this overprint depends on the concentration of magnetic particles in the sediment. In the following calculations, the remanent magnetizations that were added to each u-channel measurement were proportional to the intensity of ARM. It is assumed that the intensity of the drilling remanence is proportional to the ARM, which is a measure for the amount of magnetic particles. The intensities of NRM20mT, 1% of ARM and the ratio of 0.01·ARM/NRM20mT, are shown in Figure 11 for the upper Jaramillo transition. The ratio of 0.01·ARM/NRM20mT varies around 0.05-0.10 for samples before and after the transition and increases to 0.4 for transitional samples. These varying amounts of 0.01·ARM are subtracted or added to the NRM20mT intensities.

The VGP path is plotted in Figure 12A for the upper Jaramillo transition from u-channel measurements at a demagnetization step of 20 mT. There is a large similarity between the VGP path calculated from the ChRM directions of the u-channels (Fig. 4) and this NRM20mT record (Fig. 12A). A shift of the VGP path occurs when 1% of the ARM intensity is added to the x-component of the NRM after AF demagnetization at 20 mT (NRM20mT). The model calculations were conducted after correcting the declination to 0° for normal directions and to 180º for inverse directions, so that the x-axis points north. The positive z-direction points downward along the core. After adding 1% of ARM to the x-axis, the transitional VGPs move westward, and the VGPs in the Northern and Southern Hemispheres move to higher latitudes. The westward motion of the transitional VGPs becomes even larger when 2% of the ARM intensity is added to the x-component of NRM20mT (Fig. 12C). This example shows that an incompletely removed radial overprint could strongly affect the VGP path during the field reversal. In addition to a radial overprint, a steep vertical overprint is frequently observed. In Figure 13 we see a combination of 1% of ARM subtracted from the x-direction and 1% of ARM (Fig. 13A), 2% of ARM (Fig. 13B), and 3% of ARM (Fig. 13C) subtracted from the z-direction. The transitional VGPs now move eastward with increasing overprint compared with the original track (Fig. 12A), and the VGPs in the Southern Hemisphere tend to lie closer to the geographic South Pole. These effects become more pronounced as we increase the overprint from Figure 13A through 13C. These simulations of synthetic overprints demonstrate that a CIM that cannot be fully removed may affect the VGP path considerably.

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