Paleomagnetic analyses conducted during Leg 178 are described in the Leg 178 Initial Reports volume (Barker, Camerlenghi, Acton, et al., 1999). The shipboard paleomagnetic data from discrete and split-core sections are presented in tables in that volume or can be obtained via the World Wide Web from the Ocean Drilling Program (ODP). U-channel samples, which are strips of sediment, each 2 cm x 2 cm in cross section and up to 1.5 m long (Tauxe et al., 1983, Nagy and Valet, 1993; Weeks et al., 1993), were collected and measured postcruise. U-channel samples from Cores 178-1101A-1H through 10H were measured at the Institut de Physique du Globe de Paris, and all other U-channel samples were measured at the University of Florida. The U-channel magnetometers in both laboratories are Model 755R systems from 2-G Enterprises.
U-channel data have several advantages over split-core data. First, U-channel samples are collected from the center of split-core sections, which is the region least affected by coring disturbance and magnetic overprints related to coring with the advanced hydraulic piston corer (APC). In contrast, split-core sections from APC cores may contain a significant fraction of sediment by volume that is affected by core deformation. The deformation is most severe near the periphery of the core where the sediment is bent downward owing to friction as the piston corer cuts through the sediment and as the sediment slides into the core liner. This deformation can result in significant biases in paleomagnetic directions amounting to several degrees (Acton et al., 2002), though such deflections are insignificant for determination of polarity for the high-latitude sites cored during Leg 178. Second, relative to the shipboard magnetometer, U-channel magnetometers have smaller diameter sensor coils that are nearer the sample as the sample passes through the sensor region, which results in this magnetometer having greater sensitivity and higher spatial resolution (<7 cm) than the shipboard magnetometer (<10 cm). The sensitivity is further improved for the U-channel magnetometers used in this study because they reside in magnetically shielded rooms, whereas the shipboard magnetometer is on a metal ship that is constantly moving in the geomagnetic field. Because the resolution for the U-channel magnetometers is higher and because more time is available for measurement postcruise, we measured the U-channel samples at 1- to 2-cm intervals compared with 4- to 5-cm intervals for the split-core sections. Third, unlike the split-core sections, which were generally measured after alternating-field (AF) demagnetization of 0, 10, 20, and 25 or 30 mT, U-channel samples were subjected to progressive demagnetization up to 70-80 mT in steps of 5 or 10 mT. This allows principal component analysis (PCA) (Kirschvink, 1980) to be used to estimate the remanent magnetization direction from multiple demagnetization steps at peak AFs above the 10- to 25-mT fields needed to remove drilling overprints. The higher-quality U-channel data, along with the mcd scale, allow us to resolve conflicting results between holes at a site, as well as to identify intervals that are strongly affected by drilling overprints.
U-channel data from Holes 1095A, 1095B, 1095D, 1096A, 1096B, and 1101A are available in Tables T4, T5, T6, T7, T8, T9, respectively. Given the high latitude of the sites, the inclination is sufficient to assess the polarity of an interval. Declination and intensity information is, however, included in the paleomagnetism data tables of Barker, Camerlenghi, Acton, et al. (1999) and for the U-channel data from this study in Tables T4, T5, T6, T7, T8, T9.
Large fluctuations in the intensity over intervals comparable to or narrower than the sensor region for the long-core magnetometers can result in measurement artifacts (e.g., Nagy and Valet, 1993; Roberts et al., 1996). Although it is not practical to present the intensity data on a scale to compare with all directional changes, we have used the intensity data in our interpretation to avoid interpreting artifacts as reversals. Because we have archived the data, other investigators can also further evaluate the details of the paleomagnetic signal in intervals of interest.
For the tables giving the paleomagnetic results from U-channel samples (this study) and split-core samples (Shipboard Scientific Party, 1999a, 1999b, 1999c), data from intervals highly disturbed by drilling where such deformation was visually obvious were removed. In addition, data obtained within 5 cm of the ends of the U-channel or split-core samples were removed because these are biased by magnetic edge effects. These potentially erroneous data are not used in this study. U-channel samples from Cores 178-1095A-1H through 3H and from Cores 178-1096B-1H through 7H were taken from archive-half core sections. During Leg 178, these archive-half core sections were subjected to peak demagnetizing fields of 20 mT. In some cases, we remeasured these U-channel samples prior to beginning progressive demagnetization. In the tables, we retain these data as the 0-mT step because they contain information about the acquisition of viscous magnetization components (Fig. F2), although the user should be aware that any sample taken from the archive halves has been previously subjected to some level of demagnetization. All other U-channel samples are from working-half core sections, which were not subjected to any demagnetization prior to sampling.
As illustrated with a number of orthogonal vector demagnetization plots from AF and thermal demagnetization of discrete samples, which are shown in the "Paleomagnetism" sections of the "Site 1095," "Site 1096," and "Site 1101" chapters in the Leg 178 Initial Reports volume (Shipboard Scientific Party, 1999a, 1999b, 1999c) of Barker, Camerlenghi, Acton, et al. (1999), the sediments from the continental rise sites generally give stable univectorial directions after removal of a drilling overprint with 5- to 25-mT demagnetization. Similar behavior was seen on AF demagnetization of U-channel samples, as shown in Figures F2, F3, and F4.
The PCA results for each interval along the U-channel samples are available for each hole in Tables T10, T11, T12, T13, T14, and T15. For the PCA direction, we find the best-fit line that passes through the vector demagnetization data without the constraint that the line pass through the origin of vector demagnetization plots. To avoid contamination by drilling overprints, we do not use demagnetization steps <20 mT in the PCA. We also use an iterative search program to find and delete data from demagnetization steps that are outliers, where an outlier is defined as a datum that degrades the fit of the line relative to all other demagnetization data used. We require that data from at least three steps are used and on average use directions from five or more steps to find the best estimate of the PCA direction.
The PCA results give precise directions and a clear indication of polarity in all but a few intervals, as discussed below for each site. A measure of how well the observations fit a line is provided by the maximum angular deviation (MAD), which is generated as part of PCA (Kirschvink, 1980). MAD values <10° are typically considered to provide lines that fit the observations well. Of the 13,439 intervals measured, 13,359 intervals (99.4%) have MAD values <10°, with 12,565 intervals (93.5%) having MAD values <3°.
For comparison, we also compute a Fisherian mean (Fisher, 1953) of the highest three or four demagnetization steps for each interval from a U-channel sample. This is referred to as the stable end point direction. Typically only data from the highest three demagnetization steps are used in the average, unless the mean of these three directions has a dispersion (precision) parameter <200, in which case the data from the fourth highest demagnetization step is included. When the dispersion parameter is <200, we also use an iterative search to find and remove the direction that is the largest outlier. Comparison of the stable end joint with the PCA direction can be useful for indicating where unremoved or partially unremoved magnetization components exist or where progressive demagnetization has been ineffective in revealing linear demagnetization paths. For the Leg 178 sediments, the stable end point and PCA directions are virtually identical (Fig. F5), which indicates that a single magnetization component exists after removal of the drilling overprint and that this component can be isolated and accurately estimated using PCA or stable end points. The paleomagnetic direction obtained from a single demagnetization step at 20 or 30 mT from split-core sections similarly agrees well with the PCA direction or stable end point direction obtained from detailed progressive AF demagnetization of U-channel samples for most intervals (Fig. F5), with the few exceptions discussed below.
We interpret the component that is equally well resolved by PCA or by single-step demagnetization at 20 mT or higher as the characteristic remanent magnetization acquired by the sediment during deposition or shortly thereafter. The inclinations are therefore representative of the paleomagnetic field inclination at the time of deposition. Owing to the steepness of the paleomagnetic field vector at these high-latitude sites, the inclination gives a clear indication of polarity, with negative inclinations during normal polarity intervals and positive inclinations during reversed polarity intervals.