Shipboard paleomagnetic measurements for Holes 1144A, 1144B, and 1144C consisted of long-core measurements of the natural remanent magnetization (NRM) at intervals of 4-8 cm before and after alternating field (AF) demagnetization, usually up to 20 mT (see "Paleomagnetism" in the "Explanatory Notes" chapter). Measurements were carried out on the archive halves of all APC and XCB cores for Hole 1144A. In addition, discrete samples were collected from the working halves of Hole 1144A (APC and XCB cores) at a spacing of one sample per section (1.5 m). Some of these samples were subjected to progressive AF demagnetization up to 70 mT. Only the archive halves of APC cores from Holes 1144B and 1144C were measured. Cores 184-1144A-3H through 25H, 184-1144B-3H through 22H, and 184-1144C-3H through 21H were oriented using the Tensor tool.
The nonmagnetic cutting shoe was used with a standard core barrel on every second core, starting with Core 184-1144A-5H. Long-core measurements were carried out at 4-cm intervals with AF demagnetization steps at 10 and 20 mT for Cores 184-1144A-1H through 5H, then at 8-cm intervals with one step of demagnetization at 20 mT down to Core 184-1144A-47X.
The intensity of magnetization after demagnetization at 20 mT is on the order of 10-3 A/m down to ~430 mcd. A progressive increase of about two orders of magnitude is present below this depth. This trend, as well as other short-term characteristics, correlates with changes in magnetic susceptibility measured by the multisensor track (Fig. F12; see "Physical Properties"). This change in the concentration of magnetic minerals is evidence that major changes occurred in the source of this material and/or in the mechanisms of transport to the site.
The direction of the NRM (after demagnetization at 20 mT and correction using the Tensor tool data) is shown in Figure F13. Above 260 mcd (APC cores) the declination oscillates around 0°, and the inclination is about the value expected for a geocentered dipole field at this latitude (38°). The amplitude of the fluctuations in declination and inclination is consistent with the secular variation of the geomagnetic field. In Core 184-1144A-3H at 24 mcd, we observed a clear swing of the declination to reverse direction and then back to normal, with very high correlative values of inclination (Fig. F14). Although no negative inclinations were observed, we tentatively suggest that this represents a (partial) record of the Laschamp Event. High inclinations indicate that excursional virtual geomagnetic poles are close to the site, which is not inconsistent with the hypothesis of transitional preferred longitudinal bands during polarity reversals. We did not observe any other directional change—in particular, a change that could be associated with the Blake Event (~110 ka) was not found. No significant difference in the direction of magnetization was noted between the nonmagnetic cutting shoe used on odd cores and the standard cutting shoe used on even cores.
Below 260 mcd (XCB cores), the declinations reveal a tendency to lie between 0° and 90° with a large scatter. Inclinations are also scattered and are significantly steeper than expected at the site latitude. This high value reflects a large overprint in all XCB cores directed along the z-axis. This overprint was not removed after demagnetization at 20 mT; therefore, the direction of the primary magnetization could not be retrieved from long-core analyses.
We have attempted to obtain at least the magnetic polarity from discrete samples from the bottom of Hole 1144A (Cores 184-1144A-45X through 48X). Alternating field demagnetization of these samples to 70 mT, however, yielded somewhat ambiguous results: a few samples changed to negative polarity upon demagnetization, whereas others did not show evidence for reverse directions (Fig. F15A, F15B). Often the direction became erratic after the 45-mT step, possibly indicating some acquisition of anhysteretic magnetization in the demagnetizing coils. Therefore, the presence of reverse directions (suggested by some reverse discrete samples) needs additional postcruise confirmation.
Only long-core measurements were made on cores from Holes 1144B and 1144C, with readings spaced by 8 cm and one demagnetization step at 20 mT. Only normal polarities were observed.
Normalized magnetization is obtained by dividing the measured magnetization by a suitable normalizer to compensate for changes in the concentration of remanence-carrying grains (usually anhysteretic remanent magnetization, isothermal remanent magnetization, or, less frequently, magnetic susceptibility). Under restrictive conditions of uniformity of magnetic mineralogy (magnetite with uniform granulometry in the pseudosingle domain state), normalized magnetization may represent a proxy for changes of the geomagnetic field intensity. Although all these restricting hypotheses remain to be proven true for Site 1144 by shore-based studies, we have tentatively divided the intensity after demagnetization at 20 mT by magnetic susceptibility. The results, shown in Figure F16 for the three holes, are still contaminated by high-frequency peaks. Nevertheless, some similarities are observed between the smoothed records from the three holes, particularly between Holes 1144A and 1144B. Identification of the lows in the Hole 1144A record with known lows in the geomagnetic field intensity (Lehman et al., 1996; Guyodo and Valet, 1996) leads to the age assignment indicated in Figure F16. This speculation, which agrees with the biostratigraphic results (see "Biostratigraphy"), suggests that records of normalized field intensity might be obtained from Site 1144, provided the magnetic mineralogy is proven to be uniform. This will be tested in a postcruise study.