The median magnetizations for the three sites range from 0.01 to 0.07 A/m after 20-mT demagnetization, which is several orders of magnitude greater than the noise level of the magnetometers used in this study. Low-temperature rock magnetic analyses indicate that the main remanence-carrying mineral has a Verwey transition of 103-120 K, consistent with the presence of magnetite, slightly oxidized magnetite, or magnetite with some degree of cation (Ti, Al, or Mn) substitution (Brachfeld et al., Chap. 14, this volume; Guyodo et al., 2001). Thermal demagnetization experiments reveal a high unblocking temperature component that unblocks from 500° to 600°C, although they also show that more than one-half the magnetization unblocks between 280° and 420°C (e.g., fig. F26 from Shipboard Scientific Party, 1999a). Both the medium and high unblocking temperature components record the same paleomagnetic direction. Hysteresis analyses indicate the sediments have coercivities of remanence generally between 20 and 40 mT, except for the upper 10-20 m of all three sites, where values are generally 30-90 mT (Brachfeld et al., Chap. 14, this volume; Guyodo et al., 2001, unpubl. data). Furthermore, the samples from all three sites plot dominantly in the pseudo-single-domain region of Day plots (Day et al., 1977), with fewer samples falling in the multidomain region.
Together, these above observations suggest that most of the paleomagnetic signal is carried by magnetite and titanomagnetite, although we cannot preclude the presence of other carriers like pyrrhotite, greigite, maghemite, and hematite, particularly in the upper 10-20 m of sediment at all three sites. Most importantly, the magnetic minerals with medium to high coercivities and medium to high unblocking temperatures all appear to record a characteristic magnetization consistent with a depositional remanent magnetization (DRM) or postdepositional remanent magnetization (pDRM) that is acquired shortly after deposition.
In some relatively narrow intervals, the susceptibility drops by roughly an order of magnitude below that typical of the surrounding sediment (Fig. F6). These intervals are commonly <0.5 m thick but can be several meters thick. They are present at all three sites and can be correlated between holes at a site (Barker, Chap. 6, this volume). The susceptibility lows are not associated with any obvious changes in lithology. As can also be seen in Figure F6, intervals of low intensity coincide with the susceptibility lows (Shipboard Scientific Party, 1999b). These lows are obviously associated with changes in the composition or concentration of the magnetic minerals, probably both. We do not know if they are primary (deposition) or secondary (alteration) features. Most importantly, the lows do not appear to affect the polarity for most intervals (Fig. F6). In some low-susceptibility and low-intensity intervals, however, the paleomagnetic inclination is shallower than would be expected for a high-latitude site. This may occur if the paleomagnetic signal decreases to near the noise level of the magnetometer (~10-4 A/m) because the x-axis sensor is slightly less sensitive than that of the z-axis. Such a scenario may explain some of the difficulty in interpreting the magnetostratigraphy from 60 to 200 mcd at Site 1096, which contains an abundance of susceptibility lows. More commonly, there is little or no notable variation in the paleomagnetic direction across the susceptibility lows and therefore these lows do not adversely affect the magnetostratigraphic interpretation.