The natural remanent magnetization (NRM) of archive-half sections from Holes 1241A, 1241B, and 1241C was measured then remeasured after alternating-field (AF) demagnetization at selected levels. Sections within Cores 202-1241A-1H through 5H were then AF demagnetized at peak alternating fields of 10, 15, and 20 mT and measured. All other cores from Holes 1241A, 1241B, and 1241C were demagnetized at 20 mT and then measured. A few sections from Cores 202-1241A-9H through 11H were also demagnetized at 30 or 40 mT to further evaluate the strength of the drilling overprint. Sections obviously affected by drilling disturbance were not measured.
The NRM intensity before and after demagnetization shows the same general downhole trend with ~1 to 1.5 orders of magnitude less intensity after 20-mT AF demagnetization than before (Fig. F25). NRM intensities after AF demagnetization are ~0.004 A/m at the top and decrease to <10-4 A/m by ~100 mcd (Fig. F25). NRM intensities remain low until ~275 mcd, but at greater depths they rise to values similar to those observed at the surface (~310 mcd), before decreasing again by a factor of five. Throughout the section, intensities vary by a factor of five on a meter scale with many ~10-cm order-of-magnitude spikes in intensity resulting from ash layers or turbidites (see "Lithostratigraphy").
The NRM intensities before and after AF demagnetization at 20 mT decrease by one and two orders of magnitude, respectively, over the uppermost 15 m (Fig. F26). The first order of magnitude loss in NRM (20 mT) occurs in the uppermost 5 m and may be associated with magnetic mineral dissolution resulting from reducing conditions. All three parameters display quasi-cyclic variations of ~2-m scale from 0 to 13 mcd, which is consistent with the dominant ~100-k.y. climate cycle of the late Pleistocene based on sedimentation rates of 21 m/m.y. (see "Age Model and Mass Accumulation Rates"). The NRM (20 mT) also displays an order of magnitude intensity decrease between 13 and 15 mcd. The NRM (0 mT) and magnetic susceptibility also decrease by a factor of five at this depth. This change may be partially related to changes in sediment flux, as the dramatic intensity cyclicity of the uppermost 15 m also disappears at this depth.
Inclinations before AF demagnetization are steeply positive, characteristic of a drill string-induced magnetic overprint (Fig. F27). For the uppermost 15 mcd, inclination values after 25-mT AF demagnetization are closer but still more positive than expected for the Site 1241 latitude (~12° for an axial geocentric dipole) (Fig. F27). Below 20 mcd, sediments are more strongly overprinted, with steep positive inclinations present throughout after 20-mT AF demagnetization and even after selective demagnetization at 30 or 40 mT.
Declinations within individual cores from the upper 20 mcd are generally quite similar in value. In Core 202-1241A-3H, declinations change significantly in direction, suggesting the presence of polarity transitions (Fig. F28). Such polarity transitions are seen in Cores 202-1241A-3H (~15, 20, and 22 mcd), 202-1241B-2H (~15 mcd), and 202-1241B-3H (~20 mcd). The interpretation of these declination changes of ~180° that occur within single cores as polarity transitions is unambiguous, despite the lack of the Tensor orientation tool corroboration (the tool is not used in the uppermost three cores for operational reasons). The ages of these interpreted transitions are consistent with biostratigraphic age estimates (Table T10; see "Biostratigraphy"). The Brunhes/Matuyama (0.78 Ma) boundary is present between 14.7 and 15.0 mcd, the upper Jaramillo transition (0.99 Ma) is present at 19.7-20.0 mcd, and the lower Jaramillo transition (1.07 Ma) is present at 21.5-21.9 mcd. No other polarity boundaries below this interval could be discerned.