All core archive halves from Hole 1202A and whole-round sections from Holes 1202B and 1202D (Cores 195-1202B-1H through 10H and 195-1202D-1H through 10H) were measured on the shipboard pass-through superconducting rock magnetometer. Cores from Hole 1202C and the XCB section of Hole 1202D were not measured during Leg 195 because of time constraints. The XCB cores from Hole 1202D were left on board after Leg 195 and measured during Leg 196. Results are included in this report. Natural remanent magnetization (NRM) and remanent magnetization after one 20-mT alternating-field (AF) demagnetization step were measured at 5-cm intervals on Hole 1202A cores and Cores 195-1202B-1H through 5H. All other sections were AF demagnetized at 20 mT only. In situ orientation data were collected with the Tensor tool for APC cores from Holes 1202A and 1202C from below 30 mbsf. In addition, six oriented discrete samples (standard 8-cm3 plastic cubes) were collected from the working halves of Hole 1202A samples for progressive AF demagnetization. Because of the suspected high concentrations of organic material, the magnetic signal is prone to be affected by the dissolution of fine-grained magnetite during core storage, as has been found on cores from the upwelling regions along the Californian and African margins (Richter et al., 1999; Yamazaki et al., 2000). For this reason, the discrete samples were also used for the short- and long-term monitoring of the stability of the magnetic intensity. All discrete samples were demagnetized at successive peak fields of 5, 10, 15, 20, 25, 30, 35, 40, 50, 60, 70, and 80 mT to verify the reliability of the split-core measurements. All samples yielded good principal component analyses (Kirschvink, 1980) with fits having an average maximum angular deviation of 2°. After the AF demagnetization, a 100-mT field was imparted and the magnetic intensity and susceptibility was measured. This procedure was repeated daily, and samples were left on board for continued monitoring of the magnetic intensity and susceptibility at weekly intervals. The magnetic intensity of the uncooled samples decreased to 70% to 85% of the initial magnetization within 2 months (Fig. F6).
Magnetic susceptibility measurements were made on whole cores from all four holes as part of the multisensor track measurements program (see "Physical Properties"). Magnetic susceptibility ranges from 1.23 x 10-4 to 1.12 x 10-3 (SI volume units) (Fig. F7) and shows very distinct features that allow hole-to-hole correlation (Fig. F8) and appear to be related to climate change.
A primary magnetic component is preserved in sediments from all four holes. All discrete samples were easily demagnetized by AF techniques and revealed an excellent demagnetization behavior (Fig. F9). The intensity of NRM after 20-mT demagnetization from all four holes is similar in magnitude and trend, ranging from to 8 x 10-4 to 2 x 10-2 A/m (Fig. F7).
Magnetic inclinations and declinations from all four holes indicate that only the Brunhes (C1n) normal polarity chron (Berggren et al., 1995) is recorded in these sediments (Fig. F7). The geomagnetic field at the latitude of Site 1202 (24.8°) has an inclination of 42.2°, assuming a geocentric axial dipole model, which is sufficiently steep to determine magnetic polarity in APC and XCB cores that lack a horizontal orientation. Whether the Brunhes Chron is complete cannot be determined from the magnetostratigraphy. However, preliminary biostratigraphic datums (see "Biostratigraphy") indicate an age of <127 ka in Hole 1202D.
The intensity of NRM is controlled by the strength of the geomagnetic field, the concentration of magnetic minerals, and other rock-magnetic characteristics of sediments, including grain size, composition, and interaction of magnetic minerals. If the sediments prove to be magnetically uniform, variations of remanent intensity identified after normalization for the abundance of magnetic minerals using rock-magnetic parameters could be interpreted as relative changes of past geomagnetic field strength (paleointensity). Because relative paleointensity variations during the last 800 k.y. are relatively well understood (Guyodo and Valet, 1996, 1999), paleointensity can be used as a tool to correlate and date the age of sediments (paleointensity stratigraphy). Preliminary rock magnetic data indicate that the recovered material from Site 1202 is magnetically homogenous enough for a relative paleointensity determination, which can potentially be used to obtain a high-resolution age model within the Brunhes Chron (Guyodo and Valet, 1996, 1999).
The magnetic inclinations show a potentially excellent paleosecular variation record in the APC section but become noisy in the XCB cores (Fig. F7) because of the core disturbance and a steep vertical magnetic overprint imparted by the coring process. An interval of negative polarity in Hole 1202A occurs between 105 and 108 mbsf. The same interval in Hole 1202B reaches shallow positive inclinations between 101 and 104 mbsf. It is possible that this interval represents one of the short geomagnetic excursions within the Brunhes Chron. Several excursions are well documented, such as the Mono Lake and Laschamp reversal excursions. The Mono Lake Excursion is only found in western North America and in the Arctic Seas and has an age of ~25 ka (Nowaczyk et al., 1994). The Laschamp Excursion is well established and radiometrically dated in France and Iceland (Levi et al., 1990), Hawaii (Holt et al., 1996), and the Arctic Sea (Nowaczyk et al., 1994). The age of the Laschamp Event ranges from 40 to 45 ka. It is possible that the polarity change at 110 mbsf represents the more globally occurring Laschamp Event, but more detailed studies are needed to confirm this interpretation.