Shipboard paleomagnetic measurements for Holes 1146A, 1146B, and 1146C consisted of long-core measurements of the natural remanent magnetization (NRM) at 8-cm intervals before and after alternating field (AF) demagnetization (usually up to 20 mT) carried out on the archive halves of all APC cores. Measurements of XCB cores were pursued until the overprint was so severe that no information could be retrieved after demagnetization at 30 mT. In addition, 380 discrete samples were collected from the working halves of Hole 1146A (APC and XCB cores), at an average spacing of two samples per section (1.5 m). Half of these samples were subjected to progressive AF demagnetization with 10 steps up to 50 mT; further analysis will be carried out postcruise. For Hole 1146B, the nonmagnetic cutting shoe was used with a standard core barrel on every second core. Cores 184-1146A-6H through 21H, 184-1146B-3H through 21H, and 184-1146C-3H through 17H were oriented using the Tensor tool.
Long-core measurements of NRM and subsequent demagnetization steps were carried out at 8-cm intervals. Cores 184-1146A-1H through 4H and 13H were measured with two demagnetization steps at 10 and 20 mT; the remaining cores were measured with only one AF demagnetization step at 20 mT.
The direction of the NRM (after demagnetization at 20 mT and correction using the Tensor tool data where available) is shown in Figure F12. Above 115 mcd, declination and inclination generally oscillate around values expected for a geocentered dipole field at this latitude (0° and 38°, respectively), with an amplitude consistent with the secular variation of the geomagnetic field. At 116.7 mcd, the Brunhes/Matuyama boundary is identified by a sudden swing of the declination to 180°.
Farther downcore, the data are highly scattered, possibly as a result of incomplete removal of the overprint resulting from the coring process. As noted at previous sites, this increasing contribution of the overprint to the magnetization is revealed by positive inclinations: although some core intervals are undoubtedly reversed according to the declination, inclinations remain positive, showing that the overprint along the z-axis has not been entirely removed. Because of this, the occurrence of the Jaramillo and Olduvai Subchrons was largely obscured and could not be precisely located.
Every second discrete sample was AF demagnetized with 10 steps of demagnetization up to 50 mT. This treatment proved to be very efficient in removing the overprint in discrete samples from APC cores. An initial look at the data suggests it is consistent with the long-core measurements; further postcruise analysis should resolve some of the problems below the Brunhes/Matuyama boundary.
Only long-core measurements were made at Hole 1146B at 8-cm intervals and two demagnetization steps at 10 and 20 mT. Hole 1146B yielded better results, with the Jaramillo and possibly the Olduvai Subchrons evident (Fig. F13).
Between 99.7 and 101.3 mcd, a swing in declination with very low correlative inclinations could document a geomagnetic event. From its estimated age, we suggest that this could be a record of the Big Lost geomagnetic Event (dated at 510 to 650 ka). The Brunhes/Matuyama boundary was found just below at a depth of 114.9 mcd. Farther downcore, the upper Jaramillo transition is situated at a depth of 132.5 mcd; the lower Jaramillo, at 137.7 mcd. Additional declination swings are present farther downcore. Their identification with a known polarity chron, however, is made difficult by the large overprint present on inclination data, even after demagnetization at 20 mT. A transition from reverse declinations to normal occurs between 160.5 and 165.8 mcd, possibly marking the Olduvai Subchron. Comparison with the magnetic polarity time scale and with subsequent biostratigraphic results (see "Biostratigraphy") yields the age-depth relation shown in Table T10.
Long-core measurements in Hole 1146C were conducted with only one step of demagnetization at 15 mT down to Core 184-1146C-10H. Based on the results of Hole 1146B, Cores 184-1146C-11H through 17H were then demagnetized with two steps at 10 and 15 mT. The one-step procedure was then resumed for the XCB cores from Cores 184-1146C-18X through 28X. For this last core, both the archive and working halves of Section 184-1146C-28X-4 were measured after demagnetization. Although the AF treatment removed >90% of the magnetization, the archive and the working halves still have the same declination, indicating that the radial overprint is still dominating. Therefore, measurements were stopped at this level.
The Brunhes/Matuyama boundary was found at a water depth of 114.8 mcd. The upper Jaramillo transition is located at a depth of 132.5 mcd; the lower Jaramillo, at 138.1 mcd. At 162.3 mcd, a change from reverse to normal declination and inclination could mark the start of the upper Olduvai Subchron (Fig. F14). Unfortunately, Core 184-1146C-17H suffered a horrible fate, and the last three sections had to be extruded from the core barrel. Therefore, interpretations from this core are tentative at best.
The switch to XCB for Core 184-1146C-18X and below along with the increasing strength of the overprint made identification of other geomagnetic chrons below this level impossible.