Shipboard paleomagnetic measurements in Holes 1148A and 1148B consisted of long-core measurements of the natural remanent magnetization (NRM) at 8-cm intervals before and after alternating field (AF) demagnetization at 15 mT or 10 and 20 mT, carried out on the archive halves of all APC cores. In addition, discrete samples were collected from the working halves of Hole 1148A, at an average spacing of two samples per section (1.5 m) from Cores 184-1148-2H through 8H and one sample per section farther downcore. The ordinary cutting shoe was used with a standard core barrel for Holes 1148A and 1148B. Cores 184-1148A-4H through 16H and 184-1148B-3H through 15H were oriented using the Tensor tool.
Long-core measurements of NRM were carried out at 8-cm intervals before and after one demagnetization step at 15 mT from Cores 184-1148-2H through 5H. A two-step demagnetization routine (10 and 20 mT) was then used for Cores 184-1148A-6H through 7H, where we were expecting the Brunhes/Matuyama boundary based on the results from Site 1147. The one-step procedure (with one demagnetization at 20 mT) was then resumed for all other cores.
The direction of the NRM (after demagnetization at 15 or 20 mT and correction using the Tensor tool data where available) is shown in Figure F16. The Brunhes/Matuyama boundary can be tentatively placed at 55.2 mcd. The upper Jaramillo transition appears at 69.1 mcd; the lower, at 73 mcd. The upper Olduvai transition is encountered at 111.4 mcd, and the lower is tentatively placed at 118.5 mcd (Table T10). Below 67 mcd, however, the record appears somewhat unstable. Inclinations are largely negative down to 110 mcd, where a clear transition to positive inclination is observed in connection with a 180° declination swing to north-seeking directions.
Fourteen discrete samples were demagnetized with a 10-step routine up to a 50-mT AF field. Unfortunately, after the very first steps the magnetization in most cases fell to values comparable to or even lower than the signal of the tray used for discrete samples. Measurements were therefore stopped and will be resumed on shore, where a small-access cryogenic magnetometer will provide the necessary sensitivity.
Long-core measurements were done at a spacing of 8 cm using a two-step demagnetization routine at 10 and 20 mT for all cores from 184-1148B-1H through 15H. The direction of the NRM (after demagnetization at 20 mT and correction using the Tensor tool data where available) is shown in Figure F17. The Brunhes/Matuyama boundary was identified at 55.1 mcd by a 180° swing in declination and a jump to negative inclination. Unfortunately, just below the boundary some of the liners were crushed and the core highly disturbed, which strongly affected the direction of the magnetization. It was impossible to identify the Jaramillo Subchron and the sequences of magnetic reversals below, although changes in inclination and declination were suggested by the general trends of the record. We did not attempt to measure the XCB cores.