Shipboard paleomagnetic measurements for Holes 1145A, 1145B, and 1145C consisted of long-core measurements of natural remanent magnetization (NRM) at intervals of 8 cm before and after alternating field (AF) demagnetization, usually up to 20 mT, carried out on the archive halves of all APC and XCB cores. In addition, 116 discrete samples were collected from the working halves of Hole 1145A (APC and XCB cores), at a spacing of one sample per section (1.5 m). These samples were subjected to progressive AF demagnetization with 10 steps up to 50 mT. Cores 184-1145A-3H through 14H, 184-1145B-3H through 13H, and 184-1145C-3H through 13H were oriented using the Tensor tool.
The nonmagnetic cutting shoe was used with a standard core barrel on every second core starting with Core 184-1145A-5H. Long-core measurements were carried out at 8-cm intervals 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 93.0 mcd, declination oscillates around 0° and inclination around ~38°, which is the value expected for a geocentered dipole field at this latitude. The amplitude of the fluctuations in declination and inclination is consistent with the secular variation of the geomagnetic field. At 93.0 mcd, the Brunhes/Matuyama boundary is clearly identified by a sudden swing of the declination to 180°. Farther downcore at 110.6 mcd, a change to northerly declinations marks the upper Jaramillo transition. The lower Jaramillo transition is situated at ~116.1 mcd.
Farther downcore, the overprint resulting from the coring process increases and is not completely removed by the one-step demagnetization procedure imposed by the rate of core flow. This increasing contribution of the overprint to the magnetization is revealed by high positive inclinations, particularly at the reversals: although inclinations do display changes of ~70° as expected at this latitude, these changes are centered around 40°, not 0°. Also, the scatter of the inclinations is increased in XCB cores. Because the overprint was not completely removed by the long-core AF demagnetization, the occurrence of the Olduvai Event was obscured. Long-core measurements did reveal the Gauss/Matuyama boundary at 190 mcd as a jump of the inclination from ~25° to ~45° (no declination is obtained from XCB cores).
Demagnetization of the discrete samples proved to be more efficient in removing the overprint, probably because of the combined effect of reduced overprint in the center of the core together with the more careful demagnetizing procedure (10 steps of demagnetization up to 50 mT). Examples of demagnetization diagrams clearly show that a reverse (I = <0°) primary component is recovered after demagnetization (Fig. F13). Inclinations obtained from discrete samples are reported in Figure F12 as open squares. The discrete sample results are quite consistent with long-core measurements, and the Olduvai Event is clearly revealed from 154.6 to 164.7 mcd. Further, the inclinations around the Gauss/Matuyama boundary are closer to their expected values.
From 190 mcd to the bottom of Hole 1145A, some discrete samples appear to be heavily overprinted (I = >50°), but two samples yield unequivocal reverse inclinations. This indicates that the upper Kaena reversal (3.04 Ma) may have been reached in this hole at 210 mcd. If this hypothesis is correct, the change in sedimentation rate from 37.3 m/m.y. to 28.0 m/m.y. at 2.6 Ma suggested by biostratigraphic results (see "Biostratigraphy") might not exist.
Comparison with the magnetic polarity time scale yields the age-depth relationship shown in Table T10.
Only long-core measurements were made on cores from Hole 1145B at an interval of 8 cm and one demagnetization step at 20 mT. As discussed above in "Hole 1145A," one demagnetization step at this level is not sufficient to fully demagnetize the overprint, particularly in XCB cores.
The Brunhes/Matuyama boundary was found at a depth of 93.1 mcd. Farther downcore, the upper Jaramillo transition is placed at 110.5 mcd, whereas the lower Jaramillo transition is situated at a depth of 115.26 mcd. In Hole 1145B, the Olduvai is obscured by overprinting in the XCB cores.
Long-core measurements for Hole 1145C were conducted as in Hole 1145B. The only difference was for XCB cores, where two steps of demagnetization at 20 and 30 mT were done to reduce the overprint.
The Brunhes/Matuyama boundary was found at a depth of 92.8 mcd. The upper Jaramillo is located at a depth of 110.3 mcd; the lower Jaramillo, at 115.3 mcd. As in Holes 1145A and 1145B, the Olduvai was obscured by the overprint even at higher demagnetization levels.