All cores from Holes 1130A and 1130B with sufficient recovery to make long-core measurements feasible were measured as half cores using the 2-G 760-R magnetometer. Measurements were made of natural remanent magnetization (NRM) and after 20-mT demagnetization. The experimental nonmagnetic shoe was used for odd-numbered cores in Hole 1130A from Core 182-1130A-3H to 7H, and the experimental nonmagnetic APC core barrel assembly, which was also used at Site 1128, was again used for the odd-numbered cores from Core 182-1130B-3H to 7H. The nonmagnetic shoe and the core barrel assembly both made reductions in the "radial" component contamination, although the effects were not as dramatic as at Site 1128 (see "Appendix: Magnetics Experiment"). Discrete samples were taken from both soft sediments cored by the APC and from biscuits in the XCB and RCB cores for NRM and rock magnetic analysis. The analyses were performed using the methods described in "Paleomagnetism" in the "Explanatory Notes" chapter.
The long-core measurements revealed an extensive record of normal magnetization and a sharp switch to reverse polarity at 200 mbsf in both Holes 1130A and 1130B. Below this was ~10 m of reversed polarity, and then mixed polarities were found. Intensities of magnetization varied from 0.01 to 1 mA/m with an overall decreasing trend throughout the core. Superposed on this decreasing trend are higher frequency fluctuations with dominant wavelengths of a few tens of meters (Fig. F8). The overall decreasing trend is consistent with dissolution of the fine-grained magnetite that carries the bulk of the paleomagnetic record. Toward 350 mbsf, the intensity approaches the noise level of the 2-G 760-R. Fluctuations in intensity are similar to those seen at Site 1127, although they do not show the same correlation with field intensity fluctuations. They are, however, accompanied by inclination features with comparable wavelength.
Discrete samples were taken for demagnetization analysis of the NRM and for rock magnetic investigations. Zijderveld plots from APC and XCB cores yielded high-quality plots with good convergence to the origin over the major part of the demagnetization and well-determined characteristic directions of magnetization (Fig. F9). However, other samples from deeper in the hole were noisy, and the magnetization directions were less well determined.
Rock magnetic analysis revealed interesting downhole trends in the magnetic material. As noted above, the intensity of magnetization decreases with depth downhole. The ratio of anhysteretic remanent magnetization (ARM) to isothermal remanent magnetization (IRM), which is a measure of the relative importance of single-domain to multidomain behavior, decreases downcore (Fig. F10). This indicates that the role of biogenic magnetite is reduced downhole and, because this corresponds to a decrease in magnetization, it strongly suggests that the fine magnetite of biogenic origin is undergoing preferential dissolution.
Magnetic susceptibility from multisensor track (MST) measurements is shown in Figure F8. These data do not display any downhole trend comparable to the magnetization intensity trend. However, a decrease in the negative, diamagnetic-dominated susceptibility values between 200 and 320 mbsf is consistent with increased multidomain magnetite at this depth.
The Brunhes/Matuyama boundary is very clearly seen in both Hole 1130A and Hole 1130B at 200 mbsf (Fig. F11). It occurs at Section 1130A-22X-3, 30 cm, and at Section 1130B-22X-4, 70 cm. These depths then yield sedimentation rates of 255 m/m.y. The depth of reversed magnetization below is only between 10 and 15 m before dominantly normal magnetizations occur again. This is too short a length of core to represent the ~200,000 yr between the Brunhes/Matuyama reversal and the termination of the Jaramillo; thus, there must either be a major change in sedimentation rate or missing sediment. Tentative depth assignments have been made for the Jaramillo and Olduvai but require testing with onshore analysis. Below this, no useful magnetostratigraphy could be established.