The natural remanent magnetization (NRM) of archive sections from Site 1192 was measured at 5-cm intervals using the pass-through cryogenic magnetometer. Initially a 20-mT alternating-field demagnetization was used, but this was increased to 30 mT in an attempt to increase the signal to noise ratio. The Tensor tool is usually employed to orient the cores taken with the APC beginning with the third core at each hole, but the predominant zero-declination artifact in the cores of this site precluded orientation.
Discrete samples were collected from Holes 1192A and 1192B at a general sample rate of two per core. These were used to aid in the interpretation of the long-core record of magnetization by providing additional measurements of polarity and basic magnetic characterization. Most of these samples were demagnetized at 5, 10, 15, 20, 40, 60, and 80 mT to permit principal component analysis. For rock magnetic characterization, anhysteretic remanent magnetization (ARM) was measured in 0.2 direct current (DC) and 200-mT AC fields and isothermal remanent magnetization (IRM) in a DC field of 1 T. Samples were also measured after progressive magnetization in fields measuring up to 1.0 T to study the acquisition of the IRM.
The intensity of magnetization in the uppermost 100 m of Holes 1192A and 1192B was between 10-2 and 10-4 A/m (Figs. F9A, F10). Thus, all measurements were well above the noise level of the instrument, and the background noise from core liners and core top contamination was not significant. At depths below 100 mbsf in Holes 1192A and 1192B, the intensities fell to between 10-3 and 10-5 A/m (Figs. F9B, F9C, F11), so that core top contamination became potentially more important.
The NRM invariably had a strong downward overprint (Fig. F12). More than 90% of the NRM measurements from the top 150 m of Hole 1192A yield a positive inclination as a result of the downward overprint, which is reflected in the concentration of data points near the center of the stereographic projection (Fig. F12A). When the samples are demagnetized at 30 mT, the inclination concentration at the center significantly decreases (Fig. F12B). However, even after this demagnetization, the large variation in inclination and the nonrandom declination demonstrate that the samples are far from being cleaned of all secondary remagnetization.
Anomalous NRM intensities occur at the tops of most cores (Fig. F13). Some of these have associated magnetic susceptibility anomalies, suggesting that magnetic material has been introduced into the top of the core. Others do not exhibit these anomalies, indicating that remagnetization alone is involved without the addition of new magnetic material. These anomalies in remanence can degrade the magnetostratigraphy by giving false positive inclinations that may be misinterpreted as reverse polarity intervals. This is particularly problematic when the magnetization is not completely cleaned, as there will be a difference in intensity between normal and reversed intervals resulting from the effect of the persistent positive overprint.
The sequence of reversals in the uppermost 100 m of Hole 1192A recorded all of the Pliocene reversals (Table T5; Fig. F9A). However, the Brunhes/Matuyama boundary and the Jaramillo Subchron are missing, although limits could be placed on the location of the Brunhes/Matuyama boundary and on the onset of the Jaramillo. Good evidence of both the Kaena and Mammoth Subchrons was found within the Gauss. Within the Gilbert Chron, evidence for the Cochiti-Thvera sequence is also seen.
Below 100 mbsf, the intensity of magnetization falls by an order of magnitude and interpretation of the magnetostratigraphy from the reversal sequence becomes problematic. Down to 150 mbsf, an attempt has been made to interpret a magnetostratigraphy (Fig. F9B), but it is not possible to place the same confidence in the magnetostratigraphy compared with the uppermost 100 mbsf.
In general, there was insufficient continuous recovery from Hole 1192B to permit sequences of reversals to be identified (Fig. F11). However, a short normal interval in Core 194-1192B-1H is likely to represent the Jaramillo (Fig. F10). Below 180 mbsf where the cored section overlaps with Hole 1192A (Fig. F11), the same C4n C4r sequence may exist (Fig. F9C).
The rock magnetic properties of the sediments from Hole 1192A were investigated using discrete samples taken throughout the recovered length of core. Plots of demagnetization of ARM, IRM, and the acquisition of IRM were used to interpret the nature of the magnetic carriers. Variability in all measured parameters is greater in the upper 50 m of the section and significantly less in samples from below 100 mbsf (Fig. F14). However, the IRM acquisition is consistent with the dominance of relatively fine-grained magnetite throughout, with the intermittent addition of some harder coercivity magnetic material at depths of <100 mbsf.
Both saturation IRM and ARM show parallel trends with considerable variability in the first 50 mbsf and then show a decrease of about two orders of intensity magnitude to 120 mbsf (Fig. F15). ARM then shows very little variation to the bottom of the hole, whereas the IRM shows a minor increase from 200 to 250 mbsf.
Given that the dominant carrier is magnetite, the ratio of ARM:IRM is a measure of grain size. The variability in the top 50 mbsf includes such high values of intensity that it is unlikely to be detrital in origin but rather could be indicative of a single domain magnetosome contribution (Fig. F14). However, the low value of the crossover of IRM demagnetization and acquisition seen in Figure F13 reveals that these single domain magnetosomes are not positively interacting and so must be disaggregated. Elsewhere in the section the ARM:IRM value is typical pseudo-single domain grain size, with a slight increase in magnetic grain size toward the bottom of the hole.
The patterns of variation downcore in rock magnetic properties are consistent with those reported for magnetic susceptibility and density (see "Core Physical Properties"). They record a change from variable concentrations of magnetic material above 100 mbsf to more consistent but significantly lower concentrations below 100 mbsf.
The magnetostratigraphy interpreted for Site 1192 is given in Table T5. It appears from the rock magnetism that at ~100 mbsf, close to the Pliocene/late Miocene boundary, a predominantly negative diamagnetic susceptibility gave way to a positive susceptibility. This is consistent with the input of siliciclastic material into a carbonate depositional environment. The recovery from Hole 1192B was not sufficient to permit interpretation of a useful magnetostratigraphy.