Core archive halves from Holes 1120B and 1120D were measured on the shipboard pass-through cryogenic magnetometer. Declination, inclination, and intensity of natural remanent magnetization (NRM) and 10-mT and 20-mT alternating field (AF) demagnetization steps were routinely measured at 10-cm intervals. When time permitted, some core sections were measured at 5-cm intervals. In situ Tensor tool data were collected at the APC core tops to determine azimuthal orientation of the core to 160 mbsf. The measurements, however, proved inconsistent. At least two oriented discrete samples were collected from the working half of each core interval for progressive AF and thermal demagnetization and for rock magnetic studies. Whole-core magnetic susceptibility was measured routinely on all cores using a Bartington susceptibility loop on the automated multisensor track (MST).
All cores from Site 1120 were very carbonate rich (92%-98%; see "Lithostratigraphy"), susceptibility was mostly negative (diamagnetic), and NRM intensities were very weak (10-5 A/m on average). More than 50% of the remanence intensity was lost after demagnetization in an alternating field of 10 mT. At this level, intensities were at background levels of the pass-through cryogenic magnetometer, and the signal was too noisy and random to reliably ascertain polarity. Susceptibility and NRM intensity values were much higher in the upper 1-3 m of each core interval (NRM ranged as high as 10-1 A/m and susceptibility up to 10-3 SI; Fig. F8). Such a signal is clearly an artifact of drilling as paleomagnetic inclination and declination values do not show any less scatter within these intervals of increased intensity. Thermal demagnetization of selected discrete specimens demonstrates a small component of magnetization (<20%) that persists above 600° C, indicating hematite (Fig. F9). The most plausible explanation is that the signal is from rust contamination from the drill pipe. As each new core barrel is lowered (in the case of APC) or dropped (in the case of XCB) rust may be knocked from the drill pipe walls and accumulate at the sediment interface before each core is taken. It is also possible that the rust is suspended in the water column and injected into the sediment during coring and retained in any water between the core and core-liner. This would also explain increased intensities in intervals where the sediment was more fluid and included more drilling water (e.g., 212.0-213.5 mbsf; Fig. F8). This effect is more pronounced in XCB cores than in APC cores, as might be expected from dropping of the core barrel as opposed to lowering. Unfortunately, susceptibility values also show increases into positive values in the upper 1-3 m of each core interval, reflecting the increase in hematite. Smaller changes in susceptibility throughout the core may also reflect smaller amounts of disseminated hematite, and susceptibility values should be treated with caution when used for correlation. The reason that this effect was not seen at Site 1119 may be because of the very low intensity and susceptibility values, but also because the inside of the drill-stem was not cleaned before drilling at Site 1120, but it was at Site 1119.
Intensity of magnetization was too weak to investigate demagnetization behavior of discrete samples directly. Therefore, samples were first saturated with an isothermal remanent magnetization (IRM) and backfield IRM, and the IRM then demagnetized using first AF and then thermal methods. Typical IRM acquisition curves are shown in Figure F10. When saturation and backfield saturation values are compared, saturation was not reached by 600 mT for any sample measured, and mostly saturation was not reached by 1000 mT for many of the samples. Coercivity of remanence (Bcr) values were between 25 and 125 mT. Saturation values are low (10-3 10-4 A/m), probably because of very low concentrations of magnetic minerals. AF demagnetization was not very effective in removing the saturation IRM (SIRM) and, in most cases, >50% of the remanence remained above 60 mT. Thermal demagnetization was more effective in removing the applied SIRM: in most cases 80% of remanence was lost by 300° C of heating, 90% by 400° C of heating, and all remanence was lost by 500° C (e.g., Figs. F9B, F9D). In many cases 20%-30% of remanence was lost in a single step between 150° C and 200° C (Fig. F9). The majority of the behavior described above suggests that the main carrier of remanence is a ferrimagnetic iron sulfide mineral present in very low concentrations, although characteristically different than at Site 1119. Site 1120 contained large pyrite nodules, and greigite was identified infilling foraminifer tests in smear slides ( "Site 1120 Smear Slides") (distinguished from other sulfides by its gray nonmetallic luster in reflected light). Roberts and Turner (1993) also reported high SIRM values (900-1000 mT) for greigite samples from Upton Brook in New Zealand.