Every other core at Site 1264 was recovered with a nonmagnetic core barrel until the first barrel had to be drilled over (see Table T1; "Operations"). As at other sites, we noticed no obvious differences in the magnetic data between sediments recovered with the nonmagnetic core barrel and those with a standard barrel. All APC cores taken in Holes 1264A and 1264B were successfully oriented with the Tensor tool, with the exception of the top two cores in each hole (see "Operations," Table T1).
The archive halves of 61 cores from Holes 1264A, 1264B, and 1264C were measured in the pass-through magnetometer. Natural remanent magnetization (NRM) was measured on all cores. Most cores were demagnetized at 10 and 15 mT. As at the other sites, a strong vertical overprint is largely removed in most cases by demagnetization to 10 mT. Both the archive and working halves of Section 208-1264A-24H-4 were demagnetized to 15 mT as part of an experiment to determine the source of bias in the declination data (see "Paleomagnetism" in the "Explanatory Notes" chapter).
After alternating-field (AF) demagnetization to 15 mT, the depositional remanent magnetization (DRM) of sediments varies with depth between 10–4 and 10–5 A/m in the upper 30 mcd (lithostratigraphic Unit I) and between 10–4 and 10–5 A/m from 30 to 120 mcd (lithostratigraphic Subunit IIA) and is on the order of 10–2 A/m below 120 mcd (lithostratigraphic Subunit IIB) (Fig. F20). In lithostratigraphic Subunit IIA, the average intensity of the sediments is roughly two orders of magnitude lower than the average from the remainder of the section (Fig. F20). These low intensities are just above the stated resolution of the pass-through magnetometer for split cores (~2 x 10–5 A/m), resulting in erratic directions and intensities. This interval is also characterized by negative initial MS values (Fig. F21), reflecting the dominance of diamagnetic calcite of the MS signal.
Lithostratigraphic Subunit IIB (below ~120 mcd) can be subdivided on the basis of magnetic parameters such as DRM normalized by MS (nDRM15 mT) and the differential component of magnetization (dDRM), reflecting the low-coercivity fraction (#10 or #15 mT). The interval between 120 and 164 mcd is characterized by nDRM15 mT values nearly an order of magnitude greater than the interval below 164 mcd (Fig. F22). This appears to result primarily from lower MS values in the upper interval (Fig. F21). The upper interval is also characterized by a significant decrease in the low-coercivity (<15 mT) component (Fig. F20) when compared to the interval below. Together, these observations imply a change in magnetic mineralogy and/or grain size (probably resulting from diagenesis), rather than a long-term change in the average geomagnetic field intensity.
Further evidence for diagenetic changes can be observed in the transition (~155–170 mcd) between the two intervals in Subunit IIB described above. Downhole records of the interstitial water iron and manganese concentrations, grain density, and color reflectance exhibit changes in the same interval (see Fig. F14; "Lithostratigraphy"). Diagenesis can result in the partial dissolution of iron oxide minerals (especially magnetite) and the formation of iron manganese oxides (such as jacobsite) or iron sulfide minerals (such as pyrite, pyrrhotite, or greigite). This interpretation may explain the increase in interstitial water iron combined with a decrease in manganese in this interval (see Fig. F14; "Lithostratigraphy"). However, constraints on magnetic mineralogy await shore-based analysis of discrete samples.
As at Site 1263, the magnetostratigraphy at Site 1264 is difficult to interpret. In the upper 30 m, it is possible to make preliminary chron assignments for the Pliocene–Pleistocene (Fig. F23A; Table T10). These assignments correspond (in depth) to the available biostratigraphic datums (Tables T5, T15). No magnetostratigraphic interpretations were made for the interval of low magnetization from 30 to 120 mcd, where the directional record is extremely erratic. Below this interval of low magnetization, the inclination record becomes less erratic (Fig. F23B, F23C, F23D) but shows a bias toward negative values (normal polarity), as at Site 1263. Some tentative chron assignments have been made largely based on biostratigraphic datums, but, in general, little confidence is placed in the inclination record at this site below the Pliocene.