Every other core at Site 1263 was recovered with a nonmagnetic core barrel until the first barrel had to be drilled over (see Table T1 and "Operations"). As at Site 1262, no obvious differences were noticed in the magnetic data between sediments recovered with the nonmagnetic core barrel and those recovered with the standard core barrel.
All APC cores from Holes 1263A, 1263B, and 1263C were successfully oriented with the Tensor tool, with the exception of Cores 208-1263A-1H through 3H, 208-1263B-19H and 25H, and 208-1263C-1H through 3H (see Table T1 and "Operations").
The archive halves of 93 cores from Holes 1263A, 1263B, 1263C, and 1263D 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. A strong vertical overprint was largely removed by demagnetization to 10 mT. In some cases, 15-mT demagnetization appeared to further resolve the characteristic polarity.
Section 208-1263B-2H-3 was demagnetized up to 25 mT. Although the additional demagnetization changed the inclination slightly, it did not significantly affect the polarity interpretation. Examples of demagnetization behavior of this section are shown in Figure F24. Demagnetization steps above 10 mT show quasi-univectorial decay toward the origin, indicating that the characteristic remanent magnetization was largely isolated through alternating-field (AF) demagnetization.
Both the archive and working halves of Section 208-1263B-29X-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).
Twelve pilot samples were stepwise AF demagnetized and confirmed removal of a vertical overprint in most cases by 10 mT and in some cases by 5 mT. Inclinations after demagnetization to 15 mT generally agreed with the archive-half pass-through data (Fig. F25). When the pass-through data are not well resolved, the discrete samples often provide a less ambiguous polarity determination, suggesting that shore-based discrete sample analysis will play an important role in clarifying the magnetostratigraphy. In many cases, the discrete sample directions became erratic when demagnetized to values <1 x 10–3 A/m. Below this value, intensities of the samples began to approach the stated resolution of the magnetometer for discrete samples (4 x 10–4 A/m).
In Figure F26, we plot intensities of remanent magnetization with depth both before and after 15-mT AF demagnetization. The magnitude of the initial NRM is on the order of 10–2 to 10–1 A/m, primarily reflecting the strong low-coercivity overprint, as observed at Site 1262. After demagnetization to 15 mT, lower intensity values (~10–4 A/m) are observed from 0 to 30 mcd and from 330 mcd to the bottom, with higher values (~10–3 A/m) in between. The variations of magnetization after 15-mT demagnetization are well matched between holes, as is MS (see Fig. F4). Intensity variation of the soft component (0–10 mT) is roughly proportional to that of the hard-coercivity component (>15 mT).
The very soft nannofossil ooze at Site 1263 does not carry a clean signal (Fig. F27), making the magnetostratigraphy difficult to interpret. Preliminary chron assignments (Fig. F27; Table T10) were attempted where a reversal appears to be particularly well resolved or where the same feature is seen in sediments from more than one hole. However, the assignments are based largely on biostratigraphic age estimates and not on identifiable reversal patterns. Note that none of the chron assignments should be used with any real confidence.
A relatively well resolved normal polarity interval at ~20–23 mcd in Hole 1263A is likely to be part of Chron C3n based on the top of nannofossil datum Amaurolithus spp. (4.56 Ma) at 12.07–23.29 mcd. Two normal events appear to be at least partially resolved in Holes 1263A and 1263B at ~65–69 mcd and ~88–89 mcd. The upper normal event is assigned to Chron C12n (30.479–30.939 Ma) based on the bottom of the nannofossil datum S. distentus (30.32 Ma) and the T of S. pseudoradians (30.95 Ma), which roughly coincide in depth with the normal event (see Table T5 and "Biostratigraphy"). The lower event coincides with the top of nannofossil datum E. formosa (32.9 Ma) at 85.52–86.47 mcd, and this normal event is assigned to Chron C13n (33.058–33.545 Ma).
In the interval from ~100 to 200 mcd (Fig. F27B), a few tentative chron assignments are made where the data appear to be better resolved, but little confidence is placed in these. In the interval from ~200 to 300 mcd (Fig. F27C), the inclination record appears to be biased toward negative values (normal polarity) with positive (reversed polarity) spikes within 1–2 m of core tops. Based on the biostratigraphy, this interval corresponds to the lower to middle Eocene, where several long reversed polarity intervals should be represented. The bias cannot result from an insufficiently removed drilling overprint, which would be in the opposite direction. The bias may be an artifact of soft-sediment deformation, and no attempt to interpret the record in this interval was made. Below 320 mcd, most of the record comes from XCB cores that are significantly disturbed, and again, no polarity interpretations were made.
At this site, a preliminary study of demagnetization fractions of the vertical components was attempted. Detailed analysis and evaluation of ~15,000 data points from Hole 1263A suggests that