As in previous Leg 199 sites, all APC cores from Holes 1221A and 1221B that did not show large drilling-related disturbances were measured on the shipboard pass-through cryogenic magnetometer. The natural remanent magnetization (NRM) was measured at 5-cm intervals in each core section, followed by three steps of alternating-field (AF) demagnetization up to a maximum peak field of 20 mT. The XCB cores were not measured with the exception of Sections 199-1221C-11X-1 through 11X-3 and 199-1221D-4X-2, which contained apparently undisturbed sediment. In addition to whole-core measurements, numerous discrete samples were taken from Hole 1221A to carry out more detailed progressive demagnetization.
As in most of the siliceous sediments cored at previous sites, the NRM magnetization intensities were in the order of 10-1 to 10-2 A/m and decreased to ~10-3 A/m after partial AF demagnetization (Fig. F7). The drilling-induced overprint was mostly removed by AF demagnetization at 10 mT. Some magnetic directions did not reach a stable point at 20 mT and, despite the high grouping of antipodal directions, magnetization directions of most cores did not pass a reversal test, suggesting that the characteristic remanent magnetization (ChRM) has not been fully isolated in these samples. Further shore-based stepwise demagnetization is thus required to isolate the primary magnetization, particularly if a precise estimate of paleolatitudinal changes in the Paleogene is desired for Site 1221. Samples from XCB cores show paleomagnetic inclinations much steeper than expected for this near-equatorial site. For this reason, data from these cores were excluded when interpreting the polarity reversals. The cause of the steep inclinations is unclear and might be indicative of an unrecognized deformation during the drilling process or of a stronger drilling-induced overprint not removed by AF demagnetization. The latter could require a progressive thermal demagnetization to isolate the ChRM.
Data obtained from Site 1221 have a lower signal-to-noise ratio as compared to results from previous sites, particularly in the lower half of the section. This problem, together with the discontinuous recovery, makes the interpretation of the magnetic reversals more problematic.
Correlation of the magnetic stratigraphy to the geomagnetic polarity timescale is shown in Figure F8. We were able to identify polarity Chrons C15n to C20n (late-middle Eocene). The uppermost sediments from which we obtained reliable data (~40 mcd) were thus deposited during Chron C13r, near the E/O boundary. Below 80 mcd, the polarity record is more difficult to interpret because the record is not continuous and there is a large scatter in the virtual geomagnetic pole latitudes.