All APC cores from Holes 1220A, 1220B, and 1220C 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. As at previous sites, XCB cores were not measured with the exception of Core 199-1220B-20X, which had remarkably undisturbed sediment. In addition to core measurements, numerous discrete samples were taken from Hole 1220A to carry out more detailed progressive demagnetization. Only a few core sections from APC-cored intervals at Site 1220 were too disturbed to measure with the cryogenic magnetometer.
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. F9). 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 a high grouping of antipodal directions, magnetization directions of most cores did not pass a reversal test, suggesting that the characteristic remanent magnetization has not been fully isolated in these samples. Further, shore-based stepwise demagnetization is required to isolate the primary magnetization in these sediments, particularly if a precise estimate of paleolatitudinal changes in the Paleogene is intended for Site 1220.
Site 1220 paleomagnetic data gave excellent results and a reliable record of geomagnetic reversals from the early Miocene to the early-middle Eocene. The splice of Holes 1220A, 1220B, and 1220C shows an excellent match. The VGP latitude changes were also used to splice the cores to the composite depth scale (Table T6; Fig. F10). Correlation of the obtained magnetic stratigraphy to the GPTS shows a record from Subchron C6An.1n to Chron C21n (Fig. F10). Hole 1220B is the deepest hole drilled at Site 1220, and the recovered section is correlatable to chrons from C21 (partially recovered at ~170 mcd) to C8n, at ~38 mcd. Holes 1220B and 1220C were designed to complement gaps in the Hole 1220A record, particularly in the top and middle parts of the section. Hole 1220C was particularly useful in anchoring Subchrons 6B and 6C to the GPTS, as a gap of ~3 m exists between Cores 199-1220A-3H and 4H (see also "Composite Depths"). Data retrieved from Hole 1220B reproduced and complemented results previously obtained in Hole 1220A. The composite magnetic stratigraphy for Site 1220 faithfully reproduces ~20 m.y. of geomagnetic polarity history.