All archive-half core sections from Holes 1217A, 1217B, and 1217C were measured on the shipboard pass-through cryogenic magnetometer. A total of 134 core sections were measured from the 24 cores recovered in the 3 holes. The natural remanent magnetization (NRM) was measured at 5-cm intervals in each core section, followed by four to five steps of AF demagnetization up to a maximum peak field of 20 mT. For sections with clearly disturbed sediment, a blanket demagnetization at 15 mT was applied, but the directions were not used for establishing the magnetostratigraphy. In addition, 10 discrete samples were taken from Hole 1217A cores to carry out more detailed progressive demagnetization. As at Site 1216, several of the measured cores are in poor condition mostly because of drilling disturbance.
NRM magnetization intensities were in the order of 10-2 to 10-1 A/m and decreased to about 10-3 to 10-2 A/m after partial AF demagnetization (Fig. F9). Magnetization intensity for the basal sedimentary unit (the nannofossil chalk at 128-129 mbsf) is slightly lower than in overlying Units I and II but still above the noise level of the cryogenic magnetometer. Also, a chert interval from Core 199-1217A-14H at ~108 mbsf was measured and gave a reliable paleomagnetic signature. A large group of NRM inclinations showed steep downward directions (~70°), indicative of a drilling-induced overprint. This overprint was mostly removed with AF demagnetization, typically disappearing by the 10- to 15-mT demagnetization step. Some magnetic directions did not reach a stable point between 5 and 20 mT, suggesting that the ChRM has been only partially isolated.
The Tensor tool was used to orient APC cores in Hole 1217A, starting with Core 199-1217A-3H, whereas all cores were oriented in Holes 1217B and 1217C. Although the Tensor tool provided a good first-order orientation for most cores, some of them needed to be reoriented by an additional vertical rotation. Reorientation assumes that no tectonic rotation has occurred, which seems very reasonable in this area and is based on the analysis of the paleomagnetic directions. The mean magnetization direction computed for each core was restored to the north direction and all individual magnetization directions were rotated by the same amount. This was straightforward for all cores except for Core 199-1217A-9H, where the correct declination could not be established unambiguously. For this reason, the magnetic polarity from 61.95 to 66.04 mbsf in Hole 1217A must be treated with caution.
Oriented discrete samples (8-cm3 cubic plastic boxes) from Core 199-1217A-7H were thoroughly AF demagnetized up to 80 mT to examine the stability of the remanent magnetization and compute a more faithful ChRM direction using vector analysis instead of blanket demagnetization. Results are of good quality (Fig. F10) and provide our most reliable estimate of the paleolatitude at this site and for an age corresponding to Chron C18n (38-40 Ma). The average ChRM inclination for this set of 10 samples is 17.2°, corresponding to a paleolatitude of 8.2°N (upper/lower bounds are 9.5° and 6.8°, respectively).
Except for sections that had to be discarded because of excessive drilling disturbance, Site 1217 provided a good record of geomagnetic reversals that could be easily interpreted as magnetozones (Table T3). ChRM inclinations are usually shallow, as expected in these latitudes, and a few cores gave a particularly good set of ChRMs (e.g., Core 199-1217A-7H) that passed a reversal test, which suggests that they represent a clean record of the geomagnetic field. The directions from Core 199-1217A-7H gave average declination and inclination values of 32.15° and 22°, respectively, which are rather similar to those obtained from discrete samples. The inclination in the top 15 m of Hole 1217A cannot be unambiguously interpreted, probably because these sediments had a very low accumulation rate from Oligocene to the Quaternary time. The magnetostratigraphy of Site 1217 (Fig. F11) results from a composite of the virtual geomagnetic pole latitudes of Holes 1217A, 1217B, and 1217C that were spliced via the composite depth scale (see "Composite Depths"). We identify, from bottom to top, Chron C20n followed by Chron C19r, which is only partially resolved because of sediment disturbance. Upward in the section, Chrons C18n-C12n have also been identified. The magnetic ages are corroborated by the radiolarian zones found in the middle and upper parts of the section (see "Radiolarians" in "Biostratigraphy"). Flow-in in cores 199-1217A-5H and 6H prevented the clear identification of Chrons C16 and C17, at ~40-45 mcd. Overall, the composite stratigraphic section between 14 and 90 mcd spans ~13 m.y., or from the middle Eocene to the early Oligocene.