PALEOMAGNETISM

The archive-half core sections from Holes 1216A and 1216B were measured on the shipboard pass-through cryogenic magnetometer, except sections that were clearly disturbed by drilling in Cores 199-1216A-6H through 10H and 11X. The natural remanent magnetization (NRM) was measured at 5-cm intervals in each core section, followed by alternating-field (AF) demagnetization at 5, 10, 15, and 20 mT peak field. No discrete samples were demagnetized at this stage. Most of the measured cores displayed consistent remanent inclination and declination directions. NRM intensities were in the order of 10^-1 to 10^-2 A/m and decreased to about 10^-3 to 10^-2 A/m after partial AF demagnetization at 20 mT (Fig. F5), similar to those of Site 1215. Uncleaned NRM inclinations have steep downward directions of ~80°, indicative of a strong drilling-induced overprint. This overprint was partially removed and typically disappeared after AF demagnetization at 10-15 mT. For the characteristic remanent magnetization (ChRM), we have used the directions obtained after a blanket demagnetization at 20 mT, although the magnetic directions did not reach a stable point suggesting that the ChRM has been only partially isolated. A more reliable ChRM might be obtained by vector analysis of the measured data and shore-based measurements.

The Tensor tool was used to orient cores collected from Hole 1216A starting with Core 199-1216A-3H. The single core from Hole 1216B was not oriented. The orientation was successful in aligning the declinations between most cores as shown in Figure F6. The corrected declinations of the magnetic vectors were very useful in assessing polarity changes, otherwise the inclinations by themselves would produce an ambiguous record as a result of the low paleolatitude of this site.

Bulk magnetic susceptibility (MS) was measured on discrete samples taken from Hole 1216A, but no demagnetization was attempted.

The magnetic declinations of the oriented cores are relatively well behaved and are grouped in two antipodal clusters (Fig. F6) with a prevalence of northward directions. In the measured archive halves of Hole 1216A, the 20-mT AF demagnetization directions have an average inclination value around 35° with large scatter ( = 20). Despite the average inclination value, many samples, especially in the upper 15 mbsf of Hole 1216A, have rather steep inclinations that are not compatible with either the present field inclination or the paleolatitude of the site. We speculate that coring in soft sediment might have affected the inclinations by physically reorienting the sediment close to the core edges to a nearly vertical direction. In addition, the magnetic field inside the barrel produces a secondary overprint on the sediments. These hypotheses need to be further explored in postcruise analyses comparing discrete samples taken from the middle and external parts of the core.

No paleomagnetic record was obtained from the top 5 m in Hole 1216A because of core disturbance. Furthermore, the major change in the inclination record in the upper part of Hole 1216A (at 10 mbsf) cannot be unambiguously interpreted.

In oriented cores below 19 mbsf in Hole 1216A, the magnetic polarity has been computed by the virtual geomagnetic pole (VGP) latitude that combines the information from both inclinations and declinations. Because of the quality of the declination record in this site, the VGP latitude is very effective in outlining several geomagnetic reversals (Fig. F7) although the interpretation of the magnetic chrons is not unequivocal. Based on the radiolarian zonation (Zone RP13), we believe that Chron C20n can be found at the base of Core 199-1216A-5H at ~40 mbsf.