PALEOMAGNETISM

A total of 39 cores from Holes 1219A and 1219B were measured on the shipboard pass-through cryogenic magnetometer. The NRM was measured at 5-cm intervals in each core section, followed by three to four steps of AF demagnetization up to a maximum peak field of 20 mT. XCB cores from Hole 1219A were not measured with the pass-through system because they are made up of short "biscuits" and the information obtained from a given core would not contribute any interpretable directional data. In addition to core measurements, numerous discrete samples were taken from Hole 1219A cores to conduct more detailed progressive demagnetization. Only a few core sections from Site 1219 were in poor condition, mostly because of drilling disturbance, and were not used for paleomagnetic study.

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 (Fig. F13). Cores 199-1219A-12H and 199-1219B-10H and 11H had the weakest NRM intensity, dropping to ~10^-5 A/m after AF demagnetization, which is close to the noise level of the magnetometer. The drilling-induced overprint was mostly removed with AF demagnetization, typically disappearing by 10 mT. Some magnetic directions did not reach a stable point between 15 and 20 mT, and a large group of samples from Core 199-1219A-23H retained steep inclinations, suggesting that the characteristic remanent magnetization (ChRM) has not been fully isolated in these samples.

About 250 discrete samples (8-cm³ plastic cubes) were collected from Hole 1219A, and 125 of them were AF demagnetized. The aim of these measurements was to investigate the stability of the remanent magnetization and compute a more faithful ChRM direction based on progressive demagnetization instead of blanket demagnetization. About 60% of the discrete samples gave stable ChRM directions, and 40% showed erratic behavior during demagnetization. The average inclination of discrete samples from Cores 199-1219A-23H and 24H is 3.2° (₉₅ = 2.3°). Overall, declination and inclination patterns show that at least the lower part of the section was located in the Southern Hemisphere during deposition. The obtained mean inclination indicates a time-averaged paleolatitude of 1.6°S for the site, but this result is preliminary and will require further testing. The paleolatitude inferred from the inclination is consistent with the expected latitudes as calculated from both paleomagnetic pole positions (Petronotis et al., 1994) and those based upon a fixed hotspot model.

Using narrow demagnetization steps, it is possible to distinguish several soft components in some samples (Fig. F14). A very low coercivity component with a very steep downward direction, which is removed by 2 mT, is attributed to the drill string overprint. In addition, we found a second overprint, removed between 4 and 10 mT, which most likely represents a viscous component parallel to the present-day geomagnetic field. A mean inclination of this overprint component gives a value of 24° (₉₅ = 4°), which is statistically indistinguishable from the present-day magnetic inclination at the site (~20°). The mean paleolatitude (8°) measured from discrete samples from Cores 199-1219A-1H and 2H represents the mean latitude between the present and 5 Ma. This value is very close to the expected present-day value of 7.8° and increases our confidence on the reliability of the paleomagnetic directions in these sediments. Samples displaying such a clear present-day field overprint have been used to orient the uppermost two cores from Hole 1219A that were not oriented using the Tensor tool as well as those cores that had unsatisfactory orientations.

Data from only a few sections from Hole 1219A and 1219B had to be discarded because of core disturbance. For example, no reliable direction was obtained from Cores 199-1219A-12H and 199-1219B-10H and 11H because of the weak magnetic intensity; therefore, the polarity record in this interval has not been interpreted. Directions obtained from Core 199-1219A-18H show a distribution into several clusters corresponding to the different sections with an apparent clockwise rotation downcore. We suggest that this is a consequence of coring-induced internal deformation or some sort of unrecognized distortion that occurred during coring or handling of the core before cutting it. Thereafter, a rotation was applied to the magnetization directions in an attempt to recover the original orientation. The magnetization intensity (Fig. F13) shows many large spikes that often reflect spurious changes in the direction. This has to be taken into account in the interpretation of the directional results. The composite magnetic stratigraphy at Site 1219 spans the interval from the Pleistocene (C1n or Brunhes Chron) to the middle Eocene (Chron C20r).

The uppermost part of this stratigraphy obtained from Cores 199-1219A-1H and 2H, which were oriented using the inferred present geomagnetic field soft-component overprint, is shown in Figure F15. This record gives the only available age information for this part of the section. The oldest identified Chron, C20r, has only been partially recovered (Fig. F16B). The interval between 183 and 191 mcd, comprising Core 199-1219A-18H, does not show a clear pattern of magnetozones. Chron C15n was not unambiguously recognized, and hence, the results from this interval should be taken with caution. Chron C12r is clearly recognized, although quite noisy, perhaps because of the presence of cryptochrons. We are confident that we can also recognize Chron C13n, its lower boundary occurring at 176.3 mcd (Fig. F16). Paleontological events (for NP zones see "Calcareous Nannofossils" in "Biostratigraphy") corroborate the presence of the E/O boundary at around the suggested C13n/C13r reversal. No data are available from 12 to 20 mcd and from 110 to 130 mcd because of weak magnetization or because the results were not interpretable. Some other intervals with no data reflect incomplete core recovery (see "Composite Depths").