PALEOMAGNETISM

A total of 49 cores from Holes 1218A, 1218B, and 1218C were measured with 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 were not measured because they are made up of short "biscuits," typically <7 cm long, and the information obtained from a given core would not contribute any interpretable directional data. On the other hand, individual biscuits can be measured and oriented using the direction of the modern-day field overprint. This procedure seemed to work for Core 199-1218A-24X. In addition to core measurements, ~100 discrete samples were taken from Hole 1218A cores to carry out more detailed progressive demagnetization. Only a few core sections from Site 1218 were in poor condition, mostly because of drilling disturbance, and, therefore, were not used for paleomagnetic study.

NRM magnetization intensities were in the order of 10^-2 to 10^-1 A/m and decreased to ~10^-3 to 10^-2 A/m after partial AF demagnetization (Fig. F10). These values, typical of the sediments found during this leg, are well above the magnetometer's noise level even in the most weakly magnetized carbonate units. A large group of NRM inclinations showed steep downward directions (~70°), which are indicative of a drilling-induced overprint. This overprint was mostly removed with AF demagnetization, typically disappearing at 10 to 15 mT. Some magnetic directions did not reach a stable point between 5 and 20 mT, suggesting that the characteristic remanent magnetization (ChRM) has not been fully isolated.

The Tensor tool was used to orient Hole 1218A APC cores starting with Core 199-1218A-3H. The Tensor tool provided a good first-order orientation for most of the cores. Although pass-through measurements were not done on XCB cores, some discrete samples were taken from individual biscuits in Cores 199-1218A-22X through 30X because the E/O boundary was thought to occur within Cores 199-1218A-23X to 24X. To orient the samples taken from XCB cores, we assume that a soft magnetic component directed toward present-day magnetic north is still recognizable in the samples and is only partially masked by the drilling-induced overprint. This component of the magnetization vector can possibly be recognized after a detailed demagnetization. We applied this procedure on all measured samples from XCB cores with various degrees of success. Also, a few APC cores produced abnormal directions after correction for the Tensor tool data (e.g., east-west-trending declinations). In these cases, we also used the overprint direction to help determine the orientation of the cores. In all cases, results were cross-checked among Holes 1218A, 1218B, and 1218C to produce consistent results.

About 100 oriented discrete samples (8-cm³ plastic cubes) were collected from Hole 1218A, and 55 of them were AF demagnetized. These samples were used to determine the stability of the remanent magnetization and compute a more faithful ChRM direction. We divided the samples into two sets based on their age (Fig. F11). The average Fisher mean inclination for the Miocene set of samples is 10.4° (with the 95% confidence interval ₉₅ = 12°), corresponding to a paleolatitude of 5.2°N (error margin = 2.2°/8.4°), whereas the Oligocene set has an average inclination of 3.8° (₉₅ = 6.2°), which indicates a paleolatitude indistinguishable from the paleoequator. Although the confidence cones of the two means overlap, the increase in inclination from the Oligocene to the Miocene is consistent with the expected plate motion. The overall mean inclination for the discrete samples (6°; ₉₅ = 5.9°) is significantly shallower than the mean inclination (11.7°) obtained from blanket demagnetization in the best sections of the archive halves. Apparently, either the ChRM of the archive halves is not fully isolated or the sediment's magnetization is disturbed near core edges by the coring procedure. Both these hypotheses can be addressed by shore-based studies.

Except for a very few sections that had to be discarded because of excessive drilling disturbance, Site 1218 provided an excellent record of geomagnetic reversals, which are readily interpreted as magnetozones (Table T9). Overall, the composite magnetic stratigraphy of the oriented cores from Site 1218 spans the interval from the Pleistocene (Chron 1n) to the early Oligocene (Chron 12r; 33.5 Ma). The upper 18 m of Hole 1218A was interpreted by using changes in declination after inspecting the orientation of the soft-component overprint (Fig. F12A). ChRM inclinations are very shallow as expected in these latitudes, and a few cores that were given a reversal test gave a particularly good set of ChRMs, which suggests that they represent a relatively clean record of the geomagnetic field directions. The magnetostratigraphy of Site 1218, shown in Figure F12, results from a composite of the virtual geomagnetic pole latitudes of Holes 1218A, 1218B, and 1218C that were spliced using MST data (see "Composite Depths"). The polarity reversals from Holes 1218A, 1218B, and 1218C match remarkably well with a few exceptions. In Core 199-1218C-6H, from 108 to 115 mcd (Fig. F12C), Subchron C7n.2n is shorter than that in Holes 1218A and 1218B, and Chron C7An is shifted upcore, suggesting a hiatus in sedimentation or uncertainty in the composite-depth column. We also notice that in Core 199-1218C-6H, from 123 to 126 mcd (Fig. F12C), the magnetic polarity does not agree with that of Holes 1218A and 1218B. As before, possible explanations include partial magnetic cleaning of the samples, uncertainty in the composite-depth column and possible compression of the core relative to other cores (see "Composite Depths"). It is worthwhile to point out that the directions used for the magnetostratigraphy are based on blanket demagnetization only, and consequently, it is possible that secondary components have not been completely eliminated. The short normal-polarity interval in Hole 1218C at ~175 mcd is attributable to machine error probably caused by a flux-jump during the last demagnetization step. The 20-mT demagnetization step was repeated in Section 199-1218C-11H-2 and updated in the Janus database.

In the magnetostratigraphy working upsection, we identify Chron C12 (early Oligocene) only in Hole 1218A at ~201 to 203.7 mcd. The three records from Holes 1218A, 1218B, and 1218C start overlapping in Chron 11n, which displays both Subchrons C11n.1n and C11n.2n. All subsequent geomagnetic reversals up to the mid-Pliocene (Chron C2Ar) have been successfully identified. The magnetic stratigraphy of Site 1218 is, therefore, virtually complete and offers a highly detailed history of geomagnetic reversals during the last 30 m.y.