PALEOMAGNETISM

The natural remanent magnetization (NRM) of archive sections from Site 1193 was measured at 5-cm intervals using the pass-through cryogenic magnetometer. An alternating-field demagnetization with a maximum intensity of 30 mT was used. The large diameter of the ADCB cores prevented the use of standard measurement procedures. Therefore, we readjusted core fragments that appeared to have retained their orientation in the liner by reorienting them with their downcore axis in the +y direction of the archive coordinate convention. In this way, core pieces up to 7 cm in length, which can pass through the magnetometer aperture, were run in the cryogenic magnetometer.

Discrete samples were collected from Holes 1193A, 1193B, and 1193C at a general sample rate of 2 per core. However, the special nature of the platform material required modifications in our sampling technique. Discrete samples were used to aid the interpretation of the long-core record of magnetization by providing additional measurements of polarity and basic magnetic characterization. Most of these samples were demagnetized at 5, 10, 15, 20, 40, 60, and 80 mT to permit principal component analysis. For rock magnetic properties, anhysteretic remanent magnetization (ARM) was generated using 0.2-mT DC and 200-mT AC fields and isothermal remanent magnetization (IRM) in a DC field of 1 T. Samples were also progressively magnetized in fields up to 1.0 T to study the acquisition of the IRM. In addition, a number of thermal demagnetizations were done using the Schonstedt Model TSD-1 oven. These measurements included demagnetization of NRM to help interpret the nature of the NRM magnetization, and saturation of the IRM to help identify the magnetic carriers.

Results

Natural Remanent Magnetization

The NRM intensity generally varies between 10-2 and 10-5A/m with some values decreasing to 10-6 A/m downcore (Fig. F35) with some occasional discrete peaks exceeding these bounds. When all NRM measurements are plotted in stereographic projection, the data indicate that nearly the entire data set has a downward magnetic inclination (Fig. F36A). Under normal circumstances all types of rocks have a viscous remanent magnetization (VRM) overprint due to the modern-day Earth's magnetic field. This would result in an upward inclination of the majority of NRM measurements. The downward overprint imparted by the drilling effect overwhelms the VRM signal resulting in movement of the data points toward the center of the stereonet and down (Fig. F36A). Demagnetization to 30 mT effectively cleans this downward overprint (Fig. F36B). The distribution of data points after this demagnetization becomes more random. However, full cleaning of the VRM has still not been attained in the long-core sample measurements.

Magnetostratigraphy

Declination, inclination, and intensity were routinely measured for all cores recovered. Results for the first 35 mbsf in Hole 1193A showed a clear sequence of magnetic reversals (Fig. F36A). Based on the preliminary age estimates (Pliocene-Pleistocene) (see "Biostratigraphy and Paleoenvironments"), the identified sequence appears to match well with the Gilbert Chron to the top of C3r on the recent geomagnetic polarity time scale (GPTS) (Fig. F35A; Table T6). The short-term events on the C3n (Cochiti, Nunivak, Sidufjall, and Thvera) are very distinctive in this correlation. The top normal polarity interval observed above the Gilbert Chron, however, does not seem to match with the bottom of the Gauss Chron (C3n4n) when compared with biostratigraphic data. Instead, either it was remagnetized or must have been deposited after a hiatus in the late Pliocene, in which case it should match with the Olduvai (C2n) Subchron. Between 35 and 220 mbsf, the depth interval of the carbonate platform (see "Lithostratigraphy and Sedimentology"), the recovery was low and long core measurements did not give useful results. Despite a number of data gaps, between 225 and 375 mbsf, an attempt was made to correlate some of the observed magnetic polarity zones with the GPTS. For example, between 230 and 250 mbsf the normal-reversed-normal-reversed-normal (N-R-N-R-N) polarity sequence identified was tentatively assigned to Chron C5Cn (Fig. F35B). In addition, the relatively long normal polarity interval found between 340 and 365 mbsf is likely to represent the C6N Chron (Fig. F35C) (early Miocene). The sequence of reversals observed between 380 and 500 mbsf in Hole 1193A is difficult to match with the GPTS (Fig. F35D).

Rock Magnetism

Twelve samples from the depth interval 140-384 mbsf (in Hole 1193A) were thermally demagnetized through a stepwise heating to a temperature of 680C. Three components of magnetization have been identified. The first one, often removed by heating to 100C, is most likely related to recent remagnetization. The second and third components have an antiparallel direction (segments A and B on the Zijderveld diagrams of Fig. F37). The second component has a blocking temperature between 100 and 200C, whereas the third component starts to demagnetize between 600 and 680C (Fig. F37). The magnetic moment decay curve increases, and the magnetic vector does not intersect the origin in the Zijderveld diagram (Fig. F37) for elevated temperatures. To identify the magnetic minerals carrying this remanence, ARM, IRM and IRM thermal demagnetizations were carried out on an unheated, new set of 16 selected samples in two batches. The first batch was collected from Hole 1193A, corresponding to the same interval as the one above, and the second batch was collected from 35 to 74 mbsf in Hole 1193B (four samples) and Hole 1193C (four samples).

ARM experiments performed on these samples showed straightforward demagnetization results for most samples analyzed (Fig. F38A). However, some samples showed a slope break in the magnetic moment decay curve (Fig. F38B), which is well-reflected on the Zijderveld plot as opposing components. The IRM acquisition curve (Fig. F39) demonstrates a rapid initial increase and a gradual acquisition at stronger fields (above 300 mT), indicating the presence of both low coercive (soft) and high coercive (harder) material.

After thermal demagnetization of acquired IRM, samples of the first batch showed two components with antiparallel directions (segments A and B on the Zijderveld diagrams in Fig. F40), indicating two different Curie temperatures (normalized magnetic moment decay curve in Fig. F40). The first component, characterized by a low blocking temperature of ~150 to 200C, might be due to goethite, whereas the second component has a Curie temperature of ~600C, indicating that it might be due to titanomagnetite or magnetite. The antiparallel nature of the two components that were magnetized in the same laboratory magnetic field can be best explained by a self reversal of the low Curie temperature mineral (goethite). Other samples showed two phases having similar directions: one Curie temperature of ~350C, suggestive of pyrrhotite, and a gradual decay to zero at ~350-680C suggests the presence of titanomagnetite and small amounts of hematite (Fig. F41).

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