PALEOMAGNETISM

The primary objective of the shipboard paleomagnetic study was to produce a detailed polarity stratigraphy whenever possible. At selected sites, attempts were also made to characterize the magnetic mineralogy and magnetic properties of the sediments and to make preliminary estimates of relative paleointensity. These objectives were achieved through measurements of natural remanent magnetization (NRM) and alternating-field (AF) demagnetization of all archive halves, as well as through more detailed measurements of pilot discrete samples taken from the working half.

The conventional right-handed ODP coordinate system was used throughout (+z = downcore; +x = vertical upward from the split surface of the archive half). Orientations of APC cores were recorded using the Tensor tool (Tensor Inc., Austin, Texas). The Tensor tool consists of a three-axis fluxgate magnetometer that records the orientation of the double lines scribed on the core liner with respect to magnetic north. The critical parameters for core orientation are the inclination angle (typically <2°) and the angle between magnetic north and the double line on the core liner, known as the magnetic toolface angle.

Instrumentation

Half-core measurements were made using the shipboard long-core cryogenic magnetometer (2G Enterprises model 760R) equipped with an in-line, automated AF demagnetizer. The pickup coils of the magnetometer have a width at half height of <10 cm for all three axes, although they may sense a magnetization over an interval of 30 cm. The background noise on the magnetometer was measured to be less than ~5 x 10–11 Am2 under normal conditions. The noise level increased by nearly three orders of magnitude during pipe trips and became highly periodic. This problem was especially pronounced on the y-axis, whereas the z-axis appeared nearly unaffected. These levels should not significantly affect most of the half-core measurements, which were made under normal conditions and typically had moments greater than 10–7 Am2. However, measurements of more weakly magnetized sediments with moments less than 5 x 10–8 Am2 could be affected by this noise.

Isothermal remanent magnetization (IRM) was imparted with an ASC Scientific model IM-10 impulse magnetizer. Whole-core measurements of low-field susceptibility were conducted on the MST system (see "Multisensor Track" in "Sampling" in "Physical Properties").

Measurements and Procedures

Measurements of remanent magnetization were carried out for all archive halves at 5-cm intervals with 10-cm-long headers and trailers. Time permitting, cores were also AF demagnetized at 10 and 15 mT to isolate the characteristic remanent magnetization. The low maximum peak demagnetization level ensured that the archive halves remained useful for shore-based studies. Sections that had clearly suffered from severe drilling-related disturbance were either not measured at all or were measured only at 15 mT. In an attempt to improve the quality of the data in the soft carbonate sediments, many of the cores from Hole 1266C were measured and demagnetized to 15 mT as whole rounds (see "Paleomagnetism" in the "Site 1266" chapter). This resulted in the demagnetization (up to 15 mT) of the working-half sections of these cores (Table T4). The working halves of several other sections were also demagnetized to 15 mT as part of an experiment to determine the source of bias in the declination data (Table T4).

A small set of discrete samples processed aboard the ship were AF demagnetized in steps. Following AF demagnetization, several samples were additionally imparted an anhysteretic remanent magnetization at 100 mT in a 50-µT bias field and were again stepwise AF demagnetized. An IRM was then progressively acquired, followed by a stepwise backfield IRM to determine the coercivity of remanence.

Drill String Overprint

A strong drilling overprint directed in the vertical down (+z) direction was observed at all sites, regardless of the use of a nonmagnetic core barrel. In most cases this overprint appears to be removed by a 10-mT demagnetization. In addition to the vertical overprint, a strong bias in the archive-half declination data toward 0° in the ODP coordinate system was also observed at all sites (Fig. F5A). This phenomenon has been previously observed in soft sediments and has been attributed to a radially directed drilling overprint (Fuller et al., 1998). This bias is not removed by demagnetization to 15 mT (Fig. F5A, F5C) and results in random distribution of declinations after correcting the core data with the directional information from the Tensor tool (Fig. F5B). The problem may be at least partially (though by no means fully) mitigated by the measurement of whole cores (Fig. F5C, F5D), which is consistent with a radial component of magnetization.

Sediment Core Surface Smoothing and Data Quality

The surface of the archive halves was routinely smoothed or scraped by the sedimentologists prior to measurement on the pass-through magnetometer. Questions were raised as to whether this smoothing process was contributing to the erratic or poorly defined inclination data observed at many sites. To determine whether the smoothing was affecting the magnetization of the soft-sediment split cores, several sections were measured before and after smoothing. It was determined that smoothing most likely does not affect the magnetization of the archive halves (see "Paleomagnetism" in the "Site 1265" chapter for test results and further discussion).

Magnetostratigraphy

Preliminary reversal age assignments were based on the half-core inclination data after the removal of a low-coercivity overprint. The top 50 cm of each core was routinely ignored for this purpose because of the possible presence of drill string contamination and coring deformation. Data within 5 cm of section ends were also ignored because of edge effects. The timescale used for age assignments was chosen to provide a common age model for both biostratigraphic and magnetostratigraphic observations. This scale combines the astronomically calibrated model of Lourens et al. (in press) to the Oligocene/Miocene (O/M) boundary, the astronomically tuned ages of H. Pälike et al. (pers. comm., 2003) from the O/M boundary to the base of Chron C11n, and the ages of Cande and Kent (1995) for older periods (Table T5).

We also give all chron ages as in Cande and Kent (1995). Magnetostratigraphic interpretations at many sites were hampered by poor data quality. In addition to the drill string overprint (see above), the poor data quality is attributed to unstable or weak NRM in the sediments, deformation caused by the drilling and/or core splitting procedures, and possibly spurious directional changes induced by abrupt intensity changes (Parker and Gee, 2002).

Preliminary Paleointensity Analyses

At selected sites, preliminary estimates of relative paleointensity were made by normalizing the half-core NRM data by susceptibility. This normalized depositional remanent magnetism record is intended to give a first-order approximation of relative paleointensity until a more thorough postcruise analysis can be conducted.

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