Paleomagnetic investigations conducted during Leg 190 consisted of measurements of NRM, remanent magnetization after alternating-field (AF) demagnetization of the archive half of core, thermal demagnetization of discrete samples, and acquisition of isothermal remanent magnetization (IRM) of discrete samples. One or two oriented discrete samples were routinely collected from each section of the working half of the core primarily for shore-based analysis of the anisotropy of magnetic susceptibility. For a detailed analysis of the paleomagnetic directions at the boundaries between normal and reversed polarities, paleomagnetic measurements of selected discrete samples were also conducted. The results of these measurements were used to study the magnetostratigraphy and to investigate the magnetic mineralogy of the recovered sections.
To understand core disturbance during APC coring, we conducted an experiment to compare the NRM of whole-round, working-half, and archive-half cores.
A 2-G Enterprises pass-through cryogenic magnetometer (Model 760R) equipped with direct current superconducting quantum interference devices was used to make the majority of paleomagnetic measurements during Leg 190. This magnetometer is equipped with an in-line AF demagnetizer (2-G Model 2G600) that allows for demagnetization of samples up to 80 mT. The magnetometer and AF demagnetizer are interfaced to a PC-compatible computer and are controlled by the 2-G Long Core software program by National Instruments. A Molspin spinner magnetometer is also available on the ship. Additional instruments used for the demagnetization of samples include a Schonstedt AF demagnetizer (Model GSD-1) capable of demagnetization up to 100 mT and a Schonstedt thermal demagnetizer (Model TSD-1) capable of demagnetization up to 700°C.
Magnetic susceptibility was measured for all whole-core sections with the MST (see "Physical Properties"). The susceptibility values are stored in the Janus database as raw data in units of 10-5 SI. The true susceptibility volumes can be obtained by multiplying the raw data by a correction factor. The standard correction factor for ODP cores is ~0.67 (= 1/1.5).
To investigate the magnetic mineralogy of discrete samples, IRM experiments were conducted using an Analysis Services Company impulse magnetizer (Model IM-10) capable of applying magnetic fields from 0.02 to 1.35 T.
ODP core orientation designates the positive x-axis direction as the horizontal direction ("geomagnetic north" in a global coordinate reference frame) from the center of the core to the median line between a pair of lines inscribed lengthwise on the working half of each core liner (Fig. F9). Continuous NRM and remanence measurements were made on the archive half of each core using the pass-through cryogenic magnetometer. Remanence measurements were usually made at demagnetization steps of 10, 20, and 30 mT at intervals of 5 to 10 cm.
Paleomagnetic measurements of selected discrete samples were also conducted to gain a better understanding of magnetic polarity boundaries observed in the archive-half cores. Samples were usually demagnetized from 5 to 50 mT, using 5-mT steps. If high magnetic intensity was still observed after the demagnetization of 50 mT, the maximum demagnetization level was increased to 80 mT.
To investigate the saturation of isothermal remanent magnetization (sIRM) of sediments, 12 IRM steps from 50 to 1300 mT were applied to selected discrete samples. Each sample was then demagnetized at 60 mT after the sIRM measurements. For detailed investigation of magnetic carrier in a sediment, demagnetization of multicomponent isothermal remanent magnetization (mIRM) (Lowrie, 1990) was conducted. For these experiments, orthogonally applied fields of 1.0, 0.3, and 0.1 T were used to generate the IRM components. The samples were then demagnetized using 15 thermal steps from 50° to 650°C.
One to two discrete oriented samples were acquired from each section of the working half of the core. Paleomagnetic cubes (8 cm3) were used for removal of soft sediments, whereas minicores were cut from sections of sedimentary and crystalline rock.
Core orientation of APC cores was achieved with a Tensor tool mounted on the core barrel. Inside the Tensor tool are a three-component fluxgate magnetometer and a three-component accelerometer that record hole inclination (drift), azimuth, and magnetic tool-face readings. For the upper part of the sediments (~30 m), orientation is not usually attempted until the bottom-hole assembly is sufficiently stabilized in the sediment.
The Cande and Kent (1995) revised magnetic polarity time scale was used when interpreting magnetic polarity records observed in the core (Fig. F10).