PALEOMAGNETISM

Paleomagnetic studies conducted on the JOIDES Resolution during Leg 181 consisted of remanent magnetization measurements of archive-half sections before and after alternating field (AF) demagnetization, magnetic remanence measurements on discrete samples collected from the working half of core sections, magnetic susceptibility measurements on whole-core sections, and a limited set of rock-magnetic measurements on discrete samples. Discrete samples were collected from working halves in standard 8-cm3 plastic cubes with the arrow on the bottom of the sampling cube pointing up-core. The sampling frequency was generally two samples per core at one hole per site. Intervals with drilling-related core deformation were avoided.

Instruments and Measurement Procedure

Measurements of remanent magnetization were carried out using an automated pass-through cryogenic DC-SQUID (direct current superconducting quantum interference device) magnetometer (2-G Enterprises Model 760-R) with an in-line alternating field (AF) demagnetizer (2-G Enterprises Model 2G600), capable of producing peak fields of 80 mT with a 200-Hz frequency. The background noise level of the magnetometer on-board environment is ~3 x 10-10 Am2. The large volume of core material within the sensing region of the magnetometer, which is on the order of 100 cm3, permits the accurate measurements of cores with remanent intensities as weak as ~10-5 A/m.

The standard ODP magnetic coordinate system was used (+X: vertical upward from the split surface of archive halves, +Y: left along split surface when looking upcore, +Z: downcore).

Natural remanent magnetization (NRM) was routinely measured for all archive-half sections at 5- to 10-cm intervals. Measurements at core and section ends and within intervals of drilling-related core deformation were edited during data processing. AF demagnetizations were applied at 10 and 20 mT when time permitted; otherwise, cores were only demagnetized at a single AF step at 20 mT. All discrete sample measurements were also made on the pass-through magnetometer using a sample boat that held seven discrete cube samples at 20-cm intervals.

Discrete samples were demagnetized by AF using the in-line demagnetizer installed on the pass-through cryogenic magnetometer at 5-mT steps to 30 mT and then at 10-mT steps to 60 or 80 mT, depending on remaining sample intensity. An Analytical Services Company model IM-10 impulse magnetizer, which can apply pulsed fields from 20 to 1200 mT, was used for studies of the acquisition of stepwise isothermal remanent magnetization (IRM), saturation IRM (SIRM), and backfield SIRM of selected discrete samples. When time permitted, the SIRM of discrete samples was thermally demagnetized using a Schonstedt Model TSD-1 Thermal Specimen Demagnetizer at 40 C or 50 C steps to 640 C. However, heating was stopped if sample intensity became too weak or if routine magnetic susceptibility indicated alteration of the magnetic mineralogy from heating. Spurious fields inside the oven do not exceed 100 nT and are generally <5 nT inside the cooling chamber. Temperature gradients over the central 30.5-cm length of the oven are ~10 C, with an absolute temperature accuracy within 20 C of the set value.

Magnetic susceptibility was measured for each whole-core section as part of the MST analysis (see "Physical Properties"). The MST susceptibility meter (a Bartington MS2 meter containing an MS2C sensor with a coil diameter of 88 mm and an inducing-field frequency of 0.565 kHz) was set on SI units and the values were stored in the JANUS database in raw meter units. To convert to true SI volume susceptibilities, these values should be multiplied by 10-5 and then multiplied by a correction factor to take into account the volume of material that passed through the susceptibility coils. The correction factor for a standard ODP core is about 0.66 (=1/1.5). This correction was applied for all figures illustrating magnetic susceptibilities in the "Paleomagnetism" sections in this report. The end effect of each section was not corrected.

Core Orientation

Core orientation of the advanced hydraulic piston cores was achieved with a Tensor tool mounted on the core barrel. The Tensor tool consists of a three-component fluxgate magnetometer and a three-component accelerometer rigidly attached to the core barrel. The information from both sets of sensors allows the azimuth and dip of the hole to be measured as well as the azimuth of the double-line orientation mark on the core liner. Orientation is not usually attempted for the top three cores (about 30 mbsf) until the bottom-hole assembly is sufficiently stabilized in the sediment. Core orientation by the Tensor tool was relative to magnetic north.

Magnetostratigraphy

Where AF demagnetization successfully isolated the primary component of remanent magnetization, paleomagnetic inclinations were used to assign a magnetic polarity to the stratigraphic column. Interpretations of the magnetic polarity stratigraphy, with constraints from the biostratigraphic data, are presented in the site chapters. The magnetic polarity time scale of Cande and Kent (1995) and the time scale of Berggren et al. (1995b) were used.

We encountered several types of secondary magnetization acquired during coring, which sometimes hampered magnetostratigraphic interpretation. Details of the magnetic overprints are presented in the site chapters.

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