Paleomagnetic studies performed aboard the JOIDES Resolution during Leg 149 included routine measurements of natural remanent magnetization (NRM) and of magnetic susceptibility of sedimentary, volcanic, and basement material. The measurement of magnetic remanence generally was accompanied by alternating field (AF) or thermal demagnetization to remove secondary magnetization. Details of the laboratory instruments and configurations are given in the "Explanatory Notes" chapter for Leg 145 (Shipboard Scientific Party, 1993c).
Remanence measurements of sediments and rocks were performed by passing continuous archive-half core sections through the cryogenic magnetometer. The ODP core orientation scheme arbitrarily designates the positive X-axis direction as the in-situ horizontal direction from the center of the core to the median line between a pair of lines scribed lengthwise on the working half of each core liner. The maximum AF demagnetizing field allowed by the Information Handling Panel for archive-half sections is 15 mT or the median destructive field, whichever is lower. In some cores, this field is insufficient to remove secondary remanence and isolate the primary component of remanence required for magnetostratigraphy and leg objectives. Therefore, at least one discrete shipboard paleomagnetic sample was taken from each section and from each representative lithology. These samples were demagnetized using either the Schonstedt GSD-1 AF demagnetizer or the Schonstedt thermal demagnetizer.
Discrete samples were taken from soft sediment using oriented standard plastic boxes (6 cm3). To reduce the deformation of the sediment, the core was cut using a thin stainless steel spatula before pressing the plastic sampling boxes into the sediment. Minicores were drilled from lithified sedimentary rocks and igneous rocks using a water-cooled nonmagnetic drill bit attached to the standard drill press.
Whole-core susceptibility measurements are relatively rapid to make, are nondestructive, and provide an indication of the amount of magnetizable material in the sediment, including ferrimagnetic and paramagnetic constituents. Whole-core volume magnetic susceptibility was measured using the automated MST. Measurements were performed usually every 5 cm at the low sensitivity range (1.0) and in the SI mode. The susceptibility response is a function of the mineralogy as well as the shape and volume of the magnetic particles within the rocks. Because magnetic susceptibility is a temperature-dependent property, the cores were permitted to equilibrate thermally (2-4 hr) prior to measurement. The general trend of the susceptibility curve was used to characterize both the magnetic material in the sediment cores, as well as subtle environmental and geologic changes within the sediments.
Core orientation of the advanced hydraulic piston (APC) cores was achieved with an Eastman-Whipstock multishot tool and the new Tensor multishot tool, both of which are rigidly mounted on a non-magnetic sinker bar. The Eastman-Whipstock tool consists of a magnetic compass and a small camera. The battery-operated camera takes photographs at prescribed intervals from 0.5 to 2 min from the time it leaves the deck. At the bottom of the hole, the core barrel is allowed to rest for sufficient time (2-8 min) to permit the compass needle to settle and to make sure that several photographs are taken before the corer is shot into the sediment.
The Tensor tool consists of three mutually perpendicular magnetic sensors and two perpendicular gimbals. 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 orientation line on the core liner.
Where magnetic cleaning successfully isolates the primary component of remanence, paleomagnetic inclinations are used to assign a magnetic polarity to the stratigraphic column. With the assistance of biostratigraphic data, we attempted an interpretation of the magnetic polarity stratigraphy in the site chapters. During Leg 149, we adhered to the chronostratigraphic nomenclature and geochronology of Harland et al. (1990; see Table 3 and Fig. 5, Fig. 6, Fig. 7). For the Neogene, we used the revised ages of geomagnetic reversal boundaries taken from the new magnetic polarity time scale of Cande and Kent (1992).