Paleomagnetic studies conducted on the JOIDES Resolution during Leg 184 consisted of remanent magnetization measurements of archive-half sections before and after alternating field (AF) demagnetization and measurement of discrete samples obtained from working-half sections.
Measurements of remanent magnetization were carried out using an automated pass-through cryogenic direct-current superconducting quantum interference device magnetometer (2-G Enterprises Model 760-R) with an in-line 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 onboard environment is ~3 × 10-10 Am2. The large volume of core material within the sensing region of the magnetometer, 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).
The natural remanent magnetization before and after AF demagnetization was routinely measured for all archive-half sections at 4- to 8-cm intervals. Corrections for the end-effect were not applied. Cores from the first hole of each site were generally AF demagnetized at 10 and 20 mT. Other cores were demagnetized by AF at 10, 15, and/or 20 mT, depending on core flow. Discrete samples were demagnetized by AF at five steps: 10, 20, 30, 40, and 50 mT. Seven discrete cube samples were placed on the sample boat at a time, spaced at 20-cm intervals. As many discrete samples as possible were measured given the time limits imposed by core flow. The samples were then divided into two groups and brought back to the Institute for Rock Magnetism and to the Laboratoire des Sciences du Climat et de l'Envirronement for complete paleomagnetic analysis and additional rock and environmental magnetic analysis.
Orientation of the APC 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 (~30 mbsf) until the bottom-hole assembly (BHA) is sufficiently stabilized in the sediment. Core orientation by the Tensor tool was relative to magnetic north.
Oriented discrete samples were taken from the working half of each section (one or two per section) using 8-cm3 (2 cm × 2 cm × 2 cm = 8 cm3) plastic cubes and a stainless-steel sampler. The cube fits standard sample holders of paleomagnetic and rock-magnetic instruments. The cubes were capped to prevent dehydration. Orientation arrows for the samples were marked on the bottom of the cube.
Where AF demagnetization successfully isolated the primary component of remanent magnetization, paleomagnetic inclinations and declinations relative to magnetic north derived from the Tensor tool 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 time scale of Berggren et al. (1995) was 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.