Previous   |   Next

doi:10.2204/iodp.proc.301.105.2005

Paleomagnetism

The goals of paleomagnetic studies during Expedition 301 were twofold: (1) to determine magnetic polarities of cores for correlation with the geomagnetic polarity timescale (GPTS) and (2) to measure paleomagnetic directions for tectonic and polarity studies. Paleomagnetic studies during Expedition 301 were done using the shipboard pass-through cryogenic magnetometer to measure archive-half core sections and discrete samples from the working half of the core. Archive-half measurements were made for natural remanent magnetization (NRM) and remanent magnetization after alternating-field (AF) demagnetization. Whole-section measurements were made only on APC cores because of the internal rotation of core segments in most RCB cores. In effect, this limited archive-half core measurements to the sediment column. Discrete samples were studied from the APC and RCB cores. Such samples usually avoid core deformation that occurs at core edges and are therefore considered more reliable. In the RCB cores from igneous basement, these samples were the sole source of paleomagnetic information because the paleomagnetists did not wish to create spurious data by measuring highly fractured core pieces in the pass-through mode.

Paleomagnetic instruments

A 2G Enterprises pass-through cryogenic direct-current superconducting quantum interference device (SQUID) rock magnetometer (model 760R) was used to make paleomagnetic measurements during Expedition 301. This pass-through cryogenic magnetometer is equipped with an in-line AF demagnetizer (2G model 2G600) that allows for demagnetization of samples up to 80 mT. The magnetometer and AF demagnetizer are interfaced with a PC-compatible computer that is used to archive and analyze the collected data. Sensor coils of the cryogenic magnetometer measure a width of a little more than 30 cm, although ~85% of the remanence is sensed from a 20 cm width of a core section. A background resolution limit is imposed on measurement of rock remanence by the magnetization of the core liner itself, which is ~3 × 10^–5 A/m. In almost all cases, Expedition 301 samples had magnetization intensities well above the magnetometer limit.

Magnetic susceptibility of core sections was measured with two devices. Whole-core sections were measured on the whole-core MST. This apparatus includes a Bartington model MS2 meter with an 80 mm internal diameter MS2C sensor loop (88 mm coil diameter) operating at a frequency of 565 Hz and an alternating field of 80 A/m (0.1 mT). The specified sensitivity for the MS2 susceptibility meter is 10^–5 SI per 10 cm³ volume or 10^–8 SI per 10 g mass. A second Bartington susceptibility meter is included on the AMST. This meter uses a 15 mm diameter MS2F probe capable of making measurements of susceptibility at the core surface, giving a greater resolution than the loop sensor of the whole-core MST. The susceptibility sensitivity of the split-core meter is the same as the whole-core meter, and its specified horizontal resolution is 20 mm. Most of the probe sensitivity is within less than a diameter of the probe tip. Additional instruments in the paleomagnetic laboratory include a DTECH model D-2000 AF demagnetizer capable of demagnetization up to 200 mT and a Schonstedt thermal demagnetizer (model TSD-1) capable of demagnetization up to 700°C.

Paleomagnetic measurements

Standard ODP paleomagnetic measurement conventions were used for Expedition 301 paleomagnetic studies. The x-axis is positive upward (downward) from the split face of the archive (working) half of the core. The positive y-axis is left facing upcore along the split surface of the archive half, whereas the positive z-axis is downcore (Fig. F10).

Each APC core archive half was routinely measured at 5 cm intervals using the shipboard pass-through cryogenic magnetometer. NRM was measured initially, followed by magnetization after progressive AF demagnetization at 10 mT steps from 10 to 40 mT. Demagnetization was accomplished using the in-line AF demagnetization coils built into the cryogenic magnetometer. Discrete samples were taken at intervals dictated by core recovery and scientific interest. Sediment samples were taken in 7 cm³ plastic cubes pressed into the core split face, whereas igneous samples were collected as either 2.5 cm cubes (16 cm³ volume) cut with the rock saw or 2.5 cm diameter minicores (12 cm³ volume) drilled perpendicular to the core split face. Samples were usually either AF demagnetized at 5 mT intervals from 0 to 70 mT or thermally demagnetized at 50°C steps from 0° to 700°C. Magnetic susceptibility measurements were made on APC cores with the MST and the AMST at 3 cm intervals. The quality of these results degraded in XCB and RCB sections when the core was undersized and/or disturbed. Nevertheless, the general downhole trends were useful for stratigraphic correlations. The MS2 meter measures relative susceptibilities that have not been corrected for the differences between core and coil diameters. Susceptibility values were stored in the Janus database as raw data in units of 10^–5 SI. The true SI volume of susceptibilities should be multiplied by a correction factor to account for the volume of material that passed through the coils.

Geomagnetic polarity timescale

Magnetic polarity results were correlated to the polarity reversal sequence and absolute age using the Berggren et al. (1995) geochronology (Fig. F11). This timescale incorporates the widely used calibration of age and polarity intervals derived by Cande and Kent (1995).

Top of page   |   Previous   |   Next