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doi:10.2204/iodp.proc.329.102.2011

Paleomagnetism

Samples, instruments, and measurements

Paleomagnetic investigations during Expedition 329 focused mainly on measuring the natural remanent magnetization (NRM) of archive-half sections before and after alternating-field (AF) demagnetization. We also collected one discrete sample per section from working-half sections for use in AF experiments. Discrete samples were taken from the first APC hole (because the cores from other holes were heavily sampled for microbiological and geochemical whole-round samples, precluding sequential discrete samples for paleomagnetic study) at every site and from the basaltic cores at sites where basement was recovered. We collected discrete sediment samples by inserting a hollow subcorer into the middle of split-core sections and then extruding the sediment into plastic cubes (2 cm × 2 cm × 2 cm, with an internal volume of 7 cm³) (Fig. F6) as described in Richter et al. (2007). For discrete basalt samples, we prepared cubic samples of the same size with a rock saw.

All remanence measurements were made using a 2G Enterprises Model-760R superconducting rock magnetometer (SRM) equipped with direct-current superconducting quantum interference devices (SQUIDs) and an in-line, automated AF demagnetizer capable of reaching a peak field of 80 mT. The coordinate system for the SRM is shown in Figure F7.

The SRM and other instruments in the Paleomagnetism Laboratory used during Expedition 329 are listed in Table T6. In the table, we also give quality assessment information (i.e., sensitivity, accuracy, and precision) of the instruments determined by experimentation or based on past experience and information provided by the vendors. For example, the noise level of the SRM is <1 × 10^–10 Am², based on tests conducted at the beginning of Expedition 329. For split-core samples, in which the volume of material measured is ~100 cm³, this permits samples with intensities as low as ~1 × 10^–5 A/m to be measured.

NRM was usually measured every 2.5 or 5 cm interval along the split-core sections. The measurement was also conducted over a 5 cm long interval before each core section entered the SQUID sensors, as well as after each core section had passed through the sensors. These are referred to as the header and trailer measurements; they serve the dual function of monitoring the background magnetic moment and allowing for future deconvolution analysis. Typically, we measured NRM after 0 and 20 mT AF demagnetization, with an additional step at 10 mT when time and core flow allowed. Because core flow (the analysis of one core after the other) through the laboratory dictated the available time for measurements, 2 or 3 h per core, we did not always have time for the optimal number of demagnetization steps. During Expedition 329, we were able to measure at 10 mT demagnetization and occasionally at 5 and 15 mT steps. These additional demagnetization steps did not prove to be as beneficial as using the extra time to measure the core sections at higher resolution following 20 mT demagnetization, so we opted to cease the additional demagnetization steps and increase the resolution when time permitted. Measurements at 2.5 cm spacing became standard during the expedition. We did not measure sections that were entirely visibly disturbed. Similarly, in analyzing the data, we culled measurements within 7.5 cm of section ends and within intervals with drilling-related core disturbance, such as the top few to tens of centimeters of most cores.

AF demagnetization of discrete samples was conducted at peak fields of 0, 10, and 20 mT, the same as the demagnetization sequence applied to half-core sections, in order to check whether radial-inward drilling-induced magnetizations that have occasionally been reported from previous ODP/IODP expeditions are present.

A suite of discrete samples distributed evenly downhole (typically one sample from each core) was subjected to progressive AF demagnetization and measured at 10 mT steps to peak fields of 20 mT, with basalt samples demagnetized up to 60 mT. This was done to determine whether a characteristic remanent magnetization could be resolved and, if so, what level of demagnetization was required to resolve it.

Low-field magnetic susceptibility was measured on all whole-core sections using the WRMSL (see “Physical properties”). The susceptibility meter was a Bartington loop meter (model MS2 with an MS2C sensor, coil diameter of 88 mm, and operating frequency of 0.621 kHz. The susceptibility meter has a nominal resolution of 2 × 10^–6 SI (Blum, 1997). The “units” option for the meter was set on SI units, and the values were stored in the database in raw meter units. To convert to true SI volume susceptibilities, these should be multiplied by 0.68 × 10^–5.

Coring and core orientation

Cores were collected using nonmagnetic core barrels, except at depths where hard chert or porcellanite layers were expected. In addition, the bottom-hole assembly included a Monel (nonmagnetic) drill collar, which was deployed when the Flexit core orientation tool was employed. The Flexit tool uses three orthogonally mounted fluxgate magnetometers to record the orientation of the double lines scribed on the core liner with respect to magnetic north. The tool also has three orthogonally mounted accelerometers to monitor the movement of the drill assembly and help determine when the most stable and thus useful core orientation data were gathered. The tool declination, inclination, total magnetic field, and temperature are recorded internally at a regular interval until the tool's memory capacity is filled (Fig. F8).

Magnetostratigraphy

Drill sites were located between ~24° and 46°S; typical magnetic polarity zones could be identified by distinct changes in inclination of remanence. The present-day normal field in this region, as expected by the geomagnetic axial dipole model, has a negative inclination (about –41° to –64°), so positive remanence inclinations indicate a reversed field. Magnetostratigraphies for each site were constructed by correlating obtained magnetic polarity sequences with the GPTS. Biostratigraphic age constraints significantly limit the range of possible correlations with the GPTS.

The GPTS used during Expedition 329 is based on a composite of several timescales (Table T7) (Cande and Kent, 1995; Lourens et al., 2004; Pälike et al., 2006a). Its construction follows the procedure described by Backman et al. (2008) and is excepted here.

Global Cenozoic timescales are still under development. Orbitally tuned cyclostratigraphic data are the chronological backbone of the most recent Neogene timescale, including the Quaternary (Lourens et al., 2004). Their synthesis is considered to fairly well reflect the true progress of Neogene time. The Paleogene timescale, on the other hand, is less well defined because of the lack of a continuous Milankovitch-based Paleogene cyclostratigraphy, and it will therefore continue to develop and change over some years to come. The GPTS used during Expedition 329 was the same as that used during Expedition 320/321 and was constructed as follows:

• Interval 0.00–23.030 Ma: the Lourens et al. (2004) Neogene timescale was used, which places the Paleogene/Neogene boundary at 23.030 Ma, on the basis of an astronomically derived age for the base of Chron C6n.2n (Shackleton et al., 2000), updated to the new astronomical solution of Laskar et al. (2004) by Pälike et al. (2006b). Pälike et al. (2006a) estimated an age of 23.026 Ma for this reversal boundary (i.e., 4 k.y. younger than the Lourens et al. [2004] estimate).

• Interval 23.278–41.510 Ma: the Pälike et al. (2006a) timescale was used from the top of Chron C6Cn.3n at 23.278 Ma to the base of Chron C19n at 41.510 Ma, implying that the 248 k.y. long Chron C6Cn.2r is artificially shortened by 4 k.y. (1.6%) when shifting from the Miocene to the Oligocene timescale.

• Interval 41.510–83.000 Ma: the Cande and Kent (1995) timescale was used from the top of Chron 20n to the top of Chron C34n, implying that the 1.026 m.y. long Chron C19r is artificially lengthened by 11 k.y. (1.1%) when shifting from the Pälike et al. (2006a) timescale to Cande and Kent’s (1995) timescale.

The impact of these two artificial timescale jumps (4 k.y. with the Pälike et al. [2006a] timescale and 11 k.y. with the Cande and Kent’s [1995] timescale) on the data and discussions presented here is negligible.

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