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doi:10.2204/iodp.proc.323.102.2011

Paleomagnetism

Samples, instruments, and measurements

Paleomagnetic investigations during Expedition 323 focused mainly on measuring the natural remanent magnetization (NRM) of archive-half sections before and after alternating-field (AF) demagnetization. All NRM measurements for archive-half sections 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 systems for the SRM and the archive and working halves are shown in Figure F9. A new software package was developed for the SRM during Expedition 320T and modified during Expeditions 320 and 321.

The SRM and other instruments used, tested, or available in the paleomagnetism laboratory during Expedition 323 are listed in Table T3. This table also includes quality assessment information (e.g., sensitivity, accuracy, and precision) of the instruments as determined by experimentation, past experience, or information provided by the instrument vendors (Richter et al., 2007). For example, tests conducted with the SRM at the beginning of Expeditions 320 and 321 indicate that the ambient noise level is <2 × 10^–10 A/m². This finding permits the measurement of split-core samples (where the volume of material measured is ~100 cm³) with intensities as low as ~2 × 10^–5 A/m².

NRM measurements were usually made every 2.5 cm along the split-core sections. Additionally, measurements were made over a 15 cm interval before and after the sample entered the superconducting quantum interference device (SQUID) sensors. These measurements are referred to as the header and trailer, and they serve the dual functions of monitoring background magnetic moment and allowing future deconvolution analysis. Typically, NRM was measured after 0, 10, and 20 mT AF demagnetization. Because the laboratory's core flow (the analysis of one core after the other) dictated the amount of time available for measurements (~2–3 h per core), there was not always sufficient time to perform the optimal number of demagnetization steps. During part of Expeditions 320 and 321, a 2.5 cm measurement spacing was used when extra time was available. This practice was also used during Expedition 323. Sections that were entirely visibly disturbed were not measured. Similarly, in analyzing data, measurements were culled within 5 cm of section ends and within intervals with drilling-related core disturbance (e.g., the top few to tens of centimeters of most cores).

Low-field magnetic susceptibility was measured on all whole-core sections using the WRMSL and the Special Task Multisensor Logger (STMSL) (see "Physical properties" and "Stratigraphic correlation"). The susceptibility meters are Bartington loop meters (model MS2 with an MS2C sensor with coil diameter of 88 mm); the WRMSL has an operating frequency of 0.621 kHz; the STMSL has an operating frequency of 0.513 kHz. The meters have a nominal resolution of 2 × 10^–6 SI (Blum, 1997). The "units" option for the meters was set to SI units, and the values were stored in the database in raw meter units. To convert to true SI volume susceptibilities, the recorded values should be multiplied by 0.68 × 10^–5.

Coring and core orientation

Cores were collected using nonmagnetic core barrels except at depths at which overpull was apparent during core recovery and the more expensive nonmagnetic barrel was endangered. In addition, the bottom-hole assembly (BHA) included a Monel (nonmagnetic) drill collar when the FlexIt core orientation tool was used.

The FlexIt orientation tool was used to orient all APC cores. This instrument uses three orthogonally mounted fluxgate magnetometers to record the orientation with respect to magnetic north of the double lines scribed on the core liner. The tool also has three orthogonally mounted accelerometers that monitor the movement of the drill assembly and help determine when the most stable, and thus most useful, core orientation data are gathered. Declination, inclination, and temperature are recorded internally at regular intervals until the tool's memory capacity is filled. When measurements are taken in intervals of 6 s, which is the interval used during Expedition 323, the tool can typically be run for ~24 h. However, we attempted to switch tools at least every 8–12 h. Three FlexIt tools (serial numbers 936, 937, and 938) were available.

The tool was set up following the standard operating procedure described in the IODP manual "Core Orientation Standard Operating Procedure" (20 January 2009, available from the IODP Cumulus database). Using this procedure, the instrument was synchronized to a personal computer running the FlexIt software (version 3.5), and the tool was inserted inside a pressure casing. The enclosed tool was then given to a core technician, who installed it on the sinker bars above the core barrel. The double lines on the core liner were aligned relative to the tool. Before firing the APC, the core barrel (along with the pipe and BHA) was held stationary for several minutes. During this time, data were recorded to constrain core orientation. When the APC fired, the core barrel was assumed to maintain the same orientation even though this and previous cruises have provided evidence that the core barrel can rotate and the core liner can twist as it penetrates the sediments. Generally, the core barrel was pulled out after a few minutes. However, for cores collected with the third-generation advanced piston corer temperature tool (APCT-3) (see "Downhole measurements"), the core barrel remained in the sediments for 10 or more minutes.

Because rotation between pieces of core occurs in all XCB cores collected, XCB cores were not reoriented. However, polarity can be estimated from inclination values, which are generally high in value and diagnostic.

Magnetostratigraphy

Magnetostratigraphies for each site were constructed by correlating obtained magnetic polarity sequences with the GPTS (Table T4). Biostratigraphic age constraints significantly limit the range of possible correlations with the GPTS.

The GPTS used for Expedition 323 is based on the Neogene timescale constructed by Lourens et al. (2004), who placed the Paleogene/Neogene boundary at 23.030 Ma based on the astronomically derived age for the base of Chron C6Cn.2n (Shackleton et al., 2000), which was later updated to the new astronomical solution of Laskar et al. (2004) by Pälike et al. (2006).

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