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doi:10.2204/iodp.proc.340.102.2013

Paleomagnetism

Samples, instruments, and measurements

Paleomagnetic investigations during Expedition 340 were focused on measurement of the NRM of archive-half sections before and after alternating field demagnetization. NRM measurements of entire split archive-half sections were made using a 2G Enterprises model 760R SRM optimized for IODP section halves (15.59 cm² cross-sectional area), equipped with superconducting quantum interference devices (SQUIDs) and an in-line automated alternating field demagnetizer capable of reaching a peak field of 80 mT. The SRM generates measurements of magnetic inclination, declination, and intensity for each measurement interval.

With x-, y-, and z-axis pickup coil response functions of 6.1, 6.2, and 9.9 cm, respectively, the measured area at each programmed interval is integrated over ~100 cm³ (Richter et al., 2007). Measurement of an empty tray followed by correction for background drift allows for estimation of the ambient noise level at ~2 × 10^–6 A/m. Thus, for split-core samples, the minimum measurable intensities of the SRM are around one order of magnitude greater, or ~2 × 10^–5 A/m.

NRM was measured continuously at 2.5 cm intervals along the archive-half sections. This spacing was agreed on by the physical properties specialists and those describing the cores, ensuring coeval depth data were available for multiple properties. NRM measurements were also made over a 15 cm long interval before and after each core section. These measurements, referred to as header and trailer data, serve the dual function of monitoring the background magnetic moment and allowing for deconvolution of the response function of the pickup coils. Typically, NRM was measured before and after AF demagnetization in a peak field of 20 mT, with an additional step at 10 mT (and sometimes 5 mT and 15 mT) when time and core flow allowed. With no onboard deconvolution, we removed measurements from the first and last 10 cm of each section before incorporation into figures, as these regions are most affected by volumetric edge effects associated with the response function of the instrument.

To provide a check on the SRM data and enable more detailed magnetic investigations, we sampled the working-half core sections with 8 cm³ plastic cubes. Taken from the center of the working half, these discrete samples are potentially less affected by several sources of magnetic overprint (Fuller et al., 2006; Richter et al., 2007). We measured the NRM of these samples on an Agico (JR-6A) dual-speed spinner magnetometer in six positions (sensitivity = ~2 × 10^–6 A/m) and AF demagnetized them in fields of 5, 10, 15, and 20 mT using an ASC Scientific D-2000 AF demagnetizer. For some samples we demagnetized samples in fields as high as 60 mT in 5–10 mT steps above 20 mT, and in Hole U1398B we demagnetized three samples in a peak field of 100 mT. These three samples were also imparted with an anhysteretic remanent magnetization (ARM) (100 mT AF in a 0.05 mT direct current bias field) using the D-2000 demagnetizer, and an isothermal remanent magnetization (IRM) using an ASC Scientific IM-10 Impulse magnetizer in fields of 20, 100, 300 and 1000 mT. These induced magnetizations were used to create ratios to estimate magnetic grain size and magnetic mineralogy independent of the concentration of magnetic grains. Magnetic intensity and directional data for each discrete sample were interpreted through Zijderfeld diagrams and normalized demagnetization data to assess sediment coercivity and evidence for magnetic overprinting.

We did not demagnetize any split archive-half sections in fields >20 mT. This decision was taken with a view to preserve some of the NRM for postcruise analysis, where time allows for higher resolving measurements. A field of 20 mT was deemed sufficient to remove the drilling overprint when cores were retrieved using nonmagnetic core barrels.

Coring and core orientation

The standard IODP paleomagnetic coordinate system was used (Fig. F10; Richter et al., 2007). In this system, +x is perpendicular to the split core surface and upward out of the working half, +z is downcore, and +y is orthogonal to x and z in a righthand sense (i.e., left along the split-core surface when looking upcore at the archive half). Discrete samples were taken from the working half and oriented upcore (thus +z is downcore, +x is into the working half, and +y is to the right looking upcore); rotating these cubes through 180° around the z-axis ensured the coordinates were directly comparable to the archive half.

The FlexIt orientation tool can be deployed with APC cores using nonmagnetic barrels. The FlexIt tool uses three orthogonally mounted fluxgate magnetometers to record the declination, inclination, and total magnetic field of the core barrel relative to magnetic north immediately prior to firing. This data is in reference to the double lines scribed on the core liner; thus the extruded core can be azimuthally aligned with respect to magnetic north. FlexIt data was acquired for all nonmagnetic barrel APC cores from Hole A and often acquired for Holes B and C. Site figures show uncorrected declination (gray) and, where available, true north–corrected declination (with FlexIt correction applied; red). For APC cores without Flexit data we use the discrete inclination data (SRM data from cores recovered using standard barrels often suffer a strong overprint) to determine polarity and guide declination rotation. These SRM sections can be rotated to a mean of true north for normal polarity intervals and rotated through 180° for periods of reversed polarity (orange data in figures). Because core rotation can occur during XCB coring, declination can suffer rifling through the entire core length. This effect is neither measurable nor easily predictable; thus, for all cores recovered with the XCB coring system declination remains uncorrected.

Magnetostratigraphy

Magnetostratigraphic interpretations of the paleomagnetic record will be restricted to intervals dominated by undisturbed hemipelagic sediment as these sequences are most likely to preserve reliable paleo-geomagnetic information. Before interpretation for magnetostratigraphy, data were first assessed to determine if they could be used to interpret the behavior of the geomagnetic field. These criteria include assessment of the degree of magnetic overprinting through AF demagnetization of discrete samples, comparison of site inclination and declination data to those predicted by assuming a geocentric axial dipole (GAD), and replication of data between holes. For sites cored during Expedition 340, GAD inclination ranges from ± 31.0° (Site U1393) to ± 27.0° (Site U1398); the declination of all sites from true north is approximately –14°W. Data that satisfied these criteria were available to be used to create reversal based stratigraphies through comparison to a GPTS. We chose to employ the GPTS of Cande and Kent (1995) after discussions with biostratigraphers (Table T3). Polarity reversals are identified in our data as regions where inclination transitions between positive and negative values predicted by GAD, accompanied by a 180° shift in declination. The magnetostratigraphic ages for each site are constructed by correlating these transitions with their equivalent well-dated horizon in the GPTS. Reversals are not unique and transition from either normal to reversed polarity (or vice versa), thus a continuous record is often required to make a confident interpretation of magnetostratigraphy. Where this is not possible, biostratigraphic datums can be used in tandem with the paleomagnetic results to constrain and significantly limit the range of possible correlations with the GPTS. This proved critical for Expedition 340, as continuous records were not always obtainable.

Comparable depth scales

For Sites U1395 and U1396, paleomagnetic directions from different holes are compared on a common depth scale. This first-order common depth scale was created in the program Analyseries, primarily using magnetic susceptibility measured every 2.5 cm using a 80 mm Bartington MS2C loop. The least complete record is usually transferred to the hole with the most complete recovery. Although this cannot be considered a meters composite depth scale, it does allow measurements to be transferred between holes and construction of a preliminary site record. For a more complete explanation of this procedure, see “Physical properties.”

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