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doi:10.2204/iodp.proc.309312.102.2006

Paleomagnetism

Paleomagnetic investigations during Expedition 309/312 consisted mainly of routine remanent magnetization measurements of archive-half sections and discrete samples from the working-half sections. Magnetization measurements were carried out before and after alternating-field demagnetization and, in some cases, after thermal demagnetization. Although it was reported after Leg 206 (Shipboard Scientific Party, 2003b) that most of the archive-half samples were strongly remagnetized because of a drilling-induced overprint, we measured some of the oriented specimens from the archive half. This was done to determine which portions of the recovered cores were not strongly influenced by the induced drilling overprint and to identify the possible portion suitable for future analyses.

Instruments and measurements

Remanent magnetization

Measurements of remanent magnetization were made using an automated pass-through cryogenic magnetometer with direct-current superconducting quantum interference devices (DC-SQUIDs) (2G Enterprises model 760-R). The magnetometer is equipped with an inline alternating-field demagnetizer (2G Enterprises model 2G600) capable of producing peak fields of 80 mT with 200 Hz frequency. The magnetometer is run and data are acquired by a program called LongCore (version 207.3) written by W.G. Mills (IODP) in LabView (version 6.1) programming language. This version of LongCore was last updated during ODP Leg 207. Key parameters used within the program, including calibration constants for the SQUIDs and coil response functions, are given in Table T15.

Natural remanent magnetization was routinely measured on most of the oriented pieces from the archive-half sections before demagnetization. Most of these sections were also progressively alternating-field demagnetized up to 40 mT. The remaining remanent magnetization, including orientation and intensity, was measured after each step.

Repeated tuning of the magnetometer is necessary, which often entails heating the SQUID coils to release trapped magnetic flux. If not constantly monitored and tuned, the additional magnetometer noise that results from measuring the strong intensity split-core samples can lead to erratic results. We did, however, experiment with several continuous pieces that had few or no fractures or gaps over their entire 1.5 m section length. For these sections and for the routine experiments, the sensor velocity on the magnetometer was set at 1 cm/s in order to avoid saturation of the magnetometer electronics that causes flux-jumps.

Discrete samples from working-half sections were also measured with the cryogenic magnetometer. Typically, samples were demagnetized in steps of 5 to 40–80 mT. A total of 12 samples during Expedition 309 and 8 samples during Expedition 312 from the working-half sections were also progressively demagnetized using a Schonstedt Thermal Demagnetizer (model TSD-1) in steps of 50°C from room temperature (typically 26°C) to 500°C and in steps of 25°C from 500° to 600°C. After each demagnetization step, samples were cooled down in a low–magnetic field environment (<10 nT) and the remaining magnetic intensity and orientation were measured in the SQUID magnetometer.

Calibration and instrument sensitivity

Even though results from the shipboard cryogenic magnetometer have been compared with many other laboratories and are shown to give consistent results, it is useful to check the calibration of the magnetometer against a known standard at the beginning of each expedition. We used a standard purchased from Geofyzika that is an 8 cm³ cube with an intensity of 7.62 A/m (moment = 6.096 × 10^–5 Am²). All three axes gave results that differ <2% from the standard certified results. In addition, automated tray positioning was checked by putting the standard at known positions and measuring the tray. The position indicated by the software was found to be accurate to better than ±1 cm, which is reasonable given the stretch in the pulley system used to move the sample boat.

Based on tests conducted during ODP Legs 186, 200, and 206, the background noise level of the magnetometer in the shipboard environment is ~2 × 10^–9 Am² (Shipboard Scientific Party, 2000, 2003a, 2003b). During Expedition 309, the background noise level was frequently measured on an empty split-core tray (also referred to as the sample boat). Results were similar in that the x-, y-, and z-axis moments measured on the sample boat before cleaning were less than ±2 × 10^–9 Am² (Fig. F12A). After cleaning the sample boat and demagnetization at 80 mT, the moments are all less than ±1 × 10^–9 Am² (Fig. F12B). These results include the drift correction, which only marginally changes the results (Fig. F12). During Expedition 309, tray corrections to the split-core and discrete samples were applied for every measurement sequence. The tray-corrected data are the measured magnetic moments for a sample minus those measured at the same position for the empty sample boat. The relative size of these values, however, should always be comparable to those shown in Figure F12 for the clean, empty sample boat. When the tray correction is applied to measurements made on a clean, empty sample boat, the moments drop to less than ±2 × 10^–10 Am² (Fig. F13).

To test the noise in the sample boat after cleaning and alternating-field demagnetization at 80 mT, the empty sample boat was measured every 5 cm in continuous mode. When measured in discrete mode and when tray corrected, four sample positions were used (20, 60, 100, and 140 cm tray slots) to ensure that samples were not influenced by the magnetizations of adjacent samples (Fig. F14).

The noise level of the magnetometer is sufficiently low that it is of minor significance for most samples measured. As was apparent when the sample boat was not clean, a small amount of dirt on the tray induces noise level in the 10^–9 Am² range. Noise related to dirt on the sample boat and to the magnetization will at least be in this range. We conclude that under favorable conditions the noise level will be approximately ±2 × 10^–9 Am². For discrete samples, which typically have volumes of 1 or 9 cm³, the minimum measurable remanent intensities are greater than ~10^–4 A/m.

Core orientation

The standard IODP paleomagnetic coordinate system was used. In this system, +x is perpendicular to the split core surface and into the working half, +z is downcore, and +y is orthogonal to x and z in a right-hand sense (i.e., it points left along the split-core surface when looking upcore at the archive half) (Fig. F15).

For hard rocks, it is possible to correlate images of the exterior of the core with images of the borehole wall from the Formation MicroScanner (FMS) logging tool (see “Downhole measurements”). To allow for this possibility, the outside surface of all cylindrical core pieces were scanned with the DMT CoreScanner (see “Digital imaging”). Postcruise analysis of FMS and digital images will allow reorientation of some distinctive pieces of core to true geographical north, and, by extension, absolute orientation data for magnetic declinations should be obtained.

Sampling methods and orientations for discrete samples

During both expeditions, oriented discrete samples were taken from the working halves of selected sections. Samples with ~2 cm × ~2 cm × ~2 cm exterior dimensions (volume = ~9 cm³) were typically collected per core for shipboard magnetic analysis and physical properties studies. In most of the intervals, we drew an arrow on the split-core face pointing upcore and used the rock saw to cut the sample. We also collected 1 cm × 1 cm × 1 cm (volume = 1 cm³) pieces for alternating-field and thermal demagnetization analyses. This was done in order to avoid the drilling-induced overprint, which has been reported to be stronger away from the center of the core. When measuring the samples (i.e., working half), we placed the side with the arrow down in the tray with the arrow pointing along the –z axis, or uphole, which makes the orientation the same as that of the archive half.

Magnetic overprints

Several types of secondary magnetization were acquired during coring, which sometimes hampered interpretation. The most common was a steep downward-pointing overprint attributed to the drill string, which was also observed in Leg 206 cores (Shipboard Scientific Party, 2003b). This was also seen as a bias for 0° declinations in archive-half sections, which has been shown during many previous cruises and has been interpreted as a radially inward overprint (Fig. F15).

Data reduction and analysis

During Expedition 312, characteristic remanent magnetization (ChRM) was estimated in two ways:

1. When only the highest demagnetization step was needed to remove the drilling overprint, results from the remanence measured after this single demagnetization step were interpreted as the best estimate of the ChRM. This technique was rarely used during Expedition 309.
2. When multiple demagnetization steps were measured at fields above that needed to remove the drilling overprint, the ChRM was estimated by principal component analysis (PCA) (Kirschvink, 1980) of three or more of the stable endpoint directions.

PCA analysis was conducted using a program that iteratively searches for the demagnetization steps that minimize the size of the maximum angular deviation (MAD). This angle is a measure of how well the vector demagnetization data fit a line, formally a quality index for the calculated ChRM. MAD values >15° were typically considered ill defined, and therefore such samples were rejected from the analysis (e.g., Butler, 1992). Because drilling overprint persisted beyond 15 mT demagnetization in most of the intervals, we only used results from 15 mT or higher in the PCA, and usually above 25 mT.

During some Expedition 312 alternating-field demagnetizations, a significant anhysteretic remanent magnetization was noted, especially as a tendency for multiple samples to converge toward steep positive inclinations. In some cases, we tried to correct this effect by repeating the demagnetization with the sample in a different orientation, generally rotated 180° about the x-axis. These rotated samples were distinguished in the database by adding 0.5 cm to the depth and were averaged offline.

Expedition 312 thermal demagnetizations were preceded by alternating-field demagnetization at 10 mT to partly remove the drilling overprint. Prior to measurement at each temperature step, the alternating-field demagnetization was repeated to remove the effects of laboratory fields. Because the LongCore software does not support labeling a demagnetization as both alternating-field and thermal, repeat measurements were taken with the alternating-field run labeled as such and the repeat run labeled as a thermal run with the temperature offset by 1°C from the oven temperature. Because the measurement immediately after alternating-field demagnetization should be least affected by exposure to laboratory fields, the label for this measurement was edited to indicate thermal demagnetization at the oven temperature. All measurements were saved for completeness, but temperatures at multiples of 5° are most meaningful.

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