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doi:10.2204/iodp.proc.317.102.2011 PaleomagnetismNatural remanent magnetizationNatural remanent magnetization (NRM) was routinely measured on archive section halves using a 2G Enterprises superconducting rock magnetometer (SRM; model 760R) before and after alternating-field (AF) demagnetization. The magnetometer includes three orthogonal direct-current superconducting quantum interference devices (DC SQUIDs) and is equipped with an in-line AF demagnetizer (2G Enterprises model 2G600) capable of producing peak fields of 80 mT at 200 Hz frequency. The magnetometer was run and data were acquired by a program called SRM (version 3.23), which was still in development. Two separate modules were used for continuous (section length) and discrete measurements, both reporting measurements in SI units. Previous expeditions had demonstrated that data could be acquired effectively using this software, but not all functions were available and persistent bugs slowed core flow. Testing during transit revealed that the module for discrete samples was particularly faulty and crashed often, in most instances because of communication problems with the degausser unit. The ramp up light on the front panel of the degausser unit sometimes did not light, which indicated that the magnetic field along the y-direction was not applied despite a normal display on the main personal computer (PC) screen. As a result, the discrete software module was not used during Expedition 317. Instead, discrete samples were measured on the SRM using the continuous software module. This software assumes a semicylindrical sample with an elongate z-axis when calculating magnetizations. Inclination, declination, and intensity must therefore be recalculated from the reported magnetic moments for discrete (cube) samples. IODP orientation conventions were applied to the archive halves (+x: vertically upward; +y: horizontally to the right when looking downcore; and +z: downcore). Core sections were measured at 5 cm intervals. In addition, NRM measurements were made at 5 cm intervals over 15 cm before the sample entered the SQUID sensors and again after the sample passed through. These measurements are referred to as header and trailer measurements, and they serve the dual functions of monitoring the background magnetic moment and allowing for future deconvolution analysis. The noise levels of the SRM, established from empty core tray measurements taken both during transit and during drilling operations, were ~4 × 10–10 A/m2 for the three SQUID sensors (the z-axis usually being the noisiest). For discrete samples with volumes of 6–10 cm3, this noise level equates to an intensity of 4 × 10–5 A/m. For split core samples with effective volumes of ~100 cm3, the tray noise level corresponds to an intensity of 4 × 10–6 A/m. In fact, the background resolution limit on remanence measurements of core samples in the SRM system was dictated by the magnetization of the core liner. Although measurements of an unused, empty half liner had magnetizations on the order of 10–5 A/m (the same magnitude as the weakest section-half measurements), a used, empty half liner from Site U1352 had peak intensities on the order of 10–4 A/m. The remanence of the empty, used liner had consistently steep positive inclinations and declinations clustering to the north in the archive coordinate system. Measurements from several previous cruises indicate that accurate measurements are likely to be obtained only when split-core and discrete samples have intensities greater than 10–4 A/m and 10–3 A/m, respectively (Richter et al., 2007). Two available calibration standards (an 8 cm3 cube standard with an intensity of 7.62 A/m and an 11.4 cm3 cylinder standard with an intensity of 5.98 A/m) were also measured during transit to the first site. Measured intensities were within 0.5 A/m of reported values. Flux jumps from the SQUIDs were sometimes encountered during the measurement of empty boats and of sections from Site U1354 but were seldom encountered during the measurement of sections from other sites. When a flux jump occurred, the measurement was repeated. Background tray measurements were taken when time allowed, usually once per shift. Demagnetization of section-half NRMThe remanence of each section half was measured again after AF demagnetization was applied (using the 2G Enterprises AF demagnetization coils inline with the SRM). AF demagnetization was applied to remove both natural and drilling-induced viscous remanent magnetization overprints (see Richter et al., 2007). These overprints were observed as steep positive inclinations with declinations grouped around north in the archive section-half coordinates. This direction is the same as that observed in the used, empty liner. When time permitted (Site U1351), 10 mT and 20 mT demagnetization steps were applied and measured in turn. When time was short (Site U1352), only the 20 mT step was applied. Because only ~90 min was available to completely process each core, time was usually short and measurement spacing was altered to maintain core flow. During the preceding Expedition 324, concerns that the demagnetizing coil on the SRM was malfunctioning at fields >20 mT were reported. No action was taken before the start of Expedition 317 because the nature and cause of the problems could not be determined. During transit to the first Expedition 317 site, an isothermal remanent magnetization (IRM) was imparted on a suite of discrete samples that were then demagnetized using the DTECH Model D-2000 demagnetizer. This process was then repeated using the demagnetizing coils inline with the SRM. Note that the discrete specimens were demagnetized and measured using the SRM continuous core module software rather than the problematic discrete module software. No significant change in demagnetization behavior was observed up to 80 mT. No problems with the SRM demagnetizing coils were detected during Expedition 317. It may be that the apparent degausser coil malfunction reported during Expedition 324 (samples behaving as if they were acquiring an anhysteretic remanent magnetization during the course of AF demagnetization) arose from an unnoticed software communication problem with the degausser unit that led to an unsuccessful demagnetization protocol. Paleomagnetism and rock magnetism of discrete samplesOriented discrete samples were acquired from each section for onshore analyses. In lithified sediments, 8 cm3 cubes were cut with a rock saw, and an upward arrow was drawn on the split face of the working half. In soft sediments, samples were taken using the new IODP standard plastic boxes ("French cubes") having external dimensions of 2.2 cm × 2.2 cm × 2.3 cm (internal volume = ~6.9 cm3). These plastic cubes can be filled by pressing them directly into the split face of the working half of the core. Alternatively, an extruder can be used to extract sediment from the core and extrude it into the cube. These techniques, as described in Richter et al. (2007), result in azimuthally opposite orientations of the recovered cubes. For most soft-sediment cubes, we used a modified version of the extruder technique, which gave the same orientation as that given by pressing the cubes directly into the sediment. A long, open-topped extruder was used to cut through the sediment to the core liner. The column of sediment was then pushed from the bottom of the extruder out of the open top and into the cube. The use of the extruder resulted in a clean cut into the core, which made samples easy to extract. When sediment was very sticky, it was easier to press the cubes directly into the cut face of the core. By using the modified extruder technique most of the time, cubes could still be pressed into the sediment or cut with a rock saw when necessary and the orientation conventions of all cubes remained the same. In all cases, samples were extracted from the middle of the working half of the core with the arrow on the face of the cube pointing upcore, giving an orientation convention of +x (vertically downward normal to the face marked with the arrow), +y (horizontally to the right when looking along the arrow), and –z (along the arrow). A pair of samples (one immediately above the other) was recovered from each core to compare demagnetization behavior. When time allowed, sets of discrete samples were measured in the SRM using foam blocks taped onto the core section tray at 15 cm intervals as sample holders. Paired, discrete samples were taken to allow for a fuller demagnetization of these samples in order to determine the complexity of the routinely measured NRM. One sample from each pair was demagnetized using AF demagnetization at 5 mT intervals up to 80 mT (the maximum demagnetization level of the SRM). Thermal demagnetization of the second sample was performed in the Schonstedt TSD-1 Thermal Demagnetizer. Richter et al. (2007) recommend that plastic cubes not be heated beyond 200°C; however, we found that cubes deformed at 140°C, and sediment had to be extracted from the cube and placed in a tinfoil container during heating if samples were to be demagnetized at higher temperatures. After AF demagnetization, samples were used for rock magnetic experiments. Saturation IRM and coercivity of remanence were established by imparting a stepwise IRM up to 1 T using the ASC IM-10 impulse magnetizer. A backfield IRM was then imparted in the same manner, and magnetization was measured on the SRM after each step. An IRM was again imparted with a 1 T field, and this was stepwise demagnetized using alternating fields up to 80 mT on the SRM, allowing estimation of the mean destructive field of the samples. Magnetic susceptibilityThree methods of measuring magnetic susceptibility were available on board. The physical properties group routinely measured the low-field susceptibility of whole cores with an 88 mm Bartington model MS2C loop sensor (MSL) at 2.5 or 5 cm intervals as part of the Whole-Round Multisensor Logger (WRMSL) (see "Physical properties"). A Bartington model MS2F point sensor (MSP) was also available on the SHMSL. Routine measurements were made at 5 cm intervals. Selected intervals were measured at 1–2 cm intervals. Bulk magnetic susceptibility of discrete samples was measured using a Kappabridge KLY 4S susceptibility meter, which was also used to monitor susceptibility changes during the thermal demagnetization of discrete samples. Core orientationAPC cores were recovered using a nonmagnetic core barrel until overpull rendered this no longer feasible. Flexit orientation tools were used to orient APC cores when drilling conditions allowed. The Flexit tool uses three orthogonally mounted fluxgate magnetometers to record orientation with respect to magnetic north of the double lines scribed on the core liner. 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 most useful, core orientation data are gathered. Tool declination, inclination, total magnetic field, and temperature are recorded internally at regular intervals until the tool's memory capacity is filled. For a measurement interval of 6 s, the tool can typically be run for ~24 h, although we aimed to switch tools at least every 8–12 h. Three Flexit tools (serial numbers 936, 937, and 938) were available. The standard operating procedure involved synchronizing the instrument to a PC, running the Flexit software (version 3.5), and inserting the tool inside a pressure casing. The enclosed tool was then given to a core technician, who installed it on the sinker bars that reside above the core barrel. The double lines on the core liner were aligned relative to the tool. Prior to firing the APC, the core barrel was held stationary (along with the pipe and BHA) for several minutes while data to be used in constraining core orientation were recorded. When the APC fired, the core barrel was assumed to maintain the same orientation, although previous cruises have found evidence that the core barrel can rotate and/or the core liner can twist as it penetrates the sediments. Generally, the core barrel was pulled out after a few minutes, although it was left in the sediment for ~10 min for cores collected with the third-generation advanced piston corer temperature tool (APCT-3). Once processed, data from the Flexit tool provided the azimuthal orientation of the double line of the core barrel (north in IODP coordinate systems). The azimuthal orientation was computed as follows. The magnetic toolface angle provided by the Flexit tool was corrected to true north by adding the present-day deviation of magnetic north from geographic north at the site location, as provided by the International Geomagnetic Reference Field (IGRF) (+25° at Site U1351). This corrected azimuthal orientation was then used to orient the cores by adding this value to the declination measured with the SRM. MagnetostratigraphyWith drill sites at ~45°S, typical magnetic polarity zones could be identified by distinct changes in inclination of remanence. The present-day normal field in this region, as provided by the IGRF model, has a negative inclination (about –70°), so positive remanence inclinations indicate a reversed field. When Flexit core orientation tools were used with nonmagnetic core barrels during APC coring, declination information could also be used to establish magnetic polarity. The GPTS used during Expedition 317 is the same as that used during Expedition 320/321 and is constructed as follows.
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