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doi:10.2204/iodp.proc.320321.102.2010 PaleomagnetismSamples, instruments, and measurementsPaleomagnetic investigations during Expedition 320/321 focused mainly on measuring the natural remanent magnetization (NRM) of archive-half sections before and after alternating-field (AF) demagnetization. We typically collected one discrete sample per section from working-half sections for use in AF or thermal demagnetization experiments. Discrete samples were taken from the first deep hole cored at a site (typically Hole A). These samples were only taken from APC cores during Expedition 321 but were taken from both APC and XCB cores during Expedition 320. We collected them by inserting a hollow extruder into the middle of the split-core sections and then extruding the sediments into plastic cubes (2 cm × 2 cm × 2 cm, with an internal volume of 7 cm3) (Fig. F10) as described in Richter et al. (2007). 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 inline, 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 F11. A new software package was being developed for the SRM during Expedition 320T with alterations continuing during Expeditions 320 and 321, resulting in some evolution in data acquisition as the expeditions progressed. We continuously tested and assessed the software in order to minimize any impact that these changes might have on data quality. The principal disadvantages of the current software are the limitation of the demagnetization schedule that cannot, at this time, be used to enter more than one measurement or demagnetization step, and the numerous "Cancel" and "Continue" toggles that freeze the software. Once a well-trodden path is established through the software, measurements can be made with a minimum of surprises. The SRM and other instruments used, tested, or available in the paleomagnetism laboratory during Expedition 320/321 are listed in Table T7. In the table, we also give quality assessment information (e.g., sensitivity, accuracy, and precision) of the instruments determined by experimentation or based on past experience and information provided by the instrument vendors. For example, experimentation with the SRM indicates that the noise level is <2 × 10–10 Am2 based on tests conducted at the beginning of Expedition 320/321. For split core samples, where the effective volume of material measured is ~100 cm3, this permits samples with intensities as low as ~2 × 10–5 A/m to be measured. NRM measurements were usually made every 5 or 2.5 cm along the split-core sections as well as over a 15 cm long interval before the sample passed the center of the pick-up coils of the SQUID sensors and a 15 cm long interval after the samples had passed through it. These are referred to as the header and trailer measurements and serve the dual functions of monitoring the background magnetic moment and allowing for future deconvolution analysis. In a few selected intervals and when time allowed, we increased the measurement resolution to 1 cm. Typically, we measured NRM after 0 and 20 mT AF demagnetization. Because core flow (the analysis of one core after the other) through the laboratory dictates the available time for measurements, ~2–3 h per core, we did not always have time for the optimal number of demagnetization steps. During part of Expedition 320/321, we were able to measure a 10 mT demagnetization and occasionally even 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. The 2.5 cm measurement spacing became standard during Expedition 321. We also did not measure those sections that were entirely visibly disturbed. Similarly, in analyzing the data, we culled measurements within 5 cm (Expedition 320) or 7.5 cm (Expedition 321) of section ends and within intervals with drilling-related core disturbance, usually the top few to tens of centimeters of most cores. During Expedition 320, a suite of discrete samples distributed evenly downhole (typically one sample from each core) was subjected to progressive AF demagnetization and measured at 5 mT steps to a peak field of 40 mT and then 10 mT steps to 60 mT, with a few samples demagnetized up to 80 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. We also evaluated the NRM using thermal demagnetization on 13 samples from Site U1331. After measuring their NRM at room temperature, these samples were progressively demagnetized at 90°C and then from 150°C to 600°C at 50°C intervals. During Expedition 321, 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 the radial-inward drilling-induced magnetization that has occasionally been reported from previous ODP/IODP expeditions is present or not. Low-field magnetic susceptibility was measured on all whole-core sections using the WRMSL and the STMSL (see "Physical properties" and "Stratigraphic correlation and composite section"). The susceptibility meters are Bartington loop meters (model MS2 with an MS2C sensor, a coil diameter of 88 mm, and an operating frequency of 0.565 kHz). They have a nominal resolution of 2 × 10–6 SI (Blum, 1997). The "units" option for the meters 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 (Blum, 1997). We measured the bulk magnetic susceptibility of each discrete sample using a Kappabridge KLY-4S susceptibility meter, except for samples from Site U1338. During Expedition 320, because the plastic cube that holds the discrete sample is occasionally incompletely filled with sediments, we measured the mass of each sample and then estimated the true volume by using this mass and the bulk (wet) density of nearby samples, which are part of the moisture and density (MAD) data collected by the physical property scientists. Coring and core orientationCores were collected using nonmagnetic core barrels, except at depths where overpull was apparent during core recovery and the more expensive nonmagnetic barrel was endangered. In addition, the BHA included a Monel (nonmagnetic) drill collar when the FlexIt core orientation tool was used (Sites U1331 and U1332 of Expedition 320 and Sites U1337 and U1338 of Expedition 321). Because much of the drill pipe had not been used for several years, rust was a concern. An effort was made to clean the interior of the pipe prior to the first core during Expedition 320 by running a drilling tool, called a pig, up and down the pipe. During Expedition 320, the FlexIt orientation tool appeared to yield anomalous orientation data on deployment for APC cores at Sites U1331 and U1332, and the instrument was not used at other Expedition 320 sites. During Expedition 321, the FlexIt tool was considered to perform satisfactorily and was used to orient all APC cores at Sites U1337 and U1338. In a few cases the FlexIt orientation data were accidentally lost, in part because of bugs in the FlexIt software. 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. F12). For a measurement interval of 6 s, which is what we used, 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. Standard operating procedure was to set up a tool as described in the IODP "Core Orientation Standard Operating Procedure" manual (20 January 2009, available from the IODP Cumulus database at iodp.tamu.edu/labs/ship.html). This involves synchronizing the instrument to a PC running the FlexIt software (version 3.5) and inserting the tool inside a pressure casing. The enclosed tool is then given to a core technician, who installs it on the sinker bars that reside above the core barrel. The double lines on the core liner are aligned relative to the tool. Prior to firing the APC, the core barrel is held stationary (along with the pipe and BHA) for several minutes. During this time, the data recorded are those used to constrain the core orientation. When the APC fires, the core barrel is assumed to maintain the same orientation, although this and past 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 is pulled out after a few minutes except for cores collected with the advanced piston corer temperature tool (APCT-3) (see "Downhole measurements"), for which the core barrel remains in the sediments for ≥10 min. The procedure for determining polarity differed for the two expeditions. During Expedition 321, the FlexIt tool was used for core orientation and was judged to generate reliable orientation data. During Expedition 320, azimuthal core orientation was determined indirectly by correlating distinct geomagnetic reversal patterns, as recorded by the paleomagnetic declination in each hole, with the GPTS. A feasible polarity pattern was interpreted using biostratigraphic constraints, and the mean paleomagnetic direction for stable polarity intervals from each core was then determined using (1) Fisher statistics, with data from reversed polarity intervals inverted for Sites U1331, U1333, U1334, and Hole U1336A or (2) Bingham statistics for data from Sites U1332 and U1335. By subtracting this core mean declination from each observed declination obtained from the respective core, the azimuth of the core can be approximately reoriented from "sample" coordinates into "geographic" coordinates. After this reorientation, normal polarity intervals have ~0° declination and reversed polarity intervals have ~180° declination. This orientation method can break down when significant coring gaps occur within the composite stratigraphic section, which is rare when multiple holes are cored at a site. Paleomagnetic inclination was shallow at these paleoequatorial sites and rarely useful in constraining polarity. For all cores collected with the XCB, rotation occurs between pieces of core within a single core. Hence, XCB cores are not reoriented nor can we confidently determine polarity as the inclination is generally too shallow to be diagnostic. MagnetostratigraphyMagnetic polarity zones (magnetozones) were assigned based on distinct ~180° alternations in declination and subtle changes in inclination that occur along each stratigraphic section. Magnetostratigraphy for each site was then constructed by correlating these magnetozones with GPTS. Biostratigraphic age constraints significantly limit the range of possible correlations with the GPTS. The GPTS used for Expedition 320/321 is based upon a composite of several timescales (Table T6) (Cande and Kent, 1995; Lourens et al., 2004; Pälike et al., 2006a, 2006b). Its construction follows the procedure described by Backman et al. (2008), which is excerpted here: "Global Cenozoic timescales are still under development. Orbitally tuned cyclostratigraphic data are the chronological backbone in the most recent Neogene timescale, which includes 'Quaternary' times (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 sharp and definitive, owing to the lack of a continuous Milankovitch-based Paleogene cyclostratigraphy, and it will therefore continue to develop and change over some years to come." The PEAT expedition age model is based on the following three timescales:
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