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doi:10.2204/iodp.proc.334.102.2012

Paleomagnetism

Shipboard paleomagnetism was investigated mainly to determine directions of remanence components. Routine measurements were made on archive section halves with stepwise alternating-field (AF) demagnetization. Discrete cube and minicore samples were taken from working halves and measured with stepwise AF and thermal demagnetization for selected sections. These data were used for core reorientation and magnetostratigraphic studies.

Coordinates

All magnetic data are reported relative to the IODP orientation conventions: +x is into the face of the working half, +y points toward the left side of the face of the working half, and +z is down. The relationship of the superconducting rock magnetometer (SRM) coordinate (X, Y, Z) to the data coordinate (x, y, z) for archive halves is x = X, y = –Y, and z = Z and for working halves is x = –X, y = Y, and z = Z (Fig. F14).

Magnetic measurements

Remanent magnetization was measured using a 2G SRM (2G Enterprises model 760R). The noise level of the measurement system was routinely checked using the SRM Noise Monitor software, which was originally developed by Jeff Gee. The noise has the dominant frequency of wave motion (on the order of 0.1 Hz). The magnitude of noise was ~5 × 10^–10 Am², and it was the same between tests with 1 and 10 Hz SRM electronics filters (2G model 581), because the dominant frequency of the noise was <1 Hz. This noise corresponds to ~5 × 10^–5 A/m noise for a half core. We decided to use a 10 Hz filter, instead of the commonly used 1 Hz filter, to obtain a faster response of the SRM to changes in magnetic signal. This may improve the measurements around the areas with sharp changes in magnetization (e.g., core ends), although pilot tests using a concrete core on the JOIDES Resolution revealed essentially no difference between 1 and 10 Hz filters (Fig. F15). Discrete samples were also measured using a spinner magnetometer (AGICO model JR-6A) when the cryogenic magnetometer was in use for long-core pass-through measurements. Ocean drilling cores generally carry secondary overprint remanence components. Common overprints for ocean drilling cores include natural viscous remanence and a steep downward-pointing component attributed to the rotating drill strings. To separate these from the characteristic remanent magnetization (ChRM), stepwise demagnetization experiments were performed, as described below.

Archive halves

Measurements of archive halves were conducted using the SRM software (version 3.18) with a 15.59 cm² nominal sample area parameter. The intervals between measurement points and measurement speed were selected as 2, 5, or 10 cm and 2–10 cm/s, respectively, depending on the available time and the magnetic character of the samples.

We performed successive AF demagnetization using the in-line SRM AF demagnetizer (2G Enterprises model 2G600) on all split-core archive sections. The in-line AF demagnetizer applies a field to the X-, Y-, and Z-axes of the SRM in this fixed order. We noticed that the AF demagnetizer produces anomalous demagnetization results after 50 mT demagnetization, as shown by uniform –90° inclination for the whole-core section. Previous reports suggest that higher AF demagnetization fields have produced significant anhysteretic remanent magnetization along the Z-axis of the SRM. Given this limitation, we used demagnetization steps up to 40 mT for demagnetizing tray and sections. For most of the sediment sections, we only performed steps to 15–30 mT; for long pieces from basement sections, we performed progressive AF demagnetization to 15 mT because of time constraints. The AF demagnetization results were plotted individually as vector plots (Zijderveld, 1967) as well as downhole variations with depth. The response curve from the sensor coils of the SRM corresponds to a region ~20 cm wide; therefore, only measurements taken greater than 20 cm are independent from each other (Fig. F15). Measurements at core and section ends, whole-round locations and voids, and within intervals of drilling-related core disturbance were either not measured or were removed during data processing. We then inspected the plots visually to judge whether the remanence after demagnetization at the highest AF step reflects the ChRM and geomagnetic polarity sequence.

Discrete samples from working halves

Oriented discrete samples representative of the lithology were collected during the expedition. In soft sediments, cubic samples (~8 cm³) were taken by pressing plastic cubes into the split face of the working halves. In lithified sediments and hard rocks, minicores (~11 cm³) were taken. Measurements of discrete samples were conducted using the SRM software (version 3.18). For the JR6 spinner magnetometer, we used the REMA6 software. For approximately half of the discrete samples, we performed successive AF demagnetization with the DTech AF demagnetizer (model D-2000) for the spinner measurements. The remaining half of the discrete samples were demagnetized with the in-line AF demagnetizer and used for the SRM measurements. For selected discrete samples, we also performed successive thermal demagnetization using a thermal demagnetizer (Schonstedt model TSD-1). Temperature increments of 25° or 50°C were used depending on the unblocking temperature of each sample. We analyzed the stepwise demagnetization data of the discrete samples by principal component analyses to define the ChRM (Kirschvink, 1980).

Core orientation

Core orientation of APC cores was achieved with the Flexit orientation tool mounted on the core barrel. The tool consists of three mutually perpendicular fluxgate magnetic sensors and two perpendicular gravity sensors. The information from both sets of sensors allows the azimuth and dip of the hole to be measured, as well as the azimuth of the APC core orientation. Generally, the Flexit tool has an accuracy of 20°–30°. The orientation information contributed to paleomagnetic polarity determinations and magnetostratigraphic interpretations.

The ChRM provides a reference frame to reorient cores (see “Structural geology”). Provided that the reference magnetic pole is known, the orientation of the paleomagnetic vector is then used to restore the azimuth of the core. The horizontal component of the mean ChRM makes an angle with the reference line, which specifies the rotation of the core relative to the geographic coordinates (e.g., Fuller, 1969). The relatively young expected age (Neogene) of the sediments and nearly longitudinal (westward) slow motion of the Caribbean plate during this period (e.g., Pindell et al., 1988) indicate that we can approximate the reference magnetic pole by the present-day geographic poles for the entire sediment section. The other assumptions for the reorientation are

1. The section has enough measurements to average out geomagnetic secular variation,

2. The original bedding is horizontal,

3. Core is vertical, and

4. The sedimentary unit has not experienced any vertical axis rotations.

Assumptions 2 and 3 were checked with shipboard structural geologists, seismic profiles of the drill sites, and drilling operational records.

For intervals of particular interest for structural geology, we reported ChRMs defined from discrete samples in the interval. More detailed demagnetization steps for the discrete samples allow more accurate ChRMs than those from the archive halves.

Magnetostratigraphy

Magnetostratigraphy for each site was constructed by correlating obtained geomagnetic polarity sequences with the geomagnetic polarity timescale of Gradstein et al. (2004) (Table T6). Expedition 334 drill sites are located at low latitude (~9°N), resulting in very small differences in inclinations between reversed and normal geomagnetic polarity. Consequently, defining paleomagnetic polarity and magnetostratigraphy from magnetic measurements alone was difficult for RCB and XCB cores, and biostratigraphic age constraints were incorporated to judge the correlation with the geomagnetic polarity timescale.

Whenever possible, we offer an interpretation of the magnetic polarity, following naming convention of correlative anomaly numbers prefaced by the letter C (Tauxe et al., 1983). Normal polarity subchrons are referred to suffixes (e.g., n1, n2, etc.) that increase with age. For the younger part of the timescale (Pliocene–Pleistocene), we use traditional names to refer to the various chrons and subchrons (e.g., Brunhes, Jaramillo, Olduvai, etc.). In general, polarity reversals occurring at core ends have been treated with extreme caution because of the edge effects caused by the coring-related disruptions.

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