Previous   |    Next

doi:10.2204/iodp.proc.344.102.2013

Paleomagnetism

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

Magnetic measurements

Remanent magnetization was measured using a 2G superconducting rock magnetometer (SRM; 2G Enterprises model 760R) equipped with direct-current superconducting quantum interference devices (SQUIDs) and an in-line, automated AF demagnetizer capable of reaching a peak field of 80 mT. 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 drill string. To separate overprints from the characteristic remanence (ChRM), stepwise demagnetization was performed, as described below.

Archive-half sections

Measurements of archive halves were conducted using the SRM for Section software (version 1.0; 8/15/2011) with a nominal sample area parameter of 15.59 cm². The interval between measurement points and measurement speed were selected as 2.5 cm and 10 cm/s, respectively.

We performed successive AF demagnetization using the in-line AF demagnetizer of the SRM (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. Previous reports suggest that higher AF demagnetization fields have produced significant anhysteretic remanent magnetization (ARM) along the z-axis of the SRM. With this limitation, we used demagnetization steps up to 30 or 40 mT to demagnetize the sections. For most of the sediment sections, we performed 4–7 steps from natural remanent magnetization (NRM) to 30 or 40 mT demagnetization. AF demagnetization results were plotted individually as vector plots (Zijderveld, 1967), as well as downhole variations with depth. We inspected the plots visually to judge whether the remanence after demagnetization at the highest AF step reflects the ChRM and geomagnetic polarity sequence.

Magnetic noise due to anomalous y-axis flux jumps

During Expedition 344, especially in Hole U1412A, we observed large y-axis flux jumps due to the “antenna” effect described by the Expedition 342 scientists (see “Paleomagnetism” in the “Site U1405” chapter [Expedition 342 Scientists, in press]). We rewrapped the power cable attached to the in-line degausser with aluminum foil and ensured that this foil was electrically connected to the existing shielding material of the SRM. The flux jumps decreased abruptly thereafter, and SRM demagnetization data from Hole U1412B and subsequent sites were remarkably free of the magnetic noise caused by the y-axis flux jumps. Moderate to small flux jumps still occurred at Site U1413, possibly associated with rig floor activity. At Site U1414, we observed increased flux jumps for reasons that are unclear at present.

Discrete samples

Oriented discrete samples representative of the lithology were collected from working-half sections. In soft sediment, discrete samples were taken in plastic “Japanese” Natsuhara-Giken sampling cubes (7 cm³ sample volume; Fig. F15). Cubes were pushed into the working half of the core by hand with the “up” arrow on the cube pointing upsection in the core. For indurated intervals, cubes were cut with a table saw and trimmed to fit into the plastic containers. In lithified sediments and hard rocks, minicores (~11 cm³) were taken. Measurements of discrete samples were conducted using the SRM for Discrete software (version 1.0, 8/15/2011).

For discrete samples, we performed successive AF demagnetization with the DTech AF demagnetizer (model D-2000) for the spinner magnetometer measurements to 120 mT (majority of samples) and 200 mT (for several high-coercivity samples). We also performed successive thermal demagnetization using a thermal specimen demagnetizer (ASC Scientific model TD-48SC) for several selected discrete samples up to 675°C. Temperature increments of 25°–100°C were used, depending on the unblocking temperature of each sample. We analyzed the stepwise demagnetization data of the discrete samples by principal component analysis (PCA) to define the ChRM (Kirschvink, 1980). Section half and discrete data collected on the pass-through SRM were uploaded to the LIMS.

Anisotropy of magnetic susceptibility (AMS) measurements were made on an AGICO KLY 4S Kappabridge instrument using the AMSSpin LabVIEW program designed by Gee et al. (2008) and adapted for use with the shipboard KLY 4S. The KLY 4S Kappabridge measures AMS by rotating the sample along three axes, stacking the data, and calculating the best-fit second-order tensor. It also measures volume-normalized, calibrated bulk susceptibility (χ).

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 points downsection. The relationship of the SRM coordinates (x-, y-, and z-axes) to the data coordinates (x-, y-, and z-directions) is as follows: for archive halves, x-direction = x-axis, y-direction = –y-axis, and z-direction = z-axis; for working halves, x-direction = –x-axis, y-direction = y-axis, and z-direction = z-axis (Fig. F15). The coordinate system for the spinner magnetometer (AGICO model JR-6A) and Natsuhara-Giken sampling cubes are shown in Figure F16.

Core orientation

Core orientation of APC cores was achieved with an orientation tool (FlexIT) 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 orientation tool has an accuracy of 20°–30°. The orientation information contributed to paleomagnetic polarity determinations and magnetostratigraphic interpretations.

The ChRMs also provide a reference frame to orient 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) allow us to approximate the reference magnetic pole by the present-day geographic poles for the entire sedimentary section. The other assumptions for orientation are (1) the section has enough measurements to average out geomagnetic secular variation, (2) the original bedding is horizontal, (3) the core is vertical, and (4) the sedimentary unit has not experienced any vertical axis rotation. Assumptions 2 and 3 were confirmed with shipboard structural geologists, seismic profiles of the drill sites, and drilling operational records. Assumptions 1 and 4 remain to be checked.

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

Magnetostratigraphy

Magnetostratigraphy for each site was constructed by correlating observed polarity sequences with the geomagnetic polarity timescale (GPTS) in combination with biostratigraphic datums. We adopted the GPTS of Gradstein et al. (2012) (Table T3). In this GPTS, boundary ages for Chrons C1n–C13n and C24n.1n–C34n are orbitally tuned, whereas those for Chrons C13r–C23r are spline fitted.

For azimuthally unoriented samples from sedimentary rocks that formed in low latitudes, determining the polarity of sedimentary units can be difficult. The polarity ambiguity arises when the samples are azimuthally unoriented and the inclination is shallow near the Equator (the angular distance between reversed and normal polarity inclinations is small). Because paleomagnetic inclinations from any samples will have some degree of dispersion about their mean inclination, it is likely that when the mean inclination is shallow, the sign of the inclination will not be indicative of the polarity (e.g., McFadden and Reid, 1982; Cox and Gordon, 1984). The sign of the inclination of these samples should be used carefully as a definitive estimate of magnetic polarity.

Our interpretation is compared with predicted paleolatitudes of the drill sites according to the absolute plate motion of the Cocos plate following MORVEL plate reconstruction (DeMets et al., 2010, 2011). Based on paleomagnetic records reconstructions, our drill sites have moved northward since the formation of these sediments at a velocity of ~90 km/m.y. Following the reconstructed position of the drill holes for two representative periods at 2 and 12 Ma (Fig. F17), the paleolatitudes would be ~2° and ~12° to the south of their present day positions, respectively. However, if our mean inclinations are steeper than expected (according to the model), this could be due to some unresolved combination of unaccounted for plate motion, long-term nondipole fields, or effects of hotspot motion on the plate circuit models (e.g., Barckhausen et al., 2001).

Whenever possible, we offer an interpretation of the magnetic polarity, with the naming convention following that of correlative anomaly numbers prefaced by the letter C (Tauxe et al., 1984). Normal polarity subchrons are referred to by adding suffixes (n1, n2) that increase with age. For the younger part of the timescale (Pliocene–Pleistocene) we use the traditional names to refer to the various chrons and subchrons (e.g., Brunhes, Jaramillo, Olduvai, etc.). In general, polarity reversals occurring at core section ends have been treated with extreme caution.

Top of page   |    Previous   |    Next