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doi:10.2204/iodp.proc.317.104.2011

Paleomagnetism

Paleomagnetic analyses at Site U1352 included routine measurement and partial demagnetization of natural remanent magnetization (NRM) of archive section halves and some discrete samples from the working halves of cores. Rock magnetic experiments were also performed on these discrete samples. All depths in this section are reported in m CSF-A.

Section-half measurements

NRM was measured on all archive section halves from Holes U1352A–U1352D unless the core material was too heavily disturbed by the drilling process. A single demagnetization step at peak fields of 20 mT was applied to all measured sections. The NRM intensities of the sediments typically range from 10^–4 to 10^–2 A/m and tend to decrease with depth. The short records from Holes U1352A and U1352D are comparable to and entirely overlapped by the record from Hole U1352B (Figs. F28, F29, F30) and are therefore not shown. The deepest record from Site U1352 was from Hole U1352C, which overlaps the record from Hole U1352B for ~200 m (Fig. F31).

Hole U1352B

Hole U1352B was cored to 246.2 m using the APC system with both nonmagnetic core barrels (Cores 317-U1352B-1H through 27H [246.2 m]) and magnetic core barrels (Cores 317-U1352B-28H through 36H [246.2–297.0 m]). The remainder of the hole was cored using the XCB system. Distinct changes in magnetic behavior were observed where coring systems were changed (Fig. F28). In the nonmagnetic APC cores, NRM inclinations are usually steeply positive (~70°) with occasional intervals of shallow or negative inclination. After demagnetization at 20 mT, inclinations are steep and negative (approximately –60°). Declination varies between cores but is consistent, both before and after demagnetization, within individual cores.

Declination variability between cores suggests that this signal was not strongly overprinted by the drilling process (Richter et al., 2007). The first 18 APC cores (317-U1352B-1H through 18H [0–165.7 m]) were oriented using the Flexit tool, which records the orientation of the APC core barrel. Data from this tool were used to orient the cores with respect to true north (Fig. F30). Orienting core barrels brings the declination of NRM after 20 mT alternating-field (AF) demagnetization into good agreement between cores. The mean orientation is a declination of 27.7° and an inclination of –60.1°.

APC cores were recovered using magnetic core barrels between 246.2 and 297.0 m (Cores 317-U1352B-28H through 36H). This depth interval shows an increase in NRM intensity from ~2 × 10^–3 to ~9 × 10^–3 A/m and a slight increase in susceptibility. Declinations are consistently grouped around north during this interval, which comprises nine cores. Inclinations are fairly steeply positive throughout (~60°). Remanence does not change after AF demagnetization and is interpreted to reflect a drilling overprint that was not removed.

The XCB system was used to core from 297 m to the total depth of Hole U1352B (Cores 317-U1352B-37X through 94X [830.9 m]). Intensity varies throughout this interval. Declinations cluster close to north both before and after AF demagnetization at 20 mT, suggesting unsuccessful removal of the drilling overprint. Inclinations are typically fairly steeply positive (~55°) but do vary, with some cores showing shallower or negative inclinations.

Hole U1352C

Hole U1352C was cored with the RCB system from 575 m to a total depth of 1928 m (Fig. F31). Both recovery and intensity are low downcore to ~900 m, and a low signal to noise ratio is apparent downcore to ~1200 m. Below this interval, NRM has a consistently steep positive inclination (~70°) and a northward declination before and after demagnetization. This indicates a drilling overprint that was not removed. Intensity peaks at 1 × 10^–2 A/m at ~1700 m and drops significantly between 1850 and 1870 m from ~1 × 10^–3 A/m above to 1 × 10^–4 A/m below. At this point, data again show a wide spread as the noise level of the SRM was approached. Demagnetization at 20 mT removed up to 80% of the intensity of samples from Hole U1352C.

The boundaries between low-intensity intervals, noisy intervals, and higher intensity intervals with tightly grouped orientations correlate to lithologic unit and subunit Boundaries IIA/IIB (1189 m) and IIC/III (1852.64 m; Marshall Paraconformity), as described in "Lithostratigraphy."

Discrete measurements

Pairs of cubes were extracted from the working half of each core for shipboard analyses. Ten of these pairs were selected to characterize the sediments in Hole U1352B. Seven samples from lithologic Subunit IIC, which was found only in Hole U1352C, were also demagnetized with alternating fields.

NRM demagnetization

Seventeen samples were AF demagnetized up to 80 mT. The signal to noise ratio is poor, but drilling overprints persist to ~30 mT. Some samples (e.g., 70.33 m; Fig. F32A) show a characteristic component that demagnetizes toward the origin. Others (e.g., 255.00 m; Fig. F32C) appear to acquire a gyro-remanent magnetization (GRM) after 40 mT.

Ten samples were thermally demagnetized at 30°–40° intervals up to 460°C (Fig. F32B, F32D). Susceptibility was measured after each heating step to monitor any chemical alteration resulting in new magnetic minerals (Fig. F32G). All samples increase in susceptibility by 460°C, and four samples (at 70.32, 105.84, 230.99, and 255.03 m) show a slight (5%–10%) decrease in susceptibility between 310° and 430°C. The drilling overprint is removed by ~150°–200°C, and a characteristic component can be resolved before thermal alteration occurs.

Demagnetization of discrete samples confirmed that a subvertical overprint persists in the samples beyond the 20 mT demagnetization step applied to section-half samples. This overprint was removed in most cases by further demagnetization. Magnetic mineralogy is not fully constrained, but the acquisition of a GRM in some samples at 40 mT and the thermal alteration above 310°C suggest the presence of some iron sulfides in the sediment.

IRM acquisition and demagnetization

An isothermal remanent magnetization (IRM) was imparted to 10 samples from Hole U1352B in a stepwise manner up to 1 T (Fig. F33A) followed by backfield acquisition, which revealed fairly uniform behavior. All samples appear to saturate between 400 and 600 mT with coercivities of remanence between 40 and 75 mT.

The same 10 samples then had a 1 T IRM imparted and were stepwise AF demagnetized up to 80 mT (Fig. F33B–F33C). A clear grouping of significantly higher saturation remanence is evident in four samples (at 70.33, 202.58, 221.68, and 326.96 m) (Fig. F32B). This group also shows a softer magnetization and loses a higher proportion of this magnetization at 80 mT peak fields (Fig. F32C). The presence of these two behaviors upon AF demagnetization of IRM may suggest the presence of two low-coercivity mineralogies and/or grain-size fractions. Median destructive fields are between 40 and 50 mT for all samples.

Magnetostratigraphy

A pervasive drilling overprint prevented magnetostratigraphic interpretation of much of Site U1352. Biostratigraphic evidence suggests that the Brunhes/Matuyama boundary lies between Samples 317-U1352B-24H-CC and 30H-CC (217.9–266.9 m), constrained between the presence of Fragilariopsis fossilis (diatom; HO = 0.70 Ma) and the HCO of Reticulofenestra asanoi (nannofossil; 0.91 Ma) in these two samples, respectively. Some noisy intervals were encountered within this range, where inclinations are not clearly negative (normal). From Core 317-U1352B-28H (246.2 m) downward, magnetic core barrels were used, and characteristic magnetizations could not be isolated. Therefore, it was not possible to identify the Brunhes/Matuyama boundary, despite a promising trend toward positive inclinations throughout Core 317-U1352B-27H.

No further magnetostratigraphic constraints could be given at this site because of the pervasive drilling overprint. This overprint may eventually be removed by the full demagnetization of working-half cube samples onshore.

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