Previous   |   Next

doi:10.2204/iodp.proc.317.103.2011

Paleomagnetism

Paleomagnetic analyses at Site U1351 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

Natural remanent magnetization

NRM was measured on all archive section halves from Holes U1351A and U1351B unless the core material was too heavily disturbed by the drilling process. The record from Hole U1351A is similar to but much shorter than that from Hole U1351B and is therefore not presented in Figure F28. Magnetization intensities commonly ranged from 10^–4 to 10^–2 A/m, with anomalously high values (10^–1 A/m) often observed at the tops of cores and occasionally at points within cores. High-intensity values at the tops of cores often correspond to high-susceptibility intervals of coarse and shelly sediments, interpreted as cave-in, which may contain rust from the drill pipe (Richter et al., 2007). On some occasions, intensity spikes within cores could be traced to blade fragments from core splitting or wires from loose microsphere bags (these wires were removed at subsequent sites). At other times, no obvious source of the high intensity was identified.

Alternating-field demagnetization

Archive section halves were routinely demagnetized with alternating fields at 10 and 20 mT steps to remove any unstable overprints from drilling and handling or natural viscous remanent magnetization (VRM) overprints in an attempt to isolate a stable characteristic remanence. The response to demagnetization varied throughout the core; however, on average, intensities were ~30% lower than NRM after the 20 mT step. The most systematic change in orientation with demagnetization was recorded in the uppermost 65 m of Hole U1351B (Fig. F29). The generally steep (~80°) positive inclinations of NRM became negative (about –60°) after 20 mT demagnetization. Other intervals (e.g., 150–170 m; Fig. F30) had very little variation in orientation with demagnetization. This lack of change, together with a bias toward northward declinations in the cores (which were not oriented below Core 317-U1351B-5H [32.39 m]), suggests a drilling overprint (such as that described by Richter et al., 2007) that is still stable at 20 mT. Nonmagnetic core barrels were used throughout APC coring to 94.7 m, and episodes of pervasive drilling overprints were recognized only below this depth.

In some intervals below 94.7 m, demagnetization caused variation in the orientation of the remanence, but a stable component could not be demonstrated. Higher levels of demagnetization of discrete specimens are required to determine whether these orientations are stable and characteristic.

Magnetic susceptibility

Magnetic susceptibility was measured on whole cores and archive section halves, as described in "Physical properties." Some correlation between magnetic susceptibility and NRM was observed in Hole U1351B (e.g., 150–175 m; Fig. F30), demonstrating that, at least in these intervals, ferromagnetic minerals contribute significantly to susceptibility.

Discrete measurements

A suite of measurements was applied to discrete samples to characterize the rock magnetic properties of the sediment. Reported intensities were derived from raw moments measured on the superconducting rock magnetometer (SRM) using a standard volume of 7 cm³ (see "Paleomagnetism" in the "Methods" chapter).

Thermal demagnetization of NRM

Magnetic iron sulfides were reported at nearby Site 1119 (Shipboard Scientific Party, 1999), and these were therefore expected in sediments cored at Site U1351. Because the coercivity spectra of these minerals normally overlap those of magnetite and other iron oxides, thermal demagnetization was attempted to test for the presence of magnetic iron sulfides, which have conspicuous unblocking temperatures of ~340°–360°C. Eight samples were thermally demagnetized, but the plastic cubes deformed upon heating to 140°C, and heating steps were discontinued. Susceptibility was monitored during thermal demagnetization by measuring each sample with a Kappabridge KLY 4S meter after remanence was measured. Susceptibility values ranged from 81 × 10^–6 to 246 × 10^–6 SI units. No significant changes in susceptibility were noted up to 140°C. Very little magnetization was lost by 140°C, and thus samples were further demagnetized using alternating fields up to 80 mT in the inline coils of the SRM.

Alternating-field demagnetization of NRM

A representative suite of samples was demagnetized in a stepwise fashion using alternating fields (5 mT steps up to 80 mT). Demagnetization data were noisy, and components were difficult to trace. However, most samples had an unstable subvertical positive inclination component that was removed by alternating fields of 15–25 mT and a second shallower, more stable component that tended toward the origin up to 50 mT. At higher peak fields, sample intensities increased, presumably through the acquisition of a gyro-remanent magnetization (GRM). The first component was interpreted as a subvertical result of viscous isothermal remanent magnetization (VIRM) drilling overprint and VRM natural overprint, and the second component was interpreted as the characteristic component, which confirms that in some cases the routine 20 mT demagnetization of the archive section halves revealed the characteristic magnetization of the samples.

Isothermal remanent magnetization acquisition

Stepwise isothermal remanent magnetization (IRM; up to 1 T, followed by a backfield acquisition to the same field) was applied to seven samples to establish IRM saturation and coercivity of remanence (Fig. F31A). These acquisition curves show that saturation was reached between 300 and 400 mT for the majority of samples. Coercivity of remanence was typically 40–70 mT. These values are consistent with magnetite and/or iron sulfides as the remanence carriers.

Alternating-field demagnetization of IRM

The saturation IRM of the same seven samples was also demagnetized by alternating fields up to 80 mT (Fig. F31B, F31C). The resulting demagnetization curves show a distribution of mean destructive fields (MDF) from 25 to 65 mT (samples from 809 and 11 m, respectively), with most samples showing an MDF of 35–50 mT. Approximately 20% of IRM was demagnetized by the 20 mT step. A single sample (54 m) had an anhysteretic remanent magnetization imparted by a direct field of 0.05 mT in a 100 mT alternating field. This sample was then stepwise demagnetized with alternating fields, yielding a demagnetization spectrum similar to that of the IRM of the same sample (not shown).

Magnetostratigraphy

Because of low core recovery and a pervasive drilling overprint, it was not possible to construct an extensive magnetostratigraphy for Site U1351. However, some candidates for polarity reversals were identified in intervals with better core recovery, and these may be incorporated with other dating techniques to identify chrons from the geomagnetic polarity timescale (GPTS). The most compelling of these is the distinct change in demagnetization behavior across the unrecorded interval between 65.9 and 69.7 m (Fig. F29). This interval, composed of coarse shell hash interpreted as cave-in, was not measured in the SRM. The NRM of the overlying material had a positive inclination interpreted as an overprint, but demagnetization revealed a more stable negative (normal in the southern hemisphere) component. Below 69.7 m, intensity decreased with demagnetization, suggesting that an unstable component was still being removed. However, inclinations remained positive (reversed), which suggests that this boundary represents the first polarity change in Hole U1351B—from a long normal polarity zone interpreted as the Brunhes Chron to a reversed polarity interval interpreted as the Matuyama Chron or older. An interpretation of this transition as the Brunhes/Matuyama boundary depends on how much time is represented by a potential unconformity between the two zones. Nannofossil evidence shows the HO of Pseudoemiliania lacunosa (0.44 Ma) between 65.96 and 70.98 m and the HO of Gephyrocapsa >5.5 µm (1.26 Ma) between 76.97 and 90.02 m.

Top of page   |   Previous   |   Next