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doi:10.2204/iodp.proc.334.106.2012

Paleomagnetism

We made pass-through magnetometer measurements on all archive-half cores and on discrete samples taken from the working halves recovered in Hole U1381A. Sedimentary archive-half cores were demagnetized in an alternating field (AF) up to 30 mT and measured with the pass-through superconducting rock magnetometer (SRM) at 5 cm intervals. Basalt basement split cores were demagnetized up to 10 mT and measured at 2 cm intervals. Paleomagnetic discrete samples were subjected to stepwise AF demagnetization and measured in both the SRM and JR6 magnetometer. Several discrete samples were also subjected to stepwise thermal demagnetization to 600°C. No single shipboard paleomagnetic discrete sample was taken from basement Cores 334-U1381A-16R through 26R because of time constraints for the final operation of the expedition. All archive-half sections from Hole U1381B were measured with the pass-through SRM at 5 cm intervals and AF demagnetized up to 20 mT. Both normal and reversed polarities were observed, but no further data analyses could be performed.

Natural remanent magnetization

Sedimentary cores

The uppermost 97 m of Hole U1381A consists of a sequence of yellow to green-gray silty clay. Natural remanent magnetization (NRM) of these sediments shows similar characteristics as those at Site U1380. Steep downward drilling-induced remagnetization exists in the recovered cores (Fig. F22), as evidenced by dominantly positive inclinations (mean ~45°), which is steeper than the inclinations expected for the latitude of the site (~17°). NRM has no preferred declinations (Fig. F22A), suggesting that the radial-inward remagnetization observed at Site U1379 is less severe in Hole U1381A. The mean NRM intensity for the sedimentary interval is ~10^–3 A/m and slightly decreases downhole. Between ~30 and 35 mbsf (Core 334-U1381A-4R), higher NRM intensity values are apparent.

Basement rocks

The lowermost ~70 m of Hole U1381A consists mainly of basaltic rocks. We selected pieces longer than ~5 cm in the archive halves for the pass-through measurements with an interval of 2 cm (Fig. F23). Measured declination and inclination are consistent within each coherent piece. NRM declination has no preferred direction, and inclination ranges from approximately –45° to approximately +45°. NRM intensity scatters from ~1 to 8 A/m. It appears that NRM inclination and intensity vary downhole and can be correlated to the alteration states of the rocks. The relatively fresh samples from the top of the basalt section (Cores 334-U1381A-12R to 16R) frequently show inclinations consistent with the low latitude of the site, indicating that the drilling-induced vertical remagnetization in the basaltic cores is much less severe than in the sediments drilled during this expedition. The fresh part of the basalt is also characterized by stronger NRM intensity. Below this fresh part in Cores 334-U1381A-19R to 26R, samples are relatively altered and show lower NRM intensity with inclinations of ~50°, indicating more contamination from the drilling-induced remagnetization compared with the relatively fresh part. The majority of the measured basaltic pieces have positive inclinations, but several pieces also exhibit negative inclinations (Fig. F23), possibly reflecting prolonged igneous activities at the site.

Demagnetization behavior

Sedimentary cores

Because of the relatively deep water and slower drilling rate for coring basement rocks, we had time to perform more detailed AF demagnetization on the archive halves. For the recovered sediment core sections, we employed AF demagnetization steps up to 30 mT with 5 mT increments. AF demagnetization with 5–10 mT seems to be effective in removing the drilling-induced remagnetization, as shown by inclinations shifted toward shallower values comparable to the expectation for the site. A factor of 3~4 decrease in magnetization intensity after 5 mT demagnetization and one order of magnitude decrease after 30 mT demagnetization are evident suggesting the magnetization is dominated by a low-coercivity and low-stability component (Fig. F22C).

Interestingly, inclinations from Cores 334-U1381A-5R through 6R at ~40–50 mbsf moved from positive to negative values after progressive AF demagnetization. After 30 mT AF demagnetization, the inclinations for these two cores scatter around a mean of –45° (Fig. F22C). Although biostratigraphic evidence suggests that cores below Core 6R may have a middle Miocene age (see “Paleontology and biostratigraphy”), we suspect this negative inclination behavior is an experimental artifact based on the following observations:

• A strong anhysteric remanent magnetization (ARM) effect that shows bias toward negative inclination was noticed during our test runs for AF field >40 mT by the SRM in-line demagnetizer (see “Paleomagnetism” in the “Methods” chapter [Expedition 334 Scientists, 2012]),
• The negative inclination value is scattered and steeper than the expected dipole field value (–17°) for the site, and
• Some discrete samples also show similar negative inclination values during AF demagnetization with the in-line demagnetizer after 30 mT AF demagnetization, suggesting the existence of the ARM effect.

Thus, we do not interpret the inclination behavior of the archive halves to be records of reversed polarity of magnetization. Shore-based AF and thermal demagnetization work is needed to check this interpretation.

As shown in Figure F24, AF demagnetization on discrete samples can isolate characteristic remanent magnetization (ChRM) after the removal of the overprints. However, at high demagnetization levels on the SRM in-line demagnetizer, discrete samples frequently show bias toward negative values. In addition, several samples from the lowermost part of the sedimentary sequence have remanence intensity that is too weak (<10^–4 A/m) for the shipboard experiments to determine ChRM reliably (e.g., Fig. F25C). Establishment of the magnetic polarity stratigraphy for Site U1381 must wait for shore-based studies that can utilize more sensitive instrumentation and reliably measure the weak magnetization.

Basement rocks

Based on the pilot results of AF demagnetization measurements on Section 334-U1381A-12R-1 up to 15 mT, we performed AF demagnetization at steps 5 and 10 mT for the rest of the archive halves. Generally speaking, both intensity and direction of NRM show only little change after the 5 mT demagnetization. After 10 mT AF demagnetization, remanence intensity decreased by a factor of ~2 (~1–4 A/m). Direction of NRM shows little change, even after the 10 mT step (Fig. F23).

AF demagnetization of discrete samples revealed behaviors similar to the archive halves. The discrete samples show well-defined ChRM with only slight signs of overprint at the lowest demagnetization steps (Fig. F25). AF demagnetization was very effective, and median destructive fields are between 5 and 10 mT (Fig. F26). These samples revealed inclinations consistent with the SRM measurements of the corresponding long core pieces in the archive halves, including one with negative inclination (Figs. F23, F25).

We also performed thermal demagnetization on discrete samples. Paleomagnetic minicore (10 cm³) samples were treated using the TSD-1 thermal demagnetizer from 100° to 550°C at 25°C increments, then at 580° and 600°C. Increments of 50°C were used between 300° and 450°C. Magnetization was measured using the JR-6 magnetometer. Magnetization decreases by ~40% after heating to 100°C and then remains stable up to ~250°C. Two samples show slight increases in magnetization between 250° and 300°C. After 300°C, all samples show a steady decrease of magnetization. Most of the remanence has unblocked by 525°C, and demagnetization was completed by 580°C (Fig. F27). Magnetization directions are stable throughout thermal demagnetization experiment, with minor indications of vertical component demagnetization during low-temperature heating steps (Fig. F25). The unblocking temperature indicates that titanomagnetite with low Ti content is the most likely NRM carrier. The components demagnetized by the 100°C heating step most likely represent a viscous remanence overprint. The increase in magnetization at ~275°C may result from oxidation of (titano)maghemite. All examined samples revealed positive inclination, consistent with the SRM measurements on the corresponding long pieces from the archive halves.

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