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doi:10.2204/iodp.proc.334.103.2012

Paleomagnetism

Remanent magnetization was measured on archive-half cores and on discrete samples taken from the working halves of cores recovered in Hole U1378B. All archive-half cores were demagnetized in an alternating field (AF) to 15 mT and measured with the pass-through magnetometer. Discrete samples were subjected to stepwise AF demagnetization and measured in both the superconducting rock magnetometer and JR6 magnetometer. Cores 334-U1378B-1H through 16H were cored with the APC system using a nonmagnetic cutting shoe and oriented with the Flexit orientation tool. Cores 17X through 63X were cored with the XCB system using a standard cutting shoe.

Natural remanent magnetization

Paleomagnetic data obtained at Site U1378 exhibit significant variations in demagnetization behavior among various recovered lithologies. Drilling-induced remagnetization pervasively exists in the recovered cores (Fig. F31). The mean natural remanent magnetization (NRM) intensity varies downhole in three trends:

• NRM intensity for the uppermost 80 mbsf is on the order of 10^–1 A/m;

• For sediments at ~90–190 mbsf, NRM intensity is strongest, ranging from 10^–2 to 5 × 10^–2 A/m; and

• Mean NRM intensity for 190–300 mbsf is ~10^–2 A/m, whereas intensities at 300–400 mbsf vary on the order of 10^–3 to 10^–1 A/m (Fig. F31).

NRM intensity variations through cored units are closely correlated with those in magnetic susceptibility measurements. Like the NRM record, distinct increases in magnetic susceptibility are present at ~90–190 mbsf (see “Physical properties”).

Demagnetization behavior of APC cores

The steep downward component of magnetization imparted by the coring process is easily removed by AF demagnetization (Figs. F31, F32A, F32B). Declinations determined by the pass-through measurements are uniform within each core, except for the core ends where coring-related disruptions are usually present (Fig. F33A). After making the orientation correction, using data from the Flexit tool, declinations become close to magnetic north, with exceptions in Cores 334-U1378B-4H and 10H (Fig. F33B), indicating the remanence is of geomagnetic origin.

The magnetic properties observed from the split cores were also confirmed by discrete sample measurements. The nearly vertical overprint was removed by AF demagnetization of 5–10 mT, and the stable component has a similar direction to those in the corresponding core sections (red stars in Fig. F31). No clear reversal was observed in the magnetic records of the APC cores.

Demagnetization behavior of XCB cores

Steep inclinations imparted by XCB drilling at Site U1378 are extremely stable and resistant to AF demagnetization (Figs. F31B, F32C, F32D). NRM of the XCB cores is also oriented parallel to the +x-direction, with declinations tightly clustered at 0°, indicating the existence of a radially inward magnetization induced by the coring process (e.g., Stokking et al., 1993). We performed progressive AF demagnetization experiments on discrete samples. All samples showed rapid removal of near-vertical overprint at 5–15 mT demagnetization steps, but several samples are either completely dominated by the artificial overprint or indicate scattered demagnetization behavior (e.g., Fig. F32D). Out of 119 discrete samples, 62 discrete samples revealed behavior indicating the successful removal of the radial overprint. The inclinations of the characteristic remanent magnetization (ChRM) are close to the theoretically expected latitude of this site (red stars in Fig. F31). The success of AF demagnetization appears to be dependent on stratigraphic position. For example, virtually all samples from depth intervals to ~100 mbsf revealed ChRM, while those from ~300 or ~420 mbsf completely failed to reveal ChRM.

Rock magnetic characterization

High NRM intensity values at ~90–190 mbsf may reflect a higher magnetic mineralogy content and correlations with other physical signatures. AF demagnetization experiments suggest that samples from this interval also have higher magnetic stability than other intervals in Hole U1378B (Fig. F33B). To further characterize the magnetic mineralogy in this interval, we conducted several rock magnetic experiments on selected samples within (Sample 334-U1378B-15H-3, 63–65 cm), above (Sample 5H-2, 104–106 cm), and below (Sample 32X-2, 94–96 cm) this interval. These experiments include acquisitions of isothermal remanent magnetization (IRM) and anhysteretic remanent magnetization (ARM), followed by AF demagnetization of IRM and ARM. IRM and ARM were acquired by a pulse magnetizer (ASC Scientific, IM10) and the Dtech AF demagnetizer, respectively (see “Paleomagnetism” in the “Methods” chapter [Expedition 334 Scientists, 2012]). For ARM acquisition, we used an AF with peak intensity of 100 mT and a direct current field of 100 µT. Magnetizations were measured by the JR-6 magnetometer.

As shown in Figure F34A, all three samples revealed rapid saturation of IRM with increasing applied field (<500 mT), suggesting minor contributions of high-coercivity minerals such as hematite and that the magnetic carriers are most likely fine-grained magnetite. Sample 334-U1378B-15H-3, 63–65 cm, displayed strong saturation IRM intensity that was ~30 times higher than in the other samples. This observation is consistent with the high NRM intensity for sediments at ~90–190 mbsf. AF demagnetization experiments indicate moderate stability for all samples (Fig. F34B). Interestingly, the strongly magnetized Sample 334-U1378B-15H-3, 63–65 cm, showed rapid decay of intensity for both IRM and ARM demagnetization. More experimental results are needed before we can conclude if this behavior is generally true for the sediments at ~90–190 mbsf.

Structural application of core orientation

For intervals of particular interest for structural geology at Site U1378, the declinations of the stable ChRM were used to help restore the azimuth of the core (see “Structural geology”). We used results from discrete sample demagnetization assuming the direction of stable remanent magnetization (or ChRM) represents the expected magnetic direction at Site U1378.

Magnetostratigraphy

For APC cores, neither pass-through nor discrete-sample measurements show signs of reversed polarity. In addition, declination data corrected by the Flexit tool do not show a near-180° shift in declinations. Thus, we interpret that the sediments in the APC interval (~128 mbsf) were deposited within the normal polarity Brunhes period (<0.78 Ma). This age assignment is consistent with the shipboard micropaleontological data, which suggest that the planktonic foraminifer G. ruber pink found between Cores 334-U1378B-4H and 5H may be 0.12 Ma in age (see “Paleontology and biostratigraphy”). Nannofossil records also suggest that Cores 1H through 32X should be of late Pleistocene age, further corroborating with the notion that the sediment with this depth interval is younger than 0.78 Ma.

On the basis of biostratigraphic data, Zone NN19 of the early Pleistocene is tentatively placed at XCB Cores 334-U1378B-33X through 63X (see “Paleontology and biostratigraphy”). Paleomagnetic measurements made on the XCB cores so far have not revealed any reversed polarity for sediments in Hole U1378B. Additional shore-based work is required to check or confirm the shipboard data.

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