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doi:10.2204/iodp.proc.344.103.2013

Paleomagnetism

Cores 344-U1381C-1H through 12H were cored with the APC using two nonmagnetic cutting shoes. Core 344-U1381C-13X was cored with the XCB using a standard cutting shoe. Cores 1H through 9H were oriented with the FlexIT orientation tool. These crucial orientation data facilitate the magnetostratigraphy investigation for Hole U1381C. We made pass-through magnetometer measurements on all archive-half cores and on 85 discrete samples taken from the working halves. In order to isolate the characteristic remanent magnetization (ChRM), 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 2.5 cm intervals.

Paleomagnetic discrete samples were subjected to stepwise AF demagnetization up to 120 or 200 mT and measured with the SRM. Paleomagnetic discrete samples were not taken from Cores 344-U1381C-12H and 13X because of the short pieces of the recovered core material.

Natural remanent magnetization of sedimentary cores

Downhole variations of paleomagnetic data obtained in Hole U1381C are shown in Figure F35. The variations in NRM intensity are correlated with lithology. Paleomagnetic measurements indicate that the silty clay in Unit I (0–55.93 mbsf) has a mean NRM intensity on the order of 10^–2 A/m, whereas the foraminiferal nannofossil-rich calcareous ooze in Unit II has a much lower NRM intensity (~10^–3 A/m). Many discrete peaks of higher NRM values that appear in some depth intervals in both Units I and II (e.g., ~8, ~26, and ~45 mbsf) can be tied directly to the presence of volcanic tephra in these layers (see “Lithostratigraphy and petrology”). Magnetic susceptibility data also show positive peaks at these intervals (see “Physical properties”).

Paleomagnetic demagnetization results

As with Hole U1381A cores recovered during Expedition 334, remagnetization imparted by the coring process was encountered in Hole U1381C. NRM inclinations are strongly biased toward the vertical (mostly toward +90°) in a majority of cores. For the recovered sediment core sections, we employed AF demagnetization steps up to 30 mT at 5 mT increments. AF demagnetization to 5–10 mT seems to be effective in removing the drilling overprint magnetization, as shown by inclinations shifted toward shallower values that are comparable to the expected inclination for this site (approximately ±17°) and by a factor of 3–4 decrease in magnetization intensity after 5 mT demagnetization and an order of magnitude decrease after 30 mT demagnetization (Fig. F35). For several core sections from Unit II, AF demagnetization up to 30 mT was not effective in recovering the primary remanence magnetization. Inclination shifts toward shallower values (~50°) but still seems to be dominated by the near-vertical drilling-induced remagnetization.

The NRM declinations of Cores 344-U1381C-1H through 9H before orientation correction are different from each other, as expected. Upon orientation correction using the data from the FlexIT orientation tool, declinations are close to magnetic north for the normal polarity cores and magnetic south for some cores with negative inclinations, indicating the remanence is of geomagnetic origin (Fig. F35). More discussion about the magnetic declination records is given in “Magnetostratigraphy,” below.

The magnetic properties observed from the section halves were also confirmed by discrete sample measurements (Fig. F36). The nearly vertical overprint was removed by AF demagnetization of 5–10 mT for nearly all the samples. AF demagnetization was generally successful in isolating ChRM for most discrete samples: 58 out of 85 tested samples revealed ChRM with maximum angular dispersion <15°. Most discrete samples in Unit I show straightforward demagnetization behavior and reveal the ChRM. Several discrete samples from the lower part of Unit II, however, display more complicated demagnetization paths that do not decay simply toward the origin. An intermediate component with negative inclination but northerly declination is exhibited in these samples, which may be constrained by the great circles method. A small portion of samples also displayed erratic or incoherent demagnetization behavior, with data points that could not be fit to a line using the principal component analysis method (Kirschvink, 1980).

Magnetostratigraphy

We used ChRM declinations and inclinations from discrete measurements to define magnetic polarity sequences for the oriented cores in Hole U1381C. At a low-latitude area such as the location of Site U1381, a near 180° shift in declination in the cores is a more reliable sign of a polarity reversal than a change in sign of inclination.

As shown in Figure F37, several magnetic reversals were discerned on the basis of changes in sign of both inclinations and declinations. Based on a tephra layer that is present at ~25 mbsf and is believed to be the Tiribi Tuff ash layer (Ar-Ar dated at 320 ± 2 ka; Pérez et al., 2006), the two possibly reversed polarity zones in the depth interval from ~16 to 19 mbsf (Sections 344-U1381C-2H-6 through 3H-2) are tentatively identified as Subchrons Biwa I (176–186 ka) and Biwa II (292–298 ka) (Kawai et al., 1972). Shipboard micropaleontological studies also suggest that Core 2H should be older than 120 ka. The Brunhes/​Matuyama Chron boundary (0.78 Ma) is tentatively placed at ~49 mbsf between Section 344-U1381C-6H-3, 5 cm (49.2 mbsf), and 6H-3, 83 cm (49.98 mbsf). Lower Pleistocene biostratigraphic Zone NN19 is also placed in this interval (see “Paleontology and biostratigraphy”).

Below the Brunhes/​Matuyama Chron boundary, although the directional record defines several normal and reversed polarity zones, only one relatively well defined polarity interval has been identified in the downhole magnetostratigraphic records at ~61–64 mbsf (Fig. F37). Small magnetic polarity changes after the Brunhes/​Matuyama Chron boundary were recently documented (e.g., Jovane et al., 2008; Suganuma et al., 2011). Some of these small inclination fluctuations in Figure F37 may correspond to such changes. From Section 344-U1381C-7H-7 through the top part of Section 10H-5, negative inclinations are dominant, consistent with the notion that these cores were magnetized in a reversed field. The corresponding declinations, however, are close to magnetic north, suggesting that these cores may have recorded a normal polarity. More detailed postcruise paleomagnetic studies are needed to resolve the polarity assignment and improve the magnetostratigraphy at Site U1381.

Shipboard lithologic and micropaleontological studies suggest that sediments in Cores 344-U1381C-7H through 11H have ages 9–14 Ma, inferring a ~9 m.y. hiatus between Cores 6H and 7H. If correct, the mixed polarity sequence extending from 56 to 103 mbsf (Fig. F37) should be within Chrons C5–C5AA in the middle Miocene (Stages Tortonian–Serravalian; 11.63–15.16 Ma).

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