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doi:10.2204/iodp.proc.344.106.2013

Paleomagnetism

We measured the natural remanent magnetization (NRM) of all archive-half core sections (Cores 344-U1380C-11R through 52R) and 60 discrete samples taken from the working halves. We subjected archive-half cores to alternating field (AF) demagnetization up to 30 mT and measured them with the pass-through superconducting rock magnetometer (SRM) at 2.5 cm intervals. Discrete samples were subjected to stepwise thermal and AF demagnetization up to 475°C and 120 mT, respectively, to isolate the characteristic remanent magnetization (ChRM). Discrete samples were also measured with the SRM.

Natural remanent magnetization of sedimentary cores

Downhole variations of paleomagnetic data obtained from Hole U1380C are shown in Figure F33. In general, we find that the magnetic properties of the recovered sediments are relatively uniform, with insignificant variations downhole. In Cores 344-U1380C-2R through 13R, silty clay and claystone with fine sandstone in Unit I (438.00–552.72 mbsf; see “Lithostratigraphy and petrology”) have an NRM intensity between 1.9 × 10^–4 and 8.6 × 10^–1 A/m, with a mean of 6.2 × 10^–3 A/m. NRM intensity for the clayey siltstone, coarse, shell-rich sand, and sandstone in Unit II (552.72–771.62 mbsf) is somewhat higher than in Unit I (ranging from 2.1 × 10^–4 to 3.6 × 10^–1 A/m, with a mean of 2.0 × 10^–2 A/m). Several peaks of higher NRM intensity exist in some depth intervals in Unit II. The high-intensity peaks remained even after 30 mT AF demagnetization, suggesting that it is unlikely that they are caused entirely by drilling-induced remagnetization. Paleomagnetic measurements indicate that the silty claystone with fine sandstone in Unit III (771.62–800.00 mbsf) has NRM intensity ranging from 3.2 × 10^–4 to 1.1 × 10^–1 A/m, with a mean of 1.7 × 10^–2 A/m. Variations in magnetic susceptibility generally follow variations in NRM intensity (see “Physical properties”).

Since the beginning of DSDP in 1968, paleomagnetists have been aware of remagnetization acquired during drilling and/or recovery of cores (Stokking et al., 1993). This remagnetization is characterized by NRM inclinations that are strongly biased toward a steep downward inclination that can be removed at initial stages of demagnetization. In Hole U1380C, however, we were able to use this remagnetization to correct the orientation error for one discrete sample. As shown in Figure F34A, the drilling overprint from Sample 344-U1380C-51R-2W, 65–67 cm (788.86 mbsf), has a steep upward inclination, which is opposite to the drilling overprint component from all other samples, as well as those from the corresponding section-half interval during the pass-through section measurement (i.e., steep downward; Fig. F34B, F34C). In this case, the overprint component served as a useful check. By putting this discrete sample back to its original cored position, we found that the orientation arrow in this sample was marked 180° away from the correct direction; that is, instead of the –z-direction, it was marked in the +z-direction and measured on the SRM.

Paleomagnetic demagnetization results

For the recovered sediment core sections, we employed AF demagnetization steps up to 30 mT. AF demagnetization to 5–15 mT was generally effective in removing the drilling overprint magnetization, as shown by inclinations shifting toward shallower values and by a significant decrease in magnetization intensity (Fig. F33). Although the level of AF demagnetization we applied did not always remove the overprint, ChRM directions can generally be isolated from the pass-through measurements. An example of good-quality AF demagnetization results is shown in Figure F35A.

The remanent magnetization of 60 discrete samples from the three lithostratigraphic units in Hole U1380C was investigated using stepwise AF and thermal demagnetization. In most cases, ChRM is revealed after the removal of the drilling-induced component (Fig. F35). Most thermally demagnetized samples show unblocking temperatures between 175° and 250°C, indicating that the main magnetic carriers in these samples have low Curie temperatures.

Magnetostratigraphy

Because a rotary coring technique was used to recover cores in Hole U1380C, we used ChRM inclinations from both discrete and pass-through measurements to define magnetic polarity sequences. As shown in Figure F36, several magnetic reversals were discerned on the basis of changes in inclination sign.

Samples from Unit III are dominated by normal polarity. Within Units I and II, samples mainly have reversed polarity. Some normal polarity samples are also present in the middle part of the hole. Shipboard micropaleontological studies suggest that Core 344-U1380C-50R should be older than 2.39 Ma (see “Paleontology and biostratigraphy”). Consequently, the Matuyama/Gauss Chron boundary (2.581 Ma in Gradstein et al., 2012) is tentatively placed at ~770 mbsf.

Within the Matuyama Chron (Fig. F36), the two relatively short and well-defined normal polarity intervals at ~730–764 mbsf may represent Chron C2r.1n (Reunion Subchron; 2.128–2.148 Ma). The interval (633.91–666.61 mbsf) of Sections 344-U1380C-22R-2, 125 cm, to 26R-1, 60 cm, shows normal inclinations, consistent with this interval possibly being the normal polarity Chron C2n (Olduvai Subchron; 1.778–1.945 Ma). Above 633.91 mbsf, some single normal polarity samples are recognized but do not define any consistent normal magnetozones (Fig. F36). At this time, we are not inclined to correlate them with the geomagnetic polarity timescale because they are defined by limited data points and are tentative. Shore-based paleomagnetic studies will be conducted to improve the magnetostratigraphy at Site U1380.

Anisotropy of magnetic susceptibility

For two representative samples taken from Core 344-U1380C-13R (a major shear zone; see “Structural geology”), anisotropy of magnetic susceptibility (AMS) was measured with the Kappabridge KLY 4S before and after progressive AF demagnetization (up to 180 and 200 mT, respectively). The difference in AMS results before and after AF demagnetization was checked with the two samples, and there was no significant difference found for the K_min of these samples. Mean volume magnetic susceptibilities showed only minor changes through demagnetization from 1.356 × 10^–4 to 1.381 × 10^–4 SI for Sample 344-U1380C-13R-1, 28–30 cm, and from 1.72 × 10^–4 to 1.6599 × 10^–4 SI for Sample 344-U1380C-13R-4, 62–64 cm. For both samples, the shape parameter is positive, indicating an oblate shape of the ellipsoid. Although it is recommended to make AMS measurements before AF demagnetization (e.g., Jordanova et al., 2007), our preliminary results indicate that we can use AMS after AF demagnetization on sediment layers recovered at Site U1380.

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