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doi:10.2204/iodp.proc.329.103.2011

Paleomagnetism

Archive halves of 26 APC sediment cores from Hole U1365A, 9 from Hole U1365B, 9 from Hole U1365C, 2 from Hole U1365D, and 11 RCB basement cores from Hole U1365E were measured on the three-axis cryogenic magnetometer at 2.5 cm intervals before and after demagnetization with peak alternating fields (AF) of 10 and 20 mT. The data from samples with drilling disturbance (e.g., soupy sediments in Section 329-U1365A-3H-1) and fractured chert rubble [Cores 329-U1365A-6H through 21H] in Fig. F49) were removed based on lithostratigraphy. Data from Holes U1365B–U1365D provide only a partial record because whole-round samples were taken for geochemical and microbiological analyses.

In addition, 43 discrete sediment samples (7 cm³ cubes) taken at an interval of one per section from all cores of lithologic Units I and III (Cores 329-U1365A-1H through 5H and U1365A-23H through 26H; see “Lithostratigraphy”) were analyzed. Of these, 24 were measured for natural remanent magnetization (NRM) before and after AF demagnetization at peak fields of 10 and 20 mT using the pass-through magnetometer. A discrete rock sample was also taken and measured for NRM from each of the 11 RCB basement cores from Hole U1365E. With the basement samples, NRM was measured after AF demagnetization at peak fields of 10, 20, 30, 40, 50, and 60 mT using the pass-through magnetometer and the Agico spinner magnetometer. The primary objective of the shipboard measurements was to provide chronostratigraphic constraint by determining the magnetic polarity stratigraphy.

The Flexit core orientation tool was deployed for Cores 329-U1365A-1H through 4H. For operation of the Flexit tool, see “Paleomagnetism” in the “Methods” chapter (Expedition 329 Scientists, 2011). The Flexit tool was deployed in combination with nonmagnetic core barrels. The orientation angle determined from the Flexit software is listed in Table T10. After Core 4H, cores were collected using a steel core barrel without the Flexit tool to avoid damaging the Flexit tool or the nonmagnetic barrels on hard lithologies.

Results

Paleomagnetic data for Holes U1365A–U1365E are presented in Figures F49, F50, F51, F52, and F53, together with the whole-core susceptibility data measured on the WRMSL. Magnetization directions are not interpretable throughout most of the Site U1365 cores, possibly because of a magnetic overprint acquired during coring (high positive inclination), a viscous remanent magnetization, or diagenetic changes in the sediment. However, in two intervals in Hole U1365A (0–6 and 66–75 mbsf), distinct magnetization directions can be resolved.

Our polarity zone interpretation for 0–6 mbsf in Hole U1365A begins with the Brunhes/Matuyama Chron boundary. Interpreted reversal depths are provided in Table T11, and the polarity interpretations are shown in Figure F54. This interpretation is based on the inclination record after demagnetization at peak fields of 20 mT (Fig. F54). The present-day normal field in this region, as expected from the geocentric axial dipole model at the site, has a negative inclination (approximately –41.5°), so positive remanence inclinations indicate reversed polarity. Sedimentation rates based on this polarity interpretation decrease downcore from ~1.71 to ~0.57 m/m.y. at a depth of 6 mbsf (Fig. F55).

The discrete basalt samples taken from Hole U1365E all show normal polarity at the 20 mT demagnetization step (Fig. F53). This is consistent with the interpretation that the basaltic basement at this site is within the Cretaceous Normal Superchron.

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