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doi:10.2204/iodp.proc.317.106.2011

Paleomagnetism

Paleomagnetic analyses at Site U1354 included measurement and partial demagnetization of natural remanent magnetization (NRM) of archive section halves. No discrete samples from Site U1354 were measured during shipboard analyses. All depths in this section are reported in m CSF-A.

Section-half measurements

NRM was measured on archive section halves from Holes U1354A–U1354C before and after demagnetization at 20 mT peak fields. Persistent flux jumps in the superconducting rock magnetometer (SRM) made measurements difficult at Site U1354. Many section halves were measured several times before readings were accepted. Because of the limited time available, no acceptable measurement could be obtained for some sections, whereas for others only NRM was recorded. In spite of this, a good record of all three holes from Site U1354 was acquired.

Holes U1354A and U1354B have similar total depths, and comparable records were obtained (Figs. F16, F17). These relatively shallow holes were cored using the APC system with nonmagnetic core barrels throughout. Hole U1354C was drilled without coring for 65 m, at which point the APC system was used with nonmagnetic core barrels to recover two cores overlapping the bases of Holes U1354A and U1354B. Coring continued with the XCB system to total depth (Fig. F18).

NRM intensities typically vary between 10^–2 and 10^–3 A/m, and some anomalous peaks were recorded that correspond to intervals of high magnetic susceptibility and cave-in. However, because of the difficulty of taking measurements, many of these sections were not measured in the SRM.

NRM inclinations from Site U1354 are typically steeply positive (~80°), consistent with the drilling overprint observed at previous sites. In the upper parts of Holes U1354A and U1354B, these inclinations demagnetized at 20 mT to fairly steeply negative inclinations. Declinations throughout this interval are consistent within cores and variable between cores, and they do not change direction significantly with demagnetization, suggesting a negligible radial drilling overprint. NRM declinations in Hole U1354C are clustered in the northern hemisphere and appear to display a drilling overprint associated with the standard steel core barrels used with the XCB coring system.

Reversed polarity was observed after 20 mT demagnetization toward the base of Holes U1354A (from 69.9 m) and U1354B (from 64.75 m) and at the top of Hole U1354C. Inclinations remain steeply positive after demagnetization. Where the normal/reversed boundary is observed (Cores 317-U1354A-13H and 317-U1354B-13H), declinations change within the core. In each hole, this boundary corresponds with a lithologic boundary where green, muddy very fine sand overlies gray silty mud at an irregular contact.

The polarity reversal observed at the base of Holes U1354A and U1354B is the only one observed at Site U1354. Older sediments were recovered from Hole U1354C, but the drilling overprint imparted by the XCB system was not removed by 20 mT demagnetization.

Magnetostratigraphy

Normal polarity sediments from the Brunhes Chron were identified from 0 to 69.9 m in Hole U1354A and from 0 to 64.75 m in Hole U1354B. These sediments are underlain across a lithologic boundary by reversed polarity (Matuyama) sediments. The exact amount of time represented by this boundary is unknown, but it includes the Brunhes/Matuyama boundary (0.78 Ma) and is constrained by biostratigraphic evidence. Sediments overlying the boundary are older than 0.44 Ma, as indicated by the HO of Pseudoemiliania lacunosa between 56.36 and 64.11 m in Hole U1354A and between 56.36 and 64.11 m in Hole U1354B. Underlying sediments contain a nannofossil assemblage similar to that above the boundary (suggesting that any hiatus is brief) and lack Reticulofenestra asanoi (HO = 0.91 Ma).

Further nannofossil evidence suggests that the bases of Holes U1354A and U1354B are older than 1.26 Ma (HO Gephyrocapsa >5.5 µm is between 76.20 and 80.16 m in Hole U1354A and between 73.65 and 73.89 m in Hole U1354B). The Jaramillo normal Chron (C1r.1n = 0.998–1.072 Ma) was not recorded at Site U1354 and is likely represented by a second hiatus. This concurs with the absence of Reticulofenestra asanoi (total range = 0.91–1.14 Ma).

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