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doi:10.2204/iodp.proc.307.105.2006

Paleomagnetism

Shipboard paleomagnetic measurements were conducted on cores from Holes U1318A, U1318B, and U1318C. Alternating-field demagnetization of natural remanent magnetization (NRM) was conducted up to 20 mT in 5 mT steps on Core 307-U1318A-1H. Based on this demagnetization experiment (Fig. F10), the other sections were demagnetized at 10 and 15 mT. NRM and magnetization after two-step demagnetization were measured on archive halves. Discrete samples were taken on the working halves of cores in Holes U1318A and U1318B for subsequent shore-based magnetostratigraphic and rock magnetic studies.

The inclination data are clustered around ~64° in the uppermost part of Hole U1318A (Fig. F11). Between 58 and 64 mbsf, the inclination data decrease to 0°, which is probably a magnetic drilling overprint. Below 86 mbsf, the inclination data become more scattered. In Holes U1318B and U1318C, a similar trend is observed. The inclination data centers at ~66° but becomes more scattered near 88 mbsf in Hole U1318B and 84 mbsf in Hole U1318C. The inclination data must be interpreted cautiously because some bias and background noise in the cryogenic magnetometer and magnetic overprint gathered during drilling imparts an artificial magnetic inclination pointing downward. This is especially problematic for sediments with low magnetic intensities, such as carbonates.

Declination data could only be corrected by Tensor tool measurements for Cores 307-U1318A-3H through 11H, 307-U1318B-3H through 14H, and 307-U1318C-2H through 7H (Fig. F12). Declination data could not be corrected for the other cores because of XCB coring. Below 86 mbsf in Hole U1318A, 88 mbsf in Hole U1318B, and 84 mbsf in Hole U1318C, the declination data become more scattered, similar to the inclination data.

Up to 33 mbsf in Hole U1318A and 35.05 mbsf in Hole U1318B, magnetic intensities are between 0.002 and 0.02 A/m (Fig. F13). At 33 mbsf in Hole U1318A and 35.05 mbsf in Hole U1318B, intensities show an increase to 0.03–0.04 A/m and fall to 0 A/m at 60–62 mbsf. Between 60 and 79 mbsf in Hole U1318A and between 62 and 81.35 mbsf in Hole U1318B, a doublet in magnetic intensity shows the highest values at Site U1318 (up to 0.07 A/m). Below 87.75 mbsf in Hole U1318A, 87.1 mbsf in Hole U1318B, and 85.35 mbsf in Hole U1318C, intensities decrease to extremely low values of 10^–5 A/m. The different zones identified in the intensity correspond well with the lithostratigraphic units (see “Lithostratigraphy”). We see similar trends in magnetic susceptibilities (see “Physical properties”). This suggests that concentration of magnetic minerals, and magnetic mineralogy have the primary effect on the intensities; geomagnetic intensity is secondary.

Artificial magnetic overprints gathered during drilling and limitations in the reliability of the directional data from low-intensity measurements limits correlation to the geomagnetic polarity timescale. Thus, this first magnetostratigraphic interpretation awaits further measurements on discrete samples. Nevertheless, the positive inclinations above 86 mbsf in Hole U1318A, 88 mbsf in Hole U1318B, and 84 mbsf in Hole U1318C can be interpreted as a normal polarity zone corresponding with the Brunhes Chron, which has an age <0.78 Ma. This interval corresponds to lithostratigraphic Unit 1 and thin Unit 2 (see “Lithostratigraphy”). Below the base of Unit 2, inclination and declination data are scattered because of the drop in intensity. The base of lithostratigraphic Unit 2 is a sharp erosive boundary underlain by a shell bed (see “Lithostratigraphy”) and corresponds to a large hiatus (see “Biostratigraphy”).

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