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doi:10.2204/iodp.proc.307.103.2006

Paleomagnetism

Shipboard paleomagnetic measurements were conducted on cores from Holes U1316A, U1316B, and U1316C. Alternating-field (AF) demagnetization of the natural remanent magnetization (NRM) was conducted up to 20 mT in 5 mT steps on Section 307-U1316A-1H-1. Based on this demagnetization experiment (Fig. F19), the other sections from Site U1316 were demagnetized at 15 and 20 mT. For Holes U1316A and U1316C, NRM and magnetizations after two-step demagnetization were measured on archive halves, whereas for Hole U1316B, whole-round sections were measured. Discrete samples were taken from the working halves of Hole U1316A for subsequent shore-based magnetostratigraphic and rock magnetic studies.

Inclination data are clustered around ~66° in the uppermost part and become more scattered at ~48 mbsf in Hole U1316A and ~50.5 mbsf in Hole U1316B (Fig. F20). The upper values are approaching ~68°, which is the expected inclination at the site latitude (51.4°N). Below 55.95 mbsf in Hole U1316A and 58.7 mbsf in Hole U1316B, the inclinations decline to lower values. Below 76 mbsf, the inclination data cluster again around ~66°. The inclination data between 120 and 136 mbsf in Hole U1316C could indicate a reversal event; however, inclination data have to be considered with care because the scattering of the data below 65 mbsf in Holes U1316A and U1316C can be influenced by possible core disturbance during XCB drilling (i.e., “biscuit-structure”). On the other hand, the presence of carbonate-rich materials, characterized by weak magnetic intensities, would explain the altered data in the lower parts.

Declination data could only be corrected for in Cores 307-U1316A-3H to 7H and 307-U1316B-3H to 7H. Like the inclination data, the declination data are more scattered at 48–55.95 mbsf in Hole U1316A and 50.5–58.7 mbsf in Hole U1316B. The reliability of the inclination and declination data had to be interpreted carefully because bias and background noise (due to the motion of the ship) in the cryogenic magnetometer and a magnetic overprint gathered during drilling influenced the measurements. This was especially critical for the low magnetic intensities measured in the carbonate-rich sediments. These obstacles manifest in the form of an artificial magnetic overprint pointing downward in the core.

The magnetic intensities in Holes U1316A and U1316B reached values between 15 mA/m (NRM) and 45 mA/m (20 mT intensity) in the uppermost part of the holes and slightly increased to depths of 44 mbsf in Hole U1316A and 45.45 mbsf in Hole U1316B (Fig. F21). Below these depths, intensities decreased again in different steps. Between 44 and 48 mbsf in Hole U1316A and 45.45 and 50.5 mbsf in Hole U1316B, values are again in the same range as the uppermost sections. Low values of 1–3 mA/m (20 mT intensity) are reached between 48 and 55.95 mbsf in Hole U1316A and 50.5 and 58.7 mbsf in Hole U1316B and fall in the lowermost sections within the range of the noise level (0.1–0.01 mA/m at 20 mT intensity). The intensities follow the same trend as the magnetic susceptibilities. This suggests that the concentration of magnetic minerals, magnetic mineralogy, and size of the magnetic minerals have a strong impact on the intensities. The impact of the geomagnetic intensity is secondary. The low intensity record and the low susceptibilities below 55.95 mbsf in Hole U1316A, 58.7 mbsf in Hole U1316B, and 52 mbsf in Hole U1316C are typical for carbonate-rich sediments.

The effect of the magnetic mineral concentration and composition on the intensity could be normalized by dividing the intensities by the magnetic susceptibilities for paleomagnetic measurements made on whole-round cores (Hole U1316B). Both were measured on the same whole-round sections with exactly the same volume, which makes susceptibility an excellent parameter for the normalization of the intensities (Fig. F21). Measurements on whole-round sections did not affect the results.

As a result of uncertainty in polarity, the presence of a large hiatus in the sedimentary record, and limitations in the reliability of the directional data from the shipboard cryogenic magnetometer, correlation to the geomagnetic polarity timescale was severely limited. The predominantly normal polarities above 55.95 and 58.7 mbsf are correlated with the Brunhes Chron, with an age <0.78 Ma.

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