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doi:10.2204/iodp.proc.303306.112.2006

Paleomagnetism

Archive halves of all cores recovered at Site U1313 were measured on the three-axis cryogenic magnetometer at 5 cm intervals. The natural remanent magnetization (NRM) was measured before (NRM step) and after stepwise alternating-field (AF) demagnetization in peak fields of up to 20 mT. Cores 306-U1313A-1H through 15H and 306-U1313B-1H were AF demagnetized at peak fields of 10 and 20 mT. All the other cores were AF demagnetized at a peak field of 20 mT to increase core flow through the laboratory.

Downcore variations in magnetic intensity in Holes U1313A through U1313D are shown in Figure F18. Data associated with intervals identified as physically disturbed were removed. NRM intensities after 20 mT AF demagnetization are in the range of 10^–3 to 10^–4 A/m above 150 mbsf and drop by an order of magnitude (in the range of 10^–5 A/m) in the lower part of the section. Inclination and declination data (after 20 mT AF demagnetization) are shown in Figure F19. Declination values have been corrected using Tensor tool data, which were available starting with Cores 306-U1313A-3H, 306-U1313B-3H, 306-U1313C-4H, and 306-U1313D-3H. The Tensor tool was not used in Cores 306-U1313B-31H and 32H. All four holes display similar directional changes to ~250 mbsf.

The distribution of inclination values at Site U1313, after AF demagnetization at 20 mT, is composed of two log-normal distributions centered at –52° and +63°. These values are close to the expected site values for a geocentric axial dipole (I_GAD = ±60.1°). These results indicate that most of the drill string overprint was removed at 20 mT, although a small overprint may remain in some intervals as the distributions are slightly offset toward more positive values.

The magnetostratigraphy was constructed based on the succession of polarity reversals recorded at Site U1313 (Fig. F20). The Brunhes/Matuyama reversal occurs at 34.2 ± 0.4 mbsf in Hole U1313A, 34.1 ± 0.4 mbsf in Hole U1313B, 34.1 ± 0.4 mbsf in Hole U1313C, and 33.8 ± 0.1 mbsf in Hole U1313D. A detailed list of the occurrence depths of the polarity transitions identified at Site U1313 and their possible correlation to the geomagnetic polarity timescale (Cande and Kent, 1995) is provided in Table T24. Overall, the Jaramillo and Olduvai Subchrons are well recorded in these sediments, along with shorter geomagnetic events such as Cobb Mountain and Reunion. In the region below 150 mbsf (~167 meters composite depth [mcd]), the inclination signal is noisier due to lower NRM intensity values. It is, however, possible to observe alternating intervals of normal and reversed polarities, which are consistent from one hole to the other. The resulting magnetostratigraphy can be correlated to the geomagnetic polarity timescale with good confidence to ~220 mbsf (~246 mcd) and is consistent with biostratigraphy (see “Biostratigraphy”).

In the underlying sediment, however, the link to the biostratigraphy-based age model is not straightforward. The polarity within this interval is assumed to be reversed between 222.3 and 239.2 mbsf in Hole U1313A then normal between 239.2 and 249.7 mbsf (262.4 and 277.1 mcd). The magnetostratigraphy is uncertain below this depth, as it varies from one hole to the other. In particular, it is difficult to assess whether the polarity is normal or reversed at the bottom of the section. A possible explanation for the observed discrepancies is that alloy steel core barrels, which might induce a stronger drill string overprint, were used instead of the nonmagnetic core barrels in Cores 306-U1313A-32H and 33H and 306-U1313C-31H and 32H. If we assume that the sediment recovered in Hole U1313B carries the most reliable signal, then the polarity below ~250 mbsf (~277 mcd) is reversed.

Given the noise level of the data, there are two possible ties to the global geomagnetic timescale, which are illustrated in Figure F20. A first possibility is to identify the normal polarity interval between 239.2 and 249.7 mbsf as Chron C3An (including both Subchrons C3An.1n and C3An.2n), whereas the second possibility is to identify it as Subchron C3An.1n only. In the first case, the base of this normal polarity interval would be dated at 6.56 Ma, whereas in the second case it would be dated at 6.137 Ma (Fig. F20). The second case provides an age model that is more consistent with the one based on biostratigraphy but requires a large increase in sedimentation rates at ~250 mbsf (~277 mcd), as the next polarity reversal is not present in the section.

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