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doi:10.2204/iodp.proc.307.104.2006

Paleomagnetism

Shipboard paleomagnetic measurements were conducted on cores from Holes U1317A, U1317B, U1317C, U1317D, and U1317E. Alternating-field (AF) demagnetization of natural remanent magnetization (NRM) was conducted up to 20 mT in 5 mT steps on Section 307-U1317A-1H-1. Based on this demagnetization experiment (Fig. F12), cores from Holes U1317B, U1317C, and U1317D were demagnetized at 10 and 15 mT. Cores from Hole U1317E were demagnetized at 2, 5, 7, 10, and 15 mT. NRM and magnetization after AF demagnetization were measured on whole-round sections because Holes U1317B, U1317C, U1317D, and U1317E remained closed for further macrotomographic analyses and freezing before opening. A comparative test on whole rounds and archive halves of cores from Site U1316 (see “Paleomagnetism” in the “Site U1316” chapter) showed that measuring whole-round sections does not adversely affect the measurements. On the contrary, for core sections with very weak intensities (i.e., carbonate-rich sediments from lithostratigraphic Unit 1), whole-round sections provide more precise information because the entire mass of material is measured and is undisturbed by section splitting. Moreover, intensities measured on whole-round sections can be better used for normalization by magnetic susceptibilities because the same material is measured (see “Paleomagnetism” in the “Site U1316” chapter). Discrete samples were taken on the working halves in Hole U1317A for subsequent shore-based magnetostratigraphic and rock magnetic studies.

As at Site U1316, inclination data clustered around ~66° in the uppermost sections (0–62 mbsf) in Holes U1317A, U1317B, U1317C, and U1317E (Fig. F13). Small excursions to lower values can be observed within this uppermost zone. Below 63, 62, 53, and 56 mbsf in Holes U1317A, U1317B, U1317C, and U1317E, respectively, the inclinations have a tendency to decrease to negative values, which could indicate the transition to reversed polarity. Normal polarity inclinations are generally observed between 78 and 92 mbsf in Hole U1317A, 81.8 and 96.5 mbsf in Hole U1317B, 82.3 and 100 mbsf in Hole U1317C, and 75 and 102 mbsf in Hole U1317E. These normal inclinations are underlain by predominantly reversed inclinations. Normal inclinations are then observed in the following intervals to the hiatus at the base of the mound: 101–130.5 mbsf in Hole U1317A, 105.4–142 mbsf in Hole U1317B, 111–150 mbsf in Hole U1317C, and 108–157 mbsf in Hole U1317E. From 146 to 205 mbsf in Hole U1317D, the inclination data are highly scattered but predominantly normal (Fig. F14). Deeper than 205 mbsf, the inclination values show less variation and center at ~66°. We attribute the transition from higher to lower variability to a lithologic overprint. A mainly sandy to clayey silt facies changes to silty sands over these depths (see “Lithostratigraphy”). The inclination data must be interpreted carefully because some bias and background noise (due to the motion of the ship) in the cryogenic magnetometer and a magnetic overprint gathered during drilling may influence the measurements, resulting in an artificial magnetic inclination pointing downward. This problem may be especially severe in carbonate-rich sediments that have low magnetic intensities.

Declination data could only be corrected by Tensor tool measurements for Cores 307-U1317A-3H through 7H, 307-U1317B-3H through 12H, 307-U1317C-1H through 11H, and 307-U1317E-3H through 15H. Declination data could not be corrected for Hole U1317D because of RCB drilling.

The low magnetic intensities and susceptibilities are typical for carbonate-rich sediments. Carbonate minerals in all holes are responsible for the diamagnetic character of the susceptibility measurements. The weakest intensity values are within the range of the noise level of the cryogenic magnetometer. A normalization of the intensities was carried out by dividing intensities by magnetic susceptibilities. Some normalized intensity peaks could be recognized (Fig. F15).

Correlation to the geomagnetic polarity timescale was limited by artificial magnetic overprints gathered during drilling, a major hiatus at the mound base identified in the biostratigraphic and sedimentological data (see “Lithostratigraphy” and “Biostratigraphy”), limitations in the reliability of the directional data from low-intensity measurements, and diamagnetic overprints in the mound sediments. However, a first tentative magnetostratigraphic framework could be constructed (Fig. F13) but must be interpreted carefully.

The predominantly normal polarities above 62 mbsf in Hole U1317A, 61 mbsf in Hole U1317B, 53 mbsf in Hole U1317C, and 56 mbsf in Hole U1317E probably belong to the Brunhes Chron (C1n), which has an age <0.78 Ma. The negative inclination values below these depths correspond with a reversal period, probably the Matuyama Chron (C1r.1r). We tentatively interpret the zone between 78 and 92 mbsf in Hole U1317A, 81 and 96 mbsf in Hole U1317B, 82 and 100 mbsf in Hole U1317C, and 75 and 102 mbsf in Hole U1317E as the normal Jaramillo Chron (C1r.1n), with an age from 0.990 to 1.070 Ma. This is generally consistent with the biostratigraphic interpretation (see “Biostratigraphy”). The underlying zone would therefore be associated with Subchron C1r.2r, which is characterized by an age >1.070 Ma but <1.770 Ma. A small erosion surface at the base of this zone is observed in the sedimentology (see “Lithostratigraphy”). Below this interval, positive inclinations are observed pointing out a normal polarity chron that would correspond with the Olduvai Chron (C2n). The sediments in this chron should be <1.950 Ma; however, an alternative may be to interpret the two normal polarity intervals above the mound base as the Olduvai Chron (C2n; 1.770–1.950 Ma) and the Gauss Chron (C2An.1n; 2.60–3.04 Ma).

Measurements on discrete samples must be carried out to improve and confirm this first magnetostratigraphic interpretation.

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