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doi:10.2204/iodp.proc.302.104.2006

Paleomagnetism

General magnetic properties

The major features in the magnetic properties of the recovered sediments are a sharp decrease of natural remanent magnetization (NRM) and MS between 193 and 388 mbsf, in close correspondence with a sharp change in sediment color from olive-brown to light gray, followed by a large increase in these parameters between 388 and 405 mbsf (Fig. F45). Between 193 and 385 mbsf, NRM intensity values decrease by a factor of 200 and susceptibility values decrease by a factor of 10. In the interval with low NRM values (193–385 mbsf), some discrete intervals are strongly magnetic and may be associated with iron sulfide concretions. The strong and weak magnetic zones may be linked to the effects of depositional changes and diagenetic processes. The originally deposited iron oxides may have dissolved, leading to the formation of diagenetic iron sulfides (see “Geochemistry”). Further study of discrete samples should determine the magnetic mineralogy of both the strong and weak magnetic zones.

AF demagnetization behavior

Detailed stepwise alternating-field (AF) demagnetization was performed on U-channels taken from all cores recovered during Expedition 302. The AF demagnetization behavior is notably different in the intervals with high (>10^–3 A/m) and low (<10^–4 A/m) intensity of NRM. In the more magnetic intervals (0–193 mbsf and 388–405 mbsf), AF demagnetization allows the determination of a clear characteristic magnetization with steep inclination.

In the weak magnetic interval, the demagnetization plots are usually noisy and do not show evidence of a clear characteristic component of magnetization. The median destructive field is often as low as 10 mT. The low-coercivity components of NRM that can be identified for most levels are generally directed steeply downward and are likely to be either a viscous magnetization acquired in the present-day magnetic field or a magnetization acquired during drilling. After demagnetization of this low-coercivity component, the intensity is generally too low (<10^–5 A/m) to identify a characteristic component. Postcruise thermal demagnetization of discrete samples may be able to erase the viscous overprint without affecting the characteristic magnetization. In some cases, shallow inclinations are found that may correspond to a magnetization acquired during core processing or U-channel sampling. For almost all samples in the weakly magnetic interval, there is a tendency for the NRM to behave erratically above demagnetization steps of ~40 mT. This suggests that parasitic anhysteretic remanent magnetizations have been acquired at high demagnetizing fields.

Magnetic polarity stratigraphy

The inclinations obtained in the weakly magnetized interval (193–385 mbsf) are frequently between –60° and 60°, too low for the latitude of the site. The overall distribution of inclination values after 30 mT demagnetization is clearly not satisfactory because there is no dominance of steep negative and positive inclinations. In fact, the inclination distribution is almost random, with a slight bias toward normal polarity directions, attributable to incomplete removal at 30 mT of a viscous component with normal polarity. Within this weakly magnetized interval, the largely indeterminate inclinations prevent any meaningful interpretation of the polarity.

The distribution of inclination values after 30 mT demagnetization in the strongly magnetic intervals is notably different, as steep positive and negative inclinations predominate. Between 0 and 193 mbsf, the inclination record can be interpreted in terms of normal and reversed polarity zones (Fig. F46A, F46B) (Ogg and Smith, 2004). A number of factors contribute to the difficulty in determining a unique and unambiguous magnetostratigrapy for Expedition 302 sediment. These include the tendency of Arctic sediments to record a large number of geomagnetic excursions during the Brunhes and Matuyama Chrons, sparse biostratigraphic data, incomplete core recovery, and core disturbance. Nevertheless, we have been able to narrow our interpretations to two models (Fig. F46A, F46B) and to identify a currently preferred age model within each interval. The age-depth curves that correspond to Figure F46A and F46B are shown in Figure F19 (see “Timescale and sedimentation rates”).

Between 388 and 405 mbsf, the inclination record is also interpretable in terms of paleomagnetic polarity, except for Section 302-M0004A-33X-1, which is heavily disturbed (Fig. F47). Unfortunately, the recovery is poor and only a single reversal can be identified: the polarity shifts downward from reversed to normal within Section 302-M0004A-34X-3, at 399.63 mbsf. In view of the biostratigraphic data (see “Biostratigraphy”), this corresponds to the top of Chron C25n with an absolute age of 56.6 Ma (Luterbacher et al., 2005).

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