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doi:10.2204/iodp.proc.313.104.2010

Paleomagnetism

Discrete sample measurements

A total of 457 samples from Hole M0028A were measured in the pass-through magnetometer. In addition to the standard one sample per section, Cores 313-M0028A-2R through 11R, 27R through 37R, and 152R through 171R were sampled at a denser resolution ranging from one sample every 50 cm to one sample every 20 cm. The natural remanent magnetization (NRM) as well as the remanence after sequential alternating-field (AF) demagnetization up to 15, 30, or 60 mT was measured for all samples.

Remanent magnetization

Similar to Hole M0027A, the primary magnetization in Hole M0028A is mostly carried by a low-coercivity component, and again demagnetization also indicates the presence of a high-coercivity magnetic mineral. The initial NRM moment of the sediments is typically weak, on the order of 10^–10 to 10^–8 Am², except for two clay-rich horizons in Cores 313-M0028A-7R and 8R (~239–245 mbsf) and 155R through 167R (~620–660 mbsf), which exhibit stronger magnetic moments on the order of 10^–8 to 10^–6 Am², accompanied by peaks in magnetic susceptibility (Fig. F44).

The coarser grained sequences that dominate Hole M0028A very rarely exhibit a stable NRM component after AF demagnetization. The fine-grained sediments (silt and clay) from Cores 313-M0028A-27R through 37R (~290–321 mbsf) are also very weakly magnetized. As in Hole M0027A, inclination data show prevailing normal polarity, suggesting a viscous overprint. All cores were recovered using the ALN, which, according to our observations in Hole M0027A, could induce weak and unstable NRM magnetization magnetic grain rotation and/or structural deformation. In general, data are too noisy to distinguish any polarity zones except for Cores 313-M0028A-2R through 11R (223.33–253.83 mbsf) and 152R through 171R (~612–669 mbsf) (Fig. F45).

For the lower part of Hole M0028A, we decided to limit the demagnetization procedure to a maximum alternating field of 15 mT in order to preserve the signal for more detailed and careful treatment after the OSP. This decision was made after looking in detail at the Hole M0027A results.

In Cores 313-M0028A-7R and 8R between 239 and 245 mbsf, inclination data show frequent reversals carried by stable components (with the maximum angular deviation typically around 3°–5°). However, interpreting all of these intervals as real polarity chrons is not compatible with the general age-depth constraints given by Sr ages and biostratigraphic time zones. Instead, we suspect that either the normal or the reversed parts of this interval could be caused by a chemical remanent magnetization (CRM) overprint and more analyses are required to determine this.

As in Hole M0027A, we could only isolate a stable characteristic remnant magnetization (ChRM) within intervals with high magnetic susceptibility, implying to some degree dissolution of the primary magnetic carriers within intervals with low magnetic susceptibility. In thin sections, iron sulfide and iron carbonate are commonly observed as diagenetic phases in the sediment matrix (Fig. F46).

Magnetostratigraphy

Because of the generally weak and unstable NRM of the sediments, we were only able to establish a polarity magnetostratigraphy for Cores 313-M0028A-2R through 12R (~223–255 mbsf) (Fig. F47) and 152R through 171R (~610–669 mbsf) (Fig. F45). Preliminary magnetostratigraphic age-depth tie points are given in Table T13. For Cores 313-M0028A-2R through 11R (~223–254 mbsf), polarity interpretations are based partly on principal component analysis (PCA) (red line in Fig. F47) and inclination after 20 mT AF demagnetization. Samples from Cores 313-M0028A-152R through 171R (~610–669 mbsf) were only AF demagnetized up to 15 mT, and polarity interpretations are therefore based on the inclination after 15 mT AF demagnetization in addition to studies of NRM components using orthogonal Zijderveld plots.

As in Hole M0027A, the magnetostratigraphic interpretation of inclination data from Cores 313-M0028A-2R through 12R (~223–255 mbsf) is dependent on the placement of the m4.1 surface (currently placed at ~236 mbsf), which, according to the seismic study, could represent an unconformity in Hole M0028A. In this chapter, we present two alternative interpretations (Table T13) for each reversal boundary based on the constraints given by Sr isotope ages and biohorizons (Figs. F45, F47).

For Cores 313-M0028A-2R through 11R (~223–254 mbsf), the first reversal boundary (from normal to reversed polarity [N/R]) between 242.59 and 242.30 mbsf is identified as the onset of either Chron C5ABr or C5ACr. The next reversal boundary (R/N) between 226.53 and 226.23 mbsf is identified as the onset of Chron C5AAn or C5ABn.

For Cores 313-M0028A-152R through 171R (610–669 mbsf), the first reversal boundary (N/R) between 666.41 and 665.91 mbsf is identified as the onset of Chron C6Ar. The reversal boundary (R/N) between 656.34 and 655.92 mbsf is identified as the onset of Chron C6An.2n. The reversal boundary (N/R) between 622.90 and 622.65 mbsf is identified as the onset of Chron C6An.1r, and the last reversal boundary (R/N) between 617.02 and 616.52 mbsf is identified as the onset of Chron C6An.1n.

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