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doi:10.2204/iodp.proc.313.105.2010

Paleomagnetism

Discrete sample measurements

A total of 511 samples from Hole M0029A were measured in the pass-through magnetometer. Sequences of finer grained sediments (silt and clay) were targeted for sampling at varying resolution, from one sample every 20 cm to one sample every meter, depending on grain size and predicted accumulation rates. In addition, some coarser grained sequences were sampled at one sample per section. Natural remanent magnetization (NRM) and the remanence after sequential alternating-field (AF) demagnetization up to 15 or 60 mT was measured for all samples.

Remanent magnetization

Similar to Holes M0027A and M0028A, the primary magnetization in Hole M0029A is mostly carried by a low-coercivity component, with demagnetization also indicating 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 a clay-rich horizon in Cores 313-M0029A-210R and 211R (~734–740 mbsf), which exhibits stronger magnetic moments, on the order of 10^–8 Am² (Fig. F37).

Just as for the lower half of Hole M0028A, we decided to limit the demagnetization procedure to a maximum alternating field of 15 mT (except for cores) to preserve the signal for more detailed and careful treatment after the science party. Magnetic remanence from Hole M0029A sediments generally follows a semistraight trajectory after successive AF demagnetization up to 15 mT, indicating that a single component is being demagnetized. However, this component often does not trend toward the origin, suggesting the presence of a second higher coercivity component that is not demagnetized. As in Holes M0027A and M0028A, inclination data show prevailing normal polarity, suggesting that the first component is a viscous overprint.

Magnetostratigraphy

The low level of AF demagnetization means that, in many cases, it is impossible to isolate a characteristic remanent magnetization (ChRM) for the sediments. Preliminary polarity interpretations were made for Cores 313-M0029A-61R through 73R (~310–349 mbsf) (Fig. F38) and 208R through 217R (~728–756 mbsf) (Fig. F39), which are characterized by slightly elevated magnetic susceptibility values, in addition to stronger NRM moments in the latter case. Preliminary polarity interpretations were also made for Cores 313-M0029A-74R through 160R (~350–600 mbsf), but at this stage we were not able to isolate any apparent reversal boundaries. All interpretations were based on inclination data after 15 mT AF demagnetization (up to 60 mT between 325.02 and 332.58 mbsf) and by studying the trajectory of the NRM in orthogonal Zijderveld plots. In this section, we present preliminary age estimates (Table T12) for each reversal boundary based on the constraints given by Sr isotope ages and biohorizons (Figs. F38, F39).

For Cores 313-M0029A-61R through 73R (~310–349 mbsf), the first reversal boundary (from normal to reversed polarity [N/R]) between 331.11 and 330.91 mbsf is tentatively identified as the onset of Chron C5AAr (Fig. F38). The next reversal boundary (R/N) between 327.95 and 327.53 mbsf is identified as the onset of Chron C5AAn.

A thick zone of normal polarity between ~370 and 470 mbsf may represent Chrons C5Acn through C5ADN, dating the upper surface above sequence m5 as ~14.6 Ma (Fig. F40).

For Cores 313-M0029A-208R through 217R (~728–756 mbsf), a reversal boundary (N/R) between 733.49 and 733.29 mbsf is identified as the onset of Chron C6An.1r. A thick normal magnetozone to ~747 mbsf may be wholly or in part Chron C6An.2n.

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