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doi:10.2204/iodp.proc.313.103.2010

Paleomagnetism

Sediments from Hole M0027A generally possess a weak or unstable natural remanent magnetization (NRM). Alternating-field (AF) demagnetization up to 15–30 mT, depending on lithology, typically removes a low-coercivity overprint with normal polarity, sometimes revealing a higher coercivity stable component. During AF demagnetization, a few samples acquired what was identified as gyroremanent magnetization (GRM). The effect of GRM was removed by applying an antigyroremanent demagnetization procedure. Two reversal boundaries were successfully identified in the clay sequence in upper lithologic Unit II that could be assigned to either Chron C5ACn or C5ADn according to the time constraints of Sr analyses and biohorizons.

Discrete sample measurements

A total of 744 samples from Hole M0027A were measured in the pass-through magnetometer. In addition to the standard one sample per section, Cores 313-M0027A-65R through 70R, 110R through 116R, 155R through 166R, 175R through 193R, and 209R through 224R were sampled at a denser resolution, ranging from one sample every 50 cm to one sample every 10 cm. NRM and remanence after sequential AF demagnetization up to 60 or 100 mT was measured for all samples.

Remanent magnetization

The primary magnetization in Hole M0027A is mostly carried by a low-coercivity component, but 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 (Fig. F52). Clay sediments in Cores 313-M0027A-61R through 70R (~183–209 mbsf) exhibit stronger magnetic moments on the order of 10^–7 to 10^–6 Am², coinciding with a peak in magnetic susceptibility (Fig. F52). Finer grained sequences further downcore exhibit significantly lower NRM magnetic moments, perhaps indicating regional dissolution of the primary magnetic mineral. Inclination data show prevailing normal polarity, suggesting a viscous overprint. In general, data are too noisy to distinguish any polarity zones except for Cores 313-M0027A-61R through 70R (183–209 mbsf), which exhibit stronger NRM magnetic moments. These cores are located in the high–magnetic susceptibility clays of Unit II.

The cores were recovered using the HPC, EXN, and ALN. Weak and unstable NRM magnetization was generally observed in unconsolidated layers recovered by the ALN, indicating magnetic grain rotation and/or structural deformation.

Demagnetization behavior varies considerably depending on lithology. Typically, the NRM of sediments exhibits a normal polarity overprint, which in some cases makes it hard to distinguish the primary component (Fig. F53A). AF demagnetization of some samples typically shows a double-bulge shape feature, which is interpreted as the successive removal of two overlapping low- and high-coercivity components (Fig. F53B). The characteristic remanent magnetization (ChRM) of samples exhibiting GRM behavior during demagnetization was successfully isolated using the anti-GRM technique (Fig. F53C, F53D). We could only isolate a stable 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 (see Fig. F46 in the "Site M0028" chapter). These minerals are most likely secondary features, as the minerals form framboids or aggregates along preexisting fabrics, grains, or fossils. Nevertheless, hematite and some other opaque grains are observed as detrital grains, which may carry a primary magnetization.

Magnetostratigraphy

Because of the generally weak and unstable NRM of the sediments, we were only able to establish a polarity magnetostratigraphy for Cores 313-M0027A-61R through 70R (183–209 mbsf) (Fig. F54). Preliminary magnetostratigraphic age-depth tie points are given in Table T12. Polarity interpretations are based on principal component analysis (PCA) for samples from 208.39 to 201.31 mbsf, which were demagnetized using the antigyroremanence procedure. Based on these results, the 30 mT demagnetization step was identified as the most representative of the ChRM and was therefore used, together with studies of individual orthogonal Zijderveld plots, to establish the polarity for samples from 201.21 to 182.52 mbsf.

Magnetostratigraphic interpretation of inclination data from Cores 313-M0027A-61R through 70R is dependent on the placement of the m4.1 surface (currently placed at ~195 mbsf), which, according to the seismic study, could represent an unconformity in Hole M0027A. In this section, we present two alternative interpretations (Table T12) for each reversal boundary based on the constraints given by Sr isotope ages from the core catcher of Core 313-M0027A-70R (13.7 ± 1.17 Ma) and the identification of upper dinocyst Zone DN5 (14.2–13.6 Ma) in the sediments from the core catcher of Core 313-M0027A-65R.

The reversal boundary (from reversed to normal polarity [R/N]) between 206.94 and 206.74 mbsf is identified as the onset of either Chron C5ACn or C5ADn. The next reversal boundary (from normal to reversed polarity [N/R]) between 196.09 and 195.79 mbsf is identified as the onset of either Chron C5ABr or C5ACr. Cores 313-M0027A-61R through 63R (183–189 mbsf) (arrows in Fig. F54 show the intended core length) suffered from poor sediment recovery. The actual position of the recovered sediments within the cores is unknown, and we are therefore only able to narrow down the reversal boundary (R/N) to a relatively large depth range, 191.96–189.03 mbsf. The reversal boundary is tentatively identified as the onset of either Chron C5AAn or C5ABn, taking into account the possible hiatus below the reflector m4.1 surface.

Magnetic susceptibility

MSCL magnetic susceptibility data from the mid-part of the Oligocene section in Hole M0027A display high values and large (~60 × 10^–5 SI units) variations (Fig. F55). There appear to be ~20–25 magnetic susceptibility cycles in this 45 m section (1.8–2.25 m/cycle = 0.44–0.55 cycles/m). Magnetic susceptibility cyclicity is likely due to variations in abundance of glauconite and associated minerals (Miller et al., 1996; Metzger et al., 1997) and should be evaluated through compositional and gray-scale studies. In general, magnetic susceptibility of sediments strongly depends on the amount of mainly ferrimagnetic and partly paramagnetic minerals. Based on microscopic observation and rock magnetic work, ferrimagnetism mainly resides in greigite grains and paramagnetism resides in glauconite and pyrite grains in the section. Most greigite is authigenic and may be linked to astronomical-pace-forcing with climate proxies. High susceptibility values in sequence o1 (539.5–596.3 mbsf) appear to be derived from greigite (ferrimagnetic minerals), as well as glauconite pellets and pyrite framboids (paramagnetic minerals).

One possible cause for cyclicity in glauconite abundance is variations in downslope transport on the off-apron toe-of-slope setting (see "Lithostratigraphy"). Such variations may reflect astronomical-scale eustatic changes.

Shipboard age constraints hint at astronomical control by precession and eccentricity. The sequence between 494.97 and 539.5 mbsf is dubbed sequence o1. It can be dated as ~28.5–29.0 Ma based on Sr isotopic stratigraphy and biostratigraphy (Fig. F34). Thus, these roughly 0.5 m thick cycles appear to be precessional in scale. The general increasing trend from 530 to 565 mbsf and the decreasing trend from 565 to 595 mbsf may reflect modulation by the long eccentricity cycles. Further work should test these speculative correlations to astronomical forcing and evaluate the cause. Further discussion of magnetic susceptibility variations can be found in "Downhole measurements."

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