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doi:10.2204/iodp.proc.342.107.2014

Paleomagnetism

We completed a paleomagnetism study of APC and XCB cores from Holes U1406A–U1406C with the primary objective of establishing a magnetostratigraphic age model for the site. The natural remanent magnetization (NRM) of each archive-half section was measured at 2.5 cm intervals before and after demagnetization treatment in a peak alternating field (AF) of 20 mT. Archive-half section measurement data were processed by removing measurements made within 7.5 cm of section ends and from disturbed intervals described in the Laboratory Information Management System database. Cores 342-U1406A-1H through 17H and 342-U1406B-1H through 18H were azimuthally oriented using the FlexIT orientation tool (Table T11); all other cores were not oriented.

We also collected 185 discrete samples from working-half sections to verify the archive-half measurement data and to measure anisotropy of magnetic susceptibility (AMS) and bulk susceptibility of Site U1406 sediment. Discrete samples were collected and stored in 7 cm³ plastic cubes and were typically taken from the least disturbed region closest to the center of each section in Hole U1406A. Selected samples were subjected to measurements of AMS, including bulk susceptibility, and NRM after 20 mT AF demagnetization. Sixteen samples were further selected for step-wise demagnetization at 10, 20, 30, 40, and 60 mT. All discrete sample data are volume corrected to 7 cm³.

Results

Downhole paleomagnetism data after 20 mT demagnetization are presented for Holes U1406A, U1406B, and U1406C in Figures F17, F18, and F19, respectively. Similar to paleomagnetism results from Sites U1403, U1404, and U1405 (see “Paleomagnetism” in the “Site U1403,” “Site U1404,” and “Site U1405” chapters [Norris et al., 2013b, 2013c, 2013d]), section-half measurement data from XCB cores are difficult to interpret because of biscuiting and substantial core disturbance. We chose to interpret only results obtained from APC cores, except for a few cases where XCB core disturbance is remarkably low.

We identified two principal features in the paleomagnetism data at Site U1406. First, a large decrease in magnetic intensity and susceptibility occurs over the uppermost ~10 mbsf followed by a gradual increase downhole below ~60 mbsf, with two notable exceptions at ~180–190 and ~200–210 mbsf. Second, inclination values cluster at ~60° and –30°, a trend associated with clustering of declination values at ~0° and 180°, respectively.

Downhole intensity trends

Magnetic intensity values show distinct downhole changes and trends (Figs. F17, F18, F19). Magnetic intensity decreases from ~10^–2 to ~10^–4 A/m over the uppermost ~16 m of sediment at Site U1406. Between ~16 and ~60 mbsf, intensity values remain constant at ~10^–4 A/m. From ~60 to ~180 mbsf, intensity continues to increase gradually. Below ~180 mbsf, magnetic intensity abruptly increases to ~10^–3 A/m and in some intervals, as much as ~10^–2 A/m. The amplitude of intensity variations is greatest in this interval as well. Weak-field magnetic susceptibility shows similar trends.

These magnetic trends generally correspond with lithostratigraphy. Sediment at Site U1406 changes from yellow and brown Pleistocene–Pliocene foraminiferal sand and nannofossil ooze (lithostratigraphic Unit I) to tan nannofossil ooze (Unit II) at ~2 mbsf. Unit II abruptly changes color at 16.70 mbsf in Hole U1406A from tan to green and remains green below this depth, the same depth at which the decreasing trend in magnetic intensity and susceptibility stops. Although shipboard geochemical data were not collected at a resolution sufficient to provide an independent test (see “Geochemistry”), we suggest that the decrease in magnetic intensity in the upper 16 m of Hole U1406A is caused by progressive reductive dissolution of iron oxide. At ~60 mbsf, lithostratigraphic Unit II changes color from dark green to pale green, perhaps attributable to slightly greater carbonate content (Fig. F5; also see “Geochemistry”). The magnetic intensity peak at ~180–190 mbsf corresponds to a change from Unit II to III, and the peak at ~200–210 mbsf corresponds to a transition from tan to brown nannofossil chalk in Core 342-U1406A-23H.

Inclination and declination clustering

Inclination values following 20 mT AF demagnetization often cluster around 60° or –30° (Figs. F17, F18, F19). Inclination clustering is usually associated with declination clustering at ~0° and 180°, respectively. The approximately –30° inclination is shallow with respect to the reversed polarity value expected at the ~40°N latitude of Site U1406. This shallow bias is readily attributed to a small drilling overprint that remains after 20 mT AF demagnetization. AF demagnetization at 20 mT was more effective at removing the drilling overprint than it was with sediment recovered from Sites U1403 and U1404 but less effective than that at Site U1405. Regardless of the effectiveness of AF treatments between sites, we can utilize the positive and negative polarity clustering behavior to readily identify magnetozones in recovered intervals at Site U1406.

Comparison between pass-through and discrete sample data

AF demagnetization results for the discrete samples are summarized in Table T12. Of the 16 samples treated with a peak AF demagnetization field of 60 mT, 7 reveal relatively stable components of magnetization (e.g., Fig. F20A). These samples have remanent magnetizations that are strong enough to be measured by the onboard JR-6A spinner magnetometer. The remaining samples typically display NRM intensities that decrease by an order of magnitude following AF demagnetization in a 20 mT field. This behavior indicates that the combination of drilling overprint and magnetic intensity decrease described above has probably obscured the primary magnetic signal in these stratigraphic intervals, similar to results at Sites U1404 and U1405. Nevertheless, these results are useful for verifying the polarity of the 20 mT pass-through paleomagnetism data from the archive-half sections.

In general, paleomagnetism data from archive-half sections and discrete samples from oriented APC core intervals agree well. In contrast, some discrete samples from XCB cores show single stable components that are consistent with the section-half measurement data (e.g., Fig. F20B), but others show complex behavior that is not consistent with the section-half measurement data (e.g., Fig. F20C). These results suggest that section-half measurement data from XCB-cored intervals should be interpreted with care. A similar conclusion was reached with respect to Site U1403 data.

Magnetostratigraphy

The shipboard downhole results reveal a nearly continuous series of normal and reversed magnetozones between Cores 342-U1406A-2H and 23H (~10–204 mbsf), between Cores 342-U1406B-4H and 20H (~25–178 mbsf), and between Cores 342-U1406C-8H and 23X (~66–201 mbsf). Downhole plots indicate that additional series of magnetozones are recorded higher and possibly lower in the recovered interval in all three holes, but shore-based studies are necessary to identify and fully characterize magnetozones in these intervals and provide independent age control to correlate them to the geomagnetic polarity timescale (GPTS). Magnetozones can be straightforwardly correlated between all three holes, especially between ~65 and ~200 mbsf.

By utilizing radiolarian, foraminifer, and nannofossil biostratigraphic datums from Hole U1406A (see “Biostratigraphy”), we can correlate magnetozones to the GPTS. The shipboard magnetostratigraphic age model is based on Hole U1406A, for which we have the most biostratigraphic datums. Extension of this age model to the magnetozonation observed in Holes U1406B and U1406C is contingent on the accuracy of the stratigraphic correlation between holes, which is corroborated by some lithologic horizons and physical property features (see “Stratigraphic correlation”). In some key intervals, biostratigraphic datums from each hole provide unambiguous correlation between holes and to the GPTS. Our correlation is presented in Table T13 and is shown in Figures F17, F18, F19, and F21.

In Hole U1406A, we correlate the magnetostratigraphy in Core 2H to Chrons C5Dn through C5En. We correlate magnetozones in Core 342-U1406A-3H to Chrons C6n–C6An.1r. Magnetozones in Cores 6H through 9H correlate to Chrons C6AAr.3r–C6Cn.2r. The entire interval of Core 10H is reversed polarity, which we correlate to Chron C6Cr. This correlation implies that Chron C6Cn.3n, which is only 0.062 m.y. long (~0.7 m thick given the 1.2 cm/k.y. average linear sedimentation rate [LSR] implied by the biostratigraphy and overall magnetostratigraphy), is recorded in the core catcher of Core 9H or in the core gap between Cores 9H and 10H. A discontinuous series of magnetozones in Cores 11H, 12H, and 16H can be correlated to Chrons C6Cr–C7n.2n, C7r–C7Ar, and the Chron C8r/C9n boundary, respectively. The entire interval of Core 15H shows reversed polarity, which we interpret to correspond to Chron C8r. Finally, we correlate a discontinuous series of magnetozones in Cores 18H through 23H to Chrons C10r–C15n.

Magnetozone correlations for Hole U1406B and U1406C are generally similar to those for Hole U1406A but do include some notable differences. For example, we are unable to correlate magnetozones in Cores 342-U1406B-2H through 3H to Hole U1406A, and therefore to the GPTS, with confidence. By using biostratigraphic datums from core catchers, however, we have correlated magnetozones in Core 342-U1406B-6H to Chrons C6n.2n–C6AAr.1n. In Hole U1406C, we are unable to correlate magnetozones in Cores 342-U1406C-2H through 7H to the other two holes with confidence. A strong correlation between magnetozones in Hole U1406C to Hole U1406A and the GTPS is possible because of excellent magnetic behavior in these high-quality XCB cores and additional biostratigraphic datums. Magnetozones in Cores 342-U1406C-22X through 23X correlate to Chrons 12r–C15r. We did not observe the Chron C13n/C13r boundary (33.705 Ma) in any shipboard magnetic data.

The correlations described above provide a shipboard chronostratigraphic framework for interpreting the uppermost late Eocene–middle Miocene sediment drift record at Site U1406. The most salient implications of this age model are summarized as follows.

1. The Oligocene–Miocene transition is dated by the base of Chron C6Cn.2n (23.030 Ma), which we identified in intervals 342-U1406A-9H-5, 55.0–90.0 cm (~81.16 mbsf); 342-U1406B-10H-1, 135.0 cm, through 10H-2, 40.0 cm (~82.48 mbsf); and 342-U1406C-9H-4, 85.0–107.5 cm (~75.71 mbsf).

2. The Eocene–Oligocene transition occurs just prior to the transition from Chron C13r to C13n (33.705 Ma). We did not observe this chron boundary in any core from Site U1406, but we did identify the preceding and following chron boundaries in Holes U1406A and U1406C. The Chron C13r/C15n boundary (34.999 Ma) is between Sections 342-U1406A-23H-3, 120.0 cm, and 23H-4, 37.5 cm (~203.84 mbsf), and between Sections 342-U1406C-23X-2, 85.0 cm, and 23X-2, 110.0 cm (~197.68 mbsf). The Chron C12r/C13n boundary (33.157 Ma) is between Sections 342-U1406A-22H-2, 140.0 cm, and 22H-4, 12.5 cm (194.93 mbsf), and between Sections 342-U1406C-22X-6, 105.0 cm, and 22X-7, 10.0 cm (~193.10 mbsf). We predict that the Chron C13n/C13r boundary is in Core 342-U1406B-24X. Superconducting rock magnetometer data from section halves of this core are very noisy because of the XCB-disturbed nature of this interval. Shore-based paleomagnetism studies on discrete samples coupled with high-resolution stable carbon isotope stratigraphy will help locate this critical chron boundary with confidence.

3. The shipboard magnetostratigraphic age model also indicates at least three hiatuses in Hole U1406A, none of which are longer than ~3 m.y. These hiatuses complicate the magnetostratigraphic correlation between the three holes at Site U1406. The first of these hiatuses occurs within the interval between the lower part of Core 342-U1406A-2H and upper part of Core 3H (~16 mbsf). This interval is also marked by an abrupt step in natural gamma radiation (NGR) (see “Physical properties”). Our correlation suggests another hiatus between the bottom of Core 342-U1406A-5H and the top of Core 6H (~46 mbsf), in which we can no longer match the magnetozones to the C6AA chron series on the GPTS. This interval also corresponds to a distinct change in color, porosity, and bulk density (see “Physical properties”) and a variety of biostratigraphic datums (see “Biostratigraphy”). Finally, we conclude that ~3 m.y. is missing between Cores 342-U1406A-17H and 18H. The top of Core 17H is correlated to Chron C9n, whereas the top of Core 18H is correlated to Chron C11n.1n. This interval is also characterized by a distinct change in NGR values (see “Physical properties”). These hiatuses occur at approximately the same time as those observed in the Site U1405 record.

Like the age models for the other sites from J-Anomaly Ridge, the Site U1406 shipboard age model demonstrates slow sedimentation rates (~0.2 cm/k.y.) in the late Eocene and across the EOT. Sedimentation rates gradually increase throughout the Oligocene (~1.1 cm/k.y.) and then rapidly increase just following the Oligocene–Miocene transition (~3.3 cm/k.y.) (Fig. F23).

Magnetic susceptibility and anisotropy of magnetic susceptibility

Bulk magnetic susceptibility measured on 101 discrete samples is summarized in Table T14. Downhole variation in whole-round magnetic susceptibility (WRMS) and discrete sample magnetic susceptibility (DSMS) for Hole U1406A are shown in Figure F17. The WRMS data for Hole U1406A are shown in raw form; they have not been trimmed at section ends or filtered for obvious outliers, so noise in the data probably reflects edge effects or spurious measurements. We multiplied the WRMS data, which are in instrument units, by a factor of 0.577 × 10^–5 to convert to approximate SI volume susceptibilities (see “Paleomagnetism” in the “Methods” chapter [Norris et al., 2013a]). WRMS and DSMS data agree very well after this conversion, and we attribute small absolute differences to the fact that the conversion factor applied to the WRMS data is not constant downhole because of changes in core diameter and density; only discrete samples provide calibrated susceptibility values in SI units. Both magnetic susceptibility data sets show the same first- and second-order cyclic trends, indicating that these trends are robust features of Site U1406 sediment. Although discrete samples were collected along each core section from the entire depth of Hole U1406A, we chose to measure only odd-numbered samples.

AMS results for the discrete samples are also summarized in Table T14 and are shown in Figure F22. The eigenvalues associated with the maximum (τ₁), intermediate (τ₂), and minimum (τ₃) magnetic susceptibilities at Site U1406 show prominent downhole trends. Eigenvalues are divergent from the uppermost part of Hole U1406A to ~100 mbsf, with exceptional divergence between ~20 and 80 mbsf. This trend in magnetic fabric is similar to the downhole trends in water content and porosity (see “Physical properties”). From ~100 to 187 mbsf, τ₁ and τ₃ converge and remain relatively invariant. We note a distinct zone of eigenvalue divergence centered at ~200 mbsf. Eigenvalues diverge even more beginning at ~218 mbsf, corresponding to a change from APC to XCB coring technology. These eigenvalue trends are also clearly expressed in the degree of anisotropy, which is greatest in the uppermost 100 m of Hole U1406A. In general, the shape anisotropy is triaxial, although some intervals have strong oblate or prolate fabrics, such as between ~160 and 190 and between ~110 and 120 mbsf, respectively. However, we did not observe a strong depth-dependence in the inclination of the minimum eigenvector (V₃), suggesting that there is not preferred fabric orientation.

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