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doi:10.2204/iodp.proc.342.108.2014

Paleomagnetism

We completed a paleomagnetism study of APC and XCB cores from Holes U1407A–U1407C with the primary objective of establishing a magnetostratigraphic age model for the site. The natural remanent magnetization (NRM) of each archive section half was measured at 2.5 cm intervals before and after demagnetization treatment in a peak alternating field (AF) of 20 mT for cores from Holes U1407A and U1407B. For cores from Hole U1407C, we only measured NRM after 20 mT demagnetization. Archive section half 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-U1407A-1H through 15H and 342-U1407B-1H through 11H were azimuthally oriented using the FlexIT orientation tool (Table T11). All other cores were not oriented.

We also collected 142 discrete samples from working section halves to verify the archive-half measurement data and to measure anisotropy of magnetic susceptibility (AMS) and bulk susceptibility of Site U1407 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 U1407A. Selected samples were subjected to measurements of AMS, including bulk susceptibility, and NRM measurements after 20 mT AF demagnetization. Twenty-six samples were selected for additional step-wise demagnetization at 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 U1407A, U1407B, and U1407C in Figures F24, F25, and F26, respectively. Similar to paleomagnetism results from Sites U1403–U1406 (see “Paleomagnetism” in the “Site U1403,” “Site U1404,” “Site U1405,” and “Site U1406” chapters [Norris et al., 2014c, 2014d, 2014e, 2014f]), 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 in which XCB core disturbance is remarkably low.

We report two principal features in the paleomagnetism data at Site U1407. First, we observed five discrete zones of different levels of magnetic intensity. Second, inclination values cluster at ~45° and –30°, a trend that is associated with clustering of declination values at ~0° and 180°, respectively. These features are discussed in detail below.

Magnetic intensity zonation

Magnetic intensity values can be divided into five intervals (Figs. F24, F25, F26):

  1. ~10–3 to 10–2 A/m at 0–30 mbsf,

  2. ~10–5 to 10–4 A/m at ~30–100 mbsf,

  3. ~10–4 A/m at ~130–180 mbsf,

  4. ~10–3 to 10–2 A/m at ~180–230 mbsf, and

  5. ~10–4 to 10–3 A/m at ~240–270 mbsf.

This behavior is most evident in Holes U1407B and U1407C. Exceptions to this zonation occur in two intervals. At ~80–100 mbsf, magnetization intensity is high and inclination values are close to 90°. We conclude that this interval is affected by a strong drilling overprint. Magnetic intensity is especially low (~10–6 A/m) from ~230 to 240 mbsf. This interval corresponds to OAE 2 (Cenomanian/Turonian boundary), and the strong reducing conditions associated with this event have probably dissolved most of the primary magnetic material from this interval.

These magnetic trends generally correspond with lithostratigraphy. Sediment at Site U1407 changes from pale brown lower Oligocene clay with nannofossils (lithostratigraphic Unit II) to greenish gray nannofossil ooze with foraminifers (Unit III) at ~16 mbsf. The third magnetic intensity zone corresponds to Subunit Va, which is composed of nannofossil chalk with radiolarians and foraminifers. The fourth magnetic intensity zone is restricted to the upper ~50 m of Subunit Vb, a nannofossil chalk with occasional radiolarians. The lowest magnetic intensity zone occurs in the lowermost ~30 m of Subunit Vb.

Inclination and declination clustering

Inclination values following 20 mT AF demagnetization often cluster around 45° and –30°. 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 U1407. This shallow bias is readily attributed to a small normal polarity drilling overprint that remains after 20 mT AF demagnetization. AF demagnetization at 20 mT removes most of the drilling overprint in most APC-recovered intervals and some XCB-recovered intervals, so we can utilize the positive and negative polarity clustering behavior to readily identify magnetozones in Site U1407 sediment.

Comparison between pass-through and discrete sample data

AF demagnetization results for 95 discrete samples are summarized in Table T12. Of the 26 samples treated with a peak AF demagnetization field of 60 mT, 12 reveal reasonably stable components of magnetization (e.g., Fig. F27A, F27B). 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 a drilling overprint probably obscures the primary magnetic signal. Nevertheless, these results are useful for verifying the 20 mT pass-through paleomagnetism data from the archive section halves.

In general, paleomagnetism data from archive section halves 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 archive-half measurement data (e.g., Fig. F27B), but others show stable components that do not decay toward the origin and are not consistent with the section-half measurement data (e.g., Fig. F27C). These results suggest that section-half measurement data from XCB core intervals should be interpreted with care, similar to our conclusions regarding paleomagnetism data at Sites U1403 and U1406.

Magnetostratigraphy

The shipboard downhole results reveal a series of normal and reversed magnetozones between Cores 342-U1407A-6H and 10H (~47–91 mbsf), between Cores 342-U1407B-6H and 11H (~48–91 mbsf), and between Cores 342-U1407C-6H and 10H (~43–84 mbsf). These magnetostratigraphies can be unambiguously correlated between all three holes. Downhole plots indicate that additional series of magnetozones are recorded 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 for Holes U1407B and U1407C, provide independent age control to correlate them to the geomagnetic polarity timescale (GPTS).

By utilizing radiolarian, foraminifer, and nannofossil biostratigraphic datums from Hole U1407A (see “Biostratigraphy”), we can correlate magnetozones to the GPTS. The shipboard magnetostratigraphic age model is based on Hole U1407A, for which we have the most biostratigraphic datums. Extension of this age model to the magnetozonation observed in Holes U1407B and U1407C is contingent on the accuracy of the stratigraphic correlation between holes, which is corroborated by lithologic marker horizons, biostratigraphic datums, and physical property features (see “Stratigraphic correlation”). Our correlation is presented in Table T13 and is shown in Figures F24, F25, F26, and F28.

In Hole U1407A, we correlated the magnetostratigraphy in Cores 342-U1407A-6H through 10H to lower Chron C20r (~43.4 Ma) through upper Chron C22r (~49.4 Ma). With the aid of nannofossil datums, we correlated a reversed to normal downhole polarity transition in Section 342-U1407A-19X-2 to the Chron C25r/C26n boundary (58.959 Ma). We attribute the difficulty in correlating magnetozones observed in Cores 342-U1407A-11H through 18X to the GPTS to the two long hiatuses implied by the biostratigraphic datums (see “Biostratigraphy”). Magnetozone correlations for Holes U1407B and U1407C are similar to Hole U1407A, with the exception that we did not observe the Chron C25r–C26n transition in the former holes.

The correlations described above provide a shipboard chronostratigraphic framework for interpreting the early to middle Eocene sediment record at Site U1407. The most salient implication of this age model is that sedimentation rates along Southeast Newfoundland Ridge at the paleowater depth of Site U1407 varied between ~2.0 and 8.7 cm/k.y. during the middle Eocene (Fig. F19).

Magnetic susceptibility and anisotropy of magnetic susceptibility

Bulk magnetic susceptibility measured on 78 discrete samples is summarized in Table T14. Downhole variation in whole-round magnetic susceptibility (WRMS) and discrete sample magnetic susceptibility (DSMS) for Hole U1407A are shown in Figure F24. The WRMS data for Hole U1407A 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., 2014b]). 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 U1407 sediment. Although discrete samples were collected from each core section for the entire depth of Hole U1407A, 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 F29. The eigenvalues associated with the maximum (τ1), intermediate (τ2), and minimum (τ3) magnetic susceptibilities at Site U1407 show prominent downhole variation. Eigenvalues are consistently divergent from the top of Hole U1407A to ~82 mbsf. The exceptional divergence observed from ~82 to ~102 mbsf is probably spurious. The soupy consistency of the white nannofossil ooze that characterizes lithostratigraphic Unit IV also has extremely low magnetic susceptibility values. Similarly, the large divergence between eigenvalues from ~122 to 175 mbsf may be an artifact of the very weak magnetic susceptibility in Subunit Va. The modest and more variable divergence observed below ~175 mbsf is similar to downhole magnetic anisotropy trends observed in the lowest intervals of holes at previous Expedition 342 sites. The degree of anisotropy is modest with respect to sites at J-Anomaly Ridge. In general, the shape anisotropy is triaxial. We do not observe a strong depth-dependence in the inclination of the minimum eigenvector (V3), suggesting that there is not a preferred fabric orientation of oblate particles.

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