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doi:10.2204/iodp.proc.342.110.2014

Paleomagnetism

We completed a paleomagnetism study of APC and XCB cores from Holes U1409A–U1409C 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 for all cores from Hole U1409A. For all other cores, we only measured NRM after 20 mT demagnetization. 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-U1409A-1H through 16H and 342-U1409B-1H through 14H were azimuthally oriented using the FlexIT orientation tool (Table T12). All other cores were not oriented.

We also collected 125 discrete samples from working half sections to verify the archive half section measurement data and to measure anisotropy of magnetic susceptibility (AMS) and bulk susceptibility of Site U1409 sediment. Discrete samples were collected and stored in 7 cm³ plastic cubes and typically taken from the least disturbed region closest to the center of each section from Hole U1409A. Selected samples were subjected to measurements of AMS, including bulk susceptibility, and NRM measurements after 20 mT AF demagnetization. Twenty-one samples were selected for stepwise demagnetization at 0, 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 U1409A, U1409B, and U1409C in Figures F24, F25, and F26, respectively. Similar to paleomagnetism results from previous Expedition 342 sites, section half measurement data from XCB-recovered cores are difficult to interpret because of biscuiting and substantial core disturbance. We chose to interpret only results obtained from APC cores.

We report the following principal features in the paleomagnetism data at Site U1409. First, we observed intensity zonation that correlates with lithostratigraphy except for three distinctive horizons at ~50, 70, and 90 mbsf. Second, inclination values cluster at ~60° and 0° to –45°, a trend that is associated with clustering of declination values at ~0° and 180°, respectively.

Magnetic intensity zonation

Downhole magnetic intensity values generally vary with lithostratigraphic units. Lithostratigraphic Units I and II (~0–30 mbsf), composed of Pleistocene nannofossil to foraminifer oozes and muddy sand and Oligocene silty clay and nannofossil clay (see “Lithostratigraphy”), are magnetically characterized by high intensity values (~10^–2 A/m). The ~70 m thick Unit III, composed of middle Eocene light green–gray nannofossil clay with interbedded nannofossil ooze, has background magnetic intensity values on the order of 10^–5 A/m. However, three distinctive horizons within Unit III (~50, 70, and 90 mbsf) have exceptionally high (~10^–2 A/m) magnetic intensity values. These intervals of magnetic intensity are also distinguished by peaks in magnetic susceptibility and b* color reflectance data (see “Physical properties”). Core composite images (Cores 342-U1409A-6H, 9H, and 11H) clearly reveal the change from green and gray nannofossil clay to orange-red horizons at these intervals. We conclude that these intervals contain concentrations of oxidized magnetic minerals that are high above the background levels. Finally, the ~100 m of lower Eocene to lower Paleocene nannofossil oozes and nannofossil chalk of Unit IV (Subunits IVa–IVc) have typical intensity values on the order of 10^–3 A/m, with an order of magnitude variability over a wavelength of several meters.

Inclination and declination clustering

Inclination values following 20 mT AF demagnetization often cluster around 60° and –45°. Inclination clustering is usually associated with declination clustering at ~0° and 180°, respectively. The –45° inclination is shallow with respect to the reversed polarity value expected at the ~40°N latitude of Site U1409. 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 is most effective at removing the drilling overprint in APC-recovered intervals, mainly from lithostratigraphic Unit III; it is less effective for other intervals but sufficient to reveal reversed polarity magnetozones. We can utilize the positive and negative polarity clustering behavior to readily identify magnetozones in Site U1409 sediment.

Comparison between pass-through and discrete sample data

AF demagnetization results for 73 discrete samples are summarized in Table T13. Of the 21 samples treated with a 60 mT peak AF demagnetization field, 5 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 20 mT field AF demagnetization. This behavior indicates that a drilling overprint probably obscures the primary magnetic signal. An example of such an overprint can be seen in Figure F27C. Nevertheless, these results are useful for verifying 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, discrete samples from XCB cores do not always show results that are consistent with the archive half section measurement data (Fig. F24). This discrepancy suggests that archive half section measurement data from XCB core intervals should be interpreted with care, similar to our conclusions regarding paleomagnetism data from previous Expedition 342 sites.

Magnetostratigraphy

The shipboard downhole results reveal a series of normal and reversed magnetozones between Cores 342-U1409A-1H and 13H (~0–115 mbsf), between Cores 342-U1409B-1H and 13H (~0–120 mbsf), and between Cores 342-U1409C-1H and 13H (~0–115 mbsf). These magnetostratigraphies can be easily correlated among all three holes.

By utilizing radiolarian, foraminifer, and nannofossil biostratigraphic datums from Hole U1409A (see “Biostratigraphy”), we can correlate magnetozones to the geomagnetic polarity timescale (GPTS). The shipboard magnetostratigraphic age model is based on Hole U1409A, for which we have the most biostratigraphic datums. Extension of this age model to the magnetozonation observed in Holes U1409B and U1409C is contingent on the accuracy of the stratigraphic correlation between holes, which is corroborated by lithologic horizons, biostratigraphic datums, and physical property features (see “Stratigraphic correlation”). Our correlation is presented in Table T14 and is shown in Figures F24, F25, F26, and F28.

We correlate the normal to reversed polarity transition in Core 342-U1409A-2H at ~9 mbsf to the Bruhnes (C1n)/Matuyama (C1r.1r) boundary (0.781 Ma). The magnetostratigraphy observed in Sections 342-U1409A-3H-2 through 5H-2 is correlated to lower Chron C6Cr (~23.962 Ma) through upper Chron C13r (~33.7 Ma). We did not observe the distinctive Chron C9n–C9r magnetozonation in any hole at Site U1409 in this interval. In Hole U1409A, we also did not observe the Chron C12r–C13n transition; however, it is recorded in sediment in Hole U1409B, suggesting that this chron boundary is in the core gap between Cores 342-U1409A-4H and 5H. We correlate the well-resolved magnetozonation observed in Sections 342-U1409A-6H-5 to 13H-4 (~46 to ~111 mbsf) to lower Chron C19r (~42 Ma) continuously downhole to upper Chron C22r (~49.4 Ma). Paleomagnetism data from archive half sections and discrete samples suggest that a magnetostratigraphy can be developed in deeper intervals at Site U1409, but this magnetostratigraphy cannot be resolved in sufficient detail with shipboard data to correlate with confidence to the GPTS. Magnetozone correlations for Holes U1409B and U1409C are similar to Hole U1409A, with the exception that we did not observe the Oligocene magnetostratigraphy as clearly in Holes U1409B and U1409C as we did in Hole U1409A.

The correlations described above provide a shipboard chronostratigraphic framework for interpreting the Oligocene and middle Eocene sediment record at Site U1409. The most salient implications of this age model are as follows:

• Average linear sedimentation rates during the Oligocene at Site U1409 are very low (0.29 cm/k.y.) (Fig. F30). The condensed nature of this Oligocene interval is consistent with the abundant manganese nodules that are as large as 10 cm in diameter.

• We do not observe either the Chron C9n–C9r transition (27.439 Ma) or the Chron C9r–C10n.1n transition (27.859 Ma) in any hole at Site U1409. Notably, we observed a hiatus over the same time interval at Site U1406. Site U1409 at Southeast Newfoundland Ridge and Site U1406 at J-Anomaly Ridge are at similar midwater depths (~3500 and 3800 mbsl, respectively). Notably, deep-sea benthic foraminiferal δ¹⁸O values increase distinctly during Chron C9 (Zachos et al., 2008). Therefore, we tentatively suggest that the regional Chron C9 hiatus observed at J-Anomaly Ridge and Southeast Newfoundland Ridge is linked to regional oceanographic changes spurred by changes in global ice volume.

• We tentatively correlate the downhole normal to reversed transition in Sections 342-U1409A-6H-5, 342-U1409B-6H-3 and 6H-4, and 342-U1409C-6H-5 and 6H-6 to the Chron C13n–C13r transition (33.705 Ma). If this correlation is robust, then the Eocene–Oligocene transition may be recorded in the sediment at Site U1409. Thorough shore-based paleomagnetism and chemostratigraphic studies are necessary to verify this shipboard correlation. We note that the magnetic record, distinctive in both downhole directional and magnetic intensity behavior during the transition, is remarkably similar to the paleomagnetism record correlated to this transition at Site U1404 (see Fig. F22 in the “Site U1404” chapter [Norris et al., 2014c]).

• Average linear sedimentation rates during the middle Eocene at Site U1409 are similar to those calculated for Sites U1407 and U1408. Sedimentation rates are highest during Chron C20r (1.31 cm/k.y. from ~43.4 to 45.7 Ma) and are ~52% and 65% lower before and after this interval, respectively (Fig. F30).

Magnetic susceptibility and anisotropy of magnetic susceptibility

Bulk magnetic susceptibility measured on 71 discrete samples is summarized in Table T15. Although discrete samples were collected from each core section from the entire depth of Hole U1409A, we chose to measure only odd-numbered samples. Downhole variation in whole-round magnetic susceptibility (WRMS) and discrete sample magnetic susceptibility (DSMS) for Hole U1409A are shown in Figure F24. The WRMS data for Hole U1409A 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., 2014a]). 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 U1409 sediment.

AMS results for the discrete samples are also summarized in Table T15 and are shown in Figure F29. The eigenvalues associated with the maximum (τ₁), intermediate (τ₂), and minimum (τ₃) magnetic susceptibilities at Site U1409 show some downhole variations that correspond to changes in lithostratigraphy. We also observed a change in AMS values at the transition from APC to XCB coring technologies, with greater consistent divergence between principal eigenvalues indicative of more oblate fabrics in XCB cores. We do not observe consistently steeper inclinations of the minimum principal eigenvector (V₃) in this same interval however, suggesting that although there is a stronger oblate AMS in XCB-recovered intervals, this does not correspond to a strong fabric orientation.

Triaxial fabrics are greatest, and most variable, at the top of lithostratigraphic Subunit IVa, just below the contact with Unit III (~100–110 mbsf). Magnetic fabrics, reflected in the degree of anisotropy (P), lineation (τ₁/τ₂), and foliation (τ₂/τ₃) values, are the most developed of any observed at previous Expedition 342 sites. This interval corresponds to an increase in radiolarian abundance and a distinct lithostratigraphic transition from pink nannofossil ooze below to alternating layers of pink and white nannofossil oozes with varying amounts of foraminifers and radiolarians within the zone of high AMS (see “Lithostratigraphy”). The highest AMS values also correspond to an interval in which the sediment was noticeably softer than intervals above and below it. Although the degree of anisotropy and fabric parameters are high, the minimum principal eigenvector does not show a systematic trend, suggesting that whatever produced the strong AMS was not simply compaction from overburden.

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