Previous   |    Next

doi:10.2204/iodp.proc.342.109.2014

Paleomagnetism

We completed a paleomagnetism study of APC and XCB cores from Holes U1408A–U1408C 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 U1408A, Cores 342-U1408B-1H through 18H, and Cores 342-U1408C-6H through 8H. For all other cores, we only measured NRM after 20 mT demagnetization. Archive half section measurement data were processed by removing data points from within 7.5 cm of section ends and from disturbed intervals described in the Laboratory Information Management System database. Cores 342-U1408A-1H through 20H, 342-U1408B-1H through 5H, and 342-U1408C-6H through 8H were azimuthally oriented using the FlexIT orientation tool (Table T11). All other cores were not oriented.

We also collected 168 discrete samples from working half sections to verify the archive half section measurement data and to measure the anisotropy of magnetic susceptibility (AMS) and the bulk susceptibility of Site U1408 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 U1408A. 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 U1408A, U1408B, and U1408C in Figures F19, F20, and F21, 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 except for a few cases in which discrete samples provide additional constraints on magnetozone identification in XCB-cored intervals.

We report the following three principal features in the paleomagnetism data at Site U1408. First, we observed intensity zonation that correlates with lithostratigraphy. 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. Third, an exceptionally detailed record of two successive Eocene geomagnetic field transitions is recorded over ~6 m of sediment.

Magnetic intensity zonation

Downhole magnetic intensity values vary with lithostratigraphic units. The uppermost ~13 m of sediment comprises Pleistocene–Oligocene clayey silt and nannofossil and foraminiferal ooze (see “Lithostratigraphy”) and is characterized by high magnetic intensity values (~10^–2 A/m). The ~10 m thick Oligocene silty clay (with nannofossils) of lithostratigraphic Unit II has magnetic intensity values on the order of 10^–4 A/m. The ~202 m of Oligocene to middle Eocene nannofossil clay, claystone, and nannofossil ooze of Unit III has intensity values that vary between 10^–4 and 10^–2 A/m. In detail, Unit III exhibits three main trends:

1. A consistently high intensity of ~10^–2 A/m at ~30–110 mbsf, with high-amplitude variability from these mean values;

2. A steady downhole decrease in intensity from ~10^–3 to ~10^–4 A/m at ~110–150 mbsf, with second-order low-amplitude variability from the mean downhole trend; and

3. Intensity values of ~10^–4 A/m at ~150–230 mbsf, with low-amplitude variability.

Finally, the lower Eocene to upper Paleocene nannofossil chalk with intermittent radiolarian-rich and chert-rich intervals of lithostratigraphic Unit IV (~225–246 mbsf) are characterized by consistently higher magnetic intensity values of ~10^–3 A/m.

Downhole trends in magnetic intensity are similar to downhole trends in redox-sensitive interstitial water chemistry, particularly Mn and Fe (see “Geochemistry”). The peak in magnetic intensity in Unit III is observed in the same interval in which interstitial water iron concentrations are highest; decrease in intensity below this broad peak also follows a downhole decrease in interstitial water Fe concentrations. Sedimentation rate (see below), carbonate content, and lithostratigraphy (see “Lithostratigraphy”) remain constant over this interval, so we conclude that the first-order magnetic trends are a function of downhole dissolution of magnetic minerals rather than a change in provenance or a function of carbonate dilution. The higher frequency, lower amplitude variability, however, may reflect changes in carbonate content, sediment provenance, magnetic field strength, or a combination of these processes. Finally, we note that the first-order magnetic intensity at Site U1408 is high compared to previous sites studied during Expedition 342. This may be because magnetic dissolution is not as aggressive in sediment recovered from Site U1408 as it is at other sites; interstitial water sulfate concentrations remain high throughout the entire recovered interval (see “Geochemistry”).

Inclination and declination clustering

Inclination values following 20 mT AF demagnetization treatment often cluster around +60° and between 0 and –45°. Inclination clustering is usually associated with declination clustering at ~0° and 180°, respectively. The 0 to –45° inclinations are shallow with respect to the reversed polarity value expected at the ~40°N latitude of Site U1408. 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 was most effective at removing the drilling overprint in the APC-recovered interval from ~140 to 180 mbsf; it was less effective between ~60 and 100 mbsf but sufficient to reveal reversed polarity magnetozones. We can utilize the positive and negative polarity clustering behavior to readily identify magnetozones in Site U1408 sediment.

Comparison between pass-through and discrete sample data

AF demagnetization results for 92 discrete samples are summarized in Table T12. Of the 21 samples treated with a peak AF demagnetization field of 60 mT, 12 reveal reasonably stable components of magnetization (e.g., Fig. F22A, F22B). 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. An example of such samples can be seen in Figure F22C. 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 section half measurement data (Fig. F19). This discrepancy suggests that section half measurement data from XCB core intervals should be interpreted with care, similar to our conclusions regarding paleomagnetism data from Sites U1403, U1406, and U1407.

Magnetostratigraphy

The shipboard downhole results reveal a series of normal and reversed magnetozones between Cores 342-U1408A-4H and 25X (~29–220 mbsf), between Cores 342-U1408B-5H and 18H (~30–154 mbsf), and between Cores 342-U1408C-5H and 19H (~35–165 mbsf). These magnetostratigraphies can be straightforwardly correlated among all three holes.

By utilizing radiolarian, foraminifer, and nannofossil biostratigraphic datums from Hole U1408A (see “Biostratigraphy”), we can correlate magnetozones to the geomagnetic polarity timescale (GPTS). The shipboard magnetostratigraphic age model is based on Hole U1408A, for which we have the most biostratigraphic datums. Extension of this age model to the magnetozonation observed in Holes U1408B and U1408C is contingent on the accuracy of the stratigraphic correlation between holes, which is corroborated by some lithologic horizons, biostratigraphic datums, and physical property features (see “Stratigraphic correlation”). Our correlation is presented in Table T13 and in Figures F19, F20, F21, and F23.

In Hole U1408A, we correlated the magnetostratigraphy in Cores 342-U1408A-4H through 16H to lower Chron C17n.3n (~38.3 Ma) through upper Chron C20r (~43.4 Ma). Chron C20r is continuously recognized downhole to the bottom of the APC-recovered interval (i.e., bottom of Core 342-U1408A-20H). Continuous downhole negative inclinations indicative of reversed polarity are recognized in archive half section and discrete samples from XCB-cored sediment downhole to Core 342-U1408A-24X. The transition from negative to positive inclinations between 211.59 and 212.60 mbsf in Core 24X is correlated to the Chron C20r/C21n boundary (45.724 Ma). Magnetozone correlations for Holes U1408B and U1408C are similar to Hole U1408A, with the exception that we did not interpret the XCB-cored intervals because we did not have paleomagnetism data from discrete samples to verify the data from archive half sections.

The correlations described above provide a shipboard chronostratigraphic framework for interpreting the middle Eocene sediment record at Site U1408. The most salient implication of this age model is that sedimentation rates along the Southeast Newfoundland Ridge at the paleowater depth of Site U1408 varied between ~1.5 and 3.14 cm/k.y. across the middle Eocene (Fig. F15). Average linear sedimentation rates were higher before the MECO (~41.5 Ma) than after.

Detailed record of a geomagnetic field transition

The geomagnetic field transitions from Chrons C18n.1n to C18n.1r to C18n.2n are recorded in exceptional detail in all three holes at Site U1408. In Hole U1408A, the Chron C18n.1n–C18n.1r transition is recorded from ~47 to 51 mbsf. In Hole U1408C, the Chron C18n.1r–C18n.2n transition is recorded from ~50 to 55 mbsf. Both transitions are recorded in Hole U1408B from ~50 to 57 mbsf. See Table T13 for precise locations of the transitions in each hole. Variations in declination, inclination, and intensity during the transitions are illustrated in Figure F24. Magnetic field behavior is very coherent between all three holes.

Detailed sediment records of the most recent magnetic field transition (i.e., the Matuyama–Brunhes transition [0.781 Ma]) show that magnetic directions flip several times and intensity values oscillate while the geomagnetic field transitions between stable polarity states (e.g., Yamazaki and Oda 2001; Valet et al., 2005). We observe similar paleomagnetism behavior during the Chron C18 series transitions recorded in all three holes (Fig. F24). We note that some of the variation in NRM that remains after 20 mT demagnetization may reflect not only geomagnetic field intensity but also changes in magnetic mineralogy, concentration, grain size, or a combination of these factors. A thorough shore-based paleomagnetism and rock magnetic study is necessary to fully characterize and understand the geomagnetic field transition behavior. Cyclostratigraphic analysis of physical properties in this interval may provide constraints on the duration of these Eocene field reversals.

Magnetic susceptibility and anisotropy of magnetic susceptibility

Bulk magnetic susceptibility measured on 94 discrete samples is summarized in Table T14. Although discrete samples were collected from each core section from the entire depth of Hole U1408A, we chose to measure only odd-numbered samples. Downhole variation in whole-round magnetic susceptibility (WRMS) and discrete sample magnetic susceptibility (DSMS) for Hole U1408A are shown in Figure F19. The WRMS data for Hole U1408A 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 (IU), 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 U1408 sediment.

AMS results for the discrete samples are also summarized in Table T14 and are shown in Figure F25. The eigenvalues associated with the maximum (τ₁), intermediate (τ₂), and minimum (τ₃) magnetic susceptibilities at Site U1408 show some downhole variations that correspond to changes in lithostratigraphy. We also observed a distinct change in AMS values at the transition from APC to XCB coring technologies, with greater divergence between principal eigenvalues, consistently greater anisotropy, and a trend toward steeper inclinations of the minimum principal eigenvector (V₃) in XCB-recovered intervals.

Triaxial fabrics are greatest, and most variable, in lithostratigraphic Unit III. Magnetic fabrics, reflected in the V₃ inclination, lineation (τ₁/τ₂), and foliation (τ₂/τ₃) values, are greatest between ~90 and 180 mbsf. This interval also corresponds to the greatest sedimentation rates calculated for Site U1408 (3.14 cm/k.y.) (Fig. F15) and a zone of nannofossil clay with lower carbonate content compared to the nannofossil ooze lithostratigraphy above and below this interval.

Top of page   |    Previous   |    Next