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doi:10.2204/iodp.proc.342.111.2014

Paleomagnetism

We completed a paleomagnetism study of APC and XCB cores from Holes U1410A–U1410C 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 all cores from Hole U1410A. For all other cores, we only measured NRM after 20 mT demagnetization. Archive-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-U1410A-1H through 16H and 342-U1410B-1H through 18H were azimuthally oriented using the FlexIT orientation tool (Table T12). All other cores were not oriented.

We also collected 178 discrete samples from working section halves to verify the archive section half measurement data and to measure anisotropy of magnetic susceptibility (AMS) and bulk susceptibility of Site U1410 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 U1410A. Selected samples were subjected to measurements of AMS, including bulk susceptibility, and NRM after 20 mT AF demagnetization. Twenty-two samples were selected for step-wise 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 U1410A, U1410B, and U1410C in Figures F15, F16, and F17, respectively. Similar to paleomagnetism results from previous Expedition 342 sites, section-half measurement data from cores recovered using the XCB 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-recovered intervals.

We report the following principal features in the paleomagnetism data at Site U1410. First, we observed several trends in downhole magnetization intensity. Second, inclinations are biased toward positive values, whereas declinations alternate around 180° and cluster at ~0° and ~180°. Third, the geomagnetic field transitions correlated to Chrons C18n.1n–C18n.1r–C18n.2n are recorded in exceptional detail in Hole U1410B. The same reversal transitions were observed in similar detail at Site U1408; the two paleomagnetism records are remarkably coherent.

Downhole trends in magnetic intensity

Downhole magnetic intensity values show downhole trends that are consistent among all three holes and generally correspond to lithostratigraphic units. Magnetic intensity is invariant and high (~10^–2 A/m) in the uppermost ~35 m. This interval corresponds to the reddish brown to green Pleistocene nannofossil oozes, nannofossil and foraminifer oozes, and silty clay of lithostratigraphic Unit I (see “Lithostratigraphy”). From ~40 to ~100 mbsf, magnetic intensity generally remains high (~10^–2 A/m) but is distinguished from the interval above by frequent horizons of low magnetic intensity (as low as ~10^–4 A/m). This interval corresponds to the upper Miocene to Oligocene greenish gray clay and nannofossil clay of Unit II and the uppermost part of the middle Eocene green and greenish gray nannofossil clay and nannofossil ooze of Unit III. Although the contact between Units II and III is clearly distinguished by magnetic susceptibility, it is not obvious in the magnetic intensity record. From ~100 to 150 mbsf, magnetic intensity gradually decreases from 10^–2 to 10^–4 A/m. This interval is entirely within Unit III. Below ~150 mbsf, Holes U1410A and U1410B have two distinctive intervals that show gradual increases in intensity, from ~10^–5 to ~10^–4 A/m at ~150–220 mbsf and from ~10^–5 to ~10^–3 A/m below ~220 mbsf. In contrast, Hole U1410C shows almost constant intensity (~10^–4 A/m) below ~150 mbsf. Downhole trends in magnetic susceptibility below ~150 mbsf are similar among all three holes. The difference in downhole magnetic intensity trends among the three holes in this lowest interval is unclear, but it may be attributable to different degrees of core disturbance in these XCB-recovered intervals.

Inclination bias and declination clustering

Inclination bias indicates that there is a substantial drilling overprint even after 20 mT AF demagnetization. Because of this strong inclination bias, we often cannot identify paleomagnetism polarity solely based on shipboard inclination data. However, azimuthally oriented APC cores show ~180° alternation of declination, and these values cluster at ~0° and ~180°. We interpret intervals with declination values of ~0° (~180°) to indicate normal (reversed) magnetozones. A magnetozone with a primary normal polarity should not display inclinations less than ~40°, barring sedimentary inclination shallowing biases. Notably, intervals with ~180° declination almost always correspond to inclination values that are more shallow than those in the intervals with declination of ~0°. Thus, the drilling overprint mainly obscures remanent inclination but not declination, similar to the paleomagnetism results from Sites U1403 and U1404.

Comparison between pass-through and discrete sample data

AF demagnetization results for 109 discrete samples are summarized in Table T13. Of the 22 samples treated with a peak AF demagnetization field of 60 mT, 13 reveal reasonably stable components of magnetization (e.g., Fig. F18A, F18B). 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 a magnetic overprint can be seen in Figure F18C. Nevertheless, these results are useful for verifying the 20 mT pass-through paleomagnetism data from the archive section halves.

Magnetization intensity and declination are generally consistent between the discrete samples and the archive section half samples from APC-recovered intervals (Fig. F15). In contrast, inclinations measured in discrete samples from XCB-recovered intervals are often more shallow than their counterpart values in the archive section half samples from APC core intervals. These observations indicate that cores from XCB-recovered intervals have a relatively severe overprint.

Magnetostratigraphy

The shipboard downhole results reveal a series of normal and reversed magnetozones between Cores 342-U1410A-1H and 24X (~0–218 mbsf), between Cores 342-U1410B-1H and 15H (~0–121 mbsf), and between Cores 342-U1410C-2H and 13H (13–118 mbsf). These magnetostratigraphies can be easily correlated among all three holes.

By utilizing radiolarian, foraminifer, and nannofossil biostratigraphic datums from Hole U1410A (see “Biostratigraphy”), we can correlate magnetozones to the geomagnetic polarity timescale (GPTS). The shipboard magnetostratigraphic age model is based on Hole U1410A, for which we have the most biostratigraphic datums. Extension of this age model to the magnetozonation observed in Holes U1410B and U1410C is contingent on the accuracy of the stratigraphic correlation between holes, which is corroborated by lithostratigraphic horizons, biostratigraphic datums, and physical property features (see “Stratigraphic correlation”). Our correlation is presented in Table T14 and Figures F15, F16, F17, and F19.

We correlate the polarity transitions from the top of Core 342-U1410A-1H to 4H at ~32.95 mbsf to Chron C1n (Brunhes) to upper Chron C2An.1n (~2.6 Ma). We do not observe Chrons C2r.1r and C2r.1n (Réunion) in any hole at Site U1410 in this interval. We do not interpret this as a hiatus, but rather the inability of the wide-bore pass-through magnetometer to resolve these extremely short and rarely observed chrons in this Pleistocene sediment. We correlate the well-resolved magnetozonation observed in Sections 342-U1410A-8H-3 (~68.68 mbsf) to 24X-1 (~218.06 mbsf) to lower Chron C18n.1n (~39.5 Ma) continuously downhole to upper Chron C21r (~47.4 Ma). Paleomagnetism data from archive section halves and discrete samples suggest a magnetostratigraphy can be developed in deeper intervals at Site U1410, but it cannot be resolved in sufficient detail with these shipboard data to correlate with confidence to the GPTS. Magnetozone correlations for Holes U1410B and U1410C are similar to Hole U1410A, with the exceptions that we did not interpret XCB-recovered intervals in the former holes and we did not observe the Chron 18n.1r/18n.2n boundary in Hole U1410C.

The correlations described above provide a shipboard chronostratigraphic framework for interpreting the Pleistocene and middle Eocene sediment record at Site U1410. The most salient implications of this age model are that

• Average linear sedimentation rates during the Pleistocene at Site U1410 range from 1.47 to 2.35 cm/k.y. (Fig. F22),

• The difficulty in correlating magnetozones between Cores 342-U1410A-5H and 7H to the GPTS can be attributed to the highly condensed nature of this interval and the likelihood of several significant hiatuses inferred from the nannofossil biostratigraphy, and

• Average linear sedimentation rates during the middle Eocene at Site U1410 are similar to those calculated for Sites U1407–U1409. Sedimentation rates are highest during Chron C20n (2.63 cm/k.y. from ~42.3 to 43.4 Ma) and are ~75% lower before and after this interval. Notably, the interval of highest average linear sedimentation rate (LSR) at Site U1410 is one chron younger than at Site U1409, the downslope conjugate site to Site U1410.

Detailed record of a geomagnetic field transition

At Site U1408, we found that the geomagnetic field transitions from Chrons C18n.1n to C18n.1r to C18n.2n are recorded in exceptional detail in all three holes over ~7 m of stratigraphic section. A very similar record is observed between ~67 and 74 mbsf in Hole U1410B (Fig. F20). The remarkable coherence in the record between the two distant sites suggests that the paleomagnetism record faithfully reflects geomagnetic field behavior rather than diagenetic artifacts. A thorough, shore-based paleomagnetism and rock magnetic study is necessary, however, to fully characterize and understand the geomagnetic field transition behavior.

Magnetic susceptibility and anisotropy of magnetic susceptibility

Bulk magnetic susceptibility measured on 118 discrete samples is summarized in Table T15. Although discrete samples were collected from each core section from the entire depth of Hole U1410A, we chose to measure only odd-numbered samples, except in the interval between Cores 342-U1410A-8H and 16H, in which we measured every sample. Downhole variation in whole-round magnetic susceptibility (WRMS) and discrete sample magnetic susceptibility (DSMS) for Hole U1410A are shown in Figure F15. The WRMS data for Hole U1410A 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 U1410 sediment.

AMS results for the discrete samples are also summarized in Table T15 and are shown in Figure F21. The eigenvalues associated with the maximum (τ₁), intermediate (τ₂), and minimum (τ₃) magnetic susceptibilities at Site U1410 show some downhole variations that correspond to changes in lithostratigraphy. We also observe 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 the XCB cores. In the clay-dominated lithostratigraphic Unit III, we observed consistently steeper inclinations of the minimum principal eigenvector (V₃) in the XCB-recovered interval, but we did not observe this trend in the nannofossil chalks of Unit IV. Thus, as expected, more lithified sediment is more resistant to coring-induced fabrics.

Triaxial fabrics are greatest and most variable in three discrete intervals: Cores 342-U1410A-1H, 17X, and 24X. These intervals are also characterized by high degrees of anisotropy (P) and stronger lineation fabrics. In Core 1H, we attribute this behavior to the high pore water content and lack of lithification in these young sediments. Core 17X is at the base of the interval of highest average LSR (Chron 20n; see “Magnetostratigraphy”) and in a highly disturbed interval described as a slump deposit (Fig. F12). Core 24X is characterized by a peak in bulk density values and a low in water content in the white nannofossil chalk of lithostratigraphic Subunit IVa (see “Physical properties”).

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