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doi:10.2204/iodp.proc.342.106.2014

Paleomagnetism

We completed a paleomagnetic study of APC and XCB cores from Holes U1405A–U1405C 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 Hole U1405A. For cores from Holes U1405B and U1405C, we only measured NRM after 20 mT demagnetization to compensate for time lost while trouble shooting a problem with the superconducting rock magnetometer (SRM). These problems with the SRM, and their solutions, are detailed below. 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-U1405A-12H through 26H, 342-U1405B-1H through 24H, and 342-U1405C-1H through 25H were azimuthally oriented using the FlexIT tool (Table T11); all other cores were not oriented.

We also collected 189 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 U1405 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 in Hole U1405A. Selected samples were subjected to AMS measurements, including bulk susceptibility, and NRM measurements after 20 mT AF demagnetization. Seven 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 paleomagnetic data after 20 mT demagnetization are presented for Holes U1405A, U1405B, and U1405C in Figures F23, F24, and F25, respectively. Similar to paleomagnetic results from Sites U1403 and U1404 (see “Paleomagnetism” in the “Site U1403” chapter and “Paleomagnetism” in the “Site U1404” chapter [Norris et al., 2014b, 2014c]), section-half measurement data from XCB cores are difficult to interpret because of biscuiting and substantial core disturbance. Therefore, we chose to interpret only results obtained from APC cores.

We report the following principal features in the paleomagnetic data at Site U1405:

• Magnetic data are noisier in Hole U1405A than in Holes U1405B and U1405C;

• Magnetic intensity and susceptibility decrease significantly over the uppermost ~20–30 mbsf, followed by a gradual decrease downhole; and

• Inclination clusters at ~60° and –45°, associated with a corresponding clustering of declinations of ~0° and 180°, respectively.

Anomalous y-axis flux jumps and magnetic noise when using the superconducting rock magnetometer

The SRM lost functionality for ~15 h during the initial phases of Hole U1405B data acquisition (1500 h, 28 June to 0600 h, 29 June 2012). Prior to complete loss of functionality, we noticed an increase in the frequency and number of magnetic flux jumps in the y-axis superconducting quantum interference device (SQUID). Initially, one or two flux jumps were recorded by the y-axis SQUID during every few measurement runs following 20 mT demagnetization. Flux jumps always began when the core section first entered the measurement region of the magnetometer from the degausser side of the SRM. The number and frequency of these flux jumps increased rapidly over the next hour, reaching hundreds of jumps per measurement run and while measuring core sections without first using the degausser. We did not observe any flux jumps on the x- and z-axis SQUIDs during any of these measurement runs, indicating that the problem was isolated to the y-axis SQUID. Remanent magnetizations calculated from these data invariably yielded inclination and declination values of 0° and 270°, respectively, and magnetic intensity values that increased linearly downhole in each section half.

Several initial attempts at troubleshooting proved fruitless. These included

• Washing the sample tray and track with antistatic solution,

• Measuring the length of the magnetometer sample track with a fluxgate magnetometer to detect stray high-magnetization material inside the magnetometer,

• Heating the SQUIDs to release any trapped flux,

• Heating the strip line to remove any circulating currents from the pickup coil structure,

• Switching the x and y SQUID electronics boxes, and

• Replacing the serial card on the magnetometer PC used to communicate with the SQUID electronic boxes.

During various test runs to isolate the source of the anomalous magnetic flux, we noticed that the problem was isolated to core sections longer than 1 m. Data collected from short (<0.7 m) core sections could be interpreted, although the quality of the data was still poor. These observations, however, led us to discover that an ~1 cm interval of a power cable attached to the in-line degausser was not properly shielded. Most of this cable is shielded with Mu-metal foil to prevent ambient electromagnetic field (EMF) from interfering with the magnetometer. The ~1 cm gap between the shielded degausser and the shielded power cable was covered with aluminum foil, but this foil was connected to the outside of the cable shielding and therefore was not electrically grounded. Moreover, this gap in cable shielding was adjacent to a join in the magnetic shielding assembly around the magnetometer track that is known to leak EMF inside the shielding assembly. We rewrapped the cable gap with aluminum foil, making sure this foil was electrically connected to the existing shielding material. The number and frequency of y-axis flux jumps decreased abruptly thereafter. We continued to observe occasional flux jumps, but the typical recurrence interval was 1 flux jump every 30–40 core sections, occurring most frequently after the 20 mT AF demagnetization run. High-quality results could be acquired immediately upon remeasurement.

We attribute the large y-axis flux jumps to the “antennae” effect, in which wet and typically carbonate-containing core sections longer than ~1–1.25 m channel stray magnetic flux from the core laboratory into the magnetometer sensing region (Fig. F26). The y-axis SQUID coil configuration is particularly sensitive to induction currents that can be conducted along the section-half surface. In this particular case during Expedition 342, the antennae effect was amplified by the gap in magnetic shielding of the degausser power cable described above. We suspect that the ungrounded foil that previously covered this gap collected, rather than shielded against, ambient EMF. This collected EMF was then transmitted to the y-axis coil through the section-half surface by an inducting current. We suspect that stray radio or electromagnetic field was exceptionally high during measurement of Hole U1405B cores because of rough seas and therefore greater use of electric thruster and drilling motors and motion of these motors onboard the R/V JOIDES Resolution.

Finally, we note that after properly shielding the gap, the practical background level for SRM measurements improved to ~1 × 10^–5 A/m. This is approximately one order of magnitude greater sensitivity than we were achieving prior to arriving at Site U1405. SRM demagnetization data from Hole U1405B and subsequent holes have been remarkably free of noise and reveal a strong square-wave reversal pattern, particularly in the inclination domain.

Downhole intensity trends

Downhole magnetic intensity and susceptibility values show distinct downhole changes and trends (Figs. F23, F24, F25). Magnetic intensity decreases from ~10^–2 to ~10^–4 A/m over the uppermost ~20 m of sediment at Site U1405. From ~20 to ~240 mbsf, intensity continues to decrease, reaching sustained values of ~10^–5 A/m. A notable exception to this trend is between ~180 and 190 mbsf in all three holes, where intensity increases to ~10^–4 A/m.

Magnetic susceptibility values also show a sharp decrease from ~50 × 10^–5 to <10 × 10^–5 SI (volume normalized) in the uppermost ~20 m of recovered sediment. Magnetic susceptibility values remain low for the remainder of each Site U1405 core.

These magnetic trends correspond with lithostratigraphy. The uppermost ~20 m of sediment at Site U1405 is composed of yellow to brown Pleistocene–Pliocene foraminiferal nannofossil ooze (lithostratigraphic Subunit Ia) and clay to silty clay (Subunit Ib). The remaining lithostratigraphy is composed of green clay with varying abundances of biosilica, nannofossil ooze, and carbonate (Unit II and division; see “Lithostratigraphy”). No abrupt changes in sulfate concentration with depth are apparent in Hole U1405A, but manganese shows a pronounced downhole increase beginning at ~20 mbsf (see “Geochemistry”). These trends suggest that the initial decrease in magnetic intensity and susceptibility values reflect the downward migration of redox fronts in the upper 2–30 m of sediment. Although lithostratigraphic Unit II is characterized by glauconitic clay, we cannot, at this time, determine if the drop in magnetic intensity is attributable to diagenetic loss of paramagnetic and ferromagnetic material, a change in initial supply of these materials, dilution of these materials by silica and/or carbonate, or a combination of these processes in the Miocene drift deposits.

Inclination and declination clustering

Inclination values following 20 mT AF demagnetization treatments often cluster around 60° and –45°. This behavior is especially pronounced for data from Holes U1405B and U1405C. Inclination clustering is usually associated with declination clustering at ~0° and 180°. The approximately –45° inclination is slightly shallow with respect to the reserved polarity value expected at the ~40°N latitude of Site U1405. This shallow bias is readily attributed to a small drilling overprint that remains after 20 mT AF demagnetization. We note, however, that 20 mT AF demagnetization treatment is much more effective at removing the drilling overprint than it was with sediment recovered from Sites U1403 and U1404. This may reflect a difference in the remanence characteristics between these sites or a change in SRM sensitivity, as described above. Regardless of the effectiveness of AF treatment between sites, we can utilize the positive and negative polarity-clustering behavior to readily identify magnetozones in recovered intervals at Site U1405.

Comparison between pass-through and discrete sample data

AF demagnetization results for discrete samples are summarized in Table T12. Of the seven samples treated with peak AF demagnetization field of 60 mT, five reveal relatively stable components of magnetization (e.g., Fig. F27). These samples have remanent magnetizations 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 obscured the primary magnetic signal in these stratigraphic intervals, similar to results from Site U1404. Nevertheless, these results are useful for verifying the 20 mT pass-through paleomagnetic data from the archive section halves. In general, paleomagnetic data from archive section halves and discrete samples from oriented core intervals agree well.

Hole U1405A data are noisier than those for Holes U1405B and U1405C, but remanence data from discrete samples provide a straightforward check on the identification of magnetozones (Fig. F23). As detailed previously, we attribute this change in data noise to a change in the sensitivity of the SRM before and after shielding against the antennae effect.

Magnetostratigraphy

Shipboard downhole magnetostratigraphy results reveal a continuous series of normal and reversed magnetozones between Cores 342-U1405A-5H and 26H (~39–242 mbsf), between Cores 342-U1405B-5H and 24H (~39–224 mbsf), and between Cores 342-U1405C-5H and 24H (~37–215 mbsf). Downhole plots indicate additional series of magnetozones are recorded higher and possibly lower in the recovered section in all three holes, but shore-based studies are necessary to identify and fully characterize magnetozones in these low magnetic intensity intervals and provide preliminary age control to correlate them to the geomagnetic polarity timescale (GPTS). Magnetozones can be straightforwardly correlated between all three holes, especially below ~90 mbsf. Although the pattern and stratigraphic thickness of magnetozones in Cores 5H through 8H in all three holes is similar, significant offsets exist between the mbsf depths of these magnetozone boundaries in Hole U1405A and those in Holes U1405B and U1405C.

By utilizing radiolarian, foraminifer, and nannofossil biostratigraphic datums from Hole U1405A (see “Biostratigraphy”), we can correlate magnetozones to the GPTS. The shipboard magnetostratigraphic age model is based primarily on Hole U1405A, for which we have the most biostratigraphic tie points. Extension of this age model to the magnetozonation observed in Holes U1405B and U1405C 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”). Our correlation is presented in Table T13 and shown in Figures F23, F24, F25, and F28.

In Hole U1405A, we correlated the magnetostratigraphy in Cores 342-U1405A-5H through 8H and 13H through 21H to chron boundaries C5Br/C5Cn.1n (15.974 Ma) through C5Cn.2r/C5Cn.3n (16.543 Ma) and C6Bn.2n/C6Br (22.268 Ma) through C6Cn.3n/C6Cr (23.295 Ma), respectively. Additionally, the identification of nannofossil Zone NN3 in Sections 9H-3 and 9H-4 indicates that the reversed-to-normal downhole polarity transitions observed around this interval correlate to the Chron C5Dr.2r/C5En (18.056 Ma) and C5Er/C6n (18.748 Ma) boundaries. We note that the series of magnetozones above and below this interval are straightforwardly correlated to a continuous series of chrons on the GPTS, but both of these continuous correlations break down in Cores 8H, 10H, and 11H.

In Hole U1405B, we correlated a series of magnetozone boundaries in Cores 342-U1405B-5H to 9H to chron boundaries C5Br/C5Cn.1n (15.974 Ma) to C5Cn.3n/C5Cr (16.721 Ma). A second series of magnetozone boundaries are correlated to the Chron C6AAr.2n/C6AAr.3r (21.688 Ma) to C6Cn.3n/C6Cr (23.295 Ma) boundaries.

Magnetozone correlations in Hole U1405C are similar to those in Hole U1405B. From Sections 342-U1405C-5H-1 to 9H-6, we correlate magnetozone boundaries to the Chron C5Br/C5Cn.1n (15.974 Ma) to C5Cn.3n/C5Cr (16.721 Ma) boundaries. We did not observe the short reversed Chron C5Cn.1r in Hole U1405C, but we note that, based on the stratigraphic correlation, the interval in which we expected it occurs within the gap between Cores 37H and 8H. Magnetozone boundaries observed in Sections 11H-4 through 21H-7 are continuously correlated to the Chron C6AAr.2n/C6AAr.3r (21.688 Ma) to C6Cn.3n/C6Cr (23.295 Ma) boundaries.

The correlations described above provide a shipboard chronostratigraphic framework for interpreting the latest late Oligocene–middle Miocene sediment drift record at Site U1405. The most salient implications of this age model are summarized here. The Oligocene–Miocene transition is dated by the base of Chron C6Cn.2n (23.030 Ma), which we identified in Sections 342-U1405A-18H-7 through 19H-1, 342-U1405B-18H-3 through 18H-4, and 342-U1405C-17H-6. The shipboard magnetostratigraphic age model also indicates at least two unconformities in Hole U1405A that could be as long as 1.5 or 3.7 m.y. for hiatuses above the Chron C5Dr.2r/C5En boundary and below the Chron C5Er/C6n boundary. Only one hiatus of ~5 m.y. is easily identified between Chrons C5Cr and C6AAr.3r in Holes U1405B and U1405C. These observations suggest highly localized sediment deposition in these sediment drifts. Finally, this age model demonstrates that exceptionally high (~ 10 cm/k.y.) deep-sea sedimentation rates during the Oligocene–Miocene transition decreased only by a factor of 2–3 (~3.3–5.7 cm/k.y.) over the early Miocene at this site (Fig. F17).

Magnetic susceptibility and anisotropy of magnetic susceptibility

Bulk magnetic susceptibility measured on 59 discrete samples is summarized in Table T14. Downhole variation in whole-round magnetic susceptibility (WRMS) and discrete sample magnetic susceptibility (DSMS) for Hole U1405A are shown in Figure F23. The raw WRMS data for Hole U1405A were trimmed at section ends to remove edge effects; obviously spurious data points were also removed. 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, where they overlap in the uppermost ~80 m of sediment, show the same first- and second-order cyclic trends, indicating that these trends are robust features of Site U1405 sediment. Although discrete samples were collected through the entire depth of Hole U1405A, we chose to cease measurements below ~80 mbsf to compensate for time lost addressing measurement problems with the SRM.

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 U1405 show prominent downhole trends. τ₁ and τ₃ are indistinguishable from the top of Hole U1405A to the base of lithostratigraphic Subunit Ib (0–17.51 mbsf) (see “Lithostratigraphy”), indicating magnetic anisotropy remains weak over the depth of these upper units. This isotropic fabric is also reflected in low and invariable P values over this interval. Divergence between τ₁ and τ₃ increases substantially and as a step function at the top of Subunit IIa (17.51 mbsf) and then remains invariant downhole to ~82 mbsf, where our shipboard measurements stop. Although the AMS data indicate a strong oblate-shape anisotropy in the upper ~60 m of Subunit IIa, the inclination of the minimum eigenvector (V₃) does not show a preferred orientation. This indicates that although there is an abundance of oblate paramagnetic (or ferromagnetic) grains in this lithostratigraphic unit, they do not form a strong, depth-dependent magnetic fabric.

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