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doi:10.2204/iodp.proc.342.112.2014 PaleomagnetismWe completed a paleomagnetism study of APC and XCB cores from Holes U1411B and U1411C with the primary objective of establishing a magnetostratigraphic age model for the site. Cores from Hole U1411A were not measured because the hole was abandoned after the first core. Sediment from this first core was too disturbed to obtain a meaningful magnetic record. 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 U1411B. For cores from Hole U1411C, 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-U1411B-1H through 20H and 342-U1411C-2H through 9H were azimuthally oriented using the FlexIt orientation tool (Table T10). All other cores were not oriented. We also collected 161 discrete samples from working section halves to verify the archive section half measurement data of Site U1411 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 U1411B. Selected samples were subjected to NRM measurements after 20 mT AF demagnetization. Seventeen 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. ResultsDownhole paleomagnetism data after 20 mT demagnetization are presented for Holes U1411B and U1411C in Figures F14 and F15, 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-recovered cores. We report the following principal features in the paleomagnetism data at Site U1411. First, we observed three intervals of general magnetic intensity behavior. Second, inclinations measured from archive section halves are biased toward positive values, whereas declinations frequently cluster at ~0° and ~180°. Magnetic intensity zonationDownhole magnetic intensity trends can be binned into three intervals. Magnetic intensity decreases rapidly from ~10–2 to ~10–4 A/m from the top of the hole to ~15 mbsf. This decrease corresponds to lithostratigraphic Unit I, which is composed of gray to reddish brown interbedded Pleistocene nannofossil ooze, muddy foraminifer sand, and foraminiferal ooze with silty clay (see “Lithostratigraphy”). This rapid decrease in magnetic intensity is commonly observed in all paleomagnetism records of Pleistocene sediment from Expedition 342 and is most likely attributed to downhole reduction of iron oxides. Magnetic intensity remains relatively constant at ~10–4 A/m from ~20 to 140 mbsf; high-frequency, low-amplitude (approximately half of an order of magnitude) oscillatory variations are superposed on this general trend. This zone occurs within the Oligocene portion of lithostratigraphic Unit II, which is composed of greenish gray silty clay with nannofossils and nannofossil ooze. Magnetic intensity values increase by two orders of magnitude to ~10–2 A/m from ~140 to ~160 mbsf and remain high below this level to the bottom of the hole. The step to higher downhole magnetic intensity values occurs over the uppermost Eocene; we observed this trend at several other sites during the expedition. The lithostratigraphic Unit II/III boundary (~213 mbsf) is not associated with a distinct change in magnetic intensity. The slightly higher intensity values recorded in Unit III probably reflect a more pervasive drilling overprint in these XCB-recovered intervals. The intensity zonation is coherent with variations in magnetic susceptibility; therefore, we conclude that these trends most likely reflect variations in the concentration of magnetic minerals, caused either by diagenesis, supply, or both. Inclination bias and declination clusteringAn inclination bias toward positive values observed in most archive-half data indicates a substantial drilling overprint even after 20 mT AF demagnetization. Because of this strong inclination biasing, we often cannot identify paleomagnetism polarity solely based on shipboard inclination data. On the other hand, oriented APC-recovered cores often show ~180° alternations in declination values, which 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 shallower than those in the intervals with declination of ~0°. Thus, the drilling overprint mainly obscures the remanent inclination but not declination. This magnetic behavior is similar to the paleomagnetism results from Sites U1403, U1404, and U1410. Comparison between pass-through and discrete sample dataAF demagnetization results for 98 discrete samples are summarized in Table T11. Of the 17 samples treated with a 60 mT peak AF demagnetization field, only one sample from the APC-recovered interval reveals a reasonably stable magnetization (e.g., Fig. F16A). This sample has a remanent magnetization intensity 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 AF demagnetization. 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 F16B. 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. F14). In contrast, inclinations measured in discrete samples from XCB-recovered intervals are often more shallow than their counterpart values in the archive section half from APC-recovered core intervals. These observations indicate that cores from XCB-recovered intervals have a relatively severe overprint. MagnetostratigraphyShipboard results reveal two series of magnetozones. The first zone is observed in Cores 342-U1411B-1H through 2H (~1–11 mbsf) and 342-U1411C-1H through 2H (~0–7 mbsf). The second series is observed between Cores 342-U1411B-4H and 20H (~20–177 mbsf), and part of this series is between Cores 342-U1411C-7H and 8H (~127–143 mbsf). These magnetostratigraphies can be correlated between both holes. By utilizing radiolarian, foraminifer, and nannofossil biostratigraphic datums from Hole U1411B (see “Biostratigraphy”), we can correlate magnetozones to the geomagnetic polarity timescale (GPTS). The shipboard magnetostratigraphic age model is based on Hole U1411B, for which we have the most biostratigraphic datums. Extension of this age model to the magnetozonation observed in Hole U1411C 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 T12 and is shown in Figures F14, F15, and F17. We correlate the magnetostratigraphy in Cores 342-U1411B-1H and 2H to Chrons C1n (Brunhes) through upper C1r.3r (~1.19 Ma at ~10.4 mbsf). The magnetostratigraphy observed in Sections 342-U1411B-4H-4 through 20H-1 is correlated to lower Chrons C8n.2n (~25.9 Ma) through upper C15n (~35.0 Ma). We did not observe the top of Chron C12r, any of Chron C12n, or the bottom of Chron C11r in any hole at Site U1411 in this interval. We also did not observe Chrons C11n.1n and C11n.1r in any hole at Site U1411. Paleomagnetism data from archive section halves and discrete samples suggest a magnetostratigraphy can be developed in deeper intervals at Site U1411, but it cannot yet be resolved in sufficient detail with these data to correlate with confidence to the GPTS. We note, however, that given the foraminifer and nannofossil biostratigraphy, we expect to observe the Chrons C16 and C17 series in Cores 342-U1411B-23X through 28X. Magnetozone correlations for the uppermost 7 m of Hole U1411C are similar to Hole U1411B. Deeper in Hole U1411C, we can only interpret the top and bottom of Chron C13n at this time. The correlations described above provide a shipboard chronostratigraphic framework for interpreting the Pleistocene, Oligocene, and late Eocene sediment drift record at Site U1411. The most salient implications of this age model are summarized below:
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