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doi:10.2204/iodp.proc.340.107.2013

Paleomagnetism

Cores 340-U1397A-1H through 17H and 340-U1397B-1H through 17H were recovered using nonmagnetic core barrels with the APC. All other APC cores were recovered using standard steel core barrels. The FlexIt core orientation tool was used on all APC cores recovered with nonmagnetic barrels; thus, between 0 and 67 mbsf in Hole U1397A declination can be corrected to true north. Where FlexIt tool data were not available, declination was guided by the discrete inclination data (see “Paleomagnetism” in the “Methods” chapter [Expedition 340 Scientists, 2013a]). Expected inclination for the site is 28.0° during normal polarity and –28.0° during reversed polarity, assuming a geocentric axial dipole (GAD). The archive halves of cores from Holes U1397A and U1397B were measured on the three-axis superconducting rock magnetometer (SRM) at 2.5 cm intervals (Table T5). Natural remanent magnetization (NRM) was measured before (NRM0) and after stepwise alternating field demagnetization at 10 mT (NRM10) and 20 mT (NRM20) for Cores 340-U1397A-1H through 4H. Only NRM0 and NRM20 were measured on all other cores to maintain workflow through the laboratory. Twenty discrete samples were collected from the center of the working half of the core to compare to the SRM data (see “Paleomagnetism” in the “Methods” chapter [Expedition 340 Scientists, 2013a]).

Sediment recovered from Site U1397 was heterogeneous in composition and varied between layers of hemipelagic sediment and volcanic tephras and turbidites (see “Lithostratigraphy”). Using the detailed core description logs we only interpreted data measured on identifiable hemipelagic layers, as these appear the least disturbed and are more likely to provide information on the behavior of the geomagnetic field.

Results

NRM0 (red) and NRM20 (blue) intensities are shown for Holes U1397A and U1397B in Figures F12 and F13. NRM0 intensity is relatively high at ~0.5–1 m/A. NRM20 is ~10% of NRM0. This value reflects both the low coercivity of the sediment and the susceptibility of the material to drill string overprint. This overprint is typically removed by alternating field demagnetization at 20 mT when using nonmagnetic barrels. Between 55 and 68 mbsf in Hole U1397A and between 55 and 60 mbsf and 64 and 68 mbsf in Hole U1397B, NRM intensity is very low. These low values are mirrored in the magnetic susceptibility profile (see “Physical properties”). This interval is a sandy mixed turbidite (see “Lithostratigraphy”) and differs from the volcanic turbidites and tephras throughout the rest of the core. The lows in NRM and magnetic susceptibility therefore likely represent reduced concentrations of volcaniclastic derived ferrimagnetic minerals in this mixed deposit.

Hemipelagic sediment is discontinuous at Site U1397, particularly below ~55 mbsf. Above ~55 mbsf, SRM and discrete inclination in Holes U1397A and U1397B show scattered but positive inclination, clustering around the expected GAD inclination. Declination shows little variation, suggesting that all sediment and tephra <55 mbsf were deposited in the Brunhes Chron (<780 ka). Below this depth the record becomes increasingly discontinuous, making confident interpretation of the paleomagnetic data difficult. This interpretation is further hindered by evidence of rifling of declination within core sections recovered with the XCB. SRM inclination data in this interval are highly scattered over a range of around –50° to almost 90° in both holes and are steeper than their corresponding discrete samples. Given the tendency of Expedition 340 cores to acquire a persistent magnetic overprint when not recovered using nonmagnetic core barrels, discrete inclination data may prove the only useful magnetostratigraphic tool below ~121 mbsf in Hole U1397A and 113 mbsf in Hole U1397B.

Excluding deposits with significant evidence of reworking, biostratigraphic ages suggest a maximum age of 400 ka at the base of Hole U1397B (~253 mbsf) (see “Paleontology and biostratigraphy”). Additional geological constraints include clasts of quartz-rich andesite indicative of the eruption of Pitons du Carbet (~330–350 ka) at ~250 mbsf in Hole U1397B (see “Lithostratigraphy”). These diverse constraints suggests that all material recovered from Site U1397 was deposited in the Brunhes Chron. The majority of discrete inclination values from ~55 to ~250 mbsf show positive inclination; however, at a few depths, notably 152, 248, and 259 mbsf in Hole U1397A and 161 mbsf in Hole U1397B, negative inclinations were recorded both in SRM and discrete data. Without declination to confirm these data, coupled with the fracturing and contortion of this sediment (see “Lithostratigraphy”), it is likely that these data from lithified hemipelagic mudstones cannot be accurately interpreted to represent the behavior of the geomagnetic field during deposition. With this assumption, paleomagnetic data support the biostratigraphic and geological constraints for Hole U1397, at least to ~240 mbsf, that all sediment was deposited within the current normal polarity chron. Using the GPTS of Cande and Kent (1995), this suggests sediment is younger than 780 ka.