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doi:10.2204/iodp.proc.340.104.2013

Paleomagnetism

Cores 340-U1394A-1H through 3H were recovered with the APC using nonmagnetic core barrels. 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 24 mbsf in Hole U1394A 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, 2013]). Expected inclination for the site is 30.0° during normal polarity and –30.0° during reversed polarity, assuming a geocentric axial dipole (GAD). Archive halves of cores from Holes U1394A and U1394B were measured on the three-axis superconducting rock magnetometer (SRM) at 2.5 cm intervals (Table T5). NRM was measured before (NRM₀) and after stepwise alternating field demagnetization (NRM_{FIELD in mT}) in peak fields as high as 20 mT. Core 340-U1394A-1H and Sections 340-U1394A-2H-1A through 3H were measured at NRM_{0, 5, 10, 15, 20}, and Cores 340-U1394B-16H through 21H were measured at NRM_0, 15, 20. All other sections were measured at NRM_{0, 10, 15, 20}. Twenty discrete samples were collected from the center of the working half of the core for to compare to the SRM data (see “Paleomagnetism” in the “Methods” chapter [Expedition 340 Scientists, 2013]).

Sediment recovered from Site U1394 is very heterogeneous in composition and varied between layers of hemipelagic sedimentation to deposits related to volcanic activity and volcanic turbidites (see “Lithostratigraphy”). Using the detailed core description logs we only interpreted data measured on identifiable hemipelagic layers (red/orange directional data; Figs. F9, F10), as these appear the least disturbed and are more likely information related to the behavior of the geomagnetic field.

Results

Cores 340-U1394A-1H through 3H show scattered but expected inclination for the site, assuming a GAD, clustering around 30° (Fig. F9). APC cores recovered with steel barrels and cores recovered with the XCB possess steeper than GAD inclinations, possibly suggesting a drilling overprint that remains after alternating field (AF) demagnetization in a field of 20 mT. Discrete samples show two magnetic components: a softer component, typically removed by demagnetization in fields of 10–15 mT, and a secondary ChRM component that trends toward the origin (Fig. F11). After demagnetization at 20 mT, the inclination of the discrete samples is shallower than the corresponding SRM inclination despite similar normalized demagnetization ratios. Discrete cubes taken in the center of the working half may be more protected from drilling-related magnetic overprints (Richter et al., 2007) than the whole-section SRM data. The data from Site U1394 suggests that the steep inclination seen in the SRM records recovered in standard steel barrels may be related to acquisition of an isothermal remanent magnetization (IRM) related to the coring process (e.g., Fuller et al., 2006) that is not removed by AF demagnetization in a field of 20 mT. Coring deformation around the edges of the APC and XCB can also potentially explain the inclination steepening (Fuller et al., 2006). However, data from nonmagnetic barrels do not show the same level of discrepancy between the SRM and discrete data, suggesting that these overprints result from the barrel type used and that the sediment in the center of the core is more protected from this overprint.

Intensity at NRM₀ and NRM₂₀, inclination at NRM₂₀, and, for Cores 340-U1394A-1H and 2H with FlexIt tool data, corrected declination at NRM₂₀ is shown in Figures F9 and F10. Inclination of the 20 discrete samples demagnetized at 20 mT are plotted as black squares. As at other sites, NRM₀ intensity shows a similar signature to magnetic susceptibility (see “Physical properties”). These parameters, particularly magnetic susceptibility, are strongly related to the concentration of ferrimagnetic minerals (e.g., magnetite and correlate well with depositional units of volcanic origin) (see “Core descriptions”).

SRM data all show steep but normal polarity; all 19 discrete hemipelagic samples spaced over both holes also record normal polarity clustering around a GAD-like inclination. Nannofossil and foraminiferal datums give the sediment a Pleistocene age (see “Paleontology and biostratigraphy”). We see no evidence for reversed polarity in the record, particularly in the lower part of the record where data coverage is better. Because of the lack of a continuous record, we cannot unequivocally say that all sediment was deposited within the Brunhes chron; however, this would indicate a late Pleistocene age for the site, which agrees with biostratigraphic ages.

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