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doi:10.2204/iodp.proc.340.109.2013

Paleomagnetism

Cores 340-U1399A-1H through 12H and 340-U1399B-1H through 14H 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 95 mbsf in Hole U1399A and between 0 and 115 mbsf in Hole U1399B, declination can be corrected to true north. Where FlexIt tool data were not available, declination was guided by discrete inclination (see “Paleomagnetism” in the “Methods” chapter [Expedition 340 Scientists, 2013a]). Expected inclination for the site is 27.2° during normal polarity and –27.2° during reversed polarity, assuming a geocentric axial dipole (GAD). The archive halves of cores from Holes U1399A and U1399B 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 in a peak field of 20 mT (NRM20). Twenty-four 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, 2013a]).

Sediments recovered from Site U1399 are a heterogeneous mixture of hemipelagic sediment and volcaniclastic tephras and turbidites. Below ~24 mbsf in Hole U1399A and at 13 mbsf in Hole U1399B, hemipelagic sediment is often contorted and deformed (see “Lithostratigraphy”). Sediment deformation in these zones is often severe, and its direction is not constant, making interpretation of paleomagnetic directions related to geomagnetic field behavior difficult. Detailed consultation of the visual core descriptions (VCDs) allowed us to extract intervals less visually affected by deformation and potentially suitable for analysis of geomagnetic field behavior. Figure F10 shows three sets of the same inclination data from Hole U1399A demagnetized at 20 mT: (1) raw inclination (gray) is data from all measured sediment, (2) hemipelagic inclination (blue) is data only from hemipelagic sediment (~40% of Hole U1399A data), and (3) nonchaotic inclination (green) is inclination data only from hemipelagic sediment with little visual evidence of deformation (~20% of Hole U1399A data). Deformation of hemipelagic sediment is evident in the inclination data; one standard deviation of the hemipelagic inclination is ~42° in Hole U1399A and 45° in Hole U1399B compared to 17° and 16° for the nonchaotic inclination data from the same core. Greater scatter in paleomagnetic directions coupled with consultation of the core description is the justification for removing these intervals from the data set and restricting interpretation of the behavior of the geomagnetic field to data from nonchaotic intervals. These nonchaotic interval data are presented in Figures F11 and F12.

Results

NRM0 (red) and NRM20 (blue) intensities are shown for Holes U1399A and U1399B in Figures F10 and F11. NRM0 intensity is moderately high at ~0.1–1 m/A. NRM0 and NRM20 of material retrieved with standard steel barrels is slightly higher than when nonmagnetic barrels were used, and NRM20 is <10% of NRM0. This reflects both the low magnetic coercivity of the sediment and the susceptibility of the material to acquire a strong drill string overprint. This overprint is typically removed by demagnetization at 20 mT when using nonmagnetic barrels, though overprinting of sediment when standard steel barrels are used is a persistent feature in Expedition 340 cores (see “Paleomagnetism” in the “Methods” chapter [Expedition 340 Scientists, 2013a]).

Rifling of the core barrel is evidenced by scattered declination values in all cores recovered with the XCB; thus, inclination is the only reliable indicator of polarity below 207.5 mbsf in Hole U1399A. Biscuiting of sediment makes determination of the degree of deformation difficult, and the recovery of several thick turbidites means that identification of nonchaotic hemipelagic layers is difficult with XCB recovery. Thus interpretation of polarity data recovered from Site U1399 is restricted to sediment recovered with the APC.

Between 0 and 207.5 mbsf in Hole U1399A and throughout Hole U1399B, SRM and discrete inclination data show scattered but positive values, clustering around the expected GAD inclination (Figs. F10, F11). Declination shows some scatter, but coupled with the positive inclination data this suggests all nonchaotic sediment was deposited under normal polarity conditions. Biostratigraphic ages from planktonic foraminifers and nannofossils suggests the base of both Holes U1399A and U1399B are no older than 400 ka (see “Paleontology and biostratigraphy”). Record continuity is very poor, but paleomagnetic data show no evidence of reversed polarity within the nonchaotic hemipelagic intervals. This conclusion can be used to support the biostratigraphic datums, suggesting that all sediment at Site U1399 was deposited within the Brunhes Chron and younger than 780 ka (Cande and Kent, 1995).