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doi:10.2204/iodp.proc.340.110.2013

Paleomagnetism

Cores 340-U1400A-1H through 9H, 340-U1400B-1H through 12H, and 340-U1400C-2H through 10H were recovered using the APC and nonmagnetic core barrels. All other APC cores were recovered using standard steel core barrels. The FlexIt core orientation tool was used with Hole U1400C APC cores recovered with nonmagnetic barrels; thus, between 15 and 95 mbsf in Hole U1400C declination can be corrected to true north. In Holes U1400A and U1400B, declination was guided by the discrete inclination (see “Paleomagnetism” in the “Methods” chapter [Expedition 340 Scientists, 2013a]). Expected inclination for the site is 27.4° during normal polarity and –27.4° during reversed polarity, assuming a geocentric axial dipole (GAD) model. The declination of Site U1400 from true north is approximately –15°. The archive halves of cores from Holes U1400A, U1400B, and U1400C were measured on the three-axis superconducting rock magnetometer (SRM) at 2.5 cm intervals. Natural remanent magnetization (NRM) was measured before (NRM₀) and after stepwise alternating field demagnetization at 20 mT (NRM₂₀). Sixty 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]).

Sediment recovered from Site U1400 is dominated by hemipelagic sediment below a cap of coarse volcanic sand, 25–50 m thick. The hemipelagic sediment is deformed and physically inclined in both positive and negative directions varying between 20° and 80° (see “Lithostratigraphy”). This deformation has implications for interpretation of these data within the context of geomagnetic field behavior for Site U1400.

Results

NRM₀ (red) and NRM₂₀ (blue) intensities are shown for Holes U1400A, U1400B, and U1400C in Figures F12, F13, and F14. NRM₀ intensity is high at ~1 m/A, with little difference in intensity between the overlying sand cap and the hemipelagic sediment below. The greatest downcore variation in these measurements occurs when sediment recovery changes from using nonmagnetic to steel barrels. When standard steel barrels are used to recover cores, there is an increase in sediment intensity that is not removed by demagnetization in a field of 20 mT and a persistent offset occurs between the inclination of the discrete samples and the SRM data. This is not replicated in the magnetic susceptibility data (see “Physical properties”). Interpreted as a magnetic overprint by the drill string, this is a persistent feature of Expedition 340 cores recovered with steel barrels.

Hole U1400A consists almost exclusively of massive turbiditic and volcanic sand (see “Lithostratigraphy”) with few to no hemipelagic layers (Fig. F12); thus, paleomagnetic interpretations are restricted to Site U1400 to Holes U1400B and U1400C. Flexit tool–oriented declination data are available between 38.8 and 84.8 mbsf in Hole U1400C. Above this depth and to 191.1 mbsf, declination is arbitrarily rotated to a mean of 130° to match the Flexit tool–oriented declination, making for easier comparison. Rifling of the core barrel is evident by scattered declination values in all cores recovered with the XCB; thus, inclination is the only indicator of polarity below 191.1 mbsf in Hole U1400C. For Hole U1400B, declination has no true north reference and is rotated to a mean of zero and thus only be used to track relative direction within core variation (see “Paleomagnetism” in the “Methods” chapter [Expedition 340 Scientists, 2013a]).

Apart from two periods of negative inclination between ~28 and 38 mbsf in both holes, the zone of inclined bedding above ~385 mbsf generally shows positive inclination, as shown by both SRM and discrete data sets. SRM inclination data from nonmagnetic barrel–retrieved cores and inclination data from the majority of the discrete samples cluster, but show significant scatter, around the GAD inclination for the site. The recording of GAD-like values is surprising, considering the visible degree of deformation in the bedding. When FlexIt tool–oriented declination is available, values average ~130° during periods of positive inclination when the expected declination for the site using FlexIt-corrected data during normal polarity should be approximately –15°. Significant declination rotation also occurs within single APC cores with no associated inclination change, suggesting the magnetic directions are acting independently of each other at the site. This behavior is not characteristic of geomagnetic field behavior and is probably related to the contorted and deformed bedding at the site. That inclination values are similar to GAD-like inclination may be coincidental, but further interpretation is required to understand the processes controlling the magnetic directions at Site U1400.

Given the visible and inconsistent degree of deformation in the angle of bedding, the inclination and declination polarity disagreements in Hole U1400C, and the high degree of scatter in both the inclination and declination data sets of Holes U1400B and U1400C, we cannot interpret data from the deformed zone of Site U1400 to represent the behavior of the geomagnetic field. Although the data set does not likely maintain a geomagnetic signal, the data are of high quality and may contain information about the degree of deformation, especially if it can be determined that the sediment was initially deposited with reference to the geomagnetic field and then subsequently deformed in situ. The declination and inclination values we see in Holes U1400B and U1400C may indicate rotation associated with slumping and sediment deformation downslope but without overturning of the beds. This, however, is beyond the scope of these shipboard measurements, and further detailed investigations are required.

390 mbsf marks the lower contact of the deformed unit and hemipelagic sediments lie flatter (see “Lithostratigraphy”). Between 390 and 400 mbsf in Hole U1400C, both SRM and discrete samples show GAD-like negative inclination before returning to GAD-like positive inclination. Without coeval declination changes to confirm this transition, we cannot be certain that it represents geomagnetic field behavior. However, it would suggest that sediment in this normally bedded and less deformed unit was deposited during reversed polarity and is therefore at least 780 k.y. old using the geomagnetic polarity timescale (GPTS) of Cande and Kent (1995). This age would be consistent with biostratigraphic ages that suggest the age of the base of Hole U1400C sediment is Pliocene in age (see “Paleontology and biostratigraphy”).

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