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doi:10.2204/iodp.proc.340.105.2013

Paleomagnetism

Cores 340-U1395A-2H through 9H and 340-U1395B-2H through 10H were recovered with the APC using nonmagnetic core barrels. All other APC cores were recovered using standard steel barrels. The FlexIt orientation tool was used on all APC cores recovered with nonmagnetic barrels from Hole U1395A; thus, between 3.1 and 72.4 mbsf in U1395A, 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 30.6° during normal polarity and –30.6° during reversed polarity, assuming a geocentric axial dipole (GAD). The archive halves of cores from Holes U1395A and U1395B 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 (AF) demagnetization at 10 mT (NRM₁₀) and 20 mT (NRM₂₀). In cores that were visibly disturbed, only NRM₀ and NRM₂₀ were measured to maintain workflow through the laboratory. Twenty-nine 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 U1395 is heterogeneous in composition and varies between layers of hemipelagic sedimentation and deposits related to volcanic activity, particularly volcanic turbidites (see “Lithostratigraphy”). Using the detailed core description logs, we only interpret directional data measured on hemipelagic layers, as these appear the least disturbed and are the most likely to hold information about the behavior of the paleogeomagnetic field.

Results

NRM₀ (red) and NRM₂₀ (blue) intensities are shown for Holes U1395A and U1395B in Figures F10 and F11. NRM₀ intensity is relatively high, >1 A/m, and exhibits structure similar to magnetic susceptibility (see “Physical properties”), suggesting NRM₀ is related to the concentration of ferrimagnetic iron oxides present within volcaniclastic material. The NRM₀ intensity of sediment recovered with steel barrels is an order of magnitude higher than those recovered with nonmagnetic barrels, a trend not mirrored in the magnetic susceptibility profile (Fig. F12; see “Physical properties”). These cores all show steep positive inclination contradicting the corresponding discrete samples which record both positive and negative inclination GAD-like values (Figs. F10, F11). Similar to sediment in Site U1394, we interpret this as a strong magnetic overprint caused by remagnetization of the sediment within the standard steel barrels, which is not removed by AF demagnetization in a field of 20 mT (e.g., Fuller et al., 2006). With the use of nonmagnetic core barrels, these effects are mitigated, and there is excellent agreement in both inclination and declination between the SRM data and the discrete data (Figs. F10, F11). These data cluster and plot around the expected GAD inclination for the site. Declination data agree well with the use of both standard and nonmagnetic barrels, suggesting the radial overprint is not as severe as the vertical overprint and potentially more useful for interpretation of polarity in sediment recovered using steel barrels.

In Hole U1395A, changes in both inclination and declination indicate the depth of the Brunhes/Matuyama (B/M) polarity reversal boundary at ~66 mbsf. A turbidite masks the B/M inclination transition in Hole U1395B; the last negative inclination values are 71.9 mbsf, and the first recorded positive inclination values are at 67.7 mbsf. When recovered using steel barrels, discrete inclination points are used to guide the polarity of the SRM declination (see “Paleomagnetism” in the “Methods” chapter [Expedition 340 Scientists, 2013a]). These data show the transition from the Jaramillo (normal) Chron into the Matuyama (reversed) Chron at ~90 mbsf in Hole U1395A and 89.5 mbsf in Hole U1395B. A sequence of unconsolidated tephras and turbidites, severely faulted and altered sediment, and poor recovery below 100 mbsf (see “Core descriptions”) makes further interpretation of polarity difficult. The lower Jaramillo transition may be recorded by three discrete inclination points around 101 mbsf in Hole U1395A and a single discreet datapoint at 116 mbsf in Hole U1395B; however, further analysis is required for confirmation.

Using the magnetic susceptibility correlation between both holes (see “Physical properties”), we can transfer depth in Hole U1395B to depth in Hole U1395A and produce an aggregated inclination record for the site (Fig. F12). Excellent agreement between the nonmagnetic barrel SRM data and all discrete data gives further confidence in the paleomagnetic interpretations. Using the geomagnetic polarity timescale (GPTS) of Cande and Kent (1995), the age of the B/M boundary is defined at 0.78 Ma and the Upper Jaramillo at 0.99 Ma. Paleomagnetic data agree well with biostratigraphic ages for the site (see “Paleontology and biostratigraphy”; Fig. F13). Sedimentation rates for the site calculated from the paleomagnetic data are therefore ~8 cm/k.y. during the Brunhes Chron with slightly higher rates at the end of the Matuyama Chron of ~11 cm/k.y. A longer term average rate from the end of the Jaramillo gives average sedimentation of 9 cm/k.y. for the whole period.

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