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doi:10.2204/iodp.proc.342.207.2018

Results

ChRM directions were typically resolved over five consecutive treatment steps after AF demagnetization of 20–40 mT. The origin of orthogonal vector plots was included as an additional data point in the PCA analysis, and the resulting maximum angle of deviation (MAD) was <10° for most discrete samples and most intervals of the U-channel samples. Representative orthogonal vector plots of the AF demagnetization results are shown in Figures F2 (U-channel) and F3 (discrete). The resultant ChRM directions and associated parameters are shown in Tables T1 and T2 for the discrete samples. Although few samples or intervals were completely demagnetized by 80 mT, the straight-line decay toward the origin of the orthogonal vector plots gives us confidence that the data accurately distinguish the ChRM.

The ChRM directions resulted in absolute inclinations and relative declinations. The relative declination is provided as an angular departure of the ChRM from the double line of a core liner in the X-Y plane of the sample coordinate (see Fig. F6 in the “Methods” chapter [Norris et al., 2014b]). We observed that changes in the inclinations between negative and positive values are usually associated with ~180° azimuthal changes in the relative declinations for each APC core. This relationship implies that intervals with opposite declinations record opposite geomagnetic polarity. If we assume that the relative declinations mainly correspond to geographic north (south) when the inclinations are positive (negative), then we can convert them into “corrected” declinations by rotating the relative declinations for each core such that the mean declination of the normal and inverted reversed polarity intervals is zero.

The FlexIt orientation tool was deployed during the recovery of some APC cores. The magnetic tool face (MTF) orientation value gives the angle between geomagnetic north and the double line on the core liner for each core (see the “Methods” chapter [Norris et al., 2014b]). The sum of the MTF angle and the mean declination for each core equals 7.9° ± 38.3° (1σ, N = 87; Table T3; Fig. F4). On average, the summed angle is indistinguishable from the present declination of approximately –17° at the drill sites according to the twelfth generation of the International Geomagnetic Reference Field model (Thébault et al., 2015). We used the inclinations and the corrected declinations to calculate virtual geomagnetic pole (VGP) latitudes at a resolution of 1 cm along each APC core section. Note that because of the response function of the magnetometer, the data are inherently smoothed over a ~10 cm stratigraphic window. Downhole variations of these results are shown in Figures F5, F6, F7, and F8 for the four sites.

These figures show that APC intervals with negative inclinations are more clearly defined in shore-based results than they are in the shipboard results. We attribute this to several factors, including more thorough removal of a viscous drilling overprint, a magnetically quieter measurement environment, and a higher sensitivity magnetometer. These better-resolved negative inclinations allow us to locate chron boundary depths more precisely and with greater confidence. We used a threshold VGP latitude of 40° to define intervals of distinct polarity and determined the boundary depths based on the downhole variations of VGP latitude.

For intervals 210–252 m CCSF at Site U1408 and 154–294 m CCSF at Site U1410 (XCB intervals), the downhole variations of inclination are somewhat different between the results obtained from the discrete samples and the shipboard results (Figs. F6, F8). This difference is probably because the shipboard results are heavy disturbed during the drilling by XCB (such as biscuits). We determined the chron boundary depths based on switching in signs of inclinations derived from the discrete samples.

We updated the shipboard magnetostratigraphy (see the “Expedition 342 summary” chapter [Norris et al., 2014a]) for each of the four sites studied here. For our correlations we relied on the 2012 geomagnetic polarity timescale (GPTS; Ogg, 2014) and on biostratigraphic datums reported in the “Expedition 342 summary” chapter (Norris et al., 2014a). The magnetostratigraphy for each site is shown in the rightmost panel in Figures F5, F6, F7, and F8 and is summarized in Tables T4, T5, T6, and T7.

In most cases, the chron boundary depths determined in the present study are very similar to those determined during Expedition 342 (differences less than ~1 m). However, there are some boundaries with large differences: +3.79 m for C18r/C19n in Hole U1403B (Table T4), –2.14 m for C18n.2n/C18r in Hole U1408C (Table T5), +3.73 m for C19r/C20n in Hole U1408B (Table T5), and +4.77 m for C20r/C21n in Hole U1409C (Table T6).

In the present study, we also identified several chron boundaries that were not identified during Expedition 342: C18n.2n/C18r in Hole U1408B (Table T5); C17n.2r/C17n.3n, C17n.3n/C17r, and C17r/C18n.1n in Hole U1408C (Table T5); C18r/C19n and C19n/C19r in Hole U1409B (Table T6); C2n/C2r.1r and C20n/C20r in Hole U1410B (Table T7); and C18n.1r/C18n.2n in Hole U1410C (Table T7).

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