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doi:10.2204/iodp.proc.340.106.2013

Paleomagnetism

Cores 340-U1396A-1H through 12H, 340-U1396B-2H, and 340-U1396C-1H through 15H were recovered using nonmagnetic core barrels with the APC. All other APC cores were recovered using standard steel barrels. The FlexIt orientation tool was used on all APC cores recovered with nonmagnetic barrels; thus, declination for 0–107.4 mbsf in Hole U1396A, 5–15 mbsf in Hole U1396B, and 0–139.5 mbsf in Hole U1396C can be corrected to true north. Where FlexIt tool data were not available in Hole U1396A, declination was guided by the discrete inclination data (see “Paleomagnetism” in the “Methods” chapter [Expedition 340 Scientists, 2013]). Expected inclination for the site is 30.7° during normal polarity and –30.7° during reversed polarity, assuming a geocentric axial dipole (GAD). The archive halves of cores from Holes U1396A, U1396B, and U1396C were measured on the three-axis superconducting rock magnetometer (SRM) at 2.5 cm intervals (Table T6). Natural remanent magnetization (NRM) was measured before (NRM₀) and after stepwise alternating field demagnetization at 10 mT (NRM₁₀) and 20 mT (NRM₂₀). Core 340-U1396B-2H was substituted into the Hole U1396A record using magnetic susceptibility correlations (see “Physical properties”) because Core 340-U1396A-2H was highly disturbed after the core liner shattered. Eighteen 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, 2013]).

Cores are dominated by hemipelagic sediment but also contain many centimeter-scale fine-grained tephra layers from numerous eruptions of the Lesser Antillies volcanic arc (see “Lithostratigraphy”).

Results

NRM₀ (red) and NRM₂₀ (blue) intensities are shown for Holes U1396A and U1396B in Figure F11 and Hole U1396C in Figure F12. NRM₀ intensity is relatively high at ~0.5 m/A. Discrete tephra layers produce a strong magnetic susceptibility response (see “Physical properties”), and similarly high values of NRM₀ related to the concentration of ferrimagnetic minerals.

A strong magnetic overprint characterized by a steep SRM inclination and divergence of SRM and discrete inclination data is restricted to Cores 340-U1396A-13H through 15H. These cores were recovered using steel barrels. Agreement of SRM and discrete declination values from these cores suggests the radial overprint is less affected than the vertical overprint and declination can still be used for interpretation of polarity.

Magnetostratigraphy

Sedimentation rates for the site were predicted to be ~2–3 cm/k.y. from analysis of a 6 m gravity core (Le Friant et al., 2008). Extrapolation of these rates over the full 140 m record of Hole U1396C suggests the site could contain a 4–5 m.y. record of sedimentation. Inclination and declination for Holes U1396A, U1396B, and U1396C are shown in Figures F11 and F12. The Hole U1396A/U1396B record contains fifteen 180° shifts in declination in which declination stabilized before again reversing. The Hole U1396C record contains sixteen such reversal horizons. Changes in inclination are coeval with declination changes moving between positive and negative GAD values during these events. However, as only a 60° transition marks an inclination reversal, some of the shorter features that are clear in 180° declination shifts are not as obvious in inclination.

Using magnetic susceptibility to correlate between Holes U1396A, U1396B, and U1396C (see “Physical properties”), Holes U1396A and U1396B was transferred onto the more complete and longer depth scale of Hole U1396C to provide an aggregated reversal record (Fig. F13). This transfer also allowed more accurate splicing of the Hole U1396B record into the Hole U1396A record. This record is now referred to as Hole U1396A/B. On a common depth scale the two declination records agree very well and suggest that the geomagnetic field was accurately recorded in the sediment at Site U1396.

Using the geomagnetic polarity timescale (GPTS) of Cande and Kent (1995), eight periods of normal polarity and nine periods of reversed polarity were determined in Hole U1396A/B and nine normal polarity and nine reversed polarity episodes in Hole U1396C. The earliest polarity reversal in Hole U1396A is the beginning of Chron C3n.1n (4.29 Ma) at 129.4 mbsf, giving Hole U1396A a basal age of 4.29–4.48 Ma. The longer Hole U1396C record contains the end of Chron C3n.2n (4.48 Ma), giving the hole a basal age of 4.48–4.62 Ma. These ages are in excellent agreement with nannofossil and foraminiferal ages, which put the base of both holes within the early Pliocene (Fig. F14; see “Paleontology and biostratigraphy”).

Using the depths of the dated reversals, an age-depth relationship can be created and sedimentation rates calculated for the longer record from Hole U1396C (Fig. F14). A linear sedimentation rate calculated from the deepest reversal gives the core an average sedimentation rate of 3.1 cm/k.y. However, this gradient steepens with depth in all three stratigraphic proxies. Pliocene sedimentation rates are double (4 cm/k.y.) the Pleistocene rates (1.7 cm/k.y.), and rates within the earliest 1 m.y. of the record from the base of the core to the beginning of the Gauss Chron (3.58 Ma) are even higher ~5.3 cm/k.y.

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