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doi:10.2204/iodp.proc.346.105.2015

Paleomagnetism

Paleomagnetic samples and measurements

Paleomagnetic investigations at Site U1424 included the measurement of magnetic susceptibility of whole-core and archive-half split-core sections and of natural remanent magnetization (NRM) of archive-half sections. NRM was measured before and after alternating field (AF) demagnetization with a 20 mT peak field for all core sections from Hole U1424A. Because of increased core flow and limited measurement time, NRM of core sections from Holes U1424B and U1424C were measured only after 20 mT AF demagnetization at every 5 cm interval. The FlexIT core orientation tool (see “Paleomagnetism” in the “Methods” chapter [Tada et al., 2015b]) was used to orient a total of 16 APC collected cores from Cores 346-U1424A-2H through 17H. The APC core orientation data for Hole U1424A are reported in Table T13.

We collected one paleomagnetic discrete cube sample (see “Paleomagnetism” in the “Methods” chapter [Tada et al., 2015b]) from the first section of each APC core in Hole U1424A and occasionally from deep sections when the first section was not suitable for collecting a discrete cube sample (Fig. F36A). Stepwise AF demagnetization on seven discrete samples from Hole U1424A was performed at successive peak fields of 0, 5, 10, 15, 20, 30, 40, 50, and 60 mT to verify the reliability of the split-core measurements and to determine the demagnetization behavior of the recovered sediment. Depth levels where the measured discrete samples were collected are marked as orange triangles along the inclination data column in Figure F36A. To avoid potential anhysteretic remanent magnetization (ARM) acquisition during the AF demagnetization, which was observed in discrete sample measurements from Sites U1422 and U1423, especially at high AF peak fields, we followed the protocol of Expedition 318 Scientists (2011) to demagnetize and measure the samples multiple times at each demagnetization level. The measurements acquired at each demagnetization step were then averaged to cancel out potential ARM acquisition during AF demagnetization (see “Paleomagnetism” in the “Methods” chapter [Tada et al., 2015b]).

We processed data extracted from the shipboard LIMS database by removing measurements collected from disturbed and void intervals and measurements that were made within 10 cm of the section ends, which are slightly biased by edge effects. For declination data from cores in Hole U1424A where FlexIT tool data are available, we corrected the declination values for each core using the estimated orientation angles. A modified version of the UPmag software (Xuan and Channell, 2009) was used to analyze the NRM data of both the split-core section and the discrete cube samples. The disturbed and void intervals used in this process are reported in Table T14. The processed NRM inclination, declination, and intensity data after 20 mT AF demagnetization are reported in Table T15 and shown in Figure F36.

Natural remanent magnetization and magnetic susceptibility

Compared to Sites U1422 and U1423, NRM intensity of cores from Site U1424 is generally higher and relatively stable. NRM intensity after 20 mT AF demagnetization in all three holes is similar in magnitude for overlapping intervals, mostly ranging from ~10^–4 to 10^–2 A/m. For sediment recovered from the uppermost ~25 m of the holes, NRM intensity after 20 mT demagnetization is on the order of 10^–2 A/m. NRM intensity then decreases downcore to the order of 10^–4 to 10³ A/m from ~25 m CSF-A until the bottom of the holes.

The AF demagnetization behavior of the seven measured discrete samples is illustrated in Figure F37. Declination and inclination values acquired from the discrete sample measurement generally agree well with the split-core measurements after 20 mT AF demagnetization. All samples exhibit a steep, normal overprint that was generally removed after AF demagnetization at peak fields of ~10–15 mT, demonstrating that the 20 mT AF demagnetization is, in general, sufficient to eliminate the overprint. Discrete sample measurements acquired using the Expedition 318 Scientists (2011) protocol show generally reduced ARM acquisition. However, some measured samples still appear to acquire remanence during AF demagnetization (e.g., Fig. F37D, F37G). This is possibly related to changes in the ambient field on the ship during the measurements and the fact that some of the samples are too weak to be measured accurately using the shipboard superconducting rock magnetometer.

Magnetic susceptibility measurements were taken on whole cores from all three holes as part of the Whole-Round Multisensor Logger (WRMSL) analysis and on archive-half sections using the Section Half Multisensor Logger (SHMSL) (see “Physical properties”). The WRMSL acquired susceptibility was stored in the database in raw meter units. These were multiplied by a factor of 0.68 × 10^–5 to convert to the dimensionless volume SI unit (Blum, 1997). A factor of (67/80) × 10^–5 was multiplied by the SHMSL acquired susceptibility stored in the database. Magnetic susceptibility varies between 10 × 10^–5 and 50 × 10^–5 SI for sediment from the uppermost ~25 m of the holes and is generally <10 × 10^–5 SI for sediment from deeper than ~25 m CSF-A (Fig. F36, fourth panel). Magnetic susceptibility measurement is consistent between the two instruments and, in general, mimics NRM intensity, suggesting that the magnetic minerals that carry NRM are the same as, or at least coexist with, those that dominate magnetic susceptibility.

Magnetostratigraphy

Paleomagnetic inclination and declination data of Site U1424 appear to show patterns that allow for the determination of magnetic polarity for all sediment sequences recovered in all holes. Both paleomagnetic declination and inclination after 20 mT AF demagnetization were used when possible for the magnetostratigraphic interpretation at this site. The geomagnetic field at the latitude of Site U1424 (40.19°N) has an expected inclination of ~59.38°, assuming a geocentric axial dipole field model, which is sufficiently steep to determine magnetic polarity in APC cores that lack horizontal orientation. We identified almost all major reversals during the Pliocene and Pleistocene at Site U1424 (Table T16).

Hole U1424A clearly recorded the Brunhes/Matuyama boundary (0.781 Ma) at ~24.8 m CSF-A, the Olduvai Subchron (C2n, 1.778–1.945 Ma) between ~54.3 and 58.8 m CSF-A, the Matuyama/Gauss boundary (2.581 Ma) at ~75.7 m CSF-A, and the Gauss/Gilbert boundary (3.596 Ma) at ~109.85 m CSF-A. In Hole U1424B, the Brunhes/Matuyama boundary (0.781 Ma) was identified at ~24.5 m CSF-A, the Olduvai Subchron (C2n, 1.778–1.945 Ma) was identified between ~54.45 and 58.65 m CSF-A, and the Matuyama/Gauss (2.581 Ma) and the Gauss/Gilbert (3.596 Ma) boundaries were identified at ~74.7 and ~107.7 m CSF-A, respectively. Hole U1424B also recorded the Mammoth Subchron (3.207–3.330 Ma) between ~94.6 and 98.2 m CSF-A, the Nunivak Subchron (4.493–4.631 Ma) between ~142.2 and 147.2 m CSF-A, and the top of the Sidufjall Subchron (4.799 Ma) at ~151.4 m CSF-A. The four APC cores (346-U1424C-4H through 7H) measured for Hole U1424C clearly recorded the Olduvai Subchron (C2n, 1.778–1.945 Ma) between ~54.45 and 59.7 m CSF-A. The polarity boundaries mentioned above are all associated with clear switches in inclination data as well as ~180° shifts in raw and oriented declination values and are mostly not from the ends of cores that are more easily affected by drilling. Our interpretations agree well with the biostratigraphic events identified in Hole U1424A (see “Biostratigraphy”) and are consistent with the limited discrete sample measurements that show good demagnetization behavior. For instance, the discrete sample from 8.09 m CSF-A in Hole U1424A (Fig. F37A) clearly shows stable and positive inclination likely acquired during the Brunhes Chron. A discrete sample from 66.62 m CSF-A in Hole U1424A (Fig. F37C) appears to record negative inclination that is consistent with the interpreted reversed Chron C2r.2r (bottom of the Matuyama).

In addition to the polarity boundaries mentioned above, many short subchrons during the Pliocene and the Pleistocene also appear to be recorded at Site U1424. The positive inclination intervals between ~31.3 and 36.6 m CSF-A in Hole U1424A, ~30.4 and 35.5 m CSF-A in Hole U1424B, and ~31.6 and 37.2 m CSF-A in Hole U1424C are interpreted as the Jaramillo Subchron (0.988–1.072 Ma). All three holes appear to record very short (~1 m) positive inclination intervals right below the Jaramillo and Olduvai Subchrons. These intervals could be interpreted as the Cobb Mountain (1.173–1.185 Ma) and Reunion Subchrons (2.128–2.148 Ma) (see Table T16 for detailed depth levels). However, higher resolution postcruise paleomagnetic investigations are needed to confirm this interpretation. Within the identified Gauss Chron, the Mammoth Subchron is recorded in Hole U1424B between ~94.6 and 98.2 m CSF-A and possibly between ~94.5 and 98 m CSF-A in Hole U1424A. The Kaena Subchron (3.032–3.116 Ma), however, is not very well recorded in both holes. The depth intervals with a slight tendency of more shallow and negative inclination values between ~89.95 and 92.5 m CSF-A in Hole U1424A and ~87.5 and 90.95 m CSF-A in Hole U1424B could be interpreted as the Kaena Subchron. These intervals appear to coincide with a low in NRM intensity in both holes, and the top and bottom of the intervals are associated with large changes in declination.

Near the bottom of Holes U1424A and U1424B, a few positive inclination intervals are recorded within the identified Gilbert Chron. The boundaries of these positive inclination intervals are better recognized in Hole U1424B and are accompanied by ~180° shifts in declinations. We interpret the ~142.2–147.2 m CSF-A interval in Hole U1424B as the Nunivak Subchron (4.493–4.631 Ma). In Hole U1424A, the Nunivak Subchron is recorded within ~141.6–145.6 m CSF-A. The bottom ~4 m interval in Hole U1424B recorded part of the Sidufjall Subchron (4.799–4.896 Ma), with the top of the Sidufjall Subchron occurring at ~151.4 m CSF-A. Hole U1424A appears to have recorded the Sidufjall Subchron between ~150 and 153.4 m CSF-A and the top of the Thvera Subchron (4.997 Ma) at ~156.5 m CSF-A. The Cochiti Subchron (4.187–4.3 Ma) is not very well recorded in either Holes U1424A or U1424B.

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