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doi:10.2204/iodp.proc.333.105.2012

Paleomagnetism

Magnetic susceptibility and remanent magnetization of 258 specimens were measured at Site C0012. Samples were subjected to stepwise alternating-field (AF) demagnetization in order to isolate the characteristic remanent magnetization (ChRM). The samples were demagnetized at 5, 10, 20, and 30 mT.

As noted in previous Deep Sea Drilling Project/Ocean Drilling Program/IODP expeditions, drilling-induced magnetization is often pervasive in cored sediments because of the drilling process (e.g., Gee et al., 1989; Zhao et al., 1994). The subsequent magnetization is characterized by natural remanent magnetization (NRM) that is strongly biased toward vertical (~90°) in most specimens. As shown in Figure F29, NRM inclination values are much steeper than inclination values after 30 mT AF demagnetization, indicating that the drilling-induced magnetization had been removed during the 20–30 mT steps.

Holes C0012C and C0012D

Inclination values at Site C0012 were scattered from –85° to +85° after 30 mT AF demagnetization, indicating that ChRM directions identified in the specimens may not be original because the inclination values are significantly steeper than expected. Structural data from the site indicating faults, folds, and steeply dipping beds emphasized the need for further investigation into the scatter of the data (see “Structural geology”). Bedding correction performed on the specimens (Fig. F30) showed the corrected inclination data to be more clustered near the expected inclination value (±52°) at this site. Although the inclination values vary with the correction, the chron and subchron boundaries are not altered by the bedding correction.

Magnetostratigraphy of Holes C0012C and C0012D

Inclination after 30 mT AF demagnetization illustrates the boundaries between normal and reversed chrons and subchrons, but because of the sampling interval, not every boundary is well defined. As shown in Figure F29 and Table T6, a number of magnetic reversals were determined based on a change in sign of the inclination.

The zone of normal polarity recognized from the top of the hole to Core 333-C0012C-2H was identified as the Brunhes Chron. The Brunhes/Matuyama Chron boundary is located at ~5 mbsf, roughly 10 m higher than at Site C0011. The provisional correlation of tephra chronology (Table T6) indicates a possible equivalent of the Azuki volcanic ash bed (0.85 Ma) in Section 333-C0012C-2H-1 (~5.8 mbsf) and a probable match to the Pink volcanic ash bed (~1.05 Ma) in Section 333-C0012C-2H-3 (~7.0 mbsf), giving an approximation for the Jaramillo Subchron (0.99–1.07 Ma) (see “Lithology”). However, the Jaramillo Subchron was identifiable with uncertainty from paleomagnetic polarity data, probably due to slow sedimentation during this interval. The estimated sedimentation rate from the two tephra layers is very low (~0.73 cm/k.y.). Therefore, the duration of the Jaramillo Subchron corresponds to only 58 cm, which is too short to identify by our shipboard measurement strategy, although we tentatively assigned 2.59 m for the Jaramillo. The provisional correlation of tephra chronology also indicates a probable match to the Ohta volcanic ash bed (~4.0 Ma) in Section 333-C0012C-6H-2, which corresponds to the lower end of the Gilbert Chron. The proximity of the Jaramillo Subchron (~12 mbsf) to the Gilbert Chron (~18 mbsf) indicates that the lower portion of the Matuyama Chron is missing. A period of normal polarity above the Gilbert Chron has been identified as the Gauss Chron. It is unclear at what age this normal polarity zone begins because of a potential hiatus. This hiatus is also suggested by changes in lithology, bedding dip, porosity, calcium carbonate content, and total sulfur data (see “Lithology,” “Structural geology,” “Physical properties,” and “Organic geochemistry”) and represents a gap of roughly 2.5 m.y. (1.07 to 3.6 Ma). This possible hiatus corresponds to a chaotic interval from interval 333-C0012C-2H-5, 53 cm (10.33 mbsf), to 2H-CC, 26 cm (14.00 mbsf) (see “Structural geology”). The nannofossil Reticulofenestra asanoi, observed in Section 333-C0012C-2H-CC, indicates an age of 0.436–0.90 Ma with good coincidence with the Jaramillo Subchron (See “Biostratigraphy”). Cores below 333-C0012C-3H show steeply inclined bedding, which is classified as another structural zone (see “Structural geology”). Therefore, we assume that the successive normal chron of the bottom of Cores 333-C0012C-2H and 3H corresponds to the Jaramillo Subchron and the Gauss Chron, respectively (Fig. F29). Another chaotic interval from 333-C0012C-10H-3, 0 cm (81.63 mbsf), to 10H-9, 121.5 cm (86.11 mbsf), is also a cause of a possible hiatus (see “Structural geology”). Adopting this possible hiatus, we can explain the fewer occurrences of normal polarity during the Gilbert Chron (Fig. F29).

Comparison of the magnetic susceptibility at Sites C0011 and C0012 as well as correlation of chrons and subchrons was performed in order to identify the missing section of the Matuyama Chron (Fig. F31). Magnetic susceptibility decreases during the upper Gilbert Chron from 3.60 to 4.19 Ma at both Sites C0011 and C0012 (Fig. F31). A gentle curve begins around 5.00 Ma and continues to 7.14 Ma, at which point abrupt variations in magnetic susceptibility begin to occur. This coincidence strengthens our interpretation below the hiatus. Consequently, the missing horizon at the hiatus spans most of the lower Matuyama Chron, starting just below the Jaramillo Subchron and continuing into the upper Gauss Chron. The age gap spans >2 m.y.

A plot of age versus depth at Site C0012 indicates a hiatus in sedimentation between ~1.0 and 3.5 Ma at a zone between 10.33 and 14.00 mbsf (Fig. F25). The apparent sedimentation rate shows considerable variation throughout Holes C0012C and C0012D (Fig. F25A). The higher sedimentation rate from 3.5 to 5.0 Ma correlates to an interval of steeply dipping beds (Cores 333-C0012C-3H to 10H, structural Zone II, see “Structural geology”). Assuming an average dip angle of 60°, the true sediment thickness in this interval is reduced by half. The corrected sedimentation rate (Fig. F25B) still shows a higher rate in this interval, but the variation is less drastic.

The most recent sedimentation rate is notably slow (0.73 cm/k.y.) for an input site near a trench, which may be a reflection of the topographic height of Site C0012. The sedimentation rate appears to slow between 5.0 and 7.5 Ma, after which the sedimentation rate increases once again. Magnetostratigraphic age correlates well with age Model A based on magnetostratigraphy from Expedition 322 (Fig. F25A). Projected age for the Subunit IA/IB, IB/IC, and IC/Unit II boundaries are 4.4, 7.2, and 7.8 Ma, respectively.

Holes C0012E, C0012F, and C0012G

Shipboard paleomagnetic samples from deeper Holes C0012E, C0012F, and C0012G are composed of hemipelagic sediments and basalts. The sediments show normal polarity (Fig. F32); however, we had no additional data to constrain the age at the time of this expedition. Paleomagnetic characterization of basalts is distinctive from that of sediments, which are characterized by higher magnetic susceptibility and paleomagnetic intensity. Drilling-induced remanent magnetization is commonly removed after AF demagnetization at ~5 mT and shows a stable magnetization component decreasing toward the origin. Paleomagnetic inclination shows notably low values near zero (Fig. F32), which implies these basalts erupted at lower latitude south of this site. The limited time of onboard measurement leaves questions on the origin of these basalts.

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