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doi:10.2204/iodp.proc.333.104.2012

Paleomagnetism

Magnetic susceptibility and remanent magnetization of 373 specimens were measured at Site C0011 during Expedition 333. Two specimens were collected for each ordinary section (~140 cm long), and one specimen was collected from each short section. Characteristic remanent magnetization (ChRM) of the specimens was analyzed using stepwise alternating-field (AF) demagnetization in the Zijderveld diagrams (see “Paleomagnetism” in the “Methods” chapter [Expedition 333 Scientists, 2012]). Only one sample (333-C0011D-40X-3, 37–39 cm) underwent the stepwise thermal demagnetization process (see “Paleomagnetism” in the “Methods” chapter [Expedition 333 Scientists, 2012]).

Drilling-induced magnetization occurs commonly during drilling operations as shown by previous Deep Sea Drilling Project/ODP/IODP expeditions. Such an induced magnetization is characterized by a high angle of natural remanent magnetization (NRM) inclination that is overwritten by strong bias of vertical (toward +90°) remagnetization. As shown in Figure F26, NRM inclination is steeper than remanent magnetization inclination after 30 mT AF demagnetization. This indicates that NRM is overwritten by drilling-induced magnetization. Based on stepwise AF demagnetization, such drilling-induced magnetizations are mostly removed by 20–30 mT AF demagnetization (Fig. F27). Therefore, we use the remanent magnetization data after 30 mT AF demagnetization as the ChRM for this site. ChRM was also characterized by the thermal demagnetization curve from 300° to 600°C thermal demagnetization at 306.69 mbsf (interval 333-C0011D-40X-3, 37–39 cm).

Characteristics of declination and inclination in HPCS cores are different from those in EPCS and ESCS samples. Declinations are random in EPCS and ESCS samples from Cores 333-C0011D-24T to 52X (Fig. F26), whereas declinations of remanent magnetization after 30 mT AF demagnetization are arranged in an array in each HPCS core from Cores 333-C0011C-1H to 333-C0011D-22H. In HPCS cores, the declination array is rotated (clockwise or counterclockwise) and indicates that the core has been twisted during drilling operation (e.g., Core 333-C0011D-12H) (Fig. F26), and the inclination of remanent magnetization after 30 mT AF demagnetization steepens downward in each core. This may indicate coring disturbances (e.g., stretch of core sediments).

The magnetic susceptibility profile is similar to the NRM intensity profile. Magnetic susceptibility is likely controlled by lithology of the sediments, which implies that the NRM intensity profile may also depend on lithology.

The inclination of remanent magnetization after 30 mT AF demagnetization clearly shows normal and reversed chrons and subchrons as shown in Figure F28 and Table T8. Because we did not measure the samples successively throughout the cores, we were not able to detect certain chron and subchron boundaries. Our data indicate that polarity seems to change gradually as a transitional zone. Therefore, the transitional zone depth was calculated using an average of the top and base boundary depths.

The Brunhes Chron is identified from a sample in Section 333-C0011C-1H-1 as the first normal chron at Site C0011 (Fig. F28; Table T8). The normal chron changes to a reversed predominant interval, the Matuyama Chron, at 17.36 mbsf. This boundary corresponds to the Brunhes/Matuyama Chron boundary and indicates an age of 0.781 Ma (Lourens et al., 2004). The Gauss Chron, a predominantly normal interval, is identified at 75.72 mbsf. The age of the Matuyama/Gauss Chron boundary is 2.581 Ma (Lourens et al., 2004). The Gauss Chron transitions to a predominantly reversed interval, the Gilbert Chron, at 126.72 mbsf with an age of 3.596 Ma (Lourens et al., 2004). The base of the Gilbert Chron is located at 282.798 mbsf with an estimated age of 6.033 Ma. Magnetostratigraphy below the Gilbert Chron is not firm because frequent magnetic reversal, especially before 7.14 Ma, is beyond our sampling resolution. We leave our tentative assignment of 7.14 Ma at 333.19 mbsf as the bottom of age assignment for Hole C0011D. Although the age for the interval overlapped with Hole C0011B (340–379.93 mbsf) that was drilled during Expedition 322 is not assigned, the age-depth profile shows good continuity to the age of 7.642 Ma at 341.44 mbsf from Expedition 322 data (Fig. F29). Our paleomagnetostratigraphy is consistent with the onland tephrochronologic correlations (see “Lithology” and Table T8).

Based on our results, the average sedimentation rate throughout these holes is ~4.63 cm/k.y. The rate changes rapidly at ~90 mbsf (~3 Ma). Above ~90 mbsf the sedimentation rate is low (~2.86 cm/k.y.), whereas the interval between 90 and 208 mbsf is characterized by a higher sedimentation rate (~8.65 cm/k.y.) (Fig. F29). Projected ages of lithologic subunit/unit boundaries are 5.32 Ma for the Subunit IA/IB boundary and 7.67 Ma for the Unit I/II boundary.

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