Previous   |    Next

doi:10.2204/iodp.proc.333.103.2012

Paleomagnetism

Magnetic susceptibility and remanent magnetization measurements were performed on 313 discrete samples from Site C0018. Samples for shipboard measurements were taken routinely at a frequency of two specimens per standard section (~140 cm), whereas we flexibly omitted one or two from shorter sections. The samples were subjected to alternating-field (AF) demagnetization to isolate the characteristic remanent magnetization (ChRM). The samples were typically demagnetized at 5, 10, 20, 30, and 40 mT.

Pervasive drilling-induced magnetization is commonly encountered, as noted during previous Deep Sea Drilling Project/Ocean Drilling Program/IODP legs (e.g., Gee et al., 1989; Zhao et al., 1994). This magnetization is characterized by a direction strongly biased downward. As shown in Figure F25, upon AF demagnetization to 30 mT, a significant decrease in intensity and an inclination shift toward negative or positive shallower directions. Figure F26A and F26B shows that AF demagnetization removes drilling-induced magnetizations and isolates ChRM directions successfully.

Throughout this site, sediments have low intensity of natural remanent magnetization (NRM) overall. Average NRM intensity (1.25 × 10^–2 A/m) and 30 mT and 40 mT demagnetized intensities (2.39 × 10^–3 and 1.76 × 10^–3 A/m) at Site C0018 are comparable with those at IODP Site C0004 and roughly one order smaller than those at Site C0008 (cf. Site C0004 NRM_Ave = 1.84 × 10^–2 A/m, Site C0004 40 mT_Ave = 1.15 × 10^–3 A/m, Site C0008 NRM_Ave = 1.30 × 10^–1 A/m, and Site C0008 40 mT_Ave = 5.58 × 10^–2 A/m). Data at 40 mT demagnetization often show incoherent behavior (Fig. F26C), likely resulting from low intensity after demagnetization that is beyond the lower sensitivity limit of the used spinner magnetometer (3 × 10^–1 to 5 × 10^–6 mAm²). We therefore employed the 30 mT demagnetization step for interpretation to minimize the effects of both drilling-induced remagnetization and sensor response constraint.

A histogram of the inclinations isolated from discrete samples excluding MTD 6 horizons is shown in Figure F27. The inclinations are concentrated in a range of +45° to +60° and –35° to –50°, which correspond to an expected inclination from the geocentric axial dipole model at the latitude of this site.

Different coring systems present a different characteristic distribution in paleomagnetic declination. Declination in cores recovered using HPCS show a clustered distribution within each core, whereas those using EPCS and ESCS demonstrate scattered distribution (Fig. F25). Cores recovered using EPCS and ESCS were heavily disturbed, occurring as pieces (biscuits) interspaced with drilling slurry (see “Structural geology”). The possibility of rotating cores, as well as pieces, may be the cause of the scattered data distribution.

For intervals of particular interest for structural geology at Site C0018, we used ChRM isolated from progressive demagnetization of the discrete samples to restore azimuthal orientation. For HPCS cores, average declination can be used for most cores because no systematic twisting of core is observed (Fig. F28). For EPCS and ESCS cores, only structures from the same coherent piece as the ChRM data can be reoriented.

Natural remanent magnetization and magnetic susceptibility

Paleomagnetic data obtained in Hole C0018A present some variations in demagnetization behavior with depth. NRM intensities of the discrete samples from Hole C0018A span more than two orders of magnitude (Fig. F25). NRM intensity from the top of the hole to Sample 333-C0018A-2H-3, 90 cm (8.913 mbsf), is relatively high (averaging 7.99 × 10^–2 A/m), whereas intensity decreases with depth by > to Core 333-C0018A-8H. In the lower part of Subunit IA, from Cores 333-C0018A-9H to 13H, NRM intensity tends to be stable at slightly higher values than the upper part, 4.63 × 10^–2 and 1.68 × 10^–2 A/m on average, respectively. In Cores 333-C0018A-13H and 14H, a significant drop in intensity associated with a transitional period of paleomagnetic polarity reversal is clearly pronounced. NRM intensity of 1.50 × 10^–2 A/m at Sample 333-C0018A-13H-13, 20.5 cm (116.543 mbsf), fades rapidly to 1.58 × 10^–3 A/m at Sample 333-C0018A-14H-7, 93 cm (125.017 mbsf), and recovers to 7.58 × 10^–3 A/m at 53.5 cm below (Sample 333-C0018A-14H-8, 5 cm). Variations in NRM intensity are enhanced after demagnetization, which implies the transient weakening during the geomagnetic pole shift. The magnetic susceptibility is nearly synchronous with the paleomagnetic intensity, suggesting that these changes are controlled by the nature and/or concentration of ferromagnetic minerals.

Magnetostratigraphy

As shown in Table T8 and Figure F29, a number of magnetic reversals are discerned on the basis of changes in sign of inclination. The horizon of the MTD 6 (see “Lithology” for details) is disregarded here because it shows dispersed declination and inclination even with HPCS coring, which implies that the initial orientation has not been maintained.

Normal polarity from the top of the hole to Core 333-C0018A-14H corresponds to the Brunhes Chron regarding its continuity from the seafloor. Reversal in Core 333-C0018A-14H transits across this core and coincides with a weakening of the intensity as mentioned above. Because MTD 6 starts at Section 333-C0018A-15H-2, 91 cm (127.545 mbsf), polarity is disturbed below this section. A tephra in Section 333-C0018A-14H-8 has been assigned a chronologic age of 0.85 Ma (Azuki volcanic ash bed; see “Lithology” and Table T8); this reversal event, therefore, corresponds to the Brunhes/Matuyama boundary (0.78 Ma). Immediately below the bottom of MTD 6 (Core 333-C0018A-23H), a short interval of normal polarity is recognized (Sections 333-C0018A-24T-1, 1 cm, to 24T-4, 72 cm [190.66–194.465 mbsf]); as the Pink volcanic ash bed is recognized (age = 1.05 Ma; see “Lithology”) just above this interval, this interval would correspond to the Jaramillo Subchron (0.99–1.07 Ma). Another normal polarity chron interval is found between Sections 333-C0018A-27T-2, 20 cm (219.66 mbsf), and 28T-2, 20 cm (230.26 mbsf), and may correspond with the Cobb Mountain Subchron (1.21–1.24 Ma). A few other normal polarities below Core 333-C0018A-29T are not interpreted because of insufficient data. Although it is only one data point, Sample 333-C0018A-8H-2, 14 cm (64.008 mbsf), shows negative polarity amid the Brunhes Chron. Assuming a constant sedimentation rate, this reversal fits the Emperor excursion at 0.42 Ma; however, we leave it pending until further postcruise research reveals additional details.

An age-depth profile for the sediments at Site C0018 was constructed based on the above determined magnetostratigraphy (Fig. F24) and the sedimentation rate was calculated, accordingly. Data sets used are the Brunhes/Matuyama boundary (0.78 Ma, 122.47 mbsf), the bottom of the Jaramillo (1.07 Ma, 193.20 mbsf), and the bottom of the Cobb Mountain (1.24 Ma, 229.58 mbsf) (Table T8). Sedimentation rates above and below MTD 6 are 15.7 and 21.4 cm/k.y., respectively. These values are relatively high compared to the sedimentation rate in the Pleistocene part of Site C0004 (2.04 cm/k.y.). A very high apparent sedimentation rate of 37.08 cm/k.y. was recognized in Hole C0008A, where possible mass transport is suggested (Expedition 316 Scientists, 2009b). The depositional environment at Site C0018 is in accordance with such a high sedimentation rate.

Top of page   |    Previous   |    Next