We measured the natural remanent magnetization (NRM) of most archive halves from Hole 1139A with the pass-through cryogenic magnetometer using measurement intervals of 5 and 2.5 cm for sediment and basement rocks, respectively. Subsequently, sediment and basement core sections were demagnetized with peak alternating fields (AF) of 20 and 50 mT, respectively. Discrete sediment samples were stepwise AF demagnetized up to 30 mT. Some archive sections with high recovery and complete basement pieces were stepwise AF demagnetized up to 60 mT. Discrete basement samples were stepwise thermally demagnetized at temperatures of up to 620°C. We determined the anisotropy of magnetic susceptibility (AMS) of discrete samples of five basement units to obtain information about the magnetic fabric.
We obtained stable and reliable paleomagnetic directions from the sediments of Hole 1139A. Correlation of polarity reversals with biostratigraphic data suggests that the reversed and normal chrons are early Miocene to early Oligocene in age. We compared magnetic properties with lithologic and basement units. The trachybasalt-trachyandesite lava flows have stronger NRM intensities than the trachyte and rhyolite units. From the stable high-temperature component of the basement magnetization, we obtained reliable paleomagnetic directions and a reversed polarity. We found a difference of ~4° between the mean inclination of the basement units (64°) and the present inclination (-68°) of Site 1139, assuming a geocentric dipole field.
We measured the remanent magnetization of all archive halves from Hole 1139A except for highly disturbed sections. One archive section for each core was demagnetized stepwise up to 30 mT. We took one or two discrete samples per section, and 11 samples were stepwise AF demagnetized up to 30 mT to confirm the reliability of the whole-core measurements. We obtained reliable results in undisturbed cores and correlated normal and reversed segments with biostratigraphic zones (see "Biostratigraphy"). The sediments of Hole 1139A generally have a stable magnetization, which was obtained after AF demagnetization at 10 mT. Most discrete samples have a high median destructive field (MDF), and the remanent direction is stable in demagnetization steps between 10 and 30 mT (Fig. F83). We, therefore, used the remanent magnetization after AF demagnetization at 20 mT to correlate the paleomagnetic record with geomagnetic chrons. Furthermore, we used the data selection criteria as described in "Paleomagnetism" in the "Explanatory Notes" chapter for magnetostratigraphic studies of Hole 1139A (Fig. F84). The selection criteria were that (1) the intensity of remanent magnetization after AF demagnetization at 20 mT was >2 × 10-4 A/m, (2) the inclination was > ±30°, (3) at least two consecutive values (which corresponds to a 10-cm length of split core) had the same polarity, and (4) there was no significant core disturbance. Characteristic inclinations from discrete samples generally agree well with selected inclinations from whole-core measurements (Fig. F84). A reliable polarity sequence with high recovery and stable remanent magnetization was recorded in lithologic Unit II (see "Lithostratigraphy").
Correlation of biostratigraphic data and polarity reversals (Fig. F84) suggests that the reversed and normal chrons between 77 and 382 mbsf are early Miocene to early Oligocene in age (see "Biostratigraphy"). In the uppermost 70 m, no continuous paleomagnetic record could be provided because of the low recovery and high drilling disturbance. We propose the following correlations with paleontological data from the core catcher of each core (see "Biostratigraphy"). We correlate the normal and reversed segments between 77 and 180 mbsf with early Miocene Chrons C5Dr to C6Cn, based on the assignment of Sections 183-1139A-9R-CC through 18R-CC to nannofossil Zone CN2-CN1 (see "Biostratigraphy"). The underlying normal and reversed segments between 182 and 200 mbsf correspond to the latest Oligocene chrons between C6Cr and C7n (Sections 183-1139A-19R-CC through 21R-CC; nannofossil Zone R. bisecta). We correlate the sequences between 200 and 363 mbsf with late Oligocene Chrons C7 to C12n (Sections 183-1139A-22R-CC through 39R-CC; nannofossil Zone C. altus). Section 183-1139A-40R-CC lies within nannofossil Zones Blackites spinosus to R. oamaruensis, suggesting that the normal and reversed polarities at ~380 mbsf correlate with early Oligocene Chrons C12n and C12r or with C12n/C12r and C13n/C13r.
We obtained a reliable paleomagnetic record from most sediments of Hole 1139A. The sediments have strong NRM intensities (average = 1.07 × 10-1 A/m) and high susceptibilities (average = 3.03 × 10-4 SI units; whole-core multisensor track (MST) measurements, see "Physical Properties"). Lithologic Unit II (clay, claystones, ooze and chalk, see "Lithostratigraphy") has the strongest average intensity and the highest average susceptibility among the sediments (Fig. F85). The strong intensity and high susceptibility are probably caused by terrigenous materials. Lithologic Unit I (foraminifer-bearing, nannofossil-bearing, diatom-bearing ooze, see "Lithostratigraphy") has weaker intensities and lower susceptibilities than the other units; however, the NRM intensity (average = 5.14 × 10-3 A/m) is strong enough to measure the remanent magnetization with the shipboard magnetometer. The drilling disturbance of lithologic Unit I resulted in unreliable paleomagnetic data.
We determined the magnetic properties of each basement unit (see "Igneous Petrology" and "Physical Volcanology") and the variation of magnetic properties within each unit (Fig. F86). Three independent types of susceptibility measurements, MST, AMST, and discrete samples, generally show consistent results. We observed no significant differences in the average susceptibility and NRM intensity within or between the trachybasalt-trachyandesite lava flows (basement Units 6 through 17) (Figs. F85, F86). Average NRM intensities range from 2.4 (Unit 8) to 0.72 (Unit 15) A/m. Average susceptibilities range from 5.9 × 10-3 (basement Unit 12) to 1.3 × 10-3 SI units (basement Unit 15). Trachyte and rhyolite lava (basement Units 4, 5, 18, and 19; see "Igneous Petrology" and "Physical Volcanology") have lower average NRM intensities and susceptibilities than the trachybasalt-basaltic trachyandesite units (Figs. F85, F86). Volcaniclastic rocks (basement Units 1, 2, and 3; see "Physical Volcanology") also have lower NRM intensities and susceptibilities. We observed significantly higher susceptibilities and stronger NRM intensities in the uppermost 2 m than in the lower part of Unit 18 (Fig. F86).
From 30 basalt samples of basement Units 7, 8, 9, 11, and 17, we measured magnetic susceptibility in 15 different directions to determine the AMS and magnetic fabric. The degree of anisotropy (ratio between maximum and minimum axes) ranged from 1.036 (Unit 9) to 1.062 (Unit 17). In Units 8, 9, and 17, we found both positive and negative shape parameters; hence, no shape predominates and no grouping of any (maximum and/or minimum) axes was observed. In Units 7 and 11, we obtained positive shape parameters, which correspond to oblate shapes (Fig. F87), and observed a grouping of the minimum axes along the horizontal plane.
Six discrete samples from basement Units 4, 7, 8, 10, 11, and 18 were stepwise thermally demagnetized up to 620°C. We measured susceptibility of the samples after each heating step to detect changes of their magnetic minerals. Stepwise AF demagnetization up to 60 mT was applied to the archive half of Section 183-1139A-69R-2 from Unit 17. We chose a rock piece (interval 183-1139A-69R-2 [Piece 5, 44-109.5 cm]) that is longer than the effective sensitivity of the pass-through magnetometer (~15 cm) and analyzed its behavior during demagnetization at 72.5 cm in the section (Fig. F88). The magnetization has a MDF of 10 mT and a single-component magnetization, as shown by the straight lines in the orthogonal vector projection.
We found two-component directions during the stepwise thermal demagnetization of Sample 183-1139A-64R-2, 41-43 cm, from Unit 7 (Fig. F89A). The low-temperature component is scattered, and the high-temperature component has a stable direction toward the origin. Sample 183-1139A-65R-3, 22-24 cm, from Unit 10 shows only small scattering in both high- and low-temperature component directions (Fig. F89B). The thermal demagnetization of Sample 183-1139A-71R-3, 98-100 cm, from Unit 18 produced only a high-temperature single component (Fig. F89C). The high-temperature phase probably corresponds to magnetite or titanium-poor titanomagnetite, and the low-temperature phase to (titano) maghemite.
From the high-temperature component, we calculated the characteristic inclinations of the discrete samples using component analysis (Table T13). Inclinations are positive, indicating a reversed polarity, and range from 44° to 80°. The variation in inclinations among samples is probably caused by geomagnetic secular variation and/or limited precision in determining the primary direction. We calculated a mean inclination of 64° with a large error of >10°. The calculated present inclination of -68° for Site 1139, assuming a geocentric dipole field, differs by 4° and is, thus, within the error.