We measured the natural remanent magnetization (NRM) of most archive halves from Hole 1140A with the pass-through cryogenic magnetometer using measurement intervals of 5 and 2.5 cm for sediments and basement rocks, respectively. Subsequently, sediment and basement core sections were demagnetized with peak alternating fields (AF) of 20 and 50 mT, respectively. We did not analyze highly disturbed sections. Discrete sediment samples were stepwise AF demagnetized up to 30 mT. Six discrete basement samples were thermally demagnetized at temperatures of up to 620°C.
We obtained stable and reliable paleomagnetic directions from the sediments of Hole 1140A. Correlation of biostratigraphic data and polarity reversals suggests that the reversed and normal chrons are middle Miocene to early Oligocene or latest Eocene in age. We obtained reliable paleomagnetic directions from basement rocks and a magnetic reversal at the boundary between basement Units 1 and 2. We compared magnetic properties with lithologic and basement units.
One archive section of most cores was stepwise demagnetized 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 whole-core measurements. We obtained reliable results from undisturbed cores and correlated normal and reversed segments with biostratigraphic zones (see "Biostratigraphy"). The sediments of Hole 1140A 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. F39). 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 discussed in "Paleomagnetism" in the "Explanatory Notes" chapter for magnetostratigraphic studies of Hole 1140A (Fig. F40). 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. F40).
The correlation of biostratigraphic data and polarity reversals (Fig. F40) suggests that the reversed and normal chrons in Hole 1140A are middle Miocene to early Oligocene or latest Eocene in age (see "Biostratigraphy"). We obtained a relatively continuous paleomagnetic record from high-recovery core in the lower part of Unit I (160-250 mbsf; see "Lithostratigraphy"). The upper part of the unit provides only a limited paleomagnetic record as a result of low recovery and highly disturbed cores. However, we propose the following correlations with paleontological data from the core catcher of each core (see "Biostratigraphy"). We suggest that the normal and reversed segments between 0 and 143 mbsf lie within middle and early Miocene chrons. The normal and reversed segments between 143 and 230 mbsf correspond to chrons of late Oligocene to early Oligocene age (nannofossil Zone Ch. altus; see "Biostratigraphy"). The normal-reversed sequence at ~230 mbsf may correlate with Chrons C12n/r or C13n/r in the early Oligocene and late Eocene (Section 183-1140A-25R-CC; nannofossil Zones CP16a/b to CP15b; see "Biostratigraphy").
We observed high but scattered susceptibilities (whole-core multisensor track [MST] measurements; see "Physical Properties") and NRM intensities in the uppermost 10 mbsf (Subunit IA; diatom nannofossil ooze and silty diatom ooze; see "Lithostratigraphy") (Fig. F40). We observed lower susceptibilities and weaker NRM intensities in Subunit IB (nannofossil ooze and chalk) than in Subunit IA (Figs. F40, F41). In Subunit IB (227-245 mbsf), we observed higher magnetic susceptibilities in the lowest part.
We observed the magnetic properties of each basement unit (see "Igneous Petrology and Geochemistry" and "Physical Volcanology") and the variation of magnetic properties within each unit (Fig. F42). Three independent types of susceptibility measurements, MST, AMST, and discrete samples, generally show consistent results. We observed no significant differences in average susceptibility and NRM intensity within or between the lava flows (basement Units 1, 2, 3, 5, and 6; Figs. F41, F42). Average NRM intensities and range from 3.14 (Unit 1) to 8.91 (Unit 3) A/m. Average susceptibilities range from 0.747 × 10-3 (Unit 2) to 1.25 × 10-3 (Unit 5) SI units. Within the lava flows, we observed constant high susceptibility in three intervals (241-244 mbsf, Unit 1; 296-299 mbsf, Unit 5; and 315-317 mbsf, Unit 6). These intervals are also characterized by a large decrease of intensity after AF demagnetization (low MDF) (Fig. F42). Susceptibility was scattered in other parts of the flow units. Unit 4 (dolomite and dolomite-nannofossil-chalk; see "Lithostratigraphy") has low susceptibility values (see Figs. F41, F42).
We obtained reliable magnetic directions from whole-core measurements after AF demagnetization at 40 mT. We found negative inclinations, indicating a normal magnetic polarity, in basement Unit 1 and positive inclinations, indicating a reversed magnetic polarity, in basement Units 2, 3, 5, and 6. Unit 1 shows a reversed magnetic overprint; hence, it is probably not caused by the Holocene (Brunhes normal epoch) magnetic field. The reversed magnetic component may be secondary, acquired during alteration or drilling. Consistent with the magnetostratigraphy of the overlying sediments, this reversal is late Eocene in age. As a result of problems with the pass-through magnetometer, we have no remanent information for basement Unit 4. Because of the very strong remanent intensities of the igneous rocks within the same core section of Unit 4, the magnetometer was out of range.
Stepwise AF demagnetization up to 60 mT was applied to the archive-half of Section 183-1140A-37R-4 from Unit 6. We chose a rock (interval 183-1140A-37R-4 [Piece 1, 0-33 cm]) that is longer than the effective sensitivity of the pass-through magnetometer (~15 cm) and analyzed its behavior during demagnetization at 7.5 cm in the section (Fig. F43). The magnetization has a MDF of 20 mT and a two-component magnetization with a stable hard component, as shown by the straight lines in the orthogonal vector projection. Six discrete samples from basement Units 1, 3, 5, and 6 were stepwise thermally demagnetized up to 620°C. We measured the susceptibility of the samples after each heating step to detect changes in their magnetic minerals. We found two types of behavior during thermal demagnetization. Three samples (Samples 183-1140A-27R-2, 83-85 cm; 28R-3, 20-22 cm; and 36R-2, 128-130 cm) show a large decrease in intensity during the low-temperature demagnetization steps. The directions of the magnetization are strongly scattered, and we could not apply component analysis to determine characteristic inclinations (Fig. F44A). Susceptibility measurements show no changes in mineralogy with heating. Three other samples (Samples 183-1140A-32R-1, 94-96 cm; 34R-1, 139-141 cm; and 34R-5, 10-12 cm) display a two-component magnetization. The high-temperature phase is characterized by an unblocking temperature of ~560°C, and the low-temperature phase by an unblocking temperature of ~350°C (Fig. F44B). The high-temperature phase probably corresponds to titanium-poor titanomagnetite, and the low-temperature phase to (titano)maghemite. The magnetization is stable and we determined characteristic inclination values of 52°, 51°, and 63° (see Table T9).