We measured the natural remanent magnetization (NRM) of most archive halves from Holes 1141A and 1142A 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) at 20 and 50 mT, respectively. We did not analyze highly disturbed sections of sediments. Discrete sediment samples were stepwise AF demagnetized up to 30 mT; however, no discrete basement samples were analyzed by stepwise demagnetization because of limited time for measurement.
Sediment was recovered only in Hole 1141A. With Cores 183-1141A-8R and 9R, we obtained reliable remanent magnetizations and correlated normal and reversed polarities with middle Miocene Chrons C5 through C5AD. However, the upper part of the sediment section from Hole 1141A provides only a limited paleomagnetic record as a result of weak magnetizations and highly disturbed cores. We obtained reliable paleomagnetic directions from basement rocks from both Holes 1141A and 1142A that showed normal polarity. We observed no significant differences in average susceptibility and NRM intensity in the lava flows of Hole 1141A. In Hole 1142A, we observed the highest average susceptibility in basement Unit 6, the strongest NRM intensity in Unit 1, and the lowest average NRM intensity and susceptibility in basement Unit 4.
One archive section of most cores was demagnetized stepwise up to 30 mT. We took one or two discrete samples per section, and 12 samples were stepwise AF demagnetized up to 30 mT to confirm the reliability of whole-core measurements. We obtained reliable results from Cores 183-1141A-8R and 9R and correlated normal and reversed segments with biostratigraphic zones (see "Biostratigraphy"). Cores 183-1141A-8R and 9R generally have a stable magnetization, which was obtained after AF demagnetization at 10 mT. Most discrete samples from Cores 183-1141A-8R and 9R have a high median destructive field (MDF), and the remanent direction is stable in demagnetization steps between 10 and 30 mT (Fig. F63). Therefore, we used the remanent magnetization, after AF demagnetization at 20 mT, to correlate the paleomagnetic record with the geomagnetic chrons. Furthermore, we used the data selection criteria as described in "Paleomagnetism" in the "Explanatory Notes" chapter for magnetostratigraphic studies of Hole 1141A (Fig. F64). 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 > ±20°, (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. F64).
Correlation of biostratigraphic data and polarity reversals (Fig. F64) suggests that the reversed and normal chrons in Cores 183-1141A-8R and 9R are middle Miocene in age (see "Biostratigraphy"). We obtained short normal and reversed segments between 66 and 86 mbsf in the lower part of Unit I (see "Lithostratigraphy"). The upper part of the unit provides only a limited paleomagnetic record as a result of weak magnetizations and highly disturbed cores. However, we propose some correlations with paleontological data from the core catcher of each core (see "Biostratigraphy"). The normal and reversed segments between 66 and 86 mbsf probably lie within middle Miocene Chrons C5 through C5AD.
We observed negative susceptibilities (whole-core multisensor track [MST] measurements; see "Physical Properties") (Fig. F65) and weak NRM intensities in Unit I (nannofossil ooze; see "Lithostratigraphy") (Fig. F64). In the lower part of Unit I, we observed stronger NRM intensities and, thus, obtained a more reliable paleomagnetic record than in the upper part.
We determined the magnetic properties of each basement unit (see "Igneous Petrology" and "Physical Volcanology") and the variation of magnetic properties within each unit of Hole 1141A (Fig. F66). Three independent types of susceptibility measurements, MST, AMST (see "Paleomagnetism" in the "Explanatory Notes" chapter), and discrete samples, generally show consistent results. We observed no significant differences in average susceptibility and NRM intensity in the lava flows (basement Units 2 to 6; Figs. F65, F66). Average NRM intensities range from 4.40 (basement Unit 5) to 0.49 (basement Unit 2) A/m. Average susceptibilities range from 1.54 × 10-3 (basement Unit 3) to 1.04 × 10-3 (basement Unit 5) SI units.
We found negative inclinations, indicating a normal magnetic polarity, in all basement units after AF demagnetization at 40 mT. We observed scattered negative inclinations in Units 2 and 3. Scattered inclinations are probably related to alteration (see "Igneous Petrology" and "Physical Volcanology"). Basement Units 4, 5, and 6 have more reliable (less scattered) negative inclinations. Within basement Unit 4, we observed lower NRM intensities and susceptibilities in the bottom part than the upper part. We found higher NRM intensities and susceptibilities at the upper part of basement Unit 5 than in the lower part of the unit. In the lower part of basement Unit 6, we observed a reversed overprint, which was probably not caused by the recent (Brunhes normal epoch) magnetic field. The reversed magnetic component may be secondary, acquired during alteration or as a result of drilling effects.
Stepwise AF demagnetization up to 50 mT was applied to all the basement archive-half sections to analyze their behavior during demagnetization. For analysis, we selected data points in rock pieces that are longer than the effective sensitivity of the pass-through magnetometer (~15 cm). We found two different types of magnetic behavior. The upper part of basement Unit 6 has a stable single-component magnetization (Fig. F67A) and a high MDF. The magnetization of the lower part of basement Unit 6 is characterized by a two-component magnetization (Fig. F67B) and a low MDF. The soft reversed component is probably related to alteration or drilling effects, as mentioned above.
We observed the magnetic properties of each basement unit (see "Igneous Petrology" and "Physical Volcanology") and the variation of magnetic properties within each unit of Hole 1142A (Fig. F68). Average NRM intensities range from 2.65 (basement Unit 1) to 0.16 (basement Unit 4) A/m. Average susceptibilities range from 6.25 × 10-3 (basement Unit 6) to 8.84 × 10-5 (basement Unit 4) SI units (Fig. F65). We observed the highest average susceptibility in basement Unit 6, the strongest NRM intensity in basement Unit 1, and the lowest average NRM intensity and susceptibility in basement Unit 4 (granule-bearing clay; see "Physical Volcanology").
We found negative inclinations, indicating a normal magnetic polarity, in all units after AF demagnetization at 40 mT. We observed scattered negative inclinations in basement Units 2 and 3. Scattered inclinations are probably related to alteration (see "Igneous Petrology" and "Physical Volcanology"). Basement Units 4, 5, and 6 have more reliable (less scattered) negative inclinations. Within basement Unit 6, we observed lower NRM intensities in the bottom part than in the upper part (Fig. F68). In the upper part of basement Unit 6, we observed a reversed overprint, which was probably not caused by the recent (Brunhes normal epoch) magnetic field. As in basement Unit 6 of Hole 1141A, the reversed magnetic component may be secondary, acquired during basement alteration or as a result of drilling effects.
Stepwise AF demagnetization up to 50 mT was applied to all basement archive-half sections of Hole 1142A to analyze behavior during demagnetization. For analysis, we selected data points in rock pieces that are longer than the effective sensitivity of the pass-through magnetometer (~15 cm). The lower part of basement Unit 6 has a stable single-component magnetization (Fig. F69A) and a low MDF. The magnetic overprint was removed by AF demagnetization at 20 mT. Piece 7 in Section 183-1142A-5R-2 (Unit 3) has a very high MDF (Fig. F69B), and the intensity of the remanent magnetization remaining after AF demagnetization at 50 mT is >95% of the NRM intensity.