Measurements of natural remanent magnetization (NRM) were made every 5 cm on the archive halves of all sediment cores from Holes 1179A, 1179B, and 1179C using the shipboard pass-through cryogenic magnetometer. The measurements were of continuous cores from Holes 1179A through 1179C, of discrete sediment samples from Holes 1179B and 1179C, and of basalt samples from Hole 1179D. Alternating-field (AF) demagnetization of the continuous sediment cores was generally performed using steps of 20, 30, and 40 mT. Discrete sediment samples were AF demagnetized by steps of 5, 10, 15, 20, 25, 30, 35, 40, 50, 60, and 70 mT. Inclination, declination, and maximum angular deviation were determined from principal component analysis of Zijderveld plots of the discrete sample AF-demagnetization measurements. The majority of discrete basalt samples were thermally demagnetized after it was determined that AF demagnetization worked poorly on these samples.
NRM intensities (Fig. F30A) of the sediments cored in the upper section at Site 1179 ranged from strong (>10-1 A/m) to moderate (7 × 10-3 A/m). The NRM was dominated by a downward-directed overprint, likely imparted to the cores by their exposure to the drill string (Fig. F31). As is typical with 10- to 20-mT AF demagnetization, the overprint was removed and measurements at higher demagnetization steps gave consistent directions. Zijderveld plots from sediment samples often showed univectorial decay toward the origin for steps >10-20 mT (Fig. F32), implying that this procedure isolated the characteristic remanent magnetization (ChRM) direction. Some samples gave magnetic directions at higher AF-demagnetization steps that appeared to diverge from univectorial decay. These measurements are thought to be affected by an anhysteretic remanent magnetization imparted on the samples by the in-line demagnetization coils at AF field levels >40-60 mT.
Magnetic intensities display a systematic variation through the section, apparently related to lithology. The variation is best seen in intensity values after AF demagnetization (i.e., with the drill-string overprint removed). In lithostratigraphic Unit I (see "Sedimentology"), the magnetic intensities of the upper part, Cores 191-1179B-1H through 6H and 191-1179C-1H and 2H through 12H, are consistent at a level of 1 × 10-3 to 1 × 10-2 A/m (Fig. F30B), with occasional spikes and dips an order of magnitude higher or lower. Many spikes seem to correspond to significant ash layers. A slow decrease by an order of magnitude is seen in the lower part of this unit, from 4 × 10-3 A/m (Core 191-1179C-12H) to 1 × 10-4 A/m (Core 191-1179C-19H). A sharp increase of nearly two orders of magnitude occurs in Sections 191-1179C-19H-5 through 20H-2, just above the boundary between Units I and II. Intensity values in deeper APC/XCB cores are consistently high at about the magnitude of the upper part of Unit I, but a small dip occurs in Core 191-1179C-24H, below which magnetic intensity rises to >10-2 A/m through Core 191-1179C-26X.
Magnetic susceptibility values are low in Units I and II, typically <50 × 10-5 SI, except for higher spikes apparently caused by ash layers and perhaps metallic contamination at some core tops (Fig. F33). These lower values evidently reflect the fact that the sediments primarily consist of siliceous ooze and clay with little magnetic material. Values near the seafloor are higher, 25 × 10-5 to 50 × 10-5 SI (Cores 191-1179C-2H through 9H), and lower in Cores 191-1179C-9H through 19H (<25 × 10-5 SI), with an increase back to ~25 × 10-5 SI in Cores 191-1179C-19H through 20H. In Unit II, susceptibility values are steady near 25 × 10-5 SI (Cores 191-1179C-20H through 21H). In Unit III, susceptibility increases to ~140 × 10-5 SI from Cores 191-1179C-22H through 23H. Susceptibility values vary through the rest of Unit III, with lower values at the bottom of Core 191-1179C-23H, at the top of Core 24H, and in Core 24H. Values >400 × 10-5 SI were measured in Cores 191-1179C-25X through 26X. The higher values in Unit III are consistent with the high iron content of these sediments.
Peaks in magnetic susceptibility are used to correlate Cores 191-1179A-1H, 191-1179B-1H and 2H, and 191-1179C-1H (Fig. F34A) and Cores 191-1179B-6H and 191-1179C-2H (Fig. F34B). The tie-point depths in each hole are listed in Table T5.
The combined sediment cores of Site 1179 contain an excellent magnetic polarity reversal sequence (Fig. F35) from the early Miocene (Chron C5Dr) to the present (Chron C1n [Brunhes]). The single core from Hole 1179A records the present normal polarity Chron C1n. The six cores from Hole 1179B record five magnetic polarity reversals, including the Cobb Mountain cryptochron (C1r.2r-1n) (Table T6). The first core of Hole 1179C was taken at the sediment/water interface and is in Chron C1n (Brunhes). Cores 191-1179C-2H through 23H span Chrons C1r.2r (Matuyama) through C5Dr (Table T6). The remaining cores of Hole 1179C, Cores 191-1179C-24H, 25X, and 26X, are predominantly of normal polarity with no recognizable magnetic polarity sequence.
Measurements of discrete samples from Holes 1179B and 1179C show excellent agreement with the whole-core measurements. These samples confirmed that two distinct polarities could be isolated after removal of a large vertical overprint with AF demagnetization (Fig. F31).
Basalt samples give NRM intensities varying from 1.1 to 9.4 A/m, similar to other such rocks recovered elsewhere. AF-demagnetization experiments on pilot samples showed irregular behavior, making polarity determination difficult (Fig. F36). Zijderveld plots of thermal demagnetization experiments indicated that the downward overprint remained to temperatures in excess of 200°-300°C. Therefore, most samples were first treated to a low AF field (15 mT) to remove the drill-string overprint before thermal demagnetization to isolate the ChRM. Even with thermal demagnetization, a significant fraction of the samples displayed irregular demagnetization behavior. Isothermal remanent magnetization acquisition experiments showed rapid saturation, implying that some samples have soft magnetizations (i.e., a small applied field induces significant magnetization). Those samples that gave apparently reliable results show what may be two polarities. One group of samples gives low, negative inclinations, typically -5° to -20°, implying reversed polarity slightly north of the equator. Other samples give positive inclinations of about the same magnitude, perhaps indicating normal polarity. Additional measurements are needed to determine whether two polarities occur in the basement section or whether the combination positive and negative inclinations are due to some other cause.