Tensor tool orientations were successfully used to correct the magnetic declinations from 16.95 to 170.60 mbsf (Cores 190-1176A-3H through 19H). After measuring the natural remanent magnetization, all sections of the archive half of the core were partially demagnetized using AF magnetization at 30 mT at 5-cm intervals to remove magnetic overprints. Inclination data obtained after AF demagnetization provide useful information for interpreting late Pliocene to Pleistocene geomagnetic polarity reversals. Magnetic polarity reversals at Hole 1176A were correlated with the Hole 1175A polarity pattern using inclination measurements after AF demagnetization at 30 mT. However, identification of geomagnetic polarity intervals based on inclination changes in the lower part of Hole 1176A was difficult because of poor core recovery.
The declinations of cores from 16.95 to 170.60 mbsf (Cores 190-1176A-3H through 19H) were corrected using Tensor tool orientation measurements. These declination changes reflect the secular variation of the geomagnetic field, although some of these may possibly be short geomagnetic excursions (Fig. F13). Two anomalous declination directions of 180° at 121.45-139.85 mbsf (top of Core 190-1176A-14H through bottom of Core 15H) and 140.45-159.65 mbsf (top of Core 190-1176A-16H through bottom of Core 17H) are also observed. These declination anomalies may be caused by core rotation, but it is unknown why the corrected declinations show opposite directions from geomagnetic north. Scattered declinations below 170.60 mbsf indicate that several pieces of core rotated individually during XCB coring.
Positive inclinations were observed from 0 to 199.55 mbsf (Fig. F13) after AF demagnetization at 30 mT. Seven short intervals of reversed polarity inclinations were observed in this positive inclination zone. These negative inclinations probably reflect geomagnetic excursions commonly observed in this normal chron. These events appear to correspond to the anomalous declination directions (Table T11) and are very similar to reversed polarity inclination events at Site 1175 (see "Paleomagnetic Results" in "Paleomagnetism" in the "Site 1175" chapter).
Magnetic intensity shows slightly low values with some high-intensity peaks from 0 to 241.40 mbsf (Section 190-1176A-27X-3, 50 cm) (Fig. F13). These high-intensity peaks closely correspond to high-susceptibility peaks measured with the multisensor track (MST) (see "Physical Properties") and likely reflect the existence of ash layers. Higher intensity was observed from 241.40 to ~300 mbsf (Section 190-1176A-27X-3, 55 cm).
Site 1176 magnetostratigraphy is based on polarity changes determined by measuring the inclination of the archive half of the core after AF demagnetization at 30 mT. A magnetic polarity change from normal to reversed at 199.55 mbsf (Section 190-1176A-23X-1, 5 cm) is interpreted as the Brunhes/Matuyama Chron boundary dated at 0.78 Ma (Cande and Kent, 1995) (Fig. F14). Seven short reversed polarity events are also observed in the Brunhes Chron at 46.70 mbsf (Section 190-1176A-6H-1, 130 cm), 61.20 mbsf (Section 7H-5, 30 cm), 64.50 mbsf (Section 8H-1, 10 cm), 140.50 mbsf (Section 16H-1, 5 cm), 159.45 mbsf (Section 18H-1, 5 cm), 163.95 mbsf (Section 19H-2, 5 cm), and 181.75 mbsf (Section 21X-2, 5 cm). These short intervals are thought to represent geomagnetic excursions in the Brunhes Chron and may be correlated with the eight major excursions of Champion and Lanphere (1988), which include Laschamp (~0.04 Ma), Blake (~0.13 Ma), Jamaica (~0.19 Ma), Lavantine (~0.29 Ma), Biwa III (~0.38 Ma), Emperor (~0.47 Ma), Big Lost (~0.56 Ma), and Delta (~0.62 Ma). A clear short excursion known as Chron C1n-1 (Cande and Kent, 1995) was also identified in the Brunhes Chron at Site 1176. However, identification of such excursions is sometimes difficult because they are usually very short events. In an effort to identify the excursions, comparisons were made with the paleomagnetic results of Site 1175. The inclination anomaly at 161.47 mbsf (Section 190-1176A-18H-2, 70 cm) may correspond to excursion C1n-1.
The top of the Matuyama Chron (0.780-2.581 Ma) is interpreted to occur at 199.55 mbsf. In this reversed chron, poor core recovery makes identification of short normal polarity geomagnetic events difficult. The nearly continuous normal polarity interval from 214.80 (Section 190-1176A-24X-4, 120 cm) to 219.35 mbsf (Section 25X-1, 65 cm) may be the Jaramillo Event (0.99-1.07 Ma).
Based on the depth and age of the Brunhes/Matuyama boundary, the sedimentation rate of lithostratigraphic Units I and II is estimated at 25.58 cm/k.y. (Fig. F15) and correlates well with biostratigraphic results (see "Biostratigraphy").