Analyses were conducted to provide an initial characterization of the paleomagnetic and mineral magnetic properties at Site 1166 and to develop a preliminary magnetic polarity zonation for the core. Forty-three RCB archive core halves from Hole 1166A were measured with the shipboard pass-through cryogenic magnetometer. Natural remanent magnetization and remanent magnetization after alternating field (AF) demagnetization were measured at 4-cm intervals. Three AF steps at 10, 20, and 30 mT were used for all core sections. A total of 111 oriented discrete samples (standard 8-cm3 cubes) were collected from the working halves, from both coarse and fine lithologies. Sixty-three discrete samples were AF demagnetized at successive peak fields of 10, 20, 30, 40, 50, 60, 70, and 80 mT to verify the reliability of the whole-core measurements on the archive halves. Best-fit lines were calculated for the progressive demagnetization data and were evaluated by principal component analysis (PCA) (Kirschvink, 1980). The maximum angular deviation was calculated to provide an estimate of the precision for each best-fit line.
Mineral magnetic analyses were conducted on a set of representative discrete samples after they had been subjected to AF demagnetization (see "Paleomagnetism" in the "Explanatory Notes" chapter). The low-field magnetic susceptibility (k) was routinely measured for 65 samples, and the data was compared with the whole-core susceptibility log. For these samples, the frequency-dependent susceptibility, fd(%), was also evaluated. An anhysteretic remanent magnetization (ARM) was imparted on 63 samples using a 100-mT AF with a superimposed 0.05-mT bias field. Twenty-seven samples were subjected to isothermal remanent magnetization (IRM) imparted in a direct-current field of 1.3 T. IRM was then demagnetized by inverting the sample and applying a backfield of 300 mT to determine the S-ratio (-IRM -0.3T / IRM 1T) (e.g., Verosub and Roberts, 1995). Moreover, on selected samples we investigated
Despite poor core recovery in Hole 1166A (18.6%), enough material was obtained to establish a rock magnetic stratigraphy for the sequence. Downhole variations of the concentration-dependent parameters, including k, ARM, and IRM intensities, reveal alternating intervals of high and low magnetic mineral concentration that mirror the main lithostratigraphic units (Fig. F21). In particular, intervals of increased magnetic mineral concentration are present within Subunit IB and the uppermost part of Unit II. Conversely, intervals of decreased ferrimagnetic concentration are found in lithostratigraphic Subunit IC, Units II (lower half), III, and IV (with the exception of an increase at ~285 mbsf in black clay).
Preliminary analyses of mineralogy-dependent parameters (e.g., S-ratio, Bcr , and thermal unblocking temperature) indicate that high-coercivity minerals (e.g., hematite or goethite) are not present in samples from much of the core. Plots of IRM acquisition have steep slopes at low magnetic induction, with saturation isothermal magnetization reached at fields of 0.2-0.3 T (Fig. F22). The thermal demagnetization of the three orthogonal IRMs confirms that most of the remanence is held by the soft coercive fraction. Above ~140 mbsf, the intensity decays in a quasi-linear fashion from room temperature to the Curie temperature of magnetite (580°C), with curve slope changes observed between 330° and 360°C (Fig. F23). During thermal treatment, the magnetic susceptibility progressively decreases from 20° to 700°C. Below ~140 mbsf, the contribution of the soft coercivity fraction is almost completely removed in the 330°-360°C range. A weak residual remanence that dissipates at 580°C was observed in Samples 188-1166A-17R-1, 127 cm, and 27R-1, 44 cm (152.67 and 248.04 mbsf, respectively) (Fig. F23). The bulk susceptibility of these samples begins to increase at 330°-360°C during thermal treatment, probably because of thermochemical alteration of clay minerals or sulfides. The presence of iron sulfides below ~140 mbsf is strongly supported by geochemical analyses that indicate a major drop in dissolved sulfate below ~150 mbsf (see "Inorganic Geochemistry"). Additional evidence supporting the presence of iron sulfides comes from the magnetic behavior of the discrete samples during AF demagnetization. For some samples (~135-146 mbsf), the AF cleaning at steps >50 mT was obscured by the simultaneous acquisition of a gyromagnetic remanence (Fig. F24). This is probably due to the irreversible flip of the single-domain grain moments during AF demagnetization (Stephenson, 1980, 1981). These features, also observed during Leg 161 (Shipboard Scientific Party, 1996) and Leg 174A (Shipboard Scientific Party, 1998), have recently been theorized to provide proof of the presence of iron sulfides (e.g., Snowball, 1997; Hu et al., 1998; Sagnotti and Winkler, 1999).
Poor core recovery makes magnetic polarity interpretation difficult for Hole 1166A. For this reason, we will only describe the magnetic polarity stratigraphy deduced for the interval between 112 and 160 mbsf, where recovery is relatively high.
As at Site 1165, the lack of azimuthal orientation does not pose a problem for magnetostratigraphic studies because the field has a steep inclination (±78.4°) at the Site 1166 latitude (67.7°S). Consequently, the magnetic polarity zonation was determined from the paleomagnetic inclinations obtained after 30-mT AF steps, from long-core measurements, and from PCA of characteristic remanence components of discrete samples.
For 50% of the discrete samples, stable paleomagnetic behavior was evident in the vector component diagram, with characteristic direction of magnetization (ChRM) generally tending toward the origin and maximum angular deviation lower than 10°. These directions are in excellent agreement with the long-core measurements. For the remaining samples, demagnetization behavior is complex and reliable ChRMs are difficult or impossible to determine.
The inclination of ChRM from discrete samples and long-core measurements has a distinct bimodal distribution that demonstrates the dominance of the two stable polarity states (Fig. F25). As expected at this high-latitude site, steep-normal and steep-reverse inclinations are dominant. In conjunction with evidence from vector component diagrams (Fig. F24), this indicates that secondary magnetization components have been removed.
A good magnetostratigraphic signal is recorded in the sediments between 80 and 160 mbsf (Fig. F26). Unfortunately, it is difficult to correlate this record with the GPTS (Cande and Kent, 1992, 1995; Berggren et al., 1995) because of limited biostratigraphical datums (see "Biostratigraphy and Sedimentation Rates"). Upper Pliocene marine diatoms, including T. kolbei (LO = 1.8-1.9 Ma) and T. vulnifica (range 2.2/2.3 to 2.8/3.2 Ma), are found in two greenish gray silt beds in Core 188-1166A-13R. The presence of these taxa constrains the age of the reversed-normal-reversed polarity pattern that occurs between 113.40-117.10 mbsf to the C2r.1r (possibly C2r.2r), C2An.1n, and C2An.1r Subchrons. The correlation of the upper reversed magnetozone to Subchron C2r.1r implies that a disconformity exists at ~114.80 mbsf, indicating a significant interval of missing time (>0.441 Ma), including Subchrons C2r.1n and C2r.2r.
In the stratigraphic interval from seafloor to 113.40 mbsf, magnetostratigraphic interpretation is difficult. In addition to extremely poor recovery, microfossils (especially diatoms) are low in abundance at intermittent intervals and are not age diagnostic. The only interval characterized by an abundant and well-preserved diatom assemblage indicating a late Quaternary age (<2.22 mbsf) (see "Biostratigraphy and Sedimentation Rates") conflicts with the reversed-polarity magnetostratigraphic evidence, indicating an age older than 0.78 Ma. Upper sections of the core show "soupy" drilling disturbance (see "Lithostratigraphy"), suggesting that the observed remanence is drilling induced and does not represent the primary remanence. Furthermore, the paleontological samples were taken from a distinct in situ sponge spicule-rich bed and are unlikely to be contaminated.
The polarity record from 117.10 to 135.65 mbsf lacks biostratigraphic constraints, which makes the correlation with the GPTS difficult.
A major change in lithology occurs within Section 188-1166A-15R-2 (at 135.63 mbsf), from dark gray diamictons (Unit I) above to olive-gray diatom-bearing claystones (Unit II) below (see "Lithostratigraphy"). Below this boundary, a thin reversed-polarity interval is followed downcore by a thick interval of normal polarity (between 142.45 and 148.25 mbsf). The age of the interval between 142.45 and 148.26 mbsf is partially constrained by the presence of the diatom H. caracteristicus, which indicates an age older than ~33 Ma, based on its LO within Chron C13n in Hole 744B on the southern Kerguelen Plateau (see "Biostratigraphy and Sedimentation Rates").