For the purpose of developing a magnetic polarity stratigraphy at Site 1165, the archive halves from Holes 1165A, 1165B, and 1165C were measured with the shipboard pass-through cryogenic magnetometer. Natural remanent magnetization (NRM) and remanent magnetization after alternating field (AF) demagnetization were measured at 4-cm intervals. Time constraints and the need to keep pace with core flow permitted only two AF steps for the majority of core sections, with an additional step (30 mT) in the lower half of the core. The presence of a strong rock-saw overprint (potentially induced by the splitting saw) between ~860 and 880 mbsf necessitated progressive demagnetization of the archive halves up to 80 mT.
A total of 614 oriented discrete samples (standard 8-cm3 plastic cubes) were also collected from the working halves. Most 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. Three hundred eighty-nine of these samples yielded good principal component analysis (PCA) (Kirschvink, 1980) fits. Six discrete samples were progressively thermally demagnetized at temperatures of 100°, 200°, 300°, 330°, 360°, 400°, 400°, 500°, 550°, 600°, 650°, and 700°C. The magnetic susceptibility was measured after each step to monitor thermal alteration of the magnetic mineralogy.
The lack of azimuthal orientation of the cores does not pose a problem for magnetostratigraphic studies because the geomagnetic field at the latitude of Site 1165 (64.4°S) has a steep inclination (±76.5°, assuming a geocentric axial dipole model). Consequently, the paleomagnetic inclinations, which were determined from the 20-30 mT AF steps from the long-core measurements and from PCA of characteristic remanence components of discrete samples, are sufficient to determine polarity. In the Southern Hemisphere, negative (upward) inclinations correspond to normal polarity and positive (downward) inclinations correspond to reversed polarity.
Mineral magnetic analyses were performed on a set of discrete samples after they had been subjected to AF demagnetization. The low-field magnetic susceptibility (k) was routinely measured for each sample, and the data were compared with the whole-core susceptibility log (see "Physical Properties"). The frequency-dependent susceptibility, fd(%), of 338 selected discrete samples was monitored. An anhysteretic remanent magnetization (ARM) was imparted on 135 samples using a 100-mT alternating field and a 0.05-mT bias field. An isothermal remanent magnetization (IRM) was imparted on 334 samples in a direct-current (DC) magnetic field of 1.3 T. The IRM was then demagnetized by inverting the sample and applying a backfield of 300 mT to determine the S-ratio (-IRM -0.3 T / IRM 1.3 T) (e.g., Verosub and Roberts, 1995). Moreover, on selected samples we investigated the following:
Based on magnetic properties and behavior during demagnetization treatment, the drilled sedimentary sequence can be divided into three main intervals that are not directly related to lithologic variations in the core. The downcore variations of k, ARM, and IRM are shown in Figure F31. The highest values of these magnetic concentration-dependent parameters are recorded in the upper 94 mbsf of the core. In this interval, preliminary analyses of magnetic mineralogy-dependent parameters (e.g., Bcr and S-ratio) and thermal unblocking characteristics are consistent with a magnetite-dominated magnetic mineralogy. During thermal demagnetization of the three orthogonal IRMs, the intensity decays in a quasi-linear fashion from room temperature to the Curie temperature of magnetite (580°C), with a weak inflection point between 320° and 350°C (Fig. F32). Several magnetic minerals experience thermal unblocking at these temperatures, including titanomagnetite (Hunt et al., 1995), iron sulfides such as greigite (Roberts, 1995), and pyrrhotite (Dekkers, 1989). Shipboard investigations did not yield enough evidence to distinguish between these possibilities. The significance of these phases becomes progressively more pronounced in the lower parts of the succession (Figs. F31, F32). The interval between 94 and 130 mbsf is a transition zone where the magnetic concentration-dependent parameters decrease significantly. Below this interval, between 130 and 362 mbsf, very low concentration-dependent values are present. For example, IRM decreases from an average of 2943 mA/m between 0 and 130 mbsf to ~17 mA/m between 130 and 362 mbsf. High coercivity values (Fig. F33) and Curie temperatures of >650°C indicate the presence of hematite in addition to minor magnetite contributions in this interval. Corresponding to this interval of low-magnetic mineral concentrations, geochemical analyses show the presence of a particularly high broad peak in silica concentration (see "Inorganic Geochemistry"). We suggest that in the presence of an amorphous silica-saturated environment, magnetite was unstable and consequently was consumed by chemical processes. Such processes would preferentially remove "free" small grains with large surface area/volume ratio, leaving inclusions of magnetite in other mineral phases unaffected. The stable iron-rich solid phases expected under such conditions are iron silicates and hematite (Garrells and Christ, 1965). The presence of a low-coercivity drilling overprint suggests that a limited amount of coarse-grained, multidomain magnetite remains in this zone.
The interval between 362 and 512 mbsf is a transitional zone and is characterized by parameters that indicate frequently changing high and low magnetic mineral concentrations. The lower section, between 512 and 999 mbsf, contains almost the same high ferrimagnetic concentrations as encountered in the uppermost 94 m.
In the following section, we describe the magnetic polarity stratigraphy above 94 and below 362 mbsf. The decreased abundance of ferrimagnetic minerals between 94 and 362 mbsf coincides with a poor-quality paleomagnetic signal that prohibited the determination of a reliable magnetic polarity stratigraphy (Fig. F34). Most of the samples analyzed suggest the presence of a nearly vertical reversed polarity overprint (Weeks et al., 1995). This overprint is soft and is always removed at peak fields of 10 mT. After overprint removal, a stable characteristic remanence component is evident for a large proportion of the analyzed samples (Fig. F35). In a few cases (samples between 860 and 880 mbsf), an additional overprint—stronger than the drilling-induced overprint—is present. This overprint has a nearly horizontal inclination and is probably related to contamination introduced by cutting the sections. In most cases, this component is parallel to the splitting plane (i.e., in sample coordinates, the overprint is in the y-z plane with x = 0) and in a few cases, is perpendicular to it (the overprint is in the x-z plane with y = 0). When present, this overprint could only be partially removed even after demagnetization to peak fields of 60-80 mT.
The preliminary magnetostratigraphic interpretation of the top 50 mbsf is well constrained by diatom datums derived from initial analysis of core-catcher samples (Fig. F36; see "Biostratigraphy and Sedimentation Rates"). The inclination record for this interval, when compared with the geomagnetic polarity time scale (GPTS) (Cande and Kent, 1992, 1995; Berggren et al., 1995), provides a near-complete record of the Pliocene-Pleistocene polarity intervals from the Brunhes Chron (C1n) to the Thvera Subchron (C3n.4n), with the Pliocene/Pleistocene boundary at 6.97 mbsf. Correlation with the GPTS suggests disconformities at ~6, 14.4, 15.6, and ~16 mbsf. Corresponding to the stratigraphical break at ~6 mbsf, the Jaramillo Subchron (C1r.1n) is missing. From this correlation, the relatively reduced thickness of the Brunhes Chron can be explained by the presence of one or more disconformities. Also, if the present correlation is correct, a new reversed polarity event is recognized within Subchron C2An.3n (at ~32.64 mbsf).
The polarity record from 50 to 94 mbsf is constrained by limited biostratigraphic control, which makes correlation with the GPTS more difficult (Fig. F37); however, following the pattern recorded in the upper 50 mbsf, the four recorded reversals can be matched with the lower part of the Thvera Subchron (C3n.4n), C3r, C3An.2n, C3Ar, and the upper part of Chron C3.Bn. In this context, the Miocene/Pliocene boundary lies at ~54.60 mbsf. The sharp polarity change at ~67 mbsf suggests a disconformity with a significant amount of missing time, including Subchrons C3An.1n and C3An.1r. Several small variations in the magnetic polarity between 69 and 100 mbsf correspond with the presence of metamorphic and/or igneous pebbles.
Some segments within the transitional band between ~94 and ~130 mbsf show changes in magnetic intensity and polarity corresponding to the alternation between black and gray horizons (Fig. F38). This provides further evidence that variations in magnetic properties below ~94.0 mbsf are related to variations in redox conditions, resulting in partial dissolution of the detrital magnetic minerals and formation of secondary magnetic phases (e.g., Florindo and Sagnotti, 1995).
The polarity pattern is obscure between 94.0 and 362.0 mbsf, which renders delineation of any magnetic polarity interval impossible. A good magnetostratigraphic signal, however, is recorded in the sediments between 362.0 and 999.1 mbsf (Figs. F39, F40). Unfortunately, it proved difficult to fit this record to the GPTS because of the scarcity of biostratigraphical datums. Four taxa, including the FCO of A. ingens (<15.9-16.4 Ma), the FO of C. kanayae (<17.5-17.7 Ma), the LO of T. praefraga (>18.3-19.1 Ma), and the FO of T. praefraga (<19.9-20.8 Ma; see "Biostratigraphy and Sedimentation Rates"), are identified in this interval. Starting from these tie points, a magnetostratigraphic correlation was attempted; however, the limited biostratigraphic control and the complex nature of the paleomagnetic signal mean that this interpretation is preliminary and its validity needs to be tested through shore-based studies. By approximating the sedimentation rate from the available biostatigraphic data, the expected lengths of chrons were calculated and an iterative approach was taken to achieve a fit. Between ~350.0 and ~419.0 mbsf, polarity is mostly reversed and is followed by a normal polarity interval characterized at the bottom (449.4 mbsf) by a sharp magnetic polarity boundary. The presence of the FCO of A. ingens (<15.9-16.4 Ma) constrains these two magnetozones to the C5Br and C5Cn Chrons. The thick interval of normal polarity between 718.4 and ~824.0 mbsf contains the FO of T. praefraga (<19.9-20.8 Ma) and is correlated with Chron C6n. Consequently, the sequence of magnetozones below 824 mbsf are correlated with Chrons C6r, C6An, and C6Ar, respectively.
Biostratigraphic data indicate a disconformity between 487 and 497 mbsf with a duration of ~1 m.y. Several sharp changes in magnetic polarity (e.g., at 718.4 and 746.4 mbsf) suggest the presence of more than one disconformity surface with a significant interval of time missing.
The ideal sediment characteristic for relative paleointensity determination is one in which the remanence is a detrital remanent magnetization determined by pseudo-single domain (PSD) magnetite, the concentration does not vary by more than a factor of 10 (King et al., 1983; Tauxe, 1993; Jacobs, 1998), and there is no correlation between normalized NRM and the concentration-dependent magnetic parameters such as k, IRM, and ARM (Tauxe, 1993).
In the uppermost part of Hole 1165B (0-94 mbsf), rock magnetic analyses demonstrate that the sediment satisfies the criteria for relative paleointensity determination. Magnetite, probably in the PSD state, is the primary magnetic carrier. Variation of bulk magnetic parameters (k, ARM, and IRM) shows that its concentration does not vary by more than one order of magnitude. Furthermore, a significant correlation between the NRM/k ratio and the normalizing factor k does not exist.
The NRM intensity record, demagnetized at 20 mT to eliminate the drilling-induced overprint, was normalized with low-field magnetic susceptibility in the uppermost 100 mbsf of Hole 1165B (Fig. F41). The NRM20 mT / k ratio vs. depth shows a series of high and low values. Polarity boundaries are characterized by low NRM20 mT / k ratios that recover immediately after the termination of the polarity boundary. This is consistent with published paleointensity records and current geodynamo models. These results indicate that a relative paleointensity record can be established in the uppermost 100 m of Hole 1165B and that comparison with previously published data sets might improve the stratigraphic resolution of the Pliocene-Pleistocene part of the section.