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doi:10.2204/iodp.proc.320321.104.2010

Paleomagnetism

We measured and analyzed the remanent magnetization of archive-half sections from 47 cores (39 APC and 8 XCB cores) collected from three holes at Site U1332, excluding core catcher sections and other sections completely disturbed during coring. The natural remanent magnetization (NRM) of each section was measured before and after alternating-field (AF) demagnetization, with AF demagnetization typically consisting of a single 20 mT step. When time permitted, NRM was also measured after 5, 10, and/or 15 mT steps.

We processed the data extracted from the Laboratory Information Management System (LIMS) database by removing all measurements collected from disturbed intervals, which are listed in Table T11, and all measurements that were made within 5 cm of the sections ends, which are biased by sample edge effects. Cleaned data are available for each hole by AF demagnetization level in Tables T12, T13, T14, T15, T16, T17, T18, T19, and T20. Curation errors occurred for Sections 320-U1332A-10H-4 and 14H-4, in which the halves that should have been treated as the working halves (with double lines along the core liner) were switched with the archive halves (with a single line along the core liner). We measured these two sections as archive halves before the errors were noted. Thus, the working halves were measured in the magnetometer instead of the archive halves and discrete samples were taken from the archive halves instead of the working halves. In Tables T12, T16, T21, and T22, we corrected the declinations of samples from these sections by flipping them by 180° (note that data in the LIMS database are not corrected). We also noticed that Section 320-U1332C-6H overlaps Section 320-U1332C-7H by ~3 m CSF. This happened because Core 6H was advanced 4 m but recovered >7 m of core. The upper 2.6 m was slurry (soupy mixed sediments). To partially fix the overlap, we subtracted 2.6 m from the Core 6H depths. This brought the top of the good part of Core 6H beneath the base of Core 320-U1332C-5H and reduced the overlap between Cores 6H and 7H to ~50 cm (Tables T19, T20).

For data from the 20 mT demagnetization step, we computed the mean paleomagnetic direction for each core using Bingham statistics (Table T23) with a program developed by Tanaka (1999). Unlike Fisher statistics, Bingham statistics can treat bipolar data sets and compute a principal axis as well as two associated semiaxes of the data set. When a data set consists of a sufficient number of paleomagnetic direction data with normal or reversed polarity, this principal axis corresponds to the orientation of the normal or reversed polarity field. We used all declination and inclination data for the computation and adopted the resultant principal axes as the mean paleomagnetic directions. These mean directions were inverted when they were interpreted to be representative of reversed polarity. By subtracting the mean declination from each observed declination, the azimuth of the core can be approximately reoriented back into geographic coordinates as discussed in "Paleomagnetism" in the "Site U1331" chapter.

In the absence of other evidence, this reorientation method has ambiguity in distinguishing magnetic north and south. By correlating downhole polarity reversal sequences among holes, using distinct reversal patterns, and taking advantage of age constraints provided by biostratigraphy, it is fairly straightforward to determine a continuous polarity stratigraphy downhole and hence to obtain the correct azimuthal orientation of the core. This only breaks down when significant coring gaps occur or when rotation occurs between pieces of core within a single core, which is the case for all cores collected with the XCB. Hence XCB cores are not reoriented, nor can we confidently determine polarity from these cores because the inclination is generally too shallow at paleoequatorial sites, like all of the sites cored during Expedition 320/321. Reoriented declinations are provided for Holes U1332A–U1332C in Tables T16, T18, and T20, respectively, for the data collected after AF demagnetization at 20 mT.

We also measured NRM, mass, and bulk magnetic susceptibility for 91 discrete paleomagnetic samples, with one sample collected about every section from Hole U1332A. Of these, 76 samples were subjected to progressive AF demagnetization up to 60 mT. Remanence measurements and characteristic remanent magnetization (ChRM) directions computed using principal component analysis (PCA) are given in Tables T21 and T22, respectively. Magnetic susceptibilities and masses, along with volumes estimated using moisture and density (MAD) data (see "Physical properties"), are given in Table T24. This table also includes whole-core magnetic susceptibilities for depth intervals corresponding to the discrete samples, which are used for checking the scale factor for converting the whole-core raw susceptibility meter measurements into true volume normalized susceptibility values (0.68 x 10^–5) (see "Paleomagnetism" in the "Methods" chapter).

Results

Downhole variations in paleomagnetic data from split-core and discrete samples and magnetic susceptibility data from whole-core and discrete samples are shown in Figures F12, F13, and F14. As is typical for cores from DSDP, ODP, and IODP (e.g., Shipboard Scientific Party, 2002a), Site U1332 cores suffer a substantial drilling overprint. The overprint is primarily a viscous isothermal remanent magnetization (IRM), which results from the sediments residing inside the relatively magnetic BHA, drill pipe, and steel core barrel (and, to a lesser extent, the nonmagnetic core barrel) for about 15–45 min from the time it is collected until it is removed from the core barrel on the rig floor.

The most obvious evidence of the overprint is the steep inclination (typically ~70°–80°) measured prior to demagnetization. After AF demagnetization at 10 to 20 mT, the inclination becomes very shallow in general, as expected for sediments deposited near the equator. The effect and removal of the drilling overprint are evident from AF demagnetization behavior of the discrete samples (Fig. F15).

Following removal of the drilling overprint, a stable component of magnetization is resolved for AF demagnetization between 10 and 60 mT (Fig. F15). We interpret this ChRM to be the primary depositional remanent magnetization. Discrete samples have ChRM directions, as determined with PCA, that commonly agree within a few degrees with those of the coeval intervals of the split-core samples (Table T22; Fig. F12), for which the 20 mT demagnetization results are used as an estimate of the ChRM. This indicates that any overprint generally is successfully removed with AF demagnetization up to 20 mT. Within some intervals, however, the inclinations remain steep even after demagnetization, indicating the drilling overprint still dominates in these intervals. For example, Core 320-U1332A-2H (4.0–13.4 m CSF) has a mean inclination of –76.09° (Table T23). Inclinations and remanent magnetization intensities from a few discrete samples from this core do not agree with those from the split-core samples (Fig. F12). These are mainly limited to the upper 20 m in Hole U1332A, and it is considered that in this interval the split-core samples were more strongly overprinted.

It is likely that a small overprint remains in many intervals even after magnetic cleaning because the inclinations are not symmetrically distributed about zero. Instead, they are biased several degrees toward positive values, which could possibly result from a Brunhes field overprint or a drilling overprint. Regardless, any overprint is sufficiently small that variations in inclination can be used to aid in determining polarity even though the mean inclination at the site is very shallow. In such cases, reversed polarity intervals consistently have slightly shallower inclination than normal polarity intervals. The declination is, however, the primary parameter used for polarity determination, as it changes by ~180° across polarity reversals (Figs. F12, F13, F14).

Magnetostratigraphy

Interpretation of the magnetostratigraphy is relatively uncomplicated, as summarized in Table T25 and Figures F16, F17, and F18. We consider that the bottom of the APC cores extended down through reversal boundaries Chron C19r/C20n (42.536 Ma) for Hole U1332A (124.70 m CSF) and Chron C18n.2n/C18r (40.084 Ma) for Holes U1332B (102.25 m CSF) and U1332C (106.95 m CSF). The chron boundary closest to the Oligocene/Miocene boundary (C6Cn.2n/C6Cn.2r; 23.030 Ma) occurs at 22.40 m CSF in Hole U1332A and at 22.88 m CSF in Hole U1332C. The Eocene/Oligocene boundary occurs just below the Chron C13n/C13r reversal (33.705 Ma), which is at 75.33 m CSF in Hole U1332A and 74.33 m CSF in Hole U1332B. The magnetostratigraphies for the three holes are compared in Figure F19.

Complications include (1) the upper few cores of each hole and (2) a slump that occurs just above the Eocene/Oligocene boundary. Paleontological age estimates from core catcher samples are Quaternary age for Core 320-U1332A-1H, 23.29–22.98 Ma for Core 3H, 22.26–22.35 Ma for Core 320-U1332B-3H, and 21.9–22.2 Ma for Core 320-U1332C-2H (see "Biostratigraphy"). No age estimates were obtained for Cores 320-U1332A-2H, 320-U1332B-1H and 2H, and 320-U1332C-1H. Age data indicate that hiatuses occur between sediments of Pleistocene–Pliocene and early Miocene age. We tentatively assign geomagnetic chrons from C1n to C2Ar for Cores 320-U1332A-1H and 2H, C1n to C2An.3n for Cores 320-U1332B-1H and 2H, and C1n to C2r.1r for Core 320-U1332C-1H. The hiatuses are considered to occur below these horizons. Below the hiatus, we identify the occurrence of Chron C6An.1r at 14.20 m CSF in Hole U1332A, Chron C6n at 13.60 m CSF in Hole U1332B, and Chron C5En at 12.30 m CSF in Hole U1332C, which agree with paleontological age constraints (Figs. F16, F17, F18).

Lithostratigraphic observations, paleomagnetic data, magnetic susceptibility data, and paleontological age estimates from core catcher sections suggest that Cores 320-U1332A-8H and 9H, 320-U1332B-8H and 9H, and 320-U1332C-9H penetrated a slump (see "Lithostratigraphy," "Biostratigraphy," and "Physical properties"). The slump occurs just above the Eocene/Oligocene boundary. As a result, the upper part of Chron C13n and the lower part of Chron C12r are repeated in the sedimentary succession as evidenced by the polarity reversal sequence and the distinct and coherent variations in remanent magnetization (Figs. F20, F21, F22). The basal surface of the slump (the décollement) is a sharp contact, with sediment above and below having sustained no visible or measurable deformation.

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