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doi:10.2204/iodp.proc.320321.103.2010

Paleomagnetism

We measured and analyzed the magnetic remanence of archive-half sections from 54 cores collected from the three holes at Site U1331, excluding core catcher sections and other sections dominated by coring disturbance. The natural remanent magnetization (NRM) of each section was measured before and after alternating-field (AF) demagnetization at 20 mT. When time permitted, NRM was also measured after 5, 10, and/or 15 mT steps.

We processed data extracted from the Laboratory Information Management System (LIMS) database by removing all measurements collected from disturbed intervals, which are listed in Table T13, and all measurements that were made within 5 cm of the section ends, which are biased by sample edge effects. Cleaned data are available for each hole by AF demagnetization level in Tables T14, T15, T16, T17, T18, T19, T20, T21, T22, T23, T24, and T25.

For data from the 20 mT demagnetization step, we computed the mean paleomagnetic direction for stable polarity intervals from each core using Fisher statistics, with data from reversed polarity intervals inverted (Table T26). By subtracting the mean declination from each observed declination, the azimuth of the core can be approximately reoriented back into geographic coordinates. After this reorientation, normal polarity intervals have ~0° declination and reversed polarity intervals have ~180° declination. Based on the Pacific apparent polar wander path (APWP) (e.g., Beaman et al., 2007; Petronotis et al., 1994), the declination for Oligocene and Eocene age sediments from Site U1331 is expected to be about 0° during normal polarity intervals. Hence, this reorientation method should provide a good estimate for placing the cores in their true geographical coordinates. Reoriented declinations are provided for Holes U1331A–U1331C in Tables T18, T22, and T25, respectively, for data collected after AF demagnetization at 20 mT.

In the absence of other evidence, this reorientation method would lead to a magnetic polarity ambiguity in which one might be unable to differentiate between magnetic north and magnetic south. By mapping the paleomagnetic declination downhole among holes, using distinct polarity reversal sequences within a single core, and taking advantage of biostratigraphic age constraints, 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 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 because the inclination is generally too shallow at paleoequatorial sites, like all of the sites cored during this expedition.

We also attempted to reorient the cores using FlexIt tool data, but tool failures and other complications resulted in these data being considered only as a secondary method for orienting cores. Additional evaluation of the FlexIt tool during Expedition 321 indicated that the relative orientation from one core to the next was consistent and typically aided with reorienting the cores. For Site U1331, FlexIt orientation data proved useful in determining the polarity for the interval from 140 to 160 m CSF in Holes U1331B and U1331C, which otherwise had ambiguous polarity.

We also measured NRM, mass, and bulk magnetic susceptibility for 101 discrete paleomagnetic samples, with one sample collected about every section from Hole U1331A. Of these, 88 samples were subjected to progressive AF demagnetization up to 60 mT with a few samples demagnetized to 80 mT. The other 13 samples were progressively thermally demagnetized at room temperature, 90°C, and then from 150° to 600°C at 50°C steps. Remanence measurements and characteristic remanent magnetization (ChRM) directions computed using principal component analysis (PCA) are given in Tables T27 and T28, respectively. Magnetic susceptibilities and masses, along with volumes estimated using moisture and density (MAD) data (see "Physical properties"), are given in Table T29. This table also gives susceptibilities from whole-core data for the intervals corresponding to where the discrete samples were taken. These are used for checking the scale factor, 0.68 × 10^–5 (see "Paleomagnetism" in the "Methods" chapter), for converting whole-core raw susceptibility meter measurements into true volume-normalized susceptibility values.

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 F15, F16, and F17. Site U1331 cores contain a substantial drilling overprint, as is typical for cores from DSDP, ODP, and IODP (e.g., Shipboard Scientific Party, 2002). 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 to 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 declinations are also overprinted, although it is often possible to discern the primary declination prior to demagnetization. The remanent magnetic intensities (prior to demagnetization) are also partially overprinted, as is evident by the manner in which they mimic magnetic susceptibility (Fig. F18). This happens because the viscous IRM magnetizes many of the magnetic grains, making the NRM prior to demagnetization a proxy for magnetic mineral concentration, similar to magnetic susceptibility. After demagnetization, the magnetically soft IRM is removed and the remaining NRM varies as a function of magnetic mineral concentration and paleomagnetic field strength, thus diverging from susceptibility, which does not depend on paleomagnetic field strength.

Following removal of the drilling overprint, a stable component of magnetization is resolved for AF demagnetization between 15 and 60 mT and for thermal demagnetization between 300° and ~580°C (Fig. F19). We interpret this ChRM to be the primary depositional remanent magnetization. Discrete samples have ChRM directions, as determined by PCA, that commonly differ by only a few degrees from the coeval intervals from the split-core samples, for which the 20 mT results are used as an estimate of the ChRM. This indicates that any overprint is successfully removed by AF demagnetization up to 20 mT, with few exceptions, and so the NRM following 20 mT demagnetization is a reliable indicator of the ChRM for most intervals. Within a few narrow intervals, the inclinations remain steep even after demagnetization, indicating the drilling overprint still dominates in these intervals. For example, the 20 mT split-core data from 78.3 to 81.3 m CSF in Hole U1331A have an average inclination of 63° and the ChRM direction cannot be reliably resolved for discrete samples from this interval. A similar result was obtained for the coeval interval in Hole U1331B, so the result cannot be attributed to drilling deformation. More likely, it is related to lithology, with the interval being extremely homogeneous as opposed to the bioturbated interval above and below it. We suggest this interval may represent a turbidite that was only weakly and incoherently magnetized during deposition. Hence the drilling overprint makes a much more significant contribution.

It is likely that some 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. For example, the mean inclination for reversed polarity sediments from Hole U1331B that are Chron C12r age is 1.4°, whereas normal polarity sediments that are Chron 13n age have a mean inclination of 16.8°. Regardless, any overprint is sufficiently small that variations in inclination can be used to aid in determining the 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, as in the example for Chron C12r and C13n. Declination is, however, the primary parameter used for polarity determination, as it changes by ~180° across polarity reversals (Figs. F15, F16, F17, F20).

Magnetostratigraphy

Interpretation of the magnetostratigraphy is relatively uncomplicated, except for a few intervals (Table T30; Fig. F20). In particular, the upper 8 m of sediment record a sequence of magnetozones (N1 to R3) (see Fig. F20 for magnetozone definitions) that is tentatively interpreted to span from Chron C2An.2r in the upper part of Zone R3 to Chron C2r in Magnetozone R1. The upper normal polarity magnetozone (N1) may be partly Chron C2n and partly Chron C1n (Brunhes) because a manganese horizon occurs in interval 320-U1331A-1H-1, 74–85 cm, indicating there was a period with no or extremely low deposition (see "Lithostratigraphy"). The interpretation of this upper sequence of magnetozones is based mainly on mixed radiolarian assemblages in the core catcher of Core 320-U1331A-1H that range in age from Pleistocene to late Miocene and on the existence of an erosional contact overlain by a turbidite(?) that occurs from 6.6 to 7.6 m CSF in the individual holes or at 8.11 m CCSF-A (see Table T30). This contact is within Magnetozone R3. Nannofossils belonging to Biozone NP23 (early Oligocene) are identified 7 cm below the contact. Below the erosional contact, the polarity sequence from the lower part of Magnetozones R3 through N6 is unambiguously correlated to Chrons C10r through C13n. Given this sequence of reversals, the erosional contact at 8.11 m CCSF-A depth represents a nearly 26 m.y. long hiatus.

Other complications result from erosional contacts with overlying units that are likely turbidites (see "Lithostratigraphy"). These mainly confound interpretation in an interval from about 28 to 38 m CSF (between Magnetozones R6 and N10, which correspond to Chrons C13r and C17n.1n).

A final complication occurs for the interval below ~130 m CSF owing to discontinuous core recovery, coring deformation, and porcellanite layers. Nonetheless, we did record long intervals of constant polarity and two reversals, one at ~149 m CSF and the other at ~179 m CSF (see Table T30). A third reversal was observed at 186 m CSF in Core 320-U1331C-17H, but we consider it a somewhat suspicious result as the core had sustained minor coring deformation and because the drilling overprint could not be fully removed. Because of the discontinuous record and the difficulty with azimuthal core orientation, the reversal at 149 m CSF could correspond to either the Chron C20r/C21n or C21n/C21r reversal. The Chron C21n/C21r interpretation is in better agreement with biostratigraphic constraints, but the FlexIt orientation supports the Chron C20r/C21n interpretation, so that is the preferred interpretation (Table T30). Using inclination data, we interpret the reversal at 179 m CSF as a reversed polarity magnetozone (inclinations that are negative to near 0°) overlying a normal polarity magnetozone (inclinations that are slightly positive). Given that the seafloor at the site is Chron C23r age (~52 Ma), the reversal at 179 m CSF must be the Chron C21r/C22n, C22r/C23n.1n, or C23n.1r/C23n.2n reversal. The older ages are more compatible with biostratigraphic constraints. Hence we correlate the reversal with Chron C23n.1r/C23n.2n. Likewise, if the apparent reversal at 186 m is real, it would most likely correspond to the Chron C23n.2n/C23r boundary.

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