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doi:10.2204/iodp.proc.320321.109.2010

Paleomagnetism

Using the pass-through magnetometer, we measured the NRM of archive-half sections of 20 APC and 14 XCB cores from Hole U1337A, 27 APC cores from Hole U1337B, 8 APC cores from Hole U1337C, and 30 APC cores from Hole U1337D. Discrete samples, collected one per section from all APC cores of Hole U1337A and Cores 321-U1337B-22H through 26H, were also measured.

The primary objective of shipboard measurements is to determine magnetostratigraphy, thereby providing chronostratigraphic constraints. To accomplish this, the standard protocol at Site U1337 was to measure the NRM of each section at 2.5 cm intervals before demagnetization and after demagnetization at peak AF of 10 and 20 mT. From Core 321-U1337B-5H to 27H and from Core 321-U1337D-3H to 30H, only one demagnetization step (20 mT) was carried out to save time and maintain laboratory core flow. Except for the uppermost two XCB cores in Hole U1337A (Cores 320-U1337A-23X and 24X), other XCB cores (Cores 321-U1337A-25X through 36X) were measured before demagnetization and after a single demagnetization step (20 mT). During processing of the paleomagnetic data, we removed measurements within 7.5 cm of section ends to avoid section-end effects. Data from visibly disturbed intervals (see "Lithostratigraphy") were also removed.

The FlexIt core orientation tool was deployed in conjunction with all APC cores at Site U1337. The orientation angle (Table T15) was determined from the FlexIt software. The FlexIt tools were typically run for 12–14 h continuously, although battery life of each unit would allow them to run for up to 24 h. The driller recorded the time on bottom (prior to APC firing) for each APC core. Arrival at the seafloor can also be determined by the temperature minimum (typically 2°–3°C) recorded by the FlexIt tool. The temperature minimum should be compatible with the drillers log and the time marker in the FlexIt software. The core barrel is held stationary by the driller for at least 5 min prior to APC firing and a period of steady FlexIt orientation should be recorded during this time interval. The orientation angle is the average angle recorded during this time interval, just prior to APC firing. The quality of the FlexIt orientation angle is a function of the uniformity of the angle during the ~5 min that the tool and core barrel are held steady prior to firing. Data quality is partly represented by the standard deviation of the orientation angle (Table T15), although other factors such as the length of time over which the orientation angle is measured by the FlexIt tool should be taken into account. In addition, shattered/split core liners are an indication that the orientation angle may not be reliable. It is noteworthy that the field intensity and inclination recorded by the FlexIt tool are larger than expected for the site location as a result, presumably, of the steeply dipping drill string magnetization. The field intensity can be as much as 50% greater than the geomagnetic field intensity expected at the site and the inclination several tens of degrees higher. We assume that the drill string does not affect the orientation angle significantly because at present it is difficult to correct for the effect. We account for the declination of the present geomagnetic field (the angle between true north and magnetic north) when applying the orientation angle to the remanence declinations.

At Site U1337, unlike most previous Expedition 320 sites, the FlexIt tool was deployed and its data applied throughout the APC sections. The results indicate that the FlexIt tool is far more reliable than its predecessor (the Tensor tool). There are 11 cores for which FlexIt data were inadvertently lost (Cores 321-U1337A-19H through 21H; 321-U1337B-17H and 18H; and 321-U1337D-25H, 26H, and 28H through 30H). The reasons for data loss are not fully understood, although on one occasion the computer was accidentally interrupted during the survey and on another occasion the 9 V battery in the radio transceiver that initializes the tool and downloads data from the tool abruptly failed at the time of download.

The remanence inclination is close to zero, as expected for a site close to the paleoequator, making inclination not diagnostic of polarity. In the absence of FlexIt core orientation data, resulting 180° alternations in declination may be correlated to the GPTS as described in "Paleomagnetism" in the "Site U1331" chapter, although this was proved to be impractical at this site.

Results

Paleomagnetic data for Holes U1337A–U1337D are presented in Figures F18, F19, F20, and F21 together with whole-core magnetic susceptibility data measured on the WRMSL. Measurements of NRM above ~93 m CSF indicate moderate magnetization intensities (on the order of 10^–3 A/m) with a patchy but generally weak VRM or IRM coring overprint (see "Paleomagnetism" in the "Site U1331" chapter), and polarity reversal sequences are usually clearly recognizable. Agreement in declinations between archive-half sections and discrete samples taken from working halves (blue symbols in Fig. F18) indicate that the radial-inward drilling-induced remanence occasionally reported from previous ODP/IODP cores is not present in these cores.

Below ~93 m CSF, remanence intensities after AF demagnetization at peak field of 20 mT approach magnetometer noise level in the shipboard environment (~2 × 10^–5 A/m). In this zone, sediment magnetizations have been partly overprinted during the coring process, and remanence inclinations are occasionally steep after AF demagnetization at a peak field of 20 mT. Nonetheless, polarity reversals are apparently recorded, although the drilling-related magnetic overprint has not been effectively removed during shipboard demagnetization (Figs. F18, F19, F20, F21). Remanent magnetization intensities and magnetic susceptibilities go hand in hand downcore with a notable recovery in remanence intensity and susceptibility in the 170–190 m CSF interval (Figs. F18, F19, F20, F21).

At depths where overpull was encountered during core recovery, cores were subsequently collected using a steel core barrel, in order not to endanger the more expensive nonmagnetic barrels. Steel core barrels were used for Cores 321-U1337A-21H and below, 321-U1337B-21H and below, 321-U1337C-4H and below, and 321-U1337D-21H and below. As can be seen from Figures F18, F19, F20, and F21, the magnetic polarity interpretation for these cores was hampered by enhanced magnetic overprint that could not be removed by AF demagnetization at a peak field of 20 mT. Particularly where steel core barrels were used, scattered inclinations remain markedly steeper than expected at the site latitude (3°50′N), even after the demagnetization. It is also notable that remanence intensity tends to be highest near the top of some cores and gradually decreases toward the base, particularly for XCB cores and for APC cores utilizing steel core barrels (Figs. F19C, F20, F21C). Inclinations, although highly scattered, also show a tendency for upward increase within each core. This indicates that the upper parts of the cores are more strongly affected by steep drill string–related magnetizations, possibly indicating that the secondary magnetizations are a function not only of the magnetic field strength within the drill string but also by the level of drilling disturbance that is generally enhanced toward the core tops.

We measured the NRM of 14 XCB cores (321-U1337A-23X through 36X). Declination changes appear to be random in the XCB cores (Fig. F18C). Inclinations are highly scattered and steeper than expected, on average, even after 20 mT AF demagnetization, which indicates that magnetic overprint acquired during coring could not be removed by AF demagnetization (Fig. F18C). The unfavorable results are most likely caused by relative rotation of core pieces ("biscuits" or "protobiscuits") and remagnetization of fluidized mud filling gaps between biscuits and between a core liner and biscuits (matrix of biscuits) under a strong magnetic field produced by the drill string and the XCB core barrel. Although some of the XCB cores appear to be in excellent condition, showing no visible drilling disturbance, relative rotation of core pieces is apparent from the remanence data of all measured XCB cores.

Magnetostratigraphy

Our correlation of polarity zones with the GPTS is based on polarity zone pattern fit to the GPTS. Reversal depths are provided in Table T16 and the polarity interpretation is shown in Figure F22. All declinations are based on FlexIt orientation information apart from those cores for which the FlexIt data were lost during data acquisition or transfer (see above). There are two intervals of apparent inconsistency in declination (Fig. F22). The first involves the declinations of Core 321-U1337D-14H (137.86–147.36 m CCSF-A), which are nearly antipodal to those of the corresponding intervals in Cores 321-U1337A-15H and 321-U1337B-14H. The orientation of Core 321-U1337D-14H with the FlexIt tool is uncertain because the stable interval before APC firing was short (Table T15). The other inconsistent interval is for Core 321-U1337D-18H (180.42–189.92 m CCSF-A). In this case, declinations in the upper part of the core contradict those in the corresponding interval of Core 321-U1337A-18H, and the declinations of the lower part of the core are opposite to those of Core 321-U1337B-19H. Because the latter core is consistent in declination with the declinations of Cores 321-U1337C-4H and 321-U1337D-19H in their common intervals, we consider the orientation of Core 321-U1337D-18H to be incorrect, although the core orientation data of the FlexIt tool for this core did not raise suspicion (Table T15).

The polarity interpretation is based on the declination record after demagnetization at a peak field of 20 mT (Fig. F22). Interpretation is hampered by weak magnetization intensities that approach magnetometer noise level in some intervals, particularly below ~100 m CCSF-A. In the lower part of the record, improved clarity of the declination record in the 190–200 m CCSF-A interval, where magnetization intensities briefly recover, illustrates that magnetization intensity is the principal control on the clarity of the directional NRM record. Without multiple holes and robust correlation among holes, using digital image scans and WRMSL data (see "Stratigraphic correlation and composite section"), a polarity interpretation at this site would be difficult or impossible. As it is, scattered data from one hole are often compensated by less scattered data from another hole, allowing the magnetostratigraphy to be pieced together. The polarity interpretation (Fig. F22), based on polarity zone pattern fit to the GPTS, should be considered tentative and provisional in some intervals, although it is consistent with site biostratigraphy and has been generated independent of it (see "Biostratigraphy"). Our conclusion from this site is that the FlexIt orientation data is generally reliable, although there is an apparent tendency for giving clockwise-deflected declinations to some extent, probably as a result of the effect of the magnetic field of the drill string. Without the FlexIt orientation data an interpretation independent of the biostratigraphy would not have been possible. Sedimentation rates based on the polarity interpretation indicate a change in mean sedimentation rate at ~3.3 Ma (~50 m CCSF-A) with mean sedimentation rates of ~20 m/m.y. below this level and ~15 m/m.y. above it (Fig. F23).

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