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doi:10.2204/iodp.proc.320321.110.2010

Paleomagnetism

Using the pass-through magnetometer, we measured the natural remanent magnetization (NRM) of archive-half sections of 26 APC cores from Hole U1338A, 42 APC cores from Hole U1338B, and 47 APC cores from Hole U1338C. Discrete samples (total = 234) were collected at one per section (for the first six sections) from all APC cores of Hole U1338A and from Cores 321-U1338B-27H through 42H in an interval not covered by APC coring in Hole U1338A. Of these cubic (7 cm³) discrete samples, the NRM of 130 samples was measured prior to demagnetization and then after demagnetization at peak fields of 10 and 20 mT using the pass-through magnetometer.

The primary objective of shipboard measurements is to determine the magnetostratigraphy, thereby providing chronostratigraphic constraints. To accomplish this, the procedure in Hole U1338A was to measure the NRM of each section at 2.5 cm intervals before demagnetization and after demagnetization at peak AFs) of 10 and 20 mT. In Holes U1338B and U1338C, the NRM of each core section was measured prior to demagnetization and after a single 20 mT demagnetization step. The curtailed treatment for Holes U1338B and U1338C was necessary to maintain laboratory core flow. No XCB core sections from Site U1338 were measured based on experience at Site U1337, where XCB cores, even those visibly undisturbed, do not yield useful paleomagnetic data. During processing of the paleomagnetic data at Site U1338, 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.

At Site U1338, as at Site U1337, the FlexIt core orientation tool was deployed and its data were applied throughout the APC sections. For operation of the FlexIt tool, see "Paleomagnetism" in the "Methods" chapter and "Paleomagnetism" in the "Site U1337" chapter. The FlexIt tool was deployed in conjunction with all APC cores at Site U1338 except for Cores 321-U1338C-45H through 47H, the final three cores in Hole U1338C. The decision not to deploy the FlexIt tool for these cores was based on the possibility of damage to the FlexIt tool from overpull/jerking required to recover the preceding core (321-U1338C-44H). A malfunction of the FlexIt tool occurred for seven cores at the base of Hole U1338B (Cores 321-U1338B-36H through 42H), where FlexIt data were not recorded because of inexplicable tool failure immediately after its redeployment after charging and data download. The FlexIt tool was typically run for 6–8 h continuously (6–7 cores), although battery life of each unit should allow it to run for up to 24 h. Data loss in Hole U1338B led to the tool being replaced and recharged more frequently (after each 4–5 cores) for Hole U1338C. The orientation angle determined from the FlexIt software (by M. Hastedt) is listed in Table T14.

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, the 180° alternations in declination may be mapped downhole and 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 U1338A–U1338C are presented in Figures F18, F19, and F20, together with whole-core magnetic susceptibility data measured on the WRMSL. Magnetization directions are dispersed and not interpretable in a large part of the Site U1338 sections because of low remanent magnetization intensities (10^–5–10^–4 A/m) that approach the noise level of the magnetometer. Three intervals, however, feature magnetization intensities close to or above 10^–3 A/m where the remanence directions can be adequately resolved. These intervals are 0–50, 180–225, and 295–395 m CSF. In these intervals, the recovery in magnetization intensity is accompanied by increased susceptibility (Figs. F18, F19, F20), although the susceptibility increase is generally muted relative to the remanence intensity change. There is satisfactory agreement between directions and intensity of remanent magnetization after AF demagnetization at peak fields of 20 mT for archive-half sections and discrete samples taken from working halves in the 0–50 (Fig. F18A) and 180–225 (Fig. F18B) m CSF intervals. This agreement indicates that the radial-inward drilling-induced remanence occasionally reported from ODP/IODP cores is not present in these intervals. In the 295–395 m CSF interval, on the other hand, small but systematic differences occur in declinations (Fig. F19C), which indicate the drilling-induced magnetization remains even after demagnetization. In this interval, we also recognize some differences in remanence intensity, with discrete samples tending to have lower magnetization intensities than archive-half sections. This suggests that the rims of cores have been overprinted more strongly than their centers.

At depths where there was a likelihood that core barrels would need to be drilled over for recovery, cores were collected using a steel core barrel in order not to damage the more expensive nonmagnetic barrels. The utilization of steel (as opposed to nonmagnetic) core barrels began with Cores 321-U1338A-25H, 321-U1338B-21H, and 321-U1338C-22H. Core 321-U1338A-25H may have been affected by enhanced magnetic overprint associated with the steel core barrel (Fig. F18B), whereas Core 321-U1338C-22H (Fig. F20B) was not markedly affected. Remanence intensity tends to be highest near the top of some cores and gradually decreases toward the base (Figs. F19A, F20B). Inclinations, although highly scattered, occasionally show a tendency for uphole increase within each core (e.g., Cores 321-U1338B-20H, 34H, and below) (Fig. F19B, F19C). 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.

Magnetostratigraphy

Our correlation of polarity zones with the GPTS is based on polarity zone pattern fit to the GPTS, being cognizant of the biostratigraphic constraints. Interpreted reversal depths are provided in Table T15, and polarity interpretations are shown in Figure F21. All declinations are based on FlexIt orientation information apart from those cores for which the FlexIt data were lost (Cores 321-U1338B-36H through 42H) or where the FlexIt tool was not deployed (Cores 321-U1338C-45H through 47H). For Cores 321-U1338B-36H through 42H (321.1–387.4 m CSF; 354.1–430.9 m CCSF-A), the cores were "oriented" by applying a uniform rotation to each core that is consistent with the FlexIt oriented data for the same interval from Hole U1338C. The polarity interpretation is based on the declination record after demagnetization at peak fields of 20 mT (Fig. F21). 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 polarity stratigraphy to be pieced together. The polarity interpretation (Fig. F21), based on a combination of polarity zone pattern fit to the GPTS and biostratigraphic constraints, should be considered tentative and provisional in some intervals, although it is consistent with site biostratigraphy, such as the top of D. hamatus (9.69 Ma) at 221.8 m CCSF-A and the top of S. heteromorphus (13.53 Ma) at 362.5 m CCSF-A (see "Biostratigraphy"). Our conclusion from this site is that the FlexIt orientation data is generally reliable. Sedimentation rates based on the polarity interpretation indicate increasing sedimentation rates downcore from ~14 m/m.y. at the top to ~30 m/m.y. at the base of the section (Fig. F22). It appears from this figure that the depth intervals of unresolved magnetic polarity with reduced remanent intensity are associated with increased sedimentation rates, implying that increased burial of organic matter and enhanced sedimentation rates may have promoted reductive dissolution of magnetite, although these intervals accompany no remarkable increase in total organic carbon content (see "Geochemistry").

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