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doi:10.2204/iodp.proc.323.105.2011

Paleomagnetism

The archive halves of all cores recovered at Site U1341 were measured on the three-axis cryogenic magnetometer. All measurements were done at 2.5 cm intervals for APC cores and 20 cm intervals for XCB cores. The natural remanent magnetization (NRM) was measured before (NRM step) and/or after (demagnetization step) stepwise alternating-field (AF) demagnetization in peak fields up to 20 mT. Cores 323-U1341A-1H through 12H and Sections 323-U1341C-5H-3 through 11H-5 were measured at NRM and 20 mT demagnetization steps; other cores from Site U1341 were measured only at 20 mT demagnetization step to keep up with core flow.

The inclination, declination, and intensity after 20 mT AF demagnetization from Holes U1341A, U1341B, and U1341C are plotted in Figures F15, F16, and F17, respectively. The inclinations average nearly 70° over the entire depth range of the cores. The site axial dipole inclination is ~72°. The inclinations show several distinct intervals of reversed inclinations interpreted to be polarity epochs.

The declinations, after correction with the FlexIt tool to orient the declination data with north, suggest that there are multiple polarity intervals in the uppermost 17 cores from Holes U1341A and U1341B. The FlexIt tool appears to show a declination change of ~180° at the Brunhes/Matuyama boundary in Hole U1341A, but it does not seem to identify older polarity changes (Jaramillo onset or termination) or the Brunhes/Matuyama boundary in Hole U1341B.

The inclinations were averaged by core and section to provide an initial guide to polarity zonation in Hole U1341B (Fig. F18). The Brunhes, Jaramillo, and Gauss normal polarity chrons are discernible. The age of each chron and preliminary defined depths are shown in Table T15. Examining directional data by core to better estimate actual polarity boundaries and comparing these data to polarity boundaries with paleontological age estimates indicate that the two data sets are generally consistent.

Although strong throughout all holes, NRM intensities noticeably decrease downhole relative to magnetic susceptibility (Figs. F15, F16, F17). There is a noticeable step in NRM and magnetic susceptibility intensity at ~205 mbsf, and NRM and magnetic susceptibility values remain at the same level deeper in each hole. The presence of pyrite downcore clearly indicates that the sediments are undergoing sulfate reduction, which may be causing some limited magnetic mineral dissolution that has caused the NRM intensity loss in Holes U1341A–U1341C. NRM and magnetic susceptibility intensities vary more than an order of magnitude on a meter scale, probably due mostly to variable flux of detrital sediment versus biogenic sediment (mostly diatoms at this site). The large changes in NRM intensity also appear to be associated with notable detrital (and presumably magnetic) grain size changes. Both of these variations make the use of relative paleointensity estimates as an indicator of field changes, which are determined by normalizing the cleaned NRM (20 mT) by magnetic susceptibility, questionable in interpretation. Our paleointensity estimate from Hole U1341B is shown in Figure F19. The relative paleointensity variability is quite large, but most of it is strongly correlated with NRM and magnetic susceptibility variability. This is the same pattern noted at Site U1340.

As at other sites, there is no notable evidence for the presence of magnetic field excursions in the Brunhes Epoch (last 781,000 y). Sedimentation rates are sufficient that excursions should be obvious if they existed at this site, but enhanced bioturbation may be masking them. However, Brunhes-aged directional (inclination and declination) variability is replicable in Holes U1341A–U1341C and is interpreted to be real geomagnetic field secular variation. Its time frequency of variability is consistent with Site U1340 and other published records of secular variation; furthermore, it does not correlate to visible sediment variability.

Several good magnetic field polarity transitions are present in these cores. Figure F20 shows the Brunhes–Matuyama transition in Holes U1341A–U1341C. The same pattern of inclination variability is present in each hole, and there is also reproducible inclination variability just before the onset of the transition. Additionally, there are good polarity transition records for the Jaramillo onset and termination and perhaps for the Olduvai onset and Gauss termination as well.

Within the Matuyama reversed polarity interval, there is evidence for perhaps a dozen previously identified excursions. Future work will assess whether these excursions can be correlated between holes and with measurements from Hole U1340A. It is clear that there is still some overprint on the data, which prevents a clear picture of the excursions in each hole. However, the relative placement of these directional anomalies relative to paleointensity estimates in the various holes may permit us to tentatively identify them.

Some changes in microfossil assemblages appear to correlate with changes in clastic percentage in the sediment, which can be represented by magnetic susceptibility or relative paleointensity (Fig. F19) variations. Figure F19 shows the change in benthic foraminifer diversity with the relative paleointensity estimate, which appears to represent a strong environmental overprint. Low diversity occurs with low magnetic susceptibility or relative paleointensity values. This may indicate more reducing conditions near the sediment/water interface, which would be less favorable to benthic foraminifers.

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