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doi:10.2204/iodp.proc.323.103.2011

Paleomagnetism

The archive halves of all cores recovered at Site U1339 were measured on the three-axis cryogenic magnetometer at 2.5 cm intervals. Natural remanent magnetization (NRM) was measured before (NRM step) and/or after (demagnetization step) stepwise alternating-field (AF) demagnetization in peak fields as high as 20 mT. Core 323-U1339A-1H through Section 4H-4 and Sections 323-U1339B-7H-2 and 7H-3 were measured at NRM step and at 10 and 20 mT demagnetization steps, but other cores from Holes U1339A and U1339B were measured at only 10 and 20 mT demagnetization steps. Cores from Holes U1339C and U1339D were measured at only 20 mT demagnetization step to keep up with core flow.

Paleomagnetic directions after 20 mT AF demagnetization are plotted in Figure F15. The mean inclinations are nearly 70° over the entire depth range of the cores, and the site axial dipole inclination is ~72°, indicating that all sediments are from the Brunhes Chron (0–781,000 y before present [BP]), although an alternative interpretation is also possible (see below). The inclinations show common negative values, but almost all of them are associated with narrow intervals of sediment diagenesis or are a consequence of ash layers. The declinations were reoriented with the FlexIt core orientation tool and also average near 0°, as expected for normal polarity of the Brunhes Chron.

There is a distinctive relationship between sediment intervals of low or negative inclination and NRM intensity. In many cases, the low/negative inclinations noted in Figure F15 are associated with anomalously strong NRM intensities that are often 10 times the NRM intensity values in the surrounding sediments. This relationship is illustrated in Figure F16. The anomalous intervals have a distinctive intensity maximum that is significantly greater than the intensity in surrounding sediments, and they also have anomalously low inclinations in exactly the same intervals. This could be attributed to the authigenic growth of greigite (Fe₃S₄) in the sediments after deposition due to sulfate reduction, methanogenesis (e.g., Blanchet et al., 2009), or the presence of ice-rafted pebbles. Geochemical data indicate that sulfate reduction starts within the uppermost 20 m and methanogenesis is present within the uppermost 50 m.

Further evidence of early diagenetic processes is noted in the overall NRM intensities, which are strong at the core top (~10^–1 A/m²) but diminish to ~10^–3 A/m² near 200 mbsf (Fig. F17). NRM intensities first noticeably drop at 20–25 mbsf, consistent with sulfate reduction. This initial drop is likely associated with magnetic mineral dissolution under anoxic conditions, which diminishes overall NRM intensity. Lower NRM intensity due to selected magnetic mineral dissolution occurs throughout the sediment column below ~20 mbsf; narrow intervals of strong NRM intensity associated with greigite (or ice-rafted pebbles) occur superimposed on this general trend. Figure F17 shows that NRM values diminish below ~20 mbsf, whereas magnetic susceptibility data stay at a somewhat constant value. Near 170 mbsf, NRM intensities diminish notably again. The lower NRM intensities with more common negative inclinations below 170 mbsf may indicate the Brunhes/Matuyama boundary and that the lower intensities are due to incomplete removal of a strong normal polarity overprint from a weaker reversed polarity intensity.

NRM after 20 mT demagnetization was normalized by magnetic susceptibility (Fig. F17) to estimate magnetic field paleointensity in these sediments, although sediment variability and the presence of magnetic mineral dissolution plus intermittent greigite authigenesis make this a problematic task. The examination of paleointensity variations above 170 mbsf indicates no notable features. Relative paleointensity in the uppermost 20 m should provide a reasonable estimate of field variability for the last 100,000 y or so, but these data will be considered later when they can be compared with other independent data sets from IODP Expedition 323.

Finally, no clear magnetic field excursions in these sediments were found. The only exception may be an interval of reversed inclinations and anomalous declinations near 51 mbsf in Holes U1339B and U1339C that are associated with low NRM intensities. This interval is tentatively associated with Excursion 7α, which is ~190,000 y BP (Lund et al., 2006).

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