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doi:10.2204/iodp.proc.323.104.2011

Paleomagnetism

The archive halves of all APC cores recovered at Site U1340 were measured on the three-axis cryogenic magnetometer at 2.5 cm intervals, except for Section 323-U1340A-34H-4 through Core 323-U1340A-42H, which were measured at 5 cm intervals; cores below Core 323-U1340A-42H were measured at 20 cm intervals. 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-U1340A-1H through 6H were measured at 10 and 20 mT demagnetization steps, and Core 323-U1340A-7H through Section 18H-4 were measured at NRM and 20 mT demagnetization steps. Other cores from Hole U1340A were measured only at 20 mT demagnetization step to keep up with core flow.

Inclination, declination, and NRM intensity after 20 mT AF demagnetization from Hole U1340A are plotted in Figure F22, and inclination and intensity from Holes U1340B–U1340D are plotted in Figure F23. 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 that we interpret 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 Hole U1340A, but the FlexIt corrections are too poor to be of much detailed use in assigning polarity boundaries. The inclinations were averaged by core and section to provide an initial guide to polarity zonation in Hole U1340A (Fig. F24). The Brunhes, Jaramillo, Olduvai, and Gauss normal polarity chrons could be discerned. The age of each chron as well as preliminary defined depths are shown in Table T14. Looking more closely at directional data by core to better estimate actual polarity boundaries and comparing polarity boundaries with paleontological age estimates (see "Biostratigraphy") indicate that these two data sets are largely consistent.

NRM intensity largely remains at the same level throughout most of Hole U1340A. The presence of pyrite downcore clearly indicates that the sediments are undergoing sulfate reduction, which may be causing some limited magnetic mineral dissolution but has not caused any significant NRM intensity loss in Hole U1340A. 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 relative paleointensity estimates (Fig. F25), which were determined by normalizing the cleaned NRM (20 mT) by magnetic susceptibility, questionable in interpretation. The relative paleointensity variability is quite large, but most of it is strongly correlated with NRM and magnetic susceptibility variability and is probably not due mostly to geomagnetic field variability.

As at Site U1339, there is no notable evidence for the presence of magnetic field excursions in any of the cores. Sedimentation rates are sufficient that excursions would be obvious if they existed at this site, although enhanced bioturbation may have masked them. There is no evidence for secular variation that is replicable in Holes U1340A–U1340D in the uppermost ~30 m. Therefore, field variability of some type is preserved in the sediments.