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doi:10.2204/iodp.proc.318.109.2011

Paleomagnetism

Paleomagnetic investigations at Site U1361 involved analysis of discrete samples from Hole U1361A and measurement of archive halves from Holes U1361A and U1361B. In general, we took 2–7 samples per core. A subset of these (45 samples) were subjected to step-wise alternating-field demagnetization. After measuring the natural remanent magnetization (NRM) of the archive halves, they were demagnetized at 20 mT. The anisotropy of magnetic susceptibility (AMS) was measured on all discrete samples. We constructed a polarity log from the inclinations of the archive halves and discrete samples and correlated this log to the geomagnetic polarity timescale (GPTS) of Gradstein et al. (2004), documenting a complete section from the top of Chron C2n to the top of Chron C3n. Below Chron C2n the recovered core is disturbed and we have not completed analysis of the discrete samples as of yet. The lower portion of Hole U1361A can plausibly be correlated to the bottom of Chron C5n to C5An.

Analysis of discrete samples

Selected discrete samples were measured and demagnetized in fields of 5, 10, 15, 20, and 30 mT using the three-step protocol described in “Paleomagnetism” in the “Methods” chapter. The data were similar to those acquired at Site U1359 and were analyzed in the same manner.

Analysis of archive halves

We measured the NRM of archive halves from both holes (Fig. F11). Based on the step-wise demagnetization experiments, archive halves were demagnetized to 20 mT. Data after demagnetization to 20 mT and edited for core disturbance are shown in Figure F12 for upper and lower parts of Hole U1361A and Hole U1361B, respectively. In general, there is excellent agreement between the best-fit lines and those from the archive halves. Unfortunately, all cores below Core 318-U1361A-17X are highly disturbed by drilling and no measurements from the archive halves were deemed reliable. Discrete samples were taken from undisturbed portions so that they can provide some guide to the polarity stratigraphy.

Anisotropy of magnetic susceptibility

In addition to remanence analyses, we measured the AMS (including the bulk susceptibility, χ_b) on all discrete samples (Fig. F13). Samples were taken to avoid to the extent possible disturbed coring however a substantial number (those with V₃ inclinations less than about 60° and distinct maximum and intermediate eigenvalues) had characteristic disturbed fabrics.

There are two pronounced zones of very low bulk magnetic susceptibility (Fig. F13C), one at ~190 mbsf, coincident with a zone of low remanent intensity (Fig. F12) and a second at ~300 mbsf. These are apparently the same as the Pliocene and Miocene low-susceptibility zones recorded in cores from Site U1359.

Correlation of magnetostratigraphy to geomagnetic polarity timescale

We constructed a polarity log for Hole U1361A, as shown in Figure F12A and F12B, that can be correlated to the GPTS of Gradstein et al. (2004), shown to the right. Our correlation tie points are listed in Table T7.

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