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doi:10.2204/iodp.proc.344.107.2013

Paleomagnetism

Cores 344-U1413A-1H through 18H were cored with the APC using two nonmagnetic core barrels and were oriented with the FlexIT orientation tool. Cores 344-U1413A-19X through 26X were cored with the XCB using a standard core barrel. Cores 344-U1413B-1H through 3H were cored with the APC without orientation. Cores 344-U1413C-2R through 43R were cored with the RCB. We measured the natural remanent magnetization (NRM) of archive section halves. In order to isolate the characteristic remanent magnetization (ChRM), the archive section halves were demagnetized in an alternating field (AF) up to 40 mT and measured with the pass-through superconducting rock magnetometer (SRM) at 2.5–5 cm intervals. In order to verify the section data, we demagnetized 119 discrete samples using a progressive AF demagnetization technique and measured them in the SRM.

Natural remanent magnetization of cores

Downhole variations of paleomagnetic data observed at Site U1413 are shown in Figures F34, F35, and F36. In Unit I, cores recovered from 0 to 44.6 mbsf are silty clay and sand layers (see “Lithostratigraphy and petrology”). The variations in NRM intensity of these sediments are correlated with lithology. For the depth intervals of 0 to ~4 mbsf and ~20 to ~25 mbsf, these sandy sediments have a mean NRM intensity on the order of ~10^–1 A/m. Sediments between ~5 and 20 mbsf are more silty and turbidite-like and have a lower NRM intensity with a mean of 10^–2 A/m (Fig. F34). This pattern is also confirmed in paleomagnetic observations in Hole U1413B (Fig. F35), serving as a regional marker for correlation of these two holes. Paleomagnetic measurements on the calcareous clayey silt with minor sandstone in Unit II (44.6–366.45 mbsf) give highly variable NRM intensity values. A significant downhole increase in NRM intensity is present between ~42 and 150 mbsf (Fig. F34), probably reflecting the higher content of sand downhole within this interval. For the depth interval of 150 to ~185 mbsf, low and high NRM intensities vary between 3.74 × 10^–4 and 8.71 × 10^–1 A/m, with a mean of 7.23 × 10^–3 A/m. In Hole U1413C, sediments in Units II and III (~178–578.8 mbsf) have fairly constant NRM intensity values (average of 3.61 × 10^–3 A/m; Fig. F36). Variations in magnetic susceptibility are consistent with the variations in NRM intensity (see “Physical properties”).

Similar to what was observed at Site U1412, the magnetic flux jumps along the y-axis superconducting quantum interference device produced large magnetic noise anomalies that affected the measurements of core sections, especially in the silty and turbidite layers of Unit I (see circled data in Fig. F35).

Paleomagnetic demagnetization results

For the sediment sections, we employed AF demagnetization steps up to 40 mT. AF demagnetization to 15 mT seems to be effective in removing the drilling overprint magnetization (e.g., Fig. F34). ChRM directions of both normal and reversed polarities can generally be isolated from the pass-through measurements (Fig. F37).

The magnetic properties observed from the section halves were confirmed by discrete sample measurements (Fig. F38). We demagnetized 119 discrete samples (57 from Hole U1413A, 8 from Hole U1413B, and 53 from Hole U1413C). AF demagnetization was successful in isolating the ChRM for most of the discrete samples. Some discrete samples, however, displayed more complicated demagnetization paths that do not simply decay toward the origin. Several representative vector plots of discrete samples are shown in Figure F38. ChRM inclinations of discrete samples obtained from principal component analysis (Kirschvink, 1980) are plotted in Figure F39.

Magnetostratigraphy

As shown in Figure F39, samples from the lower parts of Unit II and Unit III in Hole U1413C are dominated by negative inclinations (reversed polarity). At ~480 mbsf, the stable inclinations switch polarity from reversed to normal and then change back to reversed polarity at ~520 mbsf. The LO of the nannofossil species Helicosphaera sellii (1.34 Ma) is also placed around this interval (see “Paleontology and biostratigraphy”), suggesting the polarity changes in the depth range of 480–520 mbsf may correspond to the Olduvai Subchron (1.778–1.945 Ma). If this is correct, sediments in this depth interval have an average sedimentation accumulation rate of 239.52 m/m.y., similar to those in Hole U1380C. The sedimentation accumulation rate for the uppermost 480 m of sediment in Hole U1413C is 269.97 m/m.y.

Like the magnetic records at other Expedition 344 sites, several relatively well defined polarity intervals were identified in the downhole magnetostratigraphic records of Site U1413. However, it is difficult to reliably correlate these intervals to the geomagnetic polarity timescale because the biostratigraphic data are lacking in the cored interval. We tentatively place the Jaramillo Subchron (0.988–1.072 Ma) between 0.58 and 94.21 mbsf, which would suggest an extremely high sedimentation rate (824 m/m.y.) for this interval. Additional shore-based work is needed to correctly interpret the record.

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