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doi:10.2204/iodp.proc.344.104.2013

Paleomagnetism

Cores 344-U1414A-1H through 22H were cored with the APC and oriented with the FlexIT tool. Cores 344-U1414A-23X through 35X were cored using the XCB. Cores 344-U1414A-36R through 63R were cored with the RCB using a nonmagnetic core barrel. Pass-through magnetometer measurements were made on the entire set of archive-half cores at 5 cm intervals for the sedimentary cores and 2.5 cm intervals for the basement cores. Archive-half cores were subjected to stepwise alternating field (AF) demagnetization up to 40 mT and then measured in the pass-through superconducting rock magnetometer. We demagnetized 55 discrete samples collected from the working halves with AF demagnetization up to 120 mT, with the main objective of recognizing the characteristic remanent magnetization (ChRM).

Natural remanent magnetization of cores

Natural remanent magnetization (NRM) intensity, inclination, and declination were measured at demagnetization steps of 15, 30, and 40 mT. Magnetic properties of recovered sediments from Site U1414 show different coercivity components, which are probably related to consistent variations in the quality and quantity of magnetic minerals. The silty clay and sand of Subunit IA (0–78.3 mbsf; see “Lithostratigraphy and petrology”) have NRM intensities between 1.3 × 10^–3 and 5.1 × 10^–1 A/m, with a mean of 1.5 × 10^–2 A/m. We recognize several peaks of higher NRM intensity values in the upper part of Subunit IA between 0 and ~3 mbsf. The high-intensity peaks were maintained even after 40 mT AF demagnetization (Fig. F44), which means that these high-intensity layers are likely related to changes in magnetic properties instead of being caused by drilling-induced remagnetization. NRM intensity for the calcareous nannofossil–rich clay of Subunit IB (78.30–145.34 mbsf) is similar to that of Subunit IA, with the exception of the upper part, which has a peak in NRM intensity between ~75 and ~85 mbsf. NRM intensity shows a steplike increase across the Subunit IIA/IIB boundary. NRM for Subunit IB ranges from 1.0 × 10^–3 to 1.7 × 10^–1 A/m, with a mean of 7.8 × 10^–3 A/m. Paleomagnetic measurements show that the nannofossil-rich clay and ooze in Subunit IIA (145.34–200.01 mbsf) have NRM intensities ranging from 7.7 × 10^–4 to 1.5 × 10^–2 A/m, with a mean of 3.2 × 10^–3 A/m. In Subunit IIB, cores recovered from 200.01 to 309.37 mbsf are alternating nannofossil-rich calcareous ooze and sponge spicule–rich calcareous ooze. NRM intensities of these sediments vary between 1.5 × 10^–4 and 4.8 × 10^–1 A/m, with a mean of 1.5 × 10^–2 A/m, with a broad peak in NRM intensity between ~200 and ~230 mbsf. Unit III, which is composed of calcareous and siliceous cemented silt and sandstones, shows a minimum NRM intensity value of 1.2 × 10^–5 A/m and a maximum value of 4.8 × 10^–1 A/m, with an average of 6.1 × 10^–3 A/m.

We observe a strong correlation between the variations in magnetic susceptibility and the variations in NRM intensity (see “Physical properties”).

Natural remanent magnetization of basement rocks

The lowermost ~96 m of recovered materials from Hole U1414A consist mainly of basaltic rock. We selected a slower tray speed mode for the pass-through measurements at every 2.5 cm and AF demagnetized core sections to 40 mT. Basement Units 1–3 show NRM intensities ranging from 3.2 × 10^–1 to 18.7 A/m, with an average of 6.3 A/m. NRM intensity shows downhole variation that can be correlated to the alteration states of the rocks. The fresh part of the basalt is characterized by stronger NRM intensity. Both normal and reversed polarities were observed, but no further data analyses could be done because of a lack of time at the end of the expedition.

Magnetic noise in superconducting rock magnetometer

Several flux jumps appeared at various steps and lithologies and occurred in the intervals ~90–100, ~115–125, and ~240–290 mbsf. Major and persistent flux jumps also occurred for most of the 40 mT AF demagnetization steps. Therefore, the data are unreliable in the intervals ~25–110 and ~165–290 mbsf.

Paleomagnetic demagnetization results

The recovered sediment sections were AF demagnetized at steps of 15, 30, and 40 mT. The magnetization of the drilling overprint was usually successfully eliminated by the first AF demagnetization step (15 mT). ChRM directions, because of the flux-jump artifacts, cannot be reliably recognized from the pass-through measurements.

The remanent magnetization of 55 discrete samples from lithostratigraphic Units I–III was investigated using stepwise AF demagnetization. We were able to isolate the ChRM for most of the samples after removal of the drilling-induced overprint (Fig. F45). However, we were not able to demagnetize some samples, which suggests the presence of a stronger magnetic component with a higher coercivity.

Magnetostratigraphy

We were able to correct the declination of the APC cores (0–200 mbsf) to geographic coordinates using the FlexIT orientation data.

We analyzed ChRM inclinations and declinations (for APC cores) from both discrete and pass-through measurements using principal component analysis to define magnetic polarity sequences. We recognize several magnetozones defined as intervals with multiple consecutive samples with polarities that are distinctly different from neighboring intervals.

The upper part of Hole U1414A is characterized by a short interval with normal polarities and then a longer reversed interval followed by three normal intervals (Fig. F44). The Brunhes Chron, if present, is very short or condensed in the uppermost ~10 m. Between 94 and 114 mbsf, four samples with homogeneous characteristics confirm the presence of a normal polarity interval. However, most of the samples from the middle part of Hole U1414A show reversed polarity. The lower part of the hole (below 282 mbsf) includes six samples with normal polarity, separated by only one sample with reversed shallow inclination. Inclination values are very shallow, <20°, which is consistent with the expected paleolatitude for this site.

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