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doi:10.2204/iodp.proc.308.108.2006

Paleomagnetism

Archive halves were measured in the pass-through cryogenic magnetometer to a peak alternating-field (AF) demagnetization of 20 mT (see Table T9). APC coring was used from Cores 308-U1324B-1H to 50H; Cores 4H to 50H were oriented using the Tensor tool. The XCB drilling system was deployed (see “Summary of operations”) beginning with Core 308-U1324B-51X. The special geotechnical cutting shoe from Fugro Inc. was deployed in some cores (Table T9), which resulted in a considerable magnetic overprint (see discussion below).

Paleomagnetic intervals

Hole U1324B was divided into five intervals based on paleomagnetic measurements (Fig. F24).

Interval 1 (0–260 mbsf)

Paleomagnetic Interval 1 includes lithostratigraphic Subunits IA–IE. Natural remanent magnetization after 20 mT AF demagnetization (NRM_20mT) in this interval averages 0.006 mA/m (Fig. F24A). This interval can be separated into two paleomagnetic subintervals (1A and 1B) corresponding to lithostratigraphic Subunits IA–IC and ID–IE, respectively. In paleomagnetic Subinterval 1A (0–110 mbsf) NRM_20mT has a lower mean value (0.004 mA/m) than in Subinterval 1B (~0.008 mA/m).

Declination in paleomagnetic Interval 1 is scattered between 0° and 360°. The average declination in this interval changes from 90° at 0 mbsf to 270° at 260 mbsf (Fig. F24B). In general, the declination signal is very noisy and therefore unreliable. Inclination in paleomagnetic Interval 1 shows an average normal polarity of 20° (Fig. F24C). The inclination signal exhibits the same level of noise as the declination data and is therefore not considered reliable in this interval. Two trends can be observed; inclination directions decrease from 60° to –30° between 0 and 180 mbsf and revert to ~30° at 180 to 260 mbsf.

Interval 2 (260–365 mbsf)

Paleomagnetic Interval 2 spans lithostratigraphic Subunits IF–IG (260–365 mbsf) and has NRM_20mT intensity values ~2.5 times greater than those in Interval 1 (0.015 mA/m) (Fig. F24A). The average declination of 330° is comparatively stable in Interval 2 (Fig. F24B). The same stable trend is observed in inclination, showing directions with an average value of 30° (Fig. F24C).

Interval 3 (365–440 mbsf)

NRM_20mT values in paleomagnetic Interval 3 (corresponding to lithostratigraphic Subunit IIA) average 0.015 mA/m and are comparable to those of Interval 2 (Fig. F24A). However, the noise level of NRM_20mT in Interval 3 is higher than in Interval 2. This may be due to changes in lithology (see “Lithostratigraphy”) from mud and clay-rich sediments in lithostratigraphic Unit I to sand-silt interbedded facies in lithostratigraphic Unit II. Declination across Interval 2 to Interval 3 shows a sudden change from 330° to 50° (Fig. F24B), which clearly coincides with the change from APC to XCB coring. The Tensor tool is not deployed with XCB and hence declination data below Core 308-U1324B-51X could not be corrected. The inclination signal shows a relatively stable normal polarity of 40° throughout Interval 3 (Fig. F24C).

Interval 4 (440–490 mbsf)

Paleomagnetic Interval 4 includes the highest variations of NRM_20mT in Hole U1324B. The upper part of Interval 4 (440–460 mbsf) reaches a maximum of 0.03 mA/m, which is 10 times that in the lower part of the interval (460–490 mbsf; NRM_20mT = ~0.003 mA/m). This feature can be correlated with the sandy MTD of lithostratigraphic Subunit IIB. Declination in Interval 4 has a relatively stable average direction of ~50° (Fig. F24B). Inclination values have stable normal polarities of 65° throughout Interval 4 (Fig. F24C).

Interval 5 (490–608 mbsf)

Paleomagnetic Interval 5 has an average NRM_20mT intensity of 0.01 mA/m (Fig. F24A). This interval corresponds to lithostratigraphic Subunits IIC and IID. The noise level in the NRM_20mT signal is comparable to that in Interval 3. Declination shows relatively stable directions, slightly higher than 50°, and thus is similar to those in Interval 4 (Fig. F24B). Inclination values have stable normal polarities, very similar to those of Interval 4 (Fig. F24).

The Mono Lake Excursion, Lake Mungo Event, and Lachamps Excursion (Clark and Kennett, 1973; Freed and Healy, 1974; Stupavsky and Gravenor, 1984; Flood, Piper, Klaus, et al., 1995; Cisowski and Hall, 1997) were not identified in Hole U1324B.

Observed deviations of inclination and declination data in Hole U1324B often correspond to cores taken with the geotechnical cutting shoe from Fugro Inc. (applied during APC coring), which is magnetic (for more details see “Summary of operations”) (Table T9; Fig. F24). The shoe caused a considerable magnetic overprint on the recovered sediments, resulting in inclination and declination outliers. Hence, data points from cores recovered using the Fugro cutting shoe are plotted in red in Figure F24 and not included in the data analysis. The drilling overprint in inclination and declination was not removed by the applied peak field of 20 mT.

Magnetostratigraphy

In general, the major trends in the NRM_20mT intensity correlate with magnetic volume susceptibility (κ), although detailed features are often expressed differently between the two records (Fig. F25). As κ is a measure of the bulk sediment, large amounts of paramagnetic clay minerals tend to increase the values, whereas diamagnetic components, such as quartz, dilute the signal of the ferrimagnetic mineral fraction. In comparison, the NRM_20mT intensity is carried exclusively by the ferrimagnetic particle fraction of the sediment. For example, the double peak occurring in NRM_20mT between 140 and 180 mbsf is dampened in the κ signal. Therefore, magnetostratigraphic tie points (MTP1, MTP2, MTP3, and MTP4; for details see “Paleomagnetism” in the “Methods” chapter) are primarily identified using the NRM_20mT intensity peaks.

The preliminary and tentative magnetostratigraphic interpretation in Hole U1324B is achieved by correlation of the identified magnetostratigraphic tie points to MIS 1.1–3.3 (Fig. F25A, F25B, F25C, F25D) (Bassinot et al., 1994). Comparison of the κ signal of Hole U1324B with the Subtropical South Atlantic susceptibility (SUSAS) stack (von Dobeneck and Schmieder, 1999) (Fig. F25A) also supports our tentative correlation. MTP1 and MTP2 are primarily defined by NRM_20mT intensity but are not clearly identified in κ. MTP3 and MTP4 are identified in NRM_20mT intensity as well as κ.

MTP4 lies within an MTD belonging to lithostratigraphic Subunit IIB. Because this deposit is composed of interbedded sandy and silty layers (see “Lithostratigraphy”), the ferromagnetic fraction in this interval may be concentrated at ~450 mbsf and diluted at ~470 mbsf. Average values as extreme as those in paleomagnetic Interval 4 are not observed elsewhere in Hole U1324B, nor are they observed in sandy layers or other identified MTDs. Within the possible time frame estimated by biostratigraphic analysis in Hole U1324B (see “Biostratigraphy”) magnetostratigraphic tie point MTP4 is thought to coincide with MIS 3.3. This is also consistent with the age of the base of the underlying Blue Unit to MIS 4 (Winker and Shipp, 2002). MTP3 and MTP4 are the only two tie points that are clearly expressed in both magnetic parameters and were therefore included in the age model (see Fig. F22; Table T10).

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