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doi:10.2204/iodp.proc.308.106.2006

Paleomagnetism

All archive-half cores of Hole U1322B were measured to a peak alternating-field (AF) demagnetization of 20 mT using the pass-through cryogenic magnetometer (Table T11). The APC coring system was used from Cores 308-U1322B-1H to 29H; Cores 6H to 29H were oriented using the Tensor tool (see “Summary of operations”). Hole U1322B was the first hole of the expedition where a special geotechnical cutting shoe from Fugro Inc. was deployed (Table T11). As this shoe is magnetic, its use resulted in a considerable overprint in the cores where it was deployed (see discussion below).

Paleomagnetic intervals

Based on paleomagnetic measurements, Hole U1322B was divided into three paleomagnetic intervals. Unlike at previous sites, the paleomagnetic intervals in Hole U1322B were defined by the magnetic susceptibility (κ) record rather than natural remanent magnetization (NRM_20mT) because of the better correlation of κ between Sites U1324 and U1322. κ and NRM_20mT are weakly correlated in Hole U1322B. Paleomagnetic Interval 1 is characterized by a significant κ peak recognized between 0 and 37 mbsf. Paleomagnetic Interval 2, between 37 and 125 mbsf, is characterized by a steady increase in κ, whereas NRM_20mT intensity shows an interval of very low values. Lithostratigraphic Unit II corresponds to the top of paleomagnetic Interval 3, ranging from 125 to 234 mbsf. This interval indicates higher signal variance around a generally higher mean value. The paleomagnetic intervals are described in more detail below.

Interval 1 (0–37 mbsf)

Paleomagnetic Interval 1 lies within lithostratigraphic Subunit IA (see “Lithostratigraphy”). NRM_20mT averages ~0.006 mA/m, with lower intensities of 0.002 mA/m toward the bottom of the interval (Fig. F24A). κ ranges between 0 and 18 × 10^–5 SI, steadily increasing toward the bottom of Interval 1 (Fig. F24B). Declination has a relatively stable average direction of 160° (Fig. F24C). The mean inclination shows directions of ~55° between 24 and 33 mbsf (Fig. F24D). However, Core 308-U1322E-4H shows deviating inclination directions of up to –35°, coinciding with a clay lithology with minor silt laminae (see “Lithostratigraphy”). This subinterval is labeled “A” in Figure F24D but was drilled using the magnetic Fugro drill shoe.

Interval 2 (37–125 mbsf)

Paleomagnetic Interval 2 includes lithostratigraphic Subunits IB, IC, and ID (see “Lithostratigraphy”). NRM_20mT is characterized by a low mean intensity of ~0.002 mA/m throughout the whole interval (Fig. F24A), with the exception of some background noise. This interval was divided into two subintervals according to trends in κ. Between 37 and 70 mbsf, κ shows an average value of 12 × 10^–5 SI. Starting at 70 mbsf, κ increases toward peak values of 40 × 10^–5 SI at the bottom of the interval (Fig. F24B). NRM_20mT intensity shows no similar trend. Declination in Interval 2 is scattered throughout the range of possible values. In general, declination is noisy in this interval and is unreliable (Fig. F24C). Inclination has slightly shallower values on average than those in Interval 1, at ~50°. There are three exceptions to this: between 62 and 71 mbsf (up to –85°), between 80 and 90 mbsf (up to –68°), and between 116 and 125 mbsf (up to –85°) inclination deviates markedly (these subintervals are labeled B, C, and D, respectively). Peaks B and C correlate with Cores 308-U1322B-8H and 10H, respectively.

The observed deviations in the inclination data in Hole U1322B (A, B, C, and D Intervals 1 and 2) correspond to cores taken with the geotechnical cutting shoe (Fugro Inc.), which is magnetic (see “Summary of operations”). This cutting shoe seems to cause a considerable magnetic overprint on the recovered sediments, and the excursions are therefore considered to be artifacts. Hence, data points from cores recovered using the Fugro cutting shoe were plotted in red and not included in the data analysis. The Fugro cutting shoe was deployed in Cores 308-U1322B-2H, 4H, 8H, 10H, and 14H and every even-numbered core downhole (see also Table T11). Therefore, the inclination deviations in Subintervals 2A–2D are caused by drilling overprint not removed by the applied peak field of 20 mT. Inclinations in Core 308-U1322B-2H show only slightly deviating values, but the overprint effect seems to be present as well. Below Core 308-U1322B-14H (in lithostratigraphic Unit II/paleomagnetic Interval 3), this overprint effect cannot be clearly distinguished from the average inclination direction. This may be due to the increasing consolidation state of the sediments with depth or the generally higher intensity values, and thus stronger remanent magnetizations, in this interval. The latter hypothesis is supported by the fact that the strongest inclination deviations occur within the NRM_20mT interval with the lowest intensity values (Fig. F24A).

Interval 3 (125–234 mbsf)

Interval 3 spans lithostratigraphic Unit II and has relatively high average NRM_20mT intensities compared to Intervals 1 and 2 (average value = 0.02 mA/m, ranging from 0.001 to 0.050 mA/m). Changes in NRM_20mT values are very high and seem to correlate with the frequent changes in lithology (Fig. F25A). Therefore, data from this interval are not used for paleomagnetic studies. Values of κ average 18 × 10^–5 SI (Fig. F25B). κ and NRM_20mT show an even weaker correlation than in Intervals 1 and 2. Declination is less scattered but nevertheless noisy in Interval 3 (Fig. F24C). Some of the declination data seem to be offset from the general trend. Between 125 and 166 mbsf (Cores 308-U1322B-15H through 19H), declination shows decreasing directions from 140° to 10°. Below Core 308-U1322B-20H, the general direction is more or less stable at ~320°. Tensor tool data measured for the first five cores were lost to a data storage error; therefore, the orientation of these cores was not corrected. From Core 308-U1322B-6H to TD it was possible to reconstruct the orientation data from the Tensor tool, but the offsets in the record after correction seem artificial and therefore declination data are unreliable. Inclination is relatively stable (ranging between 30° and 45°) in Interval 3 in contrast to the noisy declination data (Fig. F24D).

Magnetostratigraphy

Site U1322 is characterized by various MTDs and is not appropriate for paleomagnetic studies downhole from 125 mbsf. The uppermost 125 m of Hole U1322B (spanning paleomagnetic Intervals 1 and 2) correlate to the uppermost 160 m of Hole U1324B by its signal shape and the correlation of major seismic reflectors between the two sites (see “Lithostratigraphy”). Magnetic tie points MTP1 and MTP3 are identified in κ data (Fig. F24B), corresponding to MTP1 and MTP3 in Hole U1324B. But these tie points are tentative and solely supported by the magnetic susceptibility record. The NRM_20mT intensity data do not show the same trends at these depth intervals. Therefore, no magnetic tie points were included in the preliminary age model (see Fig. F23).

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