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doi:10.2204/iodp.proc.308.104.2006

Paleomagnetism

All cores from Hole U1320A were measured on the pass-through cryogenic magnetometer using a peak alternating-field (AF) demagnetization field of 20 mT, with the exception of the unconsolidated sandy intervals between 42 and 56 mbsf (Table T7). The Tensor tool was used to orient Cores 308-U1320A-4H through 9H. The drilling system was switched from APC to XCB beginning with Core 308-U1320A-10X (see “Summary of operations”). Low core recovery in Cores 308-U1320A-8H through 14X as well as 20X and 21X resulted in measurement gaps in natural remanent magnetization intensity after 20 mT demagnetization (NRM_20mT), and in declination, inclination, and magnetic susceptibility (κ) data.

Paleomagnetic intervals

Hole U1320A was divided into three intervals based on paleomagnetic measurements (Fig. F16; Table T8).

Interval 1 (0–70 mbsf)

Paleomagnetic Interval 1 NRM_20mT intensity values range between 0.00 and 0.012 mA/m (Fig. F16A) and are similar to those observed in Hole U1319A. Deviations from the normal polarity trend of the Brunhes Chron were observed in declination and inclination. The abnormal values occur at 35–70 mbsf for declination and 40–60 mbsf for inclination within Cores 308-U1320A-5H through 8H (Fig. F16B, F16C). These remanence direction variations may indicate a polarity excursion corresponding to the geomagnetic Lake Mungo Event (denoted on Fig. F16C) dated at ~30 ka for the Gulf of Mexico (Freed and Healy, 1974; Stupavsky and Gravenor, 1984; Cisowski and Hall, 1997). However, lithostratigraphic observations (see “Lithostratigraphy”) describe the sections containing the excursion as interbedded sand and mud beds. Therefore, we caution that a change in lithology could also explain this change, and that detailed onshore paleomagnetic investigations on discrete samples are needed.

Interval 2 (117–170 mbsf)

Paleomagnetic Interval 2 has a slightly higher mean NRM_20mT intensity value of ~0.013 mA/m, but the values range from 0.002 to 0.030 mA/m. Below 117 mbsf (below lithostratigraphic Subunit IID), uncorrected declination records relatively stable directions of ~50° (Fig. F16B). Inclination data show characteristic normal polarities typical for the Brunhes Chron (Fig. F16C).

Interval 3 (185 mbsf–TD)

NRM_20mT values in paleomagnetic Interval 3 average ~0.01 mA/m. Declination data in this interval (lithostratigraphic Unit V) show relatively stable directions, similar to those in Interval 2 (Fig. F16B). Inclination data show normal polarity directions.

Magnetostratigraphy

NRM_20mT intensity, κ, and bulk density values in Hole U1320A (Fig. F17) are comparable to those observed in Hole U1319A. However, the upper section (lithostratigraphic Units I–IV) is 170 m thick at Site U1320, whereas the equivalent interval at Site U1319 is only 30 m thick. Despite this difference in sedimentation rates, several magnetostratigraphic tie points (MTPs) were correlated using NRM_20mT intensity, κ, and bulk density data (see “Paleomagnetism” in the “Methods” chapter for details). In Hole U1320A, paleomagnetic Interval 1 is comparable to the upper part of paleomagnetic Interval 1 in Hole U1319A. MTP2 and MTP3 were therefore tentatively correlated to MIS 3.3 and 4.2 (Bassinot et al., 1994). Because of poor core recovery between 70 and 117 mbsf (just below Interval 1), the interpretation of these two tie points is uncertain. MTP5–MTP7 in Interval 2 are correlated to MIS 5.2–5.4, based in part on the position of ash Layer Y8 (see “Lithostratigraphy”). The second gap in core recovery between 170 and 185 mbsf makes exact determination of the MIS 5.5 termination and onset of MIS 6 impossible at this site. Within Interval 3 (185 mbsf to TD), several peaks correlate across NRM_20mT intensity, κ, and bulk density data. These are identified as MTP10–MTP12 correlative with the previously identified peaks in Hole U1319A.

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