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doi:10.2204/iodp.proc.308.103.2006

Paleomagnetism

Archive halves from Hole U1319A were measured on the pass-through cryogenic magnetometer using a peak alternating-field (AF) demagnetization field of 30 mT (Table T8). Cores 308-U1319A-1H to 13H were cored with the APC and Cores 3H through 13H were oriented using the Tensor tool. Cores 308-U1319A-14X through 18X were cored using the XCB and were not oriented (see “Operations”). XCB drilling is less suitable for paleomagnetic studies because of drill string overprints and core disturbance. The peak field of 30 mT is assumed to have removed the viscous magnetic overprint induced by the drill string (Flood, Piper, Klaus, et al., 1995; Shipboard Scientific Party, 1996).

Paleomagnetic intervals

Hole U1319A was divided into three paleomagnetic intervals.

Interval 1 (0–37 mbsf)

Natural remanent magnetization after AF demagnetization at 30 mT (NRM_30mT) intensities in paleomagnetic Interval 1 range between ~0 and 0.015 mA/m (Fig. F23A). Between 0 and 16 mbsf, NRM_30mT values are slightly higher than those between 17 and 37 mbsf. Declination ranges between 30° and 150° throughout Interval 1 except between 5 and 15 mbsf (Core 308-U1319A-2H), where declination values center around 250°. This peak is unreliable because the Tensor tool correction could not be applied for Core 308-U1319A-2H (Fig. F23B). The inclination shows normal polarity within the Brunhes Chron throughout all of Interval 1 with the exception of spurious outlying data points (e.g., at ~15 mbsf), which are interpreted as noise (Fig. F23C).

Interval 2 (37–47 mbsf)

Paleomagnetic Interval 2 has average NRM_30mT intensity values ranging between 0.005 and 0.01 mA/m (Fig. F23A). NRM_30mT intensity in Interval 2 corresponds (see “Lithostratigraphy”) to reddish clay layers (Fig. F24A), suggesting enhanced iron oxide precipitation in this interval. These layers correspond to declination of ~120° (Fig. F23B). The inclination shows normal polarity within the Brunhes Chron throughout Interval 2 (Fig. F23C).

Interval 3 (47 mbsf–TD)

NRM_30mT values in paleomagnetic Interval 3 average 0.005 mA/m with a maximum value around 0.01 mA/m (Fig. F23A). Magnetic susceptibility values in Interval 3 (Fig. F25C, F25D) correspond to an interval with increased iron oxide content within reddish clay layers and silt laminae (see “Lithostratigraphy;” Fig. F24B). Declination directions generally range between 30° and 150°, similar to Interval 1 but with considerably more scatter (Fig. F24B). The inclination record of Interval 3 shows random orientations, which might be related to possible postdepositional iron oxide formation or a generally disturbed signal in this part of the core. These values are therefore not shown in Figure F23C and are not discussed further.

The Lake Mungo and Blake events or the Laschamps Excursion (Freed and Healy, 1974; Stupavsky and Gravenor, 1984; Flood, Piper, Klaus, et al., 1995; Cisowski and Hall, 1997) could not be identified in Hole U1319A.

Magnetostratigraphy

In general, the NRM_30mT intensity signal correlates with the magnetic volume susceptibility and magnetic point susceptibility (acquired using the magnetic susceptibility core logger [MSCL] on the multisensor track [MST] and MS2F sensors, respectively) as well as with bulk density (Fig. F25). Magnetostratigraphic tie points (MTPs) are identified using the multiparameter correlation described in “Paleomagnetism” in the “Methods” chapter. We recognized MTP1–MTP8 for Interval 1 and MTP9–MTP12 for Intervals 2 and 3 (Table T9).

Preliminary magnetostratigraphic interpretation for Interval 1 in Hole U1319A was achieved by correlating the magnetostratigraphic tie points to MIS 3.1–5.5 (Bassinot et al., 1994). In addition, ash Layer Y8 at 22.7 mbsf (see “Lithostratigraphy”) supports the correlation of MTP4–MTP8. The magnetostratigraphic tie points were included in the preliminary age model (see Fig. F22) and constrain the upper 30 mbsf (lithostratigraphic Units I–V) of the sediment column to an age range of 0 to ~122 ka (MIS 5.5 to recent). As a first approach, individual peaks of NRM_30mT intensity, magnetic susceptibility, and bulk density of the lowermost part of Hole U1319A (from ~30 mbsf downhole) were matched with MTP9–MTP12, the latter probably belonging to MIS 6 (Fig. F25). This lowermost depth interval is entirely within lithostratigraphic Unit VI.

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