Archive halves of rotary-cored sections recovered at Sites 1100 and 1103 were measured at 5-cm intervals. Only a few small rocks were recovered at Site 1102; therefore, no sections were measured. The sensor velocity on the magnetometer was set at 10 cm/s to avoid saturation of the magnetometer electronics. This saturation results from high-amplitude variations of the magnetic flux induced by the rapid motion of highly magnetized material. Sections of cores containing pebbles and cobbles (dropstones or rocks from glacial units) were not measured because their magnetization is unrelated to the ambient geomagnetic field at the time of deposition and cannot be used to construct a magnetostratigraphic record. In addition, many dropstones have a very high intensity of magnetization, which causes saturation in the cryogenic magnetometer electronics.

Measurement of discrete samples from the working halves of cores and subsequent data analysis followed the methods described in "Paleomagnetism"  in the "Site 1095" chapter. Stepwise alternating field (AF) demagnetization of samples revealed that the drill-string overprint was dominantly vertically down, showing the behavior observed at all previous sites during Leg 178. The drill-string overprint was mostly or wholly removed by partial AF demagnetization of the natural remanent magnetization (NRM) at the 10-mT level. Samples from intervals identified as diamictites (see "Lithostratigraphy") had unstable directions of magnetization (Fig. F13A). After removal of the drill-string overprint, the intensity of the signal was within the noise level of the magnetometer, and no characteristic remanent magnetization could be identified (Fig. F13B, F13C, F13D). These observations, combined with the extremely high values of magnetic susceptibility (see "Physical Properties" and "Downhole Measurements"), are consistent with a magnetic mineral assemblage dominated by coarse-grained magnetite, termed "multidomain," which is not an effective geomagnetic field recorder (see "Paleomagnetism" in the "Site 1097" chapter).

The diamictites at Site 1103 were unsorted, and the possibility exists that finer grained magnetic minerals are present. To examine whether the signals from effective (1- to 10-µm magnetite) and ineffective magnetic recorders (>10-µm magnetite) could be separated, low-temperature demagnetization (LTD) was employed to erase signals carried by multidomain grains. LTD uses the crystal structure transition that occurs in magnetite at 110-120 K, termed the "Verwey transition." At this crystallographic transition, the magnetocrystalline anisotropy goes to zero, which allows a reordering of magnetic moments within domains and domain walls. Cooling through this transition and subsequent warming is performed in zero magnetic field, which eliminates even the slightest magnetic bias. Domain walls, originally formed in response to the mineral's previous thermal history (thermal remanence) and exposure to the Earth's magnetic field through time (viscous remanence), reform in such a way that the vector sum of the magnetic moments is near zero. Fine-grained magnetite (0.03-10 µm) is unaffected by LTD because the shape anisotropy of magnetite is sufficient to keep the magnetic moments locked to the long axis of the mineral grain during the Verwey transition. By using LTD to remove the signal carried by multidomain grains, the residual signal can be examined more closely.

Three pairs of discrete samples were collected from Site 1103 for LTD. In each pair, both samples were taken from the same 2-cm interval, when possible. Two pairs were taken from diamictites, one pair from Core 178-1103-27R and one from Core 178-1103-28R, and the third pair from a finer grained interval in Core 178-1103-28R. One member of each pair was subjected to the regular AF demagnetization. The second member of each pair was measured at the 0 mT (NRM) level and was then subjected to LTD. The LTD samples were placed inside a triple-layer mu-metal shield, which reduces the intensity of the ambient laboratory field by several orders of magnitude. The samples were covered in liquid nitrogen (77 K) and then thermally re-equilibrated to room temperature for 1 hr while remaining inside the mu-metal shield. The samples were remeasured at the 0-mT step and then subjected to stepwise AF demagnetization. The sample from Core 178-1103-27R lost 75% of its signal as a result of LTD (Fig. F14A), which suggests that multidomain minerals dominate the magnetic mineral assemblage. The residual signal of the sample was very weak and dropped into the noise level of the magnetometer at the 10-mT demagnetization level. The measurable signal was oriented vertically downward, the direction of the drill-string overprint (Fig. F14B). In addition, the positive inclination measured in Sample 178-1103A-27R-1, 14 cm, is the opposite of the reversed polarity measured in this same interval in the archive half of the core. The second pair of samples from a diamictite (Section 178-1103A-28R-4) had stable directions during regular AF demagnetization and LTD. The sample from this pair lost only 30% of its signal during LTD.

The sample from the finer grained lithology lost 60% of its intensity after LTD, the same percentage of signal that its twin lost at the 10-mT demagnetization level. In this instance, LTD removed the drill-string overprint. The finer grained sample retained a strong, stable remanence after LTD, and the direction had a steep negative inclination. These observations suggest that effective geomagnetic field recorders are not consistently present in the diamictites, and the polarities measured in that lithology are not trustworthy.

One goal of the leg was to date glacial events using one of several dating methods. Unfortunately, the intermittent nature of deposition on the continental shelf, the inability of coarse-grained magnetic minerals (common in diamictites) to record the paleomagnetic field, and sparse recovery together prohibit any reasonable age assignment from magnetostratigraphic constraints alone. First, the deposition of diamictites and turbidites, the two main lithologies (see "Lithostratigraphy"), is discontinuous. Diamictite units are probably deposited in a very short time with no clear mechanism for acquisition of a depositional remanence. Turbidites also are rapidly deposited, and some have given shallower than expected remanence directions at other Leg 178 sites (see "Paleomagnetism" in the "Palmer Deep [Sites 1098 and 1099]" chapter). A hiatus between each change in lithology is possible. A more severe interpretation limitation comes from the large gaps in recovery, even within the interval with highest recovery (247-356 mbsf). Within this interval, the gaps are one to three times the size of the recovered intervals. A smaller problem is that the position of the recovered core is not known within the cored interval.

Finally, the magnetic minerals in the core have been shown to be ineffective paleomagnetic field recorders. For example, Core 178-1100C-1R is from the upper 10 m of the hole but has a reversed polarity magnetization at the 30-mT demagnetization level, contrary to the expected Brunhes normal polarity (Fig. F15). For Hole 1103A, AF demagnetization and LTD experiments show that no stable component is present in the diamictites other than a drill-string overprint. In addition, logging data from the GHMT show that the induced magnetization dominates the signal in Hole 1103A, which is consistent with a coarse-grained magnetic mineral assemblage (see "Downhole Measurements"). The GHMT signal in Hole 1103A contrasts strongly with that seen in Hole 1095B, where the induced component was negligible and the polarity signal clear.

Finer grained lithologies (silty clays, rather than diamictites) at Site 1103 gave stable paleomagnetic results (Fig. F16). However, given the above uncertainties and that the finer grained lithologies comprise only ~15 m of the recovery from Hole 1103A, a magnetostratigraphic interpretation would be unfounded. Cores 178-1103-27R through 37R may provide a sequence of two normal and two reversed polarities (Figs. F17, F18), but this interpretation ignores the complications discussed above. Furthermore, even with a short sequence of reversals in an undated stratigraphic section with unknown sedimentation rates, a unique interpretation is impossible.