ANISOTROPY OF ANHYSTERETIC REMANENT MAGNETIZATION

We analyzed samples from Lithostratigraphic Units I, II, and III for anisotropy of anhysteretic remanent magnetization (AARM) at the Institute for Rock Magnetism at the University of Minnesota (USA). We selected 25 samples from Unit I, roughly every 12.5 mbsf, from Cores 191-1179B-1H through 3H, 5H, and 6H and Cores 191-1179C-3H, 4H, 6H–8H, 10H–13H, and 15H–20H. We selected 3 samples from Cores 191-1179C-20H and 21H in Unit II and 9 samples from Cores 191-1179C-22H, 23H, 25X, and 26X in Unit III.

An AF applied with a D-Tech AF demagnetizer on each of the x-, y-, and z-axes cleaned each sample of any remanence softer than 200 mT. Next, the NRM of each sample was measured with a 2G Superconducting Rock Magnetometer (SRM) (Table T2).

An anhysteretic remanent magnetization (ARM) of 100 mT with a superimposed 100-µT direct-current bias was added to the x-axis of the sample with a Schonstedt magnetizer. For the Schonstedt magnetizer, the decay rate of the AF field is 2.5 mT per half-cycle. The sample was measured again with the SRM, and then the x-axis was cleaned with a 200-mT AF applied with the D-Tech AF demagnetizer. The process was repeated for the y- and z-axes (Table T2).

The term anisotropy of anhysteretic remanent magnetization implies the determination of the second-rank tensor defining an ellipsoid. This is done by measuring the NRM and ARM for 15 different sample orientations. Our sample cubes were too large to fit into the available rigs for applying the 100-mT ARM to the 12 orientations not normal to the faces of the sample cubes. Instead, we used the difference between the ARM and the NRM for each axis to calculate a measure of the anisotropy of the ARM:

AARM = {[(ARM_x – NRM_x) + (ARM_y – NRM_y)]/(ARM_z – NRM_z)}.

The results of the AARM calculations in Table T2 are shown in Figure F1. Some of the samples had previously been tested using IRM, and the IRM acquired in the maximum applied field (1 T) was too high to be completely cleaned before the AARM tests.

Sedimentary magnetic records may be affected by inclination shallowing if the weight of the upper layers of sediment causes lower layers to compress and the magnetic particles to rotate to a shallower inclination with the other compressed sediment grains. Figure F1 shows that the core recovery was almost always >100% and that AARM does not correlate with core recovery.

For the Unit I samples, the AARM values fluctuate between 0.871 and 1.129 with no clear depth progression. Both the Unit II and Unit III samples have AARM values >1.0, with the largest values indicating larger possible inclination shallowing, increasing with depth in Unit III.