MAGNETIC PROPERTIES

All samples were subjected to stepwise IRM acquisition and AF demagnetization of ARM and IRM (Lowrie-Fuller test, in Lowrie and Fuller, 1971). The present sediments can be divided into four types based on the characteristics of the magnetic properties as follows (Fig. 3):

Type 1 is characterized by a steep initial increase in the IRM acquisition curves and a lower coercivity of IRM than ARM.

Type 2 shows a gradual rise in the IRM acquisition curves and a higher coercivity of IRM than ARM.

Type 3 displays a relatively steep initial increase in the IRM acquisition curves and a higher coercivity of IRM than ARM.

Type 4 is defined by a steep initial increase in the IRM acquisition curves and similar AF demagnetization curves for IRM and ARM.

The stratigraphic distribution of these types and the remanent intensity of magnetization (natural remanent magnetization [NRM], ARM, and IRM) are shown in Figure 4. Type 1 occurs above 2 mbsf and is characterized by high remanent intensity and stable normal polarity of magnetization. Type 2 is observed between 2 and 12 mbsf. Low remanent intensity of magnetization prevails in this interval. Types 3 and 4 are encountered repeatedly below 12 mbsf, and the intensity of remanent magnetization changes with them. The intensities of remanent magnetization within Type 4 tend to be higher than within Type 3.

IRM is saturated before the magnetic field reaches 0.4 T, except in Type 2 sediment. Magnetic remanent intensities of the samples placed in a magnetic field that reached 2 T were considered the saturation isothermal remanent magnetization (SIRM). IRM was also examined after application of a reversed field of 0.3 T after measuring SIRM. The S-ratio (Thompson and Oldfield, 1986) and the ratio of ARM to SIRM were calculated and are shown in Figure 4. The S-ratio reflects variation in the coercivity spectrum of the magnetic minerals (Thompson and Oldfield, 1986), and the ratio of ARM to SIRM primarily mirrors the presence of finer grained magnetic grains (King et al., 1982). Samples within Type 2 sediments showed a low S-ratio (although not saturated enough). A high S-ratio is observed within Type 4 sediments.