RESULTS
Low-field magnetic susceptibility has a maximum value near the bottom of the core of 5 x 10-7 m3/kg (Fig. F1). It shows a sharp decrease from the bottom of the core up to 440 mcd. From 440 mcd, it remains relatively constant at 1 x 10-7 m3/kg to 125 mcd, where it increases slightly. Between 10 and 20 mcd, there is a sharp low, with values of ~0.5 x 10-7 m3/kg, then an increase to >2 x 10-7 m3/kg to the core top. High-field magnetic susceptibility remains fairly constant at 1 x 10-7 m3/kg for the entire interval, with only a slight increase between 120 and 75 mcd and a low from 20 to 10 mcd (Fig. F1). By subtracting the high-field susceptibility from low-field susceptibility (
lf - hf), ferromagnetic susceptibility can be determined. Ferromagnetic susceptibility ranges from 2 x 10-7 m3/kg to 4 x 10-7 m3/kg from the bottom of the core to 440 mcd (Fig. F1). Between 440 and 100 mcd, ferromagnetic susceptibility averages 0, indicating the low-field magnetic susceptibility is mainly contributed by paramagnetic minerals. The slight offset to negative values of ferromagnetic susceptibility is due to calibration problems for sample-shape effects in the high-field susceptibility measurements, whereas the low-field measurements are not affected by sample geometry (Jackson et al., 2001). Because the sample sizes varied a bit as a result of dehydration, it was not possible to apply a correction. Above 100 mcd, ferromagnetic susceptibility increases to ~0.5 x 10-7 m3/kg, indicating an increase in ferromagnetic content. However, there is again a sharp drop in ferromagnetic content between 20 and 10 mcd.
Anhysteretic Remanent Magnetization
ARM values reach a maximum of 1.2 x 10-4 Am2/kg in the deepest sediments and decrease to an average value of ~1.8 x 10-6 Am2/kg at 440 mcd (Fig. F1). From 440 to 120 mcd, they remain fairly constant and then increase slightly to 1 x 10-5 Am2/kg between 120 and 20 mcd. Between 20 and 10 mcd, there is a sharp low that correlates to the drop in magnetic susceptibility with ARM values of 1.25 x 10-6 Am2/kg. From 10 to near the core top, ARM increases to 1 x 10-4 Am2/kg, which is similar to the core bottom.
Saturation magnetization (Ms) (Fig. F2) is a function of the concentration and composition of ferromagnetic material in the sediment. Changes in Ms can indicate either a change in the concentration of a single type of mineral or variations in mineralogy because various minerals have different saturation values. Ms is at a maximum of 0.05 Am2/kg near the core bottom and decreases down to ~1 x 10-3 Am2/kg between 440 and ~200 mcd. Between 200 and 40 mcd, there is a gradual increase to values of 0.01 Am2/kg. Between 20 and 10 mcd, Ms spikes sharply to low values of ~6 x 10-4 Am2/kg. Saturation remanence (Mrs), which can also be affected by grain size, generally shows the same trend as Ms (Fig. F2).
Coercivity (Hc) and coercivity remanence (Hcr) (Fig. F3) are affected by the grain size and mineralogy of the ferromagnetic minerals in the sample. The two parameters have, generally, the same trends with lower values (Hc = ~10 mT and Hcr = ~30 mT) from the core bottom to 440 mcd and generally higher values (Hc = ~30 mT and Hcr = ~55 mT), with more variability between 440 and ~100 mcd. From 100 mcd to the core top, there is a background value of ~10 mT for Hc and ~65 mT for Hcr . Three intervals at 90, 75, and 20-10 mcd have Hc spiking to ~32 mT and Hcr to ~70 mcd.
Ratios of hysteresis parameters can be used as an estimation of grain size if the mineralogy of the ferromagnetic fraction remains constant. Generally, Hcr/Hc ratios >4 indicate large multidomain grains (>10 µm), ratios between 2 and 4 indicate pseudo-single domain grains (~1 to 10 µm), and ratios <2 indicate single-domain grains (0.05 to ~1 µm) (Day et al., 1977; Dunlop, 1986). From the bottom of the core to 375 mcd, Hcr/Hc averages 3.5; at 375 mcd, values drop to ~2 and show more variability (Fig. F3). From 200 mcd, there is a slight increase in values, indicating that grain size is increasing. Between 75 and 20 mcd, Hcr/Hc is stable, with an average value of 3.8. Above 20 mcd, the ratio drops to between 2 and 3, indicating finer grains near the surface.
The Mrs/Ms ratio also indicates grain size, with lower values indicating larger grains (Fig. F2). Mrs/Ms ratios show a pattern similar to Hcr/Hc, with coarser grains in the bottom of the core to 420 mcd; between 420 and 75 mcd, grains sizes are smaller with more variability. From 75 to 20 mcd, the ratio again becomes stable and averages 0.1, similar to the bottom. Between 20 and 10 mcd, values increase then drop again near the core top.
The hysteresis parameters indicate that near the bottom and top of the cores the concentration of magnetic material is higher and coarser than in the middle, where the concentration is lower and finer grained. However, it is also possible that the mineralogy of the magnetic material is changing, which could also affect the hysteresis parameters.
