as a function of depth. The mass susceptibilities are low and range from 5.80 x
10-8 to 1.52 x
10-7 m3/kg. The mean value for the upper interval is 1.08 x
10-7 m3/kg and is slightly lower with 8.45 x
10-8 m3/kg in the lower interval.The low-temperature experiments (Fig. F3) show a rapid loss of magnetization between 10 (20) and 50 K, which is caused by the transition from ferromagnetic to paramagnetic properties of the clay minerals. Samples from the lower interval were cooled to 10 K and show an intensity loss of as much as 65% by the time they have been heated to 20 K. None of the investigated samples shows any of the typical and well-known phase transitions of ferri(o)magnetic minerals.
Hysteresis measurements were only performed on a subset of 20 samples, because the remanent properties of the material were too weak even after significant averaging times (up to 10 s per measurement point on the VSM) to produce measurements of the entire sample set. Eleven of the samples contained sufficient magnetic material to yield hysteresis parameters (Table T2). Figure F4 shows examples of hysteresis loops with a small but detectable ferrimagnetic contribution. Correction for the high-field slope of the loop (Fig. F4B, F4D) yields ferrimagnetic hysteresis curves with parameters that are likely to be magnetite. The coercivity Hc ranges from 3.98 to 10.4 mT (8.1 mT average) and the saturation remanence (Mr) to saturation magnetization (Ms) ratio from 0.042 to 0.144 (0.098 average), which suggests that pseudo-single-domain (PSD)-sized magnetite is probably the dominant magnetic carrier mineral.
The hysteresis loops show also that a significant difference between the high-field and the low-field susceptibility does not exist, which demonstrates that the low-field susceptibility is dominated by the paramagnetic (clay) minerals.
Under the assumption that magnetite is the only mineral contributing to room temperature hysteresis, the amount of magnetite can be calculated from the saturation magnetization Ms extrapolated to zero field. The percentage of magnetite is the Ms of the sample divided by the Ms of pure magnetite (92 Am2/kg) multiplied by 100. The magnetite concentration determined by this method is very low and varies from 5.28 to 5.53 ppm (average = 1.42 ppm) (Table T2).
Acquisition of pARMs was performed on 13 samples (Fig. F5). The shape of the curves in the upper interval (Fig. F5A) varies considerably, but the maximum pARM that can be imparted is centered between 10 and 20 mT (arrow in Fig. F5). This is typical for PSD magnetite with a grain size between 3 and 5 µm (Jackson et al., 1988, 1989). Shallow slopes at higher alternating fields represent sediments with a broader range of coercivities and, hence, grain sizes than steeper curves. A difference between samples from low-susceptibility and high-susceptibility intervals does not exist. Samples from the lower interval (Fig. F5B) display a consistently broader distribution of magnetic grain sizes indicated by the shallow slope of the curves. As in the upper interval, the maximum pARM that can be imparted is between 10 and 20 mT, indicating a magnetic grain size between 3 and 5 µm.
Figure F6
shows the shipboard volume susceptibilities for the upper and lower interval in
addition to the mass susceptibility, ARM, IRM, S-ratio (= IRM - 0.3 T / IRM 1T),
and kARM/
as a function of depth. The mass susceptibilities are low and range from 5.80 x
10-8 to 1.52 x
10-7 m3/kg. The mean value for the upper interval is 1.08 x
10-7 m3/kg and is slightly lower with 8.45 x
10-8 m3/kg in the lower interval.
The ARM is 3.70 µAm2/kg in the upper interval and 2.79 µAm2/kg in the lower interval on average. The average IRM at 1 T is 191.1 µAm2/kg in the upper interval and 184.9 µAm2/kg in the lower interval. The remanent magnetizations show variations that are apparently not correlated to the magnetic susceptibility (Fig. F6). Shaded bars in Figure F6 highlight horizons with high magnetic susceptibility values for comparison. The S-ratio stays relatively constant at 0.89 (0.88) in the upper (lower) interval. The S-ratio indicates the presence of a small high-coercivity phase, which is independent of the susceptibility variation.
The kARM/
ratio is frequently used as a magnetic grain-size indicator (e.g., Thomson and
Oldfield, 1986; Verosub and Roberts, 1995). In both depth intervals, the kARM/c
ratio is negatively correlated with the magnetic susceptibility, which could be
interpreted as an increase in magnetic grain size in intervals of high magnetic
susceptibility (Fig. F6).
We have demonstrated above that the susceptibility measures predominantly the
paramagnetic (clay) fraction, whereas the ARM as a remanence measures the
fine-grained ferrimagnetic (magnetite) fraction. Because the magnetic
susceptibility and the ARM measure different minerals, the interpretation of the
kARM/c
ratio as a magnetic grain size indicator at Site 1075 is not straightforward.
The ARM/IRM ratio is a ratio of two remanent magnetizations and thus is not
influenced by paramagnetic phases. It can be used as a magnetic grain-size
indicator because the ARM is more effective in activating finer magnetite grains
than the IRM. Figure F6
shows that the ratio is smaller on average in the lower interval (mean = 0.016)
than in the upper interval (mean = 0.021), indicating that the lower interval
contains less fine-grained magnetite than the upper interval. A relationship
between the grain size indicator and high- or low-susceptibility horizons is not
apparent.
The XRF-scanner results for K, Ca, Ti, Mn, and Fe for the two depth intervals are displayed together with the magnetic susceptibility on the left panel for reference (Fig. F7; Table T3). The Fe counts are the highest and the Mn counts are the lowest and exhibit the largest scatter. All elemental counts, except for the Ca counts, correlate well with the magnetic susceptibility record. The Ca counts correlate well with the CaCO3 concentration (Fig. F7) but do not show the same cyclicity as the other elements or the magnetic susceptibility. Differences in element counts are <5% between the two intervals for K, Ti, and Fe and 14% for Mn. The Ca counts are significantly higher in the upper interval (average = 150 cps) than in the lower interval (average = 52 cps). The downhole decrease in carbonate concentration is also evident from the data published by Berger et al. (1998a). The CaCO3 concentrations are only a few weight percent and too low to have a significant dilution effect on the magnetic susceptibility signal.
The magnetic susceptibility is an excellent proxy for element concentrations that are driven by climate cycles. The observed cycles reflect climate-driven changes in the sediment composition where warm climates are characterized by high susceptibilities (Berger et al., 1998; Dupont et al., submitted [N1]), and according to the presented data, higher Fe, Ti, Mn, and K concentrations. With the exception of the Ca concentration and the magnetic grain size, no significant differences in the properties of the upper and the lower interval were found.
