Low-field magnetic susceptibility was measured using a Kappabridge KLY-2 susceptibility bridge operating at an alternating-current (AC) field of 300 A/m at a frequency of 920 Hz. Magnetic susceptibility is a measure of the concentration of paramagnetic (e.g., clays and pyrite), diamagnetic (e.g., quartz and carbonate), and ferromagnetic (which includes ferrimagnetic minerals such as magnetite and maghemite and antiferrimagnetic minerals such as hematite) minerals in the sediment. Ferri- and paramagnetic minerals can contribute strongly to low-field susceptibility. Antiferrimagnetic minerals contribute weakly; diamagnetic minerals contribute weakly negative values and are, in most cases, negligible. A more thorough description of these and the following methods can be found in Thompson and Oldfield (1986) and Dunlop and Özdemir (1997).
Anhysteretic remanent magnetization (ARM) is a measure of the concentration of small single-domain to pseudo-single domain magnetic grains that have a size range between 0.05 and 1 µm. ARM was applied using a Dtech D-2000 alternating-field (AF) demagnetizer fitted with a DC magnetizing coil. The samples were magnetized with a 50-µT DC field superimposed on an AF field decaying from 100 to 0 mT. ARM was then measured on a 2-G three-axis cryogenic magnetometer. Predominantly strong ferrimagnetic grains such as magnetite and maghemite, with grain sizes between 0.05 and 1 µm, contribute to ARM.
Hysteresis loops were measured for every other sample on a Princeton Instruments vibrating sample magnetometer. The sample is vibrated in a magnetic field that is swept from 0 to +1 to -1 T then back to 0 T; the magnetization of the sample is picked up by a set of coils that are wound to cancel the sweeping DC field. Parameters extracted from the loop are used to determine the concentration and domain state, which is directly related to grain size of ferrimagnetic minerals. High-field magnetic susceptibility is also calculated from the hysteresis loop and is contributed by para- and diamagnetic minerals.