Magnetic susceptibility (χ) was measured at low (0.47 kHz; χlf)
and high (4.7 kHz; χhf) frequencies by using a Bartington MS2 dual
frequency sensor. The χ value is proportional to the concentration
of the ferrimagnetic minerals (magnetite and maghemite) in the sample,
and is also sensitive to the magnetic grain size (Dearing, 1999).
The percentage of frequency dependence (χfd %) was calculated by
the following equation: