Quantification of magnetic nanoparticles with broad-band-frequency magnetic susceptibility measurements_ a case study of an upper loess/palaeosol succession at Luochuan, Chinese Loess Plateau

Broad-band magnetic susceptibility (MS) measurement, a novel magnetic method capable of quantifying a narrow grain size distribution (GSD) of superparamagnetic (SP) particles by measuring low-field MS at a number of frequency steps spanning four orders of magnitude, has been tested in a loess/palaeosol section at Luochuan in the Chinese Loess Plateau. The studied succession consists of sequences from the latest palaeosol unit (S0) to the upper part of the loess unit (L2), spanning the last glacial-interglacial cycle. Reconstructed GSDs consist of volume fractions on the order of 10(-24) m(3), and the mean GSDs are modal but with distinctive skewness among the loess, the weakly developed palaeosols (weak palaeosols), and the mature palaeosols. This indicates that the mean volume of SP particles in this loess/palaeosol sequence tends to increase during the transition from loess -> weak palaeosol -> palaeosol, an indication of grain growth as pedogenesis progresses. Total frequency dependence, or TFD(per cent), the difference between chi(130) at the lowest (130 Hz) and chi(500k) at the highest (500 kHz) frequencies normalized to chi(130), is judged to be a more suitable index than previous frequency dependence parameters for the concentration of SP particles. TFD(per cent) has a strong correlation with chi(130), showing a continuous 'growth curve' with the rate of increase being highest for the loess, moderate for the weak palaeosols, and saturated for the palaeosols. The characteristic curve suggests that smaller SP particles are preferentially formed in the earlier stage of pedogenesis rather than the later phase when even larger particles are formed in mature palaeosols. These results demonstrate that the broad-band MS measurement method will be useful for the quantitative assessment of magnetic nanoparticles in soils and sediments.