Reconstruction of superparamagnetic particle grain size distribution from Romanian loess using frequency dependent magnetic susceptibility and temperature dependent Mössbauer spectroscopy

Previous studies have shown that the magnetic parameters of the Romanian loess/paleosol sequences have recorded Quaternary glacial/interglacial cycles over the last 600 kyr. Previous rock magnetic investigations pointed out that the magnetic enhancement in Romanian loess sequences is mainly due to pedogenic magnetite close to superparamagnetic/stable single domain boundary. We report the first reconstruction of grain size distribution (GSD) of the superparamagnetic (SP) particles for two Romanian loess deposits, Costineşti and Mircea Vodă, located in the Dobrogea Plateau between the Danube River and the Black Sea. The GSDs were obtained using both the wide-band frequency spectrum of magnetic susceptibility (FSMS) method and low temperature Mössbauer spectroscopy. The FSMS method shows that the SP particles are present both in loess and paleosols. The GSDs in loess are shifted to slightly higher diameters with respect to paleosols. The concentration of SP particles has an opposite trend. The largest value is reached in the forest paleosol and the lowest in the loess samples. Both loess and paleosols indicated almost the same dominant grain size (about 17 nm in paleosols and 18 nm in loess samples). However the GSD shapes in loess are different than those found in paleosols. In the paleosols, the shape of GSDs seems to be independent of climate and degree of pedogenesis. These factors control only the concentration of SP particles present in each paleosol. Temperature dependent Mössbauer spectroscopy also supports the presence of SP particles both in loess and paleosol with a mean diameter around 13-14 nm. Dispersion of the diameters in the case of Mössbauer spectroscopy varies from sample to sample being probably affected by random surface defects.