Why magnetite is not the only indicator of past rainfall in the Chinese Loess Plateau

This study investigates the magnetic mineralogy of palaeosol S5 from Xifeng (XF), Linyou (LY) and Baoji (BJ) sections with increasing annual precipitation from north to the south on the Chinese Loess Plateau. Palaeosol S5 samples from these three localities are further prepared as magnetic extracts and separation residues. Low-temperature magnetic measurements including field cooled and zero field cooled (FC/ZFC) remanence, in-phase magnetic susceptibility, thermal remanent magnetization and room temperature saturation isothermal remanence magnetization (RTSIRM), with X-ray diffraction measurements are carried out for all magnetic extracts and separation residues samples. The asymmetric rounded 'hump' in cooling curves on RTSIRM and the 'tilted' Verwey transition on ZFC/FC curves suggest that partially oxidized magnetite is the dominant magnetic contributor, not pure maghemite or magnetite. Furthermore, The Verwey transitions on cooling curves slightly decrease and the increased slope of 'tilted' Verwey transition on ZFC remanence curves show that the degree of oxidation of magnetite between localities increases in the order XF-LY-BJ. Hard isothermal remanent magnetization, X-ray diffraction data and the difference of magnetization inwarming curves of RTSIRM suggest that both hematite concentration in magnetic extracts and goethite concentration in separation residues increase from XF to BJ. Frequency-dependent susceptibility and ZFC/FC curves show that BJS5 layer formed under high palaeoprecipitation has less superparamagnetic (SP) but more single domain to pseudo-single domain particles, because SP maghemite was dissolved and transformed into goethite by temporary waterlogging. The increase in hematite concentration is interpreted as due to SP maghemite oxidation or original goethite dehydration within dry soil environment. Therefore, transformation of maghemite to goethite in waterlogged phases of the S5 palaeosol led to the loss of magnetization.