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The magnetic mineralogy of a loess section near Lanzhou, China

Abstract A collection of almost 500 hand samples from a 250 m loess section near Lanzhou, Gansu province, have been investigated to determine both the nature of the magnetic mineralogical differences between the loess and inter-bedded soils and the time variance of the magnetic mineralogy. Results indicate that the parent magnetic material, which is aeolian in origin, is cation-deficient magnetite/maghaemite and the strong magnetic susceptibility contrast, particularly in the younger material, between loess and soil, is mostly due to pedogenic enhancement of the magnetic content of soils by in situ formation of ultrafine magnetite, although we should not ignore possible changes in the nature of the available aeolian material during the warmer soil-forming periods. Compared to loess sections further to the east, the degree of enhancement in the soils is small, on average having a ferrimagnetic content which is only 17 0reater than that of the loess. Trends in the magnetic mineralogy of the section indicate a possible link with the long-term climate change of Northern China, the average ferrimagnetic grain size increasing with decreasing age in response to increased wind velocity and a colder, drier climate. At a point approximately 110 m above the base there is a dramatic change in the nature and concentration of the magnetic material. This change indicates either a rapid deterioration in the average climate or a sudden change in the provenance of the source material, although at this stage we cannot discount the possibility of some time gap at this position. The loess/palaeosol susceptibility contrast is noticeably reduced below this level, perhaps indicating less extremes of climate.