MOSSBAUER-SPECTROSCOPY ANALYSIS OF IRON-CONTAINING MINERALS IN THE CHINESE LOESS

Chinese loess has been analyzed in bulk, using Mossbauer spectroscopy, in order to investigate the effects of climate on the overall iron-containing mineralogy. Strongly weathered loess is found to differ from weakly weathered loess in two significant respects. It contains a much larger content of fine-grained (superparamagnetic) hematite and the ratio of paramagnetic Fe3+/Fe2+ is substantially increased. The suggestion is that weathering results in the liberation of Fe from silicate minerals and the formation of fine-grained hematite. Measurements of total iron content by atomic absorption spectrometry highlight that the strongly weathered loess has a higher content of iron than the weakly weathered loess, and that this is too large an effect to be explained by the leaching of carbonate. The indication is that the mineralogy of the dust input to the loess plateau varies as a function of climate. One possibility is that the grain size (and hence mineralogy) of the dust fluctuates in response to vegetational cover in proximal source areas. During interglacial times the vegetational cover would expand to stabilize these proximal sources, resulting in a bias to finer dust from more distant sources. Thus the mechanisms linking climate and mineralogy in the Chinese loess may be more complex than previously thought.