Resumo

RIVAS ORTIZ, Jorge F. et al. Magnetic mineralogy of volcanic soils in a toposequence of the Teotihuacán valley. Bol. Soc. Geol. Mex [online]. 2012, vol.64, n.1, pp.1-20. ISSN 1405-3322.

Numerous studies have demonstrated the relationship between magnetic mineralogy in soil and paleosol sequences and climatic and environmental variations as well as pollution effects affecting them. In order to analyze this relationship, we carried out a study of magnetic properties in five soils and a paleosol, all developed on volcaniclastic deposits and located throughout an altitudinal range of 2250 to 3040 m above sea level and within a toposequence from Teotihuacan valley. These soil profiles consist of two Cambisols, a Calcisol, two Fluvisols, as well as a Vertisol. The magnetic mineralogy was characterized by rock magnetism techniques in order to determine the composition, concentration and domain size distribution of magnetic minerals, and was compared and validated with other non-magnetic parameters. Results reflect a clear difference in magnetic properties which are influenced by environmental conditions for each soil and their degree of pedogenesis. These magnetic properties vary between two extremes: 1) Cambisols, with higher concentrations of magnetic minerals of larger particle size; and 2) Fluvisols with low concentrations of magnetic minerals dominated by fine particle size. The analyzed Calcisol presents variations between these two extremes, for both aspects (concentration and grain sizes), while the Vertisol shows low concentration and coarse sizes. Unlike magnetic susceptibility enhancement reported in loess-paleosol sequences, which is caused by a rise of SD and SP ferri- and antiferrimagnetic minerals, the increase reflected in a Cambisol and Fluvisol from Teotihuacan is caused by the incorporation of new volcanic and fluvial material to the upper part of the soils. By contrast, the higher SP and SD concentrations are located in the lower horizons. These present a rather weak formation of pedogenic magnetic minerals, both ferrimagnetic and antiferrimagnetic. The low intrinsic magnetization of the latter and the relatively low proportion in which they occur make identification difficult because they are masked by ferrimagnetic phases. Our analysis highlights the features of soils developed on volcanic material as well as the potential that the analysis of magnetic properties holds in combination with other pedogenetic indicators for the reconstruction of past environmental conditions.

Palavras-chave : magnetic mineralogy; volcanic soils; toposequence; Teotihuacan valley.

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