Magnetic mineralogy in Barranca Tlalpan exposure paleosols, Tlaxcala, Mexico

Abstract

The knowledge of past climatic and environmental conditions in central Mexico, interpreted extensively from lacustrine records, is restricted to the last ca. 50,000 years. The recent discovery of new localities of paleosol sequences in central Mexico and the reconnaissance of their usefulness as records of environmental change provides a new source of information about past climatic conditions for times prior to the last full glacial. In this paper, we analyze the mineral magnetic properties of a sequence of paleosols developed in volcaniclastic deposits and evaluate these data as paleoenvironmental proxy. The sequence consists of a modern Phaeozem soil and seven late Quaternary Luvisol paleosols outcropping in Barranca Tlalpan, Tlaxcala, Mexico. Rock magnetic analysis was used to study the concentration and type of magnetic mineralogy. The mineral magnetic data, supported by soil morphological and chemical data, are interpreted to indicate that the paleosols can be grouped in three sets, each with distinct characteristics. The Red set paleosols P7 and P6 are the most weathered, and present the highest magnetic concentration composed of Ti-magnetite and ultrafine magnetite (<30 nm). The Brown set of paleosols P5, P4 and P3, with few redoximorphic features, have coarser grains and lower concentrations of magnetic minerals. The Gray set, paleosols P2 and P1, have abundant redoximorphic features including Fe–Mn nodules, and have a multicomponent magnetic mineralogy of coarse size. Paleosols from the Red and Brown sets conserve evidence of neoformation of ultrafine magnetite grains in the Bt horizons, probably derived from pedogenic processes. However, direct observations by microscopy and rock magnetism parameters point to a later destruction of this ultrafine fraction. We consider that even if pedogenic processes resulted in the formation of ultrafine magnetite, redoximorphic conditions under humid climates are responsible of the destruction of magnetic minerals. Such conditions may have strongly prevailed in the Gray set paleosols, whereas the Red set was probably developed under contrasting seasonal precipitation climates, which favored the formation and preservation of pedogenic magnetic minerals.