Title: Magnetic
mineralogy in Barranca Tlalpan exposure paleosols, Tlaxcala, Mexico
Author(s): Ortega-Guerrero
B, Sedov S, Solleiro-Rebolledo E, Soler A
Source: REVISTA
MEXICANA DE CIENCIAS GEOLOGICAS 21 (1): 120-132 2004
Document Type: Article
Language: English
Abstract: The
knowledge of past climatic and environmental conditions in central
Mexico, interpreted extensively from lacustrine records, is restricted
to the last ca. 50,000years. 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 PI, 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.
Author Keywords: paleosols;
rock magnetic analysis; central Mexico
KeyWords Plus: CHINESE
LOESS; SUSCEPTIBILITY; SOIL; DEPOSITS; RECORD
Addresses: Ortega-Guerrero
B (reprint author), Univ Nacl Autonoma Mexico, Inst Geofis, Cd Univ,
Mexico City, DF 04510 Mexico
Univ Nacl Autonoma Mexico, Inst Geofis, Mexico City, DF 04510 Mexico Univ Nacl Autonoma Mexico, Inst Geol, Mexico City, DF 04510 Mexico E-mail
Addresses: bortega@tonatiuh.igeofcu.unam.mx
Publisher: CENTRO
GEOCIENCIAS UNAM, CENTRO GEOCIENCIAS, UNAM, CAMPUS JURIQUILLA,
QUERETARO, QRO 76230, MEXICO
IDS Number: 900UK
ISSN: 1026-8774
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