Jorge Gama-Castro

Paleosol-sedimentary sequences of the Glacis de Buenavista, Central Mexico: interaction of Late Quaternary pedogenesis and volcanic sedimentation

Abstract Two sections (Buenavista and Ahuatenco) of Late Quaternary paleosol-sedimentary sequences of the Glacis de Buenavista, Morelos, Central Mexico, were studied and interpreted as a regional record of climatic change and interaction of pedogenesis and volcanic sedimentation. Buenavista is the deepest section and includes seven pedostratigraphic units, with Andosol (surface unit) and Luvisol (all underlying units) types of pedogenesis. Both types indicate a humid forest environment, the divergency being related to differences in pedogenesis duration. Albeluvisol with a bleached E horizon in unit 5 indicates cooler but still moist conditions in the final Late Pleistocene, whereas strong vertic properties in unit 4 indicate a climate with contrasting seasonal variation of precipitation during the first part of the Holocene. In the upper part of the Ahuatenco section, Bt horizons of different pedogenetic events form a welded Luvisol profile instead of a set of separate paleosols due to less intensive sedimentation. Material of indurated Cm horizons (tepetates) in the lower part of the Ahuatenco section is a mixture of fresh coarse volcanoclastic components and re-deposited Luvisol clayey material, formed by mass movement associated with volcanic events. The resulting granulometric composition of these layers is favourable for structural collapse and hydroconsolidation, which caused hardening together with subsequent pedogenic illuviation and redoximorphic processes.

Andosol to Luvisol evolution in Central Mexico: timing, mechanisms and environmental setting

Abstract Surface and buried Andosols and buried Luvisols of the Nevado de Toluca Late Quaternary tephra-paleosol sequence (Central Mexico) were studied to show whether these soils present an evolutionary sequence and to determine the pedogenic mechanisms and environmental factors involved in the evolutionary process. Micromorphological observations and mineralogical composition of fine sand and clay fractions were used to detect type and succession of soil-forming process. Some of the buried Andosols, defined as “intergrade” Andosols, have a predominantly blocky structure, humus-depleted areas, redoximorphic features and thin clay coatings in Ah horizons. Clay fractions of buried Andosols contain halloysite besides amorphous components, whereas in modern Andosols, allophane is dominant. Luvisols have micro-areas with granular structure and abundant phytoliths in the groundmass of Bt horizons assumed to be the relict Andosol features. Luvisol clay fractions are dominated by halloysite and kaolinite. Primary minerals show micromorphological weathering features in all studied soils being stronger in Luvisols; however, even in Luvisols, sand fractions consist mostly of unstable volcanic silicates. We hypothesise that the studied profiles form an evolutionary sequence: Andosols–“intergrade” Andosols–Luvisols; the soil transformation is supposed to be linked to progressive crystallisation of 1:1 clay minerals. Comparing the Nevado de Toluca paleosol properties with the existing data on volcanic soil climo- and chronesequences and assessing the regional paleopedological and lacustrine records of Quaternary paleoclimates, we concluded that wet/dry climatic oscillations took place during the formation of the studied paleosols. Rapid crystallisation of 1:1 minerals occurred during dry phases, which speeded up the Andosol to Luvisol transformation and made it independent from the primary mineral weathering status. The Andosol to Luvisol transformation accelerated by climatic fluctuations is thought to be a common soil evolutionary pathway in the subtropical and tropical regions of recent volcanism, which suffered contrasting precipitation oscillations in the Quaternary.

Weathered pumice influence on selected alluvial soil properties in west Nayarit, Mexico

Parent material and pumiceous alluvial soils, located in a tropical region of Mexico, were studied to supply information on soil suitability for agricultural production in the context of sustainable agriculture. In recent alluvial soils, an understanding of how soil characteristics vary with parent material and topography provides a basis for determining land utilization type, land suitability as well as land quality. The main objective of this study was to establish the relationship of soil properties to parent material in west Nayarit, Mexico. Field studies were initiated in 1993 by a request for technical assistance from the Comision Federal de Electricidad. The studied soils were derived from pumice that has been reworked and mixed with detrital material from other sources. We found that such soils have unique physical, chemical and mineralogical characteristics that are rarely found in soils derived from other parent materials. Data for two selected alluvial soil profiles are presented. These soils were developed on Holocene volcanogenic pumiceous alluvial river terraces and river floodplains, under current udicisohyperthermic soil-climate conditions. The agronomic properties, tillage influences and fertilizer requirements of these soils have been studied extensively. To maximize their productivity and minimize deterioration, proper management must be based on an understanding of the unique physical, chemical and mineralogical properties. Results indicate that such soils have physical properties that provide a good environment for deep rooting and can supply the water necessary for vigorous plant growth. In both soils, water retention at 33 and 1500 kPa, particle surface area, calculated clay, cation exchange capacity, Al exchangeable percentage and P retention, and the occurrence of isotropic coatings on rock fragments and peds tend to increase in the presence of the large amounts of hydrolyzed pumice that are found in the 0.02‐2.0 mm fraction. Scanning electron microscope‐energy dispersive X-ray analyses demonstrate that the coatings dominantly consist of noncrystalline material, probably allophanic-like material as suggested by the Si/Al molar ratio of 1.0. Selective dissolution analysis reveals that these

Quaternary Mammals, People, and Climate Change: A View from Southern North America

The Pleistocene and modern mammal faunas of southern North America strongly differ in taxonomic makeup, distribution, and physiognomy. The former faunal complexes are part of the ancient landscape in which early peoples may have interacted. Customarily, differences between the Pleistocene and modern faunas have been attributed to climate change or human-impact driven extinctions. Mexico’s Pleistocene mammal record is analyzed in time and space, emphasizing the study of the Rancholabrean Chronofauna, which is the most recent North American Land Mammal Age fauna. Palynological and paleosol records are reviewed as an independent check of the interpretation derived from mammals. The integration of the information provides the basis for a proposal regarding Late Pleistocene climate change trends across the country, and whether people were involved in the mammalian community response to climate change in terms of extinction or biogeographic shifting within and outside the country. This approach supports an explanation of the differences between southern North America’s Pleistocene and modern mammal faunas.