Sergey Sedov

Mineral and organic components of the buried paleosols of the Nevado de Toluca, Central Mexico as indicators of paleoenvironments and soil evolution

Abstract Results of earlier studies of Quaternary tephra–paleosol sequences in Central Mexico revealed contradictions between paleopedological and lacustrine records. To settle the contradictions, selected quantitative characteristics of mineral and organic components of the paleosols PT1–PT7 from the Nevado de Toluca sequence were studied as independent paleoclimate proxies. Mineralogical composition of sand and clay fractions allows assessment of weathering and secondary mineral formation, both being climate-dependent processes. Stable carbon isotope ratios of humus and phytolith morphology indicate paleovegetation. In all paleosols, the sand fractions consist mainly of unstable volcanogenic minerals, although the grains have weathering features. Short durations of pedogenesis are responsible for limited weathering status. The clay fraction is dominated by halloysite and dehydrated halloysite in units PT1–PT4, which suggests drier periods during paleosol formation, whereas major amounts of kaolinite in the units PT5–PT7 indicate extended humid pedogenesis. δ 13C values of PT2–PT4 humus are intermediate between those characteristic of C3 and C4 plants. Phytolith assemblages which include both forms of C3 (dominant) and C4 (minor quantities) grasses agree with carbon isotope data. These data indicate plant cover changes driven by wet/dry paleoclimate oscillations. The new results, showing drier stages during the formation of Late Pleistocene paleosols PT2 and PT3, correct the primary interpretation of these units as indicating only humid pedogenesis, and settles the contradiction between paleopedological and lacustrine records.

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.

RADIOCARBON DATES FROM SOIL PROFILES IN THE TEOTIHUACÁN VALLEY, MEXICO: INDICATORS OF GEOMORPHOLOGICAL PROCESSES

Radiocarbon dates largely obtained from bulk soil samples in 24 soil profiles in the Teotihuac·n Valley, Mex- ico, are reported insofar as they represent a first step towards developing a sequence of soil formation, erosion, vegetation change, and human impact during the Holocene. Limitations of 14C dating in the area are considered, particularly the absence of charcoal in sediments and poor preservation of pollen. A broad temporal scheme is proposed to guide future research in which 4 periods are defined: ~5000-2000 BP (relative stability with short, intermittent episodes of erosion); ~2000-1500 BP (erosion-sedimentation, deforestation, and intensive agriculture); ~1500-1000 BP (relative stability, depopulation, and partial recovery of the landscape); and ~1000-500 BP (erosion-sedimentation, deforestation, and intensive agriculture). INTRODUCTION Between 1992 and 1999, 24 soil profiles were excavated in the Teotihuac·n Valley, State of Mexico (Figure 1), in order to study the formation of soils, erosion, and human impact associated with prehispanic settlement, resource extraction, and production in the region. The region, located approximately 50 km NE of Mexico City, is best known for the archaeological site of Teotihuac·n, the earliest city of its size and density in the Americas, occupied between approximately AD 1-650. Although the prehispanic urban center of Teotihuac·n represents a significant focus for our research, our initial goal was to establish a broad outline of the sequence of landscape transformation throughout the Holocene, based on the analysis of sediments and associated plant remains, including macrobotanical remains, pollen, and phytoliths (McClung de Tapia et al. 2003). In the Teotihuac·n Valley, only very limited paleoenvironmental research had been undertaken prior to our investigation, including geological surveys reported by Mooser (1968) and Barba (1995), a palynological study by Kovar (1970), and an analysis of Aztec and Colonial period landscape change in the Texcoco region, which included a portion of the southern extreme of the Teotihuac·n Valley (Cordova 1997). Only Cordovas research incorporated radiocarbon determinations of organic materials recovered from profiles. Consequently, our analysis represents the first stage of an attempt to develop a sequence of regional landscape dynamics in the area during the Holocene. Paleoenvironmental studies of lake sediments reported from other sectors of the Basin of Mexico indicate relatively dry conditions during the Late Glacial continuing into the Early Holocene (Loz- ano-Garcia and Ortega-Guerrero 1998; Caballero et al. 1999). Palynological studies of cores from lakes Texcoco and Chalco (Lozano-Garcia et al. 1993; Lozano Garcia and Ortega-Guerrero 1998) S and E of the Teotihuac·n Valley, respectively, and Lake Tecocomulco in the NE (Caballero et al. 1999), consistently reveal evidence for human impact during the Middle-Late Holocene that obscures possible indicators of episodes of climatic-induced vegetation change or other evidence for climatic variability that could have affected human settlements in prehispanic times.

The complex genesis of red soils in peninsula de Yucatán, Mexico: Mineralogical, micromorphological and geochemical proxies

The origin of red clayey soils developed on limestones has been largely questioned. We have analyzed thick red soils on Eocene-Pliocene limestones of the Yucatán peninsula (with detail in the Kantunil Kin area). The morphological, geochemical and mineralogical characteristics were interpreted as a record of pedogenesis and geomorphic dynamics during the Quaternary. Sand fraction mineralogy, the Zr/Ti ratio and ternary diagrams of Trace elements (Ti-Y-Zr, La-Th-Sc and Zr-Th-Sc) indicated a mixed origin of the soil parent material; volcanic and granitic/metamorphic components, redeposited by eolian transport. Within the soil matrix, primary minerals were transformed by weathering and caused desilication with the accumulation of kaolinitic clay. Reworked pedofeatures were originated from different soil bodies eroded. In the lower part of the profiles, we described the zone of carbonate leaching front, were the clay translocation in suspensions and posterior coagulation is more probable than the migration of elements in solutions and posterior synthesis.

Evolution of Chernozems in the Northern Caucasus, Russia during the second half of the Holocene: carbonate status of paleosols as a tool for paleoenvironmental reconstruction

Abstract A chronosequence of Chernozems buried under kurgans (funeral mounds) dated archaeologically to >5000, 3800–4000 and 1600–1700 years BP as well as the modern surface Chernozems were studied to reconstruct environmental change in the Central Northern Caucasus in the second half of the Holocene. Special emphasis was made on the detailed morphological and analytical characteristics of pedogenic carbonates and microscopic investigation of phytoliths. In the paleosols buried >5000 years BP, carbonates represented by compact micritic pedofeatures are in the uppermost part of the solum; neoformed gypsum is found below. This differs from the features of the modern soils and the paleosols buried 1600–1700 years BP in which the upper part of the profile is leached, pedogenic carbonates occur mostly in the form of diffuse pseudomycelium, impregnations and fine films, whereas gypsum is absent. The comparison of morphological features of the paleosols and the modern soils confirms the general tendency of paleoenvironment evolution from warm and dry in the Atlantic period to cooler and more humid in the Late Subboreal and Subatlantic periods. The compaction of soil mass and tonguing of humus horizon together with some dissolution of carbonate pedofeatures and signs of high biological activity in the paleosols buried 3800–4000 years BP are thought to indicate contrasting seasonal and annual variations of precipitation in early Subboreal period. Presence of calcified root cells of perfect habit in the paleosols of this chronointerval is also interpreted as evidence of contrasting seasonal climate. Hard carbonate nodules were found in all Chernozems of the chronosequence; the size of nodules, their internal porosity and recrystallization phenomena increase with age. We assume nodules to be relict pedofeatures in the Subatlantic paleosols and modern soils, which were transformed but not completely destroyed during the late Holocene humidification and enhancement of leaching. Thus, a multiphase model of carbonate pedofeatures formation is dominant in the soils studied. Phytolith assemblages indicate meadow–steppe vegetation in the Atlantic period; later in the Subboreal and Subatlantic periods, forest vegetation appears. Meadow (with some steppe elements) associations also determined in the late Holocene profiles probably reflect anthropogenic deforestation. These results are in good agreement with the interpretation of paleosols properties. We conclude that complex characteristic of pedogenic carbonates, which we define with the term “carbonate status” is one of the most valuable sources of the paleogeographical information in the chronosequences of steppe and forest–steppe soils.

Regoliths and Soils on Volcanic Ash

Abstract Soils on volcanic ash have specific physical and chemical properties, as well as typical micromorphological characteristics. They are characterised by an extensive accumulation of short-range order Si-Al compounds, such as allophane and imogolite, formed during weathering. This results in a grouping at the highest taxonomic level as Andosols (WRB) or Andisols (USDA) and in the use of ‘andic’ designations at other levels. The microstructure evolves from coarse monic in fresh ashes over chitonic and enaulic to granular in Andosols. A weakly separated intrapedal granular microstructure may persist in Bw horizons with a blocky microstructure or with a lenticular (freeze-thaw) microstructure. In Vertisols on volcanic ash a blocky microstructure is observed. The coarse material generally includes large amounts of volcanic glass. Fresh ash can already be transformed by hydrothermal alteration, resulting in the presence of secondary products. In the first stages of pedogenic weathering, short range order Si-Al compounds are formed. Pedogenic weathering of basaltic glass can produce palagonite-like alteromorphs of allophane. Magnetite and ilmenite can persist even in strongly weathered soils. Phytoliths and fungal hyphae are generally very abundant. The b-fabric is undifferentiated as long as short range order clays and halloysite are predominant, and it becomes stipple- or mosaic-speckled when kaolinite or illite appears. Micromass coatings around coarse fragments, and internal hypocoatings in the case of pumice, develop during a fist stage of soil formation. Other textural pedofeatures, except for rare allophane coatings, are generally not present in Andosols, but appear in intergrades to Luvisols, often still composed of optically isotropic compounds. In certain duripans (Tepetate), clay coatings with opaline zones are observed. Gibbsite coatings and nodules occur in conditions with strong leaching. Opal nodules are common in many soils formed on volcanic ash.

Micromorphology of a Soil Catena in Yucatán: Pedogenesis and Geomorphological Processes in a Tropical Karst Landscape

Development of the soil mantle in karst geosystems of the tropics is still poorly understood. We studied a typical soil toposequence formed over limestone in the northeastern Yucatan Peninsula of Mexico, to assess the pedogenetic and -geomorphological processes which control soil formation and distribution, as well as to understand their relation to landscape development and their influence in -ancient Maya agriculture. The soil cover is dominated by thin Leptic Phaeozems and Rendzic Leptosols in the uplands, and Leptic Calcisols in the wetlands. Upland soils have weathered groundmass containing abundant vermiculitic clay and iron oxides. The combination of thinness and high weathering status is explained by interaction between the intensive pedogenesis and vertical transport of soil material towards karst sinkholes. In wetlands, biochemical secondary calcite precipitation occurs, accompanied by surface accumulation of algal residues (periphyton crust). In the transitional area, a polygenetic profile (Calcisol over Cambisol) was developed, -indicating recent advance of wetlands. Because of specific pedogenesis, the upland soils lack many disadvantages of other soils of humid tropics, such as -acidity, low humus content, and poor structure. However, ancient land-use practices had to be adjusted to thin soils, low P availability and soil loss due to karst erosion.

Late pleistocene paleosol sequences as an instrument for the local paleographic reconstruction of the Kostenki 14 key section (Voronezh oblast) as an example

A sequence of five paleosol units (with seven individual paleosol profiles) buried in the Late Pleistocene (20–40 ka) deposits was studied at the Kostenki 14 (K14) key section in Voronezh oblast with the use of a set of morphological, physicochemical, and instrumental methods. The upper-lying paleosols differed from the lower-lying paleosols in the less pronounced gley features, stronger aggregation of the soil material, more significant accumulation of carbonates, and higher percentage of calcium humates and fulvates. These features attested to the higher aridity of the paleoclimate and the development of the upper-lying paleosols under grassy vegetation. Within the studied paleosol sequence, the most developed profiles were typical of the soils that formed 27–32 ka ago during the Bryansk interstadial. The good aggregation, the presence of features left by the soil fauna activity, the high magnetic susceptibility, and the morphology of the secondary carbonates in the studied paleosols suggest that they were formed under meadow-steppe vegetation in well-drained positions and resembled modern cryoarid soils.

Vetusols and paleosols: natural versus man-induced environmental change in the Atlantic coastal region of Morocco

Abstract Surface Terrae Rossae (Rhodoxeralfs), and a Terra Fusca (Petrocalcic Palexeralf) developed on mid-Quaternary calcarenites in the moister region of Rabat, show substantial pedogenic formation of clay minerals, particularly kaolinite, mainly in the fraction μm . For the most part, the pedogenic kaolinites show poor crystallinity of the fireclay type. They are mainly formed from feldspars and perhaps also amphiboles and pyroxenes in the fractions >2 μm , and from smectites inherited from the calcarenite-residual loam in the coarse (2– 0.2 μm ) and fine clay fractions ( μm ). In one Terra Rossa soil, formation was interrupted twice by deposition of eolian materials, and it is regarded as a polycyclic pedocomplex. Eolian input was much less important during development of the other surface Terrae Rossae. On younger calcarenites ( ⩾100 ka ) near the coast, three Rendzinas (Typic Calcixerolls) show only slight mineral weathering and pedogenic clay formation, in spite of a varied spectrum of primary minerals. In one example, kaolinite formation has just started. The Calcixerolls were probably recalcified by addition of eolian material as a result of anthropogenic thinning of the vegetation and/or due to the vicinity of the coast. These results suggest that, at least in this region, the younger Terrae Rossae of the coastal area were formed over much of the Brunhes epoch. In areas farther from the coast they have been developing for almost all of the Brunhes epoch. Because climatic fluctuations in the Quaternary were generally slight in this region, the direction of soil formation was constant. Therefore we regard the Rhodoxeralfs in the northern part of our study area as Vetusols. According to our expanded definition, Vetusols are surface soils which underwent almost constant processes of soil formation under an almost constant constellation of soil-forming factors, especially climate and vegetation, over a long period of time, including at least some part of the Pleistocene. Consequently the present patchy distribution of Terrae Rossae is hardly a result of climatically induced periods of “geomorphological activity and stability”. but the result of the removal of an earlier dense forest cover, followed by severe soil erosion. The human impact on the ecosystem is much more serious than any climatic fluctuations. In the southwestern part of the study area, between El Jadida and Agadir, the soil moisture regime is dry xeric to aridic and Terrae Rossae occur only in small areas, mostly in depressions. However, two selected Rhodoxeralfs in the vicinity of Tamanar show considerable pedogenic mineral weathering and clay mineral formation, though hardly any kaolinite. The strong weathering and clay illuviation must have been preceded by decalcification, implying a distinctly moister climate here in the past. However, micromorphological features of recent recalcification are common in these Rhodoxeralfs, suggesting that their main properties are not in equilibrium with the present climate and the soils should be regarded as nonburied paleosols or relict paleosols.