Illuvial Clay Coatings Research Articles

Soils of the coastal and lower belt of Galápagos Islands – The case of Isla Santa Fé and Isla Santa Cruz

This paper compares the development of soils in low-altitude zones of two eastern Galápagos islands having a similar composition of the volcanic rock substrate but contrasting topographical features, i.e. the small and relatively flat island of Santa Fé and the larger, mountainous, younger island of Santa Cruz. The soils of the lower, dry zone of Santa Cruz are less weathered compared to those of the higher slopes on the same island. They contain basalt-derived mineral grains, swelling 2:1 phyllosilicates, and poorly crystalline kaolinite, and they are rich in calcite features. The soils on Santa Fé, in a similar altimetric position, are less weathered, containing more weatherable basalt-derived mineral grains and a clay fraction composed of open 2:1 phyllosilicates and small amounts of mica-like components, together with likely sepiolite. Sepiolite seems to occur in part in the form of small yellowish nodules. Some soils on both islands contain significant amounts of phosphate, probably linked to populations of sea birds. Compared to the parent rock, an increase in Al, Fe, Ti and K, and a loss of Mg, Ca and Na, are observed for both islands, but this is less pronounced for Santa Fé. The observed micromorphological, mineralogical and chemical differences are explained by geomorphology and related climatological factors, since the present altitudinal setting of the soils is similar. The soils of Santa Fé receive less precipitation because the island has no central mountain chain. On both islands, the heterogeneous nature of the low-elevation soils indicates major colluvial input. Biological factors, such as nesting of seabirds and the development of phytogenic calcite, may play an important role in local soil development. Remnants of illuvial clay coatings, sometimes entrapped in calcitic features, indicate wetter conditions in the past.

Micromorphological evidence of pedogenesis and paleoclimatic change in the Holocene loess-paleosol in the south Qinling Mountains, China

Micromorphological investigations of loess-paleosol sequence offer the potential for understanding pedogenic and paleoclimate change at the microscopic scale. However, the micromorphological study is comparatively scarce in the south Qinling Mountains, China. Here, micromorphological analysis (coarse particles, pedofeatures and voids) in conjunction with physicochemical indexes of pedogenesis (i.e., clay content, CaCO3 and Fe2O3) were carried out from two Holocene loess profiles in the upper Hanjiang River. These results show that: (i) Paleosol is considered as strongly-developed soils on basis of much finer and rounder particles, channels, more reddish color and abundant illuvial clay coatings; (ii) Lessivage (clay formation, eluviation and illuviation), intensive carbonate eluviation and rubification were identified as the major pedogenic processes of paleosol; (iii) Comparative study with Chinese Loess Plateau (CLP) indicates that loess deposits in the south Qinling Mountains experienced a much stronger pedogenesis as evidenced by multiple generations of illuvial clay and almost completely eluviation of carbonate, which implies a warmer and wetter paleoclimate in the subtropical monsoon zone. The pedogenic differences across the Qingling Mountains are mainly controlled by regional paleoclimatic differences and provide a good response to monsoon climate change in the East Asian.

Textural features of saline-sodic soils affected by Urmia Lake in the Northwest of Iran

The increasing regression of Urmia Lake in the last 30 years has brought about the salinization and sodification of arid soils from the shoreline, which has aggravated land degradation in this area. The objective of this work was to assess the changes in the soils brought about by progressive and fast salinization of the region, and to determine the present- or past character of several textural pedofeatures. We studied the micromorphic, mineralogical and physico-chemical properties of four profiles of saline-sodic soils in the west of Urmia Lake. Most of voids in the studied horizons were mainly vesicles due to structure collapse, besides planes and channels, and their b-fabric were mainly crystallitic micritic and granostriated due to argilloturbation. The main pedofeatures were illuvial clay coatings and also their fragments in the groundmass, clay and silt coatings, calcite accumulations as nodules, infillings and coatings and redoximorphic accumulations as nodules and coatings. Owing to the arid climate and high amounts of calcium carbonate and soluble salts, the illuviation of pure clay in natric horizons probably took place during a wetter past climate during the lower Quaternary. Additionally, based on high values of exchangeable sodium, ESP and SAR, we believe that the high values of exchangeable sodium are the responsible for the structural collapse, dispersion and translocation of fine material (clay and silt) at varying distances within the soil profile. Alkalinisation is likely to proceed from evaporation of the supersaturated waters of Urmia Lake through carbonate precipitation and smectite formation, increasing ESP and changing previous argillic horizons to natric horizons. Increase of organic matter through the application or organic amendments seem to be the best strategy for their reclamation.

The undisclosed paleoclimatic record of northern mesoamerica: A pedostratigraphic approach for environmental reconstruction

This article presents the results of a research on paleosols formation conditions in the Aguascalientes Valley, northern Mesoamerica, where paleoenvironmental reconstructions have been scarce. Records of environmental and climatic changes, as well as the alteration of the landscape are expressed in the memory of the soil. Soil micromorphology is the most efficient method to identify these changes. In paleosols, micromorphological features reflecting a certain soil forming process can be related to environments in which those processes took place. Paleoenvironmental reconstruction of the region consisted in identifying pedofeatures and other present soil attributes, grouping features into pedogenic facies under constant environmental conditions, examining the groundmass in order to detect features caused by erosion or sedimentation, and finally establishing a hierarchy between the different pedogenic facies, including erosional and sedimentary features if necessary. Specific statistical data analysis disclosed stratigraphical relationships. The results we obtained, such as illuvial clay coatings, redoximorphic nodules, and hypocoatings, revealed the most representative pedofeatures that developed under humid climate. Under a semiarid climate, representative pedofeatures that were revealed included micritic coatings and hypocoatings. A paleosol with ferruginous nodules and hypocoatings in groundmass, and at least one phase of dusty and high-birefringence yellow clay illuviation is proposed as stratigraphic marker for this area of the Aguascalientes Valley. Evidence indicates the sequence of events in the area was: (i) a large period of humid climate; (ii) a short period of climate change, from humid to semiarid, followed by subhumid; and (iii) a large period of semiarid climate, including the current one. The paleosols of the Aguascalientes Valley to a large extent preserve major trends in history and provide further evidence to define the 11 phases of stability (pedogenesis) in 8 cycles and to detect the interlayer phases of instability (sediment transport/deposition).

Morphogenesis and quantification of the pore space in a tephra-palaeosol sequence in Tlaxcala, central Mexico

Tepetates are indurated subsoil horizons developed in tephra-derived materials in various parts of the Trans-Mexican Volcanic Belt. The term ‘tepetate’ includes fragipans, duripans, pedosediments and saprolites, derived from vitric rhyolitic tuff, Pleistocene volcanic ashes or pyroclastic flows. All soils with tepetates are at high risk of erosion and so they have been intensively studied for decades. The tepetates are common in Tlaxcala State of central Mexico, being formed under Ustic Isomesic soil climate. The aim of this investigation was to characterise the pore space of fragipan-type tepetates and the role of clay components in their formation. We studied porosity of tepetate from a tephra-palaeosol sequence in the north of Tlaxcala State, in undisturbed soil samples. Observations of pore space were made in 2D and 3D by analysing microscope images of thin sections and cross-sectional images from a computed tomography scanner. In the thin sections we also identified and described clay illuvial pedofeatures. Micromorphological observations showed two main pore types. Small rounded pores had a homogeneous distribution and were probably formed before the clay illuviation process that took place in a palaeosol formed on the tepetate material. The distribution pattern of the small pores in the studied tepetate was similar to that in the fragic horizon, which was probably formed by a hydro-consolidation process. Large crack-pores were formed during the palaeosol formation. Later these large pores were filled by illuvial clay coatings and so we conclude that each tepetate was part of the set of Bt horizons in the palaeosols of Luvisol type.

How fast are soil-forming processes in Quaternary sediments of a tropical floodplain? A case study in southeast Brazil

Floodplains are depositional features of a channel valley associated with a specific climatic and hydrologic regime of a drainage basin. In tropical conditions, sediments can be quickly weathered by soil-forming processes and tend to obliterate pre-existing sedimentary structures. Therefore, the objective of many studies on floodplains is to identify and discuss such sedimentary features, mostly those related to the morphology of channel deposits. Additionally, a qualitative approach to identify aspects of pedological evolution in these environments has been attempted in few studies. The purpose of this study was to interpret the intensity and the time of alluvial sediment weathering by pedogenetic processes in tropical humid conditions. Soil profiles were analyzed in different environments of deposition in the Pardo River floodplain (southeast Brazil) by accounting for sedimentary facies and pedological features supported by multi proxy data provided from geochemistry, macro- and micromorphology, grain size, total carbon, total nitrogen, 13C isotopic analysis and 14C/OSL (optically stimulated luminescence) dating. Based on the results, we identified that pedogenetic features and soil horizon distributions were closely related to sediment architecture, grain size and position in the floodplain. In the Pardo floodplain, Fluvic Gleysols (Loamic and Clayic) with moderately to strongly developed structures were identified in overbank fine sediments. Fluvic Cambisols (Clayic) were found in abandoned paleomeanders, Fluvic Cambisols (Loamic) in sandy scrollbars, Gleyic Acrisols (Clayic, Cutanic, Differentic) in channel deposits, which presented a 130-cm-thick Bt horizon with illuvial clay coatings, and Histosols (Fluvic) with high contents of total carbon (11.9% and 26.8% in histic horizons) in small-scale avulsive systems. Deposits were dated to between 35,000 and 700years BP. 13C isotopic data revealed a trend toward enriched values in older samples (−14.21‰) in which C4 plants dominated and a relative impoverishment of the values in more recent samples (−26.9‰ to −27.9‰). This result was consistent with the tendency of the distribution of C3 plants to increase over that of C4 plants, favored by the warmer and wetter climate since the Upper Holocene. In this study, a Fluvic Gleysol profile of <1000years and a Fluvic Cambisol of <700years were identified. Nevertheless, our primary contribution is to highlight that soil evolution in this specific situation could be more efficient than expected. Moreover, to provide consistent analyses in future studies, tools from both sedimentology and pedology are essential.

A powerful tool for assessing distribution and fate of potentially toxic metals (PTMs) in soils: integration of laser ablation spectrometry (LA-ICP-MS) on thin sections with soil micromorphology and geochemistry.

The dynamic behavior and inherent spatial heterogeneity, at different hierarchic levels, of the soil system often make the spatial distribution of potentially toxic metals (PTMs) quite complex and difficult to assess correctly. This work demonstrates that the application of laser ablation spectrometry (LA-ICP-MS) to soil thin sections constitutes an ancillary powerful tool to well-established analytical methods for tracing the behavior and fate of potential soil contaminants at the microsite level. It allowed to discriminate the contribution of PTMs in distinct soil sub-components, such as parent rock fragments, neoformed, clay-enriched or humified matrix, and specific pedogenetic features of illuvial origin (unstained or iron-stained clay coatings) even at very low contents. PTMs were analyzed in three soil profiles located in the Muravera area (Sardinia, Italy), where several, now abandoned mines were exploited. Recurrent trends of increase of many PTMs from rock to pedogenic matrix and to illuvial clay coatings, traced by LA-ICP-MS compositional data, revealed a pedogenetic control on metal fractionation and distribution, based on adsorption properties of clay minerals, iron oxyhydroxides or organic matter, and downprofile illuviation processes. The main PTMs patterns coupled with SEM-EDS analyses suggest that heavy metal-bearing mineral grains were sourced from the mine plants, in addition to the natural sedimentary input. The interplay between soil-forming processes and geomorphic dynamics significantly contributed to the PTMs spatial distribution detected in the different pedogenetic horizons and soil features.

A multi-disciplinary perspective on habitat preferences among dinosaurs in a Cretaceous Arctic greenhouse world, North Slope, Alaska (Prince Creek Formation: lower Maastrichtian)

The Prince Creek Formation of northern Alaska is the most abundant source of polar dinosaur remains in the world, and now, corroborating data from this well-studied rock unit allow for making inferences about the paleoecological preferences for these extinct polar animals. The rock unit records high-latitude, alluvial sedimentation and soil formation on a low-gradient, muddy coastal plain. Compound and cumulative andic Entisols and Inceptisols formed on levees, point bars, crevasse splays, and along the margins of floodplain lakes, ponds, and swamps. Abundant organic matter, carbonaceous root traces, Fe-oxide depletion coatings, and zoned peds indicate periodic waterlogging, anoxia, and gleying, consistent with a high water table. In contrast, Fe-oxide mottles, ferruginous and manganiferous segregations, bioturbation, and less common illuvial clay coatings indicate recurring oxidation and periodic drying out of some soils. An integrated reconstruction of pedogenic processes and biota suggests that this ancient Arctic coastal plain was influenced by seasonally fluctuating water table levels and floods, and in distal areas, marine waters. Four of the five bonebeds in this study are from more distal areas, represented by lower delta plain facies, while the fifth bonebed is from a more proximal part of the basin, represented by a somewhat better drained coastal plain facies.Bonebeds in the distal areas are dominated by Edmontosaurus sp. while the more proximal bonebed is dominated by the remains of the ceratopsian Pachyrhinosaurus perotorum. The distribution of these bonebeds, sedimentological facies, paleosols, and biota suggests that Pachyrhinosaurus may have preferred more upland environments while Edmontosaurus preferred lowland, deltaic environments. This distribution may be the result of physiological adaptation to the pronounced seasonality provided by polar terrestrial ecosystems. In contrast to a preferred habitat distribution of these large herbivores, the large predatory dinosaur Nanuqsaurus hoglundi seems to have had a more ubiquitous distribution across the landscape.

A micromorphological evaluation of pedogenesis on Isla Santa Cruz (Galápagos) .

Thin sections of about 200 horizons or layers, representing 60 soil profiles in the coastal area and on the windward slopes of Isla Santa Cruz (Galápagos Islands) were analysed. Based on the fabric and the composition of the groundmass and the presence of pedofeatures in individual layers or horizons, clusters of similar material are made and 7 units and 3 subunits distinguished. Plotting these units on a survey map gives a good insight in the spatial distribution of soil materials, expressing different combinations of parent material and precipitation. In the coastal area and the lowest slopes, with a summer dry climate, respectively reddish and greyish and brown materials with a porphyric c/f related distribution pattern and striated b-fabrics, and often with fragmented illuvial clay coatings are observed. The coarse material contains mainly holocrystalline basalt fragments, unweathered in the coastal area, or basalt derived individual minerals. The micromass has a halloysitic-smectitic composition. On the higher slopes, with a permanent moist climate, materials have also a porphyric or fine monic c/f related distribution pattern, but the b-fabric is undifferentiated and no illuvial features are present, gibbsitic features sometimes occur and the micromass consists mainly of halloysite and gibbsite. Subunits are distinguished here according to the microstructure, and the quantity and type of coarse material. Mesocrystalline basalt, often vesicular, dominates over holocrystalline, pointing to the influence of scoria. Soils on the higher slopes are more strongly weathered than those on the drier lower slopes and the coastal area. In depressions in the higher areas, materials with a yellowish or brownish grey micromass with a striated b-fabric, and well developed limpid illuvial clay coatings and impregnative iron oxide nodules occur. The micromass has a halloysite-smectite composition. Contrary to existing hypotheses the reddish soils in the coastal area are not considered as palaeosoils (roots of tropical soils), but as modern soils developed in colluvium on the lower slopes, which was deposited on totally eroded surfaces (marine terraces?). On the slopes the distribution of units is not only determined by hypsometric zones, as suggested in literature, but is rather clustered according to types of parent material. The soils on Santa Cruz are supposed to be formed after the last interglacial period, different from the red soils of San Cristóbal which are older.

A high-resolution micromorphological record of the Late Quaternary paleosols from Ganga–Yamuna interfluve: Stratigraphic and paleoclimatic implications

This study reports the micromorphological record of a continuous set of samples from paleosol-bearing intervals of two subsurface cores of ∼50 m depth representing typical pedosedimentary sequences of the Ganga–Yamuna interfluve in the western Ganga Plains. Micromorphology (microstructures, rhizocretes, Fe–Mn features, pedogenic carbonate, illuvial clay coatings and relict pedofeatures) was used to characterize pedogenic processes in the 13 paleosols in the IITK core and the 10 paleosols in the Bhognipur core. Formation of these paleosols took place through incipient to moderate weathering on relatively stable surfaces of the Ganga–Yamuna interfluve over the last 100 ka. These paleosols range in character from simple ones having weakly developed pedofeatures to mature paleosols with strongly developed pedofeatures and are similar to modern Entisols, Inceptisols, Alfisols and Vertisols of the Ganga Plains. Pedosedimentary evolution in the two interfluve successions differs in terms of stratigraphic stacking of paleosols, types of paleosols, and heterogeneous nature of sediments. In the northern part, the IITK interfluve core is marked by a thick mature paleosol with a strongly developed argillic (Bt) horizon at 41–45 m depth and weakly developed paleosols in the remaining part of the core. The mature paleosol represents a major discontinuity when prolonged pedogenic activity took place on stable surfaces with little or no deposition. On the other hand, thin-simple–thick cumulic paleosols with weakly developed pedogenic features, even with some preservation of sedimentary laminations indicate short stratigraphic breaks in rapidly aggrading sediments. In the southern part, the Bhognipur core is marked by the heterogeneous nature of sediments with a predominance of cratonic sediments in the lower part and the Himalayan sediments in the upper parts. In contrast, the IITK core shows a Himalayan source throughout. The entire interfluve succession in the Bhognipur region is marked by rapid sedimentation and short pedogenic intervals of weakly developed paleosols excepting one mature paleosol with well-developed argillic (Bt) and vertic (Bss) horizons at 10–14 m depth. This mature paleosol possibly formed synchronously with a mature paleosol from the northern part of this interfluve. The mature paleosol in both the cores represents a major stratigraphic break of 8000–10,000 years when prolonged pedogenesis occurred over large areas of the Ganga–Yamuna interfluve following regional degradation and local gullying in response to climate change from sub-humid to semi-arid conditions during the MIS 5–4 transition.

Mid‐Pleistocene pozzolanic volcanic ash in ancient Roman concretes

AbstractThe extraordinarily durable concretes of Imperial Age (27 B.C. through 3rd century A.D.) monument construction in Rome contain scoriaceous, highly potassic, altered volcanic ash from the Pozzolane Rosse ignimbrite, erupted at 456±3ka from the Alban Hills volcano as pozzolanic mortar aggregate. Stratigraphic, micromorphological, and chemical investigations demonstrate that during the relatively warm, humid period preceding marine isotope stage 11, intensive hydrolytic pedogenesis produced dense illuvial clay coatings in the upper horizon of the ignimbrite. Moderately altered ash of the Pozzolane Rosse transitional Bt to Bw soil horizon has opal coatings overlain by limpid, illuvial halloysite coatings. Less weathered ash from a lower C horizon apparently was altered in ground water. Where the ignimbrite filled certain paleovalleys, zeolitic alteration produced phillipsite and chabazite textures. Builders selected ash from these intermediate and least altered horizons for the highest‐quality mortars of the Imperial Age, as for the Forum and Markets of Trajan (A.D. 107 to 113). The alumina‐ and alkali‐rich compositions of cementitious complexes give some preliminary insights as to why the reaction of hydrated lime with the altered, alkali‐rich Pozzolane Rosse ash produced pozzolanic cements that have remained resistant to decay for nearly 2000 years. The results of the geological analyses fully confirm empirical observations made by Esther B. Van Deman in 1912 regarding the durability of the ancient mortars and the technical choices of Roman builders. © 2009 Wiley Periodicals, Inc.

Pedogenesis of a catena of the Farmdale–Sangamon Geosol complex in the north central United States

The Farmdale–Sangamon Geosol pedocomplex consists of the Sangamon Geosol and the overlying Farmdale Geosol, which form the most extensive terrestrial record of the last interglacial to glacial transition in the Midwest United States. The geosol complex formed for upwards of 100 ka, extending from the end of MIS 6 through 4 for the Sangamon Geosol, then the Farmdale Geosol for during a brief episode at the end of MIS 3 following slow accumulation and pedogenic modification of eolian silt deposited on top of the Sangamon Geosol. Our study site consists of a buried paleo-hillslope transect that forms a catena, enabling evaluation of slope effects on interglacial-scale soil formation. The Sangamon Geosol is formed in calcareous and illitic glaciogenic sediment. Along the catena the Sangamon Geosol profiles display some morphological changes, namely in terms of colors that we interpret as indicators of differences in drainage. Most thickness and horizonation characteristics are similar all along the transect, with intact upper sola horizons (AE and E horizons) that overlie clay-enriched Bt horizons. The Bt horizons contain abundant clay that exists as illuvial clay coatings, matrix infills, and as mosaic-speckled domains. The clay originated both by in situ weathering and through illuviation from the clay depleted upper sola. Slope does not appear to affect Bt characteristics beyond redder hues of the matrix and clay coatings in the upper slope position. With depth, effects of carbonate leaching and infilling of clay in the matrix decrease and clay coatings are restricted to walls of voids adjacent to aggregates. Clay mineralogy shows illite depletion, but no interstratified kaolinite-expandable minerals, indicating the degree of weathering is not as great as is typical of Sangamon Geosol profiles formed in loess or in glaciogenic sediment of the central Illinois type area. Clay mineralogy is also stratified with depth, coincident with particle size, which probably indicates sorting of layers of illitic dolomite and shale. Variation of horizon and profile characteristics appears to largely be a function of particle size variability and stratification than topographic position in the catena. The influence of hillslope position on soil redistribution during formation of the Sangamon Geosol appears negligible given the uniformity of upper solum horizon thickness and sandy particle size characteristics, so we conclude that a bioturbation and rainwash origin of the upper solum and the texture contrast in these profiles is not the best process model explanation. We suggest that the base-rich nature of these soils led to ecosystem characteristics that discouraged erosion and encouraged infiltration and a lessivage-type origin of the texture contrast. No convincing evidence of MIS 6 through MIS 4 loess occurs at this site. The Farmdale Geosol formed in the Robein Silt, which is Roxana Silt (MIS 3 loess) that was redistributed downslope. The Robein Silt is thicker and finer in the topographic low and indicates the cooler and forested environmental conditions during MIS 3 were conducive to downslope movement of soil and also produced greater differences in drainage-induced soil morphological changes in the Farmdale Geosol.

The role of human impacts and Holocene climate change in the Santuario della Madonna Cave (Calabria)

A geoarchaeological study was carried out in the Santuario della Madonna Cave, in northwestern Calabria. The site is a karst cave formed in Tertiary dolomite-limestone, reshaped by the sea during the Quaternary. Archaeological findings in the sedimentary infilling attest to human settlement since prehistoric times. We focused on the Holocene pedostratigraphic succession, comprising thinly laminated loamy-sand layers, overlying a soil with finer texture, illuvial clay coatings and hydromorphic features. The stratigraphy was characterized by physical, chemical, mineralogical and micromorphological analyses, in order to characterise human activities, landscape evolution and environmental changes. Major geomorphological stability during the Middle Neolithic climatic optimum was followed by rapid oscillations in the moisture regime and morphodynamics during the Late Neolithic to the Copper and Bronze Ages. Water dripping, CaCO3 leaching and precipitation, humification, runoff and colluviation all alternated and were interspersed by periods of human activity (stabling, in situ burning, setting of poles).

Clay coatings from a modern soil chronosequence: A tool for estimating the relative age of well-drained paleosols

Blue light optically stimulated luminescence (OSL) dates obtained from upland and terrace soil profiles in southeastern Mississippi have been used to generate a model for illuvial clay accumulation in deeply weathered, subtropical soils (e.g. Ultisols). The illuvial clays developed in upland vs. alluvial terrace soils of southeastern Mississippi are distinct in terms of thickness and total volume of translocated clay. The coatings in the Mississippi soils are also an analogue for clay coatings observed in mid-Cretaceous paleosols of North America. Many of the Cretaceous paleosols had a polygenetic history. The relative age of the well-drained phase of pedogenic development, characterized by illuvial clay accumulations, may be estimated using the model generated from the Mississippi soils. As many of the mid-Cretaceous paleosols mark unconformities, it would be beneficial to estimate the duration of the hiatuses. Three geomorphic surfaces characterize the landscape of southeastern Mississippi: uplands, stream-cut terraces, and modern alluvium. The present floodplain, has poorly developed soils, and lacks illuvial accumulations. Soils developing in the uplands have abundant and prominent illuvial clay coatings up to 2 mm thick, while the terraces have minor clay coatings less than 1 mm thick.

Clay illuviation and calcium carbonate accumulation along a precipitation gradient in Kansas

One of the significant features of loess-derived soils in Kansas is the occurrence of clay-rich subsurface horizons above a layer enriched with pedogenic carbonates. In order to examine the extent of clay increase and pedogenic carbonate enrichment in a precipitation gradient, ten soil profiles from three different precipitation regions were studied using micromorphological and mineralogical techniques. The precipitation gradient was divided into three groups: 400–550 mm, 550–750 mm, and 750–1100 mm regions. The objectives were to (1) understand the cause of clay orientation in clay-rich horizons (2) investigate the reasons for the clay increase, and (3) observe the interaction of clay and pedogenic carbonate accumulation features along a precipitation gradient in Kansas. Although clay films were identified in the field for soils in the 400–550 mm regions, illuvial clay films were not observed in thin section analysis. The clay accumulations mostly occurred as grain coatings. The rest of the clay accumulations observed were very thin, striated, and mostly associated with voids. The argillic horizons had a granostriated b-fabric, which indicates stress orientation of micromass caused by high shrink–swell activity. Thick and continuous illuvial coatings were observed in the buried horizons of paleosols. In the other two regions where precipitation exceeds 550 mm, illuvial clay coatings with strong orientation were observed along with thin and striated stress-oriented clay. Both types of clay orientations exceeded 1% of the cross-sectional area for the thin section. Although illuvial clay features and pedogenic carbonates were observed in all soils at approximately the same depth, complete obliteration of clay coatings was not observed in these horizons. In-situ weathering of biotite was one of the reasons for the clay increase in all soil profiles. In all soils studied, the clay increase and cause of clay orientation cannot be attributed to a single genetic process or event. Both illuviation and shrink–swell activity were involved in the orientation of clay. Although orientation of clay and pedogenic carbonates were observed in all soils at approximately the same depth, the decomposition of clay coatings was not observed in these horizons.

The Nature of Zeolite Occurrences in Deposits of the Olduvai Basin, Northern Tanzania

AbstractThe Plio-Pleistocene deposits of the Olduvai Basin in northern Tanzania consist of a sequence of lacustrine and fluvial sediments. They contain various amounts of zeolite minerals, the formation of which is related to an interaction of volcanic material or detrital clays with saline alkaline lake water and groundwater. Petrographic characteristics of zeolite occurrences provide information about their conditions of formation. They were studied for all four main stratigraphical units that are recognized at Olduvai (Beds I to IV), sampled in the southeastern part of the basin. In the lake-margin deposits of Bed I and the lower part of Bed II, chabazite is the dominant zeolite mineral accompanied by phillipsite and minor amounts of erionite and clinoptilolite. Chabazite commonly occurs as part of altered volcanic rock fragments, characterized by partial or complete dissolution of volcanic glass and the formation of chabazite inside vesicles, following the development of thin smectite coatings. It also formed within the sediment matrix, requiring extended periods of impregnation of the deposits by saline alkaline solutions. Chabazite also occurs extensively as coatings and infillings of pores, developed during periods of subaerial exposure which were characterized by high groundwater levels. Phillipsite formed at a later stage, from more evolved solutions, with higher K/Na ratios than during chabazite formation. The fluvial deposits of Bed IV, Bed III and the upper part of Bed II have a high analcime content. They also contain various amounts of chabazite, phillipsite and natrolite. All zeolite minerals mainly occur in pores. The predominance of analcime indicates a higher salinity and alkalinity than during the preceding period with sedimentation and diagenesis in a lake margin environment. Early development of zeolite occurrences, shortly after the deposits became exposed during breaks in sedimentation, is recorded for some intervals, where zeolites are covered by illuvial clay coatings or by sparitic carbonate cement. In most intervals, however, zeolites mainly formed at a later stage.

Pedogenesis of late Quaternary deposits, northern Kyonggi Bay, Korea: Implications for relative sea-level change and regional stratigraphic correlation

Late Pleistocene and early Holocene paleosols are described from construction pits and boreholes in the northern Kyonggi Bay, west coast of Korea. Thin-section petrography, chemistry, clay mineralogy and geotechnical properties of the paleosols were examined to infer relative sea-level fluctuations during the late Quaternary. Relict laminae and gradational contacts with underlying tidal rhythmites confirm that the late Pleistocene paleosol is a pedogenically altered tidal deposit. Illuvial clay coatings, rootlets and enrichment of kaolinite and chemically stable oxides indicate that pedogenesis occurred on a well-drained, stable surface during the last glacial period. Redoximorphic features, such as drab-colored root traces and sphaerosiderite, however, suggest that waterlogged conditions developed temporarily due to a rise of the water table at the end of the last glacial period. As sea level rose, an early Holocene paleosol formed in freshwater bog deposits, burying the late Pleistocene paleosol. The early Holocene paleosol has abundant sphaerosiderites and organic material. The presence of weakly developed illuvial clay coatings, oxidized siderites and pellets, however, indicates oxidizing condition associated with water-table fall occurred during the early Holocene. Despite the similarity in the degree of chemical and clay mineralogical weathering, the late Pleistocene paleosol is distinguished from its early Holocene counterpart by thicker and more abundant illuvial clay coatings, more Fe 2O 3 and a greater degree of consolidation. Duration of pedogenesis might be a primary cause for the difference of the pedofeatures with climate playing a secondary role. The widespread presence of similar late Pleistocene paleosols highlights its significance as a key stratigraphic marker for regional correlations along the west coast of Korea.

TRANSITIONAL SOIL CHARACTERISTICS OF ULTISOLS AND SPODOSOLS IN THE SUBALPINE FOREST OF TAIWAN

This study examined the illuviation of clay and spodic materials in three Ultic Haplorthods derived from fine-textured materials from the Alishan Mountain of central Taiwan. Soil analysis, including field description, physical and chemical analyses, and micromorphological studies, was used to study the soils. The soil colors are close to 7.5YR 5/6 in all subsurface horizons, with different degrees of spodic material accumulation. Various clay coatings and pellet-like aggregates were found in all subsurface horizons, which satisfied the morphological and chemical criteria for a spodic horizon (Bhs) as defined in Soil Taxonomy. The illuvial clay coatings have relatively weak birefringence but are masked by organo-metallic complexes. Spodosols were found locally to have a thick organic layer, which may intensify the podzolization process. We hypothesized that Fe and Al were chelated by organic matter in the surface portion of the pedon to form organo-metallic complexes. The translocation of clay occurred before the illuviation of organo-metallic complexes in the subsurface horizons during soil development. The slow accumulation of organic-metallic complexes in the Bhs horizons within the 50-cm depth is attributed to fine texture and less porosity. The soils derived from two pedogenic processes to show the transitional status between Ul-tisols and Spodosols that the argillic horizon is overlain by the spodic horizon.

Identification and differentiation of Pleistocene paleosols in the northern Pampas of Buenos Aires, Argentina

A combined macromorphological and micromorphological approach was used to identify and differentiate paleosols at a representative exposure of the Pleistocene sedimentary succession in the northern Pampas of Buenos Aires (La Plata area). Five pedological units (Pu), which apparently represent four discrete paleosols plus the surface soil, were initially differentiated on the basis of field-scale morphological properties. The succession was divided into B and C horizon with an A horizon only clearly identified in the surface soil and a weak A horizon at depth. Micromorphology suggests a complex pedosedimentary history of welding, with some degree of water reworking indicated by fragments of sorted layers and the significant grain-size heterogeneity of the parent material. The micromorphological data do not support the field differentiation of Pu4, Pu3, and Pu2 into discrete paleosols. Pedological features (i.e. excrements, secondary carbonate coatings, illuvial clay coatings) occur throughout without any obvious breaks or patterns. Pu4, Pu3, and Pu2 are therefore interpreted as an accretionary and/or welded pedocomplex. The Gorina section does not conform to the simple classical model proposed by other authors in the region of alternating loess and paleosol units associated with arid (loess deposition) and wet (soil formation) intervals, respectively.

Genesis and relationship of macromorphology and micromorphology to contemporary hydrological conditions of a welded Argixeroll from the Palouse in Idaho

Soils formed in distal loess of the Palouse region have welded profiles that make assessment of their genesis and pedostratigraphic relationships difficult. These soils typically have a mollic-cambic horizon sequence that is underlain by a well-expressed albic E horizon and a strongly developed argillic horizon. In this study, we utilized hydrological monitoring combined with micromorphological analysis and new stratigraphic data to help establish the genesis of a representative Argixeroll of the region and the relationship of its morphology to ancient and contemporary hydrological conditions. Monitoring results indicate that the Argixeroll is seasonally episaturated for approximately 5–6 months per year. Ferrimanganiferous nodules, segregations, hypocoatings, and depletion mottles are observed in thin section throughout the pedon, but dominate within, just above, and below the present-day E horizon. Measured soil Eh is sufficiently low for Fe and Mn reduction to occur for 1–3 months during the period of episaturation, suggesting that at least some of these redoximorphic features are likely to be contemporary. Other compound redoximorphic features are clearly relict, however, as they are coated by illuvial clay which is unaffected by Fe Mn redistribution processes. Thick illuvial clay coatings on interpedal fissures of the argillic horizon postdate pockets of Holocene tephra within deeply bioturbated aggregates, while other channel argillans in the same part of the pedon are clearly older. This indicates that at least two distinct phases of clay accumulation have occurred, the most recent being contemporaneous with loess aggradation during the Holocene. Our results suggest that these welded profiles consist of a mollic-cambic horizon sequence that has formed primarily in Holocene loess and an albic-argillic sequence that has formed in Wisconsinan loess. The modern-day E horizon was initiated as the clay-eluvial surface horizon of the regionally recognized Washtucna paleosol that continued forming into the late-Wisconsinan cold phase and, upon burial by Holocene loess, it has been transformed into an albic horizon and even enlarged as a result of redoximorphic processes associated with seasonal saturation.