Nodular Horizons Research Articles

Weathering profiles developed on gneisses from Batchenga and Doua areas, central Cameroon: Climate and topography controls

Two weathering profiles from Batchenga and Doua areas in Central Cameroun developed under contrasting equatorial climate conditions and located in different morphological units have been selected to constraint the variability of their morphology, mineralogy, and geochemistry. The parent rock is a biotite and amphibole gneiss with a heterogranular granoblastic texture. The weathering profiles are typical lateritic materials formed in a humid tropical climate zone. They comprise, from bottom to top, coarse saprolite, fine saprolite, nodular horizon and loose clayey horizon. The Doua weathered materials are more clay-rich as a result of intense chemical weathering (CIA: 95.0–99.5%) than those of Batchenga (CIA: 51.3–87.8%). The weathering profile of Doua is thicker (12.00 m) than that of Batchenga (6.30 m). The intense chemical weathering and the total leaching of mobile elements as a consequence of good drainage led to the formation of a hard nodular horizon in the Doua weathering profile. The mineral composition is dominated by quartz, kaolinite, muscovite, illite, hematite/goethite, gibbsite and anatase. Illite is particularly abundant in the Batchenga weathering profile. Among trace elements, Ba, V, Zr, Sr, and Cr have high contents while REE show moderate contents with the highest contents in the Batchenga weathering profile. The oxidizing conditions favor positive Ce anomalies while positive Eu anomalies are due to the resistance of Eu mineral bearers. Geochemical mass balance results suggest that climate and topographic position which influence the quality of drainage greatly influence the distribution patterns of major, trace and rare earth elements in the weathering profiles. Apart from Al, Fe, and P, major elements have been much leached than trace elements during chemical weathering. Thus, the important variations are explained by the variation in the annual rainfall and topographic conditions.

Changing depocentre environments of Palaeolake Olduvai and carbonates as marker horizons for hiatuses and lake-level extremes

Primary carbonate and marl layers and limestone nodular horizons were intersected in OGCP Boreholes 1A, 2A, 3A, 3B, drilled into the depocentre of Palaeolake Olduvai. The various carbonate types were analysed, employing petrographic (including cathodo-luminescence), stable isotope, and sequence stratigraphic techniques, and recorded important information concerning lake evolution.Primary carbonate and marl layers are preserved at the top of lake deepening cycles (lake-parasequences), marking maximum flooding, followed by lake withdrawal, and then fluvial erosion, leading to the next depositional cycle. Presence of pelagites and a meromict lake favouring anoxic conditions, show it to be not a playa lake, but a deeper fault-bound lake, akin to present day Lake Eyasi. Carbonate and marl layers were exclusively deposited when claystone facies show a geochemical magnesium anomaly associated with a phase of basaltic volcanism affecting the basin, marked by mafic tuffs and basaltic lava flows. Calcium was partitioned into evaporites, such as gypsum/anhydrite, together with the nodular horizons and carbonate soil profiles in the sequence. The resulting high Mg2+/Ca2+ ratio in the lake waters promoted dolomite deposition and replacement, and the formation of aragonite.The nodular horizons yield rainfall isotope values and mark times when the lake was empty and the lake-bed fell under meteoric conditions, precipitating limestone nodules just below the sediment surface under pedogenic vadose, groundwater interface, and groundwater phreatic conditions. The nodular limestone horizons sit below erosional/hiatal surfaces and show vadose micritic and fibrous types to phreatic sparry and other accretionary type textures. Enterolithic to chickenwire textured nodules indicate the pseudomorphing of anhydrite nodules that form beneath salt marshes.Thus, the two carbonate types, primary layers and nodular horizons, provided depth gauges, respectively for the extremes of lake expansion and emptying/drying out. In the case of nodular horizons, the maturity of the carbonate profile gives an indication of the magnitude of the hiatal time gap represented, ~ 1 kyr to 2 kyr for a single horizon to ~ 3 kyr to 9 kyr for more mature soil profiles. Emptying/drying out episodes of Palaeolake Olduvai were not uncommon, and often short-lived. A spectrum of hiatal disconformities in the Olduvai Basin vary from multimillennial hiatuses to mega-disconformities of ~ 40 kyr and ~ 75 kyr.

Major and trace elements behaviour in two weathering profiles developed on gabbroic rocks in Meka'a and Abiete (south Cameroon, central Africa)

The present work deals with the behaviour of chemical elements in two weathering profiles developed on tholeiitic gabbro of Meka'a and alkali gabbro of Abiete, in southern Cameroon. The weathering profiles were characterized based on the colour, structure, texture, mineralogical and geochemical compositions. Elements mobility and redistribution during weathering processes are evaluated. The region is under an equatorial climate with four seasons strongly influenced by oceanic monsoon in the coastal zone. Geologically, the study area is located within the Ntem complex (Congo craton). Fresh rocks display a granular texture made up of plagioclase crystals, pyroxene, amphibole, biotite, apatite, chlorite, ilmenite and magnetite. The profile at Meka'a is made up of a coarse saprolite, a fine saprolite and a loose clayey horizon while that of Abiete comprises four horizons: coarse saprolite, fine saprolite, nodular, and loose clayey horizons. Common minerals are goethite, kaolinite, quartz and anatase. Halloysite is specific to Meka'a while gibbsite and hematite occur only at Abiete.In the weathered profile of Meka'a, organic matter contents decrease downwards while pH values increase. Fe2O3, Al2O3, TiO2, P2O5, Ba and Cu are partially removed from the top of the profile and accumulated in the saprolite layers whereas Cs, Th, HFSE and LREE (La, Ce, Pr) contents increase upwards. Transition elements and HREE are all leached out of the profile.At Abiete, OM content oscillates around 3.5 from the surface to the nodular horizon parallel with the accumulation of Al2O3 and Fe2O3. The nodular horizon shows a peculiar feature where abundant OM and an excellent drainage have induced the decrease of the pH value from 4.5 to 3 with concomitant leaching of Al2O3, Fe2O3, and many trace elements such as REE and HFSE. In the saprolite horizons, the OM content oscillates with concomitant accumulation of Al2O3 and Fe2O3. Positive Ce anomaly observed in the saprolitic set of Meka'a profile and in the coarse saprolite and the superficial clayey horizon at Abiete is assigned to oxidizing conditions in these levels.

The paleoecological and paleoenvironmental importance of root traces: Plant distribution and topographic significance of root patterns in Upper Cretaceous paleosols

Rhizoliths are organosedimentary structures produced by plants and are an important proxy for paleoenvironmental reconstructions because the differences in the morphology of rhizoliths and their rooting depths can be used to interpret soil humidity regimes, overall biomass at the soil surface, and the behavior of the water table. With the objective of advancing knowledge on rhizoliths, a study was performed on the macromorphological and micromorphological characterizations of the rhizoliths preserved in the thirty-one paleosols of the Marília Formation in Minas Gerais, southeastern Brazil. Based on the topological and architectural attributes of rhizoliths, and on their possible functional adaptations, such as mycorrhizal associations, three different types of rhizoliths are described in this study and represent at least three different plants: rhizohaloes, root casts and rhizocretions. Well-developed paleosols with nodular horizons contain the higher density of long root casts, indicating a seasonally dry environment, because the root casts present two meters in length. These adaptations are associated with an exploitation of a lowered water table during most times of the year. The higher density of rhizoliths indicates an area with agglomerated arboreous arbustive vegetation cover. Paleosols with chalky horizons also show a high density of root casts, indicating a landscape with high plant density that is adapted to drained soils, but with a seasonally elevated water table or the influence of ephemeral flows, given the proximity to the flow channels. The poorly drained paleosols with redoximorphic features show the lowest density of rhizoliths, thus indicating an incipient vegetation in response to near-surface water table and waterlogged soils in a distal floodplain. The rhizogenic laminar horizons have developed near fluvial channels with frequent sedimentary input, resulting in a sequence of laminar horizons composed of rhizocretions that are separated by sandy deposits or poorly developed pedogenic horizons. The micromorphological analysis on the rhizogenic laminar calcretes showed biogenic features, such as alveolar septal structures, calcified filaments, microcodium, calcified cells and pisoliths. Despite the scarce paleobotanical findings in the Marília Formation, the results obtained indicate an ecosystem that was composed of plants varying from herbaceous to arboreous, with an environment similar to modern semiarid shrublands.

Strontium isotopes reveal weathering processes in lateritic covers in southern China with implications for paleogeographic reconstructions.

The isotope ratios of Sr are useful tracers for studying parent material sources, weathering processes, and biogeochemical cycling. Mineralogical and geochemical investigations of two lateritic weathering covers, in an area close to the Tropic of Cancer (Guangxi Province, southern China), were undertaken to study the regional weathering processes and Sr isotopic sources. We found that weathering and decomposition of Rb- and Sr-bearing minerals change the Sr isotopic composition in weathering products (lateritic soils). Weathering of illite lowered the 87Sr/86Sr ratio whereas dissolving and leaching of carbonate minerals increased the 87Sr/86Sr ratio. An Fe nodular horizon is widely developed on the top of the weathering covers in the studied area and it differs from the lateritic soil horizon in mineral composition, construction, and elemental concentration. Furthermore, both Fe2O3 and P2O5 (concentrations) are negatively correlated with the 87Sr/86Sr ratios, suggesting fixation of apatite by Fe oxides is a controlling factor of the Sr isotopic composition in the Fe nodular horizon. The 87Sr/86Sr and Nb/Sr ratios imply the contents and proportions of Fe nodules and clay are critical in controlling the changes of Sr isotopic composition in the Fe nodular horizon. The two stages of the weathering process of carbonate rocks are revealed by the87Sr/86Sr versus Nb/Sr diagram. The 87Sr/86Sr and Rb/Sr ratios suggest that Sr isotopes in the weathering covers within the studied area are derived mainly from parent rock weathering and that the contributions from allothogenic Sr isotopes are limited. A comparison of Sr isotopic composition signatures in the weathering covers of the studied area and Guizhou Province provided insight into the Sr isotopic source and paleogeographic evolution of southern China. From the Permian to the Triassic, the continental fragment sources of the South China sedimentary basin changed significantly. In the Permian, Southern China presented the paleogeographic pattern that the north was higher (in elevation) than the south.

Crevasse splay processes and deposits in an ancient distributive fluvial system: The lower Beaufort Group, South Africa

Up to 12% of the mud-prone, ephemeral distributive fluvial system stratigraphy in the Permo-Triassic lower Beaufort Group, South Africa, comprises tabular fine-grained sandstone to coarse-grained siltstone bodies, which are interpreted as proximal to distal crevasse splay deposits. Crevasse splay sandstones predominantly exhibit ripple to climbing ripple cross-lamination, with some structureless and planar laminated beds. A hierarchical architectural scheme is adopted, in which ~1m thick crevasse splay elements extend for tens to several hundreds of meters laterally, and stack with other splay elements to form crevasse splay sets up to 4m thick and several kilometers in width and length. Paleosols and nodular horizons developed during periods, or in areas, of reduced overbank flooding are used to subdivide the stratigraphy, separating crevasse splay sets. Deposits from crevasse splays differ from frontal splays as their proximal deposits are much thinner and narrower, with paleocurrents oblique to the main paleochannel. In order for crevasse splay sets to develop, the parent channel belt and the location where crevasse splays form must stay relatively fixed during a period of multiple flood events. Beaufort Group splays have similar geometries to those of contemporary perennial rivers but exhibit more lateral variability in facies, which is interpreted to be the result of more extreme fluctuations in discharge regime. Sharp-based crevasse splay packages are associated with channel avulsion, but most are characterized by a gradual coarsening upward, interpreted to represent progradation. The dominance of progradational splays beneath channel belt deposits may be more characteristic of progradational stratigraphy in a distributive fluvial system rather than dominated by avulsion processes in a trunk river system. This stratigraphic motif may therefore be an additional criterion for recognition of distributive fluvial systems in the ancient record.

The formation of representative lateritic weathering covers in south-central Guangxi (southern China)

The mineralogy and geochemistry have been studied in three representative lateritic weathering covers close to the Tropic of Cancer (Guangxi, South China) with the purpose of studying formation and evolution processes of lateritic covers in this region. The X-ray diffraction and scanning electron microscope analysis results indicate that the contents, particle sizes, shapes and structures of secondary minerals, such as iron and clay minerals, are distinctly different in ferruginous nodular horizon and mottled clay layer of lateritic profiles. The ferruginous nodules are characterized by high concentrations of iron and aluminum and low contents of silicon. The energy analysis suggests that there are element exchanges between hematite crystal and clay minerals. The mass balance calculation results show that there is exogenetic input of iron into lateritic covers, especially within the ferruginous nodular horizon, compared to the bottom layer of the profile. The scanning electron microscope images also confirm that voids and fractures in ferruginous nodules were filled with ferruginous material derived from soil solution. This could be a significant interpretation for the negative correlations between Fe2O3 and Al2O3. The iron and manganese oxides in ferruginous nodular horizon have an apparent effect on the adsorption and precipitation of cerium. Strontium, compared to other alkali metal and alkali-earth metal elements, concentrates in mottled clay layer. Variation in the He/Gt ratio reflected by the O/Fe ratio in a small ferruginous concretion indicates that climate changes altered aqueous activity of the soil environment. Finally, a model for the formation and evolution of typical lateritic weathering covers in south-central Guangxi, supported by all the data, was proposed.

Gold Behavior in Weathering Products of Quartz Vein in Mintom Area South Cameroon (Central Africa)

Gold mineralization in Mintom area, south Cameroon was studied in a tropical forest setting using X-ray diffraction, inductively coupled plasma-atomic emission spectrometry (ICP/AES) and inductively coupled plasma-mass spectrometry (ICP/MS) respectively for the mineralogical and chemical data. The mineralization occurs in quartz veins in the archean Ntem complex of the vast Congo Craton in Central Africa. Gold distribution patterns were vertically studied in the different horizons of the weathering profile and in the different grain-size fractions of the materials sampled in the pit on down slope of the interfluve where the mineralization exists. The weathering profile consists of an upper, thin loose sandy-clayey horizon (P6) covered by a light humic horizon, a nodular horizon with lateritic nodules or blocks (P5), a gravel horizon (P4), a thin spotted horizon (P3) and a saprolite (P2) up to 1.4 m thick. The specific geochemical signature of the bedrock is not recognized in each horizon of the weathering profile. Some groups of elements, e.g., high SiO2 and low REE characterize quartz vein while Cr-Ni characterizes a basic rock like gabbro. The residual gold is concentrated at the base of weathering profile. Its concentration increases from the saprolite up to the gravel horizon and decreases in the surface horizons. However, the evolution of visible Au distribution is not the same for all grain-size fractions: 1) in the finest fraction, the Au content is only regular in spotted and gravel horizons; 2) in the medium size fractions, the Au is in high content and greatly decreases from saprolite up to the spotted horizon and disappears in the upper horizons; 3) in the coarsest fraction, Au content is found in the saprolite and the maximum Au content of the weathering profile is found in this layer just above the mineralized quartz vein. This observation shows that the Mintom residual gold comes from the quartz vein.

Rare earth and other trace and major elemental distribution in a pedogenic calcrete profile (Slimene, NE Tunisia)

A calcrete profile developed on the top of a calcareous consolidated dune located in a coastal area of NE Tunisia (semi-arid climate) was studied with the aim to investigate the behavior of the chemical elements (rare earth elements—REE—and other trace and major elements) during the processes associated with calcrete formation, particularly dissolution and precipitation of carbonates in the vadose zone. The profile shows a vertical sequence, with clear zonations from the surface downwards: a reddish soil at the land surface, a nodular horizon, a laminar-structured level, and the consolidated old dune. Chemical and mineralogical analyses of samples from all levels were performed by neutron activation analysis and X-ray diffraction (bulk samples and < 2 μm fraction). Detailed studies of iron speciation and iron minerals were done by Mossbauer spectroscopy. Microfauna, mineralogical and chemical variations in the Slimene weathering profile point to a long term aerial exposure of the old dune in a semi-arid carbonated environment leading to the development of a pedogenic calcrete profile. The mineralogical variations with depth indicate carbonate dissolution at the surface and downward leaching of calcium. Secondary carbonates overgrow primary ones and precipitate as coating or concretions below. Minerals found in the upper levels correspond to original materials of the old dune and also weathering phases and atmospheric inputs. Phyllosilicates decrease while calcite and K-feldspars increase with depth. Kaolinite was the only clay mineral found in the old dune. Illite and chlorite were also found in all samples with calcrete. Authigenic smectite formation occurs above the laminar-structured calcrete due to restricted drainage conditions. The most significant chemical variations associated with the calcrete formation are (i) enrichment of Co, U, Br, and REE in calcrete; (ii) depletion of middle REE in the upper levels, particularly Eu, and enrichment of middle REE and heavy REE in calcrete; and (iii) Hf, Zr, Cr, Th, Cs, Ta, Ga, Rb, and K appear to be retained in the upper levels, where calcite has been dissolved. Here iron is more reduced. Fe3+ occurs in iron oxides (goethite and hematite), and clay minerals. The ratio goethite/hematite appears to increase down the profile; and Fe2+ is mainly present in clay minerals.

Formation and preservation of pedogenic carbonates in South India, links with paleo-monsoon and pedological conditions: Clues from Sr isotopes, U–Th series and REEs

The influence of the pedogenic and climatic contexts on the formation and preservation of pedogenic carbonates in a climosequence in the Western Ghats (Karnataka Plateau, South West India) has been studied. Along the climosequence, the current mean annual rainfall (MAR) varies within a 80 km transect from 6000 mm at the edge of the Plateau to 500 mm inland. Pedogenic carbonates occur in the MAR range of 500–1200 mm. In the semi-arid zone (MAR: 500–900 mm), carbonates occur (i) as thick hardpan calcretes on pediment slopes and (ii) as nodular horizons in polygenic black soils (i.e. vertisols). In the sub-humid zone (MAR: 900–1500 mm), pedogenic carbonates are disseminated in the black soil matrices either as loose, irregular and friable nodules of millimetric size or as indurated botryoidal nodules of centimetric to pluricentimetric size. They also occur at the top layers of the saprolite either as disseminated pluricentimetric indurated nodules or carbonate-cemented lumps of centimetric to decimetric size. Chemical and isotopic ( 87Sr/ 86Sr) compositions of the carbonate fraction were determined after leaching with 0.25 N HCl. The corresponding residual fractions containing both primary minerals and authigenic clays were digested separately and analyzed. The trend defined by the 87Sr/ 86Sr signatures of both labile carbonate fractions and corresponding residual fractions indicates that a part of the labile carbonate fraction is genetically linked to the local soil composition. Considering the residual fraction of each sample as the most likely lithogenic source of Ca in carbonates, it is estimated that from 24% to 82% (55% on average) of Ca is derived from local bedrock weathering, leading to a consumption of an equivalent proportion of atmospheric CO 2. These values indicate that climatic conditions were humid enough to allow silicate weathering: MAR at the time of carbonate formation likely ranged from 400 to 700 mm, which is 2- to 3-fold less than the current MAR at these locations. The Sr, U and Mg contents and the ( 234U/ 238U) activity ratio in the labile carbonate fraction help to understand the conditions of carbonate formation. The relatively high concentrations of Sr, U and Mg in black soil carbonates may indicate fast growth and accumulation compared to carbonates in saprolite, possibly due to a better confinement of the pore waters which is supported by their high ( 234U/ 238U) signatures, and/or to higher content of dissolved carbonates in the pore waters. The occurrence of Ce, Mn and Fe oxides in the cracks of carbonate reflects the existence of relatively humid periods after carbonate formation. The carbonate ages determined by the U–Th method range from 1.33 ± 0.84 kyr to 7.5 ± 2.7 kyr and to a cluster of five ages around 20 kyr, i.e. the Last Glacial Maximum period. The young occurrences are only located in the black soils, which therefore constitute sensitive environments for trapping and retaining atmospheric CO 2 even on short time scales. The maximum age of carbonates depends on their location in the climatic gradient: from about 20 kyr for centimetric nodules at Mule Hole (MAR = 1100 mm/yr) to 200 kyr for the calcrete at Gundlupet (MAR = 700 mm/yr, Durand et al., 2007). The intensity of rainfall during wet periods would indeed control the lifetime of pedogenic carbonates and thus the duration of inorganic carbon storage in soils.

Depth to gypsic horizon as a proxy for paleoprecipitation in paleosols of sedimentary environments

Pedogenic accumulations of crystals and nodules of gypsum are common in desert soils, especially extreme deserts such as the Atacama Desert, Chile. Some soils with both pedogenic gypsum and calcite have the gypsic (By) horizon below the calcic (Bk) horizon. Here we present a compilation of 88 gypsic soils from around the world to derive a relationship (R 2 = 0.63, standard error = ±129 mm) between depth to the By horizon (D, in centimeters) and mean annual precipitation (P, in millimeters) as follows: P = 87.593e 0.0209D . This relationship can be used to derive paleoprecipitation estimates from paleosols, once depth to the gypsic horizon is corrected for compaction due to overburden. Application of this technique to Early Permian paleosols near Gilliland, Texas, confi rms paleoprecipitation estimates in the same sequence derived from depth to the calcic horizon. Barite is another sulfate mineral that forms nodular horizons in paleosols, but not in modern soils. Miocene paleosols in Panama with both calcareous and barite nodules suggest that this weakly soluble salt forms at levels in paleosols unlike those of either pedogenic carbonate or gypsum.

Paleoclimatic implications (Late Cretaceous–Paleogene) from micromorphology of calcretes, palustrine limestones and silcretes, southern Paraná Basin, Uruguay

Abstract Sedimentologic and petrographic analyses of outcroping and subsurface calcretes, palustrine carbonates, and silcretes were carried out in the southern Parana Basin (Uruguay). The aim of this work is to describe the microfabric and interpret the genesis of these rocks through detailed analyses, since they contain significant paleoenvironmental and paleoclimatic evolution information. The main calcrete and silcrete host rock (Mercedes Formation) is represented by a fluvial thinning upward succession of conglomerate and sandstone deposits, with isolated pelitic intervals and paleosoils. Most of the studied calcretes are macroscopically massive with micromorphological features of alpha fabric, originated by displacive growth of calcite in the host clastic material due to evaporation, evapotranspiration and degassing. Micromorphologically, calcretes indicate an origin in the vadose and phreatic diagenetic environments. Micrite is the principal component, and speaks of rapid precipitation in the vadose zone from supersaturated solutions. The abundance of microsparite and secondary sparite is regarded as the result of dissolution and reprecipitation processes. Although present, brecciated calcretes are less common. They are frequent in vadose diagenetic environments, where the alternation between cementation and non-tectonic fracturing conditions take place. These processes generated episodes of fragmentation, brecciation and cementation. Fissures are filled with clear primary sparitic calcite, formed by precipitation of extremely supersaturated solutions in a phreatic diagenetic environment. The micromorphological characteristics indicate that calcretes resulted from carbonate precipitation in the upper part of the groundwater table and the vadose zone, continuously nourished by lateral migration of groundwater. The scarcity of biogenic structures suggests that they were either formed in zones of little biological activity or that the overimposed processes related to water table fluctuations produced intense recrystallization completely obliterating the biogenic fabric. Limestone beds containing terrestrial gastropods are geographically restricted. Situated at the top of the calcrete successions, they exhibit brecciated and peloidal-intraclastic textures but lack lamination, edaphic structures, aggregates and vertical rhizoliths. This indicates they correspond to low-energy palustrine deposits, generated in shallow, local and ephemeral ponds developed in topographic depressions. When water table levels dropped, the palustrine deposits were exposed. This favours the presence of terrestrial gastropods, seeds and insect nests. The combination of calcretes and palustrine carbonates indicates periods and areas with a reduced clastic input and a predominantly semiarid climate, with well-defined humid and dry seasons. Characteristics of the later developed massive and nodular horizons of silcretes, such as, preservation of the internal structure of the host rock, the small areal extent, the formation of massive lenses, the complex pore infillings and the lack of a columnar upper section, indicate that they were generated from groundwaters. Every silcretized horizon shows different positions of the groundwater table and relates to the dissection of landscape. The age of calcretization and silcretization is bracketed between the Late Cretaceous (Campanian–Maastrichtian) and the Early Eocene. Paleoclimate indicates changing conditions from warm and humid at the end of the Cretaceous (Mercedes Formation) to semiarid and seasonal during Paleocene (groundwater calcretes and palustrine deposits) and subtropical and seasonal in the early Eocene (Asencio Formation).

Genesis of subtropical soils with stony horizons in NE Argentina: Autochthony and polygenesis

Deep red soils, combining characteristics of highly weathered materials together with a significant degree of clay illuviation and with stony horizons close to the saprolite, are found in the Province of Misiones, northeastern Argentina. Two basic types of stony horizons have been found, which with the fine-grained material above are together considered autochthonous. The first one is a “ferruginous nodular horizon”, composed mainly of in situ goethitic nodules of gravel size resulting from weathering and glaebulization processes of basalt. The second are “siliceous horizons” which are also in situ, derived from pre-existing quartz veins within the basalt. In contrast, different analytical evidences, particularly the vertical and geographical variations in the SOM-stable carbon isotope ratios and the mineralogical composition of fine and coarse fractions, suggest that these red subtropical soils are polygenetic, being the result of a two-fold process linked to paleoecological fluctuations. In the framework of these results, different interpretations about the origin of these soils and their parent materials, and particularly the “tropical loess” theory, are discussed.

Chemical and physical transfers in an ultramafic rock weathering profile: Part 1. Supergene dissolution of Pt-bearing chromite

Chemical weathering and supergene dissolution processes of Pt-bearing chromite have been studied in a lateritic weathering profile developed on ultramafic rocks in New Caledonia (southwest Pacific). The chemical distributions of alkaline earth, transition metals, and precious metals (including Pt and Pd) were determined in a weathering profile varying from bedrock at the base upward through coarse and fine saprolites, and capped by a mottled zone and a lateritic colluvial nodular horizon. Chemi- cal analyses and mass-balance calculations suggest that progressive weathering of the parent rock is characterized by an enrichment of Fe, Co, and Mn, a segregation of Ni at the boundary between the bedrock and the coarse sapolite and in the lower part of the fine saprolite, and a depletion of Mg, Ca, Si, Al, and Cr. The higher concentration of transition metals at the interface between the coarse and fine saprolite is due to vertical transfer and precipitation at the base of the weathering profile. In such a lateritic environment, the Pt-bearing chromite grains are progressively dissolved and the Pt-group minerals (PGM) are released in the weathering mantle with a preferential depletion of Pd with regard to Pt.

Mobilization and redistribution of major and trace elements in two weathering profiles developed on serpentinites in the Lomié ultramafic complex, South-East Cameroon

The behaviour of major and trace elements have been studied along two serpentinite weathering profiles located in the Kongo-Nkamouna and Mang North sites of the Lomié ultramafic complex. The serpentinites are characterized by high SiO 2 and MgO contents, very low trace, rare earth and platinum-group element contents. Lanthanide and PGE contents are higher in the Nkamouna sample than in Mang North. Normalized REE patterns according to the CI chondrites reveal that: (i) all REE are below chondrites abundances in the Mang North sample; (ii) the (La/Yb) N ratio value is higher in the Nkamouna sample (23.72) than in the Mang one (1.78), this confirms the slightly more weathered nature of the Nkamouna sample. Normalized PGE patterns according to the same CI chondrites reveal a negative Pt anomaly in the Mang sample. The Nkamouna sample is characterized by a flat normalized PGE pattern. All element contents increase highly from the parent rock to the coarse saprolite. In the weathering profiles, Fe 2O 3 contents decrease from the bottom to the top contrarily to Al 2O 3, SiO 2 and TiO 2. The contents of alkali and alkaline oxides are under detection limit. Concerning trace elements, Cr, Ni, Co, Cu, Zn and Sc decrease considerably from the bottom to the top while Zr, Th, U, Be, Sb, Sn, W, Ta, Sr, Rb, Hf, Y, Li, Ga, Nb and Pb increase towards the clayey surface soil. Chromium, Ni and Co contents are high in the weathered materials in particular in the saprolite zone and in the nodules. REE contents are high in the weathered materials, particularly in Nkamouna. Their concentrations decrease along both profiles. Light REE are more abundant than heavy REE. Normalized REE patterns according to the parent rock reveal positive Ce anomalies in all the weathered materials and negative Eu anomalies only at the bottom of the coarse saprolite (Nkamouna site). Positive Ce anomalies are higher in the nodular horizon of both profiles. An additional calculation method of lanthanide anomalies, using NASC data, confirms positive Ce anomalies ([Ce/Ce ∗] NASC = 1.15 to 60.68) in several weathered materials except in nodules ([Ce/Ce ∗] NASC = 0.76) of the upper nodular horizon (Nkamouna profile). The (La/Yb) N ratios values are lower in the Nkamouna profile than in Mang site. PGE are more abundant in the weathered materials than in the parent rock. The highest contents are obtained in the coarse saprolite and in the nodules. The elements with high contents along both profiles are Pt (63–70 ppb), Ru (49–52 ppb) and Ir (41 ppb). Normalized PGE patterns show positive Pt anomalies and negative Ru anomalies. The mass balance evaluation, using thorium as immobile element, reveals that: – major elements have been depleted along the weathering profile, except for Fe, Mn and Ti that have been enriched even only in the coarse saprolite; – all the trace elements have been depleted along both profiles, except for Cr, Co, Zn, Sc, Cu, Ba, Y, Ga, U and Nb that have been enriched in the coarse saprolite; – rare earth elements have been abundantly accumulated in the coarse saprolite, before their depletion towards the top of the profiles; – platinum-group elements have been abundantly accumulated in the coarse saprolite but have been depleted towards the clayey surface soil. Moreover, from a pedogenetical point of view, this study shows that the weathering profiles are autochtonous, except in the upper part of the soils where some allochtonous materials are revealed by the presence of zircon grains.

Geoelectrical imaging of a thick regolith developed on ultramafic rocks: groundwater influence

A thick regolith with supergene nickel ore deposits, developed from ultramafic rocks in New Caledonia (southwest Pacific), was investigated using electrical resistivity tomography (ERT). This paper presents the results of an electrical survey and monitoring of the lateritic weathering mantle of Tiebaghi plateau. Resistivity pseudosections, validated by many borehole logs, contributed to defining a four-layer geoelectric model corresponding to four main weathering horizons: (i) ferricrete with soft nodular horizon; (ii) red mottled zone and yellow fine saprolite; (iii) coarse saprolite; and (iv) bedrock. All geoelectric sections reveal undulated bedrock topography, with each high representing a resistive bedrock ridge and each swale corresponding to a conductive saprolite trough. When parallel pseudosections are correlated, most ridges and troughs have a prominent northeast – southwest strike. Seasonal monitoring of a selected ERT profile shows that the geoelectric structure is well preserved through time. Local resistivity variations occur in the mottled zone – fine saprolite layer and along a major fracture zone, and are interpreted as being due to water content variations in groundwater pathways. On the basis of field and laboratory experiments, it is proposed that the resistivity variations are not due to dewatering but could result from fluctuations in groundwater chemistry. ERT appears to be a useful tool to document the geometry of the regolith and the groundwater flow pattern in weathering horizons and bedrock.

Pedogenic carbonates on Precambrian silicate rocks in South India: Origin and paleoclimatic significance

Calcrete in Alfisols on Precambrian silicate rocks is studied here in an attempt to reconstruct paleorainfall trends in semiarid South India. Facies of observed carbonate accumulation are pseudomycelium, calcrete nodules, nodular horizons, hardpan, and laminar horizons. Based on geochemical and Sr isotope tracing we relate the presence of calcrete to local geologic conditions within the Precambrian basement, notably discrete Ca-rich metamorphic outcrops, and rule out eolian input. Micromorphologic analysis reveals a multi-stage origin of the calcretes involving soil, colluvial, geochemical and biologic processes mostly in vadose environments. Preliminary U–Th ages of 190–200 ka obtained on calcrete nodules suggest a major phase of Ca release (rock weathering) and precipitation during marine oxygen isotope stage 7. The calcrete thus indicates fairly stable semiarid conditions at the ⩾200 ky time scale despite fluctuations around an annual rainfall average close to current values (∼700 mm). The stability of the Western Ghats barrier as a shield to the southwest monsoon explains the long-term average stability of the pedoclimatic environment in the rainshadow zone. The radiometric ages suggest that weathering may occur massively at discrete intervals of geologic history, during spikes of humidity in the paleoclimatic record. The rest of the time is spent recycling and redistributing the products of weathering incrementally at landscape scale, within seasonal cycles of a relatively stable semi-arid climate. Dating the calcrete therefore does not equate with dating the soil but, instead, the residence times of nodules in a dynamic soil system in which different constituents have different turnover rates.

Geochemical and mineralogical investigation of the Permian–Triassic boundary in the continental realm of the southern Karoo Basin, South Africa

Mineralogical and geochemical studies of the non-marine Permian–Triassic (P–Tr) boundary across two stratigraphically well-constrained sections (Commando Drift Dam and Wapadsberg, Eastern Cape Province) in the southern Karoo Basin, South Africa, have been undertaken to provide further input on the cause of this mass extinction event, and so has a sedimentological and geochemical evaluation of a third P–Tr boundary section at Injusiti (Kwazulu-Natal) in the eastern Karoo Basin, South Africa. The Commando Drift Dam section has been constrained by previous palaeontological and palaeomagnetic work, with a palaeomagnetic reversal positioned 5.3 m above the palaeontological P–Tr boundary. The Wapadsberg section has been constrained palaeontologically. All these P–Tr sections studied here mostly comprise mudstones, together with siltstones, sandstones, and in the southern Karoo Basin, carbonate nodular horizons. A change in colour of the mudstones from green-grey to red-brown takes place at the palaeontologically defined boundary. Variations in the major and trace element abundance profiles are usually limited to the carbonate nodular horizons, besides the overall effects of weathering. Bulk carbon isotopic studies of the Commando Drift Dam section revealed a negative δ 13C bulk excursion (background values of −15 to −20‰, with total extent of excursion to −24.9‰) 2 cm below the palaeontological boundary, followed by a gradual recovery and then another decrease in values leading towards the palaeomagnetic boundary. Above this boundary recovery to less negative δ 13C bulk values (ca. −18‰) occurs. The organic carbon record from the Commando Drift Dam (southern Karoo Basin) and Injusiti (eastern Karoo Basin) oscillates between −26.1 and −28.9‰, which is comparable to previous studies of different sections in the southern Karoo Basin. The magnitude of both the bulk and organic carbon isotopic variation can be interpreted to indicate a number of inputs (due to the fluctuating values) of organic carbon. The carbon isotope data for carbonates in the Injusiti section are different from the results on carbonates from other studies, but more work to expand this dataset is necessary. The palynological studies on the Commando Drift Dam section reveal the presence of a low diversity flora composed principally of bryophytes, lycophytes, and gymnosperms. These forms, including several Late Permian key-species, are traces of the surviving plants enduring after the major extinction-pulse. The presence of fungal palynomorphs and dearth of pollen/spores related to photosynthetic plants some metres above the palaeontological P–Tr boundary demonstrate similarities to the pattern of floral extinction at the Cretaceous–Palaeogene (K–Pg) boundary. The timing of vertebrate extinctions in the Karoo Basin has so far not been determined, but the 252.5 Ma age for a single zircon crystal from Commando Drift Dam dated here gives a maximum constraint on the age of the event bed, which is in agreement with the accepted age of the boundary. No evidence for impact-produced microdeformation features were found in quartz grains from either the Wapadsberg or Commando Drift Dam sections. Also, siderophile element data (including platinum group element (PGE) concentrations) do not support the possible presence of a meteoritic component at the boundary. Thus, a link between impact and P–Tr extinction is not indicated by the results of this study.

Pedogenic carbonate proxies for amount and seasonality of precipitation in paleosols

The depth to carbonate nodular (Bk) horizon in soils ( D in cm) is correlated with mean annual precipitation ( P in mm), so that Bk horizons are deep in subhumid regions, but shallow in semiarid regions. Previous quantifications of this relationship are unchanged by this new compilation of 807 soils: P = 137.24 + 6.45 D + 0.013 D 2 , where R 2 = 0.52, and standard error (S.E.) = ±147 mm. In most North American postglacial soils, the Bk horizon is thin and well defined, whereas in monsoonal tropical soils of Pakistan and Kenya, the Bk horizon is thick and diffuse. Data from 675 modern soils define the relationship between the thickness of soil with nodules ( T in cm) and mean annual range of precipitation ( M in mm difference between monthly means of wettest and driest months): M = 0.79 T + 13.71, where R 2 = 0.58, and S.E. = ±22 mm). The relationship between carbonate nodule size ( S in cm) and soil age ( A in ka) is quantified by 9 radiocarbon-dated soils from Las Cruces, New Mexico: A = 3.92 S 0.34 , where R 2 = 0.57, and S.E. = 1.8 k.y. These transfer functions are applied to reconstructing paleoclimatic change from Paleocene–Eocene paleosols of the North Horn Formation and Flagstaff Limestone of Axhandle Canyon, Utah. The terminal Paleocene spike of warmth recorded by fossil plants in nearby Wyoming was coincident with a brief peak in both mean annual precipitation and mean annual range of precipitation in Utah.

Evolution and alteration in situ of a massive iron duricrust in Central Africa

A soil sequence with iron duricrust is described in an area covered by tropical rain forest in South Cameroon. The dismantling of the iron duricrust is documented through a close observation of a soft duricrust, which corresponds to a transitional stage in the degradation of a massive iron duricrust into a loose nodular horizon. In the initial massive and hematitic duricrust, nodular shapes are progressively formed. The nodules and the internodular matrix remain hematitic. The internodular matrix undergoes goethitization and a pronounced deferruginisation before loosening; the primary structure of the iron duricrust is maintained, however, due to internodular bridges, relics of internodular matrix which escaped the process of goethitization. The iron is gradually released from these hematitic bridges, which become softer. This leads to the collapse of the initial structures of the iron duricrust and to the formation of a loose nodular material with a clayey matrix containing kaolinite and goethite. Many loose nodular horizons, which are found all over Central Africa, may have been formed by such alteration of a former iron duricrust.