Pedogenic Carbonate Formation Research Articles

Pedogenic Carbonate and Soil Dehydrogenase Activity in Response to Soil Organic Matter in Artemisia ordosica Community

Little attention has been paid to the role of soil organic matter (OM) in the formation of pedogenic carbonate in desert soils. The relationships among soil OM, soil dehydrogenase activity (DHA), and soil CaCO3 inaplantcommunity dominated by Artemisia ordosica, located on the eastern boundary of Tengger Desert in the Alxa League, Inner Mongolia, China, were studied to understand whether OM was directly involved in the formation of pedogenic carbonate. The results showed that DHA and CaCO3 positively correlated with OM content, and DHA, OM, and CaCO3 were correlated with each other in their spatial distribution, indicating that abundant OM content contributed to the formation of CaCO3. Therefore, the formation of pedogenic CaCO3 was a biotic process in the plant community dominated by A. ordosica.

Chemical and textural controls on phosphorus mobility in drylands of southeastern Utah

We investigated several forms of phosphorus (P) in dryland soils to examine the chemical and textural controls on P stabilization on a diverse set of substrates. We examined three P fractions including labile, moderately labile, and occluded as determined by a modified Hedley fractionation technique. The P fractions were compared to texture measurements and total elemental concentrations determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Labile P related to the absence of materials involved in P sorption. Moderately labile P was most strongly associated with high total Al & Fe content that we interpret to represent oxides and 1:1 clay minerals. The occluded P fraction was strongly associated with low total Al & Fe environments and interpreted to represent 2:1 clay minerals where ligand exchange tightly sequesters P. The results indicate that the controls on P fraction distribution are initially closely tied to the chemical and physical properties of the bedrock units that contribute to soil formation. Further, these results suggest that the progression of stabilized P forms in dryland areas differs from the progression observed in mesic environments. Soil development in dryland settings, such as the formation of pedogenic carbonates, may lead to differing controls on P availability and the proportional size of the moderately labile fraction.

Seasonal bias in the formation and stable isotopic composition of pedogenic carbonate in modern soils from central New Mexico, USA

In order to better calibrate pedogenic carbonate as a proxy for past environments, we compared the stable isotopic composition of soil CO 2 , soil water, and pedogenic carbonate in young soils from central New Mexico, USA. Seasonal changes in the δ 13 C value of soil CO 2 , the δ 18 O value of soil water, and the soil temperature were monitored to establish the timing of isotopic equilibrium with the carbonate. Calcite solubility was calculated from measured temperatures and CO 2 concentrations in the soil. This approach allowed us to determine the conditions associated with pedogenic carbonate formation. Carbon isotope equilibrium, oxygen isotope equilibrium, and minimum calcite solubility all occurred simultaneously during warm, dry conditions in May 2008 when soil CO 2 concentrations were low. It is therefore concluded that pedogenic carbonate forms during warm, dry periods and does not record mean growing season conditions as typically assumed. The seasonal bias in pedogenic carbonate formation may explain the occurrence of pedogenic carbonate in monsoon climates and its absence in regions where annual precipitation is more uniformly distributed. The implications of the seasonal bias for stable isotope–based paleoenvironmental reconstructions are that paleoelevations may have been previously over- or underestimated, paleoatmospheric CO 2 concentrations likely have been signifi cantly overestimated, and pedogenic carbonate provides a C 4 -biased record of paleovegetation, especially in dry soils.

A microbial pathway for the formation of gold-anomalous calcrete

The formation of pedogenic carbonate (calcrete) in terrestrial environments is commonly mediated by microorganisms. In Australia, Au-anomalous calcrete is an important sampling medium for geochemical exploration, but current models describing its formation do not include a confirmed microbial component. This study demonstrates that bacterial communities in calcareous sands from dunes overlying the Barns Gold Deposit in semi-arid South Australia, are capable of mediating the biomineralisation of Au-anomalous carbonates. Bacterial enrichment cultures obtained from calcareous sands at three depths (0.1, 0.64 and 2.1 m, plus abiotic control) were incubated in urea and Ca 2+-containing growth media (pH 8), unamended and amended with Au (100 parts-per-billion, ppb) as Au–aspartic-acid complex. During the incubation of the enrichment cultures urea was turned over to NH 4 + within 96 h to 220 h. The solution pH increased concurrently by approximately 1.2 units, and Au-anomalous Ca-carbonate crystallites were precipitated on cells, which functioned as nucleation sites; no carbonate precipitation was observed in abiotic controls. Compared to the medium, Au was strongly enriched in these carbonates and appeared to be uniformly dispersed in the individual crystallites, as shown using LA-ICP-MS; a similar distribution is present in naturally occurring Au-anomalous calcrete. Phylogenetic 16S rRNA PCR DGGE analyses, shotgun cloning and functional microbial analyses (BioLog, ureC quantitative PCR) demonstrated that naturally occurring and culture-enriched bacterial communities were dominated by alkaliphylic, halotolerant Bacillus spp. The indigenous bacterial communities were capable of utilising amino acids (including l-aspartic acid) and urea, which appears to lead to the destabilisation of the Au–amino acid complexes and concomitant co-precipitation of Au in the Ca-carbonates. In conclusion, a model combining geomicrobial– with evapotranspiration– and plant-based components is likely to best describe the formation of (Au-anomalous) calcrete in semi-arid and arid zones.

Biological enhancement of soil carbonate precipitation: passive removal of atmospheric CO2

AbstractSoils are the dominant terrestrial sink for carbon, containing three times as much C as above-ground plant biomass, and acting as a host for both organic and inorganic C, as soil organic matter and pedogenic carbonates, respectively. This article reviews evidence for the generation within the soil solution of dissolved C derived from plants and recognition of its precipitation as carbonates. It then considers the potential value of this process for artificially-mediated CO2 sequestration within soils. The ability of crops such as wheat to produce organic acid anions as root exudates is substantial, generating 70 mol/(y kg) of exuded C, equivalent to the plant's own ‘body weight’. This is still an order of magnitude less than measured C production from Icelandic woodlands (Moulton et al., 2000), which have no other possible source of C. Thus, there is apparently no shortage of available dissolved C, as bicarbonate in solution, and so the formation of pedogenic carbonates will be controlled by the availability of Ca. This is derived from mineral weathering, primarily of silicate minerals (natural plagioclase feldspars and pyroxenes; artificial cement and slag minerals). Within the UK, existing industrial arisings of calcium silicate minerals from quarrying, demolition and steel manufacture that are fine-grained and suitable for incorporation into soils are sufficient to account for 3 MT CO2 per year, compensating for half of the emissions from UK cement manufacture. Pursuing these arguments, it is shown that soils have a role to play as passive agents in the removal of atmospheric CO2, analogous to the use of reed beds to clean contaminated waters.

Stable isotope compositions of pedogenic carbonates and soil organic matter in a temperate climate Vertisol with gilgai, southern Russia

A Vertisol gilgai complex from the North Caucasus, Russia, was studied to evaluate the climatic and environmental conditions governing pedogenesis. The stable isotope compositions of soil organic matter (SOM) and carbonate pedofeatures in the Vertisol record a complex, but interpretable, pedogenic history. Variability in the isotopic composition of pedogenic carbonate as a function of morphology, position in gilgai microrelief, and soil depth emphasizes the importance of these parameters to interpreting pedogenic conditions. The isotopic compositions and age of SOM record evidence of pedogenesis in an earlier (> 5000 yr BP) environment that was warmer and drier than modern conditions, with a significant component of C 4 flora (up to 30%). This portion of the soil history is preserved only in the deepest portions of the microlow (> 140 cm) or in the central portion of the microhigh chimney. A change to cooler and wetter conditions was accompanied by a shift to an ecosystem dominated by C 3 vegetation (> 95%) and resulted in a wetter soil environment, particularly in gilgai microlow positions. Most pedogenic carbonate preserves isotopic compositions reflecting only the more recent climate history. The wetter environment contributed to recrystallization of pedogenic carbonate nodules and formation of soft carbonate masses. There are systematic differences in the isotopic composition of hard nodules and soft masses that indicate the influence of greater soil water evaporation, a greater proportion of xerophytic plants, and/or lower rates of soil respiration in the gilgai microhighs. Self-consistent interpretation of the pedogenic and paleoclimate history of these Vertisols requires consideration of a full suite of information, including carbonate pedofeature micromorphology, the stable isotope composition and age (absolute and relative) of these features, and other soil characteristics.

Carbon Distribution in a Hummocky Landscape from Saskatchewan, Canada

Changes in the topography influence organic and inorganic C contents and δ13C values of soil C across a landscape. The objectives of this research were to: (i) study the effect of landscape on the formation and distribution of pedogenic carbonate and organic matter distribution in a hummocky landscape, and (ii) estimate the amount of organic C and pedogenic carbonate accumulation in local scale in comparison with regional scale using the stable isotope geochemistry techniques and standard characterization analyses. A hummocky landscape, typical of 38% of Saskatchewan's land, with glacial till parent material under virgin grassland, was studied. Organic C content of A horizons range between 20 to 98 g kg−1 Both extremes occurred in level positions of the south‐facing and north‐facing slopes. The lowest δ13C value of organic C (−29.6‰) was measured in a depression and the highest (more positive) was obtained on a shoulder (−21.7‰). The δ13C values of carbonate ranged from −0.9‰ (carbonated parent material) at the 114‐cm depth in level complex to −7.9‰ at depth of 100 cm in footslope complex and depression. The amount and percentage of pedogenic carbonate was higher in north‐facing slopes than in southward slopes. The highest proportion and amount of pedogenic carbonate up to 1‐m depth was found in Calcicryolls in footslope complex position in the north‐facing slope, and likely represents a gain in carbonate through lateral flows. The lowest proportion and amount (34.4% and 33.9 kg m−2) was found in the shoulder complex segment of west‐facing slope and in footslope complex position in east‐west direction. On average, the rate of accumulation is about 1.25 g C m−2 yr−1 of inorganic C (pedogenic carbonate) and 1.25 g C m−2 yr−1 as organic C. These are close to the calculated rate of 1.4 g C m−2 yr−1 for Dark Brown, and 1.3 g C m−2 yr−1 for Black soils (Mollisols) in Saskatchewan.

Carbon Distribution in a Hummocky Landscape from Saskatchewan, Canada

Changes in the topography influence organic and inorganic C contents and δ13C values of soil C across a landscape. The objectives of this research were to: (i) study the effect of landscape on the formation and distribution of pedogenic carbonate and organic matter distribution in a hummocky landscape, and (ii) estimate the amount of organic C and pedogenic carbonate accumulation in local scale in comparison with regional scale using the stable isotope geochemistry techniques and standard characterization analyses. A hummocky landscape, typical of 38% of Saskatchewan's land, with glacial till parent material under virgin grassland, was studied. Organic C content of A horizons range between 20 to 98 g kg−1 Both extremes occurred in level positions of the south-facing and north-facing slopes. The lowest δ13C value of organic C (−29.6‰) was measured in a depression and the highest (more positive) was obtained on a shoulder (−21.7‰). The δ13C values of carbonate ranged from −0.9‰ (carbonated parent material) at the 114-cm depth in level complex to −7.9‰ at depth of 100 cm in footslope complex and depression. The amount and percentage of pedogenic carbonate was higher in north-facing slopes than in southward slopes. The highest proportion and amount of pedogenic carbonate up to 1-m depth was found in Calcicryolls in footslope complex position in the north-facing slope, and likely represents a gain in carbonate through lateral flows. The lowest proportion and amount (34.4% and 33.9 kg m−2) was found in the shoulder complex segment of west-facing slope and in footslope complex position in east-west direction. On average, the rate of accumulation is about 1.25 g C m−2 yr−1 of inorganic C (pedogenic carbonate) and 1.25 g C m−2 yr−1 as organic C. These are close to the calculated rate of 1.4 g C m−2 yr−1 for Dark Brown, and 1.3 g C m−2 yr−1 for Black soils (Mollisols) in Saskatchewan.

Paleoclimatic implications of pedogenic carbonates in Holocene soils of the Gangetic Plains, India

This study attempts to reconstruct the Holocene climatic history of the Indo-Gangetic Plains based on micromorphological characteristics and stable isotope composition of calcretes in a soil-chronoassociation between the Ramganga and Rapti rivers. The calcrete from the B2 horizon of older soils (QGH4–QGH5: 6500–13 500yearbp) consists of septaric and irregularly shaped nodules of dense micrite and diffused needles associated with the illuvial clay pedofeatures. Fabric of the calcretes of older soils show inclusion of soil constituents and precipitation of carbonates in between weathered layers of micas. Formation of this calcrete took place during the pedogenesis in an arid to semi-arid climate that also influenced vegetation and C4 type biomass such as Chenopodium and Typha angustata spread in the area. A change to warm and humid phase at 6500yearbp led to dissolution and reprecipitation of the calcretes. This led to the formation of blocky crystals and needles of calcite in voids and coarsening of the fabric in the lower horizons. The vegetation pattern also changed to woodland with Anogeissus and Tecomella spp. The younger soils (<2500yearbp) show secondary carbonate accumulation associated mainly along the voids in lower parts of the profiles. These are relatively coarse textured with little or no inclusion of soil constituents. This coarse grained calcrete results from capillary rise from a fluctuating water table in a sub-humid climate similar to present.Carbon and oxygen isotope data suggest that the formation of pedogenic carbonate in old soils (QGH4 and QGH5) by evapotranspiration during arid to semi-arid climate. In the upper horizons, the carbonates are depleted in 13C as their formation was influenced by the sparse vegetation and low soil respiration rate. In the lower horizons, the carbonates show enrichment of 13C due its extensive dissolution–reprecipitation during the humid climate. The younger soils showed the enrichment of 13C in their calcretes and support their formation from fluctuating shallow groundwater table. The oxygen isotope values similar to that of meteoric water of the area indicate calcrete precipitation in equilibrium with the soil water.

Physico‐chemical environment of pedogenic carbonate formation in Devonian vertic palaeosols, central Appalachians, USA

ABSTRACTThe morphology and geochemistry of pedogenic carbonate found in vertic claystone palaeosols in the Devonian Catskill Formation in central Pennsylvania preserve a record of the physical and chemical environment of carbonate precipitation. The carbonate is characterized by three distinct petrographic generations. Pedogenic rhizoliths and nodules are the earliest precipitated generation, and typically consist of dull red‐brown luminescent micrite. Clear, equant calcite spar cement fills voids in the centres of rhizoliths, as well as circumgranular cracks and septarian voids in nodules. Early spar cements are non‐luminescent to dull luminescent, whereas later spar cements exhibit bright yellow‐orange luminescence. Late stage pedogenic fractures are always occluded with very bright yellow‐orange luminescent spar cements. The incorporation of progressively higher concentrations of Mn (up to 34000 ppm) into successively younger calcite spar cements, without concomitant increases in Fe, suggests carbonate precipitation from an evolving meteoric water in which Mn2+ became increasingly mobile over time. The increased mobility is possibly due to decreasing Eh, resulting from oxidation of organic matter after rapid soil burial on the floodplain. The amount of Fe2+ available for incorporation into calcite was limited because most iron was immobile, having been earlier oxidized and bound to the palaeosol clay matrix as a poorly crystallized ferric oxide or oxyhydroxide mineral. Carbon isotope compositions of pedogenic carbonate correlate with the inferred depth of carbonate precipitation. Rhizoliths preserved below the lowest stratigraphic occurrences of pedogenic slickensides are consistently depleted in 13C relative to nodules, which formed stratigraphically higher, within the zone of active soil shrink and swell processes. Nodular carbonate, precipitated in proximity to deep cracks in the soil, is enriched due to increased gas exchange with isotopically heavy atmospheric CO2. Accordingly, rhizolith compositions will most accurately estimate palaeoatmospheric levels of CO2; the use of nodule compositions may result in overestimation of PCO2 by as much as 30%.

Paleosols in Permo-Triassic Continental Sediments from Prydz Bay, East Antarctica

ABSTRACT ODP Leg 119 drilled a sequence of ?Permo-Triassic continental red beds 58 m thick containing paleosols in Prydz Bay, East Antarctica. The paleosols, mainly in overbank siltstones and mudstones, are characterized by extensive destruction of sedimentary structures and the presence of mottles, rootlets, and rare chalky caliche. Because of the shallow depth of burial and limited compaction and diagenesis, the paleosols have retained most of their original microstructure and fabric. Micromorphological analysis of the paleosols reveals the presence of phreatic monopodial root systems, nodules, and a variety of soil structures and fabrics as well as a micritic root structure with alveolar texture. Most of the paleosols are weakly developed, with small, incomplete ped structures and lack of illuviation. They resemble weakly developed alluvial soils with A-C and B(C) profiles, and are similar to modern base-deficient inceptisols. The nature of the paleosols suggests relatively high permeabilities and water circulation during soil formation and a relatively mobile water table, with little reduction of iron except locally on the less well drained parts of the alluvial plain. Extensive pedogenic carbonate formation was precluded by the soil-moisture regime and relatively wet climate. During Late Permian and Early Triassic times Prydz Bay, which lay some 30° south of the Equator, experienced climatic conditions broadly similar to parts of the contemporary tropics, dominated by a ubtropical high-pressure system, easterly winds, and seasonal rainfall.

Identification of Pedogenic Carbonates using Stable Carbon Isotope and Microfabric Analyses

AbstractIn soils formed from limestone, it is often difficult to distinguish between inherited and pedogenic carbonate. In nature, both thermodynamic and kinetic factors have caused fractionation of C isotopes. While marine carbonates usually have δ13C values near zero, the processes of pedogenic carbonate formation result in considerably lower values, which are dependent only on the δ13C of the soil CO2 gas. Therefore, a simple proportionality may be employed to quantitatively estimate pedogenic carbonates in the soil.Seven pedons formed over Cretaceous limestone in Central and West Texas were studied. The δ13C values were determined for the parent carbonates, soil organic matter, and for the various carbonate‐enriched horizons. Both finely disseminated carbonates in A horizons and massive indurated petrocalcic materials were confirmed by isotopic analyses to be primarily of pedogenic origin. Characteristic microfabrics were correlated with isotopic assay of massive petrocalcic materials which may be diagnostic for identification. Although generally useful, isotopic analysis underestimates the pedogenic component in certain carbonate forms, including laminar caps and some concretions and pendants. It is postulated that a different mode or environment of formation for these forms is responsible for discrepancies with theoretical values.