Pedogenic Carbonate Research Articles

Natural and Human-Induced Factors on the Accumulation and Migration of Pedogenic Carbonate in Soil: A Review

As a principal part of the atmosphere–lithosphere interface, soil plays a key role in regulating the atmospheric CO2 concentration and global climate. Comprising two major pools (carbonate in soils and bicarbonate in groundwater), soil inorganic carbon (SIC) is deemed as the primary carbon (C) sink and source in areas with low mean annual rainfall. SIC may originate from soil parent material or from the formation of secondary carbonate when divalent cations from an extraneous source are supplied. The latter may result in pedogenic carbonate (PC) formation, increasing soil C content and sequestering atmospheric carbon. Since the sequestration of atmospheric CO2 through formation of pedogenic carbonate is gaining popularity as a method to support climate change mitigation efforts and to claim carbon credits, the mechanisms influencing the formation and migration of pedogenic carbonate need to be well understood. The present review provides an overview of the available literature on potential natural and anthropogenic factors influencing the pedogenic carbonate pool in soils. Firstly, the overall mechanisms of pedogenic carbonate formation, as well as the control factors, are described. Secondly, the impact of various land-use changes on pedogenic carbon pool modification is discussed. Then, the potential of stabilizing atmospheric CO2 through PC formation and the challenges and techniques of tracking the formation of PC through engineered pathways in soils are explored. Finally, isotopic signature as a technique for distinguishing neo-formed carbonate in soil is scrutinized.

Hunter-gatherer environments at the Late Pleistocene sites of Mwanganda's Village and Bruce, northern Malawi

Mwanganda's Village (MGD) and Bruce (BRU) are two open-air site complexes in northern Malawi with deposits dating to between 15 and 58 thousand years ago (ka) and containing Middle Stone Age (MSA) lithic assemblages. The sites have been known since 1966 and 1965, respectively, but lacked chronometric and site formation data necessary for their interpretation. The area hosts a rich stone artifact record, eroding from and found within alluvial fan deposits exhibiting poor preservation of organic materials. Although this generally limits opportunities for site-based environmental reconstructions, MGD and BRU are located at the distal margins of the alluvial fan, where lacustrine lagoonal deposits were overprinted by a calcrete paleosol. This has created locally improved organic preservation and allowed us to obtain ecological data from pollen, phytoliths, and pedogenic carbonates, producing a regional- to site-scale environmental context for periods of site use and abandonment. Here, we integrate the ecological data into a detailed site formation history, based on field observations and micromorphology, supplemented by cathodoluminescence microscopy and μ-XRF. By comparing local, on-site environmental proxies with more regional indicators, we can better evaluate how MSA hunter-gatherers made decisions about the use of resources across the landscape. Our data indicate that while tree cover similar to modern miombo woodland and evergreen gallery forest prevailed at most times, MSA hunter-gatherers chose more locally open environments for activities that resulted in a lithic artifact record at multiple locations between 51 and 15 ka.

Mineral-Soil-Plant-Nutrient Synergisms of Enhanced Weathering for Agriculture: Short-Term Investigations Using Fast-Weathering Wollastonite Skarn.

Enhanced weathering is a proposed carbon dioxide removal (CDR) strategy to accelerate natural carbon sequestration in soils via the amendment of silicate rocks to agricultural soils. Among the suitable silicates (such as basalt and olivine), the fast-weathering mineral wollastonite (CaSiO3) stands out. Not only does the use of wollastonite lead to rapid pedogenic carbonate formation in soils, it can be readily detected for verification of carbon sequestration, but its weathering within weeks to months influences soil chemistry and plant growth within the same crop cycle of its application. This enables a variety of short-term experimental agronomic studies to be conducted to demonstrate in an accelerated manner what could take years to be observed with more abundant but slower weathering silicates. This study presents the results of three studies that were conducted to investigate three distinct aspects of wollastonite skarn weathering in soils in the context of both agricultural and horticultural plants. The first study investigated the effect of a wide range of wollastonite skarn dosages in soil (1.5–10 wt.%) on the growth of green beans. The second study provides insights on the role of silicon (Si) release during silicate weathering on plant growth (soybeans and lettuce). The third study investigated the effect of wollastonite skarn on the growth of spring rye when added to soil alongside a nitrogen-based coated fertilizer. The results of these three studies provide further evidence that amending soil with crushed silicate rocks leads to climate-smart farming, resulting in inorganic carbon sequestration, as well as better plant growth in agricultural (soybean and spring rye) and horticultural (green bean and lettuce) crops. They also demonstrate the value of working with wollastonite skarn as a fast-weathering silicate rock to accelerate our understanding of the mineral–soil–plant–nutrient synergism of enhanced weathering.

Dynamics of Pedogenic Carbonate Growth in the Tropical Domain of Myanmar

AbstractPedogenic carbonate is widespread at mid latitudes where warm and dry conditions favor soil carbonate growth from spring to fall. The mechanisms and timing of pedogenic carbonate formation are more ambiguous in the tropical domain, where long periods of soil water saturation and high soil respiration enhance calcite dissolution. This paper provides stable carbon, oxygen and clumped isotope values from Quaternary and Miocene pedogenic carbonates in the tropical domain of Myanmar, in areas characterized by warm (>18°C) winters and annual rainfall up to 1,700 mm. We show that carbonate growth in Myanmar is delayed to the driest and coldest months of the year by sustained monsoonal rainfall from mid spring to late fall. The range of isotopic variability in Quaternary pedogenic carbonates can be solely explained by temporal changes of carbonate growth within the dry season, from winter to early spring. We propose that high soil moisture year‐round in the tropical domain narrows carbonate growth to the driest months and makes it particularly sensitive to the seasonal distribution of rainfall. This sensitivity is also enabled by high winter temperatures, allowing carbonate growth to occur outside the warmest months of the year. This high sensitivity is expected to be more prominent in the geological record during times with higher temperatures and greater expansion of the tropical realm. Clumped isotope temperatures, δ13C and δ18O values of tropical pedogenic carbonates are impacted by changes of both rainfall seasonality and surface temperatures; this sensitivity can potentially be used to track past tropical rainfall distribution.

Paleoenvironmental reconstruction of Saqqez River basin, western Iran: evidence from quaternary deposits and pedogenic carbonates

Paleoenvironmental reconstruction of Saqqez River basin, western Iran: evidence from quaternary deposits and pedogenic carbonates

Long-term denudation rate of karstic North Dalmatian Plain (Croatia) calculated from 36Cl cosmogenic nuclides

The development of a karst landscape results from complex interactions between lithology, climate, hydrology, soil, vegetation and tectonics. Weathering and erosion of carbonate rocks leads to denudation of karst landscapes. As dissolution of carbonate rocks is often considered to be the main process governing carbonate weathering, other processes are often overlooked. Here we present research done in the North Dalmatian Plain, a carbonate erosive surface located in the Dinaric karst region. Although the study site is composed of two different carbonate lithologies having different weathering style, there is no evident lithological impact on the topography of the erosive surface. Analyses of 36Cl concentration were performed in ten proximal bedrock samples from both lithologies and resulted in long-term denudation rates ranging from 14.7 to 22.8 m/Ma. Since no statistical significance was found between samples from different lithologies (all samples belong to a single normal population) and they have the same geomorphological context and climate features, variable denudation rates are attributed to local (sample specific) differences. In the study site there are no large outstanding rock residuals or patches with deep soil profiles. Thus, to maintain the levelled erosive surface, local differential denudation rates have to vary with time. We hypothesize that lichens and pedogenic carbonates have a significant role in modulating local differences in denudation rates. Our study shows that even at outcrop scale those differences can be significant, and the study of sample populations is preferred to single or limited number of analyses. Thus, the long-term denudation rate of the North Dalmatian Plain, including its local variability is 18.91 ± 0.81 m/Ma. Despite classical studies on karst terrains assume that dissolution is the main process responsible for development of these landscapes, our research highlights the importance of physical weathering in combination with dissolution of carbonates as main controls on the denudation of karst landscapes.

Assessment of pretreatment methods on CIA-K and CALMAG indices and the effects on paleoprecipitation estimates

The chemical compositions of paleosols are frequently used to reconstruct paleoclimatic and paleoenvironmental conditions using proxy relationships derived from modern soils. However, through time, some of the proxy relationships have been applied to paleosol types that were not part of the original training data sets, or they have been analyzed following pretreatment steps that were not employed by the original studies without critical evaluation of whether those methodological changes have impacted the interpretation of the data. Here we test pretreatment methods critically using chemical compositions of thirty-two modern soils from California and Texas to evaluate their impact on two of the most commonly employed climofunction indices to estimate mean annual precipitation, CIA-K and CALMAG. Chemical data are generated from paired soils which have underdone two different pretreatment styles, including using weak glacial acetic acid to digest calcite and using no pretreament methods to keep all calcium in the soil, including from pedogenic carbonate. There is little change in major element oxide values from untreated to acid-treated aliquots, except for calcium oxide depletion after pretreatment. In all soils with >2% CaO in the matrix, calcium oxide is significantly depleted by the acid-digestion pretreatment. This acid-digestion pretreatment induces a substantial increase in estimates of mean annual precipitation, which are not in accordance with published meteorological values. The extent to which precipitation estimates are skewed toward higher values as a function of CaO loss is parametrically correlated with calcium carbonate content of the soil material. This has important implications for the appropriate application of CIA-K and CALMAG indices as viable proxies of paleoprecipitation estimates from paleosol profiles that contain pedogenic and/or diagenetic carbonates and we suggest quantitative thresholds for CaO in untreated samples to minimize potential sources of error.

Late Cretaceous through Cenozoic Paleoenvironmental History of the Bagua Basin, Peru: Paleoelevation Comparisons with the Central Andean Plateau

Located in northern Peru, at the lowest segment of the Central Andes, the Bagua Basin contains a Campanian to Pleistocene sedimentary record that archives the local paleoenvironmental and tectonic history. We present new δ18O and δ13C signatures of pedogenic carbonate nodules from paleosols in the Campanian–Maastrichtian Fundo El Triunfo Formation and in the upper Eocene–middle Miocene Sambimera Formation to reconstruct the isotopic composition of paleo-meteoric water and the floristic biome. We compare these results to modern isotopic values from a newly obtained modern water transect to interpret the environmental evolution of this area and its relationship with the neighboring Eastern Cordillera. A ~2‰ δ18O depletion between the latest Cretaceous and the latest Eocene reflects a shift from a coastal to inland environment. A negative δ18O shift of ~3‰ from the middle Miocene to the present day reveals the establishment of the Eastern Cordillera as an orographic barrier for the moisture traveling westward, sometime after deposition of the top of the Sambimera Formation at ~13 Ma. A shift in the δ13C signature from ~−25‰ in the Campanian–Miocene deposits to ~−23‰ in modern–Holocene times suggests a change in biome from dominant C3 plants to a mixture of C3 and C4 plants. This environmental shift reflects both the late Miocene global C4 expansion and the transition to more arid conditions in the basin. The Campanian–middle Miocene environmental reconstruction of the Bagua Basin indicates a steady paleoelevation setting in the northernmost Central Andes during most of the Cenozoic and constrains the uplift of the Eastern Cordillera to the late Miocene–Pleistocene. This paleoelevation history contrasts with that of the Central Andean Plateau, which is characterized by two major episodes of surface uplift: early–middle Miocene and late Miocene–Pliocene. The contrasting modern topographic configuration of the Central Andean Plateau and the northernmost Central Andes gives rise to the question of what factors created such a dramatic difference in topographic evolution of the two regions that shared an overall common tectonic history. We discuss the possible factors responsible for this contrasting topographic configuration and suggest that the diachronous flat slab episodes are likely a major factor, resulting in greater shortening and crustal thickness and, ultimately, in earlier surface uplift episodes occurring in the Central Andean Plateau.

Causes and implications of the seasonal dissolution and precipitation of pedogenic carbonates in soils of karst regions – A thermodynamic model approach

Red Mediterranean Soils developed over carbonate rocks can precipitate pedogenic carbonates at the base of their profiles, forming an evolutionary stage of calcretes. We studied a 0.6 m deep soil profile in Dalmatia (Croatia) where the lowest section of the soil is a calcic horizon formed by diffuse calcite particles and small nodules. These nodules record different events of dissolution and precipitation. Using a 3-month period of soil environmental monitoring, where soil temperature, soil water content, soil bulk electrical conductivity and soil air CO2 were measured, we implemented a thermodynamic model for dissolution and precipitation of calcite in the soil. The simulation shows a stage dominated by calcite dissolution during spring and early summer followed by a stage dominated by calcite precipitation. Soil air CO2 is the main control of the calcite reactions, with concentration of solutes also being of some importance during the stage of calcite precipitation. Soil water content and temperature, both affect soil air CO2. Precipitation events also have high-frequency impact on soil air CO2, but the response is complex. The model enables soil water solutions to be supersaturated in relation to calcite without precipitation of that mineral. Simulated soil water reached variable values of supersaturation in relation to calcite before its precipitation was triggered, suggesting that there is no fixed threshold value for the precipitation of calcite in a particular soil. During the 3-month simulated period, 83% of the calcite dissolved was reprecipitated as pedogenic carbonate. Most of the initially dissolved calcite, was expected to be diffuse pedogenic carbonate particles, although nodules and bedrock at the base of the soil should also have contributed to the solutes dissolved. Therefore, karst landscapes where soils have pedogenic carbonates are expected to record less denudation rates than those regions without calcretes.

Eggshells assemblages and stable isotope composition in the Upper Cretaceous (Campanian) El Gallo Formation of Baja California, Mexico: Paleoenvironmental inferences

Fossil eggshells were recovered from the El Gallo Formation, Baja California, northwest Mexico, is one of the few North American Campanian continental outcrops outside the Western Interior Basin. Based on microstructure, the eggshells are referred to ornithopod (ootaxon Spheroolithus), theropods (ootaxa Prismatoolithus, two Tubercuoolithidae eggshells, Continuoolithus, Tristraguloolithus, Disperituberoolithus, Pseudogeckoolithus, Laevisoolithidae? eggshells and Enantiornithine indet.), and one crocodylomorph (cf. Neokrokolithes). While some of these ootaxa were associated with the modest, but diverse osteological record preserved there; others reveal the presence of small theropods, including as the first evidence of enantiornithine birds in the formation. Because the stable isotopic compositions of the eggshells differ from those of pedogenic carbonate nodules, we infer minimal diagenetic influence. We thus propose that the C3 plants had δ13C compositions between −27‰ and −29‰ and that warm evaporative conditions led to enriched δ18O in ingested water compared to meteoric water at least during the short period of dinosaur eggshell formation.

Pedogenesis and landscape evolution of Calcisols on Quaternary carbonatic sediments of Fernando de Noronha and Trindade volcanic islands, South Atlantic

Carbonate-rich soils are found in different materials, climates, and environments, and are generally characterized by alkaline pH values and high base saturation, dominated by exchangeable Ca2+. Fernando de Noronha and Trindade volcanic islands, in South Atlantic, register carbonate-rich soils on bioclastic sediments at coastal areas. This study aims to investigate the pedogenic evolution of these soils associated with landscape processes. Six profiles were described and sampled, four of which located on bioclastic sediments (PN1, PN2, PT1, and PT2), one on Quaternary fluvial sediments (PN3), and one developed on calcarenites overlying ankaratrite flows (PN4). Morphological, physical, chemical, mineralogical, micromorphological and microchemical investigations were carried out, and principal component (PCA) and statistical analyses were applied to all data. The soils were classified as Calcisols, Fluvisol, and Cambisol and the results suggest that the carbonatic parent material and local climate are the main drivers of soil genesis, subordinated by biogenic and landform processes. All profiles have high exchangeable Ca2+, base saturation, pH, CaCO3 equivalent, and total Ca content, with calcite and aragonite minerals. Pedogenic carbonates features occur in all soil, being more developed in Fernando de Noronha Calcisols, and partially dissolved in Trindade Calcisols. The contrasting landscape and climate evolution between these islands explain these differences. Hence, these insular Calcisols are products of varying pedological evolution, with Holocene paleoclimates as the main factor of differentiation. Our results indicate the adoption of “bioclástico-carbonático” sub-group for Calcisols and Fluvisols from Fernando de Noronha, proposing the inclusion of “bioclástico-chernossólico” sub-group in the Brazilian Soil Classification System.

Application of Geospatial Techniques in Analyst Distribution Patttern of Some Soil Properties in Erbil Province, Kurdistan Region-iraq

The study area is located in Erbil province, Kurdistan region, Iraq (that extends from Latitude 35.436151N to 37.319894N and from Longitude 43.374316E to 45.080122E) and aimed explain the distribution of some soil properties depending on geospatial techniques. Fourteen geo-referenced soil pedons have been elected. The ArcMap 10.7 software has been used in this study. Bulk density generally increases with depth in studied pedons. Interpolated map for distribution of clay particles, appears the higher values of clay particles and fine soil texture are located at the north, northwest, and decreasing into the south, southwest. Clay content has different distribution patterns. An alkaline studied soil pH with low salts content. Organic matter and total carbonate decreased with depth and an increase pedogenic carbonate that has a positive significant correlation with both slope and elevation. Active carbonate has a strong correlation with clay particles. Soil gypsum and total nitrogen decreased with depth in most pedons. The study concluded that decreasing into the south and southwest. Soil organic matter tends to increase as the clay content increases and higher values of organic matter content are located at northwest and decreasing gradually to south and southwest. Soils have different distribution patterns of total and active carbonates along pedons. CEC decreased from northwest toward a southeast

A geochronologically-constrained stable isotope record of the Upper Triassic Sonsela Member (Chinle Formation) at Petrified Forest National Park (Arizona, USA): Testing for paleoenvironmental linkages with biotic change and the Manicouagan impact

The Upper Triassic Chinle Formation, cropping out in and around Petrified Forest National Park (PFNP) in northern Arizona, U.S.A., preserves an important non-marine biotic and sedimentologic record of Late Triassic key Earth-life events. In 2013, the Colorado Plateau Coring Project (CPCP) obtained a 520-m-long core of the Triassic strata at PFNP to study this sedimentary record in unequivocal superposition and, among other goals, to test hypotheses about the paleoenvironmental and biotic changes preserved in the Sonsela Member of the Chinle Formation, and specifically their link to the Manicouagan impact and the Adamanian-Revueltian biotic turnover event (A-R transition). We sampled the Sonsela Member of CPCP core 1A for bulk organic material and for pedogenic carbonates to establish the δ13Corg, δ13Ccarb, and δ18Ocarb records. Throughout much of the Sonsela Member, the stable isotope record is characterized by a relatively narrow range of values (δ13Corg = ~ −25 to −30‰; δ13Ccarb = ~ −7 to −10‰; and δ18Ocarb = ~ −5 to −8‰). Based on these data, we estimate mean annual precipitation and correlate our isotope record to two previously developed, high resolution, multi-proxy age models for the CPCP core. Our new data set supports three main conclusions based on these observations: (1) whereas the A-R transition and the Manicouagan impact event might correlate in time, establishing a causal relationship between those two events remains challenging; (2) the Manicouagan impact as well as the A-R transition are not linked to a clear geochemical perturbation preserved in the CPCP core; and (3) multiple proxies agree the climate became more arid throughout the Sonsela Member, possibly contributing to the Adamanian-Revueltian biotic turnover.

Removal of atmospheric CO2 by engineered soils in infrastructure projects

The use of crushed basic igneous rock and crushed concrete for enhanced rock weathering and to facilitate pedogenic carbonate precipitation provides a promising method of carbon sequestration. However, many of the controls on precipitation and subsequent effects on soil properties remain poorly understood. In this study, engineered soil plots, with different ratios of concrete or dolerite combined with sand, have been used to investigate relationships between sequestered inorganic carbon and geotechnical properties, over a two-year period. Cone penetration tests with porewater pressure measurements (CPTu) were conducted to determine changes in tip resistance and pore pressure. C and O isotope analysis was carried out to confirm the pedogenic origin of carbonate minerals. TIC analysis shows greater precipitation of pedogenic carbonate in plots containing concrete than those with dolerite, with the highest sequestration values of plots containing each material being equivalent to 33.7 t C ha−1 yr−1 and 17.5 t C ha−1 yr−1, respectively, calculated from extrapolation of results derived from the TIC analysis. TIC content showed reduction or remained unchanged for the top 0.1 m of soil; at a depth of 0.2 m however, for dolerite plots, a pattern of seasonal accumulation and loss of TIC emerged. CPTu tip resistance measurements showed that the presence of carbonates had no observable effect on penetration resistance, and in the case of porewater pressure measurements, carbonate precipitation does not change the permeability of the substrate, and so does not affect drainage. The results of this study indicate that both the addition of dolerite and concrete serve to enhance CO2 removal in soils, that soil temperature appears to be a control on TIC precipitation, and that mineral carbonation in constructed soils does not lead to reduced drainage or an increased risk of flooding.

Acid spill impact on Sonora River basin. Part I. sediments: Affected area, pollutant geochemistry and health aspects

The Sonora River and its tributary streams (Tinajas, Bacanuchi) were impacted in 2014 by an acid solution spill (approximately 40,000 m3). This study aims to presents a clear and supported overview to determining the spill's consequences on the environment and the people inhabiting the area. The elements quantified were those found in the spilled solution: Al, As, Cu, Fe, Mn, Pb, and Zn. Potential Toxic Element (PTE) concentration means from 187 sediment samples were, in mg.kg−1: Al = 7,307, As = 16.6, Ba = 128 Cu = 106 Fe = 15,764, Mn = 566, Pb = 46 and Zn = 99. Differences between PTE concentrations in the most impacted sediments and those of the local baseline, sampled in streams not affected by the spill and regional baseline values, were not statistically significant. The similarity of PTE concentrations among sediments may be explained by natural geological enrichment, historical mining impacts, and a low increase of PTE in sediments after the acid spill because of natural and anthropogenic attenuation. Mainly heavy rains, natural pedogenic carbonates, and remedial work done by the mining company (retaining dam, adding lime; precipitation, collecting formed solids, and transport to the mine). The Contamination Factor (C.F.), Enrichment Factor (E.F.), and Geo-accumulation Index (Igeo) were determined. The C.F. indicated low and moderate contamination in all elements. Cu exhibited the highest E.F., from moderate to significant enrichment. The Igeo generally ranged from −0.02 to 0.15. Cu and Zn were classified as moderately to heavily contaminated. In local baseline sediments, the Cu C.F. varied from moderate to very high contamination, the Cu E.F. from moderate to significant enrichment, while the As, and Pb Igeo ranged from uncontaminated to moderately contaminated. In general, normalization demonstrated a high degree of Cu enrichment at sites 1–14. Sequential extractions indicated that only Cu was found in all fractions, including a significant exchangeable fraction in the very impacted sediments (1–14). The other PTEs were distributed between the Fe/Mn oxide fraction and the residual phase. Principal Components Analysis for PTE concentrations indicated three different groups with similar geochemical patterns and allowing to identify the PTE potentially sources: the first sediments from sites 1–14 were the impacted sediments in accordance with pH and electrical conductivity results, the second group from sites 15–20 showed characteristics of the mineralized environment, and the third from sites 21–30 were unrelated to the spillage. The area impacted by the acid solution spill reached approximately 30 km downstream, just roughly 15% of the initially considered area.

Paleoclimate instabilities during late Oligocene - Early Miocene in SW Europe from new geochemical climofunctions based on soils with pedogenic carbonate

In this study, two new climofunctions based on major elements are proposed from the study of 28 modern alluvial soils with pedogenic carbonate from NE Spain. The first climofunction relates a base ratio (CaO, K 2 O) to the mean annual temperature (MAT), and the second one the clayeness ratio (Al 2 O 3 /SiO 2 ) to the mean annual precipitation (MAP). In both climofunctions, the use of the parent material ratio as normalizer gave the best results, taking into account the natural variability of alluvial deposits. Application of these climofunctions to 86 paleosols from Oligo-Miocene alluvial successions in southern France and central Spain was performed, and combined with morphological functions previously proposed, with the aim to build detailed paleoclimatic data for this key period of climate history. This work provides a high frequency record (range of 10s to 100 s kyr) of paleoclimate fluctuations, with estimated MAP from both methods very comparable (mean ~ 550 mm/yr). Estimated MAT and MARP (mean annual range of precipitations) mainly exhibit the same trends: 1) high values followed by a rapid decrease during the early Aquitanian (from 18.8 to 13.5 °C; from 80 to 20 mm), 2) a warming trend towards the middle Aquitanian (up to 16.6 °C, 90 mm), 3) a cooling (down to 13.4 °C, 40 mm) during the late Aquitanian – early Burdigalian. Comparisons with paleoclimate data from paleontological remains indicate similar MAT and two times lower MAP estimated from paleosols. However, consistency between estimated MAP from this study to previous paleopedological investigations in NE Spain suggests a regional climate pattern (southern France and central Spain) precursor of the Mediterranean conditions that developed during middle Neogene whereas humid conditions prevailed in the western coastal basins of Europe. Periods of large amplitude fluctuations in MARP and MAT are interpreted as reflecting the interrelation between the lower Miocene climate instability (Mi1, Mi1a events) and continental climate. • geochemical climofunctions (MAP and MAT) are proposed from modern alluvial soils. • performed on paleosols with pedogenic carbonate from the Oligo-Miocene. • both functions can be applied to truncated paleosols. • high resolution climate record.

Black soils in the Araripe basin, Northeast Brazil: Organic and inorganic carbon accumulation in a Chernozem-Kastanozem-Phaeozem sequence

The Global Soil Partnership with the Food and Agriculture Organization of the United Nations (FAO) defines Chernozem, Kastanozem, and Phaeozems as black soils characterized by high natural fertility and dark surface horizons enriched in organic matter. Chernozems and Kastanozems are also characterized by carbonate accumulation. In the Araripe basin located in northeastern Brazil, black soils formed from limestone are present in the scarps of the Araripe Plateau. Despite their importance, particularly in dry climatic regions, there are no studies characterizing the soils of this region. Thus, our study aimed characterizing the macro and micromorphological, physical, and chemical attributes of three soil profiles (Chernozem, Kastanozem, and Phaeozem, respectively in the upper, middle, and lower third) in a toposequence of the Araripe region. In addition, we identified the main mechanisms of organic and inorganic C accumulation in these profiles. Our results showed that the combination of high mineral-associated organic matter (MAOM) content, high MAOM/soil organic C ratio, and low C/N ratio indicates a high degree of transformation and stabilization the organic matter in these soils. The high 2:1 clays and exchangeable Ca2+ benefits the stabilization of organic C by means of organic–mineral interactions. Additionally, the dominance of carbonates of geogenic origin (limestone particles in the sand fraction) is indicative of the low dissolution of these carbonates. Through micromorphological evaluation, pedogenic carbonates were observed at the top of the B horizon of these soils, where the carbon dioxide (CO2) pressure is lower. The variation in relief demonstrated an ostensible influence on inorganic C constrained to the Phaeozem (lower third). This favored high humidity and root activity at depth, increasing the CO2 pressure and hindering the re-precipitation of carbonates despite the high pH and abundance of exchangeable Ca2+.

Atmospheric pCO2 Reconstruction of Early Cretaceous Terrestrial Deposits in Texas and Oklahoma Using Pedogenic Carbonate and Occluded Organic Matter

Pedogenic carbonate samples collected from three Lower Cretaceous (Aptian–Albian) fossil localities in Texas and Oklahoma were analyzed to develop paleoatmospheric pCO2 estimates by measuring the stable carbon isotopes of pedogenic calcite and their co-existing occluded organic matter. Calcite δ13C values ranged from −10.9‰ to −4.4‰ while occluded organic matter δ13C values ranged from −27.3‰ to −21.1‰. These stable carbon isotope measurements combined with temperature (30 °C) and soil-respired CO2 concentration (839–6047 ppmV) values provided atmospheric pCO2 estimates ranging from 67 ppmV to over 1100 ppmV. These estimates show a significant increase in atmospheric pCO2 during the late Aptian followed by a decrease in atmospheric pCO2 during the late Aptian to early Albian transition period, roughly correlating with the OEA1b event. Given the lack of chronostratigraphic constraints of the Lower Cretaceous geologic units in the study area, these data provide further evidence for the approximate age of the units as well as pertinent paleoclimate insights into greenhouse climate conditions.

Paleoatmospheric CO2 oscillations through a cool middle/Late Cretaceous recorded from pedogenic carbonates in Africa

This study presents the first African pedogenic carbonate estimated atmospheric CO2 concentrations for the Early to Late Cretaceous transition. Pedogenic carbonates have been used extensively as proxies to reconstruct past climates, yet few studies have focused on the climatic conditions of Africa during the Mesozoic. The paucity of paleoclimate data from the region represents a significant obstacle in interpreting past climate change, and thus our ability to predict future changes. Paleosol carbonate nodules were sampled from Bk horizons of seven particularly well-exposed and well-characterized paleosols through the Cretaceous Galula Formation, including four from the lower Mtuka Member (Aptian–Cenomanian) and three from the upper Namba Member (Campanian). The Cretaceous Galula Formation is a fossiliferous continental sedimentary succession of braided fluvial deposits that are well exposed in recently incised river drainages of the Rukwa Rift Basin, Tanzania. Oxygen isotope values averaged −5.6‰ and −7.2‰ VPDB for the Mtuka and Namba members, equating to mean annual temperatures of 14.5 °C and 11.2 °C, respectively. Using the stable isotope composition of pedogenic carbonates from the Mtuka Member, an average pCO2 of 1860 ppmv was estimated for the Aptian–Cenomanian, before declining to an average of 520 ppmv in the Namba Member, during the Campanian. Atmospheric pCO2 values fluctuated, but remained above 1000 ppmv through the middle Cretaceous, and correspond to the “cool” greenhouse period that spanned the Aptian–Albian. The gradual decline in pCO2 (550–502 ppmv) recorded in the Namba Member paleosols occurred during Late Cretaceous cooling following the Cretaceous Thermal Maximum.

Vertical variations of soil carbon under different land uses in a karst critical zone observatory (CZO), SW China

Soil is a key carbon reservoir balancing global carbon budget and regulating climate change. Barren soils in karst zones have weak capacity for soil and water conservation and are readily erodible, making the carbon biogeochemical processes within karst soils potentially rapid and complex. To explore the vertical variation of soil carbon under karst conditions and its response to land-use change, this study investigated concentrations and isotopic compositions of both soil organic and inorganic carbon (SOC and SIC) in different depths of four land-use types in a typical karst region, SW China. Results show that as soil depth increases, SOC concentrations decrease, and its δ13C ratios (from −27.3‰ to −19.4‰) increase in 0–20 cm depth but decrease at depths below 20 cm. The fresh SOC is mostly sequestered and cycled within the topsoil and subject to different controls than that of the subsoil. The turnover rate of SOC in karst soils does not directly co-vary with isotopic fractionation among different land uses. Long-term cultivation causes SOC loss from karst soils, which can be alleviated or even partially restored after farming cessation. By contrast, SIC represents < 10% of soil total carbon. The vertical heterogeneity of SIC variation and the direct influence of biological factors on SIC are both weaker relative to SOC. The low δ13C ratios of SIC (from −20.4‰ to −3.0‰) indicate there is intense dissolution and reprecipitation of pedogenic carbonate within karst soil, especially in the upper cultivated layers. These results highlight that both SOC and SIC are labile and susceptible to land-use change in karst zones, which need to be considered in estimating karst carbon sink and its role in balancing global carbon budget on variable temporal scales.