Secondary Carbon Formation Research Articles

The rare earth elements of sequentially leached phases in the loess-paleosol sequence at Weinan on the southeastern Chinese Loess Plateau

The rare earth elements (REEs) in loess sediments may preserve key information of paleoenvironmental changes in East Asia but remain poorly explored. Here we present the REE compositions of sequentially leached phases of loess sediments from a 16 m long loess-paleosol sequence collected from Weinan at the southern margin of the Chinese Loess Plateau (CLP). The REEs associated with carbonates, easily reducible oxides, reducible oxides, clay minerals, and the residual silicate minerals were sequentially leached with Na-acetate, hydroxylamine-HCl, Na-dithionite/Na-citrate, 12 mol L−1 HCl and concentrated HF-HNO3 solutions. The results show that the REE concentrations of different phases vary in the loess-paleosol layers and are most enriched in the easily reducible phase (except for the residual phase), which is likely explained by the association of REEs with Mn-oxides. Significant Eu anomalies can only be found in the carbonate and easily reducible phases, with the carbonate phase being characterized by appreciable positive Eu anomalies and the easily reducible phase being characterized by negative Eu anomalies, indicating the incorporation of Eu2+ into secondary carbonates that leaves the authigenic Mn-oxides with depleted Eu relative to other REEs. The carbonate phase shows significant negative Ce anomalies, which is likely due to the preferential removal of Ce4+ by Mn oxides before the formation of secondary carbonates. Nevertheless, appreciable positive Ce anomalies for the easily reducible phase can only be found at the bottom of the paleosol S1 while significant negative Ce anomalies were found within the paleosol S1, indicating the downward leaching of Ce4+ that shows stronger complexation with the organic matters than REE3+ in the soil solution. This may partly account for the negative Ce anomalies previously found in the paleosols. Overall, our results suggest a two-stage evolution of Eu and Ce anomalies of the carbonate phase, with the first stage controlled by the formation of both Mn-oxides and secondary carbonates and the second stage dominated by the precipitation of secondary carbonates. The comparison of carbonate Eu and Ce anomalies with other paleoclimatic proxies indicates that the combination of Eu and Ce anomalies of the carbonate fraction of loess may be applied as novel proxy records of climate change in East Asia.

U-Th dating, taphonomy, and taxonomy of shell middens at Klasies River main site indicate stable and systematic coastal exploitation by MIS 5c-d

The archaeological record, particularly of shellfish, from the Klasies River main site (KRM) is important in understanding the fluctuating nature of coastal occupational patterns and changing coastal ecologies. In this paper, we provide new uranium–thorium (U-Th) dates for one of the earlier phases of coastal exploitation at KRM, and the microstratigraphic analyses generate novel information about the taphonomy of shell-bearing deposits from the Later Stone Age (LSA) to the MSA I period that, in turn, provide a broader context for middening at the site. A wide range of syndepositional taphonomic processes related to human activities and post-depositional effects include burning, fragmentation and compaction, chemical alteration, and cementation. Despite such issues influencing recovery, shellfish data are informative and are presented from three layers of the Witness Baulk: Shell Midden One (SMONE), Black Occupational Soils (BOS), and Silty Black Soils (SBLS). These coarse shell midden deposits exhibit visible decalcification coupled with cementation with secondary carbonate formation in association with conditions of high moisture and soft sedimentation deformation of the underlying sediments of SBLS. This stratigraphy section is chronologically anchored for the first time using U-Th dating of speleothems associated with a hiatus after the deposition of BOS. The three ages, 110,060 ± 1,100, 109,800 ± 970, and 106,000 ± 2,100 years, place the BOS layer as the base of the SASL sub-member at over 110 ka, making the underlying middens from the LBS member even older. The zooarchaeological analyses of the three layers indicate coastal ecological changes from more sheltered conditions prior to the hiatus, with the exploitation of alikreukel and brown mussels predominating. Before 110 ka, in BOS and SBLS, more exposed coastal conditions occurred, and the diversity of exploited shellfish increased. SMONE and BOS are associated with MSA II/Mossel Bay lower lithic technology and SBLS with MSA I technology, indicating asynchronous coastal ecological and technological changes. The MIS 5c-d evidence for early coastal occupation at KRM provides details on the period during which coastal occupation became stable and systematic on the South African coast and puts the KRM amongst the handful of sites with shell-bearing deposits, occurring prior to 110 ka in South Africa.

Soil Inorganic Carbon as a Potential Sink in Carbon Storage in Dryland Soils—A Review

Soil organic carbon (SOC) pool has been extensively studied in the carbon (C) cycling of terrestrial ecosystems. In dryland regions, however, soil inorganic carbon (SIC) has received increasing attention due to the high accumulation of SIC in arid soils contributed by its high temperature, low soil moisture, less vegetation, high salinity, and poor microbial activities. SIC storage in dryland soils is a complex process comprising multiple interactions of several factors such as climate, land use types, farm management practices, irrigation, inherent soil properties, soil biotic factors, etc. In addition, soil C studies in deeper layers of drylands have opened-up several study aspects on SIC storage. This review explains the mechanisms of SIC formation in dryland soils and critically discusses the SIC content in arid and semi-arid soils as compared to SOC. It also addresses the complex relationship between SIC and SOC in dryland soils. This review gives an overview of how climate change and anthropogenic management of soil might affect the SIC storage in dryland soils. Dryland soils could be an efficient sink in C sequestration through the formation of secondary carbonates. The review highlights the importance of an in-depth understanding of the C cycle in arid soils and emphasizes that SIC dynamics must be looked into broader perspective vis-à-vis C sequestration and climate change mitigation.

The efficacy of enhancing carbonate weathering for carbon dioxide sequestration

Enhanced weathering is a geoengineering strategy aiming to increase continental weathering rates, thereby increasing the delivery of atmospheric carbon (as HCO3-) to the oceans. Most enhanced weathering studies focus on the capacity of silicate rocks (e.g., basalt) and minerals (e.g., olivine, Mg2SiO4, or wollastonite CaSiO3) to remove atmospheric CO2. However, carbonate minerals (e.g., calcite, CaCO3) could provide an additional, rapid way to increase HCO3- export to the oceans. Recent studies suggest that 0.84 Gt C yr−1 could be removed from the atmosphere through the enhanced dissolution of calcite in soils, provided carbonic acid is the main dissolution agent. What is not clear is whether atmospheric CO2 dissolved in soils can be transported by rivers, which typically have lower [pCO2], to the oceans. This difference in calcite solubility between soils (where weathering occurs) and rivers (where HCO3- is transported) may lead to large amounts of secondary carbonate formation during transport, releasing the CO2 consumed through dissolution. Here, we present a modeling study comparing the estimated soil dissolution capacity (SDC) in 149 of Earth's largest river basins, to the potential transport capacity of carbon (PTCC) in corresponding rivers. We find the SDC can only be exported to the oceans, without secondary carbonate precipitation, if rivers are in disequilibrium with respect to calcite (i.e., SIc = 1). In this instance, 0.92 Gt C yr−1 may be sequestered above background weathering rates, which is ~20% of annual increases in atmospheric carbon. If rivers are at equilibrium with calcite (i.e., SIc = 0), approximately two-thirds of the carbon dissolved in soil waters are lost due to calcite precipitation in rivers, and just 0.26 Gt of additional atmospheric C yr−1 can be transported to the oceans. Overall, the efficacy of enhanced carbonate weathering is a function of the capacity rivers have for transporting the products from carbonate weathering to the oceans, rather than the dissolution capacity of soils. These findings have implications for the efficiency of enhancing silicate weathering for ocean alkalinity enhancement, as secondary carbonate precipitation during transport is not always considered.

Secondary magnesite formation from forsterite under CO2 sequestration conditions via coupled heterogeneous nucleation and crystal growth

Flow-only and coupled flow-diffusion experiments at 95 °C and 100 bars pCO2 carried out in micro-capillary tubes packed with forsterite mineral grains were used to constrain a coupled classical heterogeneous nucleation and crystal growth reactive transport model describing the formation of secondary magnesite. The study made use of a novel experimental setup in which one capillary tube for flow is connected via a three-way tee to a perpendicular capillary tube sealed at the distal end in which only molecular diffusion is allowed to occur—an experimental analogue of a single fracture-rock matrix system. While the high flux of CO2 bearing fluids and their low pH did not result in the formation of secondary carbonates in the flow-dominated channels, as much as 2.7% magnesite formed in a diffusion-controlled capillary tube sample after 300 hours of reaction. The precipitation of secondary magnesite was not uniformly distributed, however, but showed a distinctive peak shaped pattern along the sample reacted as quantified by RAMAN spectroscopic analysis. About 50% of the total magnesite precipitation formed within a narrow interval of a few millimeters close to the middle of the 3 cm diffusion sample. To simulate the behavior of the diffusion–reaction column, and in particular the pronounced mm scale peak at approximately 1.8 cm, an interfacial free energy of approximately 70 mJ·m−2 combined with a relatively high crystal growth rate for secondary magnesite was required. In agreement with the observations based on RAMAN spectroscopy, the simulations suggest that more than 50% of Mg2+ dissolved from the primary forsterite precipitated as secondary magnesite. The magnesite precipitation at the central peak position in the diffusion sample showed such rapid growth that it created a local minimum in pore fluids Mg2+ concentration close to the peak, creating a “nucleation shadow” that focused continued growth there as nearby regions had no nucleation seeds available for crystal growth. Continued crystal growth on the initial magnesite band acted to further increase the reactive surface area at this point, thus enhancing the spatially focused crystal growth rate and creating a positive feedback leading to pattern formation. This highlights the potentially critical role of an initial nucleation event in controlling mineral precipitation patterns in subsurface porous media, patterns that may determine how the pore structure subsequently evolves physically and chemically over time due to reactive flow and transport.

Landslides as geological hotspots of CO<sub>2</sub> emission: clues from the instrumented Séchilienne landslide, western European Alps

Abstract. This study makes use of a highly instrumented active landslide observatory (9 years of data) in the French Alps, the Séchilienne slope. Here, we use a combination of major element chemistry and isotopes ratios (87Sr / 86Sr, δ34S) measured in different water types of the stable and unstable part of the Séchilienne instability to assess the contribution of the different lithologies of the slope and the chemical weathering mechanisms. Chemical and isotopic ratios are used to characterize weathering processes and the origin of waters and their flow paths through the massif. A mixing model allows us to allocate the different major elements to different sources, to identify secondary carbonate formation as a major process affecting solutes in the subsurface waters of the instability, and to quantify the involvement of sulfuric and carbonic acids as a source of protons. We show that the instability creates favorable and sustained conditions for the production of sulfuric acid by pyrite oxidation, by opening new fractures and supplying fresh reactive surfaces. We clearly identify the contribution of the dissolution of each mineral phase to the chemistry of the waters, with a clear role of remote gypsum dissolution to the sulfate budget in the sampled waters. We are also able to refine the preexisting hydrogeological views on the local water circulation and water flow paths in the instability by showing the hydrological connectivity of the different zones. Overall, our results show that the Séchilienne landslide, despite its role in accelerating rock chemical and physical weathering, acts as a geological source of CO2 to the atmosphere. If generalizable to other large instabilities in mountain ranges, this study illustrates the complex coupling between physical and chemical erosion and their impact on the carbon cycle and global climate. The study also highlights the importance of distinguishing between sulfite oxidation and gypsum dissolution as a source of sulfate ions to rivers, particularly in mountain ranges.

Early diagenesis of foraminiferal calcite under anoxic conditions: A case study from the Landsort Deep, Baltic Sea (IODP Site M0063)

The chemical composition of foraminiferal calcite is widely used for studying past environmental conditions and biogeochemistry. However, high rates of microbial organic matter degradation and abundant dissolved metal sources in sediments and pore waters may impede the application of foraminifera-based proxies due to formation of secondary carbonates or other authigenic minerals on and/or inside of foraminiferal tests. Secondary carbonate precipitation severely alters the bulk foraminiferal geochemistry and can be difficult to eliminate through standard foraminiferal trace element cleaning procedures. We present results showing the mineral composition and formation sequence of diagenetic coatings on foraminiferal tests formed under extreme anoxic conditions in the Baltic Sea's deepest basin (the Landsort Deep, IODP Exp. 347, Site M0063). Our study focuses primarily on the diagenetic carbonates present on and in the tests of the low-oxygen tolerant benthic foraminiferal species Elphidium selseyensis and Elphidium clavatum. We applied various geochemical and imaging methods to ascertain the diagenesis processes and the authigenic mineral formation sequence on foraminifera. The authigenic carbonates were enriched in Mn, Mg, Fe and Ba, depending on the environmental redox conditions when the authigenic carbonates were precipitated. Concentrations of redox-sensitive elements such as Mn and Fe were particularly elevated in bottom waters and sedimentary pore waters under oxygen-depleted conditions in the Landsort Deep, resulting in formation of carbonates with highly elevated Mn and Fe contents. In addition to Mn- and Fe carbonates, several other types of authigenic minerals also formed on and in the foraminiferal chambers, including authigenic calcite, and non-carbonate accessory minerals. The formation sequence reveals the redox sensitivities of different elements and the preferential sequence of oxidants used by the microbes during organic matter oxidation and secondary redox reactions. This study provides a case study of extreme early diagenesis of foraminiferal calcite and may serve as a valuable guide when interpreting foraminiferal trace element records from low oxygen environments.

Biogeochemical Controls on 13CDIC Signatures from Circum-Neutral pH Groundwater in Cu–W–F Skarn Tailings to Acidic Downstream Surface Waters

Regular features of ground and surface waters affected by drainage from mine waste include their acidity and elevated concentrations of dissolved metals, with their attendant negative effects on drinking water quality and aquatic life. One parameter that aids in buffering these waters against acidity and sustains aquatic life is dissolved inorganic carbon (DIC). In this study, the chemical and isotopic (δ13C) composition of primary calcite and DIC (δ13CDIC) in groundwater and surface waters within and downstream, respectively, of abandoned Cu–W–F skarn tailings at Yxsjöberg, Sweden, were used to trace the biogeochemical processes controlling their respective δ13CDIC signatures. In addition, the δ13C signatures of the inorganic (carbonate) fractions of the tailings were used to verify the formation of secondary carbonates within the tailings. Lower average δ13C values of the carbonate fractions (δ13Ccarb = −2.7‰) relative to those of the primary calcite (δ13C = +0.1‰) from the orebodies from which the tailings originated pointed to the precipitation of secondary carbonates. These lower δ13Ccarb signatures were assumed to represent mixed-source C signals involving isotopically light CO2 from the atmosphere, the degradation of organic matter in the upper part of the tailings and HCO3− from calcite dissolution. The groundwater δ13CDIC values (−12.6‰ to −4.4‰) were far lower than the hypothetical range of values (−4.6‰ to +0.7‰) for primary calcite and secondary carbonate dissolution. These signatures were attributed to carbonate (calcite and secondary carbonate) dissolution and the degradation of dissolved organic carbon (DOC) from various organic sources such as peat underneath the tailings and the surrounding forests. Downstream surface water samples collected in May had low δ13CDIC values (−16‰) and high DOC (14 mg C/L) compared to the groundwater samples. These signatures represented the oxidation of the DOC from the wash out of the mires and forests during the snowmelt and spring flood. The DOC and δ13CDIC values of the surface waters from June to September ranged from 6–15 mg·C/L and −25‰ to −8.6‰, respectively. These signatures were interpreted to reflect mixed C sources, including carbonate dehydration by acidity from Fe3+ hydrolysis due to the mixing of groundwater with surface waters and the subsequent diffusive loss of CO2 (g), aquatic photosynthesis, photooxidation, DOC degradation, as well as microbial respiration. Although the 13CDIC signatures of the downstream surface waters seemed to be seasonally controlled and influenced by variable groundwater contributions, the lack of data with respect to DIC concentrations, coupled with multiple potential biogeochemical processes that could influence the DIC pool and 13CDIC values, made it difficult to identify the major regulating process of the 13CDIC signatures. Therefore, other complimentary isotopes and elemental concentrations are recommended in order to decipher the dominant biogeochemical process.

Tracing changes in monsoonal precipitation using Mg isotopes in Chinese loess deposits

The widespread aeolian deposits on the Chinese Loess Plateau (CLP) have been investigated intensively as an outstanding archive of information on past variations of the East Asian Monsoon (EAM). However, differentiating the impacts of precipitation and temperature on loess-based proxies is difficult and complex, which hampers our ability to understand the variability and dynamics of the EAM. In this study, we investigated the Mg isotope composition (δ26Mg) of the secondary carbonates in fine-grained loess samples from ten Holocene profiles and a loess-paleosol sequence, as well as in the primary carbonates of nine loess-source samples from the Asian inland deserts. Our aim was to explore the potential of the δ26Mg values of secondary carbonates in tracing precipitation changes. The results demonstrate that δ26Mg values are homogeneous (−2.07‰) in primary carbonates from different loess-sources, but display large variations in secondary carbonates of loess, increasing from −3.33‰ in the northwest CLP to −1.80‰ in the southeast CLP, and from −3.58‰ during the last glacial to −1.65‰ during the last interglacial. The variations were mainly controlled by EAM precipitation via the migration of isotopically light Mg, because Mg isotope fractionation during the formation of secondary carbonates is temperature insensitive, and the primary carbonate in different loess-sources has relatively constant δ26Mg values. This conclusion is further supported by high correlation (r2 = 0.84) between modern mean annual precipitation (MAP) and δ26Mg in the secondary carbonates of spatial loess samples. Based on this δ26Mg-MAP relationship, temporal variations in MAP were quantitatively estimated, ranging from 670 mm during the last interglacial to 270 mm during the last glacial at the southern CLP. Compared with previous estimates, our MAP exhibits a larger amplitude of glacial-interglacial fluctuations (∼400 mm) with a less uncertainty (53–76 mm), implying that δ26Mg values in secondary carbonates of loess can serve as a novel precipitation proxy to infer EAM variability and dynamics.

Flow-through reactor experiments on basalt-(sea)water-CO2 reactions at 90 °C and neutral pH. What happens to the basalt pore space under post-injection conditions?

Recent publications on the successful mineralisation of carbon dioxide in basalts in Iceland and Washington State, USA, have shown that mineral storage can be a serious alternative to more mainstream geologic carbon storage efforts to lock away permanently carbon dioxide. In this study we look at the pore solution chemistry and mineralogy of basaltic glass and crystalline basalt under post-injection conditions, i.e. after rise of the pH via matrix dissolution and the first phase of carbonate formation. Experimental findings indicate that further precipitation of carbonates under more alkaline conditions is highly dependent on the availability of divalent cations. If the pore water is deficient in divalent cations, smectites and/or zeolites will dominate the secondary mineralogy of the pore space, depending on the basalt matrix. At low carbonate alkalinity no additional secondary carbonates are expected to form meaning the remaining pore space is lost to secondary silicates, irrespective of the basalt matrix. At high carbonate alkalinity, some of this limited storage volume may additionally be occupied by dawsonite −if the Na concentration in the percolating groundwater (brine) is high. Using synthetic seawater as a proxy for the groundwater composition and thus furnishing considerable amounts of divalent cations to the carbonated solution, results in massive precipitation of calcite, magnesite, and other Ca/Mg-carbonates under already moderate carbonate alkalinity. More efficient use of the basaltic storage volume can thus be attained by promoting formation of secondary carbonates compared to the inevitable formation of secondary silicate phases at higher pH. This can be done by ensuring that the pore water does not become depleted in divalent cations, even after carbonate formation. Using seawater as carbonating fluid or injection of CO2 into the basaltic oceanic crust, where saline fluids percolate, can reach this goal. However, such an approach needs sophisticated reactive transport modelling to adjust CO2 injection rates in order to avoid too rapid carbonate deposition and clogging of the pore space too close to the injection well.

Archaeoanthrosol formation and evolution of the “Santana do Riacho” archaeological shelter: An old burial site in South America

AbstractThe “Santana do Riacho” shelter is one of the oldest human occupation sites in Brazil. Dated to the Late Pleistocene, this shelter contains over 40 buried individuals. The shelter is located on the edge of the Espinhaço Range in Mesoproterozoic quartzites. This study aims to investigate the relationship between human activities and in situ soil formation. To this end, chemical, physical, X‐ray fluorescence, X‐ray diffraction using synchrotron light, micromorphology, and scanning electron microscope with energy‐dispersive X‐ray spectroscopy analyses were performed. In situ anthropogenic practices, such as fires and burials, have had the greatest influence on the geochemical parameters. Pedological processes were clearly detected, including (1) the formation of secondary carbonates; (2) the formation of secondary phosphates; (3) the formation of magnetic Fe nodules, and (4) degradation of magnetite into maghemite. The mobility of P, Ca, and C resulted in the enrichment of these elements in layers stratigraphically considered “sterile.” The amounts of available P, K, Ca, total organic carbon, and the pH values can be considered indicators of human occupation. This study provides information on the pedological processes in rockshelters and represents one of the few pedological and geochemical studies on deposits in a tropical rockshelter in quartzite bedrock.

Mineral carbonation reactions under water-saturated, hydrothermal-like conditions and numerical simulations of CO2 sequestration in tholeiitic basalt of the Eastern Deccan Volcanic Province, India

Previous water-CO2 interaction studies have studied few basalt types (rich in Ca, Mg, and Fe cations) to determine pyroxene, olivine and plagioclase reaction rates; however, limited research has examined the Deccan basalt. Therefore, in this study, basalt-CO2-water-saturated interaction experiments and numerical simulations were performed under hydrothermal-like conditions. X-ray Diffraction (XRD) data revealed the appearance of calcite, aragonite, ankerite, huntite and siderite; additionally, smectite, chlorite, smectite/chlorite mixed layers and chabazite were also formed. SEM images showed that tiny calcite crystals developed over larger calcite crystals, incipient-disordered calcite formed with imperfections on its crystal faces and cubic chabazite crystals were bounded by smectite grains. Furthermore, these observations were confirmed by EDS analyses and mineral formulae calculations. Major shifts in the carbonate peaks observed at 155 and 282, 178, and 849 cm−1 on Raman spectra confirmed the formation of calcite, dolomite and aragonite, respectively. Mixing trends between basalt and Ca-Mg-Fe carbonates and chlorite/smectite were observed in the case of phyllosilicates. However, ankerite, calcite and siderite recorded the enrichment of (i) Ca, (ii) Fe-Mg and (iii) Fe. High degrees of carbonation and progressive mineral growth were observed with the increasing pH of the solution. The formation of secondary carbonates predominated over that of silicates in short-term experiments; however, with increasing reaction time, the carbonates no longer persisted in the system, as they were dissolved and replaced by silicates. However, the degree of carbonation increased with the increasing pCO2 and pH of the solution.Transition-state-theory (TST)-based numerical simulation models do not agree well with the results of experiments, as carbonate growth was greater in the former case. These models work well to predict the dissolution rates of most minerals but overpredict the growth of non-hydrous Mg carbonates at low temperatures.

Pedogenic carbonate formation: Recrystallization versus migration—Process rates and periods assessed by 14C labeling

Under arid to semihumid climatic conditions, dissolution of primary carbonate and recrystallization with carbon (C) from soil CO2 leads to accumulation of significant amounts of pedogenic (secondary) carbonate. Most soils of arid and semiarid regions contain a carbonate accumulation horizon, the depth of which is related to climatic conditions and properties of parent material. It remains unclear whether this carbonate migrates from the upper horizons before or after recrystallization with soil CO2. The aim of this study was to determine recrystallization rates during initial pedogenesis and to estimate the accumulation depth of secondary carbonate based on C isotopic exchange during secondary carbonate formation in an experiment with alternating moisture conditions. Maize grown on 1 m high loess‐filled columns was pulse labeled in14CO2 atmosphere every 3 weeks. After 6 months, portions of secondary (recrystallized) CaCO3 were determined in 5 cm segments, based on 14C respired in the rhizosphere and subsequently incorporated into newly formed secondary carbonate. More than 80% of recrystallized carbonate (Ca14CO3) was leached from the uppermost 15 cm of the loess column, and more than 70% of total secondary carbonate were accumulated in a depth between 15 and 50 cm. Based on the recrystallization rate calculated for the uppermost 15 cm of the loess column (1.77 ± 0.26 ⋅ 10−5 day−1), between 300 and more than 1,700 years are necessary for complete decalcification of the upper 15 cm. Our modeled data are consistent with formation of calcic horizons under relatively humid conditions.

Carbonate recrystallization in root-free soil and rhizosphere of Triticum aestivum and Lolium perenne estimated by 14C labeling

Under arid and semiarid conditions, pedogenic (secondary) carbonates are formed in soil by precipitation of Ca2+ from soil parent material with dissolved CO2 originating from root and rhizomicrobial respiration. δ13C values of secondary CaCO3 record the photosynthetic pathway of former vegetation and is therefore used as a tool for paleoenvironmental studies. The time scale of pedogenic carbonate formation as well as the influence of several environmental factors are crucial, yet poorly known. We estimated the recrystallization rate of pedogenic carbonate by the 14C isotopic exchange method. 14CO2 was assimilated by plants, respired into the rhizosphere and subsequently incorporated into secondary carbonate by recrystallization of primary loess carbonate. With ascending number of 14CO2 pulses, the amount of rhizosphere 14C recovered in loess CaCO3 increased linearly, leading to recrystallization rates of 3.2 × 10−5 and 2.8 × 10−5 day−1 for wheat and ryegrass, respectively. In loess close to roots, recrystallization rates more than twice as high were obtained. Extrapolating these rates we showed that several hundred years are necessary for complete recrystallization of primary loess CaCO3 in root-free substrate, assuming that both primary and secondary carbonate is recrystallized several times. In contrast, the process probably takes only decades in rhizosphere loess if carbonaceous encrustations form around the root, impeding repeated recrystallization. This indicates the importance of rhizosphere processes (e.g. respiration of roots and microorganisms, exudation) for secondary carbonate formation.

Formation of secondary carbonates and native sulphur in sulphate-rich Messinian strata, Sicily

Abstract Microbially formed authigenic carbonates accompanied by native sulphur are present in the ‘Calcare Solfifero’ below a thick succession of gypsum deposited during the Messinian salinity crisis in Sicily. We sampled these carbonates and associated sulphur in five former sulphur mines to subject them to a detailed petrographic and geochemical study in order to explore their different modes of formation. Native sulphur formed in conjunction with microbial sulphate reduction, which is reflected in its depletion in 34S (δ34S values as low as − 2‰ vs. V-CDT) and an enrichment of 34S in the residual sulphate (δ34S values as high as + 61‰). The oxidation of organic matter by sulphate reduction increased alkalinity, inducing precipitation of secondary carbonate minerals. A set of authigenic limestones lacking sulphate minerals, but characterized by pseudomorphs after gypsum and high δ18O values (as high as + 9‰ vs. V-PDB) reflects syngenetic mineral formation within evaporitic settings. Low δ13C values (as low as − 52‰ vs. V-PDB) reveal that these carbonate phases were formed by microbial sulphate reduction coupled to the oxidation of biogenic methane. Another set of authigenic carbonates that replaced sulphate minerals is typified by low δ18O values (as low as − 4‰). These carbonates formed epigenetically during later diagenesis following compaction. Dissolution of gypsum or anhydrite by meteoric waters delivered the sulphate for microbial sulphate reduction. Low carbon isotope values of these carbonates (− 29 to − 5‰) indicate that carbonate was derived from the oxidation of crude oil and possibly minor methane, partly involving different degrees of admixture of dissolved carbonate from other sources. Although the studied rocks with their vast amounts of secondary carbonate minerals and sulphur seem to indicate a similar genesis at first glance – having formed by biogeochemical transformations of sulphate and hydrocarbons – this study reveals that these processes can occur at different times in variable geological environments.

A seasonally ‘dry’ interglacial climate in eastern England during the early Middle Pleistocene: palaeopedological and stable isotopic evidence from Pakefield, UK

The pedology and the isotope geochemistry of the early Middle Pleistocene interglacial ‘rootlet bed’ at Pakefield, eastern England, record the climate when this unit accumulated. Horizons of nodular pedogenic calcrete indicate the existence of a strongly seasonal precipitation regime that produced a pronounced net soil moisture deficit during which secondary carbonate was precipitated. The isotopic composition of these nodules confirms the seasonality of annual precipitation, as this indicates that intense soil moisture evaporation was a major mechanism in secondary carbonate formation. The climatic interpretation is enhanced by the palaeoecology of the unit that indicates average summer temperatures significantly warmer than those of the present. The climate of this period was, therefore, different from that of the present day in terms of both temperature and the seasonality of precipitation. Although it has previously been shown that certain interglacial periods in northwest Europe were warmer than the Holocene, this is the first study to show that the precipitation regime of one such episode may also have been significantly different.

Sequestration of atmospheric CO2 in global carbon pools

Sequestering atmospheric CO2 is necessitated by its present concentration of 385 ppm and increasing at the rate of 2 ppm y−1. Increase in atmospheric emission of CO2 with the attendant global warming and environmental degradation are driven by global energy demand. In comparison with the emission of 300 Pg C between 1850 and 2000, total emission during the 21st century is estimated at 950 to 2195 Pg, with an annual rate of emission of 20 to 35 Pg C y−1. Reduction of CO2 atmospheric loading can be achieved by biological, chemical and technological options through either reducing or sequestering emissions. This article describes technological options of sequestering atmospheric CO2 into other global pools. Geologic sequestration involves underground storage of industrially emitted CO2 into the geosphere for long-term and secure storage. Liquefied CO2 is injected about 1000 m below the ground surface either in stable porous rocks, oil wells, coal beds, or saline aquifers. Co-injecting CO2 along with H2S and SO2 is also possible. Over time, the trapped CO2 reacts with minerals to form carbonates, enhances oil recovery, or displaces coal bed methane. Deep injection of CO2 under the ocean creates a CO2 lake which eventually penetrates into the sediments. Iron fertilization is another technique of enhancing the C pool in marine biota, notably phytoplankton. The so-called “biological pumping” is based on the “iron hypothesis” of transferring C to the ocean floor. Terrestrial C sequestration is based on the natural process of photosynthesis. Transfer of CO2 into the biotic pool and soil C pool via humification and formation of secondary carbonates has numerous ancillary benefits through enhancement of ecosystem services. Soil C sequestration is essential to improving soil quality, increasing use efficiency of agronomic input, and advancing world food security. It is also needed to improve water quality by filtration and denaturing of pollutants, and enhancing biodiversity by saving land for nature conservancy. Soil C sequestration is a low hanging fruit, and a bridge to the future until low-C or no-C fuel sources take effect.

Soil carbon stocks under present and future climate with specific reference to European ecoregions

World soils and terrestrial ecosystems have been a source of atmospheric abundance of CO2 ever since settled agriculture began about 10–13 millennia ago. The amount of CO2-C emitted into the atmosphere is estimated at 136 ± 55 Pg from terrestrial ecosystems, of which emission from world soils is estimated at 78 ± 12 Pg. Conversion of natural to agricultural ecosystems decreases soil organic carbon (SOC) pool by 30–50% over 50–100 years in temperate regions, and 50–75% over 20–50 years in tropical climates. The projected global warming, with estimated increase in mean annual temperature of 4–6°C by 2100, may have a profound impact on the total soil C pool and its dynamics. The SOC pool may increase due to increase in biomass production and accretion into the soil due to the so-called “CO2 fertilization effect”, which may also enhance production of the root biomass. Increase in weathering of silicates due to increase in temperature, and that of the formation of secondary carbonates due to increase in partial pressure of CO2 in soil air may also increase the total C pool. In contrast, however, SOC pool may decrease because of: (i) increase in rate of respiration and mineralization, (ii) increase in losses by soil erosion, and (iii) decrease in protective effects of stable aggregates which encapsulate organic matter. Furthermore, the relative increase in temperature projected to be more in arctic and boreal regions, will render Cryosols under permafrost from a net sink to a net source of CO2 if and when permafrost thaws. Thus, SOC pool of world soils may decrease with increase in mean global temperature. In contrast, the biotic pool may increase primarily because of the CO2 fertilization effect. The magnitude of CO2 fertilization effect may be constrained by lack of essential nutrients (e.g., N, P) and water. The potential of SOC sequestration in agricultural soils of Europe is 70–190 Tg C yr−1. This potential is realizable through adoption of recommended land use and management, and restoration of degraded soils and ecosystems including wetlands.

Mapping hydrocarbon‐induced mineralogical alteration in the northern Tian Shan using ASTER multispectral data

AbstractSeepage from hydrocarbon reservoirs can cause a number of chemical and/or mineralogical changes in overlying rocks and soils. These surface changes can be detected by remote sensing imaging systems. The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) is an advanced multispectral imaging system in the visible and near infrared (VNIR) to shortwave infrared (SWIR) region. Band ratios of ASTER VNIR and SWIR data to detect red bed bleaching and secondary carbonate formation induced by hydrocarbon seepage are proposed through analysis of the spectral properties of rock samples. Results show that ASTER band ratios of 2/1 and 4/8 can discriminate bleached red beds and secondary carbonate minerals due to hydrocarbon seepage in the northern Tian Shan. They are in good agreement with mineral identification using X‐ray diffraction. The ratio technique derived from ASTER data may provide a potential approach for hydrocarbon exploration in areas with sparse vegetation cover.

Nondestructive Identification of Arsenic and Cobalt Minerals from Cobalt City, Ontario, Canada: Arsenolite, Erythrite, and Spherocobaltite on Pararammelsbergite

A Ni-Co-As ore sample from Cobalt City, Ontario, Canada, was examined with scanning electron microscopy and energy dispersive X-ray analysis. In addition to cobaltian pararammelsbergite with variable cobalt content, for which Cobalt City is the type locality, and erythrite, one new mineral was observed for this locality. Well-formed crystals of arsenolite, As(2)O(3), were found embedded in what appears to be fibrous spherocobaltite, CoCO(3). Additional information was obtained by Raman microscopy, confirming the identification of the arsenolite. Both are considered to be secondary minerals formed by exposure to air resulting in oxidation and the formation of secondary carbonates.