Search Results (16)

Foraminiferal shell crystals incorporate the chemical signals of their environment during growth. The recorded information is extracted from the crystals via proxies and can be used to reconstruct paleoenvironments, paleoclimates, and the change of the latter. However, the information that is obtained from the biocrystals is often biased, due to structural and chemical modification of the crystals resulting from dissolution, precipitation, recrystallization, and overall, the transformation of the biologically formed crystals into their inorganic analogs. Electron-backscatter diffraction (EBSD) measurements and analysis render a wide range of information regarding crystallographic-structural attributes of the crystals, such as crystal-microstructure, crystal-texture, the misorientation interrelation of adjacent crystals, crystal-twin-generation and many more. We demonstrate in this study that diagenetic overprint of foraminiferal shell Ca-carbonate crystals can be identified by structural-crystallographic characteristics obtained from EBSD measurements. We investigated modern/pristine and fossil Trilobatus sacculifer shells and observed an undisturbed shell surface for both. Despite the latter, we demonstrate here that with an increase in the degree of fossilization and diagenetic overprint, there is an increase in recrystallized calcite in the shells and a decrease in twinned calcite. Twinned calcite is the hallmark of pristine T. sacculifer shells. We show that, with increasing degrees of shell overprint, crystal-microstructure, and crystal-texture, the frequency of the 60°|<001> twin misorientation is modified and propose to use structural-crystallographic attributes determined with EBSD measurements for the identification of recrystallized/overprinted foraminiferal carbonate. We discuss that disclosing low degrees of overprint is of main importance, as minor structure changes of overprinted shells are easily overlooked with SEM imaging. Nonetheless, these are readily identified with EBSD-measurements. Full article

Aminotris(methylene phosphonic acid) (ATMP) and poly(acrylic acid) sodium salt (PAA) have shown favorable results in the treatment of porous building materials against weathering damage, showing promising potential as mixed-in additives during the production of lime-based mortars. This study investigates the impact of these additives on microstructure and mechanical properties. Additives were introduced in various concentrations to assess their influence on CaCO₃ crystallization, porosity, strength, and carbonation behavior. Results revealed significant modifications in the morphology of CaCO₃ precipitates, showing evidence of nanostructured CaCO₃ aggregates and vaterite stabilization, thus indicating a non-classical crystallization pathway through the formation of amorphous CaCO₃ phase(s), facilitated by organic occlusions. These nanostructural changes, resembling biomimetic calcitic precipitates enhanced mechanical performance by enabling plastic deformation and intergranular bridging. Increased porosity and pore connectivity facilitated CO₂ diffusion towards the mortar matrix, contributing to strength development over time. However, high additive concentrations resulted in poor mechanical performance due to the excessive air entrainment capabilities of short-length polymers. Overall, this study demonstrates that the optimized dosages of ATMP and PAA can significantly enhance the durability and mechanical performance of lime-based mortars and suggests a promising alternative for the tailored manufacturing of highly compatible and durable materials for both the restoration of cultural heritage and modern sustainable construction. Full article

The Hendy Test is widely used for assessing whether isotopic equilibrium was reached in speleothems by examining the δ¹⁸O and δ¹³C correlation along a single growth layer. However, stalagmite micro-layers are typically only a few micrometers thick and taper off from the center towards the sides, making it challenging to sample within the same growth layer in practice. To address this, we selected three caves in the monsoon region of China (Shihua Cave in the north, Heshang Cave in the central, and Baojinggong Cave in the south) to verify whether the modern cave calcite has reached equilibrium fractionation with drip water. We examined the spatial variations in the δ¹⁸O and δ¹³C values of farmed calcite on glass plates, which are analogous to a single growth layer. The δ¹⁸O and δ¹³C correlations of farmed calcite from different cave sites are consistently strong, suggesting that kinetic fractionation effects are prevalent, especially at the drip sites with lower drip rates due to longer CO₂ degassing. The δ¹⁸O–δ¹³C covariations can also occur along speleothem growth axes on short time scales, while isotopic variations over longer time scales are still in response to climate change. We propose that the Hendy Test criteria might not be prerequisites to isotopic equilibrium, and a Replication Test provides a more reliable indication of the integrity of isotopic proxies in paleoclimate research. Full article

Reliance on groundwater is outpacing natural replenishment, a growing imbalance that requires detailed and multi-faceted water resource understanding. This study integrated water-stable isotopes and hydrogeochemical species to examine hydrogeochemical processes during groundwater recharge and evolution in the Lake Malawi basin aquifer systems. The findings provide insights into groundwater source provenance, with non-evaporated modern precipitation dominating recharge inputs. Grouped hydrochemical facies exhibit five groundwater water types, prominently featuring Ca-Mg-HCO₃. Modelled hydrogeochemical data underscore dominant silicate dissolution reactions with the likely precipitation of calcite and/or high-Mg dolomitic carbonate constrained by ion exchange. Isotope hydrology reinforces water resource system conceptualisation. Coupled isotopic-hydrogeochemical lines of evidence reveal a discernible spatial-seasonal inhomogeneity in groundwater chemical character, revealing a complex interplay of meteoric water input, evaporative effects, recharge processes, and mixing dynamics. Findings show that measurable nitrate across Malawi highlights a widespread human impact on groundwater quality and an urgent need for detailed modelling to predict future trends of nitrate in groundwater with respect to extensive fertiliser use and an ever-increasing number of pit latrines and septic systems arising from rapid population growth. This study not only refined the Lake Malawi basin aquifer systems conceptualisation but also provided isotopic evidence of groundwater and lake water mixing. This study sets a base for groundwater management and policy decisions in support of the Integrated Water Resources Management principles and Sustainable Development Goal 6 objectives for groundwater sustainability in the transboundary Lake Malawi basin. Full article

The “dolomite problem”, which has confused scientists for nearly two centuries, is an important fundamental geological problem. The mineralogical characteristics of carbonate minerals show that the dolomite structure consists of an ordered arrangement of alternating layers of Ca²⁺ and Mg²⁺ cations interspersed with ${\mathrm{C}\mathrm{O}}_3^{2-}$ anion layers normal to the c-axis. The dolomite structure violates the c glide plane in the calcite structure, which means that dolomite has R$\overline{3}$ space group symmetry. The ordered dolomite has superlattice XRD reflections [e.g., (101), (015) and (021)], which distinguish it from calcite and high-Mg calcite. The calculation of thermodynamic parameters shows that modern seawater has a thermodynamic tendency of dolomite precipitation and the dolomitization reaction can be carried out in standard state. However, the latest thermodynamic study shows that modern seawater is not conducive to dolomitization, and that seawater is favorable for dolomitization in only a few regions, such as Abu Dhabi, the Mediterranean and the hypersaline lagoons in Brazil. The kinetic factors of dolomite formation mainly consist of the hydration of Mg²⁺, the presence of sulfate and the activity of carbonate. Current studies have shown that the presence of microorganisms, exopolymeric substances (EPS), organic molecules, carboxyl and hydroxyl functional groups associated with microorganisms and organic molecules, clay minerals with negative charges and dissolved silica facilitate magnesium ions to overcome hydration and thus promote Mg²⁺ incorporation into growing Ca-Mg carbonates. Similarly, the metabolic activity of microorganisms is conducive to the increase in alkalinity. However, the inhibitory effect of sulfate on dolomite formation seems to be overestimated, and sulfate may even be a catalyst for dolomite formation. Combining the carbonate crystallization mechanism with thermodynamic and kinetic factors suggests that the early stage of dolomite precipitation or the dolomitization reaction may be controlled by kinetics and dominated by unstable intermediate phases, while metastable intermediate phases later transform to ordered dolomite via an Ostwald’s step rule. Full article

The exploitation of groundwater in arid and semi-arid areas, especially in basins where groundwater is intensively exploited, is likely to have adverse effects on the originally fragile ecological environment, which also greatly alter the hydrogeochemical evolution process. Over-abstraction of groundwater in the Haolebaoji watershed of the Ordos Basin located in the semi-arid regions has led to a series of changes in the groundwater system, which has attracted considerable attention from environmental protection organizations. However, the origin and geochemical evolution of groundwater in the Haolebaoji watershed have not been revealed. In this study, the Haolebaoji watershed is selected as the typical study area to investigate hydrogeochemical evolution under the intensive groundwater exploitation. Groundwater samples were collected and tested for major ions and stable isotopes (δ¹⁸O, δD). Various approaches including the ion proportional relationship diagram, chlor-alkali index, saturation index, Gibbs diagram, and principal factor analysis were used to reveal the hydrogeochemical processes regulating the groundwater geochemistry. The groundwater in the study area is divided into five hydrochemical types according to the Piper diagram. It was found that the chemical composition of groundwater in the study area is mainly controlled by the dissolution of calcite, dolomite, gypsum, and halite. The cation exchange intensity gradually increased with the flow of groundwater from the recharge to the discharge area. Rock weathering plays a controlling role in the formation of groundwater geochemistry, but it is also controlled by evaporative crystallization in some runoff and discharge areas. Groundwater is predominantly recharged by modern local atmospheric precipitation, and deep wells receive water supply during cold periods. The study findings provide important information for the development of sustainable groundwater management strategies for the Ordos Energy Base. Full article

The concentric layers of ooids from the modern environment are mostly aragonite and those from the ancient are mostly calcite and Mg-calcite. Dolomitic ooids are rare and are usually formed via the replacement of aragonite or calcite. Here, dolomitic ooids were found in the Pliocene Shizigou Formation in the Qaidam Basin, Northern Tibet Plateau. This paper focuses on whether the dolomitic ooids is a primary precipitate. Optical microscope and scanning electron microscope observation, combined with X-ray diffraction and cathodoluminescence analyses, indicate that the primary mineral of the ooid cortices is poorly ordered dolomite. Extracellular polymeric substances and pyrite were found in the ooids, indicating that the microbe participation was involved in the formation of the ooids. Firstly, the ooids grow on the offshore lake floor. Due to the involvement of sulfate-reducing bacteria and dissolved silica, the nanominerals were precipitated on extracellular polymeric substances. Then, the ooids were transported to strong hydrodynamic surf zones, where the random nanominerals were abraded to form flattened plates as a new polished layer. In addition, a comparison between the carbon and oxygen isotopic compositions and minerals of ooids from different periods indicate that the Pliocene lakes had a lower salinity and were more humid than Pleistocene lakes. Therefore, ooids may be an effective proxy for reflecting the climatic change and uplift history of the Tibet Plateau. Full article

Interpreting the paleoecosystems of ancient microbialites relies on our understanding of how modern microbialites form in relation with the bio-physico-chemical conditions of their environment. In this study, we investigated the formation of modern carbonate microbialites in the hydrothermal system of La Salsa in Laguna Pastos Grandes (Bolivia), which spans a wide range of physicochemical conditions and associated microbial communities. By combining dissolved inorganic carbon (DIC) isotope mass balance modeling, analysis of carbonates solubility diagram, and imaging of the microorganisms–mineral assemblages within microbial mats, we found that several modes of carbonate precipitation dominate in distinct portions of the hydrothermal system. (1) In high-[DIC] waters, undersaturated to slightly saturated with respect to calcite, cyanobacterial calcification is promoted by CO₂ degassing and photosynthetic activity within the microbial mats. (2) In alkaline waters undergoing sustained evaporation, the precipitation of an amorphous calcium carbonate phase seems to control the water a(Ca²⁺)/a(CO₃²⁻) ratio and to serve as a precursor to micritic calcite formation in microbial mats. (3) In saline ephemeral ponds, where the carbonate precipitation is the highest, calcite precipitation probably occurs through a different pathway, leading to a different calcite texture, i.e., aggregates of rhombohedral crystals. Full article

The strontium isotope signature (⁸⁷Sr/⁸⁶Sr) of calcite precipitated in rock fractures and faults is a frequently used tool to trace paleofluid flow. However, bedrock fracture networks, such as in Precambrian cratons, have often undergone multiple fracture reactivations resulting in complex sequences of fracture mineral infillings. This includes numerous discrete calcite crystal overgrowths. Conventional ⁸⁷Sr/⁸⁶Sr analysis of dissolved bulk samples of such crystals is not feasible as they will result in mixed signatures of several growth zonations. In addition, the zonations are too fine-grained for sub-sampling using micro-drilling. Here, we apply high spatial resolution ⁸⁷Sr/⁸⁶Sr spot analysis (80 µm) in transects through zoned calcite crystals in deep Paleoproterozoic granitoid fractures using laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) to trace discrete signs of paleofluid flow events. We compare the outermost calcite growth zone with ⁸⁷Sr/⁸⁶Sr values of the present-day groundwater sampled in the same boreholes to distinguish potential modern precipitates. We then connect our results to previously reported radiometric dating and C and O isotope signatures to understand the temporal history and physicochemical evolution of fluid flow within the fractures. Comparisons of modern calcite precipitated in a borehole over a period of 17 years with modern waters prove the concept of using ⁸⁷Sr/⁸⁶Sr as a marker for fluid origin in this environment and for how ⁸⁷Sr/⁸⁶Sr changed during marine water infiltration. Intermittent calcite precipitation over very long time spans is indicated in calcite of the currently open fractures, showing an evolution of ⁸⁷Sr/⁸⁶Sr from ~0.705–0.707—a population dated to ~1.43 billion years—to crystal overgrowth values at ~0.715–0.717 that overlap with the present-day groundwater values. This shows that high spatial resolution Sr isotope analysis of fine-scaled growth zonation within single calcite crystals is applicable for tracing episodic fluid flow in fracture networks. Full article

In Quaternary paleosciences, the rationale behind analogical inference presupposes that former processes can be explained by causes operating now, although their intensity and rates can vary through time. In this paper we synthesised the results of different modern analogue studies performed in a varved lake. We discuss their potential value to obtain best results from high resolution past records. Different biogeochemical contemporary processes revealed seasonality and year-to-year variability, e.g., calcite precipitation, lake oxygenation, production and deposition of pollen and phytoplankton growth. Fingerprints of the first two of these processes were clearly evidenced in the varve-sublayers and allow understanding related to past events. Pollen studies suggested the possibility of identifying and characterizing seasonal layers even in the absence of varves. Marker pigments in the water column were tightly associated with phytoplankton groups living today; most of them were identified in the sediment record as well. We observed that 50% of these marker pigments were destroyed between deposition and permanent burying. In another study, seasonality in the production/distribution of branched glycerol dialkyl glycerol tetraethers (brGDGTs) and derived temperature estimates were investigated in catchment soils and particles settling in the lake. The signatures of brGDGTs in depositional environments mainly were representative of stable conditions of soils in the catchment that last over decades; no brGDGTs seemed to be produced within the lake. The main contribution of this review is to show the advantages and limitations of a multiproxy modern-analogue approach in Lake Montcortès as a case study and proposing new working hypotheses for future research. Full article

The halophilic bacterium Halomonas smyrnensis from a modern salt lake used in experiments to induce biomineralization has resulted in the precipitation of monohydrocalcite and other carbonate minerals. In this study, a Halomonas smyrnensis WMS-3 (GenBank:MH425323) strain was identified based on 16S rDNA homology comparison, and then cultured in mediums with 3% NaCl concentration to induce monohydrocalcite at different Mg/Ca molar ratios of 0, 2, 5, 7, and 9. The growth curve of WMS-3 bacteria, pH values, NH₄⁺ concentration, HCO₃⁻ and CO₃²⁻ concentration, carbonic anhydrase (CA) activity, and the changes in Ca²⁺ and Mg²⁺ ion concentration were determined to further explore the extracellular biomineralization mechanism. Moreover, the nucleation mechanism of monohydrocalcite on extracellular polymeric substances (EPS) was analyzed through studying ultrathin slices of the WMS-3 strain by High resolution transmission electron microscopy (HRTEM), Selected area election diffraction (SAED), Scanning transmission electron microscopy (STEM), and elemental mapping, besides this, amino acids in the EPS were also analyzed. The results show that pH increased to about 9.0 under the influence of ammonia and CA activity. The precipitation ratio (%, the ratio of the mass/volume concentration) of the Ca²⁺ ion was 64.32%, 62.20%, 60.22%, 59.57%, and 54.42% at Mg/Ca molar ratios of 0, 2, 5, 7, and 9, respectively, on the 21st day of the experiments, and 6.69%, 7.10%, 7.74%, 8.09% for the Mg²⁺ ion concentration at Mg/Ca molar ratios 2, 5, 7, and 9, respectively. The obtained minerals were calcite, Mg-rich calcite, aragonite, and hydromagnesite, in addition to the monohydrocalcite, as identified by X-ray diffraction (XRD) analyses. Monohydrocalcite had higher crystallinity when the Mg/Ca ratio increased from 7 to 9; thus, the stability of monohydrocalcite increased, also proven by the thermogravimetry (TG), derivative thermogravimetry (DTG) and differential scanning calorimetry (DSC) analyses. The C=O and C–O–C organic functional groups present in/on the minerals analyzed by Fourier transform infrared spectroscopy (FTIR), the various morphologies and the existence of P and S determined by scanning electron microscope-energy dispersive spectrometer (SEM-EDS), the relatively more negative stable carbon isotope values (−16.91‰ to −17.91‰) analyzed by a carbon isotope laser spectrometer, plus the typical surface chemistry by XPS, all support the biogenesis of these mineral precipitates. Moreover, Ca²⁺ ions were able to enter the bacterial cell to induce intracellular biomineralization. This study is useful to understand the mechanism of biomineralization further and may provide theoretical reference concerning the formation of monohydrocalcite in nature. Full article

Anaerobic oxidation of methane (AOM) is a common biochemical process in the ocean and it plays an important role in global climate change, elemental circulation, and atmospheric evolution over geological time. In this paper, we analyzed of δ³⁴S, Fe, Mn, Ca/Ti, and Sr/Ti ratios, and the date of carbon and sulfur from the site SH3 of Shenhu area. Result showed that (1) 0–6 mbsf (meter blow the sea floor) was mainly affected by OSR (anaerobic oxidation of organic matters) and 7–15 mbsf was a paleo-SMTZ (sulfate–methane transition zone) position. The modern SMTZ was mainly distributed at 19–25 mbsf. The barium sulfate precipitation above the modern SMTZ indicating that the current methane leakage was stable and lasted longer during geological history. (2) By studying the change of magnetic and the different carbonate minerals, results showed that there were two AOM stages. During the early stage, Fe²⁺ were mainly produced by sulfide abiotic reductive dissolution. During the later stage, Fe²⁺ were mainly produced by the metal-AOM. (3) Study of the mineral characteristics of the paleo-SMTZ and the modern SMTZ showed that the modern SMTZ carbonate minerals were mainly low-Mg calcite and aragonite, while the paleo-SMTZ carbon minerals were mainly high Mg minerals. The reason for this difference is that the modern SMTZ layer was only experienced the first stage of anaerobic oxidation of methane. In the paleo-SMTZ layer, it has experienced two stage of anaerobic oxidation of methane. During the last stage of metal-AOM, the low Mg carbonate minerals were converted into high Mg carbonate minerals. This research confirms the presence of metal-driven methane anaerobic oxidation at the bottom of sulfate-driven methane anaerobic oxidation and during the metal-driven methane anaerobic oxidation, methane and metal oxides or hydroxides would couple to convert the in situ metal oxides or hydroxides into metal ions, meanwhile the phosphorus adsorbed on the surface of the metal oxides is released into adjacent pore water, and convert to new P-bearing minerals under suitable conditions. Full article

The region comprising of East Central Europe, South East Europe and Turkey contributed to the SISAL (Speleothem Isotopes Synthesis and AnaLysis) global database with stable carbon and oxygen isotope time-series from 18 speleothems from 14 caves. The currently available oldest record from the studied region is the ABA-2 flowstone record (Abaliget Cave; Hungary) reaching back to MIS 6. The temporal distribution of the compiled 18 records from the region points out a ~20 kyr-long period, centering around 100 ka BP, lacking speleothem stable isotope data. The regional subset of SISAL_v1 records displays a continuous coverage for the past ~90 kyr for both δ¹³C and δ¹⁸O, with a mean temporal resolution of ~12 yr for the Holocene, and >50 yr for the pre-Holocene period. The highest temporal resolution both for the Holocene and the pre-Holocene was achieved in the So-1 record (Sofular Cave; Turkey). The relationship between modern day precipitation δ¹⁸O (amount weighted annual and winter season mean values; 1961–2017) and climatological parameters was evaluated. The strong positive correlation found in East Central Europe reinforces the link between modern day precipitation δ¹⁸O, temperature and large-scale circulation (North Atlantic Oscillation) expected to be preserved in the speleothem δ¹⁸O record; while a negative relationship was documented between precipitation amount and oxygen isotope compositions in South East Europe. Variations of δ¹³C values are primarily interpreted as reflecting dry/wet periods across the region. Elevation gradients from three non-overlapping periods of the last ~5 kyr indicated elevation gradients around −0.26‰ per 100 m⁻¹ for calcite δ¹⁸O. Full article

The relationship between environmental conditions and the development, mineralization and preservation of modern tufa microbialites was investigated in a 1.1 km long freshwater stream in Villiers-le-Bâcle, a tributary of Mérantaise river. Detailed mapping of the tufa microbialite distribution combined with sedimentological, petrographical and mineralogical analyses were coupled with chemical measurements. Six organosedimentary structures were identified; their distribution appears heterogeneous along the stream and responds to physicochemical conditions of water and specific biological components (e.g., microorganism, exopolymeric substance). Two of the organosedimentary structures show evidence of mineralization and only one is lithified. Based on field observations and in-situ deployment of mineralization markers (bricks), three zones with increasing mineralization intensities are defined, ranging from no mineralization to thick mineralized crusts forming riverine tufa. Both biotic and abiotic processes were proposed for the tufa microbialite formation. We explained changes in mineralization intensities by the specific physicochemical conditions (e.g., calcite saturation index (SI_calc) and partial pressure of CO₂ (pCO₂) and a closed proximity of the cyanobacterial biofilm and carbonates precipitates. The physical and chemical composition of substrate impact development of microbial communities, mineralization potential of tufa microbialite. Even though the physicochemical and biological conditions were optimal for mineral precipitation, the potential of lithification depended on the presence of a suitable (physical and chemical) substrate. Full article

The paper presents the results of studying the development of calcite neoformations on the surfaces of modern buildings within the city of Tyumen. The objects of the study were carbonate crusts and stalactite-like bodies formed on the surfaces of five representative buildings in the city center. Research methods included visual diagnostics, optical microscopy, scanning electron microscopy with energy dispersive X-ray spectroscopy, confocal laser scanning microscopy and semi-quantitative determination of the mineral composition by X-ray diffraction analysis. The results of the study show that calcite is the main component of all carbonate crusts, while other minerals were found in small quantities. The microscopic studies revealed the differences in morphology of crusts developing on horizontal and vertical surfaces. The mycelium of fungi (presumably of the Penicillium group), represented by filamentous and often hollow hyphae covered with calcite, as well as relics of bacterial colonies were found in all studied samples. It was noted that the mycelium forms the structural frame of carbonate crusts and stalactites. Studies have shown that the prokaryotic–eukaryotic communities are responsible for the high rate of the urban speleothem growth and play the main role in calcite precipitation at the initial stages of their development. Full article