Abstract Ikaite is a calcium carbonate hexahydrate that forms at temperatures close to the freezing point of water; thus, its occurrence is associated with cryogenic conditions. This mineral is metastable and quickly transforms to calcite at temperatures above 5 °C. Pseudomorphs of calcite after ikaite are known as glendonite. The nanostructure of 25 000–43 000 year old glendonite from Victoria cave (Southern Ural, Russia) was investigated in search of structural features indicative of the ikaite-to-calcite transformation. Scanning electron microscope images display several micrometer- to submicrometer-size pores and indicate high intergranular porosity among the loosely aggregated grains. Transmission electron microscopy (TEM) data show evidence of 10–20 nm nanotwins [twin law (1014)] and 10–40 nm overlapping nanograins. Scanning TEM images reveal that the individual grains contain 5–10 nm long and 2–4 nm wide mesopores (sizes between 2 and 50 nm), which are aligned parallel to [1010] of calcite and might be associated with a crystallographically oriented dehydration of the precursor ikaite. Fourier transform infrared spectroscopy reveals no evidence of structural water but absorption bands related to molecular water trapped in fluid inclusions are present. Nitrogen absorption/desorption measurements show that the specific surface area of 5.78 m2/g and the pore volume of ~0.07 cm3/g for calcite, the constituent of glendonite, are comparable to those of a common natural calcite. We suggest that the aligned mesopores, frequently occurring twins, small grain size, presence of aqueous inclusions and the high micrometer- to submicrometer-size intergranular porosity arise from the ikaite-to-calcite transformation and thus may be used as criteria for the former presence of ikaite and hence for cold paleotemperatures. However, since similar features might also be common in biogenic carbonates, the diagnostic macroscopic pseudomorphs after ikaite are equally important for identifying glendonites and inferring cryogenic conditions.
Since carbonate formation is an important process linking inorganic and biological components of freshwater ecosystems, we characterized the formation of modern carbonate sediments in a large, shallow, calcareous lake (Lake Balaton in Hungary). We measured the amount of allochtonous mineral particles delivered to the lake by tributaries and through the atmosphere over a 2-year period, and estimated the mass of carbonate minerals that precipitated from lakewater. Chemical and structural features of mineral particles from various sources were also studied. Both the mineralogical character and the amount of particles delivered by streams and through the atmosphere were similar, and formed a minor fraction of the annual sediment increment (∼5%–6% by mass). Since the watercourses feeding the lake had high concentrations of Ca 2+ , Mg 2+ , and HCO 3 − (with a Mg/Ca mol ratio ranging from 1 to 4), Mg-bearing calcite (with 2–17 mol% MgCO 3 ) was found to continually precipitate in the lake. According to X-ray powder diffraction measurements, the Mg content of calcite increased from West to East, in parallel with changes in water chemistry. Dolomite was detected as a minor phase, and in the eastern part of the lake it typically produced a split 104 peak in X-ray diffractograms, suggesting two distinct sources: stoichiometric dolomite was allochtonous, whereas a Ca-rich protodolomite fraction formed in the lake. Mg-bearing calcite precipitating in the lake was found by far the largest contributor to sediment formation, with an estimated annual accumulation of about 0.75–0.9 mm consolidated sediment; thus, ∼89% of the currently forming sediment consists of autochtonous carbonate. In addition to providing new estimates for the rates of accumulation of distinct sediment fractions, our results also provide a baseline for further studies on the retention and release of nutrients by sediment minerals.
Abstract Amorphous calcium carbonate (ACC) is a precursor of crystalline calcium carbonates that plays a key role in biomineralization and polymorph evolution. Here, we show that several bacterial strains isolated from a Hungarian cave produce ACC and their extracellular polymeric substance (EPS) shields ACC from crystallization. The findings demonstrate that bacteria-produced ACC forms in water-rich environment at room temperature and is stable for at least half year, which is in contrast to laboratory-produced ACC that needs to be stored in a desiccator and kept below 10 °C for avoiding crystallization. The ACC-shielding EPS consists of lipids, proteins, carbohydrates and nucleic acids. In particular, we identified large amount of long-chain fatty acid components. We suggest that ACC could be enclosed in a micella-like formula within the EPS that inhibits water infiltration. As the bacterial cells lyse, the covering protective layer disintegrates, water penetrates and the unprotected ACC grains crystallize to calcite. Our study indicates that bacteria are capable of producing ACC, and we estimate its quantity in comparison to calcite presumably varies up to 20% depending on the age of the colony. Since diverse bacterial communities colonize the surface of cave sediments in temperate zone, we presume that ACC is common in these caves and its occurrence is directly linked to bacterial activity and influences the geochemical signals recorded in speleothems.
The effects of pre-existing mineral phases on the nucleation and growth of calcium carbonates from solution are relatively poorly understood, despite the widespread co-occurrence of carbonate minerals with clays and other silicates in rocks, soils, and sediments. Previous studies suggested that sheet silicates template calcite nucleation. Moreover, the presence of certain clay minerals appeared to enhance Mg2+ incorporation, resulting in Mg-bearing calcite or protodolomite. Here, we present the results of titration experiments with an environmentally relevant experimental setup, designed to study the roles of swelling clay minerals in the formation of Mg-bearing CaCO3 phases. We added both Mg-free and Mg-rich calcian solutions to carbonate buffers both in the presence and absence of smectite, and monitored the evolution of the solutions with pH and Ca ion selective electrodes, in order to identify nucleation and phase transition events. Initial products of the titration experiments were aged in their mother solutions for a few months. Both freshly formed and aged materials were studied using a variety of transmission electron microscopy (TEM) techniques. From Mg-free, homogeneous solutions vaterite was the first phase to precipitate. The addition of smectite triggered nucleation at lower supersaturation and generated calcite rather than vaterite. In Mg-rich solutions, aragonite was the first phase to precipitate both without and with clay minerals, and precipitation occurred at similar saturation levels in both samples. In the presence of clays, however, the aragonite nanocrystals were attached to smectite flakes. After the Mg-bearing systems were aged for several months, peculiar assemblages of protodolomite and low-magnesian calcite formed in association with smectite, whereas in the clay-free systems aragonite persisted. These observations suggest that if smectite is present in an environment where carbonates precipitate, the clay mineral has important and complex roles in the formation of Mg-bearing calcium carbonate phases. In addition to enhancing the nucleation of the first carbonate solid, smectite also triggers the formation of calcite-type structures, both at nucleation and in dissolution/reprecipitation reactions during aging.
Minerals of the sediments of shallow lakes are in continuous interaction with the biota. In order to understand the role of algae in producing carbonate minerals in shallow Lake Balaton, we used electron microscopy to study the nanoscale processes of mineral formation and transformation in association with algal blooms. In the immediate vicinity of photosynthesizing cells, the first phase to precipitate was amorphous calcium carbonate (ACC). Also associated with some blooms were spindle-shaped calcite particles that invariably contained nm-scale flakes of clay minerals belonging to the smectite group. In contrast, carbonate particles collected from the sediment and from suspended matter during non-bloom periods contained only Mg-bearing calcite particles and minor protodolomite, both associated with smectite flakes. By comparing these observations with the results of laboratory carbonate precipitation experiments, we interpret the distinct carbonate phases as representing stages of a complex process: (1) algal photosynthesis creates high supersaturation near the cells, resulting in the fast nucleation of ACC; (2) ACC particles attach to nm-scale smectite flakes where they instantly transform into calcite through partial dissolution and reprecipitation. Step (2) can result either in calcite spindles, or aggregate-looking, Mg-bearing calcite particles that are typical for Lake Balaton sediments. Spindles form exclusively near algae, for reasons not yet known. (3) Further aging of Mg-bearing calcite by dissolution/reprecipitation produces protodolomite. The entire process appears to take place within hours to days; thus, the typical Lake Balaton sediment contains mainly Mg-bearing calcite and some protodolomite particles. Significantly, the above multistep process of carbonate formation involves several dissolution and reprecipitation cycles, and in most stages clay minerals play crucial roles.
Ikaite (CaCO3*6H2O) is a cryogenic calcium carbonate phase, which forms below about 5°C. If the temperature increases above 5-7 °C ikaite transforms to calcite. Understanding the transformation process is important to interpret paleoclimatological data from glendonites, i.e., calcite pseudomorphs after ikaite in sediments. Tollefsen et al. (2020) suggested that the transformation occurs via a coupled dissolution–reprecipitation mechanism at the ikaite–calcite interface (1). In contrast, Vickers et al. (2022) proposed a quasi-solid state ikaite to calcite transformation mechanismand suggested that stable isotope data of glendonite can be used for reconstructing paleotemperatures(2). However, in sediments the majority of the ikaite to calcite transformation occurs in diagenetic environments, where ambient solutions interact with the transforming mineral.We synthesized ikaite at 2 °C in alkaline environment in order to study its transformation using organic solvents, vacuum pumping and rapid (1 min) heating from 5 to 30 °C. These experiments indicated the formation of amorphous calcium carbonate (ACC) during the ikaite to calcite transition. We also monitored the ikaite transformation by letting the 2 °C parent solution to reach room temperature (25 °C) within ~5 hours. We observed ACC and calcite formations depending on the alkalinity of the parent solution. Our experiments suggest that the ikaite to calcite transition is a two-step process consisting of the solid-state ikaite → ACC transformation and the ACC → calcite dissolution–reprecipitation mechanisms. During these transitions ikaite lost all its water but preserved its original morphology. We hypothesize that the occurrence of a transient amorphous phase during the ikaite to calcite transition implies the alteration of the isotopic data, similar to what was reported for the ACC to calcite transition (3).We acknowledge the financial support of NKFIH ANN141894 grant. References:(1) E. Tollefsen, T. Balic-Zunic, C. M. Mörth, V. Brüchert, C. C. Lee and A. Skelton, Scientific Reports, 2020, 10, 8141.(2) M. L.Vickers, M. Vickers, R. E. M. Rickaby, H. Wu, S. M. Bernasconi, C. V. Ullmann, G. Bohrmann, R. F. Spielhagen, H. Kassens, B. P. Schultz, C. Alwmark, N. Thibault and C. Korte, Geochimica et Cosmochimica Acta, 2022, 334, 201-216.(3) A. Demény, Gy. Czuppon, Z. Kern, Sz. Leél-Őssy, A. Németh, M. Z. Szabó M. Tóth, C.-C. Wu, C.-C. Shen, M. Molnár, T. Németh, P. Németh and M. Óvári, Quaternary International, 2016, 415, 25-32.
Bacteria play crucial roles in the biogeochemical cycle of arsenic (As) and selenium (Se) as these elements are metabolized via detoxification, energy generation (anaerobic respiration) and biosynthesis (e.g. selenocysteine) strategies. To date, arsenic and selenium biomineralization in bacteria were studied separately. In this study, the anaerobic metabolism of As and Se in Shewanella sp. O23S was investigated separately and mixed, with an emphasis put on the biomineralization products of this process. Multiple analytical techniques including ICP-MS, TEM-EDS, XRD, Micro-Raman, spectrophotometry and surface charge (zeta potential) were employed. Shewanella sp. O23S is capable of reducing selenate (SeO42-) and selenite (SeO32-) to red Se(-S)0, and arsenate (AsO43-) to arsenite (AsO33-). The release of H2S from cysteine led to the precipitation of AsS minerals: nanorod AsS and granular As2S3. When As and Se oxyanions were mixed, both As-S and Se(-S)0 biominerals were synthesized. All biominerals were extracellular, amorphous and presented a negative surface charge (-24 to -38 mV). Kinetic analysis indicated the following reduction yields: SeO32- (90%), AsO43- (60%), and SeO42- (<10%). The mix of SeO32- with AsO43- led to a decrease in As removal to 30%, while Se reduction yield was unaffected (88%). Interestingly, SeO42- incubated with AsO43- boosted the Se removal (71%). The exclusive extracellular formation of As and Se biominerals might indicate an extracellular respiratory process characteristic of various Shewanella species and strains. This is the first study documenting a complex interplay between As and Se oxyanions: selenite decreased arsenate reduction, whereas arsenate stimulated selenate reduction. Further investigation needs to clarify whether Shewanella sp. O23S employs multi-substrate respiratory enzymes or separate, high affinity enzymes for As and Se oxyanion respiration.
