Secondary Calcite Research Articles

Calcite Dissolution-Reprecipitation Reactions Are a Key Control on the Sr/Ca, Mg/Ca and δ88/86Sr Compositions of Himalayan River Waters

Silicate weathering on the continents is thought to play a critical part in regulating global climate on geological time scales but determination of the magnitude of silicate weathering fluxes is frustrated by the complexity of the weathering processes. Here we present analyses of stable Sr-isotopic compositions (𝛿88/86Sr) in a suite of river waters and bedloads from the Himalayas in Nepal to establish the lithological controls on 𝛿88/86Sr values and the secondary processes that impact carbonate weathering. A control on 𝛿88/86Sr values is lithology with the rivers in carbonate-dominated catchments marginally lower (∼0.07 ‰) than in silicate-dominated catchments. However, as for 87Sr/86Sr ratios, 𝛿88/86Sr values of carbonates are altered by silicate-carbonate mineral exchange during metamorphism. The major potential secondary control on 𝛿88/86Sr values in Himalayan catchments is precipitation of secondary calcite responsible for the marked elevation of Sr/Ca and Mg/Ca ratios in the waters. We re-evaluate the mechanisms for secondary calcite formation and conclude that a balanced solution and re-precipitation process, driven by the relative instability of the primary Mg-rich calcite, provides a better explanation for the elevated Sr/Ca ratios than simple precipitation from a highly over saturated solution. This solution-re-precipitation process is akin to that invoked to explain diagenesis of deep-sea sediments. The mechanism of secondary calcite formation impacts the distinction of cation inputs from carbonate and silicate minerals. The correlation between 𝛿88/86Sr water-calcite fractionations, Sr/Ca partition coefficients and precipitation rates allows the calcite re-precipitation rates to be inferred from the covariation of water 𝛿88/86Sr values and Sr/Ca ratios. These rates are very low (<10-8 mol m-2 s-1) but are consistent with those inferred from field estimates of the amount of calcite re-precipitated, the surface area of carbonate exposed to weathering and the calcite weathering flux. The low precipitation rates are also consistent with previously reported 𝛥44/40Ca isotope fractionations of ∼−0.2‰. The calcite reprecipitation rates are comparable to silicate weathering rates previously inferred from Li-isotopic compositions which is consistent with calcite re-precipitation taking place very close to equilibrium following the initial rapid saturation of the fluids by calcite.

Estimated in-situ carbon sequestration rates in a weathered silicate basin, southwestern Idaho, U.S.A.

Silicate weathering can induce calcite precipitation from groundwater, enabling carbon dioxide (CO2) sequestration in the critical zone (CZ), which acts as a net carbon sink with significant implications for the global carbon budget. In weathered silicates, secondary calcite dissolution accompanies precipitation-dissolution reactions, and it is unclear how calcite dissolution affects CO2 consumption in natural settings. At the Reynolds Creek Experimental Watershed - Critical Zone Observatory (RCEW-CZO), southwestern Idaho, USA, we estimate in-situ carbon sequestration rates in a semi-arid weathered silicate aquifer using hydrochemical compositions and age tracers from 6 springs and 10 wells. We delineate water-rock interactions by using observed groundwater chemistry to model open system carbon evolution, evapoconcentration in wells, silicate weathering, and formation of clays along groundwater flowpaths. We suggest carbonate precipitation under closed system conditions in deep groundwater, as calcite saturation is reached and CZ CO2 drops to just 41 % of initial concentrations in older waters. In a closed system, we estimate approximately 9 % of CZ CO2 would precipitate, indicating that on-going water-rock interactions in our weathered silicate system appear to drive continued carbon sequestration. Carbon sequestration rates via silicate weathering may help to explain a missing C sink observed at the RCEW-CZO and in other weathered silicate basins.

13C-enriched carbonate precipitates reveal intense methanogenic oil degradation in the upper Wuerhe Formation, Northwest China

Abstract Microbial methanogenesis from crude oil is an important source of CH4 for gas reservoirs and atmospheric greenhouse gases. However, its petrological records have not been found in natural environments, and the geological conditions under which it may occur remain unclear. Here, we provide the first petrological evidence of Fe(III)-mediated methanogenic hydrocarbon degradation in a deep subsurface oil reservoir in Northwest China. The major findings are as follows: (1) highly positive δ13C values (up to +16‰) of secondary calcite attributed to methanogenesis; (2) paragenetic relation of high-δ13C calcite to biodegraded hydrocarbon; and (3) remarkably high FeO contents (up to 8 wt%) and heavy δ56Fe ratios (up to +0.52‰) in calcite, indicative of microbial Fe(III) reduction. Our study shows that methanogenic hydrocarbon degradation can occur in Fe(III)-reducing environments. This process transformed hydrocarbons into CO2 and CH4, where the former mostly precipitated as Fe-rich calcite (the carbon sink), while the latter, representing an estimated ~1968 Tg, might have escaped into the overburden and atmosphere from the Permian reservoir during the Late Triassic to Early Jurassic, which may have acted as an important CH4 source in changing global climate in the geological past.

Calcite recrystallization and its impact on speleothem geochemistry

Speleothems are among the most important archives for past climatic and environmental change. Calcite recrystallization can modify the authigenic structure and geochemical composition of the speleothems and affect the reliability of calcite stalagmites as repositories of authigenic geochemical proxies of past climates and environments. The criteria for distinguishing primary from secondary speleothem calcite, and the conditions (open or semi-closed) of speleothem calcite recrystallization remain poorly understood. Thus, in this study, we investigated the fabric, geochemical composition, and recrystallization dynamics of a partially recrystallized calcite stalagmite (DDH-Z-2) from Didonghe Cave in Shaanxi Province, China, through petrographic observations, fluorescence microscopy, and geochemical analyses (stable isotopes, trace elements, UTh isotopes). We found that: (1) in the DDH-Z-2 stalagmite, open elongated columnar calcite recrystallized into compact elongated columnar calcite. Particulate organic matter and fulvic and humic acids were removed during recrystallization, while aromatic compounds were preserved and became incorporated into the secondary calcite; (2) calcite recrystallization was affected by multiple factors, including external fluid chemistry, primary calcite microstructure, and organic matter; (3) calcite recrystallization occurred under open, fluid-buffered conditions for alteration of the stable isotopes (δ18O and δ13C) and trace elements (Mg, Sr, U). The effect of external fluid composition on trace element (Mg, Sr) composition of secondary calcite varied across the stages of calcite recrystallization. Caution should, therefore, be exercised when using geochemical proxies in stalagmites composed of inclusion-free elongated columnar calcite: such calcite is likely to be recrystallized, and thus record the composition of reactive fluids at the time of recrystallization. Regarding the geochemical system of speleothem diagenesis, the contribution of the parent material and the sources of reactive fluids are key factors to consider.

Post-mortem recrystallization of biogenic amorphous calcium carbonate guided by the inherited macromolecular framework

In contrast to abiotically formed carbonates, biogenetic carbonates have been observed to be nanocomposite, organo-mineral structures, the basic build-blocks of which are particles of quasi-uniform size (10–100 nm) organized into complex higher-order hierarchical structures, typically with highly controlled crystal-axis alignments. Some of these characteristics serve as criteria for inferring a biological origin and the state of preservation of fossil carbonate materials, and to determine whether the biomineralization process was biologically induced or controlled. Here we show that a calcium storage structure formed by the American lobster, a gastrolith initially consisting of amorphous calcium carbonate (ACC) and amorphous calcium phosphate (ACP), post-mortem can crystallize into (thus secondary) calcite with structural properties strongly influenced by the inherited organic matrix. This secondary calcite meets many structural criteria for biominerals (thus called the biomorphic calcite), but differs in trace element distributions (e.g., P and Mg). Such observations refine the capability to determine whether a fossil carbonates can be attributed to biogenic processes, with implications for the record of life on Earth and other terrestrial planets.

Constraining the evolutionary stages of a hypogene karst system by combining morphological, geochemical and geochronological data - the example of carbonate breccia-hosted Melnička Peštera

A combination of morphological observations, geochemical data from calcite minerals and geochronology by burial age dating and U-series is used to constrain the geological and geomorphological setting and the evolutionary stages of a hypogene karst system. This methodological suit is applied to Melnička Peštera, a horizontal cave developed in carbonate breccia overlying dolomite marble in Melnica locality (N. Macedonia), where hydrothermal karst development occurred in both dolomite and calcite marble. The passage morphology of the cave, having a reverse triangle, Laughöhle cross-sections, suggests development near the water table by slowly moving waters. Calcite crusts are found throughout the cave, and based on their relationship to passage morphology, appear to pre-date main horizontal passage formation. Their carbonate stable and clumped isotope and fluid inclusion noble gas compositions indicate cooling of the hydrothermal system with an increased contribution of shallower groundwater. U-series data suggests Early Pleistocene deposition of the calcite crusts. Water table notches and convectional features carved into breccia bedrock and calcite crusts point to subsequent development at and above the water table by condensation corrosion. The solutional aggressiveness near the water table was likely related to CO2, that previously degassed from the deeper parts of the system where calcite was depositing, and redissolved in the cooler, shallower waters. Due to poor connection with the surface, the cave air above the water table likely had high pCO2, that further helped to maintain aggressiveness of the groundwater at the water-air contact, and boosted condensation corrosion above it, preventing deposition of related secondary calcite minerals. The cave-hosting carbonate breccia deposited as an alluvial fan filling up a paleovalley cut into Upper Miocene sediments. Cosmogenic nuclide burial age dating of quartz fragments confirms Early Pliocene age for the breccia, and constrains the paleovalley incision to Late Miocene, likely related to base level lowering caused by the Messinian Salinity Crisis.

Recycled materials and secondary processes controlled the chemical and isotopic compositions of bubbling gases discharged from two adjacent geothermal springs in the Northern Luzon Arc

Gas emissions from hydrothermal systems can serve as indicators of subsurface activity. In addition to gas sources, hydrothermal gas geochemistry is strongly influenced by secondary processes that occur during/after hydrothermal circulation. Here, we observed statistically significant differences in the geochemical characteristics (except for helium isotopes) of bubbling gases discharged from two adjacent vents in the Northern Luzon Arc. Helium (3He/4He = 4.25–7.09 Ra) in both vents was controlled by mixing between mantle and crustal components, where about 74% of helium was contributed by the mantle. Differences in N2/Ar ratios (∼ 300–330) of the two neighboring springs are attributed to subducted materials and seawater mixing (contributing ∼2.5% N2 and Ar), rather than phase separation in the reaction zone. Specifically, Ar was mainly supplied by atmospheric components that dissolved in the percolated seawater with only 8%–9% contributed by the excess radiogenic 40Ar. Excess N2 relative to Ar was mainly supplied by the decomposition of subducted materials (83%–92%) of the South China Sea plate beneath the Philippine Sea Plate. The Lutao gases showed low CO2 concentrations (0.07–22.2 mmol/mol), despite the high 3He/4He ratios indicating a significant contribution of magmatic components. Magmatic CO2 may have been largely consumed by the high Ca Lutao vent fluids via carbonate precipitation in the reaction zone. Alternatively, stable carbon isotope compositions (δ13C) indicate that Lutao CO2 may be supplied by microbial oxidation of alkanes (e.g., CH4 with concentrations of 14.6–173 mmol/mol in the samples), with fractionation factor ΔCO2–CH4 ranging from −15‰ to −25‰ and conversion rates of <10%. Up to 65% of the CO2 in the 2016 samples experienced secondary calcite precipitation in the discharge zone. Our results indicate that recycled subducted materials could potentially affect the geochemical characteristics of gases discharged from arc-volcanic systems. In addition, the influence of secondary processes needs to be considered before tracing the sources of hydrothermal fluids and/or gases, especially in shallow-water hydrothermal systems.

CHARACTERIZATION AND SELECTION OF MORTAR SAMPLES FOR RADIOCARBON DATING IN THE FRAMEWORK OF THE MODIS2 INTERCOMPARISON: TWO COMPARED PROCEDURES

ABSTRACT For several decades, many efforts have been dedicated to enhancing the accuracy of mortar radiocarbon dating and evaluating the reliability of the results concerning the typology of the examined specimens. Several assumptions that are fundamental for the application of the method may be in many cases not fulfilled, such as (a) complete primary limestone dissociation during calcination, (b) efficient separation of geogenic carbon contained in calcareous aggregates, (c) short carbonation time, and (d) absence of secondary calcite. Many laboratories all over the world have proposed different methods to select suitable fractions of mortar. The first intercomparison attempt, involving eight international laboratories, was organized in 2016 aiming at comparing and statistically treating the results obtained on the same materials by different laboratories with their own characterization and pre-treatment methods (Hajdas et al. 2017; Hayen et al. 2017). Following this first step, a new intercomparison experiment was proposed and set up in 2018 during the Mortar Dating International Meeting (Bordeaux, FR). A new set of three mortar samples was chosen, taking care of the selection of standardized materials (homogeneity, known mineralogical composition, absence of exogenous inclusions, known expected age). This work describes the results of two research teams involved in the intercomparison. The samples were characterized, selected, and dated depending on each laboratory strategy. The results stress the importance of the characterization of the raw material is to better understand the mineralogical and petrographical composition of the samples. Such information can support the choice of the most appropriate strategy for the extraction of CO2 and then for data interpretation.

Gagates in Crimean middle jurassic sediments (Sudak): coal petrography, paleobotany, formation conditions

Background. The coal-bearing deposits of the Kopsel formation of the Bathonian-Callovian age of the Middle Jurassic near the city of Sudak (Crimea) are studied. The conducted lithological and mineralogical analysis of rocks, mainly sandstones and siltstones, with shell detritus, plant residues and coal lenses (gagates), made it possible to determine the coastal-marine and lagoon sedimentation conditions. The studied gagates are represented by structural vitrinite and belong to the humus group. Secondary minerals in coal seams and host rocks indicate the stage of early catagenesis. The botanical analysis revealed the coal-forming plants, belonging to a group of conifers of the araucaria family.Aim. To identify the facies-climatic and landscape conditions for the formation of gagates of the Kopsel formation, along with their composition, structure, and secondary changes. To demonstrate that the material composition of coals is the wood of gymnosperms, mainly conifers, rather than algal thalli.Materials and methods. The natural outcrops of the coal-bearing deposits of the Middle Jurassic of the Bathonian-Callovian stage of the Kopsel formation in the valley of the Kopsel River near the city of Sudak (Crimea) were studied. The lenses and interlayers of brown coal — gagate — present in these deposits were of particular interest. Gagate samples were studied both macroscopically (forms and occurrence conditions in the section) and microscopically (by coal petrography methods). To this end, double-sided polished sections were examined using a polarizing microscope and a Vega3 Tescan scanning microscope. The microanalysis of chemical elements was performed using an ULTM Max (GIN) microscope attachment. Paleobotanical studies of carbonified plant residues were carried out. The structure, texture and mineral composition of the rocks were studied in thin sections using a microscope. A mineralogical analysis was carried out using a D8 Advace X-ray diffractometer (gross composition in a powder diffractogram and composition of clays in a fraction of &lt;0.001 mm) and a Vega3 Tescan (GIN) scanning microscope.Results. The comprehensive studies conducted in the new, unique location of coals in the Kopsel Formation of the Middle Jurassic (Bathonian-Callovian stage) in the valley of the Kopsel River determined the paleobotanical composition of coal-forming plants (with the predominance of conifers of the Araucariaceae type), the climatic and paleolandscape conditions for the formation of these coals on the northern outskirts of Tethys. The lithological and mineralogical features of rocks, mainly sandstones and siltstones, with shell detritus, plant residues and lenses of coals (gagates), make it possible to determine the facies conditions of sedimentation as coastal-marine and lagoonal, with fragments of avantdelta and proluvial, landslide deposits. The established secondary changes in the host rocks and the coals indicate the stage of early catagenesis, to which the stratum was subjected at the next stages of geological history.Conclusion. Peat accumulation and subsequent coal formation most likely occurred in shallow lagoons. The type of peat accumulation is paralytic, paragenetically related to the studied sediments. Post-sedimentation transformations of the rocks correspond to early catagenesis. These transformations were established by the presence of secondary calcite (by rock cracks and in the form of nodule formations), the abundance of authigenic gypsum, and the widespread replacement of pyrite framboids with iron oxide minerals. The interlayers of gagates have also undergone changes. In those places where organic matter is impregnated with carbonate solutions, the coal substance is exposed to heat, thus becoming optically opaque.

Clumped isotope analysis of (bio)apatite using a Kiel IV device in long‐integration dual‐inlet mode

RationaleClumped isotope (Δ47) analysis of bioapatite‐derived CO2 is a powerful tool to determine body temperatures of extinct vertebrates. The common acid bath technique in combination with dual‐inlet‐based mass spectrometric measurements has been the preferred method of choice for this purpose, but the large amount of material necessary and the presence of secondary calcite represent obstacles.MethodsWe analyzed the Δ47 composition of carbonate‐bearing (bio)apatites using a Kiel IV device, which – in general – allows a reduction of sample replicate size by a factor of ~40 over dual‐inlet‐based techniques. The Kiel IV device was tested in two different modes: without and with additional water sinks for improved water removal. Furthermore, we tested a pretreatment technique based on 1 M acetic acid (pH = 5) to selectively remove secondary calcite from the carbonate‐bearing (bio)apatite phase.ResultsSignificantly lower Δ47 values were obtained for a given bioapatite after the installation of the two water sinks. With this setup, Δ47 of (bio)apatites followed a temperature relationship that is indistinguishable from the unified one for pure carbonates, provided a dentine sample, rich in organic matter, was excluded. The original bioapatite Δ47 value was restored from a bioapatite/calcite mixture if the mixed material was treated for 1 h with 1 M acetic acid (pH = 5).Conclusions(Bio)apatites having low organic matter content such as enamel(oid) can be analyzed accurately for Δ47 using a Kiel IV equipped with water sinks that ensure effective removal of water. Secondary calcite can be effectively removed from carbonate‐bearing apatite by pretreatment with 1 M acetic acid (pH = 5) for 1 h.

Calcareous nannofossils across the Eocene-Oligocene transition: Preservation signals and biostratigraphic remarks from ODP Site 1209 (NW Pacific, Shatsky Rise) and IODP Hole U1411B (NW Atlantic Ocean, Newfoundland Ridge)

This work provides a detailed biostratigraphic correlation through the Eocene-Oligocene Transition (EOT), based on an integrated stratigraphic approach and the study of calcareous nannofossils, between two disparate sites, one in the NW Atlantic (IODP Hole U1411B) and one in the NW Pacific (ODP Site 1209).The precise site-to-site correlation provided by these data allows for a comparison of carbonate preservation across the EOT including identification of the main post-depositional processes that impact the calcareous nannofossil ooze at Site 1209. The main aim of this work is to understand the extent to which the bulk δ18O and δ13C records and their sources (mainly calcareous nannofossils) are altered by diagenesis. Our detailed SEM study highlights some differences before, during and after the EOT, suggesting local diagenetic dynamics.At Site 1209, a distinctive change, both in nannofossil assemblage composition and preservation state, is observed from the pre-EOT phase to the Late Eocene Event (LEE), with a shift in the dominant process from dissolution to recrystallisation. Surprisingly, despite the overall poor preservation, only the interval between 141 and 142.4 (adj. rmcd) was compromised in term of isotopic values and assemblage diversity and abundance. This interval, recorded in the upper Eocene, was characterized by severe dissolution, concomitant with deposition of secondary calcite on solution-resistant forms. Diagenetic processes have strongly biased the δ18O isotopic signal, resulting in a positive oxygen isotope anomaly through the upper Eocene that is difficult to reconcile with other published trends. For the remaining time intervals, diagenesis seems not to have altered the bulk δ18O profile, which closely resembles that of other sites across the world, and is particularly consistent with other data from the Pacific Ocean. In summary, the impact of diagenesis on nannofossil preservation even if clearly visible both in SEM and optical microscope observations does not always cause a pervasive alteration of the primary isotopic signal and can instead provide important clues on local depositional dynamics.

Crystallinity assessment of anthropogenic calcites using Raman micro-spectroscopy

Anthropogenic calcite is a form of calcium carbonate produced through pyrotechnological activities, and it is the main component of materials such as lime binders and wood ash. This type of calcite is characterized by a significantly lower degree of crystallinity compared with its geogenic counterparts, as a result of different formation processes. The crystallinity of calcite can be determined using infrared spectroscopy in transmission mode, which allows decoupling particle size effect from atomic order and thus effectively distinguish anthropogenic and geogenic calcites. On the contrary, Raman micro-spectroscopy is still in the process of developing a reference framework for the assessment of crystallinity in calcite. Band broadening has been identified as one of the proxies for crystallinity in the Raman spectra of geogenic and anthropogenic calcites. Here we analyze the full width at half maximum of calcite bands in various geogenic and anthropogenic materials, backed against an independent crystallinity reference based on infrared spectroscopy. Results are then used to assess the crystallinity of anthropogenic calcite in archaeological lime binders characterized by different states of preservation, including samples affected by the formation of secondary calcite, and tested on micromorphology thin sections in which lime binders are embedded in sediments.

Large-scale culturing of Neogloboquadrina pachyderma, its growth in, and tolerance of, variable environmental conditions.

The planktic foraminifera Neogloboquadrina pachyderma is a calcifying marine protist and the dominant planktic foraminifera species in the polar oceans, making it a key species in marine polar ecosystems. The calcium carbonate shells of foraminifera are widely used in palaeoclimate studies because their chemical composition reflects the seawater conditions in which they grow. This species provides unique proxy data for past surface ocean hydrography, which can provide valuable insight to future climate scenarios. However, little is known about the response of N. pachyderma to variable and changing environmental conditions. Here, we present observations from large-scale culturing experiments where temperature, salinity and carbonate chemistry were altered independently. We observed overall low mortality, calcification of new chambers and addition of secondary calcite crust in all our treatments. In-culture asexual reproduction events also allowed us to monitor the variable growth of N. pachyderma's offspring. Several specimens had extended periods of dormancy or inactivity after which they recovered. These observations suggest that N. pachyderma can tolerate, adapt to and calcify within a wide range of environmental conditions. This has implications for the species-level response to ocean warming and acidification, for future studies aiming to culture N. pachyderma and use in palaeoenvironmental reconstruction.

Mg and Sr Isotopes in Cap Dolostone: Implications for Oceanic Mixing after a Neoproterozoic Snowball Earth Event

The snowball Earth (SBE) describes a state of the Earth’s climate with global or near-global ice cover. The cap dolostone at the base of the Ediacaran successions serves as useful archives for studying environmental change during the Marinoan Snowball Earth deglaciation in Neoproterozoic. The characteristic compositions in dolomite provide critical information on continental weathering and coastal water mixing after glacial retreat. However, valid methods for pristine dolomite separation remain challenging. In this study, four selected cap dolostone samples from the base of the Ediacaran Lantian Formation were used for establishing a new 3-step leaching method, to remove the secondary calcite and other impurities before determination of δ26Mg and 87Sr/86Sr in dolomite. Non-destructive Raman, X-ray diffractometer (XRD) and scanning electron microscopy (SEM) were used to examine the distribution of dolomite and minor calcite/silicate in each sample. Micro-drill powders before each extraction procedure were examined in weight loss and mineralogical compositions, as well as the chemicals in the leaching solutions. Potential diagenetic artifacts were evaluated using Sr/Ca, Mn/Sr, 87Sr/86Sr and δ26Mg in solutions. By applying a simple two-end member mixing between the seawater and the silicate sources (R2 = 0.48, n = 23), the down-core variations of δ26Mg and 87Sr/86Sr in cap dolostone can be used to gain a better understand of the temporal weathering intensity changes, as well as the coastal oceanic mixing processes, after the Marinoan deglaciation.

Microstructural characterization of natural fractures and faults in the Opalinus Clay: insights from a deep drilling campaign across central northern Switzerland

The Middle-Jurassic Opalinus Clay is the foreseen host rock for radioactive waste disposal in central northern Switzerland. An extensive drilling campaign aiming to characterize the argillaceous formation resulted in a comprehensive drill core data set. The rheologically weak Opalinus Clay is only mildly deformed compared to the over- and underlying rock units but shows a variety of natural fractures. While these structures are hydraulically indistinguishable from macroscopically non-deformed Opalinus Clay today, their analysis allows for a better understanding of the deformation behaviour in the geological past. Here, we present an overview of the different fracture and fault types recorded in the Opalinus Clay and a detailed microstructural characterization of veins—natural dilational fractures healed by secondary calcite and celestite mineralizations. Macroscopic drill core analysis revealed five different natural fracture types that encompass tension gashes of various orientations with respect to bedding and small-scale faults with displacements typically not exceeding the drill core diameter. The occurrence of different fault types generally fits well with the local tectonic setting of the different drilling sites and with respect to the neighbouring regional fault zones. The microstructural investigations of the various vein types revealed their often polyphase character. Fibrous bedding-parallel veins of presumable early age were found to be overprinted by secondary slickenfibres. The polyphase nature of fibrous bedding parallel veins and slickenfibres is supported by differing elemental compositions, pointing towards repeated fracturing and mineralization events. Direct dating of vein calcites with U–Pb was unsuccessful. Nevertheless, age constraints can be inferred from structural orientations and fault slip kinematics. Accordingly, some of the veins already formed during sediment compaction in Mesozoic times, others possibly relate to Early Cenozoic foreland uplift. The youngest veins are most likely related to Late Cenozoic regional tectonic events, such as the Jura fold-and-thrust belt to the south and the Hegau-Lake Constance Graben to the northeast of the study area. During these latest tectonic events, previously formed veins acted as rheologically stiff discontinuities in the otherwise comparably weak Opalinus Clay along which deformation of the rock formation was re-localized.

Groundwater–rock interactions in crystalline rocks: evidence from SIMS oxygen isotope data

AbstractThe diffusive exchange of dissolved material between fluid flowing in a fracture and the enclosing wallrocks (rock matrix diffusion) has been proposed as a mechanism by which radionuclides derived from a radioactive waste repository may be removed from groundwater and incorporated into the geosphere. To test the effectiveness of diffusive exchange in igneous and metamorphic rocks, we have carried out an investigation of veins formed at low temperatures (&lt;100°C), comparing the oxygen isotopic composition of vein calcite with that of secondary calcite in the wallrocks. Two examples of veins from the Borrowdale Volcanic Group, Cumbria, and one from the Mountsorrel Granodiorite, Leicestershire, UK, have remarkably similar vein calcite compositions, ca. +20‰(SMOW) or greater, substantially heavier than the probable compositions of the host rocks, and these vein calcite compositions are inferred to reflect the infiltrating fluid and the temperature of vein formation. Calcites from the wallrocks are similar to those in veins, with little evidence for exchange with the wallrocks. The results support existing models for this type of vein which suggest low-temperature growth from formation brines originally linked to Permian or Triassic evaporites. The results are consistent with flow through fractures being attenuated through a damage zone adjacent to the fracture and provide no evidence of diffusional exchange with pore waters from wallrocks.

Origin and significance of ultra-slow calcite dissolution rates in deep sea sediments

Prediction of mineral-fluid reaction rates in geologic materials is subject to large uncertainties even though the rates are critical for many applications, from studies of past climate change to the engineering of carbon removal and storage. Deep sea carbonate sediments, which have been extensively cored and characterized at dozens of sites by ocean drilling programs, provide unique opportunities to examine how minerals and fluids react over multi-million-year timescales in natural conditions at low temperature. It has already been established that the calcite-fluid reaction in these systems is extremely slow, but the explanation for the ultra-slow rates, and the fact that non-zero rates persist for tens of millions of years, remains elusive. We extend the analysis of pore fluid Sr and Ca concentration data in the literature from ocean drilling archives to 21 drill sites in sediment sections where the fraction of carbonate is larger than 80% to systematically estimate the rates at which calcite dissolves and precipitates as a function of depth and sediment age. Pore fluid Sr/Ca used in this study, and Sr and Ca isotopes in other published analyses, provide estimates of the gross calcite dissolution rates, which are mostly balanced by secondary calcite precipitation. Pore fluid Ca concentration data provide key, previously underappreciated, information on the net calcite dissolution rates. Using these data we evaluate simple models for calcite reaction kinetics and pore fluid sediment evolution and conclude that (1) reaction rates are extremely slow because the reactive site density of the bulk sediment decreases continuously and systematically approximately as age−1, and (2) reaction persists to hundreds of meters burial depth and tens of millions of years because pore fluids never become closed due to diffusive communication with the oceans, and hence remain slightly undersaturated with respect to calcite. The systematic decrease in bulk reactive site density begins immediately upon deposition, greatly slowing the rate of dissolution and hence affecting models of calcite dissolution on the seafloor. The failure to reach equilibrium after millions of years and hundreds of meters of burial, is a consequence of extremely slow net reaction rates that cannot drive the pore fluids to equilibrium against the diffusive loss of Ca and DIC to the ocean. The existence of two or more calcite phases of different solubility is a likely source of a long-lived, but small, driving force for reaction, but its effect decreases with continued recrystallization and pore fluid evolution.

CO2 Release Driven by the Combination of Sulfide Oxidation and Carbonate Dissolution in the Upper Changjiang River: Effect of Erosion and Lithology on Chemical Weathering

AbstractThe positive link between weathering and orogeny suggests that the drawdown of atmospheric carbon dioxide (CO2) over the Cenozoic should be closely related to the enhanced silicate weathering caused by Cenozoic orogeny, especially the uplift of the Tibetan plateau. However, the uplift of orogens could also exhume carbonate and sulfide minerals, the coupling weathering of which can act as a source for atmospheric CO2 at the long‐term timescale and counteract the CO2 consumption by silicate weathering. In this study, the solute concentrations and isotopic ratios in the upper Changjiang River are determined to investigate the co‐weathering of silicate, carbonate, and sulfide minerals and its impact on the long‐term carbon cycle. An inverse mixing model incorporating elemental and isotopic ratios is established to quantitatively identify the sources of cations and weathering acids. The inversion of our samples reveals that carbonate weathering dominates the chemical composition of river water, even with the presence of secondary calcite precipitation. The sulfide oxidation widely occurs in the study area and the bulk of generated sulfuric acid is buffered by carbonate minerals. The erosion is decoupled from sulfide oxidation and chemical weathering, likely due to lithology and secondary calcite precipitation. Overall, we find that the CO2 release from sulfide oxidation coupled with carbonate dissolution exceeds the CO2 drawdown from silicate weathering in the Changjiang River basin, indicating the weathering will act as a source for atmospheric CO2 at the timescale between marine carbonate compensation (105 years) and sulfate reduction (&lt;10 Ma).

Use of bacterial binder in repair mortar for micro-crack remediation.

Micro-cracks are one of the types of stone deterioration which can propagate and lead to surface detachments and larger cracks in the long run. The present study developed a sustainable and environmentally friendly infill material-biological mortar (BM), as an alternative to conventional approaches. Using a biomineralization approach, this BM was explicitly designed for healing micro-cracks (less than 2 mm) in historic travertines. To this end, the mortar was prepared using a calcifying Bacillus sp. isolated from thermal spring water resources in Pamukkale Travertines (Denizli), stone powder gathered from travertine quarries in the vicinity, and a triggering solution specifically designed to set off calcium carbonate precipitation reaction. After setup, BM was applied to micro-cracks of artificially aged test stones for testing. Scanning electron microscopy revealed calcium carbonate-coated Bacillus sp. bodies in the BM matrix, optical microscopy showed secondary calcite minerals throughout the BM applied micro-cracks, and stereomicroscopy and nanoindentation analyses demonstrated bonding of BM with stone due to microbial calcification activities. Furthermore, BM and original material contact showed a continuous and coherent structure in all samples. Within this context, BM could be considered a promising and alternative approach for the remediation of micro-cracks of historic stones. KEY POINTS: A binder was produced by the MICP of Bacillus sp. Pamukkale. Physical, mineralogical, and nanomechanical characterization demonstrated microbial calcite precipitates in BM. A significant bond was determined between the grains and matrix of BM due to Bacillus sp. calcite production activities.

Reconstruction of hydrothermal fluid composition of the Kamthai carbonatite complex, India using lattice strain model: Implications for LREE/HREE fractionation

The composition of the fluid phase is often a poorly constrained variable in many ore deposits. In this study, we used calcite-fluid partition coefficients extrapolated to the P-T conditions of mineralization using the lattice strain model to reconstruct the composition of the hydrothermal fluids associated with different stages of the hydrothermal alteration and secondary rare earth element (REE) mineralization in the Kamthai carbonatite complex of western India. We identified three generations of calcite including magmatic, two generations of secondary/hydrothermal, as well as partially reequilibrated magmatic grains. The magmatic and partially reequilibrated calcite are unusually enriched in the REE, particularly the LREE, and Sr, which were incorporated via coupled cationic substitutions from highly differentiated brine- and incompatible element-enriched carbonatitic melts. Calcite and carbocernaite were the important primary phases that sequestered the REE from the carbonatite magma. Textural and geochemical evidence suggest that magmatic calcite and carbocernaite underwent extensive partial to complete reequilibration through dissolution and reprecipitation during hydrothermal alteration. The prominent chemical changes during the alteration of calcite involved loss of substantial REE, Sr, Fe, and Mn, and transition from LREE-dominated to LREE-poor compositions. The REE released during hydrothermal alteration were incorporated in secondary REE minerals such as REE-(fluor)carbonates, REE-(hydroxyl)carbonates, ancylite, and cerianite. The reconstructed chemistry of the carbonatite melt and the hydrothermal fluid, together with the trace element chemistry of secondary calcite, point towards progressive decrease in the REE contents. However, the HREE were conserved and ƒO2 increased from the most primitive to the most evolved fluid. The REE were redistributed as alkali‑carbonate-hydroxyl complexes, which can explain the LREE/HREE fractionation observed between the early and the late-stage hydrothermal fluid. Precipitation of the LREE-bearing secondary minerals is attributed to a decrease in temperature of the hydrothermal fluid as a consequence of mixing of the carbonatite-derived fluid with a relatively cooler and oxidized fluid. Such mixing explains the observed negative Ce-anomaly in hydrothermal calcite.