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The clumped isotope (Δ47) proxy provides specific information on formation conditions of carbonate minerals, for example the related fluid temperature. Although the clumped isotope paleothermometer often allows the direct temperature reconstruction due to the measurements being relatively free of vital effects or mineral-specific fractionation, clumped isotope geochemistry during the diagenesis and recrystallization of sedimentary rocks is understudied. This work evaluates clumped isotope data during the alteration of bivalve shells to better understand how the proxy develops under varying diagenetic conditions. Aragonitic bivalves were experimentally altered in closed vessels with variations in durations (0-16 weeks), fluid chemistry (burial and meteoric fluids) and temperature (130 to 200°C).Results generally show increasing calculated temperatures (lower measured Δ47 values) with increasing experimental temperature and duration. Different fluid chemistries (burial versus meteoric fluids) also resulted in a constant offset of values for modern samples following alteration. Fossil carbonates seem to be more resistant to alteration of Δ47 values, indicating a reduced alteration potential compared to modern samples. Lastly, fast re-equilibration during the alteration experiments seem to follow a relatively simple model, similar to the proposed solid-state isotope re-ordering model of Passey and Henkes (2012) and others, but at much higher rate constants and starting at lower temperatures.

In this study we present the first Mg isotope data that record the fate of Mg during mineralisation of atmospheric CO2 in ultramafic mine tailings. At the Woodsreef Asbestos Mine, New South Wales, Australia, weathering of ultramafic mine waste sequesters significant amounts of CO2 in hydromagnesite [Mg5(CO3)4(OH)2·4H2O]. Mineralisation of CO2 in above-ground, sub-aerially stored tailings is driven by the infiltration of rainwater dissolving Mg from bedrock minerals present in the tailings. Hydromagnesite, forming on the surface of the tailings, has lower δ26Mg (δ26MgHmgs = −1.48 ± 0.02‰) than the serpentinised harzburgite bedrock (δ26MgSerpentinite = −0.10 ± 0.06‰), the bulk tailings (δ26MgBulk tailings = −0.29 ± 0.03‰) and weathered tailings containing authigenic clay minerals (δ26MgWeathered tailings = +0.28 ± 0.06‰). Dripwater (δ26MgDripwater = −1.79 ± 0.02‰) and co-existing hydromagnesite (δ26MgHmgs = −2.01 ± 0.09‰), forming in a tunnel within the tailings, and nodular bedrock magnesite [MgCO3] (δ26MgMgs = −3.26 ± 0.10‰) have lower δ26Mg than surficial fluid (δ26Mg = −0.36‰) and hydromagnesite. Complete dissolution of source minerals, or formation of Mg-poor products during weathering, is expected to transfer Mg into solution without significant alteration of the Mg isotopic composition. Aqueous geochemical data and modelling of saturation indices, along with Rayleigh distillation and mixing calculations, indicate that the 26Mg-depletion in the drip water, relative to surficial water, is the result of brucite dissolution and/or precipitation of secondary Mg-bearing silicates and cannot be assigned to bedrock magnesite dissolution. Our results show that the main mineral sources of Mg in the tailings (silicate, oxide/hydroxide and carbonate minerals) are isotopically distinct and that the Mg isotopic composition of fluids and thus of the precipitating hydromagnesite reflects both isotopic composition of source minerals and precipitation of Mg-rich secondary phases. The consistent enrichment and depletion of 26Mg in secondary silicates and carbonates, respectively, underpins the use of the presented hydromagnesite and fluid Mg isotopic compositions as a tracer of Mg sources and pathways during CO2 mineralisation in ultramafic rocks.

The oxygen and clumped isotope signatures of Mg calcites are routinely used as environmental proxies in a broad range of surroundings, where Mg calcite forms either by classical nucleation or via an amorphous calcium magnesium carbonate (ACMC) precursor. Although the (trans)formation of ACMC to Mg calcite has been identified for an increasing number of settings, the behavior of both isotope proxies throughout this stage is still unexplored. In the present study ACMC (trans)formation experiments were carried out at constant pH (8.30 ± 0.03) and temperature (25.00 ± 0.03 °C) to yield high Mg calcite (up to 20 mol% Mg). The experimental data indicate that the oxygen isotope values of the amorphous and/or crystalline precipitate (δ$^{18}$O$_{prec}$, analyzed as Mg calcite) are affected by the (trans)formation pathway, whereas clumped isotopes (Δ$_{47prec}$ = Δ$_{47Mg-calcite}$) are not. The oxygen isotope evolution of the solid phase can be explained by the instantaneous trapping of the isotopic composition of the aqueous (bi)carbonate complexes. This entrapment results in remarkably high 10$^{3}$ln(α$_{prec-H2O}$) values of ∼33‰ at the initial ACMC formation stage. During the ACMC transformation process the oxygen isotope equilibrium is approached rapidly between Mg calcite and water (Δ$^{18}$O$_{Mg calcite-water}$ = 30.3 ± 0.4‰) and no isotopic memory of the initial to the final Mg calcite at the end of the experiment occurs. The implications for oxygen and clumped isotope signatures of Mg calcite formed via ACMC are discussed in the aspects of various scenarios of (trans)formation conditions and their use as environmental proxies.

Barium isotope fractionation during calcite and aragonite inorganic precipitation was studied in mixed flow reactors as a function of precipitation rate at 25 °C and pH = 6.3 ± 0.1. The measured Ba isotope fractionation that occurs between calcite and the forming fluid in the investigated range of calcite growth rates (10-8.0 ≤ Rp(calcite) ≤ 10−7.3 mol/m2/s) is insignificant. Barium isotope fractionation between aragonite and the fluid decreases with increasing precipitation rate from Δ137/134Baaragonite-fluid = +0.25 ± 0.06‰ for Rp(aragonite) ≤ 10−8.7 mol/m2/s to −0.10 ± 0.08‰ for Rp(aragonite) = 10−7.6 mol/m2/s, thus reflecting preferential incorporation of either heavy or light Ba isotopes in aragonite at slow and fast growth rates, respectively. The dependence of Ba isotope fractionation on aragonite growth rate is well described by the surface reaction kinetic model developed by DePaolo (2011) when the values +0.27‰ and −2.0 ± 0.2‰ are used for the equilibrium and kinetic Ba isotope fractionation factor, respectively. The enrichment of aragonite in the heavier Ba isotopes is consistent with the equilibrium fractionation factor of +0.34‰, calculated here between Ba-substituted aragonite and Ba2+ (aq), from first-principles calculations. This positive fractionation is related to a shorter average Ba[sbnd]O bond length in the aragonite structure while the coordination number does not change much (i.e. 9). The lack of isotope fractionation between the Ba aquo ions and the 6-coordinated Ba in calcite likely suggests that the coordination reduction required for the incorporation in the lattice of Ba adsorbed at calcite growing sites proceeds without isotope fractionation with the fluid. Otherwise the precipitated calcite should have been enriched in heavy isotopes by ∼0.17‰, as predicted by first-principles calculations. These results are the first experimental measurements of Ba isotope fractionation during inorganic calcite and aragonite mineral formation and set the basis for understanding the mechanisms controlling Ba isotope composition in CaCO3 minerals that is an essential perquisite for application of this isotopic system in natural samples.

The chemical and isotopic compositions of carbonates minerals allow to reconstruct the composition of the reactive solutions at the time of their formations and are thus of first importance for paleoenvironmental reconstruction over geological time. In this regard, a huge effort was addressed during the last five decades to study the incorporation, and the associated mechanisms, of traces elements in carbonates minerals. Deciphering the effect of particular physical or chemical parameters on the incorporation of traces in natural CaCO3 is not straightforward and in this respect, experimental studies under highly controlled conditions can provide important insight into our understanding of the chemical signatures of natural samples. In this study, we experimentally investigated the incorporation of Ni and Co in aragonite as a function of mineral growth rate using the constant addition technique at 25°C and 1 bar pCO2. Our results show a linear correlation between the distribution coefficients of Ni and Co and the mineral growth rate suggesting that the latter is likely an important parameter controlling the Ni and Co incorporation in aragonite. In both cases, the distribution coefficients of Ni and Co (i.e., DNi and DCo, respectively) between aragonite and the reactive solution are always lower than unity and increase with increasing growth rate following the trend of incorporation of elements incompatible with the host mineral structure. Based on the dependency of DNi and DCo with the saturation indices (SI) of the reactive solution with respect to aragonite, it was possible to estimate a distribution coefficient at equilibrium for both Ni and Co. These experimental values are several orders of magnitude lower than the theoretically estimated ones in the literature. Furthermore, as for other incompatibles elements the correlation between SI and DNi and DCo point toward the importance of the defect sites in the incorporation of these two elements in aragonite. Finally, our results suggest that DNi and DCoin aragonite could be used to rebuild the saturation state of the reactive solution.

International audience; The isotopic composition of carbonate minerals provides a record of historical geochemical and environmental conditions, but the ability to interpret these compositions as paleo-proxies hinges on their preservation over thousand to million year timescales. At chemical equilibrium, alteration of initial isotopic compositions of calcite can occur in the presence of a fluid without visible changes in morphology at the submicron scale, complicating the interpretation of stable isotope compositions of carbonates. However, the rates and mechanisms of isotope exchange at chemical equilibrium are poorly understood. To evaluate the rates and processes by which C and O isotopes are exchanged between calcite and fluid, batch reactor experiments were conducted at chemical equilibrium between calcite and a fluid enriched in 13C and 18O relative to the solid at 25 °C. Both natural and synthetic calcite of different grain sizes were investigated to evaluate the impact of mineral surface area and size on C and O isotope exchange rates. Our experimental results indicate that rapid exchange of both C and O isotopes occurs within 72 h for all calcite grain sizes studied, likely indicative of exchange of surface species in combination with a backward reaction during dissolution and Ostwald ripening of high energy surface sites. After 72 h, C and O isotope exchange rates were slower but near constant for timescales of thousands of hours. Surface-area normalized C and O isotope exchange rates were similar for all calcite grain sizes studied, and O and C were exchanged in a ∼3:1 ratio consistent with exchange of CO32–. The rates of C and O exchange were ∼4 orders of magnitude lower than far-from-equilibrium calcite dissolution rates, suggesting exchange was controlled either by dissolution-precipitation of pre-existing reactive sites alone, or a combination of dissolution-precipitation and solid state/aqueous mediated diffusion. Overall, the results of this study suggest alteration of O and C isotope compositions of calcite at ambient temperatures can proceed readily over short time scales, though the extent to which this process continues to operate over geologic time scales is difficult to predict at present. The results of this study further highlight the importance of generating a mechanistic understanding of the process of isotope exchange at chemical equilibrium, and represent a step towards this understanding

The incorporation of Mg in aragonite was experimentally investigated as a function of mineral growth rate at 5, 15 and 25 oC using the constant addition technique. Aragonite growth rate was found to be the major parameter affecting the Mg partitioning coefficient (i.e. D Mg = ( Mg Ca ) solid ( Mg Ca ) fluid ) between aragonite and fluid, whereas the effect of temperature is smaller but measurable. At similar surface normalized growth rates, DMg values decrease as temperature increases from 5 to 25 oC. The magnitude of decrease as a function of temperature is similar for all the experiments of this study where growth rate varied in the range 10-8.6 ≤ rp ≤ 10-7.1 (mol/m2/s). The combined effect of aragonite growth rate and temperature on DMg can be described by the linear equation: Log DMg = 0.583(±0.020) Log rp – 0.026(±0.001) T + 0.863(±0.153); R2 = 0.97 where T is the temperature in degrees Celsius. The increase of DMg values at decreasing temperatures in experiments conducted at similar growth rates is consistent with the increase of fluid supersaturation with respect to aragonite. Thus, it can be inferred that increased Mg incorporation at higher supersaturation is associated with the greater presence of defect sites on the growing mineral surface, similar to the incorporation of other incompatible ions in carbonate minerals. Overall, the relationship between Mg content of aragonite with the degree of saturation of the fluid with respect to this mineral phase suggests that DMg values or Mg/Ca ratio in natural aragonites can be used as a proxy for saturation degree of the formation fluid with respect to CaCO3 minerals.

The non-classical crystallization pathway of carbonate minerals proceeds through the initial formation of an amorphous precursor phase and commonly takes place during biomineralization. Although the isotopic composition of marine calcifiers is often used to reconstruct paleo-environmental conditions, the impact of the crystallization pathway followed by these organisms on the isotope composition of their skeletons has rarely been quantified. This study presents the first examination of B partitioning and isotope fractionation during CaCO3 formation via an amorphous calcium carbonate phase, which provides new insights into the incorporation of B into marine calcite and aragonite that form via this route. Boron concentrations and isotope compositions of the fluids and the formed solids were characterized during the course of experiments that lasted two years and were performed at 25 °C under controlled pH conditions. The results suggest that during Mg-ACC formation, B distribution coefficients are 1.5–4 orders of magnitude higher than those reported earlier in the literature for calcite and aragonite. Additionally, B isotope fractionation during Mg-ACC formation is strongly affected by the continuous exchange of solutes between the nanoporous solid phase and the bulk fluid. The isotope composition of the transient amorphous phase is not inherited in the crystalline carbonate mineral phase that ultimately forms, because crystallization proceeds via a continuous dissolution/re-precipitation process and/or chemical/isotope exchange between the solid surface and the fluid. Interestingly, the isotope fractionation between the fluid and the final crystalline products is different from those achieved in earlier studies where mineral growth proceeded via the standard mechanism of ion-by-ion addition of solutes to advancing steps. The B isotope fractionations measured in this study are in good agreement with results of equilibrium first principle calculations for calcite and aragonite (Balan et al., 2018) except in the case of calcite formed at pH = 8.9, suggesting that changes in aqueous speciation (i.e. increase of NaB(OH)4° and CO32− concentrations) may be responsible for slight changes of the calcite BO4 environment and isotope composition. It is expected that this study will help to better decipher the mechanisms controlling B isotope fractionation during the non-classical growth of calcium carbonates involving the formation of transient amorphous precursors.

The incorporation of magnesium and sulfate in calcite is frequently used to characterize and trace the environmental conditions occurring during carbonate mineral formation. Although both ions are simultaneously incorporated in the growing calcite, the effect of sulfate on magnesium incorporation in calcite is still under-explored. In this study, we examine the Mg incorporation in low-Mg calcite as a function of growth rate at 25 °C and 1 bar pCO2 in the presence and absence of aqueous sulfate. The obtained results suggest that high calcite growth rates induce a significant increase in the partitioning coefficient of Mg between the precipitated low-Mg calcite and aqueous solution (i.e. D Mg = n Mg / n Ca calcite [ Mg 2 + ] / [ Ca 2 + ] solution ). Obtained DMg values exhibit similar dependence to mineral growth rate for experiments performed in the presence and absence of sulfate, however a systematic shift to lower DMg values is observed for calcites formed in the presence of sulfate. The lower DMg values of calcites formed in sulfate-bearing solutions are attributed to the incorporated SO4 ions which provoke expansion in the unit cell along the c-axis of the newly formed calcite. A larger unit cell is unfavorable for the substitution of Ca by smaller Mg ions in calcite. The coupled effect of (i) sulfate uptake during calcite growth and (ii) precipitation rate on DMg can be expressed by the equation D Mg = 0.03726 - 0.02345 × - l o g r p - 7 - X S O 4 × 0.004607 + 0.002109 × - l o g r p - 7 ; (−8 ≤ log(rp) ≤ −7; 0 ≤ Xso4 ≤ 2.6; T = 25 °C) and is valid for 0.01 ≤ DMg ≤ 0.04. In analogy to Mg also SO4 ions are inhibiting calcite growth, thus promoting the formation of aragonite due to adsorption phenomena and blocking of calcite surface sites. These results improve our understanding on physicochemical parameters controlling CaCO3 composition and mineral polymorphism and are discussed in their relevance for the use of magnesium and sulfate environmental proxies in natural surroundings.

Biogenic carbonate minerals are widely used as archives in paleoenvironmental research, providing substantial information for past depositional and diagenetic regimes. However, nearly all biogenic carbonates undergo post-mortem diagenetic alteration to variable degrees. Diagenetic features are essentially caused by complex fluid-solid interaction including recrystallization and neomorphosis of shell architecture and related geochemical resetting. A common conception is that a given primary shell texture is replaced by a secondary fabric via micro-scale dissolution-reprecipitation reactions that may reach geochemical and/or isotopic equilibrium with the diagenetic fluid. Here we document that the process of petrographic and geochemical alteration of a biogenic carbonate archive progresses stepwise, and the re-equilibration processes can be separated in space and time. More specifically, attached and bound aqueous fluids within the shell organic matter and fluid inclusions likely play a crucial role in the early stages of alteration within a relatively rock-buffered system. Degradation of organics was observed via decreased sulfur concentration and decreased fluorescence. In this early stage of alteration, the conversion of micro-scale aragonite biominerals to metastable diagenetic calcite (meso)crystals is documented by RAMAN spectroscopy, electron backscatter diffraction (EBSD), and a decrease in strontium concentrations. Further stabilization to larger, neomorphosed calcite crystals resulted from re-equilibration with external fluids and was evidenced by EBSD, oxygen isotope data, and further reduction of strontium. Spatial chemical and isotope data were used to decipher the impact of fluid availability and transfer behavior, i.e. carbonate-buffered versus diagenetic-fluid buffered, and isotope and element exchange behavior, and analyzed in context to textural alteration features. These data suggest that during diagenesis, the evolution to more sustainable (diagenetically induced) fabrics is accompanied by individual reaction steps traced by elemental and isotopic signatures. Kinetics, and thus degree of diagenetic alteration of biominerals at a given exposure to physicochemical alteration conditions is initially strongly controlled by the micro- to nano-scale internal architecture governing the availability and transfer of aqueous fluids. Results shown here have significance for those concerned with biogenic carbonate archives and have wider implications for a mechanistic understanding of carbonate diagenesis.

WOS:000464851100069; International audience; The mangrove ecosystems are known to efficiently sequester trace metals both in sediments and plant biomass. However, less is known about the chemistry of rare earth elements (REE) in the coastal environments, especially in the world's largest mangrove province, the Sundarban. Here, the concentration of REE in the sediment and plant organs of eight dominant mangrove species (mainly Avicennia sp.) in the Indian Sundarban was measured to assess REE sources, distribution, and bioaccumulation state. Results revealed that light REE (LREE) were more concentrated than the heavy REE (HREE) (128-144 mg kg(-1) and 12-15 mg kg(-1), respectively) in the mangrove sediments, with a relatively weak positive europium anomaly (Eu/Eu* = 1.03-1.14) with respect to North American shale composite. The primary source of REE was most likely linked to aluminosilicate weathering of crustal materials, and the resultant increase in LREE in the detritus. Vertical distribution of REE in one of the long cores from Lothian Island was altered by mangrove root activity and dependent on various physicochemical properties in the sediment (e.g., Eh, pH, organic carbon, and phosphate). REE uptake by plants was higher in the below-ground parts than in the above-ground plant tissues (root = 3.3 mg kg(-1), leaf + wood = 1.7 mg kg(-1)); however, their total concentration was much lower than in the sediment (149.5 mg kg(-1)). Species-specific variability in bioaccumulation factor and translocation factor was observed indicating different REE partitioning and varying degree of mangrove uptake efficiency. Total REE stock in plant (above + live below ground) was estimated to be 168 g ha(-1) with LREE contributing similar to 90% of the stock. This study highlighted the efficiency of using REE as a biological proxy in determining the degree of bioaccumulation within the mangrove environment.

Zinc was co-precipitated with calcite in mixed-flow reactors at 25 °C and 6.1 ≤ pH ≤ 8.5 to quantify Zn isotope fractionation between calcite and the reactive fluid. The results suggest that the difference between the isotopic composition of the solid and the fluid (Δ66Zncalcite-fluid = δ66Zncalcite − δ66Znfluid) increases by about 0.6‰ as the solution pH decreases from 8.5 to 6.1. In contrast, based on Zn aqueous speciation and the theoretical values of the reduced partition function ratios for zinc species, lnβ, the isotopic fractionation between calcite and aqueous Zn2+, Δ 66 Z n calcite - Z n 2 + , remains constant at 0.58 ± 0.05‰ over the entire pH range investigated. The constant value of Δ 66 Z n calcite - Z n 2 + suggests that irrespective of the solution pH, the same Zn aqueous species, likely free Zn2+ ions, interacts with calcite surface sites during the growth of this mineral via ion by ion attachment. The enrichment of calcite in 66Zn is consistent with the formation of mononuclear, inner-sphere tetrahedral Zn surface complexes at the calcite surface and the increase of Zn coordination to 6 following its incorporation in the crystal lattice with no further isotopic fractionation. Overall, the results suggest that Zn isotopic composition of natural calcite has the potential to shed light on the prevailing pH at the time of calcite formation in the geological past.

International audience; The alkaline playas near Atlin, British Columbia, Canada are likely one of the few surface environments on Earth where contemporaneous formation of hydromagnesite and magnesite occurs at temperatures that do not exceed 15 °C. This environment offers a unique opportunity to examine the impact of different formation mechanisms on Mg isotope compositions of Mg-carbonate minerals at low temperature. In this study, we report the Mg isotope composition of ultramafic bedrock, Mg-carbonate sediments, and both surface and ground waters in this geological setting. The composition of hydromagnesite suggests a Rayleigh-type distillation effect on the fluid Mg isotope ratios in unsaturated sediment above the water table. Through this mechanism of formation, hydromagnesite is progressively depleted in 24 Mg obtaining δ 26 Mg values as high as +1.14‰ near the sediment surface. In contrast, magnesite formation is characterized by enrichment of the solid phase in 24 Mg. The apparent Mg isotope fractionation factor during magnesite formation at ~ 10°C ranges between ~ 0.7±0.1‰ and 1.8±0.1‰. The distinct Mg isotope composition of hydromagnesite in comparison to magnesite supports magnesite formation occurring by precipitation from the fluid, or dissolution-reprecipitation, rather than solid-phase transformation from a hydrous Mg-carbonate precursor. Overall, the results provide insights on low temperature Mgcarbonate mineral formation that has implications for long-term storage of CO2.

International audience; Quantifying the link between cyanobacterial activity and the carbon isotope signature of precipitated carbonate minerals is crucial for reconstructing the environmental conditions present at the time of carbonate mineral formation. In this study, calcite was precipitated in the presence and absence of Synechococcus sp. cyanobacteria in batch reactors. The temporal evolution of the carbon isotope composition of calcite (δ13CCalcite) and dissolved inorganic carbon (δ13CDIC) was monitored to evaluate the rate and degree to which the carbon isotope compositions in calcite are modified during and after its precipitation. The presence of cyanobacteria promoted calcium carbonate formation by increasing fluid pH and the CaCO3 saturation state. It also changed significantly the carbon isotope composition of dissolved inorganic carbon due to the preferential incorporation of 12C into the biomass. This generated an isotope disequilibrium between the calcite and the aqueous fluid phase over time after the calcite precipitated. The carbon isotope composition of the calcite evolved continuously towards mineral-fluid isotope equilibrium after its precipitation, at geometric surface area normalized rates ranging from 1.75 × 10−14 to 1.71 × 10−13 mol 13C/m2/s. These rates are sufficiently fast such that the δ13CDIC value of aqueous fluids in calcite-rich rocks would be buffered by the δ13CCalcite value of the co-existing calcite. Mass balance calculations suggest that the carbon isotope composition of calcite could change noticeably when the calcite is in isotope disequilibrium with its co-existing fluid, for example through the presence of a local 12C sink such as photosynthetic microorganisms or 12C source such as decomposition of organic material.

The fractionation of carbon and magnesium isotopes is a potentially useful tracer of natural weathering in ultramafic catchments and engineered CO2 storage. To evaluate the use of carbon and magnesium isotopes as tracers of ultramafic weathering and CO2 storage, we assessed the carbon and magnesium isotope fractionation between hydrous Mg-carbonate minerals and fluid during a mineral phase transformation from nesquehonite [MgCO3·3H2O] to dypingite [Mg5(CO3)4(OH)2· ∼ 5–8H2O], two common products of ultramafic rock weathering. Batch reactor experiments containing nesquehonite were conducted at 5 °C, 25 °C, and 35 °C and the evolution of mineralogical composition, fluid composition, and isotopic composition were tracked over time. At 5 °C, the solid remained nesquehonite throughout the experiments, and isotopic equilibrium did not appear to be achieved between the solid and the fluid phase for either carbon or magnesium. At 25 °C, and 35 °C a transformation from nesquehonite to dypingite occurred by dissolution and re-precipitation, which resulted in extensive exchange of Mg and C between solid and fluid. The phase transformation caused the initial C and Mg isotopic composition of the solid phase to be overwritten. The extensive isotopic exchange during the phase transformation suggests C and Mg isotopes likely obtained approximate isotopic equilibrium between dypingite and fluid. For dypingite, the Δ13Cdyp-DIC was 4.74 ± 0.12‰ (VPDB) and 4.47 ± 0.17‰ (VPDB) at 25 and 35 °C, respectively. The Δ meas 26 M g dyp - f l u i d between solid and the bulk fluid was −0.76‰, and −0.98 ± 0.08‰ for the 25 and 35 °C experiments, respectively. There was no clear impact of temperature on Mg or C isotope fractionation. The calculated Δ calc 26 M g dyp - M g 2 + between dypingite and the Mg2+ aquo species rather than bulk aqueous Mg values were positive. This indicates that if dypingite is formed by the incorporation of the free Mg2+ ion in the solid, the solid is preferentially enriched in the isotope of higher mass (26Mg). This is opposite to anhydrous Mg-bearing carbonate minerals, which tend to be depleted in 26Mg relative to the forming fluid. These data will help improve interpretation of carbon and magnesium isotope compositions measured in natural and engineered ultramafic weathering environments, and may help to trace the fate of anthropogenic CO2 during engineered CO2 storage efforts.

The full potential of the dolomite Ca isotope proxy only unfolds when combined with data of the other main elements (C, O, Mg) in the crystal lattice of Mg-carbonates. Data presented here reveal the level of complexity inherent to dolomite precipitation and alteration environments and add new constraints to the understanding of early diagenetic dolomite formation. Well-constrained Precambrian to Pleistocene dolomites were investigated, representing three characteristic formation and alteration environments: (i) sabkha, (ii) altered marine and (iii) lacustrine/palustrine dolomites. Primary sabkha dolomites with typically low cation ordering degree (COD), high d13C and low d44/40Ca values contrast with recrystallized sabkha dolomites with relatively high COD, low d13C and high d44/40Ca values. Both d13C and d44/40Ca values of sabkha dolomite bear witness to the relative effects of kinetic and equilibrium Ca isotope fractionation conditions. Primary sabkha dolomites display D44/40Cadolomite-fluid ranging from −0.4 to −1.30, whereas recrystallized dolomite is approaching isotopic equilibrium (D44/40Cadolomite-fluid ∼ 00). Using the fractionation factor deduced from the sabkha dolomite data set, recrystallized Precambrian dolomite points to a d44/40Caseawater/pore fluid of about 1.20 (SRM 915a), a value that is distinctively lower compared to previously suggested ones and modern seawater. Altered marine dolomites display evidence for meteoric overprint, indicated by d18O values as low as −4.620. Both Mg and Ca isotope signatures correlate with d18O values of altered marine dolomites, whereas d13C values lack correlation with the other isotope systems. We propose that freshwater circulated through silicate aquifers prior to reaching the dolostone units and Ca, Mg and O isotopes of altered marine dolomites reflect variable degrees of this meteoric overprint. Lacustrine/palustrine dolomites display a correlation between the isotope values of C, Mg and Ca. These dolomites are formed during pulses of marine ingression in swamp, playa and lake environments and are thus characterized by water-logged conditions (anaerobic) and saline, sulfate-rich fluids. Bacterial-sulfate reduction induces dolomite formation and leads to lower d13C and d44/40Ca as well as to higher d26Mg values. The Ca isotope proxy acts as a benchmark against which other proxy data can be calibrated or processes tested. Taking COD and dolomite stoichiometry into consideration, the here documented multi-proxy isotope approach is promising and provides benchmarks against which proxy signals can be calibrated and tested. Interestingly, even the limited data sets shown here point to patterns that can be interpreted in a meaningful manner that is of relevance for dolomite research.

International audience; Diagenesis of carbonate minerals is ubiquitous throughout the geologic record. Alteration is initiated immediately after deposition, or takes place in the endo- and exoskeletons as early as during the lifetime of a given carbonate-secreting biota, and can continue throughout the burial history of carbonate sediments and rocks. Variations in the diagenetic response of carbonate archives pose challenges for the reconstruction of past environmental conditions based on proxy data. This paper comparatively assesses alteration features of different aragonitic materials by experimentally-induced diagenesis. A multitude of factors lead to different reactivity and responses of a given aragonite archive, and provide insight to the interpretation of diagenetically altered material in the rock record. Chosen materials used in this study include relatively organic-rich samples such as coral skeletons and bivalve shells, and organic-lean abiotic carbonates such as speleothems and aragonite single crystals. Obtained datasets include distributions of elements, organics, carbon and oxygen isotope ratios, and crystallographic features. Observed variations in diagenetic responses include mineralogy of the diagenetic phases, rate and extent of mineral transformation, distribution of foreign ions in the crystal lattice (primarily Sr, Mg, and S), and the number of specific processes and products along diagenetic pathways. Alteration is shown to be primarily controlled by the initial diagenetic susceptibility of the sample (including porosimetry and structural characteristics, concentrations of organic material, and primary amounts of trace elements in the carbonate, such as Mg). Structural characteristics lead to initial internally “fluid-” or “rock-buffered” conditions, with low porosity and permeability resulting in a greater effect of internal fluids and organics. Differences in the amount of organic content and internal fluids affect transformation rates, secondary mineralogy, and isotope equilibria. Samples with relatively high porosity, high mineral transformation rates, and high primary Mg/Ca, may preferentially form secondary aragonite during fast equilibration with the diagenetic environment. Our results suggest that the degree and nature of diagenetic alteration of aragonite materials are strongly controlled by the micro- to nano-scale internal architecture governing the availability and transfer of aqueous fluids. Results of this study provide significant implications for the interpretation of diagenetic signals in carbonate archives, and have direct significance for the mechanistic understanding of carbonate diagenesis and (paleo)environmental conditions.

Constraining the mechanisms controlling the riverine flux of major cations and their isotopes including that of Mg to the World Ocean is one of the challenges in Earth surface isotope geochemistry. In an attempt to identify the main factors affecting the Mg isotopic composition of large rivers including vegetation, climate and lithology of the watershed, we studied the largest, in terms of discharge, Siberian river, Yenisey, and 20 of its main tributaries, during spring flood, summer flow and winter. The working hypothesis was that the influence of bedrock composition is most pronounced in winter, when the soils are frozen and the rivers are fed by deep underground waters. Thus, we anticipated that the presence of permafrost will help to distinguish the impact of surface processes, linked to biological uptake and release, and deep soil/underground transport of Mg from mineral sources. In contrast to these expectations, no sizable differences in the Mg isotope composition of the river water (±0.1‰) for both the Yenisey tributaries and its main channel has been observed between the spring flood (May) and the winter (March) period. Those two periods are characterized by the differences of discharge and degree of lithological impact on element source in the river water. Regardless of the season, there was no straightforward control of lithology (relative abundance of carbonates, basalts, granites and sedimentary rocks) on δ26Mg in the main tributaries of the Yenisey river. Our findings suggest that the use of riverine Mg isotope signature for tracing weathering mechanisms and dominant lithological impact is not straightforward at the scale of large rivers whose watersheds present multiple lithologies, variable climatic conditions and vegetation types.

The cation-to-Ca ratio in natural carbonate minerals is routinely used by the geoscientific community in order to reveal information about the conditions occurred during mineral formation in the geological past. Environmental reconstruction, however, relies on our understanding on the mechanisms controlling mineral growth but also on the physico-chemical properties of cations. In this respect experimental studies and laboratory calibrations of elemental ratios in synthetic carbonates provide important insights on the interpretation of the chemical signatures in natural samples. This holds especially truth for the chemical and isotopic signals of carbonates forming in continental environments that are characterized by the absence of bio-induced precipitation, low concentration of solutes in the forming fluid and slow growth rates (e.g. speleothems). In this study, we examine the incorporation of Li in aragonite, owing to its use as a temperature proxy and its importance in paleo-weathering reconstruction. Our preliminary results suggest that aragonite growth rate is likely the most important parameter controlling Li content in the forming phase. This finding comes in excellent agreement with the recent study by Füger et al. (2019). In addition, the experimental work suggest that temperature is also affecting the distribution of Li in aragonite but to a lesser extent than growth rate. It is anticipated that once completed this work will provide the fundamental knowledge needed for adequate interpretation of Li partitioning in aragonite and significantly improve our ability to interpret Li signatures in natural carbonates. References: Füger et al. 2019. Effect of growth rate and pH on lithium incorporation in calcite, Geochim. Cosmochim. Acta., 248, 14-24.

Carbonate minerals record, through their chemical and isotopic composition, the environmental conditions occurring at the time of their formation. Thus, the incorporation of traces/impurities in CaCO3 minerals calcite and aragonite, have been widely studied over the last five decades in order to provide the fundamental knowledge needed for the use of these traces in paleoenvironmental reconstructions. The processes controlling the uptake of traces in natural samples, however, are manifold and hard to distinguish from each other. Thus, experimental co-precipitation studies on synthetic material under strictly controlled abiotic conditions can provide fundamental understanding on the effect of each process involved in the chemical signatures of natural carbonates. In this study, we explore the incorporation of Cu in calcite and its potential as proxy of reactive fluid composition. This transition metal commonly occurs complexed with organic ligands in natural waters, however, it exhibits very high affinity for calcite. Our experiments were performed at pH 6.3 and 8.3, with varying growth rate ranging between 10-8.5 and 10-7.6 (mol/m2/s). Our first results highlight that the partitioning coefficient of Cu is positively correlated to the calcite growth rate at both pH conditions, indicating an increase of Cu entrapment at higher growth rate. These new preliminary findings could bring fundamental understanding of Cu incorporation in calcite and highlight the potential of Cu partitioning coefficient as a proxy of mineral growth rate.

Calcite and aragonite growth rate experiments have been performed in the presence of aqueous Ba at 25 °C using the constant addition technique as a function of CaCO3 mineral growth rate (mol/m2/s). The partitioning of Ba in calcite at −8.4 ≤ Logrcalcite ≤ −7.3 exhibits a weak dependence on growth rate that can be expressed as: L o g D Ba , c a l c i t e = 0.2477 ± 0.0543 × L o g r c a l c i t e - 0.2949 ± 0.4222 ; R 2 = 0.81 In the case of aragonite, Ba partitioning at −9.0 ≤ Lograragonite ≤ −7.8, exhibits a much stronger dependence on growth rate that can be described as: L o g D Ba , a r a g o n i t e = 0.4458 ± 0.0563 × L o g r a r a g o n i t e + 3.3407 ± 0.4668 ; R 2 = 0.84 The determined DBa,aragonite values are systematically lower than unity and come in contrast to previous experimental works where Ba partitioning studied during aragonite nucleation. They are, however, in excellent agreement with the theoretical free energy correlation model by Wang and Xu (2001) that considers the effect of ionic radii size on DBa,aragonite. The defined growth rate dependence of Ba incorporation in aragonite provides new insights on the role of ionic radii size during the incorporation of trace elements and the formation of solid-solutions from aqueous solutions at low oversaturation degrees. The data presented here shed light on the process controlling the elevated DBa,aragonite values occurring at high saturation degrees of natural fluids with respect to aragonite that have been recorded in natural samples. Furthermore, the trend experimentally obtained herein has the potential to record variations in growth rate regimes in natural occurring aragonites and to provide information on the environmental conditions of natural waters in the past.

The low-Mg (spine) to high-Mg (test) calcite skeletal hardparts of echinoids are exploited in carbonate archive research. These studies, however, are not coming without problems. Those rise from the solid phase metastability, the presence of amorphous precursors during biomineralization processes, the spatially variable skeletal Mg concentrations, and the porous microstructure at secretion of these biominerals. The present paper systematically evaluates the resilience of echinoid skeletal hardparts, here specifically sea urchin spines, to diagenetic alteration with a focus on magnesium isotope distribution (δ26Mg). We apply a dual approach by (i) performing alteration experiments using meteoric, marine, and burial model fluids and by (ii) comparing the experimental data with diagenetically overprinted fossil sea urchin hardparts. The degree of alteration of individual spines and an echinoid test was assessed by a combination of optical (fluorescence, cathodoluminescence (CL), scanning electron microscopy (SEM)) and geochemical tools (elemental distribution and δ26Mg). Magnesium isotope signatures of sea urchin spines are affected by experimental temperature, fluid composition, and duration of alteration experiment. Most strikingly, the recrystallization of spines from low-Mg ( 5 up to 44 mol%) is observed throughout the experiments using burial and marine fluids at 175 °C. Simultaneously, δ26Mg values of progressively recrystallized Mg calcite are increasingly enriched in 26Mg. Fossil spine material displays the highest Mg/Ca ratios and highest δ26Mg values, whereas a fossil (diagenetically overprinted) sea urchin test, formerly high-Mg and subsequently recrystallized to low-Mg calcite, displays the lowest δ26Mg values. These diagenetic features are in agreement with the data obtained from experimentally altered spines. The transformation of low- to high-Mg calcite of altered sea urchin hardparts is essentially controlled by the Mg content of the diagenetic fluid and directly affects the Mg isotope signature of the echinoderm skeleton. The experimental alteration data presented here mimic the patterns of Mg/Ca ratios and Mg isotope signatures of fossil echinoid skeletal hardparts as induced by diagenesis. Both, Mg/Ca ratios and isotope signatures of echinoderm hardparts are spatially and ontogenetically complex to start with and undergo significant and variable (fluid and carbonate composition-dependent) alteration processes in the presence of diagenetic fluids. These alteration processes include dissolution and re-precipitation reactions, but also diffusion between bulk and pore solution within the pore system of the sea urchin spine. The research shown here is significant as it documents the potential of a combined study using naturally and experimentally altered carbonate hardparts in an exemplary manner. Clearly, a critical approach to echinoid proxy archive for the reconstruction of past seawater Mg isotope composition is advised. In contrast, Mg isotope data and their changes in time and space have a significant potential to document processes and products resulting from diagenesis.

Sepiolite [Mg4Si6O15(OH)2·6H2O] is a trioctahedral 2:1 Mg-silicate that has been often used to reconstruct the evolution of sedimentary environments and facies in the geological record. To date, however, the reaction paths underlying sepiolite formation are poorly constrained and most of the existing models are based on empirical observations. In order to shed light on the mechanisms controlling the formation of this mineral phase, in the present study, sepiolite was precipitated at 25 ± 1 °C from modified seawater and MgCl2 solutions undersaturated with respect to brucite and amorphous silica. Although a suite of hydrous Mg-silicates, such as kerolite, saponite, stevensite and talc, were oversaturated in the solutions at a higher level relative to sepiolite at any time of reaction, poorly crystallized, aluminous sepiolite was the only precipitate after 91 days. The precipitated sepiolite [Mg3.4-3.8Al0.1-0.4)∑3.8-3.9(Si5.9-6.0Al0-0.1)O15(OH)2·nH2O] shares a number of structural and chemical similarities with natural sepiolite, such as a fibrous crystal shape and an atomic Si/(Si + Mg+Al) ratio of ∼0.61. The proposed reaction path for the formation of sepiolite is based on the temporal evolution of the chemical compositions of the experimental solution and solids: (i) Nucleation and growth of Al-sepiolite occurred during the first 8 days of the experimental runs via condensation and polymerization of Si OH tetrahedra onto Mg–Al–O–OH template sheets at a precipitation rate of ∼2.19 ± 0.01 × 10−10 mol s−1. (ii) At decreasing pH and in the absence of [Al]aq this intermediate phase transformed into aluminous sepiolite at a slower crystal growth rate of ∼1.08 ± 0.02 × 10−12 mol s−1. This finding explains the high abundances of sepiolite in highly alkaline, evaporitic, lacustrine and soil environments, where the growth rates of sepiolite are considered faster (10−11 to 10−10 mol s−1, Brady, 1992 ). We propose that (i) low rates of Mg2+ ion dehydration and silica condensation and polymerization at the surface of the initial precipitate, (ii) the formation of MgS 0 4 0 aquo-complexes and (iii) the reduced sorption rates of [Si]aq and [Mg]aq at the active growth sites on sepiolite surfaces at pH ≤ 8.3 retard the precipitation of sepiolite in marine-diagenetic environments.

In natural aquatic environments amorphous calcium carbonate (ACC) shows a remarkable stability, which is assumed to arise from the incorporation of foreign ions as well as organic ligands. Such impurities, like Mg, are considered to be controlling the transformation of ACC and finally occurring CaCO3 polymorphs. Although transformation of ACC with different Mg contents is extensively studied, the role of the aqueous Mg on ACC transformation is still underexplored. Therefore, experiments on the transformation behaviour of additive-free ACC reacting with Mg-bearing solutions were carried out and monitored using highly time-resolved in-situ Raman spectroscopy. The results show that while ACC transformed instantly to calcite and vaterite in pure water, it exhibited a prolonged stability of up to 11.5 h at elevated Mg concentrations in the reactive solutions. During this process ACC incorporated up to 5 mol% of Mg. The subsequent, transformation pathway of the newly formed Mg-ACC changed from forming Mg-calcite and vaterite to precipitating aragonite with nesquehonite and monohydrocalcite as intermediate phases. This indicates that the distinct ACC transformation mechanisms and thus sequences of minerals formed are controlled by the aqueous Mg concentration of the reactive solution. Hence, the finally formed minerals are not determined by the pre-structuring of the original ACC.

The calcium carbonate hexahydrate mineral ikaite (CaCO3 ⋅ 6 H2O) has been documented in aquatic environments at near-freezing temperatures. An increase of the prevailing temperature in the depositional environment, results in the transformation of natural ikaite into less soluble calcium carbonate phases occasionally leaving calcite pseudomorphs in the sediments, which are considered as an indicator for primary cold water temperatures. Detailed understanding on the physicochemical parameters controlling ikaite (trans-)formation however, such as temperature and reactive solution chemical composition, are still under debate. In order to study the formation of ikaite, we conducted precipitation experiments under controlled physicochemical conditions (pH = 8.3 ± 0.1; T = 6, 12, and 18 ± 0.1 °C) at defined aqueous molar Mg/Ca ratios. The transformation of ikaite into anhydrous calcium carbonate polymorphs was investigated in solution and at air exposure. The obtained results reveal the formation of ikaite at temperatures up to 12 °C, whereas Mg-rich amorphous calcium carbonate precipitated at 18 °C. In contact with the reactive solution ikaite transformed into aragonite at aqueous molar Mg2+/Ca2+ ratios of ≥14. In contrast, ikaite separated from the Mg-rich solution and exposed to air transformed in all cases into calcite/vaterite. The herein obtained temperature limit of ≤12 for ikaite formation is significantly higher than formerly expected and most probably caused by (i) the high saturation degree of the solution with respect to ikaite and (ii) the slow dehydration of the aqueous Ca2+ ion at low temperatures. This result questions the suitability of calcite pseudomorphs (i.e. glendonites) as a proxy for near-freezing temperatures. Moreover, our findings show that the CaCO3 polymorph formed from ikaite is strongly controlled by the physicochemical conditions, such as aqueous molar Mg2+/Ca2+ ratio of the reactive fluid and H2O availability throughout the transformation process.

Natural polycrystalline hydrothermal Sainte Colombe dolomite was dissolved in stirred titanium closed system reactors in aqueous 0.1 mol/kg NaCl solutions at 51, 75, 121, and 126 °C and a pressure of 5 bars CO2. In total, 52, 27, 16, and 12%, respectively, of the dolomite placed in the reactors dissolved into the fluid phase during these experiments. Each experiment lasted from 12 to 47 days and the fluid phase in each evolved towards, but did not exceed, ordered dolomite equilibrium at a pH of 5.9 ± 0.3. All aqueous reactive fluids were undersaturated with respect to all potential secondary phases including calcite and magnesite. The reactive fluid compositions at the end of the experiments had a molar Ca/Mg ratio equal to that of the dissolving dolomite, and the dolomite recovered after the experiments contained only pure dolomite as verified by scanning electron microscopy. The Ca and Mg isotopic ratios of the reactive fluids remained within uncertainty equal to that of the dissolving dolomite in the experiments performed at 51 and 75 °C. In contrast, the Ca isotopic composition of the reactive fluid in the experiment performed at 121 and 126 °C was significantly greater such that Δ44/42Casolid-fluid = − 0.6 ± 0.1‰, whereas that of Mg was within uncertainty equal to that of the dissolving mineral. The equilibrium fractionation factors for both divalent cations favor the incorporation of isotopically light metals into the dolomite structure. Our results at 121 and 126 °C, therefore, are consistent with the one-way transfer of Mg from dolomite to the fluid but the two-way transfer of Ca from and to dolomite as equilibrium is approached during its stoichiometric dissolution. The lack of Mg returning to the dolomite structure at these conditions is attributed to the slow dehydration kinetics of aqueous Mg. As more than 12% of the dolomite dissolved during the 121 and 126 °C closed system experiments, our observations indicate a significant change in the Ca isotopic signature of the dolomite during its stoichiometric dissolution. Moreover, as there is no visual evidence for dolomite recrystallization during this experiment, it seems likely that the resetting of Ca isotopic signatures of carbonate minerals can be readily overlooked in the interpretation of natural systems.

Calcite growth rate has been previously shown to be the dominating parameter controlling both Mg partitioning and Mg isotope fractionation during calcite growth. In natural calcite precipitation environments - characterized by abundant organic material - the presence of dissolved organic molecules may affect these two parameters. In order to assess the role of organic molecules, steady state calcite growth experiments have been performed at 25 °C, 1 bar pCO2 and constant, within analytical uncertainty growth rate (rp = 10−7.4 mol m−2 s−1) in the presence of aqueous Mg and six organic ligands in the concentration range from 0.01 to 10 mM. The organic ligands used in this study are: (i) acetic acid, (ii) citric acid, (iii) glutamic acid, (iv) salycilic acid, (v) glycine, and (vi) ethylenediaminetetraacetic acid (EDTA). These contain one or more carboxyl- and amino-groups that are commonly present in natural organic substances found in lacustrine, fluvial, soil, cave, as well as in marine and earliest diagenetic porewater environments. Results shown here indicate that the presence of these carboxyl- and amino-groups promotes an increase in the partition coefficient of Mg in calcite D Mg = ( Mg / Ca ) calcite ( Mg / Ca ) fluid that can be attributed to their adsorption onto the calcite surfaces and the subsequent reduction of the active sites of growth. This increase of DMg values as a function of the supersaturation degree of calcite in the fluid phase can be described by the linear equation: Log D Mg = 0.3694 ( ± 0.0329 ) × SI calcite - 1.9066 ( ± 0.0147 ) R 2 = 0.92 In contrast, data shown here suggest that the presence of organic ligands, with exception of citric acid, does not significantly affect the Mg isotope fractionation factor between calcite and reactive fluid. Citric acid likely exhibits larger fractionation between the Mg-ligand complexes and free aqueous Mg, compared to the other organic ligands studied in this work, as evidenced from the smaller Δ26Mgcalcite-fluid values. The main result shown here is that in Earth’s surface precipitation environments the presence of organic ligands is not a major factor affecting the Mg content or Mg-isotope composition of inorganic calcite. But in the presence of higher contents of organic macromolecules from byproducts of metabolic activities, humid, and fulvic acids and/or exopolymeric substances and matrices, an impact on Mg content or Mg isotope composition of calcite cannot be ruled out.

Various marine calcifiers form exoskeletons via an amorphous calcium carbonate (ACC) precursor phase and magnesium plays an important role in the temporary stabilization of this metastable phase. Thus, the use of Mg isotope ratios of marine biogenic carbonates as a proxy to reconstruct past seawater chemistry calls for a detailed understanding of the mechanisms controlling Mg isotope signatures during the formation and transformation of ACC to the final crystalline carbonate mineral. For this purpose we have investigated the Mg isotope fractionation between (Ca,Mg)CO 3 solids and aqueous fluids at 25 °C and pH = 8.3 during (i) the direct precipitation of crystalline Mg-calcite and (ii) the formation of Mg-rich ACC (Mg-ACC) and its transformation to Mg-calcite. The outcome documents that the small Mg isotope fractionation between Mg-ACC and reactive fluid ( Δ Mg ACC -fluid 26 = − 1.0 ± 0.1 ‰ ) is not preserved during the transformation of the ACCs into Mg-calcite. Following a pronounced isotopic shift accompanying the transformation of Mg-ACC into Mg-calcite, Δ 26 Mg calcite-fluid progressively decreases with reaction progress from ∼ − 3.0 ‰ to − 3.6 ‰ , reflecting both the approach of isotopic equilibrium and the increase of calcite Mg content (to near 20 mol % Mg). In contrast the crystalline Mg-calcite precipitated directly from the reacting fluid, i.e. lacking a discernable formation of an amorphous precursor, exhibits only small temporal variations in Δ 26 Mg calcite-fluid which overall is affected by the precipitation kinetics. The values found in this study at the onset of Mg-ACC precipitation for Mg isotope fractionation between Mg-ACC and the fluid ( Δ Mg ACC -fluid 26 = − 1.0 ‰ ) and between Mg-ACC and Mg 2+ (aq) ( Δ Mg ACC -Mg 2 + ( aq ) 26 = + 2.0 ‰ ) are consistent with the formation of a hydrated Ca nanoporous solid accommodating Mg bicarbonate/carbonate species in combination with hydrated magnesium. This material crossed by percolating channels filled with the reacting fluid easily converts to Mg-rich calcite via exchange and/or dissolution/precipitation reactions. The results of this study provide new insights on the acquisition of the Mg chemical and isotopic signatures by the skeletal marine carbonates precipitated in confined media from strongly supersaturated fluids. They also offer new guides for the interpretation of the isotopic signature of these organisms for paleo-environmental reconstructions.

Biomineralised hard parts form the most important physical fossil record of past environmental conditions. However, living organisms are not in thermodynamic equilibrium with their environment and create local chemical compartments within their bodies where physiologic processes such as biomineralisation take place. Generating their mineralized hard parts most marine invertebrates thus produce metastable aragonite rather than the stable polymorph of CaCO3, calcite. After death of the organism, the physiological conditions which were present during biomineralisation are not sustained any further and the system moves toward inorganic equilibrium with the surrounding inorganic geological system. Thus, during diagenesis the original biogenic structure of aragonitic tissue disappears and is replaced by inorganic structural features. In order to understand the diagenetic replacement of biogenic aragonite to non-biogenic calcite, we subjected Arctica islandica mollusc shells to hydrothermal alteration experiments. Experimental conditions were between 100 °C and 175 °C with reaction durations between one and 84 days, and alteration fluids simulating meteoric and burial waters, respectively. Detailed microstructural and geochemical data were collected for samples altered at 100 °C (and at 0.1 MPa pressure) for 28 days and for samples altered at 175 °C (and at 0.9 MPa pressure) for 7 and 84 days, respectively. During hydrothermal alteration at 100 °C for 28 days, most but not all of the biopolymer matrix was destroyed, while shell aragonite and its characteristic microstructure was largely preserved. In all experiments below 175 °C there are no signs of a replacement reaction of shell aragonite to calcite in X-ray diffraction bulk analysis. At 175 °C the replacement reaction started after a dormant time of 4 days, and the original shell microstructure was almost completely overprinted by the aragonite to calcite replacement reaction after 10 days. Newly formed calcite nucleated at locations which were in contact with the fluid, at the shell surface, in the open pore system, and along growth lines. In the experiments with fluids simulating meteoric water, calcite crystals reached sizes up to 200 micrometres, while in the experiments with Mg-containing fluids the calcite crystals reached sizes up to one mm after 7 days of alteration. Aragonite is metastable at all applied conditions. A small bulk thermodynamic driving force exists for the transition to calcite, which is augmented by stresses induced by organic matrix and interface energies related to the nanoparticulate architecture of the biogenic aragonite. We attribute the sluggish replacement reaction to the inhibition of calcite nucleation in the temperature window from ca. 50 °C to ca. 170 °C, or, additionally, to the presence of magnesium. Correspondingly, in Mg2+-bearing solutions the newly formed calcite crystals are larger than in Mg2+-free solutions. Overall, the aragonite-calcite transition occurs via an interface-coupled dissolution-reprecipitation mechanism, which preserves morphologies down to the sub-micrometre scale and induces porosity in the newly formed phase. The absence of aragonite replacement by calcite at temperatures lower than 175 °C contributes to explain why aragonitic or bimineralic shells and skeletons have a good potential of preservation and a complete fossil record.

The widespread formation of interstratified glauconite-smectite (Gl-S) and illite-smectite (I-S) in modern and ancient diagenetic settings records the physicochemical conditions prevailing during clay mineral authigenesis. To date, however, significant gaps in our knowledge persist in respect to the influence of interstitial solution chemistry, temperature and reaction kinetics on the evolution of composition, mineralogy and microstructure of Gl-S and I-S. Herein, we present a study on the reaction mechanisms and the physicochemical conditions that led to the precipitation of early diagenetic Gl-S and late diagenetic I-S on a stable carbonate platform during the Cenomanian at Langenstein in the Northern German Basin. The texture and the K-Ar age (95.0 ± 1.8 Ma) of the green glauconitized grains revealed that green-clay authigenesis progressed in initially organic-rich, semi-confined micromilieus, i.e., in fecal pellets and in foraminifera, close to the sediment-seawater interface. The composition of Gl-S varied in the range (K+0.20–0.74Na+0–0.10Ca2 +0–0.05)0.28–0.75 (Fe3 +0.63–1.20Fe2 +0.08–0.24Al3 +0.19–0.97 Mg2 +0.29–0.52)2.01–2.12 [Al3 +0.09–0.35Si4 +3.65–3.91O10](OH2), and depended on the rate of aqueous Fe2 + and K+ ion diffusion, the micromilieu of glauconitization and on the bulk sedimentation rate. The mineralogical, microstructural and chemical changes of the ongoing Gl-S products revealed the following reaction for green-clay authigenesis at Langenstein: Fe(III)-smectite reacted with monosilicic acid, goethite and aqueous K+, Mg2 + and Fe2 + to form glauconite and aqueous Na+, Ca2 + and H+ ions. This process considers complex mineral transformations commonly associated with glauconitization, such as early diagenetic oxidation of organic matter and microbial-catalyzed dissolution of Fe-(oxy)hydroxides, carbonates and detrital silicates. In contrast, the K-Ar age of I-S (68.0 ± 1.6 Ma) and its compositional variability, (K+0.29–0.45Na+0–0.10Ca2 +0–0.06)0.30–0.55 (Fe3 +0.16–0.29Fe2 +0–0.10Al3 +1.37–1.68Mg2 +0.18–0.43)2.00–2.12 [Al3 +0.17–0.39Si4 +3.61–3.83O10](OH2), indicate a burial diagenetic origin for this mineral phase, rather than transformation of illitic clays into I-S during weathering under warm and humid climatic conditions. The results from kinetic modelling support a diagenetic origin of I-S (50–60%I layers and 50–40%S layers) and imply its formation by the replacement of pre-existing K-feldspar at high pore-fluid activity K/Na ratios and at low Fe2 + concentrations. We propose that the substitution of Al3 + for Fe3 +, Fe2 + and Mg2 + in the octahedral sheet shifts the stability field of the kaolinite–Fe-Al-Mg-smectite–Fe-Al-Mg-illite (or glauconite) triple point to much lower monosilicic acid activities, and stabilizes the I-S (or Gl-S) structure. This reaction supports the idea that the (bio)availability of Fe is the rate-limiting factor for glauconitization, which is not the case for the diagenetic growth of I-S, whereby the pore water Fe2 + concentration may be limited by the competing formation of Fe-(oxy)hydroxides and/or Fe-sulfides.

The formation of amorphous calcium carbonate (ACC) and its transformation to crystalline phases plays a key role in the formation of carbonate minerals on Earth’s surface environments. Nonetheless, the physicochemical parameters controlling the formation of crystalline CaCO3 via an amorphous precursor are still under debate. In the present study we examine whether crystalline CaCO3 formation occurs via an ACC precursor in the pH range from 7.8 to 8.8 and at initial Mg/Ca ratios from 1/3 to 1/8. The obtained results document that the transformation of Mg-rich ACC (Mg-ACC) to a crystalline phase is strictly controlled by the prevailing ratio of the Mg2+ to Ca2+ activity, aMg2+/aCa2+, of the reactive solution after Mg-ACC was synthesized: Mg-ACC transformed to (i) Mg-calcite at 5 ≤ aMg2+/aCa2+ ≤ 8 and to (ii) monohydrocalcite at 8 ≤ aMg2+/aCa2+ ≤ 12. Our findings suggest that the formation of the crystalline phase induces undersaturation of the reactive solution with respect to the ACC and triggers its disso...

The Cr(VI) incorporation and chromium stable isotope composition in calcite has been studied in experiments performed in the pH range between 8.0 and 10.6 at constant temperature (T = 25 ± 1 °C), precipitation rate (rp = 10−7.7 ± 0.2 mol m−2 s−1) and total aqueous Cr(VI) concentration (Cr(VI) = 49.6 ± 1.3 mM). The obtained results indicate that Cr(VI) incorporation in calcite is pH-dependent and it is likely significantly affected by the formation of the aqueous CaCrO40 species. The experimental findings suggest that during calcite growth at pH O bond lengths in aqueous CaCrO40 compared to CrO42− species the lighter 52Cr(VI) isotope is preferentially abundant in the aqueous CaCrO40. The preferential uptake of the isotopically lighter CaCrO40 in the growing calcite results in Cr(VI) isotope fractionation, Δ53Crcalcite-solution = δ53Cr calcite – δ53Crsolution, as low as −0.7‰ at pH 8. In contrast, at pH > 9.4 the smaller contributions of CaCrO40 in the total concentration of Cr(VI) in calcite yields in a Δ53Crcalcite-solution value close to 0‰. Our results imply that the chromium isotope tracer system applied to calcite, as an environmental proxy for the reconstruction of ocean redox conditions, is not solely a mirror of redox effects in the aqueous fluid from which the carbonates precipitate, but additionally is controlled by the pH of the forming fluid and consequently by the relative stability and the distribution of aquo-complexes. Speciation calculations for calcite precipitated from seawater and include the presence of CaCrO0 complex, predict isotope fractionation values that lay within −0.67‰

International audience; Amorphous precursors are known to occur in the early stages of carbonate mineral formation in both biotic and abiotic environments. Although the Mg content of amorphous calcium magnesium carbonate (ACMC) is a crucial factor for its temporal stabilization, to date little is known about its control on ACMC solubility. Therefore, amorphous CaxMg1−xCO3·nH2O solids with 0 ≤ x ≤ 1 and 0.4 ≤ n ≤ 0.8 were synthesized and dispersed in MgCl2–NaHCO3 buffered solutions at 24.5 ± 0.5 °C. The chemical evolution of the solution and the precipitate clearly shows an instantaneous exchange of ions between ACMC and aqueous solution. The obtained ion activity product for ACMC (IAPACMC = “solubility product”) increases as a function of its Mg content ([Mg]ACMC = (1 − x) × 100 in mol%) according to the expression: log(IAPACMC) = 0.0174 (±0.0013) × [Mg]ACMC − 6.278 (±0.046) (R2 = 0.98), where the log(IAPACMC) shift from Ca (−6.28 ± 0.05) to Mg (−4.54 ± 0.16) ACMC endmember, can be explained by the increasing water content and changes in short-range order, as Ca is substituted by Mg in the ACMC structure. The results of this study shed light on the factors controlling ACMC solubility and its temporal stability in aqueous solutions.

Hydrated Mg-carbonate minerals form during the weathering of ultramafic rocks, and can be used to store atmospheric CO2 to help combat greenhouse gas-fueled climate change. Optimization of engineered CO2 storage and prediction of the composition and stability of Mg-carbonate phase assemblages in natural and engineered ultramafic environments requires knowledge of the solubility of hydrated Mg-carbonate phases, and the transformation pathways between these metastable phases. In this study, we evaluate the solubility of nesquehonite [MgCO3·3H2O] and dypingite [Mg5(CO3)4(OH)2·(5 or 8)H2O] and the transformation from nesquehonite to dypingite between 5 °C and 35 °C, using constant-temperature, batch-reactor experiments. The logarithm of the solubility product of nesquehonite was determined to be: −5.03 ± 0.13, −5.27 ± 0.15, and −5.34 ± 0.04 at 5 °C, 25 °C, and 35 °C, respectively. The logarithm of the solubility product of dypingite was determined to be: −34.95 ± 0.58 and −36.04 ± 0.31 at 25 °C and 35 °C, respectively, with eight waters of hydration. This is the first reported dypingite solubility product. The transformation from nesquehonite to dypingite was temperature-dependent, and was complete within 57 days at 25 °C, and 20 days at 35 °C, but did not occur during experiments of 59 days at 5 °C. This phase transformation appeared to occur via a dissolution-reprecipitation mechanism; external nesquehonite crystal morphology was partially maintained during the phase transformation at 25 °C, but was eradicated at 35 °C. Together, our results facilitate the improved evaluation of Mg-carbonate mineral precipitation in natural and engineered ultramafic mineral weathering systems that sequester CO2, and for the first time allow assessment of the saturation state of dypingite in aqueous solutions.

Carbonates are only a minor sink of oceanic lithium, yet the presence of this element and its abundance relative to other metal cations in natural carbonate minerals is routinely used as a paleo-environmental proxy. To date, however, experimental studies on the influence of physicochemical parameters that may control lithium incorporation in calcite, like pH and precipitation rate, are scarce. Therefore, we experimentally studied Li incorporation in calcite to quantify the apparent partitioning coefficient ( D Li ∗ = c Li / c Ca calcite m Li + / m Ca 2 + solution ) between calcite and reactive fluid as a function of calcite growth rate and pH. The obtained results suggest that D Li ∗ increases with calcite growth rate, according to the expression: L o g D Li ∗ = 1.331 ( ± 0.116 ) × L o g R a t e + 6.371 ( ± 0.880 ) ( R 2 = 0.87 ; 10 - 8 . 1 ≤ R a t e ≤ 10 - 7 . 1 m o l m - 2 s - 1 ) Additionally the experimental results suggest that D Li ∗ values exhibit a strong pH dependence. For experiments conducted at similar growth rates (i.e. Rate = 10−7.7±0.2 mol m−2 s−1), D Li ∗ decreases with increasing pH as described by: L o g D Li ∗ = - 0.57 ( ± 0.047 ) × p H + 0.759 ( ± 0.366 ) ( R 2 = 0.90 ; 6.3 p H 9.5 ) The positive correlation of D Li ∗ with calcite growth rate is consistent with an increasing entrapment of traces/impurities at rapidly growing calcite surfaces, although the incorporation of monovalent cations such as Li+ and Na+ does not necessarily imply a substitution of Ca2+ ions in the calcite crystal lattice. The dependence of D Li ∗ on pH can be considered as an indication that activity of aqueous HCO3− controls the incorporation of Li+ in calcite. The proposed coupled reaction can be explained by charge balance of these monovalent species, which is likely valid at least during the initial step of adsorption on the crystal surface. These new findings shed light on the mechanisms controlling Li incorporation in calcite and have direct implications on the use of Li partitioning coefficients in natural carbonates as an environmental proxy.

The Mg-isotope systematics of peridotite weathering and low-temperature carbonation have not yet been thoroughly investigated, despite their potential to provide insights into reaction pathways and mechanisms of lithosphere-hydrosphere transfer of Mg and sequestration of CO2 in carbonate minerals. Here, we present new observations of the evolution of Mg isotope ratios during subtropical ultramafic rock weathering and associated magnesite formation, including the lowest δ26Mg of magnesite reported so far. At the investigated field sites in eastern Australia, the proximity of the ultramafic Mg source rocks and associated magnesite deposits provides boundary conditions that constrain Mg isotope fractionation during low-temperature alteration. Saprolite samples from Attunga, New South Wales, show that weathering of serpentinite is accompanied by Mg loss and formation of secondary Mg-bearing clay minerals. Furthermore, Mg isotope ratios increase systematically with weathering intensity, indicating that incorporation of 26Mg into clay mineral structures controls Mg isotope fractionation during ultramafic rock weathering. The Mg-bearing clay formed by decomposition of serpentine minerals has a δ26Mg value of ∼0.35‰, which is up to ∼0.6‰ heavier than the ultramafic precursor. In contrast, nodular magnesite hosted in ultramafic rock shows δ26Mg values between −3.26‰ and −2.55‰ that are significantly lower than those of magnesite and dolomite formed by hydrothermal alteration of peridotite at higher temperature (δ26Mg = −0.69‰ and −0.62‰). The strong enrichment of 24Mg in nodular magnesite does not reconcile with simple fractionation during direct precipitation from ultramafic host rock buffered meteoric fluids and instead suggests multiple formation steps involving dissolution and re-precipitation of pre-existing carbonate accompanied by fractionation between species of dissolved Mg. Our data highlight the potential of Mg isotope studies for distinguishing the formation pathways of low temperature magnesite and for tracing Mg in low temperature alteration processes based on the distinct signatures of secondary silicate and carbonate minerals.

The formation of Mg-rich carbonates in continental lakes throughout the world is highly relevant to irreversible CO2 sequestration and the reconstruction of paleo-sedimentary environments. Here, preliminary results on Mg-rich carbonate formation at the coastal zone of Lake Vtoroe Zasechnoe, representing the Setovskiye group of water bodies located in the forest-steppe zone of Southwest Western Siberia, are reported. The Setovskiye lakes are Cl&minus, &ndash, Na+&ndash, (SO42&minus, ) type, alkaline, and medium or highly saline. The results of microscopic and mineralogical studies of microbialites from shallow coastal waters of Lake Vtoroe Zasechnoe demonstrated that Mg in the studied lake was precipitated in the form of hydrous Mg carbonates, which occur as radially divergent crystals that form clusters in a dumbbell or star shape. It is possible that hydrous Mg carbonate forms due to the mineralization of exopolymeric substances (EPS) around bacterial cells within the algal mats. Therefore, the Vtoroe Zasechnoe Lake represents a rare case of Mg-carbonates formation under contemporary lacustrine conditions. Further research on this, as well as other lakes of Setovskiye group, is needed for a better understanding of the possible role of biomineralization and abiotic mechanisms, such as winter freezing and solute concentration, in the formation of authigenic Mg carbonate in modern aquatic environments.

The magnesium isotope compositions of sedimentary carbonates are widely used to investigate the geochemical cycling of this element in seawater. Density Functional Theory techniques and experimental data from this study and literature on Mg isotope fractionation between sedimentary minerals and fluids have been used to derive equilibrium fractionation factors and to quantify the impact of aqueous magnesium speciation on the isotopic composition of its aquo ion and Mg in the precipitated carbonates. Although aqueous Mg2 + undergoes hydrolysis to a lesser extent than divalent transition metals, it nevertheless forms relatively strong complexes with inorganic and organic ligands including bicarbonate/carbonate, sulfate and carboxylate. Furthermore, aqueous Mg2 + undergoes a significant contraction of its coordination sphere when it reacts with bicarbonate and carbonate ions to form the MgHCO3+ and MgCO3° complexes which favors the preferential partitioning of heavy Mg into these species. Calculated values of the reduced partition function ratios for Mg2 +(aq) and Mg2 + bound to a number of inorganic and organic ligands show a strong enrichment of 26Mg in MgCO3°, MgHCO3+ and MgSO4° compared to Mg2 + (i.e. 1000ln?26/24MgCO3°-1000ln?26/24Mg2 + = 5.2 at 25 °C), and either a significant enrichment (Mg(oxalate)2 ?, Mg(oxalate)22 ?, Mg(citrate)?) or depletion (Mg(EDTA)2 ?, Mg(citrate)24 ?) of 26Mg in Mg-carboxylate complexes compared to Mg2 +. Analysis of experiments from this study and the literature on Mg isotope fractionation between aqueous solution and Mg carbonate and hydroxide minerals validate the reduced partition functions for Mg2+, Mg bicarbonate/carbonate and carboxylate couples generated in the present work and confirms the significant impact of carbonate and carboxylic ligands on the isotope composition of precipitated Mg-bearing minerals. This study thereby provides new insights into the parameters controlling the isotope composition of aqueous Mg2+ in natural fluids as well as improved tools to reconstruct paleo-environmental conditions from the magnesium isotope compositions recorded in carbonate sediments.