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Mechanisms of olivine dissolution by rock-inhabiting fungi explored using magnesium stable isotopes
Abstract
To unravel the dissolution mechanisms of olivine by a rock-inhabiting fungus we determined the stable isotope ratios of Mg on solutions released in a laboratory experiment. We found that in the presence of the fungus Knufia petricola the olivine dissolution rates were about seven-fold higher (1.04 × 10−15 mol cm−2 s−1) than those in the abiotic experiments (1.43 × 10−16 mol cm−2 s−1) conducted under the same experimental condition (pH 6, 25 °C, 94 days). Measured element concentrations and Mg isotope ratios in the supernatant solutions in both the biotic and the abiotic experiment followed a dissolution trend in the initial phase of the experiment, characterized by non-stoichiometric release of Mg and Si and preferential release of 24Mg over 26Mg. In a later phase, the data indicates stoichiometric release of Mg and Si, as well as isotopically congruent Mg release. We attribute the initial non-stoichiometric phase to the rapid replacement of Mg2+ in the olivine with H+ along with simultaneous polymerization of Si tetrahedra, resulting in high dissolution rates, and the stoichiometric phase to be influenced by the accumulation of a Si-rich amorphous layer that slowed olivine dissolution. We attribute the accelerated dissolution of olivine during the biotic experiment to physical attachment of K. petricola to the Si-rich amorphous layer of olivine which potentially results in its direct exposure to protons released by the fungal cells. These additional protons can diffuse through the Si-rich amorphous layer into the crystalline olivine. Our results also indicate the ability of K. petricola to dissolve Fe precipitates in the Si-rich amorphous layer either by protonation, or by Fe(III) chelation with siderophores. Such dissolution of Fe precipitates increases the porosity of the Si-rich amorphous layer and hence enhances olivine dissolution. The acceleration of mineral dissolution in the presence of a rock-dissolving fungus further suggests that its presence in surficial CO2 sequestration plants may aid to accelerate CO2 binding
... Previous studies [10][11][12][13] stated that mineral dissolution rate is a key factor restricting olivine carbon dioxide sequestration. The dissolution mechanism and activation method of olivine are still a research hotspot in the fields of geochemistry, mineralogy, and materials science [11,12,[14][15][16]. ...
... Previous studies [10][11][12][13] stated that mineral dissolution rate is a key factor restricting olivine carbon dioxide sequestration. The dissolution mechanism and activation method of olivine are still a research hotspot in the fields of geochemistry, mineralogy, and materials science [11,12,[14][15][16]. Various approaches have been used to improve its dissolution rate and extent, mainly referring to increasing reaction temperature and time [12,17,18], reducing pH [2,12]and particle size [12,17], improving the CO 2 pressure [17,18], and/or using various additives such as organic acids [2,17,[19][20][21][22]. ...
Olivine-type minerals are the most probable minerals for vast carbon dioxide sequestration since their abundant and wide distribution around the world. Nevertheless, mineral dissolution rate is the key factor restricting its carbon dioxide sequestration process. In this study, first principles calculation combined with the scanning electron microscope (SEM) were used to investigate different types of organic acids that promoted mineral dissolution mechanism of Mg2SiO4, including the adsorption configuration, adsorption energy, and electronic properties. Thermodynamics calculations on the complexation paths between organic acid
molecules and the Mg(OH)2 or Si(OH)4 groups are studied. Results show that cations on the olivine surface prefer bonding with carbonyl O of the carboxyl group after the organic acid molecule was adsorbed onto the surface with charge transferred and orbital hybridization. The H - O bond in the carboxyl group will be broken up, resulting in the mineral surface hydroxylation. A greater number of carboxyl groups will be better for the mineral dissolution with more etch pits appearing. Multiple types of complexes formed by ionized organic acid molecules and Mg2+ coexist with different reaction paths and Gibbs free energy from thermodynamics calculations. The strong interaction between the dissolved cations and carbonyl O of the carboxylate radical was the first time to predicted, which could be useful to find reasonable additives to improve the mineral dissolution kinetics
... Many bacteria and fungi promote dissolution of alkaline minerals by altering the chemical microenvironment at the mineral surface, in some cases for liberation of essential nutrients (Rogers and Bennett, 2004). Mechanisms include acidification via secretion of protons and weak organic acids, generation of carbonic acid as a byproduct of respiration, production of strong acids by chemolithotrophs, and production of polymeric and small molecule organic acids that act as chelators that catalyze mineral dissolution (Barker et al., 1997;Drever and Stillings, 1997;Nordstrom and Southam, 1997;Bennett et al., 2001;Lazo et al., 2017;Pokharel et al., 2019;Gerrits et al., 2021). Fungi can also increase the surface area of rocks by exerting mechanical forces that induce cracking (Bechinger et al., 1999). ...
... Acceleration of silicate dissolution, e.g., 7x increase in olivine dissolution by Knufia petricola in benchtop experiment (Pokharel et al., 2019). ...
Over the previous two decades, a diverse array of geochemical negative emissions technologies (NETs) have been proposed, which use alkaline minerals for removing and permanently storing atmospheric carbon dioxide (CO2). Geochemical NETs include CO2 mineralization (methods which react alkaline minerals with CO2, producing solid carbonate minerals), enhanced weathering (dispersing alkaline minerals in the environment for CO2 drawdown) and ocean alkalinity enhancement (manipulation of ocean chemistry to remove CO2 from air as dissolved inorganic carbon). CO2 mineralization approaches include in situ (CO2 reacts with alkaline minerals in the Earth's subsurface), surficial (high surface area alkaline minerals found at the Earth's surface are reacted with air or CO2-bearing fluids), and ex situ (high surface area alkaline minerals are transported to sites of concentrated CO2 production). Geochemical NETS may also include an approach to direct air capture (DAC) that harnesses surficial mineralization reactions to remove CO2 from air, and produce concentrated CO2. Overall, these technologies are at an early stage of development with just a few subjected to field trials. In Part I of this work we have reviewed the current state of geochemical NETs, highlighting key features (mineral resources; processes; kinetics; storage durability; synergies with other NETs such as DAC, risks; limitations; co-benefits, environmental impacts and life-cycle assessment). The role of organisms and biological mechanisms in enhancing geochemical NETs is also explored. In Part II, a roadmap is presented to help catalyze the research, development, and deployment of geochemical NETs at the gigaton scale over the coming decades.
... They include members of the genera Acidithiobacillus, Bacillus, Burkholderia, Enterobacter, and Sphingomonas, among others, as well as some fungi, such as Cladosporium (Meena et al., 2016), which all act by producing organic and inorganic acids (Maurya et al., 2014;Meena et al., 2015;Uroz et al., 2009;Sheng et al., 2008). Pokharel et al. (2019) used Mg stable isotopes to unravel the mechanism of dissolution of olivine by the fungus, Knufia petricola. They demonstrated a non-stoichiometric initial phase, in which Mg and Si were released, Mg 2+ being replaced with H + in the olivine with simultaneous polymerization of Si tetrahedra, followed by a stoichiometric phase associated with the accumulation of an Si-rich amorphous layer. ...
... The reactions depend on both environmental conditions and microbial species, as well as the actual composition of the EPS, which contains not only polysaccharides but also other polymers, such as proteins and nucleic acids; research workers have shown that biofilms containing living bacteria and EPS increase the release of K, Si and Al from shale (Man et al., 2014) and induce the weathering of primary Fe-bearing minerals such as biotite in iron ore tailings (Yi et al., 2021). Gerrits et al. (2020), using the same fungus-olivine system as Pokharel et al. (2019), mentioned in Section 3.1.1, showed that EPS produced by the fungus, Knufia petricola, prevents the oxidation and precipitation of abiotically released Fe at the mineral surface and thus allows faster dissolution than in fungus-free solution. ...
Fundamental processes for the biodeterioration of stone and metal involve many of the same microbially mediated reactions – oxidation, reduction, acid dissolution and elemental cycling – resulting from the activities of many of the same groups of environmental microorganisms. Differences depend on the nature of the substratum – stone vs. metal – and the composition of the surroundings, whether terrestrial (stone) or aquatic (stone and metal). Reactions within surface-related biofilms dominate the biodeterioration of metals and contribute greatly to the biodeterioration of stone. In the latter, phototrophic organisms, and especially cyanobacteria, are important first participants, while metal biodeterioration is almost entirely associated with bacteria, archaea and fungi. Biofilms on metal surfaces can produce chemical and electrochemical responses. While electrochemical responses are absent in stone, extracellular electron transfer can be a biodeterioration mechanism in some iron-rich rocks. Microorganisms in biofilms can penetrate and create fissures or cracks in stone and metals. However, the most obvious differences in the reactions of built stone and metal structures are related to the definition of failure, length of time required for a defined failure of the substratum, the area over which the failure occurs and the consequences of failure. Time and space are, similarly, quite distinct for biological breakdown and mineral cycling of metal and stone, with stone/rock cycling potentially occurring over thousands of years and kilometers.
... However, there are serious controversies, whether such an effect is largely direct (metabolically driven nutrient mobilization and specific ligand production) or indirect (passive pH lowering and cation complexation via EPS excretion), given that it is largely dependent on the nature of mineral and the identity of microorganism 9,45-48 . Furthermore, considering basic silicates, some studies reported nil or inhibiting effect of bacteria on olivine 32,49,50 or wollastonite 28 dissolution, whereas other claimed strong accelerating effect of fungi on olivine 21,22,51 . However, in the latter works, all the effects of microorganisms were limited to their ability to sequester rateinhibiting Fe(III) thus preventing the slowdown of dissolution, rather than directly promoting the breaking of structural bonds in the mineral lattice. ...
... npj Materials Degradation (2021)51 Published in partnership with CSCP and USTB ...
Assessment of the microbial impact on mineral dissolution is crucial for a predictive understanding of basic (Ca, Mg bearing) silicate weathering and the associated CO2 consumption, bioerosion, and CO2 storage in basaltic rocks. However, there are controversies about the mechanism of microbial effect, which ranges from inhibiting via nil to accelerating. Here we studied diopside interaction with the heterotrophic bacterium Pseudomonas reactants and the soil fungus Chaetomium brasiliense using a combination of mixed-flow and batch reactors and in situ (AFM) and ex situ (SEM) microscopy. The results provide new nano-level insights into the degree to which microorganisms modify silicate dissolution. Taking into account negligible effects of organic ligands on diopside dissolution as reported earlier, we conclude that the microbial effect on Ca-Mg silicates is weak and the acceleration of dissolution of “basic” silicate rocks in the presence of soil biota is solely due to pH decrease in porewaters.
... No other silicate's reactivity has been studied so often and under more experimental conditions as olivine's (Oelkers et al., 2018). And even though there is a general consensus regarding olivine's abiotic dissolution (Rimstidt et al., 2012;Oelkers et al., 2018), the biotic impact on its dissolution is far from agreed upon; ranging from an enhancement (Shirokova et al., 2012;Seiffert et al., 2014;Pokharel et al., 2019;Gerrits et al., 2020) to an inhibition of olivine dissolution (Garcia et al., 2013;Oelkers et al., 2015). ...
... The black meristematic fungus Knufia petricola A95 is able to accelerate olivine dissolution (Seiffert et al., 2014;Pokharel et al., 2019;Gerrits et al., 2020). In a previous study, we have shown how K. petricola enhances the dissolution of olivine through attachment to the mineral surface (Gerrits et al., 2020). ...
Many microorganisms including free-living and symbiotic fungi weather minerals through the formation of biofilms on their surface. Weathering thus proceeds not only according to the mineral's chemistry and the environmental conditions but also according to the local biofilm chemistry. These processes can be dissected in experiments with defined environmental settings and by employing genetic tools to modify traits of the fungal biofilm. Biofilms of the rock-inhabiting fungus Knufia petricola strain A95 (wild-type, WT) and its melanin-deficient mutant (ΔKppks) were grown on polished olivine sections in subaerial (air-exposed) and subaquatic (submerged) conditions. After seven months of interaction at pH 6 and 25 °C, the fungus-mineral interface and abiotic olivine surface were compared using high resolution transmission electron microscopy (HRTEM). The abiotic, subaquatic olivine section showed a 25 nm thick, continuous amorphous layer, enriched in Fe and depleted in Si compared to the underlying crystalline olivine. This amorphous layer formed either through a coupled interfacial dissolution reprecipitation mechanism or through the adsorption of silicic acid on precipitated ferric hydroxides. Its thickness was likely enhanced by mechanical stresses of polishing. Directly underneath a fungal biofilm (WT and mutant alike), the surface remained mostly crystalline and was strongly etched and weathered, indicating enhanced olivine dissolution. The correlation between enhanced olivine dissolution and the absence of a continuous amorphous layer is a strong indication of the dissolution-inhibiting qualities of the latter. We propose that the fungal biofilm sequesters significant amounts of Fe, preventing formation of the amorphous layer and driving olivine dissolution onwards. The seemingly similar olivine surface underneath both WT and mutant biofilms illustrates the comparably insignificant role of specific biofilm traits in the weathering of olivine once biofilm attachment is imposed. Under subaerial conditions, the absence of water on the abiotic surface prohibited olivine dissolution. This was overcome by the water retention capacities of both the WT and mutant biofilm: the olivine surface underneath subaerial fungal biofilms was as weathered as the corresponding subaquatic olivine surface. Under the studied environmental settings, the effect of fungal biofilms on olivine weathering seems to be universal, independent of the production of melanin, the composition of extracellular polymeric substances (EPS) or air-exposure.
... complete) dissolution of the frustule. This is most clearly seen in the mineral dissolution literature for a range of isotope systems, including Si (Ziegler et al., 2005), but also Fe (Kiczka et al., 2010), Mg (Pokharel et al., 2019) and Zn (Weiss et al., 2014), among others. If the same is true for diatom bSi, it is only the early stages of dissolution that will induce a fractionation. ...
Silicon stable isotope ratios (expressed as δ30Si) in biogenic silica have been widely used as a proxy for past and present biogeochemical cycling in both marine and lacustrine settings, in particular for nutrient utilization reconstructions. Yet an analysis of publication trends suggests a significant decline in the application of δ30Si to Quaternary science questions in the last five years. At the same time as δ30Si proxy applications have decreased, we are learning more about its complexities: an expanding body of work is highlighting biases, caveats or complications involved in the application of δ30Si-based approaches to the sediment record. These include the demonstration of species-specific silicon isotope fractionation factors (i.e. 'vital effects') or the potential for Fe or other trace metals to influence silicon isotope fractionation. Others have inferred the potential of biogenic silica dissolution to alter an initial δ30Si value, or questioned the preservation of the initial δ30Si through early diagenetic processes more generally. Another challenge receiving more attention is centered around deconvolving a δ30Si-value into a signal reflecting biological productivity and a signal reflecting changes in the δ30Si of dissolved silicon driven by whole-system and/or circulation changes. Finally, a number of studies focus on analytical difficulties, especially during sample preparation related to achieving and demonstrating a contaminant free biogenic silica. These challenges lead us to posit that the Quaternary science community is moving away from silicon isotope proxies because they are losing confidence in their reliability and usefulness. Here, focusing on the diatoms-the dominant biosilicifiers in both lakes and the ocean-we synthesize progress in understanding nuances and caveats of δ30Si-based proxies in order to answer whether the fall-off in δ30Si-based Quaternary research is warranted. We suggest that with some simple steps that can be readily implemented, and with the closing of key knowledge gaps, there is no reason to believe silicon isotopes do not have a promising future in the Quaternary sciences.
... The abiotic dissolution of silicate minerals (such as olivine) can also result in the formation of amorphous Si layers and Fe (III) oxide precipitates, both of which can further imped underlaying mineral dissolution. These layers have been shown to be susceptible to microbial weathering, which can remove developed layers and prevent their development, allowing for continued mineral dissolution (Pokharel et al., 2019;Gerrits et al., 2020). Inoculation of the kimberlitic material to create a FRD-biofilm composite, is essential to both enhance weathering, enhance mineral carbonation and to aid in dewatering via clay adsorption. ...
The observation of photosynthetic biofilms growing on the Fine Residue Deposit (FRD) kimberlite produced by the Venetia Diamond Mine, Limpopo, South Africa suggests that processed kimberlite supports bacterial growth. The presence of this biofilm may aid in the acceleration of weathering of this ultra-mafic host material – a process that can sequester CO 2 via carbon mineralization. Laboratory and field trial experiments were undertaken to understand the microbe–mineral interactions occurring in these systems, and how these interactions impact geochemical cycling and carbonate precipitation. At laboratory scale it was discovered that using kimberlite as a growth supplement increased biomass production (up to 25-fold) and promoted microbiome diversity, while the inoculation of FRD systems aided in the aggregation, settling, and dewatering of kimberlitic slurries. Field trial studies combining photosynthetic biofilms (cultured in 3 × 1,000 L bioreactors) with FRD material were initiated to better understand microbially enhanced mineral carbonation across different depths, and under field environmental conditions. Over the 15-month experiment the microbial populations shifted with the kimberlitic environmental pressure, with the control and inoculated systems converging. The natural endogenous biosphere (control) and the inoculum accelerated carbonate precipitation across the entire 40 cm bioreactor depth, producing average 15-month carbonation rates of 0.57 wt.% and 1.17 wt.%, respectively. This corresponds to an annual CO 2 e mine offset of ~4.48% and ~ 9.2%, respectively. Millimetre-centimetre scale secondary carbonate that formed in the inoculated bioreactors was determined to be biogenic in nature (i.e., possessing microbial fossils) and took the form of radiating colloform precipitates with the addition of new, mineralized colonies. Surficial conditions resulted in the largest production of secondary carbonate, consistent with a ca. 12% mine site CO 2 e annual offset after a 15-month incubation period.
... Mg is one of the most important rock-derived nutrients, such that its incorporation in the biomass is associated with enrichment in light Mg isotopes in residual waters (Black et al., 2008;Bolou-Bi et al., 2009;Pokharel et al., 2019). In the Yellow River basin, crops growth rates are high during the monsoon seasons due to temperate climate and intense rainfall (Chen et al., 2003). ...
... Prerequisite i) is violated as the deposition of precipitation, and the solubilisation of soil constituents (e.g., SOM, soil minerals, organic and inorganic fertilizer, agricultural lime) and atmospheric dry deposition continues during crop growth. Even though mineral dissolution takes place without isotope fractionation beyond the stages of incipient weathering (Maher et al., 2016;Pokharel et al., 2019;Ryu et al., 2016Ryu et al., , 2011Wimpenny et al., 2010), which is also assumed for other soil constituents, the additional Mg would bias fuptake, fuptake,i and the 26 Mginitial exch.,i value. ...
A sustainable use of soil resources is urgently required to cope with the increasing demand for agricultural products during climate change. To inspire farmers on new soil cultivation methods like subsoil management requires not only yield measures but also nutrient use efficiency measures for which analytical tools are still missing. Stable isotopes of the macronutrient magnesium (Mg) are a potential novel subsoil management evaluation tool in agronomy and soil/plant sciences because its isotope ratios shift considerably during Mg uptake by crops. The feasibility of Mg stable isotopes was first demonstrated conceptually by simulating subsoil management on soils with low, middle, and high inventories of bioavailable Mg and crop plants typically cultivated in Germany. This simulation showed that the magnitude of Mg isotope shifts among crops and the exchangeable fraction of Mg in soil is resolvable from the long-term external precision of Mg isotope analyses only if three conditions are met. First, the crop uptake-related Mg isotope fractionation factor should be at the upper end of hitherto published fractionation factors. Second, a high Mg uptake flux of crop plants (e.g., sugar beets) is matched by a low Mg supply from the exchangeable fraction in soil (e.g., sandy soils). Third, subsoil management causes a considerable deepening of the rooting system (e.g., flipping the topsoil root cluster below 30 cm depth). If these conditions are met, Mg stable isotopes can be used in a qualitative manner to identify the main Mg uptake depth, and in a quantitative manner by calculating the Mg use efficiency, defined here as the ratio of Mg uptake versus Mg supply, solely from Mg isotope ratios. This concept was tested for Alfisols on field trials by conducting deep loosening with and without the incorporation of compost. Magnesium isotope shifts in crops and the exchangeable fraction of Mg in soil were mostly unresolvable from the long-term external precision of Mg isotope analyses, which positively tested the Mg isotope concept for well nurtured soils. However, systematic Mg isotope shifts among bulk crops cultivated on and beside a melioration strip were found and attributed to the uplift of isotopically distinct compost-derived Mg on the melioration strip and root restricting layers beside the melioration strip. Given that the Mg isotope composition of the exchangeable fraction barely varies with depth, field-based crop uptake-related ‘apparent’ Mg isotope fractionation factors of winter wheat and spring barley could be determined, which differed from one another (Δ²⁶Mgwheat-rem.exch. = 0.63 ± 0.05‰, Δ²⁶Mgbarley-rem.exch. = 0.37 ± 0.12‰). Nonetheless, the quantitative approach of Mg isotopes was violated when calcareous fertilizer was applied to the field as differences in the isotope-derived Mg use efficiency could be attributed to the uneven distribution of lime-derived Mg with depth. Using Mg stable isotopes as a new geochemical routine for agronomy and soil/plant sciences requires future work focussing on isotope fractionation factors related to crop uptake and intra-plant translocation of Mg – which may depend on species, environmental conditions, and nutrient status – to allow minimally invasive sampling of the soil-plant system and to reduce sample sets.
... Glucose is usually used as a carbon source in this type of study. The content of glucose ranged from $0.2 to 8% in previous liquid-culture studies of fungimineral interactions (Crawford et al., 2000;Adeyemi and Gadd, 2005;Balogh-Brunstad et al., 2008;Lian et al., 2008;Daghino et al., 2009;Pokharel et al., 2019;Yu et al., 2019). An amount of 20 g l À1 of D-glucose was added to the fresh medium because our aim was to investigate the secondary metabolites produced during fungus-mineral interactions, and these metabolites are produced by the oxidation of glucose (Plassard and Fransson, 2009). ...
Fungus-mineral interactions influence a variety of geochemical and biological processes on and near the Earth’s surface. The secondary metabolites produced by fungi during these interactions control mineral weathering and affect geochemical element cycling; however, the extent to which minerals exert their influence on fungal metabolism has not been quantitatively determined in detail. Here, we used mass spectroscopy to investigate the exometabolome of Talaromyces flavus, a serpentine soil-inhabiting fungus, during interactions with antigorite, chlorite, hornblende, lizardite, magnetite, and quartz at 28 °C under aerobic conditions. Our experimental results showed that all the minerals used promoted spore germination and hyphal growth. The glucose consumption rate during the initial stages of the fungus-mineral interactions indicated that the metabolic activity of T. flavus was enhanced by a factor of ∼7 by lizardite, antigorite, chlorite, hornblende, and magnetite and by a factor of ∼3 by quartz. A total of 1198 secondary metabolites were detected at 166 h, and 977 (81.55%) of them were common to the T. flavus interactions with antigorite, chlorite, hornblende, lizardite, and magnetite. Statistical and comparative analyses of the 28 most common organic acids revealed that the production of these secondary metabolites by T. flavus was highly mineral-specific and that their content varied significantly in response to the different minerals. Moreover, the component composition of the secondary metabolites of T. flavus interacting with lizardite was more similar to chlorite and magnetite than to antigorite, hornblende, and quartz. Strong correlations were found between the production of gluconic, oxalic, citric, and itaconic acid and the bioavailability of certain elements. Fumaric and orsellinic acid production were stimulated by magnetite and chlorite, respectively. These results demonstrate that fungi can significantly regulate their metabolic behavior during interactions with minerals and that their metabolic regulation is primarily related to the chemical composition, weatherability, and surface properties of the minerals. Such biotic and abiotic responses have important implications for the fungus-induced geochemical transformation of minerals and metals.
... Deviations in the δ 88/86 Sr value of the bio-available fraction with respect to bedrock cannot be attributed to isotopically incongruent release of Sr by preferential mineral dissolution (Fig. 7). We can also exclude release of isotopically light Sr from the mineral's surface layer into the fluid during fast initial dissolution as reported for other isotope systems (Brantley et al., 2004;Pokharel et al., 2019; J o u r n a l P r e -p r o o f Journal Pre-proof Wimpenny et al., 2010). That is because in the EarthShape sites the rocks weather at quasi steadystate, thereby obliterating any fingerprint of early fast mineral dissolution. ...
Weathering and ecosystem nutrition are intimately linked through the supply of fresh mineral nutrients from regolith and bedrock (the "geogenic nutrient pathway"). However, the prominence of this link is dependent on the efficiency of nutrient recycling from plant litter (the "organic nutrient cycle"). Isotope ratios of strontium (Sr), an element that behaves similarly to Ca in ecosystems, confer two types of information: radiogenic Sr isotopes inform as to the sources of Sr and the degree of weathering, while stable Sr isotopes constrain partitioning between compartments of the Critical Zone (bedrock, water, secondary solids, and plants). To date, however, neither the reactions nor the mass balance between compartments that fractionate Sr isotopes, nor the fractionation factors involved, are well understood. Here, we present geochemical budgets of Sr (using radiogenic and stable Sr isotopes, and Ca/Sr ratios) at four sites along a substantial climate and primary production gradient in the coastal mountains of Chile. We found that Sr release through weathering is isotopically congruent, and released Sr is not strongly isotopically fractionated either during secondary mineral formation or transfer into the exchangeable pool. Despite this, the 88 Sr/ 86 Sr ratio of bio-available Sr, which should reflect the ratio of dissolved Sr, is higher than that of rock and regolith. We propose that this offset is caused by plants: while 88 Sr/ 86 Sr in plant organs at the four study sites systematically increased from roots towards their leaves, whole-plant Sr isotope compositions indicate preferential uptake of light Sr into plants (with a fractionation of up to-0.3 ‰ relative to the bio-available pool). Despite this strong biological fractionation, 88 Sr/ 86 Sr ratios in bio-available Sr do not covary with biomass production across our study sites, because with greater plant growth Sr is recycled more times after release by weathering-an isotope-neutral process. Rather, the loss of Sr from the ecosystem in solid organic material sets the isotope ratio of dissolved or bio-available Sr. Organic solids thus appear to constitute a significant export path of elements released during weathering, with the removal of solid plant debris reducing the recycling factor of Sr, and possibly that of other mineral nutrients too. Keywords J o u r n a l P r e-p r o o f Journal Pre-proof weathering Chile Coastal Cordillera nutrient uptake stable isotope fractionation stable and radiogenic metal isotopes EarthShape
Catalyzing mineral weathering for permanent, safe and cost-effective carbon storage
Martin Van Den Berghe, CEO of Cytochrome, discusses catalyzing mineral weathering for permanent, safe, and cost-effective carbon storage. Earth is unique as it is the only planet we know that harbours life. The geological record informs us that both life and liquid water have been inalienable components of our planet for over 3 billion years. Yet three billion years ago, the sun was emitting only about 75% of its current solar radiation, which was not enough heat to sustain liquid water under present atmospheric conditions. Earth was able to sustain liquid water through a very strong greenhouse effect from very high atmospheric CO2 concentrations. Over the eons the sun gradually got brighter and hotter, and atmospheric CO2 concentrations, in turn, gradually decreased through continuous weathering. (1, 2)
The velocity effect was studied in the synthesis of nanopores with a noncircular cross section by etching tracks of swift heavy ions in olivine. The developed atomistic model for the etching of olivine irradiated with swift heavy ions predicts the possibility of synthesizing nanopores with a noncircular cross section in it. The model consists of connected blocks that describe the sequential stages of track formation and etching. The TREKIS Monte Carlo model describes the initial electronic and lattice excitations in the nanoscale vicinity of the trajectory of an incident ion. These results are used as initial conditions for molecular dynamics simulation of structural changes along the ion trajectory. The obtained atomic coordinates after cooling of the structurally damaged area serve as the initial data for the original atomistic model of track etching in olivine. The results of the model application show that it is possible to control the cross section of these pores by changing the orientation of the crystal relative to the direction of irradiation. The presented simulation results for Xe ions demonstrate that the size of the resulting pores depends on the velocity of the incident ion, and not only on its linear energy loss.
We present a multiscale model describing wet chemical etching of swift heavy ion tracks in olivine on the atomic scale. The approach combines the Monte-Carlo code TREKIS and molecular dynamics for simulations of excitation and structure transformations of the material after an ion impact. Analysis of the obtained atomic positions in the proximity of the ion trajectory allows to build up the algorithm of atom-by-atom removal from the etching surface. The model describes primary etching of the highly damaged cylindrical track core followed by dissociation of the peripheral region and formation of etched channels, which cross section shapes depend on the crystal orientations relative to the incident ion beam.
Soil supports life by serving as a living, breathing fabric that connects the atmosphere to the Earth's crust. The study of soil science and pedology, or the study of soil in the natural environment, spans scales, disciplines, and societies across the planet. The scope of soil science continues to grow by extending across spatial and temporal scales given advancements in analytical tools, capabilities, and a growing emphasis on the need to integrate research across disciplines. This review demonstrates the urgent need to integrate the study of soil, and soil scientists, by presenting research areas centered on three important yet seemingly unrelated questions of central interest to scientists, students, and government agencies alike: 1) How do the properties of soil influence the selection of habitat and survival by organisms, especially threatened and endangered species struggling in the face of climate change and habitat loss during the Anthropocene? 2) How do we disentangle the heterogeneity of abiotic and biotic processes that transform minerals and release life-supporting nutrients to soil, especially at the nano to microscale where mineral-water-microbe interactions occur? and 3) How can soil science advance the search for life and habitable environments on Mars and beyond- from distinguishing biosignatures to better utilizing terrestrial analogs on Earth for planetary exploration? This review also highlights the tools, resources, and expertise that soil scientists bring to interdisciplinary teams focused on questions centered belowground, whether the research areas involve conservation organizations, industry, the classroom, or government agencies working to resolve global challenges and sustain a future for all.
Peridotite and serpentinites can be used to sequester CO2 emissions through mineral carbonation. Olivine dissolution rate is directly proportional with temperature, presence of CO2, surface area of mineral particles and presence of ligands and is inversely proportional to pH. Olivine dissolution is better under air flow and increases seven times when rock-inhibiting fungus (Knufia petricola) is used. Olivine dissolution retards as silica layers form during reaction. Sonication, acoustic and concurrent grinding using various grinding medias have been used to artificially break these silica layers and achieve high magnesium extraction. Wet grinding using 50 wt.% ethanol enhanced CO2 uptake of dunite 6.9 times and CO2 uptake of harzburgite by 4.5 times. The best economical process is single-stage concurrent grinding at 130 bar, 185 °C, 15 wt.% solids and 50 wt.% grinding media (zirconia) using 0.64 M NaHCO3. Ratio of grinding media to feed should not be less than 3:1. Yield increases with temperature, pressure, time of reaction, pH and rpm and using additives and grinding media and reducing particle size. This review aims to investigate the progress from 1970s to 2021 on aqueous mineral carbonation of olivine and its naturally available rocks (harzburgite and dunite). This paper comprehensively reviews all aspects of olivine carbonation including olivine dissolution kinetics, effects of grinding and concurrent grinding, thermal activation of olivine feedstock (dunites and harzburgites) as well as chemistry of olivine mineral carbonation. The effects of different reaction parameters on the carbonation yield, role of mineral carbonation accelerators and costs of mineral carbonation process are discussed.
In a small, forested catchment underlain by gneiss (Conventwald, Black Forest, Germany), we found that the magnesium isotope composition (δ²⁶Mg) of creek water did not show seasonal variability, despite variations in dissolved Mg concentrations. To investigate the potential controlling factors on water δ²⁶Mg values, we studied the Mg isotope composition of solid samples (bedrock, bulk soil, clay-sized fraction of soil, separated minerals, the exchangeable fraction of regolith) and water samples comprising time series of creek water, groundwater and subsurface flow. Subsurface flow from 0-15 cm depth (-0.80 ± 0.08 ‰) and 15-150 cm depth (-0.66 ± 0.17 ‰), groundwater (-0.55 ± 0.03 ‰), and creek water (-0.54 ± 0.04 ‰) are all depleted in heavy Mg isotopes compared to bedrock (-0.21 ± 0.05 ‰). Subsurface flow samples have similar δ²⁶Mg values to the regolith exchangeable fraction at the respective sampling depths. Also, groundwater and creek water show δ²⁶Mg values that are identical to those of the exchangeable fraction in the deep regolith. We suggest, therefore, that cation-exchange processes in the regolith control Mg concentrations and δ²⁶Mg values of creek water at our study site. This assumption was further verified by batch adsorption-desorption experiments using soil samples from this study, which showed negligible Mg isotope fractionation during adsorption-desorption. We propose that the exchangeable fraction of the regolith buffers dissolved Mg concentrations by adsorbing and storing Mg when soil solutions are high in concentration in the dry season and desorbing Mg when rainfall infiltrates and percolates through the regolith in the wet season. This mechanism may explain the near chemostatic behavior of Mg concentrations and the invariance of δ²⁶Mg values in creek water. In addition, the depth distribution of exchangeable Mg concentration and isotope composition in the regolith reflects mineral dissolution and secondary mineral formation in deep regolith (> 3 m) and biological cycling in shallower depth (0 – 3m). Magnesium stable isotopes thus provide an accurate snapshot of the geogenic (weathering) and the organic (bio-cycled) nutrient cycle.
In a controlled growth experiment we found that the cyanobacterium Nostoc punctiforme has a bulk cell ²⁶Mg/²⁴Mg ratio (expressed as δ²⁶Mg) that is -0.27‰ lower than the growth solution at a pH of ca. 5.9. This contrasts with recently published δ²⁶Mg value that was 0.65‰ higher than growth solution for the black fungus Knufia petricola at similar laboratory conditions, interpreted to reflect loss of ²⁴Mg during cell growth. By a mass balance model constrained by δ²⁶Mg in chlorophyll extract we inferred the δ²⁶Mg value of the main Mg compartments in a cyanobacteria cell: free cytosolic Mg (-2.64‰), chlorophyll (1.85‰), and the non-chlorophyll-bonded Mg compartments like ATP and ribosomes (-0.64‰). The lower δ²⁶Mg found in Nostoc punctiforme would thus result from the absence of significant Mg efflux during cell growth in combination with either a) discrimination against ²⁶Mg during uptake by desolvation of Mg or transport across protein channels; or b) discrimination against ²⁴Mg in the membrane transporter during efflux. The model predicts the preferential incorporation of ²⁶Mg in cells and plant organs low in Mg, and the absence of isotope fractionation in those high in Mg, corroborated by a compilation of Mg isotope ratios from fungi, bacteria, and higher plants.
This study provides experimental evidence of the resetting of the magnesium (Mg) isotope signatures of hydromagnesite in the presence of an aqueous fluid during its congruent dissolution, precipitation, and at equilibrium at ambient temperatures over month-long timescales. All experiments were performed in batch reactors in aqueous sodium carbonate buffer solutions having a pH from 7.8 to 9.2. The fluid phase in all experiments attained bulk chemical equilibrium within analytical uncertainty with hydromagnesite within several days, but the experiments were allowed to continue for up to 575 days. During congruent hydromagnesite dissolution, the fluid first became enriched in isotopically light Mg compared to the dissolving hydromagnesite, but this Mg isotope composition became heavier after the fluid attained chemical equilibrium with the mineral. The δ²⁶Mg composition of the fluid was up to ∼0.35‰ heavier than the initial dissolving hydromagnesite at the end of the dissolution experiments. Hydromagnesite precipitation was provoked during one experiment by increasing the reaction temperature from 4 to 50 °C. The δ²⁶Mg composition of the fluid increased as hydromagnesite precipitated and continued to increase after the fluid attained bulk equilibrium with this phase. These observations are consistent with the hypothesis that mineral-fluid equilibrium is dynamic (i.e. dissolution and precipitation occur at equal, non-zero rates at equilibrium). Moreover the results presented in this study confirm (1) that the transfer of material from the solid to the fluid phase may not be conservative during stoichiometric dissolution, and (2) that the isotopic compositions of carbonate minerals can evolve even when the mineral is in bulk chemical equilibrium with its coexisting fluid. This latter observation suggests that the preservation of isotopic signatures of carbonate minerals in the geological record may require a combination of the isolation of fluid-mineral system from external chemical input and/or the existence of a yet to be defined dissolution/precipitation inhibition mechanism.
Magnesium is the metal at the center of all types of chlorophyll and is thus crucial to photosynthesis. When an element is involved in a biosynthetic pathway its isotopes are fractionated based on the difference of vibrational frequency between the different molecules. With the technical advance of multi-collectors plasma-mass-spectrometry and improvement in analytical precision, it has recently been found that two types of chlorophylls (a and b) are isotopically distinct. These results have very significant implications with regards to the use of Mg isotopes to understand the biosynthesis of chlorophyll. Here we present theoretical constraints on the origin of these isotopic fractionations through ab initio calculations. We present the fractionation factor for chlorphyll a, b, d, and f. We show that the natural isotopic variations among chlorophyll a and b are well explained by isotopic fractionation under equilibrium, which implies exchanges of Mg during the chlorophyll cycle. We predict that chlorophyll d and f should be isotopically fractionated compared to chlorophyll a and that this could be used in the future to understand the biosynthesis of these molecules.
Plants and soil microbiota play an active role in rock weathering and potentially couple weathering at depth with erosion at the soil surface. The nature of this coupling is still unresolved because we lacked means to quantify the passage of chemical elements from rock through higher plants. In a temperate forested landscape characterised by relatively fast (∼ 220 t km-2 yr-1) denudation and a kinetically limited weathering regime of the Southern Sierra Critical Zone Observatory (SSCZO), California, we measured magnesium (Mg) stable isotopes that are sensitive indicators of Mg utilisation by biota. We find that Mg is highly bio-utilised: 50–100 % of the Mg released by chemical weathering is taken up by forest trees. To estimate the tree uptake of other bio-utilised elements (K, Ca, P and Si) we compared the dissolved fluxes of these elements and Mg in rivers with their solubilisation fluxes from rock (rock dissolution flux minus secondary mineral formation flux). We find a deficit in the dissolved fluxes throughout, which we attribute to the nutrient uptake by forest trees. Therefore both the Mg isotopes and the flux comparison suggest that a substantial part of the major element weathering flux is consumed by the tree biomass. The enrichment of 26Mg over 24Mg in tree trunks relative to leaves suggests that tree trunks account for a substantial fraction of the net uptake of Mg. This isotopic and elemental compartment separation is prevented from obliteration (which would occur by Mg redissolution) by two potential effects. Either the mineral nutrients accumulate today in regrowing forest biomass after clear cutting, or they are exported in litter and coarse woody debris (CWD) such that they remain in “solid” biomass. Over pre-forest-management weathering timescales, this removal flux might have been in operation in the form of natural erosion of CWD. Regardless of the removal mechanism, our approach provides entirely novel means towards the direct quantification of biogenic uptake following weathering. We find that Mg and other nutrients and the plant-beneficial element Si (“bio-elements”) are taken up by trees at up to 6 m depth, and surface recycling of all bio-elements but P is minimal. Thus, in the watersheds of the SSCZO, the coupling between erosion and weathering might be established by bio-elements that are taken up by trees, are not recycled and are missing in the dissolved river flux due to erosion as CWD and as leaf-derived bio-opal for Si. We suggest that the partitioning of a biogenic weathering flux into eroded plant debris might represent a significant global contribution to element export after weathering in eroding mountain catchments that are characterised by a continuous supply of fresh mineral nutrients.
New developments in Geochemistry during the last two decades have revolutionized our understanding of the processes that shape Earth's surface. Here, complex interactions occur between the tectonic forces acting from within the Earth and the exogenic forces like climate that are strongly modulated by biota and, increasingly today, by human activity. Within the Helmholtz Laboratory for the Geochemistry of the Earth Surface (HELGES) of the Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences, it is our goal to quantify the rates and fluxes of these processes in detail and to develop new techniques to fingerprint them over various temporal and spatial scales. We use mass spectrometry facilities to analyze metal stable isotopes, element concentrations and cosmogenic nuclides to fingerprint and quantify geomorphological changes driven by erosion and weathering processes. We use these novel geochemical tools, to quantify, for example, the recycling of metals in plants after their release during weathering of rocks and soils, soil formation and its erosion rates, and mechanisms and speed of sediment transport through drainage basins. Our research is thus dedicated towards understanding material turnover rates at the Earth's surface by using geochemical fingerprints.
A biological impact on weathering was recognized already at the beginning of the twentieth century, when biochemical influence of the lichen growth on rocks was convincingly demonstrated. Later it was shown that the progress of solid rock weathering initiated by biological colonization was affected by the initial porosity system and sensitivity of mineral association. In the meantime a considerable amount of diverse scientific data confirm the importance of biological rock colonizers (lichens and free-living rock biofilms) in mineral material dynamics as they occur at the atmosphere-exposed rock surfaces on local as well as global scale. Subaerial rock biofilms—microbial ecosystem including free-living heterotrophic and phototrophic settlers of bare rock surfaces—are characteristic for the first stage of primary succession of terrestrial ecosystems on mineral substrates. These cultivable and free-living communities are dominated by fungi and set the stage for the later development of a lichen cover, but in comparison to lichens also represent a new tool for laboratory experimentation and thus open a new stage of work in geomicrobiology. The minerals sensitivity to microbially induced biological weathering can be demonstrated by studies of natural samples as well as by the laboratory mesocosm experiments.
Sub-aerial biofilms (SAB) are ubiquitous, self-sufficient microbial ecosystems found on mineral surfaces at all altitudes and latitudes. SABs, which are the principal causes of weathering on exposed terrestrial surfaces, are characterized by patchy growth dominated by associations of algae, cyanobacteria, fungi and heterotrophic bacteria. A recently developed in vitro system to study colonization of rocks exposed to air included two key SAB participants - the rock-inhabiting ascomycete Knufia petricola (CBS 123872) and the phototrophic cyanobacterium Nostoc punctiforme ATCC29133. Both partners are genetically tractable and we used them here to study weathering of granite, K-feldspar and plagioclase. Small fragments of the various rocks or minerals (1–6 mm) were packed into flow-through columns and incubated with 0.1% glucose and 10 μM thiamine-hydrochloride (90 μL min⁻¹) to compare weathering with and without biofilms. Dissolution of the minerals was followed by: (i) analysing the degradation products in the effluent from the columns via Inductively Coupled Plasma Spectroscopy and (ii) by studying polished sections of the incubated mineral fragments/grains using scanning electron microscopy, transmission electron microscopy and energy dispersive X-ray analyses. K. petricola/N. punctiforme stimulated release of Ca, Na, Mg and Mn. Analyses of the polished sections confirmed depletion of Ca, Na and K near the surface of the fragments. The abrupt decrease in Ca concentration observed in peripheral areas of plagioclase fragments favored a dissolution-reprecipitation mechanism. Percolation columns in combination with a model biofilm can thus be used to study weathering in closed systems. Columns can easily be filled with different minerals and biofilms, the effluent as well as grains can be collected after long-term exposure under axenic conditions and easily analyzed.
Chemical weathering of silicate minerals consumes atmospheric CO2 and is a fundamental component of geochemical cycles and of the climate system on long timescales. Artificial acceleration of such weathering (“enhanced weathering”) has recently been proposed as a method of mitigating anthropogenic climate change, by adding fine-grained silicate materials to continental surfaces. The efficacy of such intervention in the carbon cycle strongly depends on the mineral dissolution rates that occur, but these rates remain uncertain. Dissolution rates determined from catchment scale investigations are generally several orders of magnitude slower than those predicted from kinetic information derived from laboratory studies. Here we present results from laboratory flow-through dissolution experiments which seek to bridge this observational discrepancy by using columns of soil returned to the laboratory from a field site. We constrain the dissolution rate of olivine added to the top of one of these columns, while maintaining much of the complexity inherent in the soil environment. Continual addition of water to the top of the soil columns, and analysis of elemental composition of waters exiting at the base was conducted for a period of five months, and the solid and leachable composition of the soils was also assessed before and after the experiments. Chemical results indicate clear release of Mg2+ from the dissolution of olivine and, by comparison with a control case, allow the rate of olivine dissolution to be estimated between 10-16.4 and 10-15.5 moles cm-2 s-1. Measurements also allow secondary mineral formation in the soil to be assessed, and suggest that no significant secondary uptake of Mg2+ has occurred. The olivine dissolution rates are intermediate between those of pure laboratory and field studies and provide a useful constraint on weathering processes in natural environments, such as during soil profile deepening or the addition of mineral dust or volcanic ash to soils surfaces. The dissolution rates also provide critical information for the assessment of enhanced weathering including the expected surface-area and energy requirements.
The long-term security of geologic carbon storage is critical to its success and public acceptance. Much of the security risk associated with geological carbon storage stems from its buoyancy. Gaseous and supercritical CO2 are less dense than formation waters, providing a driving force for it to escape back to the surface. This buoyancy can be eliminated by the dissolution of CO2 into water prior to, or during its injection into the subsurface. The dissolution makes it possible to inject into fractured rocks and further enhance mineral storage of CO2 especially if injected into silicate rocks rich in divalent metal cations such as basalts and ultra-mafic rocks. We have demonstrated the dissolution of CO2 into water during its injection into basalt leading to its geologic solubility storage in less than five minutes and potential geologic mineral storage within few years after injection [1], [2] and [3]. The storage potential of CO2 within basaltic rocks is enormous. All the carbon released from burning of all fossil fuel on Earth, 5000 GtC, can theoretically be stored in basaltic rocks [4].
We established a protoplast-based system to transfer DNA to Knufia petricola strain A95, a melanised rock-inhabiting microcolonial fungus that is also a component of a model sub-aerial biofilm (SAB) system. To test whether the desiccation resistant, highly melanised cell walls would hinder protoplast formation, we treated a melanin-minus mutant of A95 as well as the type-strain with a variety of cell-degrading enzymes. Of the different enzymes tested, lysing enzymes from Trichoderma harzianum were most effective in producing protoplasts. This mixture was equally effective on the melanin-minus mutant and the type-strain. Protoplasts produced using lysing enzymes were mixed with polyethyleneglycol (PEG) and plasmid pCB1004 which contains the hygromycin B (HmB) phosphotransferase (hph) gene under the control of the Aspergillus nidulans trpC. Integration and expression of hph into the A95 genome conferred hygromycin resistance upon the transformants. Two weeks after plating out on selective agar containing HmB, the protoplasts developed cell-walls and formed colonies. Transformation frequencies were in the range 36 to 87 transformants per 10 μg of vector DNA and 10(6) protoplasts. Stability of transformation was confirmed by sub-culturing the putative transformants on selective agar containing HmB as well as by PCR-detection of the hph gene in the colonies. The hph gene was stably integrated as shown by five subsequent passages with and without selection pressure.
Silicate minerals represent an important reservoir of essential nutrients at Earth's surface. Due to the slow kinetics of primary silicate mineral dissolution and the potential for nutrient sequestration by secondary mineral precipitation, the bioavailability of many silicate-bound nutrients may be limited by the ability of microorganisms to actively scavenge these nutrients via organic ligand production. In this study, the effectiveness of ligand production as a means to scavenge Fe from Fe-silicates is addressed through targeted laboratory experiments using olivine as a model mineral.
Preliminary results show that microbial Fe-binding ligands (i.e. siderophores) can accelerate olivine dissolution rates stoichiometrically by almost an order of magnitude in experiments buffered at circumneutral pH. In addition to higher reaction rates, organic Fe-binding ligands fostered the accumulation of dissolved Fe in solution, which was below detection in the abiotic experiments due to the precipitation of secondary Fe minerals in the presence of O2. Accelerated olivine dissolution rates in the presence of microbial Fe-binding ligands is somewhat unexpected because these ligands are known to be highly selective towards Fe3+ whereas olivine dominantly contains Fe2+. Spectrophotometric analysis of the ligand complexes produced during reaction with olivine reveals the dominance for Fe3+ -ligand complexes in solution.
The mechanisms and rates of olivine carbonation reactions have been the object of a number of studies, but the thermodynamic limitations and the kinetics of the elementary processes that control the overall reaction are still poorly understood and characterized.The main objective of this study is to probe the effect of Fe on the measured rates of olivine carbonation and its role in the formation of Si-rich surface layers, which can significantly inhibit olivine dissolution and limit the extent of the carbonation reaction. A series of batch and flow-through reactor experiments was conducted in pure water at 90 and 150 °C and under a CO2 partial pressure of 100 and 200 bar, using both a natural sample of Fe-bearing olivine (Fo88) and a synthetic sample of pure forsterite (Fo100). Experimental results show that Fe plays an ambivalent role in the carbonation rates of olivine. On one hand, the presence of Fe favors the formation of Fe–Si-rich protective layers at the interface between olivine and aqueous solution, slowing down the dissolution reaction and limiting the extent of carbonation, whereas pure silica coatings have little to no inhibiting effect on measured carbonation rates. On the other hand, Fe enhances olivine to carbonate conversion rates at low degrees of supersaturation, by promoting the formation of fast precipitating Mg–Fe carbonate solid solutions. The passivating properties of Fe–Si-rich layers originate from the strong Fe(III)–Si interaction and are linked to the permanence of oxidizing conditions in the aqueous fluid. As a consequence, under reducing conditions, olivine carbonation rates can be significantly increased by higher extents of dissolution and by the formation of ferroan magnesites (Mg,Fe)CO3, which nucleate faster than the pure Mg end-member.Forsterite and olivine carbonation reactions can be hindered by the formation of secondary Mg sheet-silicates but, at the conditions studied, the formation of such silicate phases was observed to be transitional and not affecting significantly the rates of carbonation at the end of one-month long experimental runs.This work presents new measurements of olivine carbonation rates and delivers relevant information that suggest new reference criteria for the assessment of the sequestration potential of CO2 repositories and the optimization of the mineral carbonation process in mafic and ultramafic rocks.
Storms are responsible for up to ~50 % of total annual rainfall on tropical islands and result in rapid increases in discharge from rivers. Storm events are, however, notoriously under-sampled and their effects on weathering rates and processes are poorly constrained. To address this, we have undertaken high-frequency sampling of Quiock Creek catchment, a Critical Zone Observatory located in Guadeloupe, over a period of 21 days, encompassing several storm events. Chemical and isotopic (Li and Mg) analyses of different critical zone reservoirs (throughfall, soil pore water, groundwater and river water) were used to assess the interactions between rock, water and secondary minerals. The Li concentrations and δ7Li values of these different reservoirs range from 14 to 95 nmol/kg and 1.8 to 16.8‰, respectively. After several rain events, the average δ7Li value (13.3‰) of soil solutions from the lower part of the soil profile (> ~150 cm below the surface) was unchanged, whereas in the upper part of the profile δ7Li values increased by ~2 - 4‰ due to increased contribution from throughfall. By contrast, the δ26Mg value of soil waters in the upper part of the soil profile were not significantly affected by the rain events with an average value of - 0.90‰. The δ26Mg values of the different fluid reservoirs were generally close to the value of throughfall (~ -0.90‰), but higher δ26Mg values (up to -0.58‰) were measured in the deeper parts of the soil profile, whereas groundwaters that have a long residence 28 time had lower δ26Mg values (down to -1.48‰). These higher and lower values are attributed to, respectively, adsorption/desorption of light Mg isotopes on/from the surface of clay minerals. The δ7Li value of the river waters was ~9.3‰, with a Li concentration of 60 μmol/kg, but during a storm these values decreased to, respectively, 7.8‰ and 40 μmol/kg. This change in δ7Li is consistent with an increased contribution of Li from the soil solution. Thus, even in highly weathered catchments, changes in hydrological conditions can have a significant impact on weathering processes and therefore the composition of river waters delivered to the ocean.
Phlogopite weathering experiments were carried out under aerobic conditions in a closed system to study Mg isotopic fractionation during the dissolution and processes involved in the presence of bacterial strains. Four different bacterial strains with different metabolisms were chosen. Biotic experiments were performed in batch reactors at 24 °C for 12 days. In parallel, abiotic phlogopite weathering experiments were also performed using organic and nitric acids. Citric and gluconic acids were used to model the effects of chelating agents produced by bacteria, and nitric acid was used to model the effects of acidifying agents. The results indicate that both decreases in pH and the production of metabolites during heterotrophic bacterial activities significantly accelerate the release of Mg and Si in solution. Also, at a given pH, the fraction of elements released in solution is greater in the presence of bacteria or citric acid compared to that released in the presence of nitric and gluconic acids. Magnesium isotopic analyses indicate that the solutions obtained using nitric acid and acidifying bacterial strains display, on average, δ²⁶Mg values that are close to or slightly heavier (by 0.4‰) than those of fresh phlogopite (−1.2‰ ± 0.08). In contrast, in experiments performed with citric acid, the Mg leached into solution is slightly enriched in light isotopes by −0.3‰ relative to the initial phlogopite, while its binding to organics is expected to be related to its preferential enrichment in heavy isotopes. Despite its small range of variations, the δ²⁶Mg values of solutions vary inversely with pH, thus suggesting that secondary phases preferentially enriched in ²⁶Mg at the highest pH values may play a key role.
The dissolution rates of olivine have been considered by a plethora of studies in part due to its potential to aid in carbon storage and the ease in obtaining pure samples for laboratory experiments. Due to the relative simplicity of its dissolution mechanism, most of these studies provide mutually consistent results such that a comparison of their rates can provide insight into the reactivity of silicate minerals as a whole. Olivine dissolution is controlled by the breaking of octahedral M²⁺-oxygen bonds at or near the surface, liberating adjoining SiO4⁴⁻ tetrahedra to the aqueous fluid. Aqueous species that adsorb to these bonds apparently accelerate their destruction. For example, the absorption of H⁺, H2O and, at some conditions, selected aqueous organic species will increase olivine dissolution rates. Nevertheless, other factors can slow olivine dissolution rates. Notably, olivine dissolution rates are slowed by lowering the surface area exposed to the reactive aqueous fluid, by for example the presence and/or growth on these surfaces of either microbes or secondary phases. The degree to which secondary phases decelerate rates depends on their ability to limit access of the reactive aqueous fluids to the olivine surface. It seems likely that these surface area limiting processes are very significant in natural systems, lowering olivine surface reactivity in many environments compared to rates measured on cleaned surfaces in the laboratory. A survey of the literature suggests that the major factors influencing forsteritic olivine dissolution rates are 1) pH, 2) water activity, 3) temperature, and 4) mineral-fluid interfacial surface area. Evidence suggests that the effects of aqueous inorganic and organic species are relatively limited, and may be attributed at least in part to their influence on aqueous solution pH. Moreover, the observed decrease in rates due to the presence of secondary mineral coatings and/or the presence of microbes can be attributed to their ability to decrease olivine surface area directly exposed to the reactive aqueous fluid. As the reactivity of forsterite surfaces are spatially heterogeneous, its surface area normalized rates will tend to decrease as it dissolves even in the absence of a mineral or bacterial coating. Each of these factors limits and or influences the application of forsterite dissolution to 1) enhanced weathering efforts, 2) mineral carbonation, and 3) the low temperature generation of hydrogen or hydrocarbons via the oxidation of its divalent iron.
A Mg isotope study of sugar maple (Acer saccharum Marsh.) in a field site in southern Québec, Canada, and seedlings grown in sterile soil substrate in the laboratory, both demonstrate per mil level within-tree Mg isotope fractionation. However, only sugar maple seedlings grown in the laboratory fractionate Mg isotopes during uptake into fine roots, favoring heavy isotope enrichment in the plant compared to the growth medium. Absence of uptake-related Mg isotope fractionation in field stands of sugar maple is tentatively attributed to the activities of the arbuscular mycorrhizal fungi that colonize fine roots of the trees in the field, but were absent from the laboratory grown specimens. The fungi facilitate nutrient uptake for the tree, while the tree provides valuable carbohydrates to the fungi. Without the symbiotic fungi, pot-grown trees in the laboratory are visibly stressed and often die. The mechanisms responsible for Mg isotopic fractionation in stressed trees remain to be elucidated. Rivers are isotopically light compared to bedrock weathering sources of Mg, and this has bearing on the δ²⁶Mg value of the continental weathering flux of Mg to the oceans, which is an important parameter in studies of ocean Mg cycling in the geological past. If uptake-related fractionation is negligible in many other naturally growing tree species, as it is in sugar maple, then forest growth will exert little or no influence on the δ²⁶Mg value of the export flux of Mg to first-order streams and rivers, and in turn the ocean Mg cycle. Above the tree line, preferential retention of heavy Mg isotopes in clay minerals formed during silicate weathering has been linked to the low δ²⁶Mg values in rivers. In the forested catchment of this study there is no clear evidence for these effects. The 1 N HNO3 leach of the Bf-BC and C mineral soils, which are often used to identify minerals that may be releasing Mg and other base cations to plant-available pools, have the same average δ²⁶Mg value (−0.66‰, n = 2) as the litter layer and exchangeable leach of the forest floor, all soil solutions, and the stream (−0.63 ± 0.17‰ 2σ, n = 23). More revealing is the molar Mg/Ca ratio of the 1 N HNO3 treatment (0.17), which is nearly identical to the bulk Mg/Ca ratio of the aboveground biomass (0.14). We conclude that the 1 N HNO3 leach in this setting releases Mg from secondary minerals, such as vermiculite, other clays, and amorphous phases, which have taken up plant-recycled Mg and Ca that has filtered down through the soil from the litter layer of the forest floor. A single mineral, chlorite, with an estimated δ²⁶Mg value of −0.78‰, appears to be responsible for supplying most of the Mg that is circulating between the forest and soils in this setting, which is weathered and cached over timescales of thousands of years.
The model rock-inhabiting microcolonial fungus Knufia petricola fractionates stable Mg isotopes in a time- and pH-dependent manner. During growth, the increase of 26Mg/24Mg in the fungal cells relative to the growth media amounted to 0.65 ± 0.14 ‰ at pH 6 and 1.11 ± 0.35 ‰ at pH 3. We suggest a constant equilibrium fractionation factor during incorporation of Mg into ribosomes and ATP as a cause of enrichment of 26Mg in the cells. We suggest too that the proton gradient across the cell wall and cytoplasmic membrane controls Mg2+ transport into the fungal cell. As the strength of this gradient is a function of extracellular solution pH, the pH-dependence on Mg isotope fractionation is thus due to differences in fungal cell mass fluxes. Through a mass balance model we show that Mg uptake into the fungal cell is not associated with a unique Mg isotope fractionation factor. This Mg isotope fractionation dependence on pH might also be observed in any organism with cells that follow similar Mg uptake and metabolic pathways, and serves to reveal Mg cycling in ecosystems.
Geomycology investigates the impact of fungi on geological processes, including the alteration and weathering of rocks and minerals, mediated both by biomechanical and by biochemical activities. Several functional groups of fungi are involved in mineral bioweathering, including saprotrophic and mycorrhizal fungi. Serpentine sites provide interesting environmental cases for geomycology, because they are naturally enriched in heavy metals and can bear asbestos-rich rocks. An otherwise uncommon species, Verticillium leptobactrum, was found to be abundant in several serpentine sites in the Western Alps. V. leptobactrum and other fungal strains were shown to be able to extract different amounts of iron and magnesium from asbestos in vitro. The amount of iron mobilized from the fibres depended on the fungal activity but also on the physical characteristics of the mineral. Iron is of particular importance because one of the reactions determining asbestos toxicity is the iron-catalyzed generation of free radicals leading to biomolecules oxidation. Asbestos fibres modified in vitro by fungi were less chemically reactive and lost their oxidative activity against DNA.
We investigated the iron isotope signatures of dissolved Fe in the water of the Wettinquelle mineral spring (Bad Brambach, Germany) by time-series sampling covering seismically active periods related to tectonic activity near the Eger Rift system in central Europe. Our objective was to test whether Fe isotopes trace earthquake-induced abiotic and biotic changes in the fluid/rock interaction of the deep, fissured, granitic aquifer. We found that the dissolved Fe isotope signatures in spring water are distinct from the granitic source signature (δ56Fe = +0.09 ‰). Particularly, we discovered that water δ56Fe values are remarkably stable (-0.01 ± 0.11 ‰, 2SD, n = 4) before and during a strong seismic swarm period in 2000 (local magnitudes ML > 3), while O2 and H2 concentrations in water decrease and dissolved Fe content increases. Later, recurring events of lower δ56Fe values down to -0.3 ‰ are observed in the period from 2001 to 2003 with intermittent seismic events (1 < ML < 2.6). The observations indicate a time lag between tectonic forcing and Fe isotope response. The role of abiotic fluid/rock interaction and Fe-utilizing bacteria identified in the mineral spring water on Fe isotope fractionation is discussed. We explain recurring changes towards isotopically lighter values by a combination of Fe dissolution from deep granite and admixture of isotopically light Fe generated by a complex combination of abiotic and biotic processes operating in the aquifer when disturbed by swarm earthquakes events. We propose a conceptual model scenario of earthquake-triggered changes in biogeochemical processes.
Symbiotic ectomycorrhizal fungi mobilise nutrients from both organic and inorganic substrates and supply them to their host plants. Their role in mobilising base cations and phosphorus from mineral substrates through weathering has received increasing attention in recent years but the processes involved remain to be elucidated. We grew selected ectomycorrhizal and nonmycorrhizal fungi in axenic systems containing mineral and organic substrates and examined their capacity to fractionate and assimilate stable isotopes of magnesium. The mycorrhizal fungi were significantly depleted in heavy isotopes with the lowest Δ(26) Mg values (the difference between δ(26) Mg in fungal tissue and δ(26) Mg in the substrate) compared with nonmycorrhizal fungi, when grown on mineral substrates containing granite particles. The ectomycorrhizal fungi accumulated significantly higher concentrations of Mg, K and P than the nonmycorrhizal fungi. There was a highly significant statistical relationship between δ(26) Mg tissue signature and mycelial concentration of Mg, with a clear separation between most ectomycorrhizal fungi and the nonmycorrhizal fungi. These results are consistent with the idea that ectomycorrhizal fungi have evolved efficient mechanisms to mobilise, transport and store Mg within their mycelia. This article is protected by copyright. All rights reserved.
A wide variety of fungi and bacteria are known to contaminate fuels and fuel systems. These
microbial contaminants have been linked to fuel system fouling and corrosion. The fungus
Hormoconis resinae, a common jet fuel contaminant, is used in this study as a model for developing
innovative risk assessment methods. A novel qPCR protocol to detect and quantify H. resinae in, and
together with, total fungal contamination of fuel systems is reported. Two primer sets, targeting the
markers RPB2 and ITS, were selected for their remarkable specificity and sensitivity. These primers
were successfully applied on fungal cultures and diesel samples demonstrating the validity and
reliability of the established qPCR protocol. This novel tool allows clarification of the current role of
H. resinae in fuel contamination cases, as well as providing a technique to detect fungal outbreaks
in fuel systems. This tool can be expanded to other well-known fuel-deteriorating microorganisms.
To link to this article: http://dx.doi.org/10.1080/08927014.2016.1177515
Fungus-mineral interactions play unparalleled roles in shaping the planet Earth but are underappreciated relative to bacterial in uences. Unique to fungus, but largely unknown, are the interfacial processes and extensiveness of hypha- versus spore-mineral interactions given the associated turgor pressure differences and the vast contact areas between mycelia and minerals in the critical zone. Here we examine lizardite [Mg3Si2O5(OH)4] dissolution by single cells of a native fungal strain using confocal laser scanning microscopy, atomic force microscopy, and transmission electron microscopy–energy dispersive X-ray spectroscopy to explore the mechanism, driving force, and magnitude of the interfacial reactions. Results from our inspection showed (1) signi cant pH reduction in the vicinity of cells upon mineral surface attachment, (2) exclusive Fe loss from the mineral at the cell-mineral interfaces, and (3) destruction of the mineral crystal structure below the area colonized by hyphae but not that by spores. Compared to the results from bulk experiments and at the mineral-water interface, these observations indicate that (1) only attached cells release siderophores and (2) biomechanical forces of hyphal growth are indispensable for fungal weathering and strong enough to breach the mineral lattice. Estimated mineral mass loss at the interface suggests that cellular dissolution can ultimately account for ~40%–50% of the overall bio-weathering, signi cantly larger than the previous estimate of ~1% contribution.
Chemical breakdown of rocks, weathering, is an important but very slow part of the carbon cycle that ultimately leads to CO2 being locked up in carbonates on the ocean floor. Artificial acceleration of this carbon sink via distribution of pulverized silicate rocks across terrestrial landscapes may help offset anthropogenic CO2 emissions. We show that idealized enhanced weathering scenarios over less than a third of tropical land could cause significant drawdown of atmospheric CO2 and ameliorate ocean acidification by 2100. Global carbon cycle modelling driven by ensemble Representative Concentration Pathway (RCP) projections of twenty-first-century climate change (RCP8.5, business-as-usual; RCP4.5, medium-level mitigation) indicates that enhanced weathering could lower atmospheric CO2 by 30-300 ppm by 2100, depending mainly on silicate rock application rate (1 kg or 5 kg m-2 yr-1) and composition. At the higher application rate, end-of-century ocean acidification is reversed under RCP4.5 and reduced by about two-thirds under RCP8.5. Additionally, surface ocean aragonite saturation state, a key control on coral calcification rates, is maintained above 3.5 throughout the low latitudes, thereby helping maintain the viability of tropical coral reef ecosystems. However, we highlight major issues of cost, social acceptability, and potential unanticipated consequences that will limit utilization and emphasize the need for urgent efforts to phase down fossil fuel emissions.
The development of complex alteration layers on silicate mineral surfaces undergoing dissolution is a widely observed phenomenon. Given the complexity of these layers, most kinetic models used to predict rates of mineral–fluid interactions do not explicitly consider their formation. As a result, the relationship between the development of the altered layers and the final dissolution rate is poorly understood. To improve our understanding of the relationship between the alteration layer and the dissolution rate, we developed a spatially resolved surface kinetic model for olivine dissolution and applied it to a series of closed-system experiments consisting of three-phases (water (±NaCl), olivine, and supercritical CO2) at conditions relevant to in situ mineral carbonation (i.e. 60 °C, 100 bar CO2). We also measured the corresponding δ26/24Mg of the dissolved Mg during early stages of dissolution. Analysis of the solid reaction products indicates the formation of Mg-depleted layers on the olivine surface as quickly as 2 days after the experiment was started and before the bulk solution reached saturation with respect to amorphous silica. The δ26/24Mg of the dissolved Mg decreased by approximately 0.4‰ in the first stages of the experiment and then approached the value of the initial olivine (−0.35‰) as the steady-state dissolution rate was approached. We attribute the preferential release of 24Mg to a kinetic effect associated with the formation of a Mg-depleted layer that develops as protons exchange for Mg2+.
San Carlos forsterite was dissolved in initially pure H2O in a batch reactor in contact with the atmosphere for 5 years. The reactive fluid aqueous pH remained relatively stable at pH 6.7 throughout the experiment. Aqueous Mg concentration maximized after approximately 2 years time at 3 × 10−5 mol/kg, whereas aqueous Si concentrations increased continuously with time, reaching 2 × 10−5 mol/kg after 5 years. Element release rates closely matched those determined on this same forsterite sample during short-term abiotic open system experiments for the first 10 days, then slowed substantially such that the Mg and Si release rates are approximately an order of magnitude slower than that calculated from the short-term abiotic experiments. Post-experiment analysis reveals that secondary hematite, a substantial biotic community, and minor amorphous silica formed on the dissolving forsterite during the experiment. The biotic community included bacteria, dominated by Rhizobiales (Alphaproteobacteria), and fungi, dominated by Trichocomaceae, that grew in a carbon and nutrient-limited media on the dissolving forsterite. The Mg isotope composition of the reactive fluid was near constant after 2 years but 0.25‰ heavier in δ26Mg than the dissolving forsterite. Together these results suggest long-term forsterite dissolution in natural Earth surface systems maybe substantially slower than that estimated from short-term abiotic experiments due to the growth of biotic communities on their surfaces.
Industrial waste acids such as sulfuric and hydrochloric acid are produced in large quantities. This note deals mainly with waste sulfuric acid, although similar results have been obtained with hydrochloric acid. A method is described by which these acids can be neutralized using crushed olivine rock. This process produces silica. The heavy metals present can be precipitated from the resulting solution. The remaining clean magnesium-sulfate solution can be dumped at sea without adverse environmental consequences. The process has been patented (Dutch patent PCT NL 85/00026), and is being further developed in cooperation with Dutch industries. Silica, one of the major products of the process, has been tested as an additive to concrete. An addition of between 5 and 10% silica greatly decreases the permeability of concrete, thereby increasing the resistance of concrete constructions under chemically aggressive conditions.
Weathering rates of silicate minerals observed in the laboratory are in general up to five orders of magnitude higher than those inferred from field studies. The differences between experimental conditions in the laboratory and natural conditions in the field have been thoroughly discussed in previous studies, however, the discrepancy was never fully resolved. It has been shown in past work that if the field conditions are fully simulated in standard laboratory experiments, it is not possible to measure the slow rates of mineral dissolution that are observed in the field using standard laboratory experiments. Therefore, a novel method that uses the change of Si isotopes ratio in spiked solutions is used in the present study to measure weathering rates of feldspar under close-to-natural conditions.
To unravel the Mg isotope fractionation pathways within the continuous permafrost zone in the larch deciduous forest of Central Siberia, we measured the Mg isotopic composition of two large Siberian rivers (Nizhnaya Tunguska and Kochechum, which flow into the Yenisey), a small forested stream, and the major fluid and solid sources of Mg in the watershed: atmospheric precipitates, surface suprapermafrost flow, interstitial soil solutions, plant biomass, litter and mineral soils. The obtained results indicate a significant seasonal variation in riverine water Mg isotope signatures. During the winter baseflow, the Mg isotope composition of large rivers is significantly lighter than the source basaltic rocks and the atmospheric depositions. These differences support the presence of fluids enriched in lighter Mg isotopes, such as those affected by the mineral precipitation of secondary silicates or fluids that dissolve sedimentary carbonate rocks. During the spring flood and in the summer and fall seasons, the river fluid δ26Mg values increased by 0.2–0.3‰ and approached the Mg isotope composition of the ground vegetation (dwarf shrubs, mosses) and the soil organic horizon. Overall, the riverine waters were 0.3–0.7‰ lighter than the unaltered bedrock and the deep minerals soil horizons.
Terrestrial enhanced weathering, the spreading of ultramafic silicate rock flour to enhance natural weathering rates, has been suggested as part of a strategy to reduce global atmospheric CO2 levels. We budget potential CO2 sequestration against associated CO2 emissions to assess the net CO2 removal of terrestrial enhanced weathering. We combine global spatial datasets of potential source rocks, transport networks and application areas with associated CO2 emissions in an optimistic and a pessimistic scenario. The results show that the choice of source rocks and material comminution technique dominate the CO2-efficiency of enhanced weathering. CO2 emissions from transport amount on average 0.5-3% of potentially sequestered CO2. The emissions of material mining and application are negligible. After accounting for all emissions, 0.5-1.0 t CO2 can be sequestered on average per tonne of rock, translating into a unit cost of 1.6 to 9.9 GJ per tonne CO2 sequestered by enhanced weathering. However, to control or reduce the atmospheric CO2 concentrations substantially with enhanced weathering would require very large amounts of rock. Before enhanced weathering could be applied at large scales, more research is needed to assess weathering rates, potential side effects, social acceptability, and mechanisms of governance.
In order to evaluate the chemistry and kinetics of mineral carbonation reactions under conditions relevant to subsurface injection and storage of CO2, olivine alteration was studied at 60 °C and 100 bar CO2 pressure, including olivine dissolution and the formation of carbonate minerals. Batch experiments were performed with olivine (Fo92), water, CO2, and NaCl inside gold cells contained within rocking autoclaves. Two reproducible experiments yielded an initial (1 hour) dissolution rate of 9.50 ± 0.10 x 10- 11 and a long-term (10–70 days) rate of 1.69 ± 0.23 x 10- 12 mol cm- 2 s- 1. The long-term rate is consistent with previously published rate laws at 4.5 < pH < 5.5. The dissolution rates presented here are constant with increasing pH in the same range, suggesting a pH-independent dissolution mechanism at elevated CO2(aq) and SiO2(aq) at 60 °C. A Si-rich surface layer forms on olivine grains within 2 days of reaction and appears to slow dissolution by passivating the surface. The olivine dissolution rate decreases by 2 orders of magnitude over 4 days as the system approaches amorphous silica saturation but remains constant thereafter based on a linear increase in Mg concentrations. Secondary phases consist of amorphous silica and magnesite, with up to 7 mol% olivine converted to Mg-carbonate over 94 days of reaction. Magnesite precipitation rates could not be precisely quantified due to experimental limitations. However, our minimum estimate of 1.40 x 10- 13 mol cm- 2 s- 1 suggests that the precipitation rate is several orders of magnitude greater than predicted by previous studies. Finally, the presence of 0.5 M NaCl resulted in a decrease in olivine dissolution rate at reaction times of less than 4 days, but a significant enhancement of the reaction rate at reaction times greater than 4 days relative to electrolyte-free experiments. Our results suggest that geochemical models developed to predict the behavior of subsurface CO2 storage systems in mafic and ultramafic rocks should incorporate the effects of dissolved species, including SiO2 and NaCl.
We present elemental and isotopic data detailing how the Mg isotope system behaves in natural and experimentally synthesized clay minerals. We show that the bulk Mg isotopic composition (δ26Mg) of a set of natural illite, montmorillonite and kaolinite spans a 2‰ range, and that their isotopic composition depends strongly on a balance between the relative proportions of structural and exchangeable Mg. After acid leaching, these natural clays become relatively enriched in isotopically heavy Mg by between 0.2 and 1.6‰. Results of exchange experiments indicate that the Mg that has adsorbed to interlayer spaces and surface charged sites is relatively enriched in isotopically light Mg compared to the residual clay. The isotopic composition of this exchangeable Mg (-1.49 to -2.03‰) is characteristic of the isotopic composition of Mg found in many natural waters. Further experiments with an isotopically characterized MgCl2 solution shows that the clay minerals adsorb this exchangeable Mg with little or no isotopic fractionation, although we cannot discount the possibility that the uptake of exchangeable Mg does so with a slight preference for 24Mg. To characterize the behaviour of Mg isotopes during clay mineral formation we synthesized brucite (Mg(OH)2), which we consider to be a good analogue for the incorporation of Mg into the octahedral sheet of Mg-rich clay minerals or into the brucitic layer of clays such as chlorite. In our experiment the brucite mineral becomes enriched in the heavy isotopes of Mg while the corresponding solution is always relatively enriched in isotopically light Mg. The system reaches a steady state after 10 days with a final fractionation factor (αsolid-solution) of 1.0005 at near-neutral pH. This result is consistent with the general consensus that secondary clay minerals preferentially take up isotopically heavy Mg during their formation. However our results also show that exchangeable Mg is an important component within bulk clay minerals and can have an important influence over the bulk clay δ26Mg value. Modeling shows that in certain soils or sediments where the percentage of exchangeable Mg is >40% and the isotopic composition of the exchangeable Mg is around -2‰, the generation of bulk δ26Mg values of <-0.5‰ is likely. On a broader scale, Mg-rich minerals such as smectite and illite are likely to impart a stronger control over the Mg budget in clay rich sediments, and their high structural Mg component is likely to result in bulk sediment δ26Mg values that are closer in composition to the UCC. Despite this, results of modeling, together with experimental observation suggests that the uptake of exchangeable Mg into these clay rich sediments could cause a decrease in the bulk δ26Mg value by up to ∼0.3-0.4‰. This should be accounted for when assessing the δ26Mg value of sediments on a crustal scale.
Understanding the biogeochemical cycle of magnesium (Mg) is not only crucial for terrestrial ecology, as this element is a key nutrient for plants, but also for quantifying chemical weathering fluxes of Mg and associated atmospheric CO2 consumption, requiring distinction of biotic from abiotic contributions to Mg fluxes exported to the hydrosphere. Here, Mg isotope compositions are reported for parent basalt, bulk soils, clay fractions, exchangeable Mg, seasonal soil solutions, and vegetation for five types of volcanic soils in Iceland in order to improve the understanding of sources and processes controlling Mg supply to vegetation and export to the hydrosphere. Bulk soils (δ26Mg = −0.40 ± 0.11‰) are isotopically similar to the parent basalt (δ26Mg = −0.31‰), whereas clay fractions (δ26Mg = −0.62 ± 0.12‰), exchangeable Mg (δ26Mg = −0.75 ± 0.14‰), and soil solutions (δ26Mg = −0.89 ± 0.16‰) are all isotopically lighter than the basalt. These compositions can be explained by a combination of mixing and isotope fractionation processes on the soil exchange complex. Successive adsorption–desorption of heavy Mg isotopes leads to the preferential loss of heavy Mg from the soil profile, leaving soils with light Mg isotope compositions relative to the parent basalt. Additionally, external contributions from sea spray and organic matter decomposition result in a mixture of Mg sources on the soil exchange complex. Vegetation preferentially takes up heavy Mg from the soil exchange complex (Δ26Mgplant-exch = +0.50 ± 0.09‰), and changes in δ26Mg in vegetation reflect changes in bioavailable Mg sources in soils. This study highlights the major role of Mg retention on the soil exchange complex amongst the factors controlling Mg isotope variations in soils and soil solutions, and demonstrates that Mg isotopes provide a valuable tool for monitoring biotic and abiotic contributions of Mg that is bioavailable for plants and is exported to the hydrosphere.
When rock is converted to weathering products, the involved processes can be fingerprinted using the stable isotope ratios of metals (for example Li, Mg, Ca, Fe, Sr) and metalloids (B, Si). Here we construct a framework for interpreting these “novel” stable isotope ratios quantitatively in the compartments of the weathering zone in a geomorphic context. The approach is applicable to any novel stable isotope system and is based on a simple steady-state mass balance model that represents the weathering zone from the scale of a soil column to that of entire continents. Our model is based on the assumption that the two main processes associated with isotope fractionation are formation of secondary precipitates such as clays, and uptake of nutrients by plants.
The model results show that the isotope composition of a given element in the weathering zone compartments depends on (1) the ratio between the release flux to water through primary mineral dissolution and the erosion flux of isotopically fractionated solid material, consisting of secondary precipitates and organic matter; (2) the isotope fractionation factors associated with secondary mineral precipitation and uptake by plants. A relationship is established between isotope ratios, isotope fractionation factors, and indexes for chemical weathering [such as chemical depletion fractions (CDF) and elemental mass transfer coefficients (τ)] derived from simple elemental concentration measurements. From this relationship, isotope fractionation factors can be calibrated from chemical and isotope data measured on field material. Furthermore, we show how the ratio of solid export to dissolved export of a given element from the weathering system can be estimated from the comparison of the isotope composition between bedrock, water, and sediment. This calculation can be applied to samples from soils, from rivers, and from the sedimentary record, and does not require knowing the isotope fractionation factors involved in the reactions. Finally, we apply the model to the oceanic Li isotope record reconstructed from marine carbonate sediments in order to discuss changes in global geomorphic regimes through the Cenozoic.
Li, B, Mg and Ca isotopes became of increasing interest during the last decade due to their potential for better constraining the carbon cycle and nutrient cycling. At the soil-water-plant scale, Li and B isotopes are powerful tools for the understanding of processes leading to clay mineral formation in soils. Ca and Mg isotopes allow, for their part, to identify plant-mineral interactions and recycling by vegetation. At the scale of monolithological silicate watersheds, Li and B isotope fractionations are mainly controlled by the degree of mineral leaching and the amount of clay mineral formation. Ca and Mg isotope signatures in soil and waters vary seasonally, depending on the vegetation growth cycle and rain events. In mixed-lithology basins, B and Li isotopes are controlled by alteration rates of silicate minerals and the residence time of waters within the watershed. Ca and Mg isotope ratios of river waters appear to be also lithology-controlled.
This study provides direct experimental evidence of magnesium (Mg) isotope fractionation between an aqueous fluid and magnesite during its congruent dissolution, precipitation and at equilibrium. Closed-system batch reactor experiments were performed at temperatures from 120 to 200 °C and at 15 or 30 bar CO2 pressure. During congruent magnesite dissolution the fluid became enriched in isotopically heavy Mg, with a steady-state δ26Mgfluid composition that was 0.4‰ higher than the dissolving magnesite at 15 bar of CO2 pressure and 0.15‰ higher at 30 bar of CO2 pressure. Magnesite precipitation was provoked by increasing the reactor temperature after equilibrium had been attained via dissolution. Rayleigh isotope fractionation effects were observed immediately after the reactor temperature was increased and rapid magnesite precipitation occurred. However, isotopic exchange continued as the system equilibrated, eradicating this Rayleigh signal. The equilibrium 26Mg/24Mg fractionation factors (αeqm) for the magnesite-fluid system were found to be 0.99881 at 150 °C and 0.99912 at 200 °C. Taken together, these observations (1) support the theoretical hypothesis that mineral-fluid equilibrium is dynamic (i.e. dissolution and precipitation occur at equal, non-zero rates at equilibrium), and (2) indicate that isotopes will continue to exchange and equilibrate even if the mineral surfaces and co-existing fluids are in chemical equilibrium. The fact that isotopes continue to exchange at chemical equilibrium will tend to eradicate both kinetic and paleo-environmental isotopic signatures, and the degree to which such signatures are completely eradicated depends on how deep into the surface the isotopic exchange process penetrates.
This paper demonstrates a method for systematic analysis of published mineral dissolution rate data using forsterite dissolution as an example. The steps of the method are: (1) identify the data sources, (2) select the data, (3) tabulate the data, (4) analyze the data to produce a model, and (5) report the results. This method allows for a combination of critical selection of data, based on expert knowledge of theoretical expectations and experimental pitfalls, and meta-analysis of the data using statistical methods.
The influence of soil type and soil horizon on the distribution of microfungi in five natural grassland soils at Lakenheath Warren has been investigated. The soils, which are all very dry and sandy, vary from a shallow, highly alkaline soil to a deep, highly acid podsol.
One hundred and forty-eight species of fungi have been isolated from these soils by the soil-plate method.The most common genera are Penicillium and Mortierella, followed by Absidia, Cephalosporium, Fusarium, Gliocladium, Gliomastix, Mucor, Thielavia, Trichoderma and Zygorrkynchus. The occurrence of fungal species in twenty profile samples of each soil has been recorded, and the distribution of the more important species is discussed in detail.
It has been shown that the number of fungal species and colonies in a profile falls off with depth, the rate of reduction being dependent on the depth of the soil and the nature of the soil horizons. Most species occur most abundantly in the surface layers of the soil. A few species were commoner in subsurface layers, and a few were abundant near the chalky boulder clay of the C horizons. Fungi show different distributions in the five soils, and two large groups may be distinguished: those common in the acid soils and those common in the alkaline soils. Species of Penicillium were particularly abundant in the acid soils.
Microscopic examination of soil has shown that fungi are present both as spores and mycelium. Study of incubated soil plates stained with lactophenol cotton blue has shown that most of the colonies develop from humus particles. Viable spores have also been found in these soils, especially the acid ones.
The dissolution rates of Fo100 and Fo91 in aqueous solutions in the pH range 2–12.4 at 25°C have been measured using fluidized bed and batch reactors. Rates depend upon the pH, the partial pressure of CO2, and the presence of organic ligands. At low PCO2 (≤10−4.5 atm) with no organic ligands present, the rate of olivine dissolution, R, is given by , where aH+ is the activity of H+ in solution. However in basic solutions, when the partial pressure of CO2 is equal to atmospheric levels (PCO2 = 10−3.5 atm), olivine dissolution rates are nearly pH independent throughout the pH range 6–12 and are about equal to the minimum rate of dissolution under CO2 purged conditions. At pH 11 the presence of atmospheric levels of CO2 reduces the dissolution rate by over an order of magnitude (to 10−14.1 mol cm−2 s−1). Apparently, positive charge on the olivine surface can be neutralized by increasing PCO2. In contrast, experiments conducted in the acidic and near neutral pH ranges indicate that organic ligands chelate surface Mg causing an increase in the olivine dissolution rate when present. Organic ligand effects are greatest in the near neutral pH domain. For example, at pH 4 dissolution rates are increased by 0.75 log units (to 10−12.25 mol cm−2 s−1) in solutions of 10−3 molar ascorbic acid or 0.05 molar potassium acid phthalate over rates measured in organic free solutions. The chelation effect becomes less important as pH decreases. Rates at pH 2 in the presence of these organic ligands are indistinguishable from those measured without organics.
This study investigates the potential of Mg isotopes as tracers of biogeochemical processes in a small-forested catchment located on sandstones extremely poor in Mg-bearing minerals. The average d 26 Mg is À0.63 ± 0.12& and 0 ± 0.14& for local rainwater and bedrock, respectively. From the C horizon to the upper eluvial (E) horizon, soil d 26 Mg (from 0.0 ± 0.14& to 0.25 ± 0.14&) is close to the bedrock value, while more than 70% of Mg is lost, suggesting a small isotopic shift during illite dissolution. The surface soil horizon (A h) d 26 Mg is close to plant d 26 Mg, and especially to the grass d 26 Mg value (À0.49&). The bulk d 26 Mg of trees and grass (À0.32& and À0.41&, respectively) are higher than the average d 26 Mg values of the soil exchangeable fraction (À0.92& to À0.42&), and of rainwater (À0.65&). Within plants, roots are enriched in heavy isotopes, whereas light isotopes are preferentially translocated and stored in the above ground parts. In Norway spruce, the older nee-dles, forming the annual litterfall, are isotopically lighter and strongly depleted in Mg compared to more recent needles. Soil solution d 26 Mg shifts seasonally, from low values, lower than rainwater and close to litterfall during a high rainfall period in spring, to higher values, close to soil d 26 Mg in dryer periods of winter or summer. At the watershed scale, streamwater d 26 Mg varies between À0.85 ± 0.14& and À0.08 ± 0.14& and d 26 Mg values decrease linearly with discharge. The high streamwater d 26 Mg at low flow, close to bedrock d 26 Mg, most likely reflects dissolution processes in the deep saprolite in relation to the very long water residence time. Conversely, we suggest that low stream level d 26 Mg values are at least partly related to the contribution of surface flows from wet areas. Using a simple mass and isotopic balance approach, we compute that mineral dissolution rates in the soil (0.35 kg Mg ha À1 year À1) presently compensate for Mg losses from the soil.
A surface reaction kinetic model is developed for predicting Ca isotope fractionation and metal/Ca ratios of calcite as a function of rate of precipitation from aqueous solution. The model is based on the requirements for dynamic equilibrium; i.e. proximity to equilibrium conditions is determined by the ratio of the net precipitation rate (Rp) to the gross forward precipitation rate (Rf), for conditions where ionic transport to the growing crystal surface is not rate-limiting. The value of Rp has been experimentally measured under varying conditions, but the magnitude of Rf is not generally known, and may depend on several factors. It is posited that, for systems with no trace constituents that alter the surface chemistry, Rf can be estimated from the bulk far-from-equilibrium dissolution rate of calcite (Rb or kb), since at equilibrium Rf=Rb, and Rp=0. Hence it can be inferred that Rf≈Rp+Rb. The dissolution rate of pure calcite is measureable and is known to be a function of temperature and pH. At given temperature and pH, equilibrium precipitation is approached when Rp (=Rf−Rb)≪Rb. For precipitation rates high enough that Rp≫Rb, both isotopic and trace element partitioning are controlled by the kinetics of ion attachment to the mineral surface, which tend to favor more rapid incorporation of the light isotopes of Ca and discriminate weakly between trace metals and Ca. With varying precipitation rate, a transition region between equilibrium and kinetic control occurs near Rp≈Rb for Ca isotopic fractionation. According to this model, Ca isotopic data can be used to estimate Rf for calcite precipitation. Mechanistic models for calcite precipitation indicate that the molecular exchange rate is not constant at constant T and pH, but rather is dependent also on solution saturation state and hence Rp. Allowing Rb to vary as Rp1/2, consistent with available precipitation rate studies, produces a better fit to some trace element and isotopic data than a model where Rb is constant. This model can account for most of the experimental data in the literature on the dependence of 44Ca/40Ca and metal/Ca fractionation in calcite as a function of precipitation rate and temperature, and also accounts for 18O/16O variations with some assumptions. The apparent temperature dependence of Ca isotope fractionation in calcite may stem from the dependence of Rb on temperature; there should be analogous pH dependence at pH
For mitigating against rising levels of atmospheric CO2, carbonation of M2+-bearing silicates has been proposed as a possible option for sequestering CO2 over long time spans. Due to its rapid far-from-equilibrium dissolution rate and its widespread occurrence in mafic and ultramafic rocks, olivine has been suggested as a potentially good candidate for achieving this goal, although the efficacy of the carbonation reaction still needs to be assessed. With this as a goal, the present study aims at measuring the carbonation rate of San Carlos olivine in batch experiments at 90°C and pCO2 of 20 and 25MPa.When the reaction was initiated in pure water, the kinetics of olivine dissolution was controlled by the degree of saturation of the bulk solution with respect to amorphous silica. This yet unrecognized effect for olivine was responsible for a decrease of the dissolution rate by over two orders of magnitude. In long-term (45days) carbonation experiments with a high surface area to solution volume ratio (SA/V=24,600m−1), the final composition of the solution was close to equilibrium with respect to SiO2(am), independent of the initial concentration of dissolved salts (NaCl and NaClO4, ranging between 0 and 1m), and with an aqueous Mg/Si ratio close to that of olivine. No secondary phase other than a ubiquitous thin (≤40nm), Si-rich amorphous layer was observed. These results are at odds with classic kinetic modeling of the process. Due to experimental uncertainties, it was not possible to determine precisely the dissolution rate of olivine after 45days, but the long term alteration of olivine was indirectly estimated to be at least 4 orders of magnitude slower than predicted.Taken together, these results suggest that the formation of amorphous silica layers plays an important role in controlling the rate of olivine dissolution by passivating the surface of olivine, an effect which has yet to be quantified and incorporated into standard reactive-transport codes.
In the Earth's lithosphere, fungi are of fundamental importance as decomposer organisms, animal and plant pathogens and symbionts (e.g. lichens and mycorrhizas), being ubiquitous in sub-aerial and subsoil environments. The ability of fungi to interact with minerals, metals, metalloids and organic compounds through biomechanical and biochemical processes, makes them ideally suited as biological weathering agents of rock and building stone. They also play a fundamental role in biogeochemical cycling of nutrients, (e.g. C, N, P and S) and metals (e.g. Na, Mg, Ca, Mn, Fe, Cu, Zn, Co and Ni) essential for the growth of living organisms in the biosphere. In addition they play an integral role in the mobilization and immobilization of non-essential metals (e.g. Cs, Al, Cd, Hg and Pb). Most studies on mineral-microbe interactions and microbial involvement in geological processes have concentrated on bacteria and archaea (Prokaryota): fungi (Eukaryota) have, to a certain extent, been neglected. This article addresses the role of fungi in geomicrobiological processes, emphasizing their deteriorative potential on rock, building stone and mineral surfaces and involvement in the formation of secondary mycogenic minerals. Such roles of fungi are also of importance for the global carbon reservoir and have potential biotechnological applications, e.g. in the bioremediation of xenobiotic-, metal- and/or radionuclide-contaminated soils and wastes, and metal/radionuclide recovery.
Fungal colonization of sandstone and granite from Antarctica was studied. Granite from a church, sandstones from a monument and a courthouse in Germany, glazed bricks from a German cathedral, and some other stone types were also examined. All samples contained fungi and heterotrophic bacteria, often also cyanobacteria or algae. Fungal genera identified were Alternaria, Aspergillus, Aureobasidium, Candida, Cladosporium, Paecilomyces, Phoma, Penicillium, and Sporobolomyces. Scanning electron microscopy revealed fungal bridging of open spaces with their hyphae or clse contact between fungal hyphae and coccal cells believed to be algae. -from Authors