Simulated Diagenesis Research Articles

Simulated diagenesis of the iron-silica precipitates in banded iron formations

Abstract Banded iron formations (BIF) are chemically precipitated sediments that can record Archean ocean geochemistry. BIFs are laminated silica- and iron-rich deposits that host a range of iron(II, III) minerals, including hematite, magnetite, siderite, greenalite, minnesotaite, and stilpnomelane. This diverse mineralogical assemblage reflects secondary mineralization reactions due to diagenesis and/or post-depositional alteration. While petrographic observations of BIFs sparingly contain the iron silicate greenalite, recent evidence of greenalite nanoparticles preserved in early-mineralizing BIF chert suggest this mineral was a primary phase in BIF progenitor sediments. Therefore, it is critical to investigate the formation and alteration of greenalite to constrain the Archean ocean environment and help unravel post-depositional processes. To examine how iron silicates precipitate and then crystallize and/or transform during diagenesis, we simulated these two processes under Archean ocean conditions. We first precipitated a poorly ordered Fe-rich serpentine with subsidiary ferrihydrite at neutral pH by performing in situ Fe(II) oxidation experiments at 25 °C in the presence of silica. Subjected to simulated diagenesis at 80 °C, the rudimentary Fe-phyllosilicate transformed into a crystalline phyllosilicate characterized as 30% cronstedtite and 70% greenalite accompanied by magnetite and persistent ferrihydrite. At temperatures ≤150 °C, we continued to observe ferrihydrite, increased magnetite formation, and elevated incorporation of Mg into the phyllosilicate as it further recrystallized into Mg-greenalite. Our findings demonstrate a possible formation mechanism of early silicates through partial Fe(II) oxidation and support petrographic observations that magnetite likely mineralizes during diagenesis. Additionally, we suggest that Mg contents in BIF iron phyllosilicates could serve as a tracer for diagenesis, with Mg signaling phyllosilicate-fluid interactions at elevated temperatures. Ultimately, our experiments help reveal how initial iron-silica coprecipitates are altered during diagenesis, providing novel insights into the interpretation of greenalite and magnetite in ancient BIF assemblages.

Simulated Diagenesis of the Iron-Silica Precipitates in Banded Iron Formations

Simulated Diagenesis of the Iron-Silica Precipitates in Banded Iron Formations

Effects of simulated diagenesis and mineral amendment on the structure, stability and imidacloprid sorption properties of biochars produced at varied temperatures

In order to successfully apply biochar to soil remediation such as immobilizing contaminants and sequestering carbon, the prerequisite is to maintain its strong sorption capacity for contaminants and stability after long-term aging. Simulated diagenesis was performed here to mimic real geochemical aging process of biochars with different mineral content. The results showed that biochars exhibited increasing aromaticity, thermal and chemical oxidation resistance, microporosity, but declined surface and total polarity after direct diagenesis. The involvement of minerals during diagenesis further increased the surface Fe, Al, and Si content, as well as surface polarity and thermal stability compared to direct diagenesis. Other property changes induced by mineral involvement were dependent on original biochar structure and mineral type. Diagenetic treatment slightly improved the sorption capacity of 450 and 600 °C biochars, regardless of mineral amendment. Only marginal effects were observed relating to their sorption capacity, while the structure of 250 °C biochars was most profoundly affected during diagenesis. Consequently, the simulated geochemical aging had positive effects on biochars long-term stability and ability to immobilize pollutants.

Effect of simulated diagenesis on the compositions, chemical stability and sorption properties of natural and engineered organic matter with different mineral contents

Despite the abundance of diagenetically derived organic materials in the environment, the effects of diagenesis on the structure and some of the associated properties (e.g., stability and sorption behavior) of organic matter (OM) remain unclear. Here, subcritical water treatment, suggested previously to mimic the diagenesis process, was chosen to further investigate the impact of diagenesis on OM compositions, stability and sorption of selected hydrophobic organic compounds. Humic acids and biochars were selected as representatives of natural and engineered OM, respectively. To examine the impact of mineral constituents on the diagenesis of OM, de-ashed samples, as well as samples amended with minerals (kaolinite, calcite and calcium dihydrogen phosphate), were included in the diagenesis treatment system. Comparison of OM composition before and after treatment indicated that simulated diagenesis resulted in lower bulk polarity, higher surface polarity and aromaticity and greater microporosity. Thermal analysis and chemical oxidation suggested that as a result of the increase in aromaticity and decrease in O/C ratio of OM, the resistance of OM to thermal and chemical oxidation was enhanced after simulated diagenesis. Moreover, our diagenesis treatment of OM induced stronger sorption nonlinearity and higher sorption capacity, for phenanthrene. Additionally, minerals protected the structure of OM from being changed by simulated diagenesis. Consequently, with regard to the susceptibility of OM to oxidative decay, the presence of minerals mitigated the increase in chemical stability imparted by simulated diagenesis but, on the other hand, protected OM from degradation.

Limited Zn and Ni mobility during simulated iron formation diagenesis

Iron formations (IF) are iron- and silica-rich chemical precipitates that were deposited during the Precambrian. Several recent studies have demonstrated how the trace metal abundances in IF can be used as proxies for the bioavailability of trace metals in ancient seawater; with the ultimate goal being to understand first-order controls on the composition of the ancient biosphere. However, the utility of IF as proxies depends on the immobilization of trace metals during diagenesis. Here, we assess the mobility of Zn and Ni from ferric oxyhydroxides (ferrihydrite) in the absence and presence of organic matter (glucose) during simulated diagenesis (170°C, 1.2kbar); similar to what some Precambrian IF experienced. Quantitative concentration data, coupled with X-ray diffraction analysis and electron microprobe element mapping, demonstrate that both metals are relatively immobile during simulated diagenesis. Additionally, the mechanism for initial Ni sorption is examined using X-ray adsorption spectroscopy. For the initial sorption of trace elements in abiotic ferrihydrite experiments, 93.38% Zn and 65.95% Ni were initially sorbed. In experiments utilizing biogenic ferrihydrite, 97.03% of Zn and 93.38% of Ni were initially sorbed. Following the diagenetic capsule treatments, more than 99% of Zn and more than 91.9% of Ni were retained under the varied conditions considered here. Capsule experiments suggest the strong retention of Zn and Ni following the diagenesis of either abiotic or biogenic ferrihydrite. Overall, our results indicate that paleomarine Zn and Ni concentrations are likely to be faithfully recorded in well-preserved IF deposits.

Impact of the Simulated Diagenesis on Sorption of Naphthalene and 1-Naphthol by Soil Organic Matter and its Precursors

Soil organic matter (SOM) in a peat soil, humic acid, and humin and their precursors (i.e., cellulose and lignin) were treated at high temperature (250 and 400 °C) with high pressure in a sealed platinum reaction kittle to simulate the influence of diagenesis on their composition and structure, and impact of the simulated diagenesis on sorption behaviors of hydrophobic organic compounds (HOCs) (i.e., naphthalene and 1-naphthol) by these samples was investigated. High temperature and pressure treatment greatly influenced chemical composition and physical properties of the original samples and their sorption for both naphthalene and 1-naphthol. Sorption of naphthalene by all samples was jointly regulated by hydrophobic and π-π interactions with their alkyl and aromatic carbon moieties, which was derived from the positive correlation between total hydrophobic carbon content of all sorbents and their organic carbon content-normalized sorption coefficients (Koc) for this compound (p = 0.075). However, sorption of 1-naphthol by the tested sorbents was governed by hydrogen bonding with their O-containing polar functionalities, as derived from the positive correlation between Koc values of 1-Naph and their polarity index ((O+N)/C). Difference in sorption mechanisms of naphthalene and 1-naphthol by the original and treated samples noted the great influence of chemical composition of sorbates on their interaction and essential roles of specific interactions (e.g., hydrogen bonding) in sorption of polar compound (i.e., 1-naphthol) to these sorbents. Surface area (SA) and porosity data of sorbents obtained from N2 sorption-desorption isotherms at 77 K showed that new SA and pores were created during the diagenetic process of all original samples, which provided substantial sorption sites and thus enhanced sorption of naphthalene and 1-naphthol. Among all tested samples, physicochemical properties of cellulose were most strongly affected by the simulated diagenetic process, and impact of such a process on its sorption intensity for the tested compounds was the most significant. The characterization data of the treated sorbents showed that the high temperature and pressure treatment similarly simulated the naturally occurring diagenesis of SOMs and their precursors, which is a first attempt. These findings are valuable for better understanding of the sorption behaviors of HOCs to SOM and its precursors as affected by diagenesis, which in turn is critical for elucidating the transport and fate of HOCs in the environment.

Formation of a polyalkyl macromolecule from the hydrolysable component within sporopollenin during heating/pyrolysis experiments with Lycopodium spores

The most resistant component of Lycopodium spores is the macromolecule sporopollenin. The recent and fossil representatives of this material are structurally distinct and the transformations that bring about this chemical discord are poorly understood. To investigate the diagenesis of spores and their biopolymer, solvent extracted and saponified examples of Lycopodium clavatum underwent simulated diagenesis by heating (100–400°C) under vacuum for 48h. Following simulated maturation, spores were analysed by pyrolysis-gas chromatography–mass spectrometry (Py-GC–MS) and thermochemolysis-GC–MS. Py-GC–MS data clearly demonstrate that there is an increase in the polyalkyl hydrocarbon material in the pyrolysable component with increasing anhydrous maturation temperature. Hydrous pyrolysis of spores leads to similar changes but with an increased response from aliphatic relative to aromatic material. If the spores are hydrolysed prior to heating the generation of the polyalkyl portion of the macromolecule is markedly reduced. It appears, therefore, that the polyalkyl portion of fossil sporopollenin may be formed by maturation-induced polymerisation of the ‘labile’ hydrolysable component to form a recalcitrant polyalkyl network.

Stable Isotope Values of Bone Organic Matter: Artificial Diagenesis Experiments and Paleoecology of Natural Trap Cave, Wyoming

The presence of original organic matter and retention of an indigenous isotopic signal in fossils have been disputed for years. An experiment was conducted to evaluate the influence of diagenesis on bone-protein isotope values, analyses were conducted on Holocene and Pleistocene fossils from Natural Trap Cave (NTC), Wyoming. Modern cow, Bos taurus, bone was heated with and without excess water for up to 195 hours at 100°C in an inert atmosphere. Collagen and non-collagenous proteins (NCP) were extracted and analyzed isotopically. Under dry conditions, carbon and nitrogen isotope values change by less than 0.4‰ during the 0 to 195 hour interval. In the presence of excess water, carbon and nitrogen isotope values change by no more than 1.0‰ and 0.5‰, respectively, over 192 hours. The relative abundance of amino acids of collagen from heated bone differs by less than 10% from that of unheated collagen. Protein preservation is indicated by matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) that strongly indicates a portion of the original osteocalcin exists intact in samples heated for 195 hours. Good preservation of collagen in NTC fossils is implied by high collagen yields, C:N, and realistic trophic structures based on isotope values. Carbon and nitrogen isotope values of ancient collagen increase with trophic level, allow dietary assessments to be made, and differentiate between ruminant and non-ruminants. The results indicate that isotope values are resilient during simulated diagenesis and suggest that an indigenous isotopic signal can exist in well-preserved fossils such as those from NTC.

Simulated diagenesis of plant cuticles — implications for organic fossilisation

The possible origin of the characteristic compositions of pyrolysates of organic fossil leaf cuticles has been investigated by heat treatment of cuticles of recent Kalanchoe grandiflora followed by flash pyrolysis–gas chromatography. Dewaxed cuticles, whether heat-treated or not, yield mainly cyclic compounds from adventitious cell-wall material together with cutin monomers. The pyrolysate of waxy cuticle includes the same compounds but is dominated by wax components which are simply desorbed unchanged under the conditions of pyrolysis. However prior heat treatment of waxy cuticle transforms the pyrolysate so that its chromatogram shows a prominent series of alkene–alkane peaks from C 8 to C 34, typical of fossil leaf cuticles. It is concluded that leaf waxes, instead of or in addition to cutan, are a principal source of the aliphatic constituents of organic fossil leaf cuticles.

A distributed reactivity model for sorption by soils and sediments 13. Simulated diagenesis of natural sediment organic matter and its impact on sorption/desorption equilibria.

Subcritical water treatment was used to effect rapid compositional and functional changes to peat organic matter that mimic those of the natural diagenesis process. Elemental, solid state 13C NMR, FTIR, and calorimetry analyses all indicated that the organic matter of the artificially aged peat was chemically similar to that of geologically mature coal kerogens. This paper extends the work of the previous paper in this series, which investigated the effects of subcritical water treatment of humic topsoil on subsequent phenanthrene sorption and desorption equilibria. As opposed to the previous study, however, changes in sorptive reactivity herein were unequivocally related to changes in organic matter rather than other soil constituents, and organic matter functional changes due to the simulated diagenesis were more accurately characterized. Phenanthrene sorption capacity and isotherm nonlinearity both increased with increasing degrees of artificial aging, supporting the viewpoint that hydrophobic organic contaminant sorption equilibrium properties can be directly related to the degree of diagenesis of geosorbent organic matter. In addition, this work investigated effects of subcritical water treatment of a geologically mature, kerogen-containing shale sample. In contrast to the peat, the functional characteristics of the shale were unchanged by this treatment, and subsequent phenanthrene sorption equilibria were altered far less.

A Distributed Reactivity Model for Sorption by Soils and Sediments. 12. Effects of Subcritical Water Extraction and Alterations of Soil Organic Matter on Sorption Equilibria

Subcritical water extraction was used as a tool to remove the carboxylic, aliphatic, and carbohydrate types of organic carbon from a humic soil. The rates and extents of soil organic carbon removal were quantified as functions of superheated water temperature, phase, and exposure time. The experimental data suggest that superheated water effects deoxygenation/aromatization reactions of soil organic matter that mimic those of geologically slow, natural diagenesis processes. Phenanthrene sorption and desorption equilibrium isotherms for the altered soils were measured. The sorption isotherms were characterized by increasing capacity and nonlinearity with increasing degree of polar functionality removal and simulated diagenesis of the soil organic matter.

Oxygen index as an indicator of early organic maturity

Most maturity indices can be used during catagenesis and metagenesis but there are only a few indicators to show the degree of organic maturation within the diagenesis zone. A previous study has already demonstrated that the oxygen index (OI) measured by Rock-Eval analysis provides reliable information about the early evolution stages of coals and type III kerogens. The main purpose of this study was to investigate the conditions of OI application in both natural and artificial evolution. The results of experiments presented here revealed that the accuracy of OI in estimating the evolution level depended on the amount and chemical composition of the organic matter. Natural series of low-rank coals showed OI to be a valuable rank parameter not only during diagenesis but in early catagenesis also. OI appeared to be a sensitive indicator during both simulated diagenesis and the entire oil window for gas-prone coal having a substantial humic contribution to its precursor material. In contrast, the OI of oil-prone coal and coal with mixed oil and gas potential proved to be a valuable rank parameter only up to the beginning of the oil window. Though it could be utilized successfully for describing depth profiles in organic-rich intervals of early maturity in wells, its use in determining natural maturity was restricted by the organic-richness of rocks, as well as by the effect of the mineral matrix. Providing humic material is present, OI can be used to follow the artificial thermal maturation of type II kerogen in the absence of mineral matrix.

Simulated diagenesis and catagenesis of marine kerogen precursors: Melanoidins as model systems for light hydrocarbon generation

Amino acids and sugars are principal constituents of marine organisms. The condensation of amino acids and sugars is one possible nonenzymatic, early diagenetic pathway for the incorporation of these compounds into more complex geopolymers. In this study, aqueous solutions consisting of l-lysine, l-histidine, l-arginine and d-(+)-glucose were heated (100°C) for up to 288 h. Portions of the melanoidin polymer isolated after heating were reheated in the presence of water (hydrous pyrolysis) for 72 h at 325°C. Reaction products were identified by GC and GC/MS. Stable isotopic ( δ 13C) and elemental analyses were used to follow thermal evolution. While the initial melanoidin was synthesized from a simple, four component system, the products generated during hydrous pyrolysis are of considerable complexity, and include straightchain and branched alkanes, alkadienes, alkynes, indole, dimethyl indane, ethyl phenol, quinoline, and xylenes, in addition to a multitude of as yet, unidentified components. Stable carbon isotopic values for the reactants and products correspond to trends observed for naturally generated geopolymers and light gases. Elemental analyses of the melanoidin prior and subsequent to hydrous pyrolysis indicate a kerogen evolution pathway similar to that observed for natural samples. Considering the intractable nature of kerogen, laboratory simulation studies of simple systems can provide an alternative approach for elucidating the origins of geopolymers and their potential for hydrocarbon generation.

Analysis and simulated diagenesis of kerogen in a Recent bottom mud from Mono Lake, California: a comparison with selected ancient kerogens

The insoluble organic matter, or kerogen, in a Recent bottom mud (<1000yr old) from Mono Lake, California, has been analyzed by vacuum pyrolysis-GC-MS and compared with the kerogen from several Precambrian rocks, including the Belingwe and Transvaal stromatolites. The Mono Lake kerogen consists mainly of cyclic and acyclic aliphatic components with lesser amounts of aromatics present. It is less aromatic and more susceptible to thermal degradation than the Precambrian kerogens, and its products show a much greater diversity especially among the unsaturated aliphatics and the heteroatomic components. The presence of these compounds, most notably 2,5-dimethylfuran, in the Mono Lake kerogen indicates a relatively rapid formation and incorporation into a young kerogen and suggests that their presence in the Precambrian kerogens, such as the Belingwe stromatolite, may be consistent with an ancient biological origin. In simulated diagenesis experiments the Recent mud was heated at 150°C for 3 months or 225°C for 8 months. The former was insufficient to affect the kerogen pyrolysis products. The latter, however, caused a large decrease in the heteroatomic components and a slight increase in the abundance of n-alkanes relative to that of the cyclic and branched alkanes. This suggests that the presence of some of these components in Precambrian rocks should be consistent with a high degree of preservation of these rocks, as appears to be the case for the Belingwe and Transvaal stromatolites.