Volume 30, Issues 9–10,
, Pages 959-967
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Mathematical modeling of sediment early diagenesis always involves choosing and describing a set of chemical reactions and processes that is both self-consistent and sufficient for the problem at hand. This critical choice is always a compromise between describing the system's complexity in all details and using a manageable set of reactions with known or obtainable parameters such as equilibrium and rate constants. We present a database tool for modeling sediment early diagenesis (dSED) that is designed to help modelers and sediment geochemists in this difficult conceptual step. The database should facilitate the development of state-of-the-art spatially continuous reaction-transport models (RTM), as well as simpler interacting-compartment (box) models. It allows one to explore: available kinetic and thermodynamic information, alternative descriptions of the same major processes, different degrees of completeness in description, processes and reactions that could be added or modeled differently, published solutions used by previous workers, and other information. The database is searchable and allows viewing reactions by specific products or reactants, types of processes, or other customized criteria. It operates under Microsoft Access™ and can be added to, modified and programmed by the user. The latest version of dSED and its user manual can be downloaded at http://www.science.uottawa.ca/LSSE/dSED.
Sediment early diagenesis reaction-transport models (RTMs) are based on complex and interconnected sets of chemical reactions and transport processes that describe spatially continuous dynamics of both solid and dissolved species in the sediment (e.g., Berner, 1980; Boudreau, 1997). They have been employed to model a broad range of aquatic environments, including marine (Boudreau, 1996; Klump and Martens, 1989; Dhakar and Burdige, 1996; Wang and Van Cappellen, 1996; Tromp et al., 1995), lacustrine (Furrer and Wehrli, 1996), estuarine (Regnier et al., 1997), ground water (Mayer et al., 2002; Hunter et al., 1998), and potentially other environments such as rivers and wetlands.
Sediment early diagenesis is, to a large degree, controlled by the kinetics of the involved biogeochemical and physical transformations. Whereas thermodynamic equilibrium calculations have benefited from several good database/program combinations (e.g., Meeussen, 2003; Herberlin and Westall, 1999; Parkhurst and Appelo, 1999; Wolery, 1992a, Wolery, 1992b; Allison et al., 1991; Ball and Nordstrom, 1991), kinetic modeling of diagenesis is still in the early stages of a systematic integration of biogeochemical knowledge (Regnier et al., 2002a, Regnier et al., 2002b; Meysman et al., 2003a, Meysman et al., 2003b). At present, there exist no established database tools to classify and analyse the kinetic data that is essential to the development of sediment diagenesis RTMs.
An expandable web-distributed Knowledge Base (KB) (Regnier et al., 2002a, Regnier et al., 2002b, Regnier et al., 2003) has recently been introduced to amend this situation. It is an ambitious attempt to synthesize and organize conceptual and quantitative biogeochemical information relevant to diagenetic modeling. The KB is currently designed to be integrated with a reaction-transport modeling software that is being developed by the same group at Utrecht University. A user of such software will choose from a collection of reaction pathways, rate law formulations and parameter values for a particular site or problem, or will have an option of accepting the proposed default values. The MEDIA code (Meysman et al., 2003a, Meysman et al., 2003b) is another recently developed software tool that is based on object-oriented programming concepts. Its object database was designed to facilitate adding reactions, species, and parameters (objects) to the associated MEDIA modeling software. The database is a library of ASCII files that contain descriptions of reactions, species, and parameters in a pre-defined format and that can be selected by the user in constructing a model. Both MEDIA and the KB significantly facilitate integration of the required biogeochemical knowledge into diagenetic modeling and will increase the universality of RTMs. At the same time, we feel that there is a further need for a more general database tool (and at this stage more complete) that is free from restrictions imposed by integration with simulation software.
In this paper, we present our “database for reaction-transport modeling of sediment early diagenesis”, (dSED). It characterizes a diverse range of coupled biocatalytic, mineralogical, rheological, biological, photochemical, and other processes. Being complementary to KB and sharing much of its philosophy, our stand-alone tool offers flexibility provided by customizable search filters. Its main features, in comparison to those of KB and MEDIA, are presented in Table 1. dSED alleviates the historical bias in the data, as the database content is not selected based on subjective quality criteria or software formatting requirements. dSED can be used alone or in parallel with MEDIA or KB. Unlike KB, dSED does not provide in-depth reviews of current approaches, nor does it propose default parameter values and process formulations. dSED can also be used in parallel with equilibrium reaction databases, as we did not intend to include all available equilibrium data or all known aquatic speciation reactions. The database can also serve as a bibliographic tool.
As the field of diagenetic reaction-transport modeling is itself relatively new (Steefel and Van Cappellen, 1998), dSED is intended to be a work in progress that interested workers can contribute to. We feel that this searchable collection of experimental and conceptual data will facilitate the development of both spatially continuous RTMs and simpler (e.g., box-type or interacting compartment) models. It will also benefit sediment geochemists who are concerned with interpretations of molecular and larger scale processes without necessarily performing model simulations. The database and its user manual are freely available on the Web at http://www.science.uottawa.ca/LSSE/dSED
Note that, in this paper, we use the term ‘reaction’ to include all processes that can be represented by a chemical reaction equation (including precipitation, dissolution, sorption, and surface complexation).
dSED motivation and philosophy
The philosophy, goals and possible uses of dSED have been elaborated with the potential for future development in mind and can be summarized as follows.
The database should include as many reactions and processes as possible. In contrast to expert systems like KB, dSED strives to include not only a “most important set” of established reactions and processes but also all those that may be relevant in aquatic environments, including those that have not yet been formulated for modeling in the form
dSED description: content
dSED contains the following types of information:
1. Reactions, including aqueous chemical reactions, solid phase precipitation and dissolution, and sorption. The reactions are characterized by their stoichiometries and by thermodynamic (Gibbs free energy change or equilibrium constant) and kinetic (rate law, rate constant) parameters. Sorption substrates are characterized by their surface properties. Parameters are given in the format and in the units of the original research articles and it is
dSED description: implementation
The database is implemented in Microsoft Access™, an easily available database software. The main entries in the database are individual reactions. For each reaction, the corresponding database record contains the reaction equation and, in many cases, its alternative forms (that is, reactions differing in the stoichiometries of their reactants, the ionic speciation, or the mineral species). The database also provides a short descriptive name of the reaction, values of thermodynamic parameters,
dSED is a valuable database tool for RTM, box-model, and interpretation development in sediment early diagenesis. We invite all interested researchers to contribute to it with experimental, theoretical, or bibliographic data. The dSED database is maintained by the Lake Sediment Structure and Evolution (LSSE) group at the University of Ottawa. Contributions to the database can be made by e-mail to the corresponding author.
The authors would like to thank Frederick Ghogomu, Darryl Roberts and Claar van der Zee for their helpful suggestions and Filip Meysman and an anonymous reviewer for constructive criticism. This work was funded by a Strategic Project Grant from the Natural Sciences and Engineering Research Council of Canada.
- B.P. Boudreau
A method-of-lines code for carbon and nutrient diagenesis in aquatic sediments
Computers & Geosciences
- G. Furrer et al.
Microbial reactions, chemical speciation, and multicomponent diffusion in porewaters of a eutropic lakeSee AlsoAnoxic survival potential of bivalves: (arte)factsMeHg production in eutrophic lakes: Focusing on the roles of algal organic matter and iron-sulfur-phosphorus dynamicsMineralogy and geochemistry of fine-grained clastic rocks in the Eocene Huadian Basin (NE China): Implications for sediment provenance, paleoclimate and depositional environmentHydrocarbon potential of organic-rich sediments in the Ukrainian Outer Carpathians and its Foreland
Geochimica et Cosmochimica Acta
- K.S. Hunter et al.
Kinetic modeling of microbially-driven redox chemistry of subsurface environmentscoupling transport, microbial metabolism and geochemistry
Journal of Hydrology
- G.W. Luther et al.
Interactions of manganese with the nitrogen cyclealternative pathways to dinitrogen
Geochimica et Cosmochimica Acta
- F.J.R. Meysman et al.
Reactive transport in surface sediments
I. Model complexity and software quality, Computers & Geosciences
- F.J.R. Meysman et al.
Reactive transport in surface sediments
II. Mediaan object-oriented problem-solving environment for early diagenesis, Computers & Geosciences
- J.W. Morse et al.
The chemistry of the hydrogen sulfide and iron sulfide systems in natural waters
- P. Regnier et al.
Long-term fluxes of reactive species in macrotidal estuariesestimates from a fully transient, multicomponent reaction-transport model
- P. Regnier et al.
Modeling complex multi-component reactive-transport systemstowards a simulation environment based on the concept of a Knowledge Base
Applied Mathematical Modeling
- D. Rickard
Kinetics of FeS precipitationPart 1. Competing reaction mechanisms
Geochimical et Cosmochimica Acta
Quenching of labile functionalised lipids by inorganic sulphur speciesevidence for the formation of sedimentary organic sulphur compounds at the early stages of diageneis
Geochimica et Cosmochimica Acta
A review of the source, behaviour and distribution of arsenic in natural waters
A global model for the early diagenesis of organic carbon and organic phosphorus in marine sediments
Geochimica et Cosmochimica Acta
A multicomponent reactive transport model of early diagenesisapplication to redox cycling in coastal marine sediments
Geochimica et Cosmochimica Acta
Early diagenetic influences on iron transformations in a fresh-water lake sediment
Geochemistry, Groundwater and Pollution
Early DiagenesisA Theoretical Approach. Princeton Series in Geochemistry
Manganese oxides, Earth surface oxygenation, and the rise of oxygenic photosynthesis
2023, Earth-Science Reviews
Oxygenic photosynthesis is arguably the most important biological innovation in Earth's history, facilitating the transition to a habitable planet for complex life. Dating the emergence of oxygenic photosynthesis, however, has proven difficult with estimates spanning a billion years. Sedimentary manganese (Mn) enrichments represent a potentially important line of evidence given the high redox potentials necessary to oxidize Mn in natural environments. However, this view has been challenged by abiotic and anaerobic Mn oxidation pathways that decouple Mn enrichments from oxygenation. With these in mind, we review Mn oxidation pathways and Mn enrichments and evaluate their relation to Earth's oxygenation. We argue that despite possible alternative pathways, shallow oxygenated seawater is a prerequisite for generating and, importantly, preserving significant sedimentary Mn enrichments (and associated geochemical signals). This implies that Mn enrichments indeed track Earth's oxygenation and oxygenic photosynthesis emerged 100s of millions of years prior to irreversible atmospheric oxygenation.
Significant role of organic sulfur in supporting sedimentary sulfate reduction in low-sulfate environments
2017, Geochimica et Cosmochimica Acta
Citation Excerpt :
Sulfate also can be incorporated into authigenic minerals such as calcium carbonates and be subsequently released upon their dissolution in sediments. The available data on the dissolution kinetics of typical sulfur minerals (Katsev et al., 2004) suggest that these processes may supply sulfate to porewaters for up to several thousand years. In lakes with high abundances of sulfur-bearing minerals in the catchment, such as Lake Michigan (Black, 1997), their dissolution is thus likely to amplify the sub-interface peaks in porewater sulfate (e.g. Thomsen et al., 2004).
Dissimilatory sulfate reduction (DSR) is a major carbon mineralization pathway in aquatic sediments, soils, and groundwater, which regulates the production of hydrogen sulfide and the mobilization rates of biologically important elements such as phosphorus and mercury. It has been widely assumed that water-column sulfate is the main sulfur source to fuel this reaction in sediments. While this assumption may be justified in high-sulfate environments such as modern seawater, we argue that in low-sulfate environments mineralization of organic sulfur compounds can be an important source of sulfate. Using a reaction-transport model, we investigate the production of sulfate from sulfur-containing organic matter for a range of environments. The results show that in low sulfate environments (<500μM) the in-sediment production of sulfate can support a substantial portion (>50%) of sulfate reduction. In well-oxygenated systems, porewater sulfate profiles often exhibit sub-interface peaks so that sulfate fluxes are directed out of the sediment. Our measurements in Lake Superior, the world’s largest lake, corroborate this conclusion: offshore sediments act as sources rather than sinks of sulfate for the water column, and sediment DSR is supported entirely by the in-sediment production of sulfate. Sulfate reduction rates are correlated to the depth of oxygen penetration and strongly regulated by the supply of reactive organic matter; rate co-regulation by sulfate availability becomes appreciable below 500μM level. The results indicate the need to consider the mineralization of organic sulfur in the biogeochemical cycling in low-sulfate environments, including several of the world’s largest freshwater bodies, deep subsurface, and possibly the sulfate-poor oceans of the Early Earth.
Modeling of decadal scale phosphorus retention in lake sediment under varying redox conditions
2013, Ecological Modelling
Phosphorus (P) releases from lake sediments are controlled in the long term by P burial into the deep sediment and on shorter time scales by the redox conditions at the sediment–water interface. In Lake Sempach (Switzerland), hypolimnetic oxygen concentration was increased by artificial aeration after two decades of nearly anoxic conditions. Using diagenetic reaction-transport modeling and sediment core analysis, we investigated the effects that this change, as well as variations in the organic carbon loadings, had on the long-term mobility of sediment P. During low-oxygen conditions, the reducible iron pool in the sediment was depleted, resulting in the release of previously accumulated P. The remobilization of iron-bound P affected phosphate effluxes from the sediment on the time scale of the sediment iron cycle (several years). On longer time scales, P effluxes followed the sedimentation fluxes of organic matter. Mass balance calculations indicate that, despite the dominance of internal P loading in Lake Sempach, over the long-term phosphorus content in the water column was controlled by the external P inputs. The results suggest that, whereas short-term decreases in sediment P releases may be achieved by preventing sediment anoxia, long-term solutions should involve reductions in the external P inputs.
Comparative survey of potential nitrate and sulfate reduction rates in aquatic sediments
2012, Geochimica et Cosmochimica Acta
Citation Excerpt :(Video) S2S20-16- Sediment Carbon Delivery Preservation- the Amazon, G-B, Irrawaddy (Steve Kuehl, 10/14/20)
In addition, on the time scale of the experiments, only small fractions of the total organic matter are degraded (Abell et al., 2009). In most early diagenetic transport–reaction models, the consumption of TEAs is controlled by the supply and reactivity of organic matter (Berner, 1980; Van Cappellen and Gaillard, 1996; Boudreau, 1997; Soetaert et al., 2000; Katsev et al., 2004). As the organic matter oxidatively degrades, the resulting flow of electrons reduces the available TEAs, which in turn fuels a complex network of secondary reactions.
Nitrate and sulfate are two major terminal electron acceptors of anaerobic respiration in nearshore sediments. Potential nitrate and sulfate reduction rates (NRR and SRR) were determined on surficial sediments sampled at 14 sites representing a wide range of shallow-water depositional environments. The rates were obtained by supplying undisturbed slices of sediments with nitrate, sulfate or both using a flow-through reactor technique. No external electron donor was added to the sediments. The results indicate that all studied sediments harbored viable and coexisting nitrate- and sulfate-reducing communities, which were able to instantaneously consume the electron acceptors supplied to the reactors. On average, NRR exceeded SRR by about one order of magnitude (309±180nmol cm−3h−1 versus 37±29nmol cm−3h−1). The NRR:SRR molar ratio, however, varied significantly from site to site, with values ranging from 1.7 to 59. Nitrite production, indicative of incomplete nitrate reduction, was observed in all studied sediments and, on average, accounted for 45% of NRR (range 3–80%). Production of sulfate under nitrate-reducing conditions was observed in 10 out of 14 of the studied sediments, suggesting a common occurrence of sulfide oxidation coupled to nitrate reduction. Oxidation of sulfide accounted for 0 to 40% of NRR in the nitrate-only experiments. When both electron acceptors were supplied simultaneously, net sulfate consumption decreased on average by 45%. The effect of nitrate on SRR was highly variable, however, ranging from near complete inhibition to a 25% enhancement of SRR. Overall, the results of this study point to the need to critically reassess the model formulations used to represent anaerobic respiration processes and their interactions in early diagenetic models.
A reaction-transport model of periodic precipitation of pyrite in anoxic marine sediments
2011, Chemical Geology
Periodic precipitation patterns (Liesegang bands) constitute good manifestations of self-organization in geochemical systems. Examples of periodic pyrite bands have been previously reported in anoxic sediment systems, such as sapropels from the Eastern Mediterranean. In this contribution, we present a model of Liesegang pyrite band formation in anoxic marine sediments. The model is based on early diagenetic reactive-transport processes and incorporates nucleation and growth of pyrite crystallites. The simulation results of pyrite band formation are compatible with the experimental pyritization pattern observed in a sediment analog, as well as the observations in sapropel sediments.
Factors controlling long-term phosphorus efflux from lake sediments: Exploratory reactive-transport modeling
2006, Chemical Geology
Citation Excerpt :
Because of an inherent freedom in the choice of those levels, and because parameters such as reaction rate constants can vary by as much as several orders of magnitude, the analysis identified factors that are potential phosphorus controls when varied across this selected range of possible conditions, rather than at a particular system or site. The parameter ranges were determined by searching the literature for typical species concentrations and polling the dSED database (Katsev et al., 2004) for typical ranges of kinetic constants. For the sake of simplicity, we choose a single response function: The steady-state diffusive phosphorus efflux Φout from the sediment into the water column (Fig. 1).
We perform, for the first time, a global sensitivity analysis on a generic diagenetic reaction-transport model (RTM) to elucidate the effects of environmental, thermodynamic, and kinetic factors on the magnitudes of phosphorus release fluxes from aquatic sediments under a range of conditions typically found in lake sediments. Our model includes the processes that describe redox-sensitive phosphorus releases (the so-called classical model) and processes by which phosphorus mobilization is affected by porewater sulfate. On decadal and longer time scales, sediment phosphorus effluxes are primarily determined by: sedimentation flux of reactive organic matter, sedimentation flux of iron oxyhydroxides, concentrations of dissolved oxygen and sulfate at the sediment–water interface, and the rate at which phosphate is immobilized in reduced sediment. We show that the effects of these factors on phosphorus effluxes are interdependent and discuss the mechanisms of such interactions. The dominant pathways by which dissolved sulfate increases phosphorus efflux in iron-rich hypoxic sediments are discussed in detail. In contrast to short-term phosphorus releases, such as described by the classical model, long-term phosphorus retention is controlled by phosphorus removal to deep, reduced, sediment, rather than processes at the sediment–water interface. Hence, the results of short-term laboratory or in-situ studies (such as sediment incubations experiments) cannot be unequivocally extended to long time scales. Our results provide explanations to the reports that lake restoration measures such as restricting phosphorus inputs to a lake or oxygenating the lake's hypolimneon (or both) in the long-term often fail to decrease sediment phosphorus effluxes. The re-deposition of sediment substances after their release into the water column (the feedback often overlooked in sediment RTMs) can critically affect the magnitude and dynamics of phosphorus efflux from sediments. Depending on sediment history, the same set of external (boundary) conditions can generate diagenetic regimes with either high or low phosphorus effluxes (bistability).
Vibrational spectroscopic characterization of the phosphate mineral kulanite Ba(Fe2+,Mn2+,Mg)2(Al,Fe3+)2(PO4)3(OH)3
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, Volume 115, 2013, pp. 22-25
The mineral kulanite BaFe2Al2(PO4)3(OH)3, a barium iron aluminum phosphate, has been studied by using a combination of electron microscopy and vibrational spectroscopy. Scanning electron microscopy with EDX shows the mineral is homogenous with no other phases present. The Raman spectrum is dominated by an intense band at 1022cm−1 assigned to the symmetric stretching mode. Low intensity Raman bands at 1076, 1110, 1146, 1182cm−1 are attributed to the antisymmetric stretching vibrations. The infrared spectrum shows a complex spectral profile with overlapping bands. Multiple phosphate bending vibrations supports the concept of a reduction in symmetry of the phosphate anion. Raman spectrum at 3211, 3513 and 3533cm−1 are assigned to the stretching vibrations of the OH units. Vibrational spectroscopy enables aspects on the molecular structure of kulanite to be assessed.
The molecular structure of the phosphate mineral kidwellite NaFe93+(PO4)6(OH)11⋅3H2O – A vibrational spectroscopic study
Journal of Molecular Structure, Volume 1074, 2014, pp. 429-434
The mineral kidwellite, a hydrated hydroxy phosphate of ferric iron and sodium of approximate formula NaFe93+(PO4)6(OH)11⋅3H2O, has been studied using a combination of electron microscopy with EDX and vibrational spectroscopic techniques.
Raman spectroscopy identifies an intense band at 978cm−1 and 1014cm−1. These bands are attributed to the PO43− ν1 symmetric stretching mode. The ν3 antisymmetric stretching modes are observed by a large number of Raman bands. The series of Raman bands at 1034, 1050, 1063, 1082, 1129, 1144 and 1188cm−1 are attributed to the ν3 antisymmetric stretching bands of the PO43− and HOPO32− units. The observation of these multiple Raman bands in the symmetric and antisymmetric stretching region gives credence to the concept that both phosphate and hydrogen phosphate units exist in the structure of kidwellite. The series of Raman bands at 557, 570, 588, 602, 631, 644 and 653cm−1are assigned to the PO43− ν2 bending modes. The series of Raman bands at 405, 444, 453, 467, 490 and 500cm−1 are attributed to the PO43− and HOPO32− ν4 bending modes.
The spectrum is quite broad but Raman bands may be resolved at 3122, 3231, 3356, 3466 and 3580cm−1. These bands are assigned to water stretching vibrational modes. The number and position of these bands suggests that water is in different molecular environments with differing hydrogen bond distances. Infrared bands at 3511 and 3359cm−1 are ascribed to the OH stretching vibration of the OH units. Very broad bands at 3022 and 3299cm−1 are attributed to the OH stretching vibrations of water. Vibrational spectroscopy offers insights into the molecular structure of the phosphate mineral kidwellite.(Video) Life in Oxic, Suboxic, & Anoxic Sediments (Yes, Some Life Doesn't Require Oxygen!) | GEO GIRL
Iron isotopes in an Archean ocean analogue
Geochimica et Cosmochimica Acta, Volume 133, 2014, pp. 443-462
Iron isotopes have been extensively used to trace the history of microbial metabolisms and the redox evolution of the oceans. Archean sedimentary rocks display greater variability in iron isotope ratios and more markedly negative values than those deposited in the Proterozoic and Phanerozoic. This increased variability has been linked to changes in either water column iron cycling or the extent of benthic microbial iron reduction through time. We tested these contrasting scenarios through a detailed study of anoxic and ferruginous Lac Pavin (France), which can serve as a modern analogue of the Archean ocean. A depth-profile in the water column of Lac Pavin shows a remarkable increase in dissolved Fe concentration (0.1–1200μM) and δ56Fe values (−2.14‰ to +0.31‰) across the oxic–anoxic boundary to the lake bottom. The largest Fe isotope variability is found at the redox boundary and is related to partial oxidation of dissolved ferrous iron, leaving the residual Fe enriched in light isotopes. The analysis of four sediment cores collected along a lateral profile (one in the oxic layer, one at the redox boundary, one in the anoxic zone, and one at the bottom of the lake) indicates that bulk sediments, porewaters, and reactive Fe mostly have δ56Fe values near 0.0±0.2‰, similar to detrital iron. In contrast, pyrite δ56Fe values in sub-chemocline cores (60, 65, and 92m) are highly variable and show significant deviations from the detrital iron isotope composition (δ56Fepyrite between −1.51‰ and +0.09‰; average −0.93‰). Importantly, the pyrite δ56Fe values mirror the δ56Fe of dissolved iron at the redox boundary—where near quantitative sulfate and sulfide drawdown occurs—suggesting limited iron isotope fractionation during iron sulfide formation. This finding has important implications for the Archean environment. Specifically, this work suggests that in a ferruginous system, most of the Fe isotope variability observed in sedimentary pyrites can be tied to water column cycling—foremost to the oxidation of dissolved ferrous iron. This supports previous suggestions that enhanced iron isotope variability in the Archean may record a unique stage in Earth’s history where partial ferrous iron oxidation in upwelling water masses was a common process, probably linked to oxygenic or anoxygenic photosynthesis.
Numerical research on the mechanism of contaminant release through the porous sediment-overlying water interface
Journal of Hydrodynamics, Ser. B, Volume 26, Issue 6, 2015, pp. 971-979
After the pollutant discharged into the river or lake has been reduced, the release of the contaminant from the sediment to the overlying water may cause the river and lake be contaminated again. On the condition that the overlying water flow does not lead to sediment suspension, numerical researches are carried out for the mechanism of contaminant release through the sediment-overlying water interface. The overlying water flow is calculated as turbulence. The sediment is regarded as isotropic homogeneous porous medium, therefore the seepage field in the porous sediment layer is obtained by solving Darcy's equations. Coupled two dimensional steady flows of the overlying water and the pore water in the sediment are calculated. Based on the flow fields obtained, the unsteady contaminant solute transportation process in the pore water in the sediment and the overlying water is numerically simulated, as the shapes of the sediment-overlying water interface are flat or periodic triangular respectively. Numerical results show that the exchange of the pore water and the overlying water is an important factor which decides the release flux of the contaminant from the sediment to the overlying water. The pressure distribution produced by the overlying water flow along the sediment-overlying water interface, as it is not flat, may induce the seepage of the pore water in the sediment and through the sediment-overlying water interface, which may increase the release flux of the contaminant from the sediment to the overlying water.
Case studies on the utility of sequential carbonate leaching for radiogenic strontium isotope analysis
Chemical Geology, Volume 497, 2018, pp. 88-99
Radiogenic strontium isotopes (87Sr/86Sr) have been extensively used as a tool to explore a diversity of Earth system problems, including long-term global weathering rates and global sequence correlation. Strontium isotopes are measured on a range of geological materials (e.g., calcite fossils, barites, limestone micrites), but whole-rock limestones are by far the most abundant of these materials within the geological record for paleo-seawater 87Sr/86Sr work. Whole-rock limestones, however, have a poor track record of recording primary seawater 87Sr/86Sr values. Alteration of the limestone during diagenesis and contamination from detrital aluminosilicate phases during carbonate extraction have been consistent problems. Various preparation and quality control methods have been applied to whole-rock 87Sr/86Sr work, yet there remains no consistent framework used to separate and identify both contamination and alteration simultaneously. The lack of consistent and systematic methods has made it difficult to gauge the accuracy and fidelity of much of the previously generated whole rock limestones 87Sr/86Sr data, especially for Precambrian sequences. Building on previous work, we explore a sequential leaching method designed to systematically isolate least-altered carbonate phases from detrital aluminosilicate Sr contamination and present several case studies that demonstrate the advantages of this approach. In the first case study, we use the Mid-Carboniferous Bird Spring Formation to empirically validate the accuracy of this sequential leaching method. Comparing least-altered sequentially leached whole-rock 87Sr/86Sr values with well-preserved calcite brachiopod 87Sr/86Sr values from the same section, we find near identical values. Following this first case, we studied the Neoproterozoic Dhaiqa Formation and the mid-Proterozoic Jixian Group and Muskwa Assemblage to outline a framework for identifying least-altered leachate fractions in Proterozoic carbonate samples. As a whole, we find that with this method it is possible to better identify whole-rock samples primary or least-altered carbonate fractions, and better account for alteration, providing a means to back-calculate a samples primary and least-altered marine 87Sr/86Sr value.
Simulating Precambrian banded iron formation diagenesis
Chemical Geology, Volume 362, 2013, pp. 66-73
Post-depositional diagenetic alteration makes the accurate interpretation of key precipitation processes in ancient sediments, such as Precambrian banded iron formations (BIFs), difficult. While microorganisms are proposed as key contributors to BIF deposition, the diagenetic transformation of precursor Fe(III) minerals associated with microbial biomass had not been experimentally tested. We incubated mixtures of ferrihydrite (proxy for biogenic ferric oxyhydroxide minerals) and glucose (proxy for microbial biomass) in gold capsules at 1.2kbar and 170°C. Both wet chemical analysis and mineralogical methods (microscopy, X-ray diffraction and Mössbauer spectroscopy) were used to analyze the reaction products. Under these conditions, ferrihydrite (FeIII(OH)3) transforms to hematite (Fe2IIIO3), magnetite (FeIIFe2IIIO4), and siderite (FeIICO3). Silica-coated ferrihydrite prepared at conservative Si:Fe ratios (as predicted for the Precambrian oceans) and mixed with glucose yielded hematite and siderite, whereas magnetite could not be identified microscopically. Our results show that electron transfer from organic carbon to Fe(III) minerals during temperature/pressure diagenesis can drive the production of key BIF minerals. Our results also demonstrate that the post-depositional mineralogy of BIF does not directly archive the oceanic or atmospheric conditions present on Earth during their lithification. As a consequence, atmospheric composition regarding concentrations of methane and CO2 during the time of BIF mineral deposition cannot be directly inferred from BIF mineralogical data alone.(Video) ADCPs and suspended sediments - David Velasco
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