Constraints on the formation of sedimentary dolomite

Proto-dolomite spherulites with heterogeneous interior precipitated in brackish water cultivation of freshwater cyanobacterium Leptolyngbya boryana

Primary dolomite is believed to be formed through cyanobacterial calcification, yet the details and mechanisms of this process are not fully understood. In this study, a freshwater filamentous cyanobacterium, Leptolyngbya boryana, was cultured and domesticated in artificial freshwater and brackish solutions with various Mg/Ca ratios. The hydrochemistry, the extracellular polymeric substance (EPS) composition, and precipitate mineralogy in the medium were monitored. The results showed that the L. boryana induced proto-dolomite precipitation in brackish medium with salinity of 1.5 % and Mg/Ca ratio of 5. The proto-dolomite in this study has a "double spherical" appearance and a hollow core, which may have originally been filled by the complex composed of EPS and amorphous CaMg carbonate. With regard to elemental composition, the cyanobacterially-induced proto-dolomite is rich in calcium inside and magnesium-rich on the surface, and cyanobacterial organic matter is sealed inside the particles during spherulite growth. In this study, the accelerators for Mg2+ to enter the carbonate lattice mainly include extracellular acidic amino acids and polysaccharides. The changes of these promoters among different cultures were related to the growth state of cyanobacteria under salinity stress. The polysaccharides concentration has a significant increasing in the dolomite-precipitating medium, indicating that it may be the main promoter of proto-dolomite precipitation and significantly increases the amount of Mg2+ precipitation. At the meantime, the amount of precipitated Ca2+ was suppressed by increasing salinity and Mg2+, thus leading to the precipitation of proto-dolomite in this shifting process. This study can potentially provide a reference for explaining the dolomite (proto-dolomite) precipitation in aerobic brackish environment where cyanobacteria thrive.

Dissolution enables dolomite crystal growth near ambient conditions

Crystals grow in supersaturated solutions. A mysterious counterexample is dolomite CaMg(CO3)2, a geologically abundant sedimentary mineral that does not readily grow at ambient conditions, not even under highly supersaturated solutions. Using atomistic simulations, we show that dolomite initially precipitates a cation-disordered surface, where high surface strains inhibit further crystal growth. However, mild undersaturation will preferentially dissolve these disordered regions, enabling increased order upon reprecipitation. Our simulations predict that frequent cycling of a solution between supersaturation and undersaturation can accelerate dolomite growth by up to seven orders of magnitude. We validated our theory with in situ liquid cell transmission electron microscopy, directly observing bulk dolomite growth after pulses of dissolution. This mechanism explains why modern dolomite is primarily found in natural environments with pH or salinity fluctuations. More generally, it reveals that the growth and ripening of defect-free crystals can be facilitated by deliberate periods of mild dissolution.

Metalloenzyme signatures in authigenic carbonates from the Chukchi Borderlands in the western Arctic Ocean

Migration of methane-rich fluids at submarine cold seeps drives intense microbial activity and precipitation of authigenic carbonates. In this study, we analyzed microbially derived authigenic carbonate samples recently recovered from active gas hydrate mounds on the southwestern slope of the Chukchi Borderlands (CB), western Arctic Ocean. Our main aim was to characterize the distribution patterns of trace elements in carbonate-hosted lipid fractions to assess metalloenzyme requirements of microbes involved in anaerobic oxidation of methane (AOM). We measured stable isotopes, trace elements, lipid biomarkers, and genomic DNA, and results indicate the dominance of AOM-related lipid biomarkers in studied carbonate samples, as well as a predominant occurrence of the anaerobic methanotrophic archaea (ANME)-1. We also report evidence for significant preferential enrichments of various trace elements (Li, Ni, Co, Cu, Zn, and Mo) in the total lipid fractions of CB carbonates, relative to elemental compositions determined for corresponding carbonate fractions, which differ from those previously reported for other seep sites. We hypothesize that trace element enrichments in carbonate-hosted lipid fractions could vary depending on the type of AOM microbial assemblage. Additional work is required to further investigate the mechanisms of lipid-bound trace elements in cold seep carbonates as potential metalloenzymes in AOM.

High Mg/Ca Molar Ratios Promote Protodolomite Precipitation Induced by the Extreme Halophilic Bacterium Vibrio harveyi QPL2

Bacterial activities have been demonstrated as critical for protodolomite precipitation in specific aqueous conditions, whereas the relationship between the various hydrochemical factors and bacterial activity has not been fully explored. In this study, biomineralization experiments were conducted using a newly isolated extreme halophilic bacterium from salina mud, Vibrio harveyi QPL2, under various Mg/Ca molar ratios (0, 3, 6, 10, and 12) and a salinity of 200‰. The mineral phases, elemental composition, morphology, and crystal lattice structure of the precipitates were analyzed by XRD, SEM, and HRTEM, respectively. The organic weight and functional groups in the biominerals were identified by TG-DSC, FTIR, and XPS analysis. The amounts of amino acids and polysaccharides in the EPS of QPL2 cultured at various Mg/Ca molar ratios were quantified by an amino acid analyzer and high-performance liquid chromatography. The results confirm that disordered stoichiometric protodolomite was successfully precipitated through the activities of bacteria in a medium with relatively high Mg/Ca molar ratios (10 and 12) but it was not identified in cultures with lower Mg/Ca molar ratios (0, 3, and 6). That bacterial activity is critical for protodolomite formation as shown by the significant bacterial relicts identified in the precipitated spherulite crystals, including pinhole structures, a mineral coating around cells, and high organic matter content within the crystals. It was also confirmed that the high Mg/Ca molar ratio affects the composition of the organic components in the bacterial EPS, leading to the precipitation of the protodolomite. Specifically, not only the total EPS amount, but also other facilitators including the acidic amino acids (Glu and Asp) and polysaccharides in the EPS, increased significantly under the high Mg/Ca molar ratios. Combined with previous studies, the present findings suggest a clear link between high Mg/Ca molar ratios and the formation of protodolomite through halophilic bacterial activity.

Carbon Geochemistry of the Active Serpentinization Site at the Wadi Tayin Massif: Insights From the ICDP Oman Drilling Project: Phase II

A large part of the hydrated oceanic lithosphere consists of serpentinites exposed in ophiolites. Serpentinites constitute reactive chemical and thermal systems and potentially represent an effective sink for CO₂. Understanding carbonation mechanisms within ophiolites are almost exclusively based on studies of outcrops, which can limit the interpretation of fossil hydrothermal systems. We present stable and radiogenic carbon isotope data that provide insights into the isotopic trends and fluid evolution of peridotite carbonation in ICDP Oman Drilling Project drill holes BA1B (400-m deep) and BA3A (300-m deep). Geochemical investigations of the carbonates in serpentinites indicate formation in the last 50 kyr, implying a distinctly different phase of alteration than the initial oceanic hydration and serpentinization of the Samail Ophiolite. The oldest carbonates (∼31 to >50 kyr) are localized calcite, dolomite, and aragonite veins, formed between 26°C and 43°C and related to focused fluid flow. Subsequent pervasive small amounts of dispersed carbonate precipitated in the last 1,000 years. Macroscopic brecciation and veining of the peridotite indicate that carbonation is influenced by tectonic features allowing infiltration of fluids over extended periods and at different structural levels such as along fracture planes and micro-fractures and grain boundaries, causing large-scale hydration of the ophiolite. The formation of dispersed carbonate is related to percolating fluids with δ ¹⁸O lower than modern ground and meteoric water. Our study shows that radiocarbon investigations are an essential tool to interpret the carbonation history and that stable oxygen and carbon isotopes alone can result in ambiguous interpretations.

Taphonomy of marine vertebrates of the Pisco Formation (Miocene, Peru): Insights into the origin of an outstanding Fossil-Lagerstätte

The Miocene Pisco Formation, broadly exposed in the Ica Desert of southern Peru, is among the most outstanding Cenozoic marine Fossil-Lagerstätten worldwide. It is renowned for its exceptional preservation and abundance of vertebrate fossils, including a rich assemblage of whales and dolphins (Cetacea). Here, we integrate taphonomic data on 890 marine vertebrate fossils, gathered through 16 different localities. Our observations range from the taxonomic distribution, articulation, completeness, disposition and orientation of skeletons, to the presence of bite marks, associations with shark teeth and macro-invertebrates, bone and soft tissue preservation, and the formation of attendant carbonate concretions and sedimentary structures. We propose that the exceptional preservation characterising many Pisco vertebrates, as well as their exceptionally high abundance, cannot be ascribed to a single cause like high sedimentation rates (as proposed in the past), but rather to the interplay of several favourable factors including: (i) low levels of dissolved oxygen at the seafloor (with the intervention of seasonal anoxic events); (ii) the early onset of mineralisation processes like apatite dissolution/recrystallisation and carbonate mineral precipitation; (iii) rapid burial of carcasses in a soupy substrate and/or a novel mechanism involving scour-induced self-burial; and (iv) original biological richness. Collectively, our observations provide a comprehensive overview of the taphonomic processes that shaped one of South America’s most important fossil deposits, and suggest a model for the formation of other marine vertebrate Fossil-Lagerstätten.

Modern dolomite formation caused by seasonal cycling of oxygenic phototrophs and anoxygenic phototrophs in a hypersaline sabkha

The "Dolomite Problem" has been a controversy for over a century, owing to massive assemblages of low-temperature dolomite in ancient rocks with little dolomite forming today despite favorable geochemical conditions. Experiments show that microbes and their exopolymeric substances (EPS) nucleate dolomite. However, factors controlling ancient abundances of dolomite can still not be explained. To decode the enigma of ancient dolomite, we examined a modern dolomite forming environment, and found that a cyclic shift in microbial community between cyanobacteria and anoxygenic phototrophs creates EPS suited to dolomite precipitation. Specifically, EPS show an increased concentration of carboxylic functional groups as microbial composition cycles from cyanobacterial to anoxygenic phototroph driven communities at low-and high- salinity, respectively. Comparing these results to other low-T forming environments suggests that large turnover of organic material under anoxic conditions is an important driver of the process. Consequently, the shift in atmospheric oxygen throughout Earth's history may explain important aspects of "The Dolomite Problem". Our results provide new context for the interpretation of dolomite throughout Earth's history.

New insights into the role of solution additive anions in Mg2+ dehydration: implications for mineral carbonation

Computer simulations of the Mg²⁺ dehydration mechanism show that solution additives can stabilise undercoordinated Mg²⁺ hydration configurations, opening up coordination sites on the central Mg²⁺ ion, promoting Mg-carbonates nucleation and growth.

The influence of Ca–Mg disorder on the growth of dolomite: a computational study

A new growth model for primary dolomite is proposed, in which a disordered dolomite/calcite interface seems to promote dolomite nucleation on a newly formed calcite crystal.

Formation of large carbonate concretions in black cherts in the Gufeng Formation (Guadalupian) at Enshi, South China

The formation of carbonate concretions is a cementation process which passively infills the pore spaces within sediments. They record the original environments of deposition and diagenetic conditions of the host rocks. Little is known about the precise mechanisms responsible for the precipitation of carbonate concretions. The most common host rocks are mudstones/shales, sandstones, and limestones. This study presents an example of large carbonate concretions from an unusual host rock, the black bedded cherts of the Gufeng Formation (Guadalupian) at Enshi on the northern Yangtze Platform, South China. Petrographic observations (X-ray diffraction, optical microscopy, scanning electron microscopy) and multiple geochemical analyses (pyrite- and carbonate-associated-sulfate (CAS)-sulfur isotopes, carbon isotopes) indicate that (a) the studied carbonate concretion are mainly composed of micritic calcite with subordinate dolomite; (b) the concretions may have been mainly formed in the bacterial sulfate reduction (BSR) zone during very early diagenesis near the sediment-water surface; (c) the paleo-bottom water overlying the sediments during formation of the concretions was mainly euxinic; and (d) the growth of the studied concretions proceeded via a pervasive model, where later cementation phase initiated in the lower part of the concretions and progressed upward.

The role of marine sediment diagenesis in the modern oceanic magnesium cycle

The oceanic magnesium cycle is largely controlled by continental weathering and marine authigenic mineral formation, which are intimately linked to long-term climate. Uncertainties in the magnesium cycle propagate into other chemical budgets, and into interpretations of paleo-oceanographic reconstructions of seawater δ²⁶Mg and Mg/Ca ratios. Here, we produce a detailed global map of the flux of dissolved magnesium from the ocean into deeper marine sediments (greater than ∼1 meter below seafloor), and quantify the global flux and associated isotopic fractionation. We find that this flux accounts for 15–20% of the output of magnesium from the ocean, with a flux-weighted fractionation factor of ∼0.9994 acting to increase the magnesium isotopic ratio in the ocean. Our analysis provides the best constraints to date on the sources and sinks that define the oceanic magnesium cycle, including new constraints on the output flux of magnesium and isotopic fractionation during low-temperature ridge flank hydrothermal circulation.

The oceanic magnesium cycle is closely linked to Earth’s carbon cycle and long-term climate change, due to processes such as continental weathering and authigenic mineral formation. Here, the authors update the global oceanic magnesium budget by quantifying the flux of magnesium from oceans to marine sediments and the associated isotopic fractionation.

Can Primary Ferroan Dolomite and Ankerite Be Precipitated? Its Implications for Formation of Submarine Methane-Derived Authigenic Carbonate (MDAC) Chimney

Microbes can mediate the precipitation of primary dolomite under surface conditions. Meanwhile, primary dolomite mediated by microbes often contains more Fe2+ than standard dolomite in modern microbial culture experiments. Ferroan dolomite and ankerite have been regarded as secondary products. This paper reviews the process and possible mechanisms of microbial mediated precipitation of primary ferroan dolomite and/or ankerite. In the microbial geochemical Fe cycle, many dissimilatory iron-reducing bacteria (DIRB), sulfate-reducing bacteria (SRB), and methanogens can reduce Fe3+ to Fe2+, while SRB and methanogens can also promote the precipitation of primary dolomite. There are an oxygen respiration zone (ORZ), an iron reduction zone (IRZ), a sulfate reduction zone (SRZ), and a methanogenesis zone (MZ) from top to bottom in the muddy sediment diagenesis zone. DIRB in IRZ provide the lower section with Fe2+, which composes many enzymes and proteins to participate in metabolic processes of SRB and methanogens. Lastly, heterogeneous nucleation of ferroan dolomite on extracellular polymeric substances (EPS) and cell surfaces is mediated by SRB and methanogens. Exploring the origin of microbial ferroan dolomite may help to solve the “dolomite problem”.

Acceleration of dolomitization by zinc in saline waters

Dolomite (CaMg(CO₃)₂) plays a key role in the global carbon cycle. Yet, the chemical mechanisms that catalyze its formation remain an enigma. Here, using batch reactor experiments, we demonstrate an unexpected acceleration of dolomite formation by zinc in saline fluids, reflecting a not uncommon spatial association of dolomite with Mississippi Valley-type ores. The acceleration correlates with dissolved zinc concentration, irrespective of the zinc source tested (ZnCl₂ and ZnO). Moreover, the addition of dissolved zinc counteracts the inhibiting effect of dissolved sulfate on dolomite formation. Integration with previous studies enables us to develop an understanding of the dolomitization pathway. Our findings suggest that the fluids’ high ionic strength and zinc complexation facilitate magnesium ion dehydration, resulting in a dramatic decrease in induction time. This study establishes a previously unrecognized role of zinc in dolomite formation, and may help explain the changes in dolomite abundance through geological time.

The reason for dolomite being widespread in ancient rocks remains an unsolved conundrum and artificial attempts to form well-ordered dolomite at ambient conditions have proven very challenging. Here, the authors provide laboratory experiments that show the acceleration of dolomite formation via dissolved zinc.

Halophilic Archaea Mediate the Formation of Proto-Dolomite in Solutions With Various Sulfate Concentrations and Salinities

In the past several decades, sulfate concentration and salinity have been considered to be the two essential hydrochemical factors in the formation of dolomite, yet arguments against this hypothesis have existed simultaneously. To clarify the effects of sulfate concentration and salinity in the mineralization of dolomite, we conducted experiments on dolomite precipitation mediated by a halophilic archaeon, Natrinema sp. J7-1 with various sulfate concentrations and salinities. This strain was cultured in a series of modified growth media (MGM) with salinities of 140, 200, and 280‰. Cells in the post-log phase were harvested and used to mediate the formation of dolomite in solutions with various sulfate concentrations of 0, 3, 29.8, and 100 mM and salinities of 140, 200, and 280‰. X-ray diffraction (XRD) spectra showed that proto-dolomite, monohydrocalcite, and aragonite formed in samples with cells, yet only aragonite was detected in samples without cells. Proto-dolomite was found in all biotic samples, regardless of the variation in salinity and sulfate concentration. Moreover, the relative abundances of proto-dolomite in the precipitates were positively correlated with the salinities of the media but were uncorrelated with the sulfate concentrations of the solutions. Scanning electronic microscopy (SEM) and energy dispersive spectroscopy (EDS) results showed that all the proto-dolomites were sphere or sphere aggregates with a mole ratio of Mg/Ca close to 1.0. No obvious variations in morphology and Mg/Ca were found among samples with various sulfate concentrations or salinities. This work reveals that a variation of sulfate concentration in solution (from 0 to 100 mM) does not affect the formation of dolomite mediated by halophilic archaea, but an increase of salinity (from 140 to 280‰) enhances this process. Our results indicate that under natural conditions, an increase in salinity may be more significant than the decrease of sulfates in microbe-mediated dolomite formation.

Estimation of coastal residence time of submarine groundwater discharge using radium progenies

A methodology based on γ-spectrometry measurements of untreated coastal water samples is proposed for the direct estimation of coastal residence time of submarine discharged groundwater. The method was applied to a submarine spring at Stoupa Bay covering all seasons. The estimated residence time exhibited an annual mean of 4.6±1.7 d. An additional measurement using the in situ underwater γ-spectrometry technique was performed, in the same site. The in situ method yielded a value of 2.8±0.2 d that was found consistent with the corresponding value derived using the developed lab-based method (3.4±2.0 d) for the same period.

Testing the cation-hydration effect on the crystallization of Ca-Mg-CO3 systems

Dolomite and magnesite are simple anhydrous calcium and/or magnesium carbonate minerals occurring mostly at Earth surfaces. However, laboratory synthesis of neither species at ambient temperature and pressure conditions has been proven practically possible, and the lack of success was assumed to be related to the strong solvation shells of magnesium ions in aqueous media. Here, we report the synthesis of MgCO3 and MgxCa(1-x)CO3 (0 < x < 1) solid phases at ambient conditions in the absence of water. Experiments were carried out in dry organic solvent, and the results showed that, although anhydrous phases were readily precipitated in the water-free environment, the precipitates' crystallinity was highly dependent on the Mg molar percentage content in the solution. In specific, magnesian calcite dominated in low [Mg(2+)]/[Ca(2+)] solutions but gave way to exclusive formation of amorphous MgxCa(1-x)CO3 and MgCO3 in high-[Mg(2+)]/[Ca(2+)] and pure-Mg solutions. At conditions of [Mg(2+)]/[Ca(2+)] = 1, both nanocrystals of Ca-rich protodolomite and amorphous phase of Mg-rich MgxCa(1-x)CO3 were formed. These findings exposed a previously unrecognized intrinsic barrier for Mg(2+) and CO3(2-) to develop long-range orders at ambient conditions and suggested that the long-held belief of cation-hydration inhibition on dolomite and magnesite mineralization needed to be reevaluated. Our study provides significant insight into the long-standing "dolomite problem" in geochemistry and mineralogy and may promote a better understanding of the fundamental chemistry in biomineralization and mineral-carbonation processes.

Surface chemistry allows for abiotic precipitation of dolomite at low temperature

Although the mineral dolomite is abundant in ancient low-temperature sedimentary systems, it is scarce in modern systems below 50 °C. Chemical mechanism(s) enhancing its formation remain an enigma because abiotic dolomite has been challenging to synthesize at low temperature in laboratory settings. Microbial enhancement of dolomite precipitation at low temperature has been reported; however, it is still unclear exactly how microorganisms influence reaction kinetics. Here we document the abiotic synthesis of low-temperature dolomite in laboratory experiments and constrain possible mechanisms for dolomite formation. Ancient and modern seawater solution compositions, with identical pH and pCO2, were used to precipitate an ordered, stoichiometric dolomite phase at 30 °C in as few as 20 d. Mg-rich phases nucleate exclusively on carboxylated polystyrene spheres along with calcite, whereas aragonite forms in solution via homogeneous nucleation. We infer that Mg ions are complexed and dewatered by surface-bound carboxyl groups, thus decreasing the energy required for carbonation. These results indicate that natural surfaces, including organic matter and microbial biomass, possessing a high density of carboxyl groups may be a mechanism by which ordered dolomite nuclei form. Although environments rich in organic matter may be of interest, our data suggest that sharp biogeochemical interfaces that promote microbial death, as well as those with high salinity may, in part, control carboxyl-group density on organic carbon surfaces, consistent with origin of dolomites from microbial biofilms, as well as hypersaline and mixing zone environments.

Sedimentology of part of the Lower Oil-Shale Group (Dinantian) sequence at Granton, Edinburgh, including the Granton “shrimp-bed”

The Dinantian Lower Oil-Shale Group of the Edinburgh area records the evolution of a SW–NE trending basin within an area of fluvial-dominated deltaic sedimentation, with rare marine incursions from the E. The sequence at Granton records the abandonment of a delta lobe, with deltaic sandstones overlain by lagoonal mud-shales. These thinly-laminated non-bioturbated mud-shales record low-energy, stagnant conditions. Fish, ostracodes and conchostracans inhabited the lagoon at this time.

Within the mud-shale sequence is the dolomitic Granton “shrimp-bed” which contains fenestrae, possible gypsum pseudomorphs and in situ shrinkage breccias suggesting periods of emergence. The “shrimp-bed” contains a number of laminae packed with soft-bodied fossils including marine forms, which resulted from brief marine incursions into the stagnant lagoon. The fluctuating salinity conditions seem to have favoured the development of an unusual fauna of shrimp-like crustaceans, fish, conchostracans, ostracodes and bellerophontid gastropods. This fauna appears to have suffered periodic mass mortalities during marine incursions which brought orthocone cephalopods, polychaete worms and conodonts into the lagoon, according to our current knowledge of the distribution of these marine taxa in the sequence.

The Granton sequence displays a variety of ductile and brittle deformation features most of which can be attributed to local SW-directed extension related to regional dextral strike-slip across the Midland Valley of Scotland.

Dolomites, a review of origins, geometry and textures

Dolomites form in a range of environments by processes able to drive large volumes of magnesium-rich waters through existing carbonate sediments or rocks. These fluids need not be of unusual composition but there are kinetic barriers to crystallisation which is influenced by factors such as the Mg/Ca ratio, salinity, temperature, the /Ca2+ ratio and time. Dolomite is able to form at near-surface temperatures and pressures within a few thousand years.

Textures in dolomitic rocks are controlled by their conditions of formation. A large proportion are replacive but few of these are mimetic, preserving primary structures. Crystals vary from euhedral to anhedral with boundaries ranging from planar to consertal. Solution chemistry and temperatures influence the density and distribution of nuclei together with growth rates and crystal morphology. There is still doubt whether irregular crystal faces are products of high or low temperatures or saturation. Dolomite cements are more important than has previously been realised in massively dolomitised rocks. Differential dissolution of aragonite, calcite, or evaporite minerals leaves space for these cements to occupy. Dolomitisation may also be allied to compaction, generating stylolitic rocks which are progressively enriched in dolomite. Dolomite may be replaced by calcite or it may be dissolved and the resulting pores filled with a calcite cement.

There is no general correlation between any set of petrographic features and particular geological models for dolomitisation. Similar physicochemical conditions are reproduced in a range of environments and the most effective guides to origin are in the geometry and regional petrographic variation of dolomite bodies.

The effect of submarine groundwater discharge on the ocean

The exchange of groundwater between land and sea is a major component of the hydrological cycle. This exchange, called submarine groundwater discharge (SGD), is comprised of terrestrial water mixed with sea water that has infiltrated coastal aquifers. The composition of SGD differs from that predicted by simple mixing because biogeochemical reactions in the aquifer modify its chemistry. To emphasize the importance of mixing and chemical reaction, these coastal aquifers are called subterranean estuaries. Geologists recognize this mixing zone as a site of carbonate diagenesis and dolomite formation. Biologists have recognized that terrestrial inputs of nutrients to the coastal ocean may occur through subterranean processes. Further evidence of SGD comes from the distribution of chemical tracers in the coastal ocean. These tracers originate within coastal aquifers and reach the ocean through SGD. Tracer studies reveal that SGD provides globally important fluxes of nutrients, carbon, and metals to coastal waters.

Precipitation of low-temperature dolomite from an anaerobic microbial consortium: the role of methanogenic Archaea

Here we report precipitation of dolomite at low temperature (30 degrees C) mediated by a mixed anaerobic microbial consortium composed of dissimilatory iron-reducing bacteria (DIRB), fermenters, and methanogens. Initial solution geochemistry is controlled by DIRB, but after 90 days shifts to a system dominated by methanogens. In live experiments conditions are initially saturated with respect to dolomite (Omega(dol) = 19.40) and increase by two orders of magnitude (Omega(dol) = 2 330.77) only after the onset of methanogenesis, as judged by the increasing [CH(4)] and the detection of methanogenic micro-organisms. We identify ordered dolomite in live microcosms after 90 days via powder X-ray diffraction, while sterile controls precipitate only calcite. Scanning electron microscopy and transmitted electron microscopy demonstrate that the precipitated dolomite is closely associated with cell walls and putative extra-cellular polysaccharides. Headspace gas measurements and denaturing gradient gel electrophoresis confirm the presence of both autotrophic and acetoclastic methanogens and exclude the presence of DIRB and sulfate-reducing bacteria after dolomite begins forming. Furthermore, the absence of dolomite in the controls and prior to methanogenesis confirm that methanogenic Archaea are necessary for the low-temperature precipitation of dolomite under the experimental conditions tested.

2. The composition of clay materials

Clay materials are composed of solid, liquid and vapour phases. The solid phases are of mineral and organic phases that make up the framework of the clay materials. The mineralogy can be broadly subdivided into the clay and non-clay minerals, including poorly crystalline, so-called ‘amorphous’ inorganic phases. By definition, minerals are crystalline solids with well-ordered crystal structures but clay minerals and other inorganic phases in clay materials are often poorly crystalline compared to minerals such as quartz and feldspar.

Some clay materials may be dominated by one mineral phase, e.g. smectite in bentonites, opal in diatomaceous earths. However, most clay materials are composed of heterogenous mineral mixtures. Based on the bulk mineral analysis of over 400 samples, Shaw & Weaver (1965) reported the modal mineralogical composition of siliciclastic mudrocks to be:

In soils, mineral and organic compositional variations reflect the weathered parent rocks and the physical, chemical and biological factors controlling the soil forming processes (see Chapter 3).

The liquid and vapour phases, of which water is usually the

Saline systems of the Great Plains of western Canada: an overview of the limnogeology and paleolimnology

In much of the northern Great Plains, saline and hypersaline lacustrine brines are the only surface waters present. As a group, the lakes of this region are unique: there is no other area in the world that can match the concentration and diversity of saline lake environments exhibited in the prairie region of Canada and northern United States. The immense number of individual salt lakes and saline wetlands in this region of North America is staggering. Estimates vary from about one million to greater than 10 million, with densities in some areas being as high as 120 lakes/km².

Despite over a century of scientific investigation of these salt lakes, we have only in the last twenty years advanced far enough to appreciate the wide spectrum of lake types, water chemistries, and limnological processes that are operating in the modern settings. Hydrochemical data are available for about 800 of the lake brines in the region. Composition, textural, and geochemical information on the modern bottom sediments has been collected for just over 150 of these lakes. Characterization of the biological and ecological features of these lakes is based on even fewer investigations, and the stratigraphic records of only twenty basins have been examined.

The lake waters show a considerable range in ionic composition and concentration. Early investigators, concentrating on the most saline brines, emphasized a strong predominance of Na⁺ and SO₄^-2 in the lakes. It is now realized, however, that not only is there a complete spectrum of salinities from less than 1 ppt TDS to nearly 400 ppt, but also virtually every water chemistry type is represented in lakes of the region. With such a vast array of compositions, it is difficult to generalize. Nonetheless, the paucity of Cl-rich lakes makes the northern Great Plains basins somewhat unusual compared with salt lakes in many other areas of the world (e.g., Australia, western United States). Compilations of the lake water chemistries show distinct spatial trends and regional variations controlled by groundwater input, climate, and geomorphology. Short-term temporal variations in the brine composition, which can have significant effects on the composition of the modern sediments, have also been well documented in several individual basins.

From a sedimentological and mineralogical perspective, the wide range of water chemistries exhibited by the lakes leads to an unusually large diversity of modern sediment composition. Over 40 species of endogenic precipitates and authigenic minerals have been identified in the lacustrine sediments. The most common non-detrital components of the modern sediments include: calcium and calcium-magnesium carbonates (magnesian calcite, aragonite, dolomite), and sodium, magnesium, and sodium-magnesium sulfates (mirabilite, thenardite, bloedite, epsomite). Many of the basins whose brines have very high Mg/Ca ratios also have hydromagnesite, magnesite, and nesquehonite. Unlike salt lakes in many other areas of the world, halite, gypsum, and calcite are relatively rare endogenic precipitates in the Great Plains lakes. The detrital fraction of the lacustrine sediments is normally dominated by clay minerals, carbonate minerals, quartz, and feldspars.

Sediment accumulation in these salt lakes is controlled and modified by a wide variety of physical, chemical, and biological processes. Although the details of these modern sedimentary processes can be exceedingly complex and difficult to discuss in isolation, in broad terms, the processes operating in the salt lakes of the Great Plains are ultimately controlled by three basic factors or conditions of the basin: (a) basin morphology; (b) basin hydrology; and (c) water salinity and composition. Combinations of these parameters interact to control nearly all aspects of modern sedimentation in these salt lakes and give rise to four 'end member' types of modern saline lacustrine settings in the Great Plains: (a) clastics-dominated playas; (b) salt-dominated playas; (c) deep water, non-stratified lakes; and (d) deep water, "permanently" stratified lakes.

Evidence for low sulphate and anoxia in a mid-Proterozoic marine basin

Many independent lines of evidence document a large increase in the Earth's surface oxidation state 2,400 to 2,200 million years ago, and a second biospheric oxygenation 800 to 580 million years ago, just before large animals appear in the fossil record. Such a two-staged oxidation implies a unique ocean chemistry for much of the Proterozoic eon, which would have been neither completely anoxic and iron-rich as hypothesized for Archaean seas, nor fully oxic as supposed for most of the Phanerozoic eon. The redox chemistry of Proterozoic oceans has important implications for evolution, but empirical constraints on competing environmental models are scarce. Here we present an analysis of the iron chemistry of shales deposited in the marine Roper Basin, Australia, between about 1,500 and 1,400 million years ago, which record deep-water anoxia beneath oxidized surface water. The sulphur isotopic compositions of pyrites in the shales show strong variations along a palaeodepth gradient, indicating low sulphate concentrations in mid-Proterozoic oceans. Our data help to integrate a growing body of evidence favouring a long-lived intermediate state of the oceans, generated by the early Proterozoic oxygen revolution and terminated by the environmental transformation late in the Proterozoic eon.

Oceanic minerals: their origin, nature of their environment, and significance

The chemical and isotopic compositions of oceanic biogenic and authigenic minerals contain invaluable information on the evolution of seawater, hence on the history of interaction between tectonics, climate, ocean circulation, and the evolution of life. Important advances and greater understanding of (a) key minor and trace element cycles with various residence times, (b) isotopic sources and sinks and fractionation behaviors, and (c) potential diagenetic problems, as well as developments in high-precision instrumentation, recently have been achieved. These advances provided new compelling evidence that neither gradualism nor uniformitarianism can explain many of the new important discoveries obtained from the chemistry and isotopic compositions of oceanic minerals. Presently, the best-developed geochemical proxies in biogenic carbonates are 18O/16O and Sr/Ca ratios (possibly Mg/Ca) for temperature; 87Sr/86Sr for input sources, Cd/Ca and Ba/Ca ratios for phosphate and alkalinity concentrations, respectively, thus also for ocean circulation; 13C/12C for ocean productivity; B isotopes for seawater pH;, U, Th isotopes, and 14C for dating; and Sr and Mn concentrations for diagenesis. The oceanic authigenic minerals most widely used for chemical paleoceanography are barite, evaporite sulfates, and hydrogenous ferromanganese nodules. Marine barite is an effective alternative monitor of seawater 87Sr/86Sr, especially where carbonates are diagenetically altered or absent. It also provides a high-resolution record of seawater sulfate S isotopes, (evaporite sulfates only carry an episodic record), with new insights on factors affecting the S and C cycles and atmospheric oxygen. High-resolution studies of Sr, Nd, and Pb isotopes of well-dated ferromanganese nodules contain invaluable records on climate driven changes in oceanic circulation.

Geochemistry of diagenetic non-silicate minerals: kinetic considerations

The early stages of burial diagenesis involve the reactions of various oxidizing agents with organic matter, which is the only reducing agent buried with the sediment. In a system in which a local equilibrium is established, thermodynamic principles indicate that, inter alia , manganese, iron and sulphate should each be consumed successively to give rise to a clearly characterized vertical zonation. However, ferric iron may not react fast enough and the relative rates of reduction of Fe III and sulphate not only control the formation of iron sulphide and associated carbonate but also may lead to extreme chemical and isotopic dis-equilibrium. This produces kinetically controlled ‘micro -environments’. On a larger scale, sulphide will diffuse upward to a zone in which its oxidation leads to a reduction of pH. The various dramatic changes in chemical environment across such an interface cause both dissolution and precipitation reactions. These explain common geological observations: the occurrence of flint nodules (and their restriction to chalk hosts) and the association of phosphate with glauconite.