Limestone Layers and the Flood

Nosy: Can you describe this in more detail please?edge: Not sure what Rox means here.

ABSTRACT

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We report low-temperature microbial precipitation of dolomite in dilute natural waters from both field and laboratory experiments. In a freshwater aquifer, microorganisms colonize basalt and nucleate nonstoichiometric dolomite on cell walls. In the laboratory, ordered dolomite formed at near-equilibrium conditions from groundwater with molar Mg:Ca ratios of <1; dolomite was absent in sterile experiments. Geochemical and microbiological data suggest that methanogens are the dominant metabolic guild in this system and are integral to dolomite precipitation. We hypothesize that the attached microbial consortium reacts with the basalt surface, releasing Mg and Ca into solution, which drives dolomite precipitation via nucleation on the cell wall. These findings provide insight into the long-standing dolomite problem and suggest a fundamental role for microbial processes in the formation of dolomite across a wide range of environmental conditions.citation: Roberts, Jennifer A, Bennett, Philip C, Gonzalez, Luis A, Macpherson, G L, Milliken, Kitty L, 2004, Microbial precipitation of dolomite in methanogenic groundwater, Geology, vol.32, no.4, pp.277-280.

ABSTRACT

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The failure to precipitate dolomite experimentally at low temperatures or from seawater in which it is both supersaturated and the most thermodynamically favoured carbonate phase, together with its unequal distribution through geological time relative to limestone, are all aspects of the "dolomite problem", a subject of continuing controversy. A plethora of physicochemical models has been invoked to explain sedimentary dolomite formation, none of which satisfactorily addresses the basic problem of how kinetic barriers are overcome. These barriers are related to the disproportionate distribution of the component ions of dolomite, cation hydration and ion complexing in seawater. Competing claims for the effectiveness of sulphate as an inhibitor to dolomite formation further confuse the debate, although there are many reports of modern dolomite associated with bacterial sulphate reduction. The uppermost sediments in some lakes of the Coorong region of South Australia comprise almost 100% dolomite, and afford an ideal opportunity to study this association. Samples of lake waters taken during late evaporative stages of several shallow hypersaline dolomitic lakes showed high initial sulphate concentrations, high pH and high carbonate alkalinities. Pore waters from unlithified lake sediment cores directly below the lake-water sample sites showed a substantial and progressive decrease in sulphate concentrations with depth, coupled with an exponential increase in carbonate concentrations, through the sulphate-reduction zone. By the end of the evaporative cycle, sulphate was entirely removed. High bacterial counts on cultures from the sediment cores, and sulphur isotope values consistent with "bacterial" fractionation in lake waters, indicate that the chemical changes in ambient water chemistry can be related to active bacterial sulphate reduction. Laboratory experiments using sulphate reducers cultured from the lake sediments and simulating the anoxic microbiogeochemical environment of the lakes, have resulted in the precipitation of dolomite, demonstrating that bacterial sulphate reduction in the Coorong lakes modifies lake-water and pore-water chemistry so that dolomite precipitation is kinetically favoured. Given the wide spatial and temporal distribution of sulphate-reducing bacteria, and their frequent association, both past and present, with cyanobacteria, it is likely that this process was more widespread in the geological past when dolomite was found in far greater abundance than limestone. Bacterial sulphate reduction may thus have played an important role in dolomite formation throughout the geological record.citation: Wright, David T, Wacey, David, 2004, Sedimentary dolomite; a reality check, Monograph: The geometry and petrogenesis of dolomite hydrocarbon reservoirs, Geological Society Special Publications, vol.235, pp.65-74.

In some waters the concentration of calcium carbonate in the sea water is high enough to precipitate directly out of solution onto the seafloor. This is much more common in groundwater environments such as caves, but it apparently can happen in the ocean, too.

Thus, apart from (probably mythical) purely evaporitic and autotrophic ones, most limestones must be considered as principally of heterotrophic bacterial origin. As the carbon of limestones is issued from organic matter, bacterial heterotrophic carbonatogenesis appears as a fundamental phenomenon in the relationships between atmosphere and lithosphere during the biogeological evolution of the Earth.SOURCE: Camoin, Gilbert F, 1999, Ca-carbonates precipitation and limestone genesis; the microbiogeologist point of view, in Special issue: Microbial mediation in carbonate diagenesis,Sedimentary Geology, vol.126, no.1-4, pp.9-23.

AbstractExperiments show that the production of carbonate particles by heterotrophic bacteria follows different ways. In heterotrophy, the passive carbonatogenesis is generated by modifications of the medium that lead to the accumulation of carbonate and bicarbonate ions and to the precipitation of solid particles. It is induced by several metabolic pathways of the nitrogen cycle (ammonification of amino-acids, degradation of urea and uric acid, dissimilatory reduction of nitrates) and of the sulphur cycle (dissimilatory reduction of sulphates). The active carbonatogenesis is independent of the mentioned metabolic pathways. The carbonate particles are produced by ionic exchanges through the cell membrane following still poorly known mechanisms. In autotrophy, non-methylotrophic methanogenesis and cyanobacterial photosynthesis also may contribute to the precipitation of carbonates (autotrophic carbonates). As carbonatogenesis is neither restricted to particular taxonomic groups of bacteria nor to specific environments, it has been an ubiquitous phenomenon since Precambrian times. Carbonatogenesis is the response of heterotrophic bacterial communities to an enrichment of the milieu in organic matter. After a phase of latency, there is an exponential increase of bacterial numbers together with the accumulation of metabolic end-products. These induce a pH increase and an accumulation of carbonate and hydrogenocarbonate ions in the medium. This phase ends into a steady state when most part of the initial enrichment is consumed and there is a balance between death and growth in bacterial populations. Particulate carbonatogenesis occurs during the exponential phase and ends more or less after the beginning of the steady state. The active carbonatogenesis seems to start first and to be followed by the passive one which induces the growth of initially produced particles. In eutrophic conditions, the first solid products are patches that appear on the surface of the bacterial bodies and coalesce until forming a rigid coating and/or particles excreted from the cell. All these tiny particles assemble into biomineral aggregates which often display "precrystalline" structures. These aggregates grow and form biocrystalline build-ups which progressively display more crystalline structures with growth. In oligotrophic conditions, the primary solid products are rapidly smoothed in the crystalline structure and leave no trace. In present aqueous environments, apart from deep ocean, the potential efficiency of heterotrophic bacterial carbonatogenesis in Ca-carbonate sedimentation is much higher than autotrophic or abiotic processes. It much more likely accounts for extensive apparently abiotic limestone formation than any of the latter. As far as biodetrital particles are concerned, it may be observed that the shells and tests of organisms are built from the activity of cellular organites which are nowadays considered by a number of biologists as endosymbiotic bacteria. Thus, apart from (probably mythical) purely evaporitic and autotrophic ones, most limestones must be considered as principally of heterotrophic bacterial origin. As the carbon of limestones is issued from organic matter, bacterial heterotrophic carbonatogenesis appears as a fundamental phenomenon in the relationships between atmosphere and lithosphere during the biogeological evolution of the Earth.