David T. Wright

Nonphotosynthetic Bacteria and the Formation of Carbonates and Evaporites Through Time

Abstract Nonskeletal sedimentary carbonate rocks are an important component of the Precambrian geological record, but consensus on their origin is lacking. Phanerozoic carbonates are almost exclusively biogenic products of shelly fossils, but it has generally been assumed that carbonate rocks deposited before a shelly biota evolved in the marine environment formed by direct precipitation from supersaturated solution in seawater. However, there is no unequivocal empirical evidence that calcium carbonate or dolomite precipitates directly from modern seawater, and it has been suggested that kinetic inhibitors to carbonate precipitation, related to the low concentration and activity of the carbonate ion, cation hydration and ion complexing, are especially effective in saline waters. On the other hand, there is increasing evidence that these inhibitors can be overcome through microbial mediation. Bacteria have been implicated in calcium carbonate precipitation since the Archaean, and though best known in seas and lakes, microbial carbonates are also important in fluviatile, spring, cave, and soil environments. The mechanisms of microbial mineral precipitation appear diverse, but many bacteria exhibit an ability to change solution chemistry and control pH at the microscale, passively or actively, thereby creating the ambient conditions for both oversaturation of Ca2 + and CO3 2 − ions, and removal of kinetic inhibitors. Bacteria dominated the ecosystems of Precambrian shallow marine environments, enhancing their potential involvement in widespread carbonate formation. Chemical precipitation of evaporite minerals is generally accepted, but the involvement of microbes may be significant and underestimated. This review evaluates current knowledge and attempts to define some of the many questions that await resolution.

Sedimentary dolomite: a reality check

Abstract 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.

Heavy minerals solve structural and stratigraphic problems in Ordovician strata of the western Irish Caledonides

Heavy minerals in Ordovician successions in western Ireland record, in the Upper Arenig Sheeffry Formation, the erosion of an ophiolite/island arc complex. The appearance of staurolite and garnet at a basin-wide horizon in the Lower Llanvirn Upper Derrylea Formation signals the unroofing of the Dalradian metamorphic complex. Parts of the Ordovician sequence on Inishturk and in two small inliers are correlated with the standard sequence with unexpected results. The garnet-, sillimanite-, and staurolite-bearing Letter Formation correlates with the Upper Derrylea Formation and, on Inishturk, heavy minerals in south-younging turbidites reveal a sinistral ramp zone, that places the Sheeffry Formation structurally above and to the south of the younger Derrylea Formation.

Finance and the fuel cell industry: a review of the current financing climate

The substantial price decline seen on the worlds stock markets since the peak of 2000 has resulted in a significant rise in the level of risk aversion displayed by both the corporate and financial sectors with respect to new investments. The owner/managers of emerging fuel cell companies must come to terms with these post-boom conditions and realise that investment by external investors (or venture capitalist) often involves their active participation in the business. Furthermore, financial investors will expect owner/managers to be prepared to delegate a degree of control to professional managers when appropriate. The ownership of the company assets will also come under scrutiny with venture capitalists expecting to invest in an entity that owns the intellectual property. Start-up companies would be well advised to strive to understand the pressures faced by professional investors. Those with the greatest sense of financial and commercial realism stand a good chance of technical and commercial success.

Chapter 16 High-Resolution Heavy Mineral Analysis (HRHMA): A Brief Summary

Abstract High-resolution heavy mineral analysis (HRHMA) is the identification and categorisation of different varieties of individual heavy mineral species, based on the recognition that the majority of rock-forming and accessory minerals form in a diversity of size and habit and are represented by several chemical, structural, colour and optical varieties, controlled primarily by petrogenetic conditions. Because a wide range of lithologies provide detritus to siliciclastic sediments, their heavy mineral assemblages are complex and an individual species may comprise several kinds of varieties. Variables that prove most informative are grain morphology, colour and internal structure. These can be provenance-diagnostic, facies-independent or facies-sensitive. Failure to appreciate such information by grouping the diverse varieties of a particular species into one single mineral category can generate misleading information and often incorrect interpretations, while the full history of a sediment and accurate reconstruction of provenance remains concealed. HRHMA can be easily conducted under the polarising microscope, it is internally consistent and categories can easily be defined and kept without errors. Categorisation and point counting of the varieties of a single species also eliminates problems caused by density-controlled sorting during transport and dissolution processes that affect chemically unstable species.