Microbially mediated carbonate precipitation in a hypersaline lake,Big Pond (Eleuthera,Bahamas)

Microbial metabolism impacts the degree of carbonate saturation by changing the total alkalinity and calcium availability; this can result in the precipitation of carbonate minerals and thus the formation of microbialites. Here, the microbial metabolic activity, the characteristics and turnover of the extracellular polymeric substances and the physicochemical conditions in the water column and sediments of a hypersaline lake, Big Pond, Bahamas, were determined to identify the driving forces in microbialite formation. A conceptual model for organomineralization within the active part of the microbial mats that cover the lake sediments is presented. Geochemical modelling indicated an oversaturation with respect to carbonates (including calcite, aragonite and dolomite), but these minerals were never observed to precipitate at the mat–water interface. This failure is attributed to the capacity of the water column and upper layers of the microbial mat to bind calcium. A layer of high Mg‐calcite was present 4 to 6 mm below the surface of the mat, just beneath the horizons of maximum photosynthesis and aerobic respiration. This carbonate layer was associated with the zone of maximum sulphate reduction. It is postulated that extracellular polymeric substances and low molecular weight organic carbon produced at the surface (i.e. the cyanobacterial layer) of the mat bind calcium. Both aerobic and anaerobic heterotrophic microbes consume extracellular polymeric substances (each process accounting for approximately half of the total consumption) and low molecular weight organic carbon, liberating calcium and producing inorganic carbon. The combination of these geochemical changes can increase the carbonate saturation index, which may result in carbonate precipitation. In conclusion, the formation and degradation of extracellular polymeric substances, as well as sulphate reduction, may play a pivotal role in the formation of microbialites both in marine and hypersaline environments.     

Past changes in the geomagnetic field caused by glaciations and deglaciations

Contributions have been made to the theory of the relation between terrestrial magnetism and glaciations. The results are applied to the ice sheets of high-latitude regions. A review of Pleistocene palcomagnetic dating is given.     

Editorial

Abstract

The analysis of geographical information is compared with other production processes in which a user can only accept an end-product if it meets certain quality requirements. Whereas users are responsible for defining the levels of quality they need to use the results of the analyses of geographical information systems in their work, database managers, experts and modellers could greatly assist users to achieve the quality of results they seek by formalizing information on: (1) data collection, level of resolution and quality; (2) the use of the basic analytical functions of the geographical information system; and (3) the data requirements, sensitivity and error propagation in models. These meta-data could be incorporated in a knowledge base alongside the geographical information system where, together with procedures for on-line error propagation, a user could be advised on the best way to achieve a desired aim. If the analysis showed that the original constellation of data, methods and models could not achieve the aim with the desired quality, the intelligent geographical information system would present a range of alternative strategies—better methods, more data, different data, better models, better model calibration, or better spatial resolution—and their costs by which the user's aims could reasonably be achieved.     

Mineralogy and Petrology of a Tactite near Helena, Montana

The aluminous pyroxene, fassaite, occurs in two small tabularbodies within mafic plutonites of the Boulder Batholith nearits north-east margin twelve miles east of Helena, Montana.First described by Knopf & Lee (1957), the bodies are contact-metasomatizedlimestone septa, now magnesian-tactites, consisting chieflyof fassaite, spinel, garnet, vesuvianite, and clintonite. Lesscommon minerals include pargasite, diopside, wollastonite, sphene,perovskite, anorthite, forsterite, calcite and chlorite. Sometwenty-five microprobe analyses of the fassaite show it is variablein composition and largely consists of the components CaMgSi₂O₆(53–83 per cent), CaAl₂SiO₆ (7–25 per cent), CaFeAlSiO₆(8–28 per cent), and CaTiAl₂O₆ (0–7 per cent). Thestoichiometry generally requires that most of the iron is ferric,consistent with Mössbauer data taken on a typical sample.If fassaite analyses from these and other contact metamorphicrocks are plotted on a triangular diagram with Ca(Mg,Fe)Si₂O₆,CaAl₂SiO₆ and CaFeAlSiO₆ as end-members, the distribution ofpoints offers no positive evidence for a solvus gap betweenfassaite and diopside as proposed by Ginzburg (1969). The mostaluminous fassaites occur with spinel-clintonite ± grossularand have 25 per cent of the Si replaced by Al, making them truepolymorphs of a garnet (i.e. Gr₄₂And₂₃Pp₃₅). No unusual cationordering is detected in these fassaites by single-crystal X-rayphotographs or Mössbauer measurements. Smede's (1966) estimate of 3–4 km of stratigraphic coverfor the Boulder Batholith indicates pressures of approximately1 kb, in agreement with the occurrence of andalusite + K-feldsparin a hornfels at the Kokaruda Ranch complex. The partial assemblagesof grossular, epidote, perovskite, anorthite-wollastonite, anorthite-calcite,and fassaite-calcite require X_CO2 = 0·12 ± 0·08and T = 570 ± 10 °C at these pressures. These pressuresand temperatures place this occurrence in the upper portionsof the hornblende-hornfels facies after Turner (1968), althoughthe low pressures and water-rich fluids permit assemblages (wollastonite,calcite-forsterite-diopside) that Turner lists as characteristicof the pyroxene-hornfels facies.     

A Megatheropod Tooth from the Early Cretaceous of Fusui,Guangxi, Southern China

A nearly complete right maxillary or left dentary tooth（NHMG 10858） from the Lower Cretaceous Xinlong Formation of the Napai Basin, Fusui County, Guangxi, southern China, is described. The tooth is large in size, with a CBL of 37 mm. Given the geological age and its crown morphology, including the size, it is most likely that the tooth belongs to a carcharodontosaurid. The recovered specimen represents one of the largest theropod teeth hitherto reported from the Early Cretaceous of Asia.     

Magnetic mineral transport and sorting in the swash‐zone: northern Lake Erie,Canada

A combined field and laboratory study in northern Lake Erie has provided new insights into the origin and dynamics of heavy mineral placer deposits on beaches consisting primarily of non‐magnetic sediment. Work was conducted on the cross‐shore and longshore transport of heavy magnetic minerals using magnetic susceptibility and fluorescent paints to trace the movement, in the field, of samples of magnetic (magnetite) and non‐magnetic (quartz and calcite) grains, respectively. Laboratory experiments examined how the burial of small, dense magnetic minerals is affected by the grain size of the non‐magnetic host material, and how grain burial affects magnetic susceptibility measurements at the surface. The field experiments demonstrated that the magnetic mineral tracers were buried rapidly beneath coarser, non‐magnetic grains under low to moderate wave conditions, and subsequently were unable to move in the longshore or cross‐shore directions. The laboratory experiments showed that the magnetic susceptibility rapidly decreased with the rate and depth of burial of the magnetic minerals, and that magnetic grain burial was most effective beneath coarser rather than finer non‐magnetic sand and, for the latter sediments, under less rather than more energetic conditions. The results imply that magnetic mineral concentrations develop in this area through magnetic grain burial under fairly mild conditions, and subsequent settling, exposure and concentration in the upper swash zone during more energetic periods, when the non‐magnetic grains are eroded. It is probably during these erosional periods, when the magnetic minerals are exposed in fairly homogeneous deposits, that longshore and cross‐shore transport takes place.