Boulder, Colo., USA - The Geological Society of America has posted a new batch of Geology papers online ahead of print (23 May 2012). Locations studied include the Rainy Lake zone, Ontario, Canada; the Black Sea;
Boulder, Colo., USA - The Geological Society of America has posted a new batch of Geology papers online ahead of print (23 May 2012). Locations studied include the Rainy Lake zone, Ontario, Canada; the Black Sea; Lake Matano, Indonesia; the Great Bahama Bank; Lake Eyre, Australia; the Burullus Lagoon of the Nile Delta, Egypt; and Tungurahua volcano, Ecuador. Studies involve work both in the field and in the lab.
Highlights are provided below. GEOLOGY articles published ahead of print can be accessed online at http://geology.
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The tectonic significance of dikes of irregular fold-like shape
Elena Druguet et al., Dept. de Geologia, Universitat Autonoma de Barcelona, 08193 Bellaterra, Barcelona, Spain. Posted online 23 May 2012; doi: 10.1130/G32960.1.
Dikes and veins are common features in orogenic regions, and they may often deform subsequent to their emplacement. Deformation of dikes results in folded (shortened) or boudined (stretched) geometries, depending on the relative strengths of the dike and host rock and the relative orientation between the dikes and strain. The resultant geometries are often used to analyze the tectonic history, but analysis is typically predicated on the assumption that the dikes had planar shapes prior to deformation. This assumption is not always valid, particularly where dikes intrude layered or banded rocks with strength contrasts. In this study, Elena Druguet and colleagues test the effects of post-emplacement deformation on originally non-planar dikes experimentally and compare them to field data from the Rainy Lake zone (Ontario, Canada), where irregular fold-like structures are observed. The analyses reveal that initial geometrical irregularities can be amplified or subdued with deformation depending on the strength contrast between the dike and host rock, the layer geometries, and the orientation of the imposed strain, resulting in either over- or under-estimation of strain using standard procedures. Relevant criteria are given to distinguish between intrusive and deformation structures in intensely deformed areas.
Microbial nucleation of Mg-rich dolomite in exopolymeric substances under anoxic modern seawater salinity: New insight into an old enigma
Stefan Krause et al., Dept. of Marine Biogeochemistry, Helmholtz Centre for Ocean Research Kiel (GEOMAR), 24148 Kiel, Germany. Posted online 23 May 2012; doi: 10.1130/G32923.1.
The origin of the dolomite formations observed in the rock record has been debated for more than a 100 years. The present laboratory study revealed that biofilms of sulfate-reducing bacteria are causative for the formation of dolomite under modern anoxic seawater conditions. The findings support modern theories of microbially mediated dolomite formation, suggesting that microbial dolomite has been produced in the world's ocean in former geological periods. During culturing experiments with a pure bacteria strain, dolomite nano-crystals nucleated exclusively within extracellular polymeric substances (EPS) of the biofilm, subsequently intercalating to micrometer-scale crystals. With regards to the surrounding bulk liquid, EPS accumulated relatively more calcium than magnesium ions, causing a considerable decrease of the magnesium/calcium ratio within the biofilm. In addition, light calcium isotopes accumulated within the biofilm, creating a pre-fraction calcium reservoir, from which the dolomite crystals appeared to precipitate. The sequential increase of light calcium isotopes from the bulk liquid to the biofilm, and to the crystals suggests a two-step fractionation process during the sequestration of calcium from the bulk liquid into the crystal. The observed involvement of microbiologically-produced substances during isotopic fractionation might contribute to the interpretations of rock record calcium isotopy to further constrain paleo-environmental conditions.
Extreme change in sulfide concentrations in the Black Sea during the Little Ice Age reconstructed using molybdenum isotopes
G.L. Arnold et al., Max Planck Institute for Marine Microbiology, 28359 Bremen, Germany. Posted online 23 May 2012; doi: 10.1130/G32932.1.
A novel approach allows G.L. Arnold and colleagues to use the molybdenum (Mo) isotope signature of sediments as a quantitative paleo-sulfide concentration proxy. Applying this technique in the Black Sea, they reconstruct historical sulfide concentrations in the area of the Bosporus Inle