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No abstract available.
A lack of stratigraphical, sedimentological and geochemical data for sediment accumulation rates and indicators of productivity and anoxia means that causative models for ancient black shales are largely inferred from modern settings. Coastal upwelling has been suggested as a general hypothesis for Ordovician black shale deposition within the Iapetus Ocean, but has not been directly tested. Despite anchizone metamorphism we utilize a suite of geological and geochemical environmental proxy data (TOC wt.%, δ13Corg, Ba/Al, P) to elucidate the mechanism for the origin of a single grey-black shale cycle within the upper Katian succession of the Welsh Basin. Here we interpret organic carbon (OC)-rich deposition to be suggestive of a period of high photic productivity (higher TOC wt.%), with 12C and Ba enrichment, compar...
Earthworms of the family Lumbricidae, which includes many common species, produce and secrete up to millimeter-sized calcite granules, and the intricate fine-scale zoning of their constituent crystals is unique for a biomineral. Granule calcite is produced by crystallization of amorphous calcium carbonate (ACC) that initially precipitates within the earthworm calciferous glands, then forms protogranules by accretion on quartz grain cores. Crystallization of ACC is mediated by migrating fluid films and is largely complete within 24 h of ACC production and before granules leave the earthworm. Variations in the density of defects formed as a byproduct of trace element incorporation during calcite crystal growth have generated zoning that can be resolved by cathodoluminescence imaging at ultraviolet to blue wavelengths and using the novel ...
The initial steps of silica polymerization and silica nanoparticle formation have been studied in-situ and in real-time. The experiments were carried out in near neutral pH (7–8) solutions with initial silica concentrations of 640 and 1600 ppm ([SiO2]) and ionic strengths (IS) of 0.02, 0.05, 0.11 and 0.22 M. The polymerization reactions were induced by neutralizing a high pH silica solution (from pH 12 to 7) and monitored by the time-dependent depletion in monosilicic acid concentration over time. The accompanied nucleation and growth of silica nanoparticles (i.e., change in particle size over time) was followed in-situ using time-resolved synchrotron-based Small Angle X-ray Scattering (SAXS) and conventional Dynamic Light Scattering (DLS) combined with scanning and (cryo)-transmission electron microscopy (SEM/...
No abstract available.
Avalon-type fossils are crucial to understanding the origin of Phanerozoic ecosystems, but their usual occurrence in volcaniclastic and siliciclastic facies greatly limits their paleobiological resolution. The unique carbonate-hosted assemblage of the Khatyspyt Formation, on the Olenek uplift of north-central Siberia, promises a much enhanced anatomical and paleo-ecological view of these enigmatic organisms. Avalon-type fossils are preserved by authigenic carbonate cementation in intervals of finely laminated nodular limestones (Khatyspyt-type taphonomic window). Interbedded silicified calcareous mudstones yield diverse carbonaceous compressions, occasionally with taphonomic phantoms of Avalon-type taxa (Miaohe-type taphonomic window). Styles of moldic preservation do not appear to be taxon selective, and therefore cannot alone be resp...
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