506 research outputs found
Post-fossilization processes and their implications for understanding Ediacaran macrofossil assemblages
Fossil assemblages from Newfoundland's Avalon Peninsula preserve diverse examples of the enigmatic Ediacaran macrobiota, offering some of the earliest evidence for large and complex multicellular life. These fossils are exposed on extensive coastal bedding planes in extraordinary abundances, permitting palaeoecological studies based on census data from spatially extensive palaeocommunities. Such studies have been used to constrain the reproductive strategy and phylogenetic placement of Ediacaran organisms. Geological mapping and stratigraphic correlation in the Mistaken Point Ecological Reserve reveal that some fossil-bearing surfaces can be tracked over distances of several kilometres. These laterally extensive surfaces reveal that the modern processes by which the sediment overlying a fossil surface is removed may impose important controls on the observed composition of fossil assemblages. Weathering and erosion â along with factors associated with tectonics, metamorphism and discovery â are here grouped as âpost-fossilization processesâ and introduce biases that are often not explicitly accounted for in palaeoecological studies. Specifically, post-fossilization processes may differentially influence the preservational fidelity of individual specimens on a given surface and generate features that could be mistaken for original morphological characters. We therefore recommend that post-fossilization processes must be considered when undertaking palaeoecological studies in Ediacaran successions in Newfoundland and, potentially, elsewhere
Condensates beyond mean field theory: quantum backreaction as decoherence
We propose an experiment to measure the slow log(N) convergence to mean-field
theory (MFT) around a dynamical instability. Using a density matrix formalism,
we derive equations of motion which go beyond MFT and provide accurate
predictions for the quantum break-time. The leading quantum corrections appear
as decoherence of the reduced single-particle quantum state.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
Great Canadian LagerstÀtten 6. Mistaken Point Ecological Reserve, Southeast Newfoundland
Mistaken Point Ecological Reserve (MPER) World Heritage Site, on the southeastern coast of Newfoundland, Canada, is one of the foremost global Ediacaran fossil localities. MPER contains some of the oldest known assemblages of the soft-bodied Ediacaran macrobiota, and its fossils have contributed significantly to Ediacaran paleobiological research since their initial discovery in 1967. Preservation of multiple in situ benthic paleocommunities, some comprising thousands of specimens, has enabled research into Ediacaran paleoecology, ontogeny, taphonomy, taxonomy and morphology, offering insights into the possible phylogenetic positions of Ediacaran taxa within the tree of life. Meanwhile, a thick and continuous geological record enables the fossils to be placed within a well-resolved temporal and paleoenvironmental context spanning an interval of at least 10 million years. This article reviews the history of paleontological research at MPER, and highlights key discoveries that have shaped global thinking on the Ediacaran macrobiot
Dynamics of a two-mode Bose-Einstein condensate beyond mean-field theory
We study the dynamics of a two-mode Bose-Einstein condensate in the vicinity
of a mean-field dynamical instability. Convergence to mean-field theory (MFT),
with increasing total number of particles , is shown to be logarithmically
slow. Using a density matrix formalism rather than the conventional
wavefunction methods, we derive an improved set of equations of motion for the
mean-field plus the fluctuations, which goes beyond MFT and provides accurate
predictions for the leading quantum corrections and the quantum break time. We
show that the leading quantum corrections appear as decoherence of the reduced
single-particle quantum state; we also compare this phenomenon to the effects
of thermal noise. Using the rapid dephasing near an instability, we propose a
method for the direct measurement of scattering lengths.Comment: 17 pages, 9 figures, Phys. Rev. A 64, 0136XX (2001
Petrological evidence in support of the death mask model for Ediacaran soft-bodied preservation in South Australia
Microbially mediated early diagenetic pyrite formation in the immediate vicinity of organic material has been the favoured mechanism by which to explain widespread preservation of soft-bodied organisms in late Ediacaran sedimentary successions, but an alternative rapid silicification model has been proposed for macrofossil preservation in sandstones of the Ediacara Member in South Australia. We here provide petrological evidence from Nilpena National Heritage Site and Ediacara Conservation Park to demonstrate the presence of grain-coating iron oxides, framboidal hematite, and clay minerals along Ediacara Member sandstone bedding planes, including fossil-bearing bed soles. SEM and petrographic data reveal that framboids and grain coatings, which we interpret as oxidized pyrite, formed before the precipitation of silica cements. In conjunction with geochemical and taphonomic considerations, our data suggest that anactualistically high concentrations of silica need not be invoked to explain Ediacara Member fossil preservation: we conclude that the pyritic âdeath maskâ model remains compelling.AGL is funded by the Natural Environment Research Council [grant number NE/L011409/2]. SM acknowledges support from the European Unionâs Horizon 2020 Research and Innovation Programme under Marie SkĆodowska-Curie grant agreement 747877 ... JJM recognises support from Mitacs ..
The arrangement of possible muscle fibres in the Ediacaran taxon <i>Haootia quadriformis</i>
Haootia quadriformis from Newfoundland, Canada, is one of the most unusual impressions of a soft-bodied macro-organism yet described from the late Ediacaran Period. Interpreted as a metazoan of cnidarian grade, the body impression of H. quadriformis possesses features interpreted as fibrous structures that represent possible evidence for muscular tissue. Evidence both in support of and against a relationship between H. quadriformis and the Staurozoa, one of the cnidarian groups to which Haootia was compared in Liu et al., is outlined by Miranda et al.. Our intention in our original paper was to illustrate the staurozoan body plan for comparative purposes, rather than suggest homology or direct ancestry. Nevertheless, fresh insights from workers with expertise in the biology of extant cnidarians are welcomed
Nonlinear Josephson-type oscillations of a driven, two-component Bose-Einstein condensate
We propose an experiment that would demonstrate nonlinear Josephson-type
oscillations in the relative population of a driven, two-component
Bose-Einstein condensate. An initial state is prepared in which two condensates
exist in a magnetic trap, each in a different hyperfine state, where the
initial populations and relative phase between condensates can be controlled
within experimental uncertainty. A weak driving field is then applied, which
couples the two internal states of the atom and consequently transfers atoms
back and forth between condensates. We present a model of this system and
investigate the effect of the mean field on the dynamical evolution.Comment: 4 pages, 3 fig
Excitation of a Dipole Topological Mode in a Strongly Coupled Two-Component Bose-Einstein Condensate
Two internal hyperfine states of a Bose-Einstein condensate in a dilute
magnetically trapped gas of Rb atoms are strongly coupled by an
external field that drives Rabi oscillations between the internal states. Due
to their different magnetic moments and the force of gravity, the trapping
potentials for the two states are offset along the vertical axis, so that the
dynamics of the internal and external degrees of freedom are inseparable. The
rapid cycling between internal atomic states in the displaced traps results in
an adiabatic transfer of population from the condensate ground state to its
first antisymmetric topological mode. This has a pronounced effect on the
internal Rabi oscillations, modulating the fringe visibility in a manner
reminiscent of collapses and revivals. We present a detailed theoretical
description based on zero-temperature mean-field theory.Comment: 10 pages, 8 eps figures included; submitted to PR
Late Ediacaran occurrences of the organic-walled microfossils Granomarginata and flask-shaped Lagoenaforma collaris gen. et sp. nov.
New occurrences of flask-shaped and envelope-bearing microfossils, including the predominantly Cambrian taxon Granomarginata, are reported from new localities, as well as from earlier in time (Ediacaran) than previously known. The stratigraphic range of Granomarginata extends into the Cambrian System, where it had a cosmopolitan distribution. This newly reported Ediacaran record includes areas from Norway (Baltica), Newfoundland (Avalonia) and Namibia (adjacent to the Kalahari Craton), and puts the oldest global occurrence of Granomarginata in the Indreelva Member (< 563 Ma) of the StĂĄhpogieddi Formation on the Digermulen Peninsula, Arctic Norway. Although Granomarginata is rare within the assemblage, these new occurrences together with previously reported occurrences from India and Poland, suggest a potentially widespread palaeogeographic distribution of Granomarginata through the middleâlate Ediacaran interval. A new flask-shaped microfossil Lagoenaforma collaris gen. et sp. nov. is also reported in horizons containing Granomarginata from the StĂĄhpogieddi Formation in Norway and the Dabis Formation in Namibia, and flask-shaped fossils are also found in the Gibbett Hill Formation in Newfoundland. The GranomarginataâLagoenaforma association, in addition to a low-diversity organic-walled microfossil assemblage, occurs in the strata postdating the Shuram carbon isotope excursion, and may eventually be of use in terminal Ediacaran biostratigraphy. These older occurrences of Granomarginata add to a growing record of body fossil taxa spanning the EdiacaranâCambrian boundary
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