Rounding of pumice clasts during transport: field measurements and laboratory studies

Volcanic clasts in many pyroclastic density current deposits are notably more round than their counterparts in corresponding fall deposits. This increase in roundness and sphericity reflects different degrees of comminution, abrasion and breakup during transport. We performed experimental measurements to determine an empirical relationship between particle shape and mass loss caused by particle–particle interactions. We consider, as examples, pumice from four volcanoes: Medicine Lake, California; Lassen, California; Taupo, New Zealand; Mount St Helens, Washington. We find that average sample roundness reaches a maximum value once particles lose between 15% and 60% of their mass. The most texturally homogeneous clasts (Taupo) become the most round. Crystal-rich pumice abrades more slowly than crystal-free pumice of similar density. Abrasion rates also decrease with time as particles become less angular. We compare our experimental measurements with the shapes of clasts in one of the May 18, 1980 pyroclastic density current units at Mount St Helens, deposited 4–8 km from the vent. The measured roundness of these clasts is close to the experimentally determined maximum value. For a much smaller deposit from the 1915 Lassen eruption, clast roundness is closer to the value for pumice in fall deposits and suggests that only a few volume percent of material was removed from large clasts. In neither field deposit do we see a significant change in roundness with increasing distance from the vent. We suggest that this trend is recorded because much of the rounding and ash production occur in proximal regions where the density currents are the most energetic. As a result, all clasts that are deposited have experienced similar amounts of comminution in the proximal region, and similar amounts of abrasion as they settle through the dense, near-bed region prior to final deposition.     

Movement of logperch—the obligate host fish for endangered snuffbox mussels: implications for mussel dispersal

Unionid mussels are highly imperiled and the survival of their local populations is linked to the availability and dispersal potential of their host fish. We examined the displacement distance of logperch (Percina caprodes), which are obligate host fish for endangered snuffbox mussels (Epioblasma triquetra), to determine the dispersal potential by fish. Logperch in the Sydenham River, Ontario, Canada, were electrofished and marked with visible implant elastomer on five sampling dates during the gravid period of E. triquetra. The majority of all recaptures (82%) of P. caprodes occurred within 30 m of their original capture location, with a mean displacement distance of 13 ± 3 m (mean ± standard error, n = 28). These results were consistent with a review of movement studies of small benthic host fish (i.e., darters and sculpins), which revealed average fish displacement distances of 37 ± 19 m (n = 14 species; range: 4–275 m). However, significantly greater movement distances were also found and the maximum displacement distance increased significantly with the spatial extent of the study and with the duration of the study. These results indicate that many P. caprodes remain in a small area, which could restrict the dispersal and (re)colonization potential of E. triquetra. Further studies are needed to determine the dispersal potential of mussels via host fish, which may be important for maintaining the connectivity among unionid populations.     

Optimization of interpolation parameters when deriving DEM from contour lines

Digital elevation models (DEMs) are still an important and current source of information for digital soil mapping and the modeling of soil processes. The grid DEM is often interpolated from contour lines. The contour sampling step becomes an additional interpolation parameter which can play an important role. The objective of this paper is to optimize the interpolation parameters of the Regularized spline with tension (RST) method, in order to prepare a DEM suitable as an input for erosion modeling. Two contrasting cases, with and without a reference DEM, were investigated. If a reference DEM was available, good results of interpolation were reached both by small and larger sampling steps. In the second case, it was found that small sampling steps should be avoided. The influence of the sampling was demonstrated by topographic potential for erosion and deposition.     

Microbial community and activity shifts after soil transplantation in a glacier forefield

The majority of Alpine glaciers are currently receding because of global warming. Their forefields have become interesting sites to study primary microbial colonisation and microbial adaptation. Here, the structure and enzyme activity of microbial communities in exposed rock substrates and their changes in a gradient of temperature and soil moisture conditions within the forefield of the Damma glacier in the Swiss Central Alps are discussed. The temperature at the sites differed in the course of a day and also showed differing mean temperatures over the summer. Distinct bacterial communities inhabit the differing sites at the beginning of the experiment and even after transplantation they stay distinct. But a seasonal shift in the communities could be observed, which followed the same pattern for all the samples. Interestingly, microbial enzyme activity was highest at the site with the smallest temperature shifts.     

Dust Features of Visually Bright AGB Variables as Seen by ISO and IRAS

ISO-SWS spectra of two Mira variables (R Cas and T Dra), taken at two different pulsational phases, are presented. We describe the reduction of the data and the encountered problems. The dust features observed by ISO at different phases and with the Low Resolution Spectrometer of IRAS are compared. We briefly discuss the results in terms of current models for the circumstellar dust shells of AGB variables. This revised version was published online in September 2006 with corrections to the Cover Date.     

Observations of Jet Diameter, Density and Dynamics

Collimated jets are believed to be an essential ingredient of the star formation process, and we are now able for the first time to test observationally the theories for their formation and propagation. The major advances achieved in recent years are reviewed, regarding the observed morphology, kinematics and excitation properties of jets, from the parsec-scale `giant outflows' down to the `microjets' from T Tauri stars. High angular resolution images and spectra have provided valuable estimates of jet diameter, space velocity, temperature, ionization fraction, electron and total density, both along and across the flow. We can thus calculate key physical quantites, as the shock excitation parameters, or the mass and momentum fluxes in the flow. The results obtained appear to validate the popular magneto-centrifugal models for jet launching, although some important issues are still under debate, as to the cause of knotty structures, observed wind thermal properties, and the dynamical relationship between jets and molecular outflows. Among the most interesting recent findings, we mention the observed indications for jet rotation, with inferred toroidal velocities consistent with the prescribed angular momentum balance between infall and outflow.     

Synthetic radii and visibilities for pulsating red giants

We present synthetic radii and visibilities in the IR, based on hydrostatic and dynamic model atmospheres of carbon stars. The differences between these two sets of input models and to uniform disc models are discussed and compared to the expected capabilities of the VLTI. This revised version was published online in July 2006 with corrections to the Cover Date.