Measurements of the vertical correlation in turbulence under broken waves

Turbulence measurements have been carried out in the surf zone of a wave flume. The purpose of the measurements is to determine the length scale of the turbulence generated by the wave breaking. The length scale of the turbulence is estimated on basis of the correlation between simultaneous measurements of the vertical turbulent fluctuations, taken at different levels above the bed.     

Dynamical passage to approximate equilibrium shapes for spinning, gravitating rubble asteroids

Many asteroids are thought to be particle aggregates held together principally by self-gravity. Here we study — for static and dynamical situations — the equilibrium shapes of spinning asteroids that are permitted for rubble piles. As in the case of spinning fluid masses, not all shapes are compatible with a granular rheology. We take the asteroid to always be an ellipsoid with an interior modeled as a rigid-plastic, cohesion-less material with a Drucker-Prager yield criterion. Using an approximate volume-averaged procedure, based on the classical method of moments, we investigate the dynamical process by which such objects may achieve equilibrium. We first collapse our dynamical approach to its statical limit to derive regions in spin-shape parameter space that allow equilibrium solutions to exist. At present, only a graphical illustration of these solutions for a prolate ellipsoid following the Drucker-Prager failure law is available [Sharma, I., Jenkins, J.T., Burns, J.A., 2005a. Bull. Am. Astron. Soc. 37, 643; Sharma, I., Jenkins, J.T., Burns, J.A., 2005b. Equilibrium shapes of ellipsoidal soil asteroids. In: García-Rojo, R., Hermann, H.J., McNamara, S. (Eds.), Proceedings of the 5th International Conference on Micromechanics of Granular Media, vol. 1. A.A. Balkema, UK; Holsapple, K.A., 2007. Icarus 187, 500-509]. Here, we obtain the equilibrium landscapes for general triaxial ellipsoids, as well as provide the requisite governing formulae. In addition, we demonstrate that it may be possible to better interpret the results of Richardson et al. [Richardson, D.C., Elankumaran, P., Sanderson, R.E., 2005. Icarus 173, 349-361] within the context of a Drucker-Prager material. The graphical result for prolate ellipsoids in the static limit is the same as those of Holsapple [Holsapple, K.A., 2007. Icarus 187, 500-509] because, when worked out, his final equations will match ours. This is because, though the formalisms to reach these expressions differ, in statics, at the lowest level of approximation, volume-averaging and the approach of Holsapple [Holsapple, K.A., 2007. Icarus 187, 500-509] coincide. We note that the approach applied here was obtained independently [Sharma, I., Jenkins, J.T., Burns, J.A., 2003. Bull. Am. Astron. Soc. 35, 1034; Sharma, I., 2004. Rotational Dynamics of Deformable Ellipsoids with Applications to Asteroids. Ph.D. thesis, Cornell University] and it provides a general, though approximate, framework that is amenable to systematic improvements and is flexible enough to incorporate the dynamical effects of a changing shape, different rheologies and complex rotational histories. To demonstrate our technique, we investigate the non-equilibrium dynamics of rigid-plastic, spinning, prolate asteroids to examine the simultaneous histories of shape and spin rate for rubble piles. We have succeeded in recovering most results of Richardson et al. [Richardson, D.C., Elankumaran, P., Sanderson, R.E., 2005. Icarus 173, 349-361], who obtained equilibrium shapes by studying numerically the passage into equilibrium of aggregates containing discrete, interacting, frictionless, spherical particles. Our mainly analytical approach aids in understanding and quantifying previous numerical simulations.     

Simulation of the Magnetothermal Instability

In many magnetized, dilute astrophysical plasmas, thermal conduction occurs almost exclusively parallel to magnetic field lines. In this case, the usual stability criterion for convective stability, the Schwarzschild criterion, which depends on entropy gradients, is modified. In the magnetized long mean free path regime, instability occurs for small wavenumbers when (∂ P/∂z) (∂ ln T/∂ z) > 0, which we refer to as the Balbus criterion. We refer to the convective-type instability that results as the magnetothermal instability (MTI). We use the equations of MHD with anisotropic electron heat conduction to numerically simulate the linear growth and nonlinear saturation of the MTI in plane-parallel atmospheres that are unstable according to the Balbus criterion. The linear growth rates measured from the simulations are in excellent agreement with the weak field dispersion relation. The addition of isotropic conduction, e.g. radiation, or strong magnetic fields can damp the growth of the MTI and affect the nonlinear regime. The instability saturates when the atmosphere becomes isothermal as the source of free energy is exhausted. By maintaining a fixed temperature difference between the top and bottom boundaries of the simulation domain, sustained convective turbulence can be driven. MTI-stable layers introduced by isotropic conduction are used to prevent the formation of unresolved, thermal boundary layers. We find that the largest component of the time-averaged heat flux is due to advective motions as opposed to the actual thermal conduction itself. Finally, we explore the implications of this instability for a variety of astrophysical systems, such as neutron stars, the hot intracluster medium of galaxy clusters, and the structure of radiatively inefficient accretion flows. J. M. Stone: Program in Applied and Computational Mathematics, Princeton University, Princeton, NJ 08544     

The role of bioindicators in estuarine management

Estuarine organisms can be considered three ways as bioindicators—first, as indicators of a defined set of environmental conditions, second, as indicators of contaminant loads on the system, and third, as indicators of the overall health of the system. By their very nature, estuarine organisms are eurytolerant and this, along with the various stratagems that have evolved to cope with the problems of life in estuaries results in a confusing picture for the estuarine manager. The recommended bioindicators for estuarine conditions are plankton; for contaminant loads, a range of species with different feeding types; and for system status, a eurytolerant species, such as the musselMytilus edulis for laboratory monitoring or a “target” species such as the clamMacoma balthica for field monitoring. However, it would be unwise to rely on a single indicator, and the recommendation is that the final choice, and preferably choices, be tailored to the desired goal.     

Influence of IHSS Standard and Reference Materials on Copper and Mercury Toxicity to Vibrio fischeri

The reduction in light emission of the marine bacterium Vibrio fischeri used in the standard Microtox^® bioassay was measured for the metals copper and mercury. The concentration at which the light emission was reduced by 50% (EC₅₀) was determined to be (3.43 ± 0.83) μmol/L for Cu²⁺ and (0.66 ± 0.01) μmol/L for Hg²⁺. The reduction of the toxicity of these metals by humic and fulvic acids were studied using IHSS Standard and Reference Materials. Copper toxicity was reduced 17...20% by the soil and peat fulvic acids and 9...20% by the aquatic fulvic acids. While there appeared to be little difference in the reduction of Cu toxicity by fulvic acids from soils, peats, or aquatic systems, Hg toxicity was reduced 3.6...7.3% by the soils and peats, while aquatic fulvic acids reduced Hg toxicity 14...16%. Soil fulvic acids appear to have significantly less capacity to reduce Hg toxicity than Cu toxicity. Humic acids had much higher reductions of Cu toxicity (44...124%) compared to the fulvic acids, with little difference between aquatic and soil or peat humic acids, 44...124% and 67...100%, respectively. However, humic and fulvic acids, regardless of origin, had approximately the same effect on Hg toxicity with 3.5...16% reduction by fulvic acids and 8...20% reduction by humic acids. Unlike the fulvic acids, no clear trend was observed relative to origin of the humic acids. There was no correlation between percent reduction of Cu or Hg toxicity by the organic compounds and copper binding capacity (CuBC), C/N ratio, or carboxyl content of the materials. Examination of natural organic matter (NOM) isolated by reverse osmosis techniques from three water sources had reductions of both Cu and Hg toxicity that were most similar to the Suwannee River and Nordic fulvic acids.     

Association Between <Emphasis Type="BoldItalic">Fusarium</Emphasis> spp. on <Emphasis Type="BoldItalic">Spartina alterniflora</Emphasis> and Dieback Sites in Connecticut and Massachusetts

Sudden vegetation dieback (SVD) is defined as the loss and lack of recovery of smooth cordgrass (Spartina alterniflora) in salt marshes. A new species of a moderately pathogenic fungus called Fusarium palustre is consistently found in SVD sites, but greenhouse tests revealed that it is not capable of causing mortality of healthy plants. Similarly, root-knot nematodes (Meloidogyne spartinae) are also found in SVD sites, but their incidence in marshes affected by SVD is not known. To understand more about the ecology of F. palustre and M. spartinae, salt marshes along Connecticut’s Long Island Sound and Massachusetts’ Cape Cod that exhibited SVD and those that did not, were visited during the summers of 2007, 2008, and 2009. Belowground and aboveground tissues of smooth cordgrass plants from 18 marshes were removed, washed, and assayed for Fusarium spp. to determine if patterns between the incidence of the different species of Fusarium, their virulence on S. alterniflora, root-knot nematodes (M. spartinae), and the health of the marsh could be revealed. There were significantly more colonies of Fusarium growing from plants in SVD sites (6.1%) than in healthy marshes where no SVD was present (<1.0%). The incidence of Fusarium spp. from plants at the perimeter of the SVD site was not statistically different from asymptomatic plants 10–20 m from the SVD edge. The majority of isolates could be assigned to one of two species, F. palustre or another slightly pathogenic group called Fusarium cf. incarnatum (88% in 2007, 62% in 2008, and 96% in 2009). The ratio of F. palustre to F. cf. incarnatum was 6.7, 2.7, or 2.1 for 2007, 2008, or 2009, respectively. Greenhouse tests on healthy S. alterniflora revealed that isolates of F. palustre were more virulent than F. cf. incarnatum, regardless of whether they were recovered from plants in healthy marshes or in SVD sites. Root-knot nematodes were found sporadically and could not be associated with SVD. Factorial greenhouse experiments did not demonstrate any interaction between F. palustre and M. spartinae providing no experimental evidence that combining Fusarium and root-knot nematodes could cause mortality. The presence of Fusarium on S. alterniflora in healthy marshes also suggests an endophytic relationship that may subsequently function in the breakdown of tissue when plants are compromised.     

Evolution of a Carboniferous carbonate-hosted sphalerite breccia deposit,Isle of Man

A newly discovered, extensive sphalerite-bearing breccia (~7.5 wt.% Zn) is hosted in dolomitised Carboniferous limestones overlying Ordovician–Silurian metasedimentary rocks on the Isle of Man. Although base metal sulphide deposits have been mined historically on the island, they are nearly all quartz vein deposits in the metamorphic basement. This study investigates the origin of the unusual sphalerite breccia and its relationship to basement-hosted deposits, through a combination of petrographic, cathodoluminescence, fluid inclusion, stable isotope and hydrogeologic modelling techniques. Breccia mineralisation comprises four stages, marked by episodes of structural deformation and abrupt changes in fluid temperature and chemistry. In stage I, high-temperature (T _h > 300°C), high-salinity (20–45 wt.% equiv. NaCl) fluid of likely basement origin deposited a discontinuous quartz vein. This vein was subsequently dismembered during a major brecciation event. Stages II–IV are dominated by open-space filling sphalerite, quartz and dolomite, respectively. Fluid inclusions in these minerals record temperatures of ~105–180°C and salinities of ~15–20 wt.% equiv. NaCl. The δ³⁴S values of sphalerite (6.5–6.9‰ Vienna-Canyon Diablo troilite) are nearly identical to those of ore sulphides from mines in the Lower Palaeozoic metamorphic rocks. The δ¹⁸O values for quartz and dolomite indicate two main fluid sources in the breccia’s hydrothermal system, local Carboniferous-hosted brines (~0.5–6.0‰ Vienna standard mean ocean water) and basement-involved fluids (~5.5–11.5‰). Ore sulphide deposition in the breccia is compatible with the introduction and cooling of a hot, basement-derived fluid that interacted with local sedimentary brines.     

Si,Al ordering in leucite by high-resolution 27Al MAS NMR spectroscopy

High-resolution ²⁷Al MAS NMR spectra of natural leucite recorded at H ₀=11.7T contain three resolvable resonances at ²⁷Al δ _i = 69.2, 64.7, and 61.0±0.5 ppm. These three resonances are assigned to the three inequivalent framework positions of leucite: T3, T2, and T1, respectively. Fitting the observed spectra yields a Si,Al distribution for leucite in which approximately one-half of the Al is in T1 and one-quarter in each of T2 and T3. This Si,Al distribution differs substantially from those obtained by previous workers using ²⁹Si NMR spectroscopy and X-ray diffraction. New ²⁹Si NMR spectra and revision of previously reported ²⁹Si NMR peak assignments, however, make the ²⁷Al and ²⁹Si NMR results consistent. The ²⁷Al δ _i correlate linearly with the mean T-O-T′ bond angles of the average structure, which allows the peak assignments to be made. However, this correlation lies distinctly toward higher frequency and larger bond angles than correlations for Si,Al ordered aluminosilicates, suggesting that the mean T(Al)-O-T′(Si) bond angle for each site in leucite is smaller than the mean bond angle of the average structure, which is averaged over T(Al)-O-T′(Si) and T(Si)-OT′(Si,Al) angles.