Modest Disturbance Research Articles

True-Triaxial Experimental Study on Mechanical Behaviours and Acoustic Emission Characteristics of Dynamically Induced Rock Failure

Dynamic disturbance is an important factor for the failure of surrounding rocks in underground engineering projects, particularly under high static stress conditions. In this study, three dynamic disturbances are considered: a single-pulse dynamic load with a large stress amplitude (a mild disturbance), a low-frequency cyclic dynamic load with a moderate stress amplitude (a modest disturbance), and a high-frequency cyclic dynamic load with a small stress amplitude (a weak disturbance). Based on the true-triaxial experiments of coupled static and dynamic loads, the mechanical behaviors and acoustic emission characteristics of the dynamically induced rock failure are analyzed. The test results show that the dynamically induced rock failure depends not only on the initial static stress level but also on the dynamic disturbance type. In the rock failure induced by the mild disturbance, the increasing initial static stress produces the dual effect of an initial increase and then a decrease in the failure strength of the rock specimens. In the rock failure induced by the modest disturbance, a sufficiently high initial static stress is the basic condition to trigger rock failure. In the rock failure induced by the weak disturbance, the initial static stress threshold value needed to trigger the rock failure is higher than that needed for the modest disturbance. The rock failure induced by the mild disturbance is mainly contributed by the large input of disturbance energy and the slight impact effect. The rock failure induced by the modest disturbance is mainly contributed by the fatigue damage effect, which reduces the ultimate energy storage capacity of the rocks. The rock failure induced by the weak disturbance is related to not only the fatigue damage effect intensifying the specimen damage, but also the dynamic disturbance effect inducing the release of elastic strain energy. Additionally, the fracture mode for rocks subjected to the mild disturbance is predominantly the shear fracture mode. More tensile cracks are detected for rocks subjected to the modest disturbance than those subjected to the mild disturbance. More shear cracks and fewer tensile cracks are detected for rocks subjected to the weak disturbance than those subjected to the modest disturbance.

Disturbance intensity, timing and history interact to affect pasture weed invasion

Past disturbance events shape future community trajectories if their ecological legacies interact with contemporary disturbance regimes to affect recruitment and reproduction. In pastures, large-scale disturbance events can become undetectable in aboveground community components but persist as augmented weed seedbanks, suggesting disturbance history may be a predictor of weed invasion magnitude following subsequent disturbance. Using a randomized block factorial design, we investigated how soil disturbance intensity, timing and history interact to affect weed recruitment, diversity and reproduction by implementing a range of disturbance intensities in April and June 2004 in pastures of differing disturbance history. Response variables were followed for two years. Disturbance timing interacted with disturbance intensity and history to affect recruitment, with some June treatment combinations having ≥48% higher weed abundance than those in April. Relative to undisturbed controls, low to intermediate disturbance intensity facilitated recruitment by ≥117% but high intensity disturbance had highest recruitment, particularly in previously disturbed pastures (≥543% increase). In both years, weed species richness was highest in intense disturbances in previously disturbed pastures. Importantly, weed reproduction was nearly one order of magnitude higher in intense disturbance patches, especially in previously disturbed pastures, and increased through time by ≥243%. These findings indicate weed recruitment, diversity and reproductive output are seed- and microsite-limited, and that intense soil disturbance may result in high long-term weed abundance. Although moderate disturbance facilitated recruitment, reproduction here was low, suggesting modest disturbance will not appreciably increase weed abundance. The most common species were Taraxacum officinale and Poa pratensis, both of which can benefit forage-livestock production, but most establishing species were of low palatability. We recommend grassland managers explicitly integrate disturbance history into dynamic management planning, consider augmenting seedbanks with ‘desirable’ weed species to help communities recover following inevitable disturbance events and do not rely exclusively on aboveground characters to evaluate their system's vulnerability to undesirable plant establishment and persistence.

Disproportionate importance of nearshore habitat for the food web of a deep oligotrophic lake

In large deep oligotrophic lakes, multiple lines of evidence suggest that the shallow nearshore water provides disproportionately important feeding and breeding habitat for the whole-lake food web. We examined the trophic importance of the nearshore environment, human impacts nearshore, and several approaches to disturbance detection in a deep (190 m) oligotrophic lake with relatively modest residential development. In Lake Crescent, on the Olympic Peninsula of Washington (USA), stable isotope analysis demonstrated that apex salmonid predators derived more than 50% of their carbon from nearshore waters, even though this nearshore water accounted for only 2.5% of total lake volume. Unfortunately, it is this land–water interface that is initially degraded as shorelines are developed. We hypothesised that under these conditions of relatively modest disturbance, the effects of residential development would be strongly localised near to shore. Indeed, we found striking differences between developed and undeveloped sites in periphyton and associated organic matter, though there were no offshore signals of human impact in water nutrient analysis or paleolimnological investigations. Together, these results suggest that nearshore biological monitoring should be integrated in lake management plans to provide ‘early warning’ of potential food-web repercussions before pollution problems are evident in open water and comparatively intractable.

Eye Dryness Sensations After Refractive Surgery: Impaired Tear Secretion or "Phantom" Cornea?

The cornea is richly innervated by various functional types of sensory nerve fibers. When stimulated, these fibers evoke conscious sensations of different quality including ocular dryness, discomfort, and pain. Refractive surgery involves a variable degree of damage to corneal nerves. This leads to an altered expression of membrane ion channels at the injured and regenerating nerve fibers, giving rise to aberrant spontaneous and stimulus-evoked nerve impulse firing. It is speculated that these abnormal sensory discharges are read by the brain as ocular surface dryness. This would explain the high incidence of eye dryness sensations after photorefractive surgery, which are experienced by a large number of patients despite the often modest disturbance of tear secretion. Therefore, drugs that reduce abnormal activity in injured nerves may represent a therapeutic alternative for eye dryness sensations after refractive surgery.

Huntington’s disease

Although available treatments for Huntington's disease (HD) are imperfect, thoughtful application can positively impact quality of life. Dopamine antagonists can provide control of the troublesome hyperkinetic movements. These agents can also diminish the frequency of hallucinations and delusions when symptoms of psychosis occur. Classical neuroleptics have the widest utilization, although atypical antipsychotics are being increasingly used. Suppression of choreiform movements has also been reported with amantadine and tetrabenazine, which is not currently approved in the United States but under investigation. Alteration in mood can be successfully managed with a variety of antidepressant medications. Superior tolerability and value in the management of a variety of behavioral disturbances have lead to extensive use of serotonin reuptake inhibitors. Modest disturbance of mood can sometimes be addressed with anticonvulsant medications. Considered a manifestation of advanced disease, dementia is less commonly addressed therapeutically. However, gathering experience suggests improved cognitive function can occur with cholinesterase inhibitor therapy. Frequently overlooked is the value of rehabilitation services in the management of diverse symptoms. Although the value of a dysphagia evaluation is apparent, the benefit to be derived from physical and occupational therapy involvement cannot be overstated. Current therapeutic trials will undoubtedly provide additional therapies to moderate symptoms, but once the mechanism(s) of selective striatal projection neuron degeneration are delineated, a revolution in the management of HD will occur.

Interleukin-2 decreases accumbal dopamine efflux and responding for rewarding lateral hypothalamic stimulation

Systemic administration of interleukin-2 (IL-2) provoked marked alterations of responding for rewarding brain stimulation from the medial forebrain bundle (MFB). In particular, when animals were tested for ICSS immediately following IL-2 treatment only a modest disturbance of responding was evident. However, if animals were subsequently exposed to repeated daily ICSS sessions (24–168 h) in the drug-free state, rightward shifts in the rate intensity functions and significant increases in reward thresholds were apparent. These results were dependent upon the presence of IL-2 during the initial ICSS session. If animals were tested for ICSS 24 h after IL-2 administration, without an intervening test, performance was unaffected. Evaluation of nonreinforced behavior after IL-2 treatment revealed that ICSS remained under stimulus control and the cytokine did not provoke reward-unrelated performance deficits. Dopamine (DA) activity in the nucleus accumbens has been implicated in goal-directed responding to positively reinforcing stimuli and in the present investigation, using in vivo microdialysis, it was observed that IL-2 markedly reduced DA release from this region. It was suggested that the protracted consequences of IL-2 on ICSS likely do not involve motoric, soporific, attentional or cognitive changes, but may be attributable to its specific actions on motivational arousal, possibly engendered by the cytokine-induced diminution of accumbal DA efflux.

Interleukin-2 decreases accumbal dopamine efflux and responding for rewarding lateral hypothalamic stimulation

Systemic administration of interleukin-2 (IL-2) provoked marked alterations of responding for rewarding brain stimulation from the medial forebrain bundle (MFB). In particular, when animals were tested for ICSS immediately following IL-2 treatment only a modest disturbance of responding was evident. However, if animals were subsequently exposed to repeated daily ICSS sessions (24–168 h) in the drug-free state, rightward shifts in the rate intensity functions and significant increases in reward thresholds were apparent. These results were dependent upon the presence of IL-2 during the initial ICSS session. If animals were tested for ICSS 24 h after IL-2 administration, without an intervening test, performance was unaffected. Evaluation of nonreinforced behavior after IL-2 treatment revealed that ICSS remained under stimulus control and the cytokine did not provoke reward-unrelated performance deficits. Dopamine (DA) activity in the nucleus accumbens has been implicated in goal-directed responding to positively reinforcing stimuli and in the present investigation, using in vivo microdialysis, it was observed that IL-2 markedly reduced DA release from this region. It was suggested that the protracted consequences of IL-2 on ICSS likely do not involve motoric, soporific, attentional or cognitive changes, but may be attributable to its specific actions on motivational arousal, possibly engendered by the cytokine-induced diminution of accumbal DA efflux.

Partial night sleep deprivation reduces natural killer and celhdar immune responses in humans

Prolonged and severe sleep deprivation is associated with alterations of natural and cellular immune function. To determine whether alterations of immune function also occur after even a modest loss of sleep, the effects of early-night partial sleep deprivation on circulating numbers of white blood cells, natural killer (NK) cell number and cytotoxicity, lymphokine-activated killer (LAK) cell number and activity, and stimulated interleukin-2 (IL-2) production were studied in 42 medically and psychiatrically healthy male volunteers. After a night of sleep deprivation between 10 P.M. and 3 A.M., a reduction of natural immune responses as measured by NK cell activity, NK activity per number of NK cells, LAK activity, and LAK activity per number of LAK precursors (CD16,56, CD25) was found. In addition, concanavalin A-stimulated IL-2 production was suppressed after sleep deprivation due to changes in both adherent and nonadherent cell populations. After a night of recovery sleep, NK activity returned to baseline levels and IL-2 production remained suppressed. These data implicate sleep in the modulation of immunity and demonstrate that even a modest disturbance of sleep produces a reduction of natural immune responses and T cell cytokine production.

Impact ages of meteorites: A synthesis

Abstract—Isotopic ages of meteorites that indicate chronometer resetting due to impact heating are summarized. Most of the ages were obtained by the39Ar‐40Ar technique, but several Rb‐Sr, Pb‐Pb, and Sm‐Nd ages also suggest some degree of impact resetting. Considerations of experimental data on element diffusion in silicates suggest that various isotopic chronometers ought to differ in their ease of resetting during shock heating in the order K‐Ar (easiest), Rb‐Sr, Pb‐Pb, and Sm‐Nd, which is approximately the order observed in meteorites. Partial rather than total chronometer resetting by impacts appears to be the norm; consequently, interpretation of the event age is not always straightforward.Essentially all39Ar‐40Ar ages of eucrites and howardites indicate partial to total resetting in the relatively narrow time interval of 3.4–4.1 Ga ago (1 Ga = 109years). Several disturbed Rb‐Sr ages appear consistent with this age distribution. This grouping of ages and the brecciated nature of many eucrites and all howardites argues for a large‐scale impact bombardment of the HED parent body during the same time period that the Moon received its cataclysmic bombardment. Other meteorite parent bodies such as those of mesosiderites, some chondrites, and IIE irons also may have experienced this bombardment. These data suggest that the early bombardment was not lunar specific but involved much of the inner Solar System, and may have been caused by breakup of a larger planetismal.Although a few chondrites show evidence of age resetting ∼3.5–3.9 Ga ago, most impact ages of chondrites tend to fall below 1.3 Ga in age. A minimum of ∼4 impact events, including events at 0.3, 0.5, 1.2, and possibly 0.9 Ga appear to be required to explain the younger ages of H, L, and LL chondrites, although additional events are possible. Most L chondrites show evidence of shock, and the majority of39Ar‐40Ar ages of L chondrites fall near 0.5 Ga. The L chondrite parent body apparently experienced a major impact at this time, which may have disrupted it.The observations (1) that lunar highland rocks experienced major impact resetting of various isotopic chronometers ∼3.7–4.1 Ga ago; (2) that the HED parent body experienced widespread impact resetting of the K‐Ar chronometer but only modest disturbance of other isotopic systems, during a similar time period; (3) that ordinary chondrite parent bodies show much more recent and less extensive impact resetting; and (4) that impacts, which initiated cosmic‐ray exposure of most stone meteorites almost never reset isotopic chronometers, may all be a consequence of relative parent body size. Greater degrees of isotopic chronometer resetting occur in larger and warmer impact ejecta deposits that cool slowly. The relatively greater size of bodies like the Moon and Vesta (assumed to be the parent asteroid of HED meteorites) both permit such favorable ejecta deposits to occur more easily compared to smaller parent bodies (generally assumed for chondrites) and also protect parent objects from collisional disruption. Thus, impacts on larger bodies would tend to more easily reset chronometers, consistent with the observed relative ease of resetting of Moon (easiest), HED, chondrites and of K‐Ar (easiest), Rb‐Sr, other chronometers. In contrast, the more recent impact ages of chondrites are postulated to represent collisional disruption of smaller parent objects whose fragments are more readily removed from the meteorite source reservoirs. Impacts that initiate cosmic‐ray exposure are mostly small in scale and produce little heating.

Effects of a large sewage spill on a kelp forest community: Catastrophe or disturbance?

San Diego's sewage outfall broke during winter 1992, spilling 7.1 × 10 8 litres/d of treated effluent in kelp forest depths for a two month period during an El Niño event. The ecological implications for the Point Loma kelp forest community were studied by comparing long term data with conditions during and after the spill. Surface ammonium concentrations within 1 km of the break were at potentially toxic levels, and light levels were reduced enough to have inhibited kelp germination and growth. However, because of El Niño conditions, it is unlikely that kelp would have germinated in the absence of a spill. Beyond 1 km, high ammonium concentrations benefitted the nutrient-depleted surface canopy of giant kelp ( Macrocystis pyrifera). Measured sedimentation rates were significantly higher near the outfall during the spill and were strongly related to wave height; water motion, however, prevented sediment accumulation. Bioassays were conducted on a grid of stations surrounding the outfall. There were significant reductions in the density and growth of microscopic sporophytes of Macrocystis outplanted near the outfall during the spill, but this pattern disappeared in samples collected 11 d after the repair was completed and was not observed again. Sediments collected near the outfall during the spill significantly reduced Macrocystis germ tube elongation; a post repair assay showed no differences with respect to the outfall. No significant effects were observed in outplants of juvenile Macrocystis sporophytes, cup corals, and juvenile abalones. Video transects during the spill and subsequent diving observations provided no evidence of sediment accumulation or negative impacts on established animal populations. Kelp population dynamics at the permanent sites were predictable from existing population structure and El Niño conditions. Damage to kelps, apparently resulting from a combination of low light and nutrient conditions with mechanical damage from storms, construction activity, and barge anchor cables, was observed along the outfall immediately adjacent to the break point. Shortly after the outfall was repaired, upwelling improved conditions for kelp germination and growth, and the zone of maximum impact developed into a dense kelp forest. Suspension feeders, detritivores and sea urchins, whose natural history indicates they could have been affected by the spill, showed no unusual population changes. In the context of the continuum of disturbances observed in two decades of population studies at Point Loma, the spill was a modest disturbance similar to the natural vagaries of kelp recruitment. We emphasize that this spill was an intense but not chronic impact during an El Niño event that also stressed control areas. However, it is representative of massive spills in coastal regions, and the fact that a sewage spill of this magnitude had no lasting effects on a kelp forest community is of general interest.

A three-dimensional, time-dependent simulation of the global dynamical response of the thermosphere to a geomagnetic substorm

Some of the major features of the atmospheric circulation predicted to occur following a geomagnetic substorm in the earth's thermosphere are described on the basis of a three-dimensional, time-dependent simulation. The circulation changes and the gravity wave oscillatory phenomena are traced at two levels of the thermosphere, 120 and 240 km, over a period of 4 1 2 h following the substorm onset. For both aspects of the response, and for each height, the characteristics, magnitudes and propagation are quite distinct, and the three-dimensional model shows that very large differences occur between one local time and another. The night-time period, and particularly that between 0000 and 0600 LT, shows, at middle latitudes, the largest wind and wave amplitudes, and the most complex response. In general, the response at any location to an individual substorm can be considered as a series of wave-like disturbances. This is due to the nature of the impulsive disturbance which is concentrated in an annular ring corresponding to the auroral oval, of which each part generates both poleward-travelling as well as equatorward-travelling components, with approximate circular symmetry. Observing the response at 240 km, it appears that focusing occurs of the initial poleward wavefront, and in subsequent poleward wavefronts, the latter arising from the apparent ‘reflection’ of waves initially propagating sunward as they interact with the mean anti-sunward flow on the dayside. Three major waves can therefore be identified in the night-time mid-latitude thermosphere—the initial wave from the nearby auroral oval, a second wave following about 90 min later originating in the dayside auroral oval. This wave is stronger than the first and, being focused and guided by the enhanced mean anti-sunward flow, is capable of generating larger waves. About another 90 min later a third wave arrives due to the dayward propagating wave being reflected, refocused and guided by the mean flow, which can also cause comparable disturbances throughout the thermosphere. There is a strong enhancement of the contra-rotating vortices of the high latitude region—providing sunward flow in the morning and evening auroral ovals with a strong return anti-sunward flow over the polar cap, which extends well into the mid-latitude regions in the midnight and early morning period. In general, amplitudes of the wind and wave disturbances increase continuously with altitude above about 100 km, even allowing for the omission of propagating tidal winds from our model (generated by the lower atmosphere). For the modest disturbance simulated here—a substorm we consider typical of a period when K p ~ 6-winds of 200–250 m s −1 occur at 120 km, increasing to 500 m s −1 at 240 km, and near 400 km the winds exceed 600 m s −1. While at 240 km the response is dominated by oscillations superimposed on an enhancement of the general thermospheric circulation, at 120 km there is strong evidence that two long-lasting vortices may be created by the dynamical and energetic effects of the substorm, while there is yet no direct observational counterpart of such vortices—an anticyclone centred between 2200 and midnight, and a cyclone centred between 1000 and midday both in the auroral oval—they may be quite characteristic features of the slow recovery of the thermosphere from the impulsive effects of the additional energy input during geomagnetic substorms.

Ground‐based photometric observations of the magnetospheric dayside cleft

A ground‐based meridian‐scanning photometer was operated at Cape Parry, N.W.T., Canada (70°N, 125°W geographic) in November and December, 1973, to observe the atomic oxygen emissions at 6300 Å and 5577 Å, and the H‐beta emission at 4861 Å. A 6300 Å behaviour pattern that could be identified with the dayside cleft was observed during a quiet afternoon and during a magnetically disturbed period. During quiet times a 1 kR 6300 Å feature was observed at 78° invariant latitude while during a modest disturbance the intensity increased to 4 kR and the feature moved to 73° invariant. This confirmed that the 6300 Å signature is readily identifiable from the ground, and that Cape Parry is one of the few relatively accessible Northern hemisphere sites where the optical cleft emissions can be observed from the ground.