Instability in the 3rd Dimension of the Extended Störmer Problem

The problem of motions of charged particles or charged grains of matter in the field of two magnetic dipoles in rotation about a common center are studied by means of computing families of periodic solutions and their stability. Ten such families are found and their stability study shows that only two out of the ten have short arcs of stable members, all with low inclination to the plane of the two dipoles, while the rest of the families consist exclusively of unstable solutions. Hence the impact of adding one dipole to the configuration of the Störmer problem, leads to the conclusion that in the modified system the three-dimensional motions are basically unstable unlike those of the one-dipole case. The model magnetic field used in this study is a first approximation of the Z3model of the magnetic field of Saturn as computed from the Pioneer 11 and Voyager 1 and 2 measurements.     

Subtidal flow and its variability at the entrance to a subtropical lagoon

Spatial and temporal variability of the subtidal exchange flow at West Pass, an inlet at the entrance to a subtropical lagoon (St. Andrew Bay, Florida), was studied using moored and towed current velocity profiles and hydrographic data. Towed and hydrographic measurements were captured over one diurnal tidal cycle to determine intratidal and spatial changes in flow. Hydrographic profiles over the tidal cycle showed that tidal straining modified density stratification asymmetrically, thus setting up the observed mean flow within the inlet. During the towed survey, the inlet's mean flow had a two-layer exchange structure that was moderately frictional and weakly influenced by Coriolis accelerations. Moored current profiles revealed the additional contribution to the dynamics from centrifugal accelerations. Along channel residual flows changed between unidirectional and exchange flow, depending on the forcing from the along-estuary wind stress and, to a lesser extent, the spring–neap tidal cycle. Increases in vertical shear in the along channel subtidal flow coincided with neap tides and rain pulses. Lateral subtidal flows showed the influence on the dynamics of centrifugal accelerations through a well-developed two-layer structure modulated in magnitude by the spring–neap tidal cycle.     

Tidal and residual circulation in the St. Andrew Bay system,Florida

Two 24-h surveys were conducted in St. Andrew Bay, Florida, during spring and neap tides to describe the tidal and non-tidal circulation patterns and to determine the factors that affect these patterns. In particular, the effect of tidal forcing in modulating such circulation patterns was explored. Observed velocities were fitted to diurnal and semidiurnal harmonics separating tidal motions from sub-tidal motions. Residual flows were compared with an analytic model that allowed variations in the relative contributions from Coriolis acceleration and friction using the Ekman number. A solution with an Ekman number of 0.04 resembled the observations best and indicated that the hydrodynamics were governed by pressure gradient, Coriolis and friction. Locally, advective accelerations became important around headlands in sub-estuaries in the system. The consistency of the mean pattern from October to March suggests that tides play a minor role in modulating the exchange flow. Deviations from the long-term mean are mainly caused by wind-driven coastal setup and setdown.     

Structural composition and sediment transfer in a composite cirque glacier: Glacier de St. Sorlin,France

This paper considers the links between structure, sediment transport and sediment delivery at Glacier de St. Sorlin, France. Sediment transported by the glacier is concentrated at flow‐unit boundaries as medial moraines, controlled by the position of bedrock outcrops in the accumulation area. Rockfall entrained within primary stratification is tightly folded at flow‐unit boundaries under high cumulative strains and laterally compressive stress. High cumulative strains and laterally compressive stresses lead to the development of longitudinal foliation from primary stratification. Folding elevates subglacial sediments into foliation‐parallel debris ridges, which are exposed in the ablation area. Crevasses and shear planes within the glacier have little control on sediment transport. Debris stripes in the proglacial area are morphologically similar to foliation‐parallel debris ridges; however, they are not structurally controlled, but formed by fluvial erosion. The conclusion of this study is that at Glacier de St. Sorlin proglacial sediment‐landform associations are subjected to intense syn‐ and post‐depositional modification by high melt‐water discharges, hence their composition does not reflect that of sediments melting out at the terminus. The action of melt water limits the potential of the sedimentary record to be used to constrain numerical models of past glacier dynamics in debris‐poor glacierized Alpine catchments. Copyright © 2008 John Wiley & Sons, Ltd.     

Dissolved Aluminum in the Upper St. Lawrence Estuary

The concentrations of dissolved aluminum (Al) in the upper St. Lawrence Estuary were determined during periods of high and intermediate river-discharge. Laboratory experiments simulating estuarine processes were also conducted in order to examine possible mechanisms controlling the Al distribution. During the high river-discharge, the Al concentration at river end-member was 1.63 µM and decreased exponentially with increasing salinity. An almost complete removal of dissolved Al was observed in the low salinity area up to 10 with an intensive removal in the turbidity maximum zone. Principal mechanisms responsible for the Al removal inferred from the laboratory experiments were flocculation and adsorption onto suspended particulate matter (SPM). During the intermediate river-discharge, the Al concentration was 0.72 µM at the river end-member and again decreased with increasing salinity. However, the removal was less pronounced, being only about 25%. Good fits with model predictions and laboratory experiments suggest that principal removal mechanisms were authigenic aluminosilicate formation and adsorption onto SPM. In the upper St. Lawrence Estuary, Al distribution is controlled by a combination of three removal mechanisms: flocculation, authigenic aluminosilicate formation, and adsorption. Each mechanism can become a dominant factor depending on the concentration level and speciation of dissolved Al in the river water.     

Dissolved Iodate and Total Iodine During an Extreme Hypoxic Event in the Southern Benguela System

Dissolved iodate and total iodine were studied in St Helena Bay, South Africa, during a period of acute hypoxia, following upwelling off Cape Columbine. Despite the generally high concentrations of chlorophyll α (10–30 mg m⁻³) total iodine concentration was essentially constant in the main part of the Bay, and similar to that found elsewhere in the oceans. Occasional, lower concentrations of total iodine (0·28 to 0·42 μM) were found with exceptionally high chlorophyll α concentrations (500 mg m⁻³) in shallow waters. In contrast, iodate was found to be reduced to iodide at both the surface and the bottom of the Bay. The implications of these changes are discussed, given that the surface waters reflect sustained eutrophication while the bottom waters are hypoxic as a result of the organic-rich sediment from the waters above.     

云南地区强震前的应变面积异常

详细计算和分析了云南省几个地区的五次ＭＬ≥６．５级强震前的应变面积。结果表明,在震前５～８年的时间内,归一化为４级地震的Ｓｔ值会出现明显异常变化。Ｓｔ值空间分布表现为二种特征：（１）未来震中区域从Ｓｔ值变小,周转地区增大,形成空区。（２）Ｓｔ值有由零星分布逐渐向震中区收缩、集中、增强的趋势。此外,从Ｓｔ值空间动态变化图像的演化规律得出结论,强震前５～８年中小地震活动是由时间稍长的平稳、随机分布到时     

Thirty years of tephra erosion following the 1980 eruption of Mount St. Helens

Repeated measurement of tephra erosion near Mount St. Helens over a 30-year period at steel stakes, installed on 10 hillslopes in the months following the 1980 eruption, provides a unique long-term record of changing processes, controls and rates of erosion. Intensive monitoring in the first three post-eruption years showed erosion declined rapidly as processes shifted from sheetwash and rilling to rainsplash. To test predictions about changes to long-term rates and processes made based on the 3-year record, we remeasured sites in 1992, 2000 and 2010. Average annual erosion from 1983 to 1992 averaged 3.1 mm year⁻¹ and ranged from 1.4 to 5.9 mm year⁻¹, with the highest rate on moderately steep slopes. Stakes in rills in 1983 generally recorded deposition as the rills became rounded, filled and indistinct by 1992, indicating a continued shift in process dominance to rainsplash, frost action and bioturbation. Recovering plants, where present, also slowed erosion. However, in the second and third decades even unvegetated hillslopes ceased recording net measurable erosion; physical processes had stabilized surfaces from sheetwash and rill erosion in the first few years, and they appear to have later stabilized surfaces against rainsplash erosion in the following few decades. Comparison of erosion rates with suspended sediment flux indicates that within about 6 years post-eruption, suspended sediment yield from tephra-covered slopes was indistinguishable from that in forested basins. Thirty years after its deposition, on moderate and gentle hillslopes, most tephra remained; in well-vegetated areas, plant litter accumulated and soil developed, and where the surface remained barren, bioturbation and rainsplash redistributed and mixed tephra. These findings extend our understanding from shorter-term studies of the evolution of erosion processes on freshly created substrate, confirm earlier predictions about temporal changes to tephra erosion following eruptions, and provide insight into the conditions under which tephra layers are preserved. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.     

Maritime clusters in diverse regional contexts: The case of Canada

This paper presents the main results of a project on Canada's regional clusters in the maritime industry. The objectives of this paper are twofold: firstly, the driving forces and the development processes of the different maritime clusters in Canada are described. Secondly, the role of cluster policy in promoting maritime competitiveness in Canada and the key institutional and geographic barriers that affect their growth and development are discussed. The empirical work consists of analyses and assessment of three clusters in different provinces in Canada: Quebec, Newfoundland and Labrador, and British Columbia.     

Macrobenthic invertebrate responses to prolonged drought in South Africa's largest estuarine lake complex

St Lucia, the largest estuarine lake complex of its type in Africa, is characterised by natural physico-chemical spatial and temporal fluctuations. This estuary functions as an important nursery area making use of a number of different habitat types and associated environmental conditions. The system has been subject to a number of natural episodic events such as cyclonic flooding and cyclical droughts, the most current has persisted since 2001, resulting in almost permanent mouth closure. Furthermore, high levels of evaporation have reduced lake levels and resulted in compartmentalisation of the system for up to several years at a time. St Lucia is sensitive to evaporation and therefore to vast salinity fluctuations and hypersaline conditions. The upper reaches in North Lake are particularly sensitive to drying out and extended hypersaline events. The macrobenthos has been well studied, but no studies have concentrated on the entire system for such an extent of time. Macrobenthic samples were initially collected in 2004, again in 2005 and biannually from 2006 to 2008. This study was part of a larger multidisciplinary programme aimed at determining the effects of long term drought conditions on the ecology of the St Lucia system, and increasing current understanding of the system response to future catastrophic climatic events. Three areas were sampled (North and South Lake and Estuary) at five sites per area, over five years. Data were analysed by representing communities through time series depictions and discriminating sites and sampling periods to test multidimensional relationships. Linkages between biological distribution and selected environmental forcing variables were investigated. The system was found to be highly variable and supporting a large number of different habitats and co-occurring environmental conditions. This variability makes it difficult to discern spatial or temporal patterns in environmental conditions and macrobenthic ecology. However, some level of resilience to wide ranging environmental changes associated with drought is maintained. This study was able to determine that there exists a core of taxa able to persist even under shallow depth conditions and prolonged hypersalinity. These taxa clearly have a self recruiting strategy and are therefore not reliant on mouth opening to re-establish local populations. This finding alone has important bearing for future studies and management of the system under similar adverse conditions.