The kink instability in infinite cylindrical flux tubes: Eigenvalues for power-law twist profiles

We develop simple accurate methods of calculating ideal MHD instability eigenvalues for infinitely-long cylindrical tubes, with twist functionT(r)=B /rB _z . A complete theoretical treatment is presented for force-free magnetic equilibria with arbitraryT(r), and detailed semi-analytic results for the kink instability are given for the particular case of a power-law twistT(r)=r , where the index is non-negative. Our results show that the most rapidly growing and energetic instabilities occur in the Gold-Hoyle =0 field, with the instability progressively weakening with increasing . However, the maximum force eigenvalue is always small, so that even in the Gold-Hoyle case (where =O(10^–2) in dimensionless units) only a small proportion of the available magnetic energy can be released in the linear phase. Our results also confirm that the linear pinch (=) is remarkably weak (=O(10^–3)) yet relatively resistant to line-tying! It is shown that the weakness of the force eigenvalue implies that the influence of uniform gas pressure on stability is negligible. Implications for the energy-release mechanism in solar flares are discussed.     

Toxicity of sediments from around a North Sea oil platform: are metals or hydrocarbons responsible for ecological impacts?

Discharges of contaminated drill cuttings have caused appreciable ecological change of the benthos adjacent to many oil and gas platforms in the North Sea. Many platforms have large piles of cuttings lying beneath them and these probably present the greatest potential hazard to the environment during platform decommissioning and removal. There is, however, a lack of consensus on which aspects of drill cuttings are responsible for the adverse ecological effects. This hinders risk assessment of management options. Here we report data on the toxicity of sediments from around the North West Hutton platform to the amphipod Corophium volutator, the polychaete Arenicola marina and the Microtox" acute test system. Sediment was acutely toxic to Corophium out as far as 600 m from the platform. Sediment from 100 m from the platform remained acutely toxic to Corophium when 3% contaminated sediment was mixed with clean sediment. A 10% dilution of this sediment also inhibited Arenicola feeding almost completely. Sediment elutriates did not inhibit Microtox light output, but organics extracted by dichloromethane were very toxic. Fifteen minute EC50 values were as low as 0.25 mg ml(-1) and were strongly correlated with hydrocarbon concentrations. Metal concentrations in whole sediments were correlated with their toxicity to Corophium but the relationship was much weaker when data on dilutions were included. Except at sites immediately adjacent to the platform, metal concentrations were well below ERL values from the literature, so were too low to explain sediment toxicity. Toxicity of sediments to Corophium was closely correlated with their hydrocarbon content, even when tests on dilutions were included in the analysis. We conclude that hydrocarbons are the most significant cause of toxicity in these sediments contaminated with oil based drill cuttings and that polar organics, sulphide. ammonia and other water soluble substances are of much lower significance. Applying OSPAR guidelines to our data on the toxicity of cuttings pile material to Corophium data would give a maximum allowable concentration of 0.03% in clean sediments. The Microtox data indicate that sediments from deeper in the pile would require an even greater dilution than this.     

Paraglacial Modification of Glacigenic Sediment

The sedimentological characteristics of in situ and paraglacially reworked till are compared at recently deglaciated sites in Norway. Glacigenic deposits reworked by debris flows are shown to retain many of the characteristics of the parent sediments, and cannot readily be distinguished from in situ till in terms of fabric strength or type, clast imbrication, shape, angularity or texture, matrix granulometry or packing. Paraglacially re-worked sediments appear to differ from in situ tills only in terms of preferred clast orientation (down flow rather than downvalley) and their structural and lithofacies characteristics. These criteria are employed to differentiate paraglacial sediments from unre-worked tills exposed in valley-side sections. The stratigraphic relations between these indicate glacial reworking of earlier paraglacial sediments as well as paraglacial remobilisation of glacigenic deposits, indicating cyclic alternation of glacial and paraglacial sediment transport.     

A GIS-based approach to site vegetated buffer strips for erosion control within an agricultural catchment in southern England

Agricultural soil erosion is largely attributed to arable intensification and increased mechanization. Runoff from arable land and intensively managed grassland transports sediment and contaminants across the landscape and into watercourses, causing crop loss, land degradation, and water quality issues. One low-cost and low-maintenance nature-based mitigation approach is the implementation of vegetated buffer strips (VBS): grassland sited along field margins to trap sediment and contaminants, reducing transportation and diffuse pollution rates. GIS modelling using remotely sensed landscape indices and land parcel data can provide an efficient means of identifying priority areas for intervention at sub-catchment or farm system scales. We develop and test a scalable runoff risk model in the lower Rother catchment, West Sussex. The model uses the Normalized Difference Vegetation Index (NDVI) applied to satellite images as an erodibility proxy and identifies locations along pathways that are conceivably at greatest risk of sediment accumulation and transfer, guided by field observations. We assess current and historical field boundaries near high-risk locations, evaluating the potential capacity of their margins to contribute to runoff risk reduction using an innovative ranking system. Recommendations are made for VBS implementation and the value of historical field boundary and margin restoration is discussed. Our method offers a rapid approach with minimal data requirements to identify high-risk sediment runoff locations and priority sites for intervention. The tool has the potential to guide decision-makers responsible for targeting and implementing soil erosion and runoff control measures such as VBS, while also maximizing agri-environmental and cultural benefits.     

Deep‐Sea Diversity: Overlooked Messages from Shallow‐Water Sediments

Abstract. Much of the discussion of high deep‐sea diversity has assumed that asymmetric inter‐specific competition will rapidly lead to the elimination of many species unless restrained by extensive differentiation of niches, or the action of predation and/or environmental disturbance. This is true for some habitats, including rocky shores. However, experimental studies indicate that marine soft sediment communities do not function like this. In shallow‐water sediments, competition is usually symmetric and relatively weak. Asymmetric competition that leads to the elimination of one species by another on time scales shorter than one generation is rare, apart from interactions that involve large‐scale modification or disturbance of the sediment. Competition is therefore relatively unimportant as a process structuring communities and the impact of predation is usually to reduce rather than enhance diversity. These results have been largely ignored by the literature on deep‐sea diversity. If deep‐sea communities function in similar ways, coexistence of many species within small areas on short time scales does not need further explanation. We do , however, need to explain why abundances of all species remain bounded on large spatial scales and time scales of several generations. The model of diversity maintenance proposed by Huston (1979) and applied to the deep sea by Rex (1983) achieves this by implicit intra‐specific, density‐dependent processes that increase the population growth rate of species that have become locally rare. This would give robust maintenance of diversity, but there is no evidence for density dependence of this type in the deep sea, and no plausible mechanisms by which it could occur. Alternative models require either spatial heterogeneity on a scale much larger than that envisaged by the grain‐matching hypothesis or the placing of a cap on the abundance of common species, perhaps by frequency‐dependent predation. Arbitrating between these possibilities will require assessments of the population dynamics and spatial distribution of individual species on spatial and temporal scales much greater than those usually considered in the deep sea.     

Eutrophication In Lake Rotorua. 2. Using ROTAN and OVERSEER to model historic,present and future nitrogen loads

OVERSEER is used in New Zealand to estimate nutrient losses from farmland, but does not quantify subsequent movement through the catchment, or attenuation. This paper uses the ROTAN model, based on the Scandinavian HBV-N model, to route nitrogen losses from 1900–2015 to Lake Rotorua where groundwater age ranges from 14 to 170 years. ROTAN conceptualises three delivery pathways (quickflow, groundwater and streamflow) with different attenuation. When calibrated to measured stream and groundwater concentrations, several combinations of attenuation gave equally good fits largely because of sparse and uncertain input and calibration data. Nevertheless, lake N loads were predicted for current land use (754?±?39?t y^?1) and with proposed N loss reductions (431?±?26?t y^?1). Probabilities were also calculated that the reductions are more (12%–18%) or less (82%–88%) than required to meet the target lake N load (405?t y^?1). ROTAN shows promise for calculating nitrogen movement in catchments dominated by groundwater where there is limited data.     

Selection of integration time intervals for quartz OSL decay curves

In quartz optically stimulated luminescence (OSL) dating protocols, an initial integral of the OSL decay curve is used in the calculation of equivalent dose, once a background integral has been subtracted. Because the OSL signal commonly contains a number of exponentially decaying components, the exact choice of time intervals used for the initial-signal and background integrals determines the composition of the net signal. Here we investigate which combination of time intervals will produce the net signal most dominated by the fast OSL component, while keeping an acceptable level of precision. Using a three-component model of OSL decay, we show that for a specified level of precision, the net signal most dominated by the fast component can be obtained when the background integral immediately follows the initial signal and is approximately 2.5 times its length. With this ‘early-background’ approach, the contribution of slow components to the net signal is virtually zero. We apply our methods to four samples from relatively young deposits. Compared to the widely used ‘late-background’ approach, in which the background integral is taken from the last few seconds of OSL, we find less thermal transfer, less recuperation and a higher proportion of aliquots yielding an equivalent dose in agreement with expectations. We find the use of an early background to be a simple and effective way of improving the accuracy of OSL dating, and suggest is should be used in standard protocols.     

Paraglacial modification of slope form

The morphological consequences of paraglacial modification of valley-side drift slopes are investigated at six sites in Norway. Here, paraglacial slope adjustment operates primarily through the development of gully systems, whereby glacigenic sediment is stripped from the upper drift slope and redeposited in debris cones downslope. This results in an overall lowering of average gradient by up to 4·5° along gully axes. In general, slope profile adjustment appears to be characterized by a convergence of slope profiles towards an ‘equilibrium form’ with an upper rectilinear slope gradient at 29°± 4° and a range of concavities of approximately 0·0 to 0·4. After initial rapid incision, further gully deepening is limited, but gullies become progressively wider as sidewall gradients decline to c. 25°, after which parallel retreat appears to predominate. The final form of mature paraglacial gully systems consists of an upper bedrock-floored source area, a mid-slope area of broad gullies whose sidewalls rest at stable, moderate gradients, and a lower slope zone where gullies discharge onto the surfaces of debris cones and fans. Some gullies appear to have attained this final form and have stabilized following exhaustion of readily entrainable sediment within decades of gully initiation. At most sites, paraglacial activity has transformed steep drift-mantled valley sides into gullied slopes where an average of c. 2–3 m of surface lowering has taken place. At the most active sites, these average amounts imply minimum erosion rates averaging c. 90 mm a⁻¹ since gully initiation, which highlights the extreme rapidity of paraglacial erosion of deglaciated drift-mantled slopes. Copyright © 1999 John Wiley & Sons, Ltd.