An Assessment of Performance in the Routine Analysis of Silicate Rocks Based on an Analysis of Data Submitted to the GeoPT Proficiency Testing Programme for Geochemical Laboratories (2001–2011)

Data reported by laboratories contributing to the GeoPT proficiency testing programme for geochemical laboratories over the period from 2001 to 2011 have been assessed to identify the elements and concentration ranges over which analytical performance can be considered satisfactory. Criteria developed in the paper indicated that performance in the content determination of the elements/constituents SiO₂, Al₂O₃, MnO, Cs, Dy, Er, Eu, Ga, Hf, Ho, Lu, Nd, Pr, Sm, Sr, Tb, Tl, Tm, U, Y, Yb and Zn was satisfactory over the full concentration range assessed. The elements/constituents TiO₂, Fe₂O₃(T), MgO, CaO, Na₂O, K₂O, P₂O₅, Ba, Be, Cd, Ce, Co, Gd, La, Li, Nb, Rb, Sb, Sc, Sn, Ta, Th, V and Zr showed some degradation in performance at lower concentration levels (approaching the detection limit of some techniques). Performance in determining LOI, As, Bi, Cr, Cu, Ge, Mo, Ni, Pb and W was in general unsatisfactory over the full concentration range assessed. Other elements (especially Fe(II)O, H₂O⁺, CO₂, Ag, Au, B, Br, Cl, F, Hg, I, In, Ir, N, Os, Pd, Pt, Re, Rh, S, Se, Te) could not be evaluated as they were not routinely reported by laboratories participating in the GeoPT programme, often because they are present in silicate rocks at sufficiently low concentrations to require a pre‐concentration stage. Some suggestions are made for the causes of unsatisfactory performance, but further progress will require a detailed assessment of the methods used by participating laboratories, which will form the subject of a further paper.     

Tracking of TRACE Ultraviolet Flare Footpoints

Solar flares produce bright, compact sources of UV emission in the lower atmosphere, identified as flare footpoints. Observed at high time cadence with the Transition Region and Coronal Explorer, groups of UV footpoints define flare `ribbons' which move as the flare progresses. We have developed a procedure to track individual bright kernels within flare ribbons, enabling us to study the motion of these sites of excitation through the solar chromosphere. We have applied this to a flare observed by TRACE in the 1600 Å passband at 2-s cadence. In this event, the footpoints have an average speed of 15 km s<sup>–1</sup>, with a superposed random `meandering' component, consistent with the footpoint magnetic field being anchored around the edges of granular cells. Examining the brightness as a function of time, we find that the timing of peaks in brightness is significantly correlated with the timing of peaks in the product of the footpoint speed with the line-of-sight magnetic field strength at the footpoint location; in other words with a measure of the coronal reconnection rate.     

Automatic Recognition and Characterisation of Supergranular Cells from Photospheric Velocity Fields

We have developed an exceptionally noise-resistant method for accurate and automatic identification of supergranular cell boundaries from velocity measurements. Because of its high noise tolerance the algorithm can produce reliable cell patterns with only very small amounts of smoothing of the source data in comparison to conventional methods. In this paper we describe the method and test it with simulated data. We then apply it to the analysis of velocity fields derived from high-resolution continuum data from MDI (Michelson Doppler Imager) on SOHO. From this, we can identify with high spatial resolution certain basic properties of supergranulation cells, such as their characteristic sizes, the flow speeds within cells, and their dependence on cell areas. The effect of the noise and smoothing on the derived cell boundaries is investigated and quantified by using simulated data. We show in detail the evolution of supergranular cells over their lifetime, including observations of emerging, splitting, and coalescing cells. A key result of our analysis of cell internal velocities is that there is a simple linear relation between cell size and cell internal velocity, rather than the power law usually suggested. Electronic Supplementary Material The online version of this article () contains supplementary material, which is available to authorized users.     

‘All my hopes and dreams are shattered’: Urbanization and migrancy in an imploding African economy - the case of Zimbabwe

During the 1990s, urban livelihoods in Zimbabwe began to suffer a series of economic stresses, which accelerated in 1997 and then accelerated again, with the inception of fast-track land reform, from 2000. This has reduced urban living standards significantly and devastated real urban income levels. After a discussion of the economic and political parameters of this period, this paper links these to empirical changes in the urban livelihoods and perceptions of urban living standards of recent in-migrants to Harare. This draws on a longitudinal database of four surveys of recent migrants to the city conducted in 1985, 1988, 1994 and 2001. Among a range of qualitative issues explored in these surveys has been the question of migrants’ future plans in relation to their intended length of stay in town. It is shown that migrants have been feeling increasingly negative or unsure about their urban experience since structural adjustment began in the early 1990s, and that these perceptions had greatly strengthened by 2001. By the last survey only a small minority felt they would remain permanently in town and most of the migrants from rural areas felt that their living standards in Harare were either worse than rural living standards, or no better. While Zimbabwe’s current political and economic crisis is exceptional, serious urban poverty is a feature across sub-Saharan African countries and it is argued that this has had a depressing effect on net rural-urban migration rates.     

Using Field-Portable XRF to Assess Geochemical Variations Within and Between Dolerite Outcrops of Preseli, South Wales

Eight dolerite outcrops in Preseli, south Wales were measured in situ using field-portable XRF analysis, in order to compare two different analysis ("sampling") strategies, and to investigate geochemical variability within and between outcrops. A sampling strategy of two (neighbouring but independent) measurements at each of a maximal number of locations dispersed over an outcrop was the more effective in indicating the overall chemical variance of that outcrop. Analysis of variance indicated that much of the observed variance within individual outcrops stems from real geochemical variability rather than from sampling and analytical factors. Standard ANOVA F tests showed that several of the studied outcrops are heterogeneous at the 5% significance level for one or more elements. Geochemical distinctions between some outcrops were demonstrated using discriminant analysis. PXRF analysis offers an alternative approach to conventional characterisation of outcrops, which is often based on laboratory analysis of small numbers of samples. However, PXRF data are affected by rock weathering and may require correction for this if they are to be compared with analyses of fresh rock.     

Marine flooding in the Thames Estuary and tidal river c.1250–1450: impact and response

Marine flooding, particularly that associated with North Sea storm surges, has posed a recurrent threat to human use of the Thames Estuary and its shorelines. During the later Middle Ages a growing frequency of surges placed increasing burdens upon the resources of coastal communities. This is reflected in expenditure upon sea-walls and related defences and in the frequent issuing of commissions of sewers, the mechanism by which the Crown intervened in matters of flood prevention and drainage. In the era of labour shortages initiated by the Black Death of 1349, the defence of some reclaimed marshlands around the Estuary and tidal river was abandoned in a precursor of managed retreat or realignment.     

Coarse bed load transport in an alluvial gravel bed stream,Dupuyer Creek,Montana

Coarse bed load was sampled in a gravel/cobble bed stream during two major floods in the snowmelt runoff season. The channel is characterized by high rates of bank erosion and, therefore, high rates of sediment supply and bed load flux. Peak discharge reached four times bank‐full, and bed load was sampled at flows 0·7–1·7 times bank‐full. A large aperture bed load sampler (1 m by 0·45 m) captured the largest particles in motion, and specifically targeted the coarse bed load size distribution by using a relatively large mesh (32 mm or D₂₅ of streambed surface size distribution). Bed load flux was highly variable, with a peak value of 0·85 kg/s/m for the coarse fraction above 38 mm. Bed load size distribution and maximum particle size was related to flow strength. Entrainment was size selective for particles D₇₀ and larger (88–155 mm), while particles in the range D₃₀–D₇₀ (35–88 mm) ceased to move at essentially the same flow. Bed load flux was size selective in that coarse fractions of the streambed surface were under‐represented in or absent from the bed load. Painted tracer particles revealed that the streambed surface in the riffles could remain stable even during high rates of bed load transport. These observations suggest that a large proportion of bed load sediments was sourced from outside the riffles. Repeat surveys confirmed major scour and fill in pools (up to 0·75 m), and bank erosion (>2 m), which together contributed large volumes of sediment to the bed load. Copyright © 2007 John Wiley & Sons, Ltd.     

Crustal structure of Atlantic fracture zones–II. The Vema fracture zone and transverse ridge

Summary. The crustal structure beneath the Vema fracture zone and its flanking transverse ridge was determined from seismic refraction profiles along the fracture zone valley and across the ridge. Relatively normal oceanic crust, but with an upwarped seismic Moho, was found under the transverse ridge. We suggest that the transverse ridge represents a portion of tectonically uplifted crust without a major root or zone of serpentinite diapirism beneath it. A region of anomalous crust associated with the fracture zone itself extends about 20 km to either side of the central fault, gradually decreasing in thickness as the fracture zone is approached. There is evidence to suggest that the thinnest crust is found beneath the edges of the 20 km wide fracture zone valley. Under the fracture zone valley the crust is generally thinner than normal oceanic crust and is also highly anomalous in its velocity structure. Seismic layer 3 is absent, and the seismic velocities are lower than normal. The absence of layer 3 indicates that normal magmatic accretionary processes are considerably modified in the vicinity of the transform fault. The low velocities are probably caused by the accumulation of rubble and talus and by the extensive faulting and fracturing associated with the transform fault. This same fracturing allows water to penetrate through the crust, and the apparently somewhat thicker crust beneath the central part of the fracture zone valley may be explained by the resultant serpentinization having depressed the seismic Moho below its original depth.     

Fip Enhancement by Alfvén Ionization

Alfvén ionization is offered as a possible mechanism underlying the enhanced population of low first ionization potential (FIP) species in the solar corona. In this process, the photospheric flow impinging on the magnetic structure of a coronal flux tube collides with, and displaces, ions in the magnetised plasma within the flux tube. This leads to pockets of charge imbalance that persist due to the impeded electron transport perpendicular to the magnetic field. The localised electric field then energises electrons to the impact ionization energy threshold of low-FIP components in the surface flow. Such species remain trapped in the plasma, and drift up the magnetic structure, causing a localised population enhancement compared to photospheric levels. We find that this mechanism successfully accounts for observed biases for flow speeds known to exist in the photosphere, and moreover explains certain anomalous abundances which do not fit into existing theories.