Tracking a non-dipole geomagnetic anomaly using new archaeointensity results from north-east China

A collection of ceramics and samples, collected from north-east China with ages ranging from 1000 to 7000 years, have been investigated using a modified version of the Shaw palaeointensity techniques (Shaw 1974; Rolph & Shaw 1985) in which only partial NRMs and TRMs (PNRMs and PTRMs) with blocking temperatures (T_b) above 300 C are used after pre-selection of samples by mineral magnetic analysis. A secular variation curve obtained from this study is quite consistent with previous results from other areas of China (Wei et al. 1987; Tang et al. 1991), as well as with the global model of McElhinny & Senanayake (1982). Comparison of the Chinese results with contemporaneous results from Greece (Aitken et al. 1989) has allowed us to track the movement of a large non-dipole anomaly as it drifts westwards.     

Residential differentiation in nineteenth‐century Fremantle: the antipodean case

The study of residential differentiation in nineteenth‐century cities has generally been pursued without reference to Australian examples due to a lack of suitable data sources and a tendency to assume ‘New World’ models for the antipodean case. This study of late nineteenth‐century Fremantle, Western Australia, utilises data drawn from contemporary municipal ratebooks in a comprehensive analysis of residential space at the micro‐scale of individual lots. Conducted at three time intervals during a period of in‐migration and urban transformation, the analysis reveals the distinctiveness of one Australian urban experience during an era of ‘compressed development’. Residential differentiation is shown to exist both between and within occupational status groups, reflecting subtle differences in tenancy status, dwelling type and lot densities. With population increase and the onset of industrialisation, many of these differentials can be seen to disappear, particularly between middling status groups. Drawing upon additional evidence from other researchers, it is possible to construct a schematic model of antipodean urban development to represent more precisely the period in Australian cities prior to large‐scale suburbanisation.     

Using project food aid to help alleviate urban poverty - experience and issues

As urban poverty in the Third World worsens, an increasing proportion of World Food Programme (WFP) food aid is being used to support poor urban dwellers. WFP food aid has typically been used in three main ways in urban areas: as free relief after disasters (such as floods or earthquakes) have struck urban centres; as part of institutional feeding projects; and as support to urban renewal. While there have been some creative approaches to using food aid to help overcome urban poverty, the benefits have usually only been temporary. It has proved difficult to design food aid interventions for the urban poor that help to overcome the causes of poverty in sustainable ways. The challenge is to use food aid to help make permanent as well as temporary improvements in the lives and livelihoods of the poor.     

Retrieval of turbulent pressure fluctuations at the ground surface beneath a forest

The relationship between surface pressure fluctuations and the velocity field associated with turbulent coherent structures is examined for flow within and above a deciduous forest. Measurements were taken with tower-mounted sonic anemometer/thermometers at six heights, Lyman-alpha humidiometers at three heights, and a pressure sensor at the forest floor. We find a strong, near-linear relationship between the mean square turbulent velocity and the standard deviation of the high-pass-filtered pressure fluctuations. Lagged cross-correlations between vertical velocity fluctuations and those of pressure show maximum correlations of ± 0.5 but with a phase offset. Examination of surface pressure during the passage of coherent structures, which are characterized by a transition from ejection to sweep, reveals a period of overpressure about 20 s in duration roughly centered on the time of passage of the scalar microfront at the top of the canopy. Pressure patterns associated with coherent structures appear to be largely responsible for the form of the correlations stated above.Pressure patterns calculated from an integrated Poisson equation, using observed velocity and temperature signals during coherent structures, match the main features of the observed pressure. Retrieval of the pressure fluctuations in this manner reveals that the mean wind shear/turbulence interaction term is dominant, but that important contributions arise from two other terms in the equation. Buoyancy effects are negligible. We show that the surface pressure signal is mainly created by the velocity field near the top of the forest, and present evidence to suggest that features of the sub-crown air movement result directly from this pressure field.     

Trace element behaviour during anatexis in the presence of a fluid phase

If anatexis takes place in the presence of an immiscible volatile-rich fluid phase, the behaviour of a trace element depends not only on partition coefficients D^{i ?} between mineral i and silicate liquid, but also on coefficients D^{i f} between mineral i and fluid. The limited experimental data available, as well as theoretical arguments, suggest that for common minerals these coefficients differ, in some cases (REE) notably.Theory has been developed to permit calculation of concentration variations with the fraction F of rock melted, if D-values are known. In its present state this theory neglects the influence of T, P and composition variations in liquid and fluid on the D-values: if such variations can be expressed as functions of F, appropriate modifications can be made. The theory also neglects the effects of T and P on the solubility of fluid in liquid and the resulting modifications to phase petrology, by assuming simply that a given mass of rock melts in the presence of a mass proportion v of fluid. By choosing different values of v, from 0 (dry melting) to 100 (large fluid excess), the response of the trace element concentrations can be followed, beginning with concentration c_o in the unmelted rock. Such treatment is highly idealised, but serves to indicate some limits on what can occur.     

Double-averaging methodology and its application to turbulent flow in and above vegetation canopies

Double averaged equations for atmospheric boundary layer flows are introduced as natural extensions of single averaged Reynolds equations. We show that in circumstances where double averaged equations are needed, the two fundamental properties of Reynolds averaging are violated. First, we consider double-averaging in free air turbulence, where the aim is to separate coherent motions from background turbulence. We illustrate the different properties of the main operators that have been used and the physical meaning of the terms that result. Second, in canopy flows, the multiply connected nature of the canopy airspace leads to a different set of departures from the standard Reynolds equations. We establish the physical meaning of the extra terms that arise. Finally we briefly discuss the problems, both practical and theoretical, that arise when we use double averaged equations to interpret real data.     

The effect of experiment geometry on the mechanism and rate of dissolution of quartz in basanite at 0.5 GPa and 1350 °C

Mineral dissolution is an important factor in many magmatic processes such as melting, assimilation and magma mixing. Since it is not possible to determine dissolution rates or mechanisms from natural samples, experimental measurements are very useful. However, the geometry of the crystal–melt system can have a large effect on the measured rate, depending on whether the contaminated melt formed during dissolution is gravitationally stable or unstable. This study examines the effects of the crystal–melt geometry on the dissolution rate and mechanism. The experiments were performed using basanite melt and cylinders and spheres prepared from a single crystal of natural quartz. All of the experiments were performed in the piston cylinder apparatus at 0.5 GPa and 1350 °C. Four crystal–melt geometries were used: (1) quartz cylinders on top of a column of melt; (2) quartz cylinders beneath a column of basanite melt; (3) quartz cylinders in the middle of column of melt; (4) quartz spheres on top of a column of basanite melt. These geometries allow an examination of non-convective, convective and mixed non-convective/convective dissolution. Sphere experiments were included, as this has been the most commonly used geometry in previous experimental studies. In all of the experiments quartz dissolves directly into the basanite without formation of cristobalite or tridymite. Quartz on top of a column of melt dissolves at a rate almost proportional to the square root of time and forms a silica-rich compositional boundary layer that is gravitationally stable. All of the samples show well-defined compositional gradients in the boundary layer; however, the melt at the interface varies in composition with time and plots of concentration as a function of distance normalized to time show that the diffusion rate of SiO₂ increases with time. These data suggest that the rate-controlling step during quartz dissolution is interface reaction rather than cation diffusion. Quartz on the bottom of a column of basanite dissolves much more quickly than in the quartz-on-top experiments and the dissolution rate is linear, due to the periodic gravitational instability and resultant convection of the boundary layer. Even though interface kinetics are the rate-controlling step in quartz dissolution, convection causes an increase in dissolution rate because it replenishes the boundary layer with new, silica-undersaturated melt, which dissolves the quartz more quickly than the contaminated melt. These data suggest that the interface reaction rate is controlled by the degree of undersaturation of the solvent melt in the dissolving component. Both quartz-in-middle and quartz sphere experiments dissolve at a rate intermediate between the two extremes and both show a power law rate. Both dissolve by a combination of convective and non-convective dissolution but the sphere experiments are affected by an additional factor. During the experiment the sphere can sink through the capsule causing forced convection which adds another complication to the interpretation of the dissolution rate data. The results of this study indicate that the choice of experiment geometry plays a major role in determining the observed dissolution rate. Mineral spheres, which have been widely used in the past, are not ideal for dissolution studies. Instead, dissolution rates and mechanisms are best determined in the absence of convection. These experiments have an additional advantage in that for diffusion-controlled dissolution, they allow determination of cation diffusivity. Received: 2 March 2000 / Accepted: 11 April 2000     

A new concept in Euler deconvolution of isolated gravity anomalies

Euler's homogeneity equation has been used to develop a new technique to interpret the gravity anomalies over some simple geometrical sources, namely a finite horizontal line/vertical line, a finite vertical ribbon, a semicircular dome/basin and an isosceles triangle approximating an anticline/syncline. A linear over-determined system of equations has been solved to compute the depth, the horizontal location and the structural index, all treated as free parameters. The concept of a variable structural index provides better depth estimates and helps to identify the source geometry. Nomograms have been prepared to compute an additional model parameter, namely the horizontal/vertical extent of a line, the vertical extent of a ribbon and the radius of a dome/basin. The efficacy of the proposed method has been evaluated using two real field examples.     

Variations in the geomagnetic dipole moment over the last 12 000 years

An analysis has been made of archaeointensity data for the past 12 000 years. There are 3243 results from different areas of the world covering the past 12 000 years. Of these, 2203 are from the European region and 1040 are from the rest of the world. The archaeointensity data set analysed in the present study is almost three times larger than that used by McElhinny & Senanayake (1982 ). Although there is no major difference between our global data and the earlier data, the data for the non-European region have been improved and we now have a data set for Asia.