Self-Similar Fold Evolution Under Prescribed End-Shortening

The evolution of single-layer folds under prescribed end-shortening conditions displays folds of varying wavelength. We investigate a simple model of this kind and characterize the long-term behaviour of fold profiles. In particular we determine the evolution of the axial load and the variation of the wavelength, and we show that fold profiles are highly self-similar.     

A master equation for reactive solute transport in porous media

The mean value of a density of a cloud of points described by a generalized Liouville equation associated with a convection dispersion equation governing adsorbing solute transport yields a joint concentration probability density. The general technique can be applied for either linear or nonlinear adsorption; here the application is restricted to linear adsorption in one-dimensional transport. The equation generated for the joint concentration probability density is in the general form of a Fokker-Planck equation, but with a suitable coordinate transformation, it is possible to represent it as a diffusion equation with variable coefficients.     

Invasive species and climate change: an agronomic perspective

In the current review we wish to draw attention to an additional aspect of invasive species and climate change, that of agricultural productivity and food security. We recognize that at present, such a review remains, in part, speculative, and more illustrative than definitive. However, recent events on the global stage, particularly in regard to the number of food riots that occurred during 2008, even at a time of record harvests, have prompted additional interest in those factors, including invasive species, which could, through climatic uncertainty, alter food production. To that end, as agricultural scientists, we wish to begin an initial evaluation of key questions related to food production and climate change including: how vulnerable is agriculture to invasive species?; are current pest management strategies sufficient to control invasive outbreaks in the future?; what are the knowledge gaps?; can we provide initial recommendations for scientists, land managers and policy makers in regard to available resources? Our overall goals are to begin a synthesis of potential impacts on productivity, to identify seminal research areas that can be addressed in future research, and to provide the scientific basis to allow agronomists and land managers to formulate mitigation and adaptation options regarding invasive species and climate change as a means to maintain food security.     

Natural climatic variability and the Norse settlements in Greenland

A multi-millennial simulation with the CSIRO Mark 2 coupled global climatic model has been used to determine whether climatic conditions approximate to those experienced by the medieval Norse settlers in Greenland could be identified. The aim of this analysis was to see whether such conditions could be replicated by the natural climatic variability in this unforced simulation, in order to counteract claims that the current observed global warming is merely another example of this type of climatic regime. This view has been expressed in the media in an attempt to refute the existence of a CO₂-induced global warming. A 291-year period of above-average temperature followed by a 41-year cooler period were identified in one millennium of the simulation, and subsequently used as an analogue of conditions representative of the time of the Norse settlements. Considerable interannual variability existed in both these periods, but with noticeable positive and negative surface temperature anomalies in the warm and cold periods respectively. Thus the warm period was not a time of uniform benign conditions. Above-average precipitation was also associated with the warm period, and these climatic conditions would have enhanced pasture growth and hay production (the only crop the Norse produced) thereby sustaining the livelihood of the Norse Greenlanders. The climatic conditions associated with the cold period in the model were probably sufficient to limit the survival prospects of the settlers, especially when other, probably more critical, deleterious factors are taken into account. The temperature anomalies replicated in the simulation are similar to the limited proxy data, but may be smaller in magnitude: nevertheless they appear to be sufficiently large to have affected the viability of the Norse Greenlanders. After considering possible climatic mechanisms that could have contributed to these warm and cold periods it was concluded that they are simply a consequence of stochastic influences generated by nonlinear processes in the simulation. Thus this simulation provides no support for the contention that the current global warming is a manifestation of conditions prevailing during the Norse settlements in Greenland.     

Stochastic control of Indian megadroughts and megafloods

A multi-millennial run of the CSIRO Mark2 coupled climatic model has been used to investigate megadroughts and megafloods during the Indian summer monsoon (June–September). These extreme events were defined as having rainfall anomalies at least two standard deviations from normal. More than ten megafloods and more than twenty megadroughts, so-defined, were found to occur in a 5,000-year period of the simulation. The simulation replicated most of the major features of the observed summer monsoon, but a comparison of observed and simulated probability density functions suggests that the limited observed rainfall time series to date does not adequately sample the possible range of Indian monsoonal rainfall. An investigation of causal mechanisms of Indian rainfall variability reproduced the observed negative correlation with ENSO events, but it was found that neither extreme ENSO events or extremes of a range of other climatic phenomena coincided with the simulated, extreme megadroughts and megafloods. This disconnect between these events is succinctly illustrated with examples related to ENSO events in particular. Autoregressive and FFT analysis of observed and simulated Indian summer monsoon rainfall time series revealed them to consist of white noise. Since these time series therefore consist of random outcomes, it is apparent that these Indian megadroughts and megafloods are the consequence of stochastic influences. Thus, it is concluded that the interannual variability of Indian summer monsoonal rainfall cannot be predicted in general, nor can megadroughts and megafloods in particular.     

Localized folding of viscoelastic layers

Naturally occurring fold systems are typically irregular. Although such systems may sometimes be approximated by a periodic geometry, in reality they are commonly aperiodic. Ord (1994) has proposed that naturally occurring fold systems may display spatial chaos in their geometry. Previous work has indicated that linear theories for the formation of fold systems, such as those developed by Biot (1965), result in strictly periodic geometries. In this paper the development of spatially chaotic geometries is explored for a thin compressed elastic layer embedded in a viscoelastic medium which shows elastic softening. In particular, it is shown that spatially localized forms of buckling can develop and the evolution of these systems in the time domain is presented. A nonlinear partial differential equation, fourth order in a spatial variable and first order in time, is found to govern the evolution. A related nonlinear fourth-order ordinary differential equation governs an initial elastic phase of folding. The latter equation belongs to a class with spatially chaotic solutions. The paper reviews the implications of localization in the geological framework, and draws some tentative conclusions about the development of spatial chaos. Crudely arrived-at, yet plausible, evolutionary time plots under the constraint of constant applied end displacement are presented. Emphasis throughout is on phenomenology, rather than underlying mathematics or numerics.     

Pre-seismic electromagnetic phenomena in the framework of percolation and fractal theories

Understanding the electromagnetic response to geodynamic processes occurring in the earth's upper crust, in particular pre-seismic and seismic processes, is a challenging task in modern geophysics. There is increasing evidence that seismo-electromagnetic (SEM) phenomena are difficult to describe quantitatively by “linear” models using “averaged” parameters of the medium, such as electrical and hydraulic conductivities. Because the upper crust is highly inhomogeneous (at all scales), porous, and can be fully or partially water-saturated, the most natural way to describe its parameters is via fractal-theoretic and percolation-theoretic models.Recent studies indicate that the electrokinetic effect is the most likely driving mechanism for the various types of SEM signals. Here we considered the hydraulic, electric, and electrokinetic conductances of a porous water-saturated medium as a function of porosity () and moisture content (θ), utilizing a percolation/fractal approach. We show that the electric conductivity and electrokinetic current in such a medium are both proportional to (–_c)² and (θ–θ_c)², where _c and θ_c are the critical values of porosity and moisture content, respectively. This behavior admits the possibility of a relatively large change in the respective electric and electrokinetic parameters due to a small change in the mechanical strain field. This is significant because it may account for the appearance of some types of SEM signals at large distances from the earthquake origin, which is the main deficiency of most models. Indeed, the anticipated strain changes related to pre-earthquake processes is usually very small except near the focal area. The expected ‘averaged’ electromagnetic response also would be very small, unless a local underground water system exists, not necessarily near the focal area, but which is close to critical point(s).We discuss the conditions under which electrotelluric and geomagnetic variations can accompany mechanical disturbances in the earth's crust.     

Pathogen transport in groundwater systems: contrasts with traditional solute transport

Water quality affects many aspects of water availability, from precluding use to societal perceptions of fit-for-purpose. Pathogen source and transport processes are drivers of water quality because they have been responsible for numerous outbreaks resulting in large economic losses due to illness and, in some cases, loss of life. Outbreaks result from very small exposure (e.g., less than 20 viruses) from very strong sources (e.g., trillions of viruses shed by a single infected individual). Thus, unlike solute contaminants, an acute exposure to a very small amount of contaminated water can cause immediate adverse health effects. Similarly, pathogens are larger than solutes. Thus, interactions with surfaces and settling become important even as processes important for solutes such as diffusion become less important. These differences are articulated in “Colloid Filtration Theory”, a separate branch of pore-scale transport. Consequently, understanding pathogen processes requires changes in how groundwater systems are typically characterized, where the focus is on the leading edges of plumes and preferential flow paths, even if such features move only a very small fraction of the aquifer flow. Moreover, the relatively short survival times of pathogens in the subsurface require greater attention to very fast (<10 year) flow paths. By better understanding the differences between pathogen and solute transport mechanisms discussed here, a more encompassing view of water quality and source water protection is attained. With this more holistic view and theoretical understanding, better evaluations can be made regarding drinking water vulnerability and the relation between groundwater and human health.     

A Comparison of the Geochemical Features of Some Loess Sections in New Zealand and China

A comparison of the geochemical features of loesses of New Zealand and China indicates that the distributions of the elements and their variations reflect the fluctuations of climate which can be compared with the oxygen isotope stages and glacial periods.New Zialand loess is different in source from Chinese loess.Therefore ,some differences are also noticed in their chemical compositions .Loess accumulation in New Zealand is later than that in China.Because of more rainfall in New Zealand and different distributions of loess the elements in loess have suffered stronger leaching than in China.