Quantification of the relation between surface morphodynamics and subsurface sedimentological product in sandy braided rivers

This study uses digital elevation models and ground‐penetrating radar to quantify the relation between the surface morphodynamics and subsurface sedimentology in the sandy braided South Saskatchewan River, Canada. A unique aspect of the methodology is that both digital elevation model and ground‐penetrating radar data were collected from the same locations in 2004, 2005, 2006 and 2007, thus enabling the surface morphodynamics to be tied explicitly to the associated evolving depositional product. The occurrence of a large flood in 2005 also allowed the influence of discharge to be assessed with respect to the process–product relationship. The data demonstrate that the morphology of the study reach evolved even during modest discharges, but more extensive erosion was caused by the large flood. In addition, the study reach was dominated by compound bars before the flood, but switched to being dominated by unit bars during and after the flood. The extent to which the subsurface deposits (the ‘product’) were modified by the surface morphodynamics (the ‘process’) was quantified using the changes in radar‐facies recorded in sequential ground‐penetrating radar surveys. These surveys reveal that during the large flood there was an increase in the proportion of facies associated with bar margin accretion and larger dunes. In subsequent years, these facies became truncated and replaced with facies associated with smaller dune sets. This analysis shows that unit bars generally become truncated more laterally than vertically and, thus, they lose the high‐angle bar margin deposits and smaller scale bar‐top deposits. In general, the only fragments that remain of the unit bars are dune sets, thus making identification of the original unit barform problematic. This novel data set has implications for what may ultimately become preserved in the rock record.     

Saturn's magnetospheric interaction with Titan as defined by Cassini encounters T9 and T18: New results

We present new results of Cassini's T9 flyby with complementary observations from T18. Based on Cassini plasma spectrometer (CAPS) and Cassini magnetometer (MAG), compositional evidence shows the upstream flow for both T9 and T18 appears composed of light ions (H⁺ and H₂⁺), with external pressures ∼30 times lower than that for the earlier TA flyby where heavy ions dominated the magnetospheric plasma. When describing the plasma heating and sputtering of Titan's atmosphere, T9 and T18 can be considered interactions of low magnetospheric energy input. On the other hand, T5, when heavy ion fluxes are observed to be higher than typical (i.e., TA), represents the limiting case of high magnetospheric energy input to Titan's upper atmosphere. Anisotropy estimates of the upstream flow are 1<T_⊥/T_‖<3 and the flow is perpendicular to B, indicative of local picked up ions from Titan's H and H₂ coronae extending to Titan's Hill sphere radius. Beyond this distance the corona forms a neutral torus that surrounds Saturn. The T9 flyby unexpectedly resulted in observation of two “wake” crossings referred to as Events 1 and 2. Event 2 was evidently caused by draped magnetosphere field lines, which are scavenging pickup ions from Titan's induced magnetopause boundary with outward flux ∼2×10⁶ ions/cm²/s. The composition of this out flow is dominated by H₂⁺ and H⁺ ions. Ionospheric flow away from Titan with ion flux ∼7×10⁶ ion/cm²/s is observed for Event 1. In between Events 1 and 2 are high energy field aligned flows of magnetosphere protons that may have been accelerated by the convective electric field across Titan's topside ionosphere. T18 observations are much closer to Titan than T9, allowing one to probe this type of interaction down to altitudes ∼950 km. Comparisons with previously reported hybrid simulations are made.     

Detection of fluorescent compounds in the environment using granular activated charcoal detectors

Granular activated charcoal (GAC) acts as a general adsorptive medium for organic molecules, and is widely used to capture fluorescent tracer dyes. To investigate the capability of GAC in contaminant screening and tracer adsorption, a number of detectors were deployed for 2-6 days in a range of urban surface waters, and the adsorbed compounds eluted in an alkaline alcohol solution. Simultaneous water samples showed a diverse range of fluorescence environments, ranging from relatively clean, steady groundwater discharge, to highly concentrated and variable treated municipal sewage. A wide variety of organic compounds and dyes were found in the waters, as chronic and acute contaminants. The relationship between charcoal and water spectra depended on exposure time and loading. Short exposure times emphasized short wavelengths, longer exposure times emphasized longer wavelengths at the expense of shorter wavelengths. The magnitude of the effect depended on loading, being greater in enriched waters. In general, charcoal eluent shows a significant gain in fluorescence intensity over water. However, there may be an apparent loss at shorter wavelengths for samples with long exposure times and high loading. A similar bias was also discovered with the elution time of activated charcoal. Short wavelength fluorescence intensity peaked after a few minutes of elution; longer wavelength fluorescence increased over many days. These results show that charcoal is a reasonably effective material for adsorption of longer wavelength compounds. However, the ubiquity of many fluorescent dyes in the environment, and the complex relationship between the waters and the eluent spectrum, suggest that considerable care is required in the interpretation of eluent spectra.     

Critical reflections on practice: the changing roles of three physical geographers carrying out research in a developing country

To date, discussions on positionality and the relationship with research collaborators have been very much in the human geography realm. In this paper, we explore issues of expertise, positionality, collaboration and participation from our perspective as physical geographers working in a developing country context. We trace our journey from identifying ourselves as top-down 'experts' to participatory 'facilitators', and the difficulties and dilemmas encountered during this journey as we coped with the contrasting challenges of academic demands and local necessities. Our experiences highlight the many assumptions we make about doing research in developing countries and the real lack of capacity in these places to undertake typical short-term research projects designed in the developed world. We conclude with a call for a longer term and deeper commitment by physical geographers to the people that we engage with in our research .     

River spacing and drainage network growth in widening mountain ranges

Drainage networks in linear mountain ranges always display a particular geometrical organisation whereby the spacing between the major drainage basins is on average equal to half the mountain width (distance from the mountain front to the main drainage divide), independent of climate and tectonics. This relationship is valid for mountains having different widths and is thus usually thought to be maintained by drainage reorganisation during mountain belt widening. However, such large‐scale systematic drainage reorganisation has never been evidenced. In this paper, we suggest an alternative explanation, namely that the observed drainage basin relationships are an inherent property of dendritic river networks and that these relationships are established on the undissected, lowland margins outside mountain ranges and are progressively incorporated and quenched into uplifted topography during range widening. Thus, we suggest that the large‐scale geometry of drainage networks in mountain ranges is mainly antecedent to erosion. We propose a model in which the large‐scale drainage geometry is controlled mainly by the geometrical properties of the undissected surfaces (in particular, the ratio of the regional slope to the local slope related to roughness) over which rivers are flowing before uplift, and is therefore independent of climate and tectonics.     

On the role of “hot towers” in tropical cyclone formation

Summary The probabilistic approach to tropical cyclogenesis is advanced here by examining the role of convection in the early stages. The development of hot towers, that is tall cumulonimbus towers which reach or penetrate the tropopause, and their role in tropical cyclogenesis is investigated in two well-documented cases of formation. namely hurricane Daisy (1958) in the Atlantic and Tropical Cyclone Oliver (1993) in the Coral Sea. The hot towers in Daisy had been intensively studied by Malkus and Riehl three decades ago but remained mainly unpublished. The dynamics of Oliver genesis by merging mesoscale vortices has been recently reported, but much of the aircraft data remained. This paper adds the evolving contribution of cumulus-scale events and their associated electrification, which was made possible by the addition of an electric field mill, a numerical cloud model and other remote sensors.In their genesis stages, Daisy and Oliver appeared very different because Daisy resulted from a deepening tropical wave in the Atlantic and the pre-Oliver vortex emerged eastward from the Australian monsoon trough. However, the vertical profiles of _E in the rain areas were nearly identical, with the characteristic concave shape showing substantial midlevel minima. Therefore, both required increasing upflux of high _E subcloud air in order to accomplish the formation stage, with about two hot towers each in the nascent eyewall. In both cases, partial eyewalls developed at the edge of the convection, permitting subsidence in the forming eye, which was shown to contribute to the pressure fall. The probabilistic concept proposes that any contribution to early pressure fall raises the probability of success. When the incipient storm goes through those fragile phases more rapidly, the risk of death by the onset of unfavorable large-scale factors such as wind shear or upper-level subsidence is reduced. Daisy developed in an inactive, moist environment with light, variable winds throughout the troposphere while in Oliver, strong divergent upper outflow apparently outweighed strong wind shear, although the latter was responsible for a slow and messy development of a closed, circular eye.In both storms, the hot towers in the major rainband were taller and stronger than those in the early eyewall. Onedimensional time-dependent model runs were used to simulate both in Oliver with two important results: 1) the taller rainband clouds permitted greater high level heating, if it could be retained; and 2) greater electrification and more lighting occurred in the rainband although the partial eyewall clouds also showed strong electrification. Airborne radar, electrification measurements and models are fitted together to understand their relationship. An important result is the clear inference that fairly deep mixed phase regions existed in both eyewall and rainband, in which the DC-8 aircraft experienced liquid water at temperatures colder than –40°C below freezing. These results show that the claims of no supercooled liquid water in tropical cyclones require re-examination with the proper measurements of electricification that are now feasible.With 13 Figures     

Trace metals associated with deep-sea tailings placement at the Batu Hijau copper–gold mine,Sumbawa, Indonesia

The Batu Hijau copper–gold mine on the island of Sumbawa, Indonesia operates a deep-sea tailings placement (DSTP) facility to dispose of the tailings within the offshore Senunu Canyon. The concentrations of trace metals in tailings, waters, and sediments from locations in the vicinity of the DSTP were determined during surveys in 2004 and 2009. In coastal and deep seawater samples from Alas Strait and the South Coast of Sumbawa, the dissolved concentrations of Ag, As, Cd, Cr, Hg, Pb and Zn were in the sub μg/L range. Dissolved copper concentrations ranged from 0.05 to 0.65 μg/L for all depths at these sites. Dissolved copper concentrations were the highest in the bottom-water from within the tailings plume inside Senunu Canyon, with up to 6.5 μg Cu/L measured in close proximity to the tailings discharge. In general, the concentrations of dissolved and particulate metals were similar in 2004 and 2009.     

The retention of heavy metals in sewage sludge applied to a freshwater tidal wetland

The hypothesis that freshwater tidal wetlands act as sinks for heavy metals was tested using sewage sludge applied biweekly from March to October 1981 at low treatment (25 g m^?2 wk^?1) and high treatment (100 g m^?1) levels. No differences in aboveground macrophyte standing crop were found except in June when high and low treatment sites had significantly higher (p=0.05) standing crops than control sites. Except for chromium, metal standing stocks in the vegetation on treatment sites did not increase as a result of sludge application. The March litter had significantly higher (p=0.05) concentrations of chromium, copper, lead, and nickel at all sites than the October vegetation, but only high and low treatment litter chromium levels were significantly higher (p=0.05) than control litter. When sludge application terminated in October, the top 5 cm of soil at the high and low treatment sites had retained, respectively, 47 and 43% of the cadmium, 53 and 28% of the chromium, 52 and 0% of the copper, 51 and 0% of the zinc, 31 and 0% of the lead, and 0 and 0% of the nickel applied; only cadmium (15 and 46%, respectively) and chromium (12 and 28%, respectively) were still retained the following March. Thus, freshwater tidal wetlands can retain significant quantities of heavy metals associated with sewage sludge. The vegetation and litter play minor roles while the soil plays a major role in heavy metal retention.