Geochemical modelling of a Zn–Pb skarn: Constraints from LA–ICP–MS analysis of fluid inclusions

The Bismark deposit (northern Chihuahua, Mexico) is one of several base metal-rich high-temperature, carbonate-replacement deposits hosted in northern Mexico. Previous fluid inclusion studies based on microthermometry and PIXE have shown that the Zn-rich, Pb-poor Bismark deposit formed from a moderate salinity magmatic fluid [Baker, T. and Lang, J.R., 2003. Reconciling fluid inclusion types, fluid processes, and fluid sources in skarns: an example from the Bismark Deposit, Mexico. Mineralium Deposita 38(4), 474–495; Baker, T., van Achterberg, E., Ryan, C.G. and Lang, J.R., 2004. Composition and evolution of ore fluids in a magmatic-hydrothermal skarn deposit. Geology 32(2), 117–120]. The exact precipitation mechanisms are unclear and may have due to cooling, salinity decrease and wall rock reaction. Furthermore, PIXE data suggested that Pb and Zn concentrations were comparable and inconsistent with the Zn-rich nature of the ore. However, Pb was commonly below the limit of detection for PIXE and the data presented by Baker et al. [Baker, T., van Achterberg, E., Ryan, C.G. and Lang, J.R., 2004. Composition and evolution of ore fluids in a magmatic-hydrothermal skarn deposit. Geology 32(2), 117–120] are regarded as the maximum concentrations of Pb in the fluid. In this study new LA ICP MS analysis was carried out on the same fluid inclusion population to compare with the PIXE data in order to constrain the uncertainty related to the Pb data and the new results are used to model possible ore deposition mechanisms. The new laser ablation data reveal overall lower concentrations of Pb in the ore fluid (average value ~ 285 ppm) than previously indicated by PIXE analysis (average value ~ 713 ppm). Chemical modelling using the new laser ablation data tested the following ore deposition processes: 1) cooling; 2) fluid–rock reaction at constant temperature; 3) cooling and simultaneous fluid–rock interaction. Modelling results show that the gangue and ore minerals observed at Bismark are best reproduced by fluid–rock interaction and simultaneous cooling. Results from the simulations strongly indicate that ore deposition was mainly driven by a pH increase due to the neutralization of the acidic ore fluid (pH = 3.9) as the result of the reaction with the limestone. Modelling results also suggest that the deposit likely formed under cooling conditions, but do not support the hypothesis of a temperature decrease as the principal ore-forming process.     

Pyroclastic dune bedforms: macroscale structures and lateral variations. Examples from the 2006 pyroclastic currents at Tungurahua (Ecuador)

Pyroclastic currents are catastrophic flows of gas and particles triggered by explosive volcanic eruptions. For much of their dynamics, they behave as particulate density currents and share similarities with turbidity currents. Pyroclastic currents occasionally deposit dune bedforms with peculiar lamination patterns, from what is thought to represent the dilute low concentration and fluid‐turbulence supported end member of the pyroclastic currents. This article presents a high resolution dataset of sediment plates (lacquer peels) with several closely spaced lateral profiles representing sections through single pyroclastic bedforms from the August 2006 eruption of Tungurahua (Ecuador). Most of the sedimentary features contain backset bedding and preferential stoss‐face deposition. From the ripple scale (a few centimetres) to the largest dune bedform scale (several metres in length), similar patterns of erosive‐based backset beds are evidenced. Recurrent trains of sub‐vertical truncations on the stoss side of structures reshape and steepen the bedforms. In contrast, sporadic coarse‐grained lenses and lensoidal layers flatten bedforms by filling troughs. The coarsest (clasts up to 10 cm), least sorted and massive structures still exhibit lineation patterns that follow the general backset bedding trend. The stratal architecture exhibits strong lateral variations within tens of centimetres, with very local truncations both in flow‐perpendicular and flow‐parallel directions. This study infers that the sedimentary patterns of bedforms result from four formation mechanisms: (i) differential draping; (ii) slope‐influenced saltation; (iii) truncative bursts; and (iv) granular‐based events. Whereas most of the literature makes a straightforward link between backset bedding and Froude‐supercritical flows, this interpretation is reconsidered here. Indeed, features that would be diagnostic of subcritical dunes, antidunes and ‘chute and pools’ can be found on the same horizon and in a single bedform, only laterally separated by short distances (tens of centimetres). These data stress the influence of the pulsating and highly turbulent nature of the currents and the possible role of coherent flow structures such as Görtler vortices. Backset bedding is interpreted here as a consequence of a very high sedimentation environment of weak and waning currents that interact with the pre‐existing morphology. Quantification of near‐bed flow velocities is made via comparison with wind tunnel experiments. It is estimated that shear velocities of ca 0·30 m.s^?1 (equivalent to pure wind velocity of 6 to 8 m.s^?1 at 10 cm above the bed) could emplace the constructive bedsets, whereas the truncative phases would result from bursts with impacting wind velocities of at least 30 to 40 m.s^?1.     

Landforms and landscape evolution in the Skardu,Shigar and Braldu Valleys,Central Karakoram

The Central Karakoram, which includes K2 in Pakistan, is one of the most rapidly rising areas on Earth and exhibits complex topography and extreme relief. Impressive valley fills and glacial landforms are present throughout the valleys. The dynamics of landscape evolution of the region are currently not well understood. Consequently, the landforms were mapped and assessed in the Skardu, Shigar, and Braldu valleys, to elucidate the spatio-temporal scale dependencies of surface processes active in the region. These valleys were examined using geomorphic field methods, remote sensing, geomorphometry, and terrestrial cosmogenic nuclides (TCNs) surface exposure dating. The glaciers in this region have oscillated considerably throughout the Late Quaternary, and four glacial stages have been recognized including at least six glacial advances. Surface processes readjusted after glacier retreat, and ubiquitous mass movements and catastrophic landsliding transported material from steep slopes to valley bottoms, while glaciofluvial meltwater and glacier outburst floods redistributed sediment down valley. Glacier geochronology and late Holocene ages of the outburst flood deposits indicate that landscape evolution has been dominated by glaciation and paraglaciation during the late Quaternary.     

Parameterization of the logarithmic layer of double-averaged streamwise velocity profiles in gravel-bed river flows

The logarithmic layer of double-averaged (in time and space) streamwise velocity profiles obtained from field measurements made in the Swiss rivers, Venoge and Chamberonne is parameterized and discussed. Velocity measurements were made using a 3D Acoustic Doppler Velocity Profiler. Both riverbeds are hydraulically rough, composed of coarse gravel, with relative submergences (h/D₅₀) of 5.25 and 5.96, respectively. From the observations, the flow may be divided into three different layers: a roughness layer near the bed, an equivalent logarithmic layer and a surface or outer layer. It was found that a logarithmic law can describe the double-averaged profiles in the layer 0.30 < z/h < 0.75. The parameterization of the logarithmic law is discussed. Special emphasis is given to the geometric parameters roughness and zero-displacement heights and to the equivalent von Karman constant.     

Intercomparison of Formaldehyde Measurements in Clean and Polluted Atmospheres

Three different techniques used tomeasure atmospheric formaldehyde were compared duringa field campaign carried out at a clean maritime siteon the West coast of Ireland. Two spectroscopictechniques Differential Optical AbsorptionSpectroscopy (DOAS) and Tunable Diode Laser AbsorptionSpectroscopy (TDLAS), together with a glass coil/Hantzschreaction/fluorescence technique, wereemployed for measurements of atmospheric formaldehydeof the order of a few hundred pptv. The betteragreement was observed between the fluorescence andDOAS instruments.Two DOAS instruments were compared to the glasscoil/Hantzsch reaction/fluorescence technique at asemi-polluted site on the North Norfolk coast, U.K.,where concentrations of formaldehyde were observed atlevels up to 4 ppbv. A very good agreement wasobserved between the two instruments.The glass coil/Hantzsch reaction/fluorescence and theTDLAS instruments were also deployed simultaneously inorder to measure indoor air inside a mobile laboratorylocated at the Imperial College Silwood Park site nearAscot, U.K. The doors of the mobile laboratory wereleft open in order to obtain the backgroundformaldehyde concentrations. Closing them afterwardsallowed us to observe the increase in concentrationsas a result of indoor emissions. The agreement betweenthe two instruments was outstanding (correlationcoefficient was 99%).The results from this study showed that of the fourinstruments included in this intercomparison the glasscoil/Hantzsch reaction/fluorescence technique provedthe most suitable for continuous measurements offormaldehyde in the background atmosphere.     

The added value of participatory modelling in fisheries management – what has been learnt?

How can uncertain fisheries science be linked with good governance processes, thereby increasing fisheries management legitimacy and effectiveness? Reducing the uncertainties around scientific models has long been perceived as the cure of the fisheries management problem. There is however increasing recognition that uncertainty in the numbers will remain. A lack of transparency with respect to these uncertainties can damage the credibility of science. The EU Commission's proposal for a reformed Common Fisheries Policy calls for more self-management for the fishing industry by increasing fishers' involvement in the planning and execution of policies and boosting the role of fishers' organisations. One way of higher transparency and improved participation is to include stakeholders in the modelling process itself. The JAKFISH project (Judgment And Knowledge in Fisheries Involving StakeHolders) invited fisheries stakeholders to participate in the process of framing the management problem, and to give input and evaluate the scientific models that are used to provide fisheries management advice. JAKFISH investigated various tools to assess and communicate uncertainty around fish stock assessments and fisheries management. Here, a synthesis is presented of the participatory work carried out in four European fishery case studies (Western Baltic herring, North Sea Nephrops, Central Baltic Herring and Mediterranean swordfish), focussing on the uncertainty tools used, the stakeholders' responses to these, and the lessons learnt. It is concluded that participatory modelling has the potential to facilitate and structure discussions between scientists and stakeholders about uncertainties and the quality of the knowledge base. It can also contribute to collective learning, increase legitimacy, and advance scientific understanding. However, when approaching real-life situations, modelling should not be seen as the priority objective. Rather, the crucial step in a science–stakeholder collaboration is the joint problem framing in an open, transparent way.     

Evaluation of Seasonal Factors on Petroleum Hydrocarbon Vapor Biodegradation and Intrusion Potential in a Cold Climate

A detailed seasonal study of soil vapor intrusion at a cold climate site with average yearly temperature of 1.9 °C was conducted at a house with a crawlspace that overlay a shallow dissolved‐phase petroleum hydrocarbon (gasoline) plume in North Battleford, Saskatchewan, Canada. This research was conducted primarily to assess if winter conditions, including snow/frost cover, and cold soil temperatures, influence aerobic biodegradation of petroleum vapors in soil and the potential for vapor intrusion. Continuous time‐series data for oxygen, pressure differentials, soil temperature, soil moisture, and weather conditions were collected from a high‐resolution monitoring network. Seasonal monitoring of groundwater, soil vapor, crawlspace air, and indoor air was also undertaken. Petroleum hydrocarbon vapor attenuation and biodegradation rates were not significantly reduced during low temperature winter months and there was no evidence for a significant capping effect of snow or frost cover that would limit oxygen ingress from the atmosphere. In the residual light nonaqueous phase liquid (LNAPL) source area adjacent to the house, evidence for biodegradation included rapid attenuation of hydrocarbon vapor concentrations over a vertical interval of approximately 0.9 m, and a corresponding decrease in oxygen to less than 1.5% v/v. In comparison, hydrocarbon vapor concentrations above the dissolved plume and below the house were much lower and decreased sharply within a few tens of centimeters above the groundwater source. Corresponding oxygen concentrations in soil gas were at least 10% v/v. A reactive transport model (MIN3P‐DUSTY) was initially calibrated to data from vertical profiles at the site to obtain biodegradation rates, and then used to simulate the observed soil vapor distribution. The calibrated model indicated that soil vapor transport was dominated by diffusion and aerobic biodegradation, and that crawlspace pressures and soil gas advection had little influence on soil vapor concentrations.     

Spatial Distribution of Incrustations around a Water Well after 38 Years of Use

The spatial distribution of iron incrustations around an abandoned water well after 38 years of use was studied by analyzing the iron content of soil samples from 12 core borings. Three of these were performed inside the gravel pack, the rest at different distances in the adjacent aquifer. Incrustations had preferentially accumulated at the bottom of the screen coinciding with a zone of higher aquifer permeability. As expected, all cores from the annulus show buildup of iron oxides but the ones outside also showed elevated iron content up to 4 m away from the screen. Such distant incrustations are very difficult to remove, by mechanical or chemical methods. The common procedure of removing tubing and annulus material to install a new well at the same location could therefore give the replacement well a hydraulic disadvantage as the incrustations in the aquifer remain in place.