Epistemic and ethical trade-offs in decision analytical modelling

Designing decision analytical models requires making choices that can involve a range of trade-offs and interactions between epistemic and ethical considerations. Such choices include determining the complexity of a model and deciding what types of risk will be assessed. Here, we demonstrate how model design choices can involve trade-offs between the epistemic benefits of representational completeness and simplicity, which interact with ethical considerations about fairness and human life. We illustrate this point by focusing on modeling studies that assess flood risks in New Orleans, Louisiana. Addressing the ethical and epistemic implications of model design choices can help clarify the scope of factors necessary to inform ethically sound and economically efficient decision-making.     

Use of alternative conceptual models to assess the impact of a buried valley on groundwater vulnerability

A buried valley incised into a sequence of pre-Quaternary sediments is shown to seriously affect the vulnerability of groundwater. Often the existence of buried valleys is not known or is not described explicitly in a hydrogeological model. In the present study, two numerical groundwater models, representing two alternative conceptual models, were produced to help quantify the effect of the valley on groundwater vulnerability. One model included the buried valley and the other did not. Both models were subjected to calibration and were found to describe hydraulic head and river discharge equally well. Even though the two models showed similar calibration statistics; fluxes, travel paths and travel times were affected by the inclusion of the buried valley. The recharge area and the groundwater age of potential abstraction wells placed in the pre-Quaternary deep aquifers surrounding the buried valley were different for the two models, with significantly higher vulnerability when the valley was included in the model. Based on the results of the present study, it is concluded that a buried valley may not always be detectable when calibrating a wrong conceptual model. If reliable results should be obtained a good geological model has to be constructed.     

Hydrogen incorporation in enstatite in the system MgO–SiO2–H2O–NaCl

The incorporation of hydrogen in enstatite in a hydrous system containing various amounts of NaCl was investigated at 25 kbar. The hydrogen content in enstatite shows a clear negative correlation to the NaCl-concentration in the system. The most favourable explanation is the reduction of water fugacity due to dilution. Other reasons for the limited hydrogen incorporation at high NaCl levels, such as a significant influence of Na⁺ on the defect chemistry or an exchange between OH^- and Cl⁻in enstatite, appear much less important. A partition coefficient D _Na ^En/Fluid = 0.0013 could be determined, demonstrating that Na is less incompatible in enstatite than H. The new results support the idea that dissolved components have to be considered when the total hydrogen storage capacity in nominally anhydrous minerals is estimated, especially in geological settings with high levels of halogens, such as subduction zones.     

Oxidized and reduced mineral assemblages in greenstone belt rocks of the St. Ives gold camp, Western Australia: vectors to high-grade ore bodies in Archaean gold deposits?

Hydrothermal sulfide–oxide–gold mineral assemblages in gold deposits in the Archaean St. Ives gold camp in Western Australia indicate extremely variable redox conditions during hydrothermal alteration and gold mineralization in space and time. Reduced alteration assemblages (pyrrhotite–pyrite) occur in deposits in the southwest of the camp (e.g., Argo, Junction deposits) and moderately to strongly oxidized assemblages (magnetite–pyrite, hematite–pyrite) occur in deposits in the Central Corridor in the northeast (e.g., North Orchin, Revenge deposits). Reduced mineral assemblages flank the Central Corridor of oxidized deposits and, locally, cut across it along E–W trending faults. Oxidized mineral assemblages in the Central Corridor are focused on gravity lows which are interpreted to reflect abundant felsic porphyritic intrusions at about 1,000 m below present surface. Hydrothermal magnetite predates and is synchronous with early phases of gold-associated albite–carbonate–pyrite–biotite–chlorite hydrothermal alteration. Later-stage, gold-associated pyrite is in equilibrium with hematite. The spatial distribution and temporal sequence of iron sulfides and oxides with gold indicate the presence of at least two spatially restricted but broadly synchronous hydrothermal fluids with contrasting redox states. Sulfur isotope constraints support the argument that the different mineral assemblages reflect differences in redox conditions. The δ ³⁴S values for pyrite for the St. Ives gold camp range between −8.4‰ and +5.1‰ with the negative values occurring in oxidized magnetite-rich domains and slightly negative or positive values occurring in reduced, pyrrhotitic domains. Preliminary spatial and paragenetic analysis of the distribution of iron sulfides and oxides in the St. Ives camp suggests that gold grades are highest where the redox state of the hydrothermal alteration assemblages switches from relatively reduced pyrrhotite–pyrite to relatively oxidized magnetite–pyrite and hematite–pyrite both in space and time. Gold deposition is inferred to have occurred where fluids of contrasting redox state mixed.     

Visualization of tectonic structures in shallow-depth high-resolution ground-penetrating radar (GPR) profiles

Ground-penetrating radar (GPR) is applied to detect subsurface tectonic structures and to map the geometry of faulted blocks. Tectonic interpretations from a profile crossing the graben fault and a grid in a second-order graben structure providing a 3D data set are correlated to the structural inventory of the outcrop. Folded layers of the roll-over anticline are identified by continuous curved reflectors and an increasing dip towards the main graben fault. Faults are indicated by arrays of reflector terminations. Variations in the water and clay content caused by karstification and brecciation on fault planes are displayed by changing amplitudes of the detected signal. The 3D visualization of the second-order graben structure with a grid of GPR profiles illustrates the local stress pattern which coincides with structural observations in the outcrop and photo lineations.     

A comparison of the climatology of a tropospherestratosphere-mesosphere model with observations

Mean fields from a perpetual January simulation of a GCM extending from the surface to 0.01 hPa (near 80 km) are compared to observations. The zonal mean temperature and wind fields correspond quite well with reality; the low stratosphere, especially in the polar night, is too cold, but warmer than in the original version of the model, with an upper boundary at 25 hPa. Mean fields at standard levels show that the major features of the troposphere are represented by the model, but rather over emphasised; the stratospheric winter polar vortex is too strong, too cold, and too barotropic; it resembles an `undisturbed' January rather than the climatology. Differences in the stationary eddy activity between the extended and orginal versions of the model are noted, and used to explain some differences between the two simulations.This paper was presented at the International Conference on Modelling of Global Climate Change and Variability, held in Hamburg 11–15 September 1989 under the auspices of the Meteorological Institute of the University of Hamburg and the Max Planck Institute für Meteorology. Guest Editor for these papers is Dr. L. Dümenil     

Tourism development on the north Yucatan coast: Human response to shoreline erosion and hurricanes

The barrier coast of north Yucatan has been evolving into a beachfront recreational landscape since the introduction of passenger rail service between Mérida and Progreso in 1881. Beginning in the 1950's, middle-class vacation home construction spread laterally outward from Progreso, and even distal fishing and salt-gathering villages became engulfed by a recreational landuse overlay. In spite of geomorphic evidence of marine transgression, such as dune scarps and washover fans, the focus of recreational urbanization has been the beachfront. And although attempts at retarding shore erosion have taken place since the 1960s, hazard perception has remained generally low until the late 1980s. When Hurricane Gilbert blew across the Yucatan peninsula in 1988, damage to beachfront structures, highways, boats, and salt ponds was extensive. The barrier environment exhibited extensive overwashing and breaching, especially in areas of human modification, and the lagoon ecosystem experienced considerable ecologic disruption. Because the north Yucatan coast is a locus of domestic tourism, and the thousands of uninsured summer homes are owned largely by members of Mérida's middle class, reconstruction since Gilbert has been slow. Incipient trends of international tourism development have slowed since 1988.     

Back-arc rifting in the Izu-Bonin Island Arc: Structural evolution of Hachijo and Aoga Shima Rifts

Abstract Multi- and single-channel seismic profiles are used to investigate the structural evolution of back-arc rifting in the intra-oceanic Izu-Bonin Arc. Hachijo and Aoga Shima Rifts, located west of the Izu-Bonin frontal arc, are bounded along-strike by structural and volcanic highs west of Kurose Hole, North Aoga Shima Caldera and Myojin Sho arc volcanoes. Zig-zag and curvilinear faults subdivide the rifts longitudinally into an arc margin (AM), inner rift, outer rift and proto-remnant arc margin (PRA). Hachijo Rift is 65 km long and 20–40 km wide. Aoga Shima Rift is 70 km long and up to 45 km wide. Large-offset border fault zones, with convex and concave dip slopes and uplifted rift flanks, occur along the east (AM) side of the Hachijo Rift and along the west (PRA) side of the Aoga Shima Rift. No cross-rift structures are observed at the transfer zone between these two regions; differential strain may be accommodated by interdigitating rift-parallel faults rather than by strike- or oblique-slip faults. In the Aoga Shima Rift, a 12 km long flank uplift, facing the flank uplift of the PRA, extends northeast from beneath the Myojin Knoll Caldera. Fore-arc sedimentary sequences onlap this uplift creating an unconformity that constrains rift onset to ～1-2Ma. Estimates of extension (～3km) and inferred age suggest that these rifts are in the early syn-rift stage of back-arc formation. A two-stage evolution of early back-arc structural evolution is proposed: initially, half-graben form with synthetically faulted, structural rollovers (ramping side of the half-graben) dipping towards zig-zagging large-offset border fault zones. The half-graben asymmetry alternates sides along-strike. The present ‘full-graben’ stage is dominated by rift-parallel hanging wall collapse and by antithetic faulting that concentrates subsidence in an inner rift. Structurally controlled back-arc magmatism occurs within the rift and PRA during both stages. Significant complications to this simple model occur in the Aoga Shima Rift where the east-dipping half-graben dips away from the flank uplift along the PRA. A linear zone of weakness caused by the greater temperatures and crustal thickness along the arc volcanic line controls the initial locus of rifting. Rifts are better developed between the arc edifices; intrusions may be accommodating extensional strain adjacent to the arc volcanoes. Pre-existing structures have little influence on rift evolution; the rifts cut across large structural and volcanic highs west of the North Aoga Shima Caldera and Aoga Shima. Large, rift-elongate volcanic ridges, usually extruded within the most extended inner rift between arc volcanoes, may be the precursors of sea floor spreading. As extension continues, the fissure ridges may become spreading cells and propagate toward the ends of the rifts (adjacent to the arc volcanoes), eventually coalescing with those in adjacent rift basins to form a continuous spreading centre. Analysis of the rift fault patterns suggests an extension direction of N80°E ± 10° that is orthogonal to the trend of the active volcanic arc (N10°W). The zig-zag pattern of border faults may indicate orthorhombic fault formation in response to this extension. Elongation of arc volcanic constructs may also be developed along one set of the possible orthorhombic orientations. Border fault formation may modify the regional stress field locally within the rift basin resulting in the formation of rift-parallel faults and emplacement of rift-parallel volcanic ridges. The border faults dip 45–55° near the surface and the majority of the basin subsidence is accommodated by only a few of these faults. Distinct border fault reflections decreases dips to only 30° at 2.5 km below the sea floor (possibly flattening to near horizontal at 2.8 km although the overlying rollover geometry shows a deeper detachment) suggesting that these rifting structures may be detached at extremely shallow crustal levels.     

Development of a Standard, Pure-Compound Base Gasoline Mixture for Use as a Reference in Field and Laboratory Experiments

In recent years, leaking underground petroleum storage tanks have become a major environmental concern. Federal, state, and local agencies are making efforts to assess, control, and remediate petroleum product leaks. Because petroleum products are mixtures of many compounds and the composition frequently changes, there exists a need for a standard reference mixture that can be used as a basis for comparison in the study of fluid transport properties in porous media and to evaluate leak detection devices. The proposed standard presented here retains the most important liquid and vapor properties (such as vapor density, air diffusion coefficient, and basic chemical constituency) of gasoline mixtures. It also reduces the complexity of the mixture to make it an acceptable standard for routine use in laboratory and field experiments.