Fluid inclusions in high-grade gneisses of the Kapuskasing structural zone,Ontario: metamorphic fluids and uplift/erosion path

Fluid inclusions in quartz grains from five samples of high-grade rocks (two paragneisses, an amphibolite, a mafic gneiss and a tonalite dike) from the 2.7 Ga Kapuskasing structural zone (KSZ), Ontario, were examined with petrographic, microthermometric and laser Raman techniques. Three types of fluid inclusions were observed: CO₂-rich, H₂O-rich and mixed CO₂-H₂O. CO₂-rich fluid inclusions are pseudosecondary or secondary in nature and are generally pure CO₂; a few contain varying amounts of CH₄·H₂O-rich fluid inclusions are secondary in nature, contain variable amounts of dissolved salts, and generally contain daughter crystals. Mixed CO₂-H₂O fluid inclusions occur where trails of H₂O-rich inclusions intersect trails of CO₂-rich inclusions. Isochores for high density (p=1.03 g/cm³) pseudosecondary, pure CO₂ inclusions intersect the lower pressure portion of the estimated P-T field for high-grade metamorphism, implying that pure CO₂ was the peak metamorphic fluid. The variable CH₄ content of CO₂ inclusions within graphite-bearing samples suggests that CH₄ was introduced locally after the formation of the CO₂ inclusions; however the origin of the CH₄ remains problematic. An aqueous fluid clearly penetrated the gneisses after the peak metamorphism (during uplift/erosion), forming secondary inclusions and contributing to the minor retrogressive hydration observed in these rocks. The presence of the pseudosecondary, high-density CO₂ inclusions in quartz crystals in the KSZ rocks constrains the uplift/ erosion path for the KSZ to one of simultaneous decrease in pressure and temperature.     

Toxic metal adsorption in the triassic sandstone aquifer of the English Midlands

Laboratory experimental results show that adsorption can be an important mechanism in taking up heavy metals contained in groundwaters in the sandstones. The adsorption can be described in terms of Freundlich isotherms and can be simply modeled mathematically. The adsorption function has been integrated into a mass balance equation, and tested using a laboratory sand tank model and in a small field study. The results show that the transport-adsorption models are not easily applied in the field because of hydrogeological complications.     

Cover folding in the Sonmartel Chain (Jura Neuchatelois)

Sonmartel forms part of the anticlinal Cornu-Foulets Chain of the Neuchâtel High Jura. The strata exposed in the district include much of the Malm (Argovien to Purbeckien), the Lower Cretaceous (Berriasien to Hauterivien) and some Tertiary rocks (Siderolithique, Helvetian Miocene). An outline account is given of the Mesozoic formations, the distribution of marker bands being shown on the map (Pl. 1). The structure of the area is discussed in terms of its constituent parts. The folding is concentric but due to erosion-breaching following the earliest movements, subsequent deformation of the Pre- and Post-Kimeridgien rocks differed. The former are gently diapiric, while the folding of the latter is, in part, a gravity effect. Faults normal to the fold axes are present everywhere. They originated as tension cracks but later permitted transcurrent movements. Tear faults are best developed in regions where the trend of the fold axes changes. Thrusts are restricted to the Grand Sonmartel area. The joint systems are thought to result from stresses due to bed-over-bed sliding during the folding. The different types of deformation combine to produce a rhomboidal pattern of dissected cross-folds. Each of these strain-phenomena show that the region has been subjected to stresses of two different orientations, with a third also affecting the vicinity of Grand Sonmartel. These owe their origins to relative movements of basement blocks beneath the area — thrusting during the Post-Stampian phase, the thrusts re-activated as sinistral shears during the Post-Pontian phases when they were joined by conjugate dextral shears. On this basis the Alpine Push theory for the origin of the Folded Jura must be rejected. The Wegmann hypothesis of basement shears accounts best for the structures observed.     

Using a One-and-a-Half Order Closure Model of the Atmospheric Boundary Layer for Surface Flux Footprint Estimation

A knowledge of the distribution of the contribution of upwind sources to measurements of vertical scalar flux densities is important for the correct interpretation of eddy covariance data. Several approaches have been developed to estimate this so-called footprint function. Here a new approach based on the ensemble-averaged Navier—Stokes equations is presented. Comparisons of numerical results using this approach with results from other studies under a range of environmental conditions show that the model predictions are robust. Moreover, the approach outlined here has the advantage of a potential wide applicability, due to an ability to take into account the heterogeneous nature of underlying surfaces. For example, the model showed that any variations in surface drag, such as must occur in real life heterogeneous canopies, can exert a marked influence of the shape and extent of flux footprints. Indeed, it seems likely that under such circumstances, estimates of surface fluxes will be weighted towards areas of highest foliage density (and therefore quite likely higher photosynthetic rates) close to the measurement sensor. Three-dimensional footprints during the day and night were also determined for a mixed coniferous forest in european Russia. A marked asymmetry of the footprint in the crosswind direction was observed, this being especially pronounced for non-uniform plant distributions involving vegetation types with different morphological and physiological properties. The model also found that, other things being equal, the footprint peak for forest soil respiration is typically over twice the distance from the above canopy measurement sensor compared to that for canopy photosynthesis. This result has important consequences for the interpretation of annual ecosystem carbon balances by the eddy covariance method.     

The role of atmosphere feedbacks during ENSO in the CMIP3 models. Part II: using AMIP runs to understand the heat flux feedback mechanisms

Several studies using ocean?Catmosphere general circulation models (GCMs) suggest that the atmospheric component plays a dominant role in the modelled El Ni?o-Southern Oscillation (ENSO). To help elucidate these findings, the two main atmosphere feedbacks relevant to ENSO, the Bjerknes positive feedback (??) and the heat flux negative feedback (??), are here analysed in nine AMIP runs of the CMIP3 multimodel dataset. We find that these models generally have improved feedbacks compared to the coupled runs which were analysed in part I of this study. The Bjerknes feedback,???, is increased in most AMIP runs compared to the coupled run counterparts, and exhibits both positive and negative biases with respect to ERA40. As in the coupled runs, the shortwave and latent heat flux feedbacks are the two dominant components of ?? in the AMIP runs. We investigate the mechanisms behind these two important feedbacks, in particular focusing on the strong 1997?C1998 El Ni?o. Biases in the shortwave flux feedback, ?? _SW, are the main source of model uncertainty in ??. Most models do not successfully represent the negative ??_SW in the East Pacific, primarily due to an overly strong low-cloud positive feedback in the far eastern Pacific. Biases in the cloud response to dynamical changes dominate the modelled ?? _SW biases, though errors in the large-scale circulation response to sea surface temperature (SST) forcing also play a role. Analysis of the cloud radiative forcing in the East Pacific reveals model biases in low cloud amount and optical thickness which may affect ?? _SW. We further show that the negative latent heat flux feedback, ?? _LH, exhibits less diversity than ?? _SW and is primarily driven by variations in the near-surface specific humidity difference. However, biases in both the near-surface wind speed and humidity response to SST forcing can explain the inter-model ??_LH differences.     

Sub-Micrometre Resolution FIB-SEM-based ToF-SIMS Used to Map Geochemical Zoning in Four Zircon Reference Materials

Zircon geochemistry can vary over micrometre scales; therefore, natural reference materials need to be well characterised before being used to calculate trace element mass fractions in unmeasured samples. Moreover, reference material homogeneity needs to be ensured with the accelerating rate of geoanalytical developments to map mineral chemistry at increasingly finer scales. Here, we investigate trace element zoning in four widely used zircon reference materials: 91500, Mud Tank, Temora and Plešovice, as well as zircon crystals from the Mount Dromedary/Gulaga Igneous Complex, Australia. Sub-micrometre resolution focused ion beam scanning electron microscope (FIB-SEM) based time-of-flight secondary ion mass spectrometry (ToF-SIMS) and 5 μm resolution LA-ICP-MS mapping show that trace elements are zoned in all reference materials, though 91500 exhibited the least zonation. We demonstrate that FIB-SEM-based ToF-SIMS can rapidly resolve variations in trace elements (e.g., U, Th, Sc, Y, Gd, Dy, Yb and Li) at sensitivities down to the μg g^-1 level with a spatial resolution of 195 nm for areas 100 × 85 μm to 959 × 828 μm. Zircon 91500 is recommended for future quantitative analyses provided that (1) the spatial distribution of elements is imaged before analysis of unknown samples and (2) it is used in conjunction with a doped glass as the primary reference material.     

Local and regional constraints on relative sea-level changes in southern Isle of Skye,Scotland, since the Last Glacial Maximum

New relative sea-level (RSL) data constrain the timing and magnitude of RSL changes in the southern Isle of Skye following the Last Glacial Maximum (LGM). We identify a marine limit at ~23 m OD, indicating RSL ~20 m above present c. 15.1 ka. Isolation basin data, supported by terrestrial and marine limiting dates, record an RSL fall to 11.59 m above present by c. 14.2 ka. This RSL fall occurs across the time of global Meltwater Pulse 1A, supporting recent research on the sources of ice melting. Our new data also help to resolve some of the chronological issues within the existing Isle of Skye RSL record and provide details of the sub-Arctic marine environment associated with the transition into Devensian Lateglacial climate at c. 14.5 k cal a bp , and the timing of changes in response to the Loch Lomond Stadial climate. Glacio-isostatic adjustment (GIA) model predictions of RSL deviate from the RSL constraints and reflect uncertainties in local and global ice models used within the GIA models. An empirical RSL curve provides a target for future research.