Basic model of water- and gas-flow through smectite clay buffers

Canister embedment of highly compacted Na bentonite forms a low-permeable medium of significant homogeneity both macro- and microscopically. However, despite the fact that a large part of the porewater is in interlamellar positions and not mobile by ordinary hydraulic gradients, there are still a number of pore passages that let water and gas through even at very high bulk densities. A preliminary model for flow and diffusion has been outlined on the basis of generalized, quantitative microstructural data and basic physical relationships. It accounts for actually recorded hydraulic and gas conductivities as well as for commonly measured swelling pressures. Qualitatively, it is in agreement also with published ion diffusion data.     

Wave refraction–diffraction effect in the wind wave model WWM

Based on the extended mild-slope equation, the wind wave model (WWM; Hsu et al., 2005) is modified to account for wave refraction, diffraction and reflection for wind waves propagating over a rapidly varying seabed in the presence of current. The combined effect of the higher-order bottom effect terms is incorporated into the wave action balance equation through the correction of the wavenumber and propagation velocities using a refraction–diffraction correction parameter. The relative importance of additional terms including higher-order bottom components, the wave–bottom interaction source term and wave–current interaction that influence the refraction–diffraction correction parameter is discussed. The applicability of the proposed model to calculate a wave transformation over an elliptic shoal, a series of parallel submerged breakwater induced Bragg scattering and wave–current interaction is evaluated. Numerical results show that the present model provides better predictions of the wave amplitude as compared with the phase-decoupled model of Holthuijsen et al. (2003).     

Analytical model for a geomorphological instantaneous unit hydrograph

The rainfall‐runoff modelling of a river basin can be divided into two processes: the production function and the transfer function. The production function determines the proportion of gross rainfall actually involved in the runoff. The transfer function spreads the net rainfall over time and space in the river basin. Such a transfer function can be modelled using the approach of the geomorphological instantaneous unit hydrograph (GIUH). The effectiveness of geomorphological models is actually revealed in rainfall‐runoff modelling, where hydrologic data are desperately lacking, just as in ungauged basins. These models make it possible to forecast the hydrograph shape and runoff variation versus time at the basin outlet. This article is an introduction to a new GIUH model that proves to be simple and analytical. Its geomorphological parameters are easily available on a map or from a digital elevation model. This model is based on general hypotheses on symmetry that provide it with multiscale versatile characteristics. After having validated the model in river basins of very different nature and size, we present an application of this model for rainfall‐runoff modelling. Since parameters are determined relying on real geomorphological data, no calibration is necessary, and it is then possible to carry out rainfall‐runoff simulations in ungauged river basins. Copyright © 2006 John Wiley & Sons, Ltd.     

Conditional estimation variances: An empirical approach

One of the tasks routinely carried out by geostatisticians is the evaluation of global mining reserves corresponding to a given cutoff grade and size of selective mining units. A long with these recovery figures, the geostatistician generally provides an assessment of the global estimation variance, which represents the precision of the overall average grade estimate, when no cutoff is applied. Such a global estimation variance is of limited interest for evaluating mining projects; what is required is the reliability of the estimate of recovered reserves or, in other words, the conditional estimation variance. Unfortunately, classical linear geostatistical methods fail to provide an easy way to estimate this variance. Through the use of simulated deposits (representing various types of regionalization)the present paper reviews and discusses the effects of changes in cutoff grade and selective mining unit size on the conditional estimation variance. It is shown that, when the cutoff grade is applied to a pointsupport (sample-size)distribution, the conditional estimation variance appears to be readily accessible by classical formulas, once the conditional semivariogram is known. However, the evaluation of the conditional estimation variance seems to be less straightforward for the general case when a cutoff is applied to the average grade distribution of selective mining units. Empirical approximation formulas for the conditional estimation variance are tentatively proposed, and their performance in the case of the simulated deposits is shown. The limitations of these approximations are discussed, and possible ways of formalizing the problem suggested.     

How fast does tufa grow? Very high-resolution measurement of the tufa growth rate on artificial substrates by the development of a contactless image-based modelling device

Accurate determination of the tufa growth rate (TGR) is required to answer the fundamental geomorphological question of tufa evolution. The TGR has been measured by various direct and indirect methods. One of the most popular direct methods uses modified micro-erosion meter (MEM), which has several drawbacks. Here, we present for the first time a coordinate measuring macro-photogrammetry device (CMD) for monitoring the TGR in a contactless manner. The CMD was applied on 28 limestone plates at 14 locations within the Skradinski buk area, Croatia, and measurements were performed in the laboratory. The TGR was derived from digital tufa high-resolution models (DTHRMs). The accuracy of the device was evaluated using state-of-the-art three-dimensional (3D) scanners and error calculation at checkpoints. Moreover, the precision was evaluated with the split test (n = 5). A total of 74 DTHRMs with a spatial resolution of 0.0236 mm were created. The TGR ranged from 0.327 to 19.302 mm a⁻¹, with an average of 5.771 mm a⁻¹. A higher TGR was observed on the limestone plates near mosses, located in fast and turbulent water rather than in stagnant water. We found that specific micro-environmental factors (e.g. proximity to moss) positively affected tufa growth. Erosion events were observed, as well as the presence of aquatic insect larvae (Simuliidae and Chironomidae), which positively affected tufa growth. The CMD is a precise and accurate device that does not suffer from the drawbacks of the MEM method and has many other advantages. It has a high capability of tufa erosion detection, enables the identification of macroinvertebrates, and multispectral or hyperspectral cameras can be mounted on the device for spectral reflectance analysis of the tufa surface. The CMD can be applied in any study requiring a sub-millimetre data quality and involving the comparison of consecutive 3D models and derivation of various parameters of smaller objects. © 2020 John Wiley & Sons, Ltd.     

Tracking hydrological responses of a thermokarst lake in the Old Crow Flats (Yukon Territory,Canada) to recent climate variability using aerial photographs and paleolimnological methods

Recent studies using remote sensing analysis of lake‐rich thermokarst landscapes have documented evidence of declining lake surface area in response to recent warming. However, images alone cannot identify whether these declines are due to increasing frequency of lake drainage events associated with accelerated thermokarst activity or to increasing evaporation in response to longer ice‐free season duration. Here, we explore the potential of combining aerial photograph time series with paleolimnological analyses to track changes in hydrological conditions of a thermokarst lake in the Old Crow Flats (OCF), Canada, and to identify their causes. Images show that the water level in lake OCF 48 declined markedly sometime between 1972 and 2001. In a sediment core from OCF 48, complacent stratigraphic profiles of several physical, geochemical, and biological parameters from ～1874–1967 indicate hydro‐limnological conditions were relatively stable. From ～1967–1989, declines in organic matter content, organic carbon isotope values, and pigment concentrations are interpreted to reflect an increase in supply of minerogenic sediment, and subsequent decline in aquatic productivity, caused by increased thermo‐erosion of shoreline soils. Lake expansion was likely caused by increased summer rainfall, as recorded by increased cellulose‐inferred lake‐water oxygen isotope compositions. Stratigraphic trends defining the lake expansion phase terminated at ～1989, which likely marks the year when the lake drained. Above‐average precipitation during the previous year probably raised the lake level and promoted further thermo‐erosion of the shoreline soils that caused the lake to drain. These are meteorological conditions that have led to other recent lake‐drainage events in the OCF. Thus, the decline in lake level, evident in the aerial photograph from 2001, is unlikely to have been caused by evaporation, but rather is a remnant of a drainage event that took place more than a decade earlier. After drainage, the lake began to refill, and most paleolimnological parameters approach levels that are similar to those during the stable phase. These findings indicate that combined use of aerial images and paleolimnological methods offers much promise for identifying the hydrological consequences of recent climatic variations on thermokarst lakes. Copyright © 2011 John Wiley & Sons, Ltd.     

Chrysophyte cysts as paleolimnological indicators of environmental change due to cottage development and acidic deposition in the Muskoka-Haliburton region, Ontario, Canada

Chrysophyte cysts preserved in recent and pre-industrial lake sediment samples from 54 Muskoka-Haliburton (Ontario) lakes were used in a paleolimnological study to determine the impact of acidic precipitation and cottage development on water quality. A total of 246 cyst morphotypes were identified. Ecological preferences of cyst morphotypes were determined using multivariate statistical analysis, cluster analysis, and species-environment correlations. Recent cyst assemblages were related to water chemistry and lake morphometric variables using Redundancy Analysis (RDA). The distribution of morphotypes was related to a gradient of acid neutralising capacity (ANC), expressed through the association of variables related to buffering (i.e. longitude, watershed area, and ionic concentration) with the first axis (₁ = 0.29). Cyst assemblages were also defined, to a lesser extent (₂ = 0.06), by a trophic status gradient, created through the combination of total nitrogen (TN), total phosphorus (TP), volume-weighted cottage density, and lake depth variables. The identification of lakewater pH and trophic status as important determinants of cyst assemblage structure allowed for the reconstruction of acidification and eutrophication related water chemistry changes using fossil cyst assemblages. The reconstruction of pre-industrial (pre-1850) water quality conditions with fossil cyst assemblages indicated that pH significantly decreased in 24.1% of the study lakes and increased in 16.7% of the lakes. Increases in pH in more alkaline drainage basins are attributed to alkalinity generation processes induced by acidic precipitation as has been shown in other studies. Total phosphorus (TP) concentrations significantly declined in 12.9% of the lakes and increased in 16.6% of lakes. Increases in [TP] were linked to cottage development. Decreases in trophic status may be due to landuse changes, the result of the acidification occurring in the area, or warmer and drier climates. A comparison of chrysophyte cyst and diatom water quality inferences show similar trends in pH changes. There is a good agreement between diatom and chrysophyte bioindicators with respect to [TP] changes in oligotrophic lakes (< 10 g/L); however, diatom inferences suggest that lakes with current [TP] values greater than 10 g/L have decreased in trophic status over time, while chrysophyte reconstructions suggest that these same lakes have become more productive systems.     

Ion microprobe (SHRIMP) dating of detrital zircon grains from quartzites of the Eckergneiss Complex, Harz Mountains (Germany): implications for the provenance and the geological history

The Eckergneiss Complex (EGC) is a geologically unique medium- to high-grade metamorphic unit within the Rhenohercynian domain of the Mid-European Variscides. A previously, poorly defined conventional lower U–Pb intercept age of about 560 Ma from detrital zircons of metasedimentary rocks has led to speculations about an East Avalonian affinity of the EGC. In order to unravel the provenance and to constrain the age of the sediment protolith, we carried out sensitive high-resolution ion microprobe U–Pb analyses on detrital zircons from five different EGC quartzite occurrences. The obtained age spectrum indicates a SW Baltica provenance of the detritus. Sveconorwegian ages between 0.9–1.2 Ga are particularly well represented by analyses from metamorphic recrystallization/alteration zones penetrating into igneous zircon. Cadomian (Pan-African) ages, which might reflect a metamorphic event, could not be substantiated. Instead, zircons of igneous origin yielded concordant Lower Devonian and Silurian ages of 410±10, 419±10, and 436±6 Ma (1), implying that sedimentation of the EG protolith must have taken place after 410±10 Ma. The lower age limit of the EGC metamorphism is constrained by 295 Ma intrusion ages of the adjacent, nonmetamorphosed Harzburg Gabbronorite and Brocken Granite. Sedimentation and metamorphism must thus have taken place between about 410 Ma and 295 Ma. Given that this time span coincides with most of the sedimentation within the virtually nonmetamorphosed (lowest grade) Rhenohercynian in the Harz Mountains, including the direct vicinity of the EGC, along with the high-grade metamorphism, the EGC can hardly be seen as uplifted local basement. A possible candidate for the root region is an easterly, concealed marginal segment of the Rhenohercynian domain of the Variscides, which is tectonically overridden and suppressed by the Mid-German Crystalline Rise during continent collision. However, based on the concept of strike-slip movement of Variscan terranes with different P–T–t histories as a result of postaccretion intraplate deformation, the EGC could also represent a fault-bounded complex with an origin located far east or south east of the present location.

Fluid flow in marbles at Jervois, central Australia: oxygen isotope disequilibrium and zoning produced by decoupling of mineralogical and isotopic resetting

The Jervois region of the Arunta Inlier, central Australia, contains para- and orthogneisses that underwent low-pressure amphibolite facies metamorphism (P = 200–300 MPa, T = 520–600 °C). Marble layers cut by metre-wide quartz + garnet ± epidote veins comprise calcite, quartz, epidote, clinopyroxene, grandite garnet, and locally wollastonite. The marbles also contain locally discordant decimetre-thick garnet and epidote skarn layers. The mineral assemblages imply that the rocks were infiltrated by water-rich fluids (XCO₂ = 0.1–0.3) at ∼600 °C. The fluids were probably derived from the quartz-garnet vein systems that represent conduits for fluids exsolved from crystallizing pegmatites emplaced close to the metamorphic peak. At one locality, the marble has calcite (Cc) δ¹⁸O values of 9–18‰ and garnet (Gnt) δ¹⁸O values of 10–14‰. The δ¹⁸O(Gnt) values are only poorly correlated with δ¹⁸O(Cc), and the δ¹⁸O values of some garnet cores are higher than the rims. The isotopic disequilibrium indicates that garnet grew before the δ¹⁸O values of the rock were reset. The marbles contain  ≤15% garnet and, for water-rich fluids, garnet-forming reactions are predicted to propagate faster than O-isotopes are reset. The Sm-Nd and Pb-Pb ages of garnets imply that fluid flow occurred at 1750–1720 Ma. There are no significant age differences between garnet cores and rims, suggesting that fluid flow was relatively rapid. Texturally late epidote has δ¹⁸O values of 1.5–6.2‰ implying δ¹⁸O(H₂O) values of 2–7‰. Waters with such low-δ¹⁸O values are probably at least partly meteoric in origin, and the epidote may be recording the late influx of meteoric water into a cooling hydrothermal system. Received: 29 April 1996 / Accepted: 12 March 1997