Methodological Considerations Regarding the Measurement of Turbulent Fluxes in the Urban Roughness Sublayer: The Role of Scintillometery

We address some of the methodological challenges associated with the measurement of turbulence and use of scintillometers in the urban roughness sublayer (RSL). Two small-aperture scintillometers were located near the roof interface in a densely urbanized part of Basel, Switzerland, as part of the Basel Urban Boundary-Layer Experiment (BUBBLE) in the summer of 2002. Eddy correlation instruments were co-located near the mid-point of each scintillometer path for data verification purposes. The study presents the first values of the inner length scale of turbulence (l ₀) and the refractive index structure parameter of air for a city and demonstrates the influence of mechanical driven turbulence on dissipation. Comparison of dissipation values determined from the two approaches show large scatter that is possibly due to the spatial inhomogeneity of the turbulence statistics within the RSL. Velocity and temperature spectra display a −2/3 slope in the inertial subrange, although the spectral ratio is less than the theoretical prediction of 4/3 expected for isotropy. Conventional Monin–Obukhov equations used to calculate fluxes from the scintillometer were replaced with urban forms of the equations. The results suggest that the scintillometer may be an appropriate tool for the measurement of sensible heat flux (Q _H ) above the rooftops given a suitable determination of the effective measurement height.     

Empirical-statistical downscaling and error correction of regional climate models and its impact on the climate change signal

Realizing the error characteristics of regional climate models (RCMs) and the consequent limitations in their direct utilization in climate change impact research, this study analyzes a quantile-based empirical-statistical error correction method (quantile mapping, QM) for RCMs in the context of climate change. In particular the success of QM in mitigating systematic RCM errors, its ability to generate “new extremes” (values outside the calibration range), and its impact on the climate change signal (CCS) are investigated. In a cross-validation framework based on a RCM control simulation over Europe, QM reduces the bias of daily mean, minimum, and maximum temperature, precipitation amount, and derived indices of extremes by about one order of magnitude and strongly improves the shapes of the related frequency distributions. In addition, a simple extrapolation of the error correction function enables QM to reproduce “new extremes” without deterioration and mostly with improvement of the original RCM quality. QM only moderately modifies the CCS of the corrected parameters. The changes are related to trends in the scenarios and magnitude-dependent error characteristics. Additionally, QM has a large impact on CCSs of non-linearly derived indices of extremes, such as threshold indices.     

Hydrogeochemical and multi-tracer investigations of arsenic-affected aquifers in semi-arid West Africa

The semi-arid Sahel regions of West Africa rely heavily on groundwater from shallow to moderately deep(100 m b.g.l.)crystalline bedrock aquifers for drinking water production.Groundwater quality may be affected by high geogenic arsenic(As)concentrations(10μg/L)stemming from the oxidation of sulphide minerals(pyrite,arsenopyrite)in mineralised zones.These aquifers are still little investigated,especially concerning groundwater residence times and the influence of the annual monsoon season on groundwater chemistry.To gain insights on the temporal aspects of As contamination,we have used isotope tracers(noble gases,~3H,stable water isotopes(~2 H,~(18)O))and performed hydrochemical analyses on groundwater abstracted from tube wells and dug wells in a small study area in southwestern Burkina Faso.Results revealed a great variability in groundwater properties(e.g.redox conditions,As concentrations,water level,residence time)over spatial scales of only a few hundred metres,characteristic of the highly heterogeneous fractured underground.Elevated As levels are found in oxic groundwater of circum-neutral pH and show little relation with any of the measured parameters.Arsenic concentrations are relatively stable over the course of the year,with little effect seen by the monsoon.Groundwater residence time does not seem to have an influence on As concentrations,as elevated As can be found both in groundwater with short(50 a)and long(10~3 a)residence times as indicated by ~3He/~4He ratios spanning three orders of magnitude.These results support the hypothesis that the proximity to mineralised zones is the most crucial factor controlling As concentrations in the observed redox/pH conditions.The existence of very old water portions with residence times10~3 years already at depths of50 m b.g.l.is a new finding for the shallow fractured bedrock aquifers of Burkina Faso,suggesting that overexploitation of these relatively low-yielding aquifers may be an issue in the future.     

A pilot-scale field experiment for the microbial neutralization of a holomictic acidic pit lake

A strategy to neutralize acidic pit lakes was tested in an upscaling process using field mesocosms of 26 to ca. 4500 m³ volume in the acidic pit Mining Lake 111 in Germany. After addition of the substrates Carbokalk and straw a neutral sediment layer formed, in which microbial sulfate and iron reduction as well as sulfide precipitation occurred. The net rate of neutralization was limited by the precipitation of iron sulfides rather than by microbial reactions. Oxidation of H₂S by ferric iron in the anoxic sediment lowered the net sulfate reduction rate. Seasonal fluctuations of iron sulfides in the sediment showed that the reaction products were not necessarily stable. The long-term success of the approach depends on the net partition of the precipitated iron-(mono-/di-) sulfide that is permanently buried in the anoxic sediment. It could be shown by field experiments that the long-term success of the neutralization depends on the spatial scale and duration of the experiments. Volumes from 26 to 4500 m³, exposition times from 4 months to 5 years, and increasingly thick coverings of the sediments with straw, from zero to 40 cm, were used. Net neutralization rates decreased from 41 meq m^− 2 d^− 1 in laboratory microcosms to a mean rate of 2.3 meq m^− 2 d^− 1 in the 4500 m³ field experiment. The results show that the success of the microbial treatment of acid pit lakes lastly depends on the limnological conditions in the lake that cannot be simulated by upscaling of simple laboratory experiments.     

Complex interplay between depositional and petrophysical environments in Holocene tidal carbonates (Al Ruwais,Qatar)

Carbonate rocks can be classified in terms of those properties relating to the pore system of lithified sediments, so‐called ‘petrophysical rock types’, or ‘depositional rock types’ which are categorized based on characteristics directly reflecting their original depositional environment. Whereas petrophysical rock types are typically used to identify and distribute rock bodies within a reservoir with similar flow characteristics, depositional rock types ignore pore types and capture sedimentary structures, lithology and fossils. Both classification systems are extensively used to describe reservoir rocks, but the degree of plurality between them remains poorly understood and is the motivation for this study. To examine the degree of congruency between the two classification schemes, a field assessment was conducted for a 175 km² area situated offshore Al Ruwais, northern Qatar, encompassing depositional environments spanning supratidal, intertidal, shallow subtidal and open marine conditions. A total of 350 surficial sediment samples were collected along 24 shore‐normal transects. Each sample was assigned a ‘petrophysical rock type’ class based on analysis of sedimentary texture (grain size and sorting). ‘Depositional rock type’ classes, by contrast, were defined with reference to faunal content and, in turn, classes of mineralogy were delimited by weighting this content against the mineralogy of each faunal category. Of course, the samples studied correspond to unconsolidated sediments and not to indurated rocks. However, considering only primary porosity and permeability preservation, it is reasonable to assume that the classified sediments would become petrophysical rock types and depositional rock types when consolidated, following their primary grain size, sorting and grain type distribution. Therefore, the term ‘rock type’ is retained here for ease of terminology but, for clarity, these are sediment samples. The discrete samples were interpolated into continuous surfaces describing the distribution of depositional rock types, petrophysical rock types and mineralogy, and spatial correspondence between those surfaces was statistically evaluated. In order to link these parameters with environment of deposition, their correlation with water depth (as audited from airborne light detection and ranging) and ecological habitat (mapped from DigitalGlobe satellite imagery) was also assessed. The data reveal that spatial distributions of sedimentary faunal, petrographic and mineralogical properties do not show exactly congruent patterns. Other meaningful trends do exist, however. For example, the occurrence of certain depositional rock types is indicative of particular petrophysical rock types, and vice versa. Further, connections between petrophysical rock types and mineralogy are emphasized and offer insight as to how the evolution of matrix porosity might be predicted via diagenetic models tuned to specific sediment textures. Useful relationships are also identified between the occurrence of petrophysical rock types and depositional rock types, and both ecological habitat and water depth. The potential of such dualities is two‐fold. Firstly, they can be applied to more realistically distribute petrophysical rock types and depositional rock types by environment of deposition in reservoir models and, secondly, the use of modern carbonate systems as subsurface analogues might be enhanced.     

Petrology, magnetostratigraphy and geochronology of the Miocene volcaniclastic Tepoztlán Formation: implications for the initiation of the Transmexican Volcanic Belt (Central Mexico)

The volcaniclastic Tepoztlán Formation (TF) represents an important rock record to unravel the early evolution of the Transmexican Volcanic Belt (TMVB). Here, a depositional model together with a chronostratigraphy of this Formation is presented, based on detailed field observations together with new geochronological, paleomagnetic, and petrological data. The TF consists predominantly of deposits from pyroclastic density currents and extensive epiclastic products such as tuffaceous sandstones, conglomerates and breccias, originating from fluvial and mass flow processes, respectively. Within these sediments fall deposits and lavas are sparsely intercalated. The clastic material is almost exclusively of volcanic origin, ranging in composition from andesite to rhyolite. Thick gravity-driven deposits and large-scale alluvial fan environments document the buildup of steep volcanic edifices. K-Ar and Ar-Ar dates, in addition to eight magnetostratigraphic sections and lithological correlations served to construct a chronostratigraphy for the entire Tepoztlán Formation. Correlation of the 577 m composite magnetostratigraphic section with the Cande and Kent (1995) Geomagnetic Polarity Time Scale (GPTS) suggests that this section represents the time intervall 22.8–18.8 Ma (6Bn.1n-5Er; Aquitanian-Burdigalian, Lower Miocene). This correlation implies a deposition of the TF predating the extensive effusive activity in the TMVB at 12 Ma and is therefore interpreted to represent its initial phase with predominantly explosive activity. Additionally, three subdivisions of the TF were established, according to the dominant mode of deposition: (1) the fluvial dominated Malinalco Member (22.8–22.2 Ma), (2) the volcanic dominated San Andrés Member (22.2–21.3 Ma) and (3) the mass flow dominated Tepozteco Member (21.3–18.8 Ma).     

Computation and Analysis of the Global Distribution of the Radioxenon Isotope 133Xe based on Emissions from Nuclear Power Plants and Radioisotope Production Facilities and its Relevance for the Verification of the Nuclear-Test-Ban Treaty

Monitoring of radioactive noble gases, in particular xenon isotopes, is a crucial element of the verification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). The capability of the noble gas network, which is currently under construction, to detect signals from a nuclear explosion critically depends on the background created by other sources. Therefore, the global distribution of these isotopes based on emissions and transport patterns needs to be understood. A significant xenon background exists in the reactor regions of North America, Europe and Asia. An emission inventory of the four relevant xenon isotopes has recently been created, which specifies source terms for each power plant. As the major emitters of xenon isotopes worldwide, a few medical radioisotope production facilities have been recently identified, in particular the facilities in Chalk River (Canada), Fleurus (Belgium), Pelindaba (South Africa) and Petten (Netherlands). Emissions from these sites are expected to exceed those of the other sources by orders of magnitude. In this study, emphasis is put on ¹³³Xe, which is the most prevalent xenon isotope. First, based on the emissions known, the resulting ¹³³Xe concentration levels at all noble gas stations of the final CTBT verification network were calculated and found to be consistent with observations. Second, it turned out that emissions from the radioisotope facilities can explain a number of observed peaks, meaning that atmospheric transport modelling is an important tool for the categorization of measurements. Third, it became evident that Nuclear Power Plant emissions are more difficult to treat in the models, since their temporal variation is high and not generally reported. Fourth, there are indications that the assumed annual emissions may be underestimated by factors of two to ten, while the general emission patterns seem to be well understood. Finally, it became evident that ¹³³Xe sources mainly influence the sensitivity of the monitoring system in the mid-latitudes, where the network coverage is particularly good.     

Reconstructing the development of sampled sites on fluvial island surfaces of the Tagliamento River,Italy, from historical sources

In parallel with research on surface sediment characteristics of evolving island features (patches) along an island‐braided reach of the Tagliamento River, Italy, the present research reconstructed the development of the sampled sites using historical information coupled with field measurements. Since any field sediment sampling programme inevitably focuses on small areas (of the order of a few metres), the historical sources were assessed bearing in mind their spatial resolution and geographical accuracy in relation to the size of the sampled sites, and the information they could provide regarding the historical evolution of those sites. The analysis combined four sources of information: (i) river stage records revealed flood events that had the potential to reset bar surfaces and float in wood and seeds to initiate vegetation colonization; (ii) oblique ground photographs identified areas of the river's active corridor where riparian vegetation colonization appeared to have been initiated by specific flood events; (iii) dendrochronology was used to estimate the age of sampled sites; (iv) field‐measured geographical locations of the sampled sites were combined with historical vertical areal imagery to further establish sampling site age and changes in vegetation cover from 1944 to 2012. A chronosequence of sampled sites (0, 2, 8, 12, 40 years) was established. Vegetation colonization and island development showed a statistically significant development trajectory among sites of each age across the 40 year period following formative floods in 1965–1966, 2000, 2004 and a flow pulse in 2010. The trajectory progressed through pioneer and building island stages until vegetated areas became part of established islands. Evidence from the younger sites indicates that the pioneer island phase lasts up to eight years. Evidence from the oldest site indicates a building island phase lasting a maximum of 30 years, but probably a lot less. Copyright © 2014 John Wiley & Sons, Ltd.     

On the repeatability and consistency of three-component ambient vibration array measurements

Ambient vibration measurements with small, temporary arrays that produce estimates of surface wave dispersion have become increasingly popular as a low-cost, non-invasive tool for site characterisation. An important requirement for these measurements to be meaningful, however, is the temporal consistency and repeatability of the resulting dispersion and spatial autocorrelation curve estimates. Data acquired within several European research projects (NERIES task JRA4, SESAME, and other multinational experiments) offer the chance to investigate the variability of the derived data products. The dataset analysed here consists of repeated array measurements, with several years of time elapsed between them. The measurements were conducted by different groups in different seasons, using different instrumentations and array layouts, at six sites in Greece and Italy. Ambient vibration amplitude spectra and locations of dominant sources vary between the two measurements at each location. Still, analysis indicates that this does not influence the derived dispersion information, which is stable in time and neither influenced by the instrumentation nor the analyst. The frequency range over which the dispersion curves and spatial autocorrelation curves can be reliably estimated depends on the array dimensions (minimum and maximum aperture) used in the specific deployment, though, and may accordingly vary between the repeated experiments. The relative contribution of Rayleigh and Love waves to the wavefield can likewise change between repeated measurements. The observed relative contribution of Rayleigh waves is generally at or below 50%, with especially low values for the rural sites. Besides, the visibility of higher modes depends on the noise wavefield conditions. The similarity of the dispersion and autocorrelation curves measured at each site indicates that the curves are stable, mainly determined by the sub-surface structure, and can thus be used to derive velocity information with depth. Differences between velocity models for the same site derived from independently determined dispersion and autocorrelation curves—as observed in other studies—are consequently not adequately explained by uncertainties in the measurement part.     

Optically stimulated luminescence dating of fluvioglacial (sandur) sediments from north-eastern Germany

This study presents first results of a project which aims to develop an absolute chronology of the Weichselian ice marginal positions in north-eastern Germany using Optically Stimulated Luminescence (OSL) dating. Owing to the special geomorphological and stratigraphical setting, many of the sampling sites were situated close to the former ice margin. At some sections the sediments showed an incomplete resetting of the OSL signal. For this study the representative sandur sediment section from the Beelitz outwash cone was chosen to elaborate a strategy to reliably detect insufficient bleaching and to choose an appropriate age model for the calculation of the true burial dose of each sample. The age determined for the formation of the Beelitz outwash cone is 34.1 ± 3.0 ka. Although this age might still be a maximum age, this study shows that by applying the proposed methodological strategy, plausible ages can be derived from insufficiently bleached, ice proximal sandur sediments.