The patchy accretion disc in HT Cassiopeiae     

Sonja Vrielmann  Frederic V. Hessman  Keith Horne 《Monthly notices of the Royal Astronomical Society》2002,332(1):176-192

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Physical parameter eclipse mapping of the quiescent disc in V2051 Ophiuchi     

Sonja Vrielmann †  Rae F. Stiening  Warren Offutt 《Monthly notices of the Royal Astronomical Society》2002,334(3):608-620

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Development of a hydrogeological conceptual wetland model in the data-scarce north-eastern region of Kilombero Valley,Tanzania     

Sonja Burghof  Geofrey Gabiri  Christine Stumpp  Romain Chesnaux  Barbara Reichert 《Hydrogeology Journal》2018,26(1):267-284

Understanding groundwater/surface-water interactions in wetlands is crucial because wetlands provide not only a high potential for agricultural production, but also sensitive and valuable ecosystems. This is especially true for the Kilombero floodplain wetland in Tanzania, which represents a data-scarce region in terms of hydrological and hydrogeological data. A comprehensive approach combining hydrogeological with tracer-based assessments was conducted, in order to develop a conceptual hydrogeological wetland model of the area around the city of Ifakara in the north-eastern region of Kilombero catchment. Within the study site, a heterogeneous porous aquifer, with a range of hydraulic conductivities, is underlain by a fractured-rock aquifer. Groundwater chemistry is mainly influenced by silicate weathering and depends on groundwater residence times related to the hydraulic conductivities of the porous aquifer. Groundwater flows from the hillside to the river during most of the year. While floodwater close to the river is mainly derived from overbank flow of the river, floodwater at a greater distance from the river mainly originates from precipitation and groundwater discharge. Evaporation effects in floodwater increase with increasing distance from the river. In general, the contribution of flood and stream water to groundwater recharge is negligible. In terms of an intensification of agricultural activities in the wetland, several conclusions can be drawn from the conceptual model. Results of this study are valuable as a base for further research related to groundwater/surface-water interactions and the conceptual model can be used in the future to set up numerical flow and transport models.  相似文献   

Proposal for applying a component-based mixture approach for ecotoxicological assessment of fracturing fluids   总被引：2，自引：2，他引：0  

Janet Riedl  Stefanie Rotter  Sonja Faetsch  Mechthild Schmitt-Jansen  Rolf Altenburger 《Environmental Earth Sciences》2013,70(8):3907-3920

Hydraulic fracturing is increasingly being used to produce gas from unconventional resource sites for energy supply. Therefore, concerns about risks of this technology related to human health and the environment have to be addressed. Among the major issues is the potential contamination of surrounding water systems by chemical additives used in fracturing fluids. In this study, the ecotoxicological hazards of fracturing fluids, both, their individual components (chemicals) as well as their mixtures (product) were assessed using a component-based mixture approach. For five exemplary fracturing fluids, 40–90 wt% of the contained substances could unambiguously be defined in their chemical identity. The concentrations used in the applied fluid mixture were considered as (maximum) exposure concentrations. For components with mass fractions between 10 and 74 wt%, the effect concentrations for acute and chronic toxicity of fish, daphnia and algae were retrieved from experimental databases and through predictive modeling. The hazard indices calculated from the ratio of exposure to effect concentration were >1 for all fracturing fluids, using different scenarios. This indicated a hazard from the undiluted fracturing fluids. The assessment framework presented in this study allows for dealing with data gaps and uncertainties in a tiered fashion and in particular accommodates for combined effects resulting from chemical mixtures. It might be employed for ecotoxicological risk assessment of products containing chemical mixtures and optimization of their environmental performance.  相似文献   

A chromate-contaminated site in southern Switzerland – Part 1: Site characterization and the use of Cr isotopes to delineate fate and transport     

Christoph Wanner  Urs Eggenberger  Daniel Kurz  Sonja Zink  Urs Mäder 《Applied Geochemistry》2012

The risk of groundwater contamination by chromate at a former chromite ore processing industrial site in Rivera (Switzerland) was assessed by determining subsoil Cr(VI) concentrations and tracking naturally occurring Cr(VI) reduction with Cr isotopes. Using a hot alkaline extraction procedure, a total Cr(VI) contamination of several 1000 kg was estimated. Jarosite, KFe₃((SO₄)_x(CrO₄)_1−x)₂(OH)₆, and chromatite (CaCrO₄) were identified as Cr(VI) bearing mineral phases using XRD, both limiting groundwater Cr(VI) concentrations. To track assumed Cr(VI) reduction at field scale δ⁵³Cr values of contaminated subsoil samples in addition to groundwater δ⁵³Cr data are used for the first time. The measurements showed a fractionation of groundwater δ⁵³Cr values towards positive values and subsoil δ⁵³Cr towards negative values confirming reduction of soluble Cr(VI) to insoluble Cr(III). Using a Rayleigh fractionation model, a current Cr(VI) reduction efficiency of approximately 31% along a 120 m long flow path was estimated at an average linear groundwater velocity of 3.3 m/d. Groundwater and subsoil δ⁵³Cr values were compared with a site specific Rayleigh fractionation model proposing that subsoil δ⁵³Cr values can possibly be used to track previous higher Cr(VI) reduction efficiency during the period of industrial activity. The findings strongly favor monitored natural attenuation to be part of the required site remediation measures.  相似文献   

Decoupled crystallization and eruption histories of the rhyolite magmatic system at Tarawera volcano revealed by zircon ages and growth rates   总被引：2，自引：2，他引：0  

Sonja?StormEmail author  Phil?Shane  Axel?K.?Schmitt  Jan?M.?Lindsay 《Contributions to Mineralogy and Petrology》2012,163(3):505-519

We obtained U–Th disequilibrium age data on zircons from each of the four rhyolite eruptions that built Tarawera volcano in the last 22 ka within the Okataina Volcanic Center (OVC), caldera, New Zealand. Secondary ion mass spectrometry analyses on unpolished euhedral crystal faces that lack resorption features show that crystal growth variously terminated from near-eruption age to ~100 ka prior to eruption. Age-depth profiling of crystals reveals long periods of continuous (~34 ka) and discontinuous growth (~90 ka). Growth hiatuses of up to ~40 ka duration occur, but do not all relate to obvious resorption surfaces. Age differences up to similar magnitude are found on opposing faces of some crystals suggesting episodes of partial exposure to melts. These features are best explained by periodic, complete, or partial, sub-solidus storage and/or inclusion in larger crystal phases, followed by rapid liberation prior to eruption. This is supported by high abundances of U and Th (~500 − >2,000 ppm) in some zircons consistent with periods of high crystallinity (>70%) in the magmatic system, based on crystal/melt partitioning. Contemporaneous but contrasting rim-ward trends of these elements within crystals, even in the same lava hand sample, require synchronous growth in separate melt bodies and little connectivity within the system, but also significant crystal transport and mixing prior to eruption. Many crystals record continuity of growth through the preceding ~60 ka OVC caldera-collapse and subsequent eruptions from Tarawera. This demonstrates a decoupling between eruption triggers, such as shallow crustal extension and mafic intrusion, and the crystallization state of the OVC silicic magmatic system. The data highlights the need to distinguish between the time for accumulation of eruptible magma and the long-term magma residence time based on the age of crystals with high closure temperatures, when assessing the potential for catastrophic eruptions.  相似文献   

Paleolimnological evidence of mining and demographic impacts on Lac Dauriat, Schefferville (subarctic Québec, Canada)     

Laurence Laperrière  Marie-Andrée Fallu  Sonja Hausmann  Reinhard Pienitz  Derek Muir 《Journal of Paleolimnology》2008,40(1):309-324

Qualitative and quantitative analysis of fossil diatoms and geochemical signals preserved in the sediments of Lac Dauriat (subarctic Quebec) were performed to evaluate the impacts of nearby mining activity and the expansion of the town of Schefferville on the water quality of the lake, and to reconstruct the changes of its trophic status. The presence of taxa typical of nutrient-enriched environments (e.g., Cyclostephanos invisitatus, Nitzschia gracilis, Nitzschia perminuta) and the low percentages of chrysophytes were indicative of the advanced state of eutrophication of the lake during the peak of mining activity, and were evidence of the negative impacts of municipal waste on the water quality of Lac Dauriat. Sedimentary analysis of metals, notably lead, mercury, cadmium, bismuth, cobalt, copper and zinc, showed maximum concentrations between 1940 and 1960 with mining era to pre-development enrichment factors ranging from 4.5 to 7.9. The changes seen in recent sediments reflected 3 distinct stages in the recent history of this ecosystem: (a) the non-perturbed, pre-mining (1882–1939), (b) the perturbed, mining period (1939–1977) with accelerated eutrophication, and (c) the post-mining period (1977–1999) with indications of natural recovery of the system after the installation of a water treatment plant in 1975, the closing of the mine in 1983, and the subsequent exodus of the town’s population. Despite the trajectory towards a return to the lake’s natural conditions, water resource managers and (paleo-)limnologists should be alarmed that the impacts of past human disturbance are still in evidence more than 20 years after the closure of the mines, and that Lac Dauriat has yet to reach its natural state of the period preceding extreme anthropogenic impact.  相似文献   

Winter climate change at different temporal scales in Vaccinium myrtillus, an Arctic and alpine dwarf shrub     

Christian Rixen  Christoph Schwoerer  & Sonja Wipf 《Polar research》2010,29(1):85-94

Snow cover strongly influences plant growth in Arctic and alpine ecosystems. Snow characteristics and snowmelt timing are likely to change in a warmer climate. We studied year rings and shoot growth of the dwarf shrub bilberry ( Vaccinium myrtillus ), and species abundances of the vegetation, in response to early or late snowmelt at a study site in the Central Alps, near Davos, Switzerland. Snowmelt was manipulated on experimental plots for 3 and 30 years. Additional plots were set up along a natural snowmelt gradient, and at high and low elevation. Growth ring data showed an increasing trend in annual growth increment over the last 20 years, especially in the extraordinarily hot summer of 2003. Comparing high and low elevation sites, growth rings were wider at low elevation, but only in cold years. In years with relatively cold summers, however, xylem ring width was greater in plots with late rather than early snowmelt along the natural snowmelt gradient, possibly indicating drought stress in early snowmelt plots. Snow cover had a strong influence on species abundances along the natural snowmelt gradient, and change (not yet significant) was beginning to be seen in plots with 30 years of snow manipulation. Our results indicate that beneficial effects of early snowmelt for shrub growth may be offset in cold summers. Although early snowmelt prolongs the growing season, harsh conditions and frost events early in the growing season may become more likely, and hamper plant growth, and this could affect plant growth in all Arctic and alpine snow-dominated ecosystems.  相似文献   

New insights into Late Pleistocene glacial and postglacial history of northernmost Ungava (Canada) from Pingualuit Crater Lake sediments     

Hervé Guyard  Guillaume St-Onge  Reinhard Pienitz  Pierre Francus  Bernd Zolitschka  Garry K.C. Clarke  Sonja Hausmann  Veli-Pekka Salonen  Patrick Lajeunesse  Grégoire Ledoux  Michel Lamothe 《Quaternary Science Reviews》2011,30(27-28):3892-3907

The Pingualuit Crater was formed by a meteoritic impact ca. 1.4 million years ago in northernmost Ungava (Canada). Due to its geographical position near the center of successive North American ice sheets and its favorable morphometry, the Pingualuit Crater Lake (water depth = 246 m) promises to yield a unique continuous sedimentary sequence covering several glacial/interglacial cycles in the terrestrial Canadian Arctic. In this paper, we suggest the existence of a subglacial lake at least during the Last Glacial Maximum (LGM) by hydraulic potential modeling using LGM ice-surface elevation and bed topography derived from a digital elevation model. These results support the hypothesis that the bottom sediments of the Crater Lake escaped glacial erosion and may contain a long-term continental sedimentary sequence. We also present the stratigraphy of a 9 m-long core retrieved from the deep basin of the lake as well as a multiproxy reconstruction of its deglacial and postglacial history. The base of the core is formed by very dense diamicton reflecting basal melt-out environments marking the end of subglacial conditions at the coring site. The overlying finely laminated silt are related to the onset of proglacial conditions characterized by extremely low lacustrine productivity. Infra Red Stimulated Luminescence and AMS ¹⁴C dating, as well as biostratigraphic data indicate sediment mixing between recent (e.g. Holocene) and much older (pre- to mid-Wisconsinan) material reworked by glacier activity. This process prevents the precise dating of these sediments that we interpret as being deposited just before the final deglaciation of the lake. Two finer grained and organic-rich intervals reflect the inception of lacustrine productivity resulting from the cessation of glacial meltwater inputs and ice-free periods. The lower organic interval corresponds to the early postglacial period (6850–5750 cal BP) and marks the transition between proglacial and postglacial conditions during the Holocene Thermal Maximum, while the uppermost organic-rich core section represents late Holocene sediments (～4200–600 cal BP). The organic intervals are separated by a basin-scale erosive slide occurring around 4200 cal BP and likely related to 1) a seismic event due to the glacio-isostatic rebound following the last deglaciation or 2) slope instabilities associated with rapid discharge events of the lake.  相似文献   

No significant ice-sheet expansion beyond present ice margins during the past 4500 yr at Rauer Group, East Antarctica     

Sonja Berg  Bernd Wagner  Martin Melles 《Quaternary Research》2010,74(1):23-25

The history of glacial advances and retreats of the East Antarctic ice sheet during the Holocene is not well-known, due to limited field evidence in both the marine and terrestrial realm. A 257-cm-long sediment core was recovered from a marine inlet in the Rauer Group, East Antarctica, 1.8 km in front of the present ice-sheet margin. Radiocarbon dating and lithological characteristics reveal that the core comprises a complete marine record since 4500 yr. A significant ice-sheet expansion beyond present ice margins therefore did not occur during this period.  相似文献