Baroclinically-driven flows and dynamo action in rotating spherical fluid shells     

Radostin D. Simitev  Friedrich H. Busse 《地球物理与天体物理流体动力学》2017,111(5):369-379

The dynamics of stably stratified stellar radiative zones is of considerable interest due to the availability of increasingly detailed observations of Solar and stellar interiors. This article reports the first non-axisymmetric and time-dependent simulations of flows of anelastic fluids driven by baroclinic torques in stably stratified rotating spherical shells – a system serving as an elemental model of a stellar radiative zone. With increasing baroclinicity a sequence of bifurcations from simpler to more complex flows is found in which some of the available symmetries of the problem are broken subsequently. The poloidal component of the flow grows relative to the dominant toroidal component with increasing baroclinicity. The possibility of magnetic field generation thus arises and this paper proceeds to provide some indications for self-sustained dynamo action in baroclinically-driven flows. We speculate that magnetic fields in stably stratified stellar interiors are thus not necessarily of fossil origin as it is often assumed.  相似文献   

Late Pleistocene outburst floods from Issyk Kul,Kyrgyzstan?          下载免费PDF全文

Swenja Rosenwinkel  Angela Landgraf  Wolfgang Schwanghart  Friedrich Volkmer  Atyrgul Dzhumabaeva  Silke Merchel  Georg Rugel  Frank Preusser  Oliver Korup 《地球表面变化过程与地形》2017,42(10):1535-1548

Elevated shorelines and lake sediments surrounding Issyk Kul, the world's second largest mountain lake, record fluctuating lake levels during Quaternary times. Together with bathymetric and geochemical data, these markers document alternating phases of lake closure and external drainage. The uppermost level of lake sediments requires a former damming of the lake's western outlet through the Boam gorge. We test previous hypothesised ice or landslide dam failures by exploring possible links between late Quaternary lake levels and outbursts. We review and recompile the chronology of reported changes in lake site, and offer new ages of abandoned shorelines using ¹⁴C in bivalve and gastropod shells, and plant detritus, as well as sand lenses in delta and river sediments using Infrared Stimulated Luminescence. Our dates are consistent with elevated lake levels between ~45 ka and 22 ka. Cosmogenic ¹⁰Be and ²⁶Al exposure ages of fan terraces containing erratic boulders (>3 m) downstream of the gorge constrain the timing of floods to 20.5–18.5 ka, postdating a highstand of Issyk Kul. A flow‐competence analysis gives a peak discharge of >10⁴ m³ s^–1 for entraining and transporting these boulders. Palaeoflood modelling, however, shows that naturally dammed lakes unconnected to Issyk Kul could have produced such high discharges upon sudden emptying. Hence, although our data are consistent with hypotheses of catastrophic outburst floods, average lake‐level changes of up to 90 mm yr^–1 in the past 150 years were highly variable without any outbursts, so that linking lake‐level drops to catastrophic dam breaks remains ambiguous using sedimentary archives alone. This constraint may readily apply to other Quaternary lakes of that size elsewhere. Nonetheless, our reconstructed Pleistocene floods are among the largest reported worldwide, and motivate further research into the palaeoflood hydrology of Central Asia. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

Mega-monsoon variability during the late Triassic: Re-assessing the role of orbital forcing in the deposition of playa sediments in the Germanic Basin   总被引：1，自引：0，他引：1  

André Bahr  Gilles Kolber  Stefanie Kaboth-Bahr  Lutz Reinhardt  Oliver Friedrich  Jörg Pross 《Sedimentology》2020,67(2):951-970

The formation of the supercontinent Pangaea during the Permo–Triassic gave rise to an extreme monsoonal climate (often termed ‘mega-monsoon’) that has been documented by numerous palaeo-records. However, considerable debate exists about the role of orbital forcing in causing humid intervals in an otherwise arid climate. To shed new light on the forcing of monsoonal variability in subtropical Pangaea, this study focuses on sediment facies and colour variability of playa and alluvial fan deposits in an outcrop from the late Carnian (ca 225 Ma) in the southern Germanic Basin, south-western Germany. The sediments were deposited against a background of increasingly arid conditions following the humid Carnian Pluvial Event (ca 234 to 232 Ma). The ca 2·4 Myr long sedimentary succession studied shows a tripartite long-term evolution, starting with a distal mud-flat facies deposited under arid conditions. This phase was followed by a highly variable playa-lake environment that documents more humid conditions and finally a regression of the playa-lake due to a return of arid conditions. The red–green (a*) and lightness (L*) records show that this long-term variability was overprinted by alternating wet/dry cycles driven by orbital precession and ca 405 kyr eccentricity, without significant influence of obliquity. The absence of obliquity in this record indicates that high-latitude forcing played only a minor role in the southern Germanic Basin during the late Carnian. This is different from the subsequent Norian when high-latitude signals became more pronounced, potentially related to the northward drift of the Germanic Basin. The recurring pattern of pluvial events during the late Triassic demonstrates that orbital forcing, in particular eccentricity, stimulated the occurrence and intensity of wet phases. It also highlights the possibility that the Carnian Pluvial Event, although most likely triggered by enhanced volcanic activity, may also have been modified by an orbital stimulus.  相似文献   

Sand particle velocities over a subaqueous dune slope using high-frequency image capturing     

Renske C. Terwisscha van Scheltinga  Giovanni Coco  Heide Friedrich 《地球表面变化过程与地形》2019,44(10):1881-1894

This work presents measurements and analysis of sand particle velocities over a subaqueous dune with median sand diameter of 0.85 mm. Time-lapse images of the mobile bed and an automated particle image velocimetry (PIV)-based cross-correlation method are used to obtain mean velocity of sand particles. This technique is shown to be consistent with measurements obtained with manual tracing. The measurements indicate an increase in mean particle velocity over a dune slope. Three regions are distinguished over the dune slope: (1) region of fluctuating particle velocity, (2) region of increasing particle velocity, and (3) region of maximum particle velocity. The observations are aligned with experimental and numerical modelling studies, indicating fluctuations in flow velocity over a dune stoss slope. We furthermore show that the standard deviation of the mean particle velocity is affected by the slope location and decreases from the lower slope towards the upper slope. The particle velocity variability is discussed in the context of general onset and cessation of sediment transport, the effect of the reattachment zone, sweep-transport events, and the existence of superimposed bedforms. With this work we bridge the gap between measurements of bedload transport at the particle-scale and at the bedform-scale. © 2019 John Wiley & Sons, Ltd.  相似文献   

Non-Energy Resources,Connecticut Coastal Waters,Year 9 and 10 Activities     

Ralph S. Lewis  Mary L. Digiacomo-Cohen  Nancy Friedrich Neff  Richard Hyde 《Marine Georesources & Geotechnology》2013,31(2-3)

The Connecticut effort in year 9 of the Continental Margins Program concentrated on a 13.6-mi2 area south of the Housatonic River. This area was chosen for a side-scan sonar survey because it lies on and adjacent to the bathymetric expressions of two fairly large, subcropping, potential sources of coarse material. Previous seismic work in the area indicated that outcrops or subcrops of these potential source deposits could be delineated using their bathymetric expression. Owing to the limited resolution of the seismic data, a correlation between bottom type and underlying source deposits could not be made with the seismics alone. Results from the November 1993 side-scan survey show that although the source deposits have discernible bathymetric expressions, they are not cropping out as much as expected. As a result, bottom type is not necessarily determined by subcropping deposits in this particular area. Year 10 work was concentrated in Fishers Island Sound, where three areas of potential interest for near-shore gravel resources had previously been identified. These areas were surveyed, during the spring of 1996, using the RoxAnn Seabed Classification System. A small video camera and a Van Veen grab sampler were used to calibrate and verify the RoxAnn data. Although previous sampling had indicated the presence of gravel or gravelly sediment in all of the survey areas, the RoxAnn results showed less gravel than anticipated. Vibrant eelgrass beds and other habitat indicators were detected in all of the survey areas. Given the variable sediment results, and the high habitat potential of the areas surveyed, the likelihood of developing a sand and gravel supply from the near shore of Fishers Island Sound appears quite low.  相似文献   

Year-round variability of ambient noise in temperate freshwater habitats and its implications for fishes     

Sonja Amoser  Friedrich Ladich 《Aquatic Sciences - Research Across Boundaries》2010,72(3):371-378

Changes in habitat acoustics over the year can potentially affect fish hearing and orientation to sound, especially in temperate climates. This is the first study where year-round changes in ambient noise in aquatic habitats were assessed. Seven different European freshwater habitats were chosen for this study. Sound pressure level (SPL) and spectral composition of the ambient noise varied in both quiet stagnant habitats (lakes, backwaters) and in flowing habitats (streams, rivers). Linear equivalent SPL (L_{Leq, 60s}) tended to be lower in stagnant habitats (means: 91.6–111.7 dB) than in flowing habitats (means: 111.2–133.4 dB). The changes in SPL were smallest in the river (means: 4.2–4.4 dB, maxima: 8.5–10.1 dB), whereas significantly higher values were measured in stagnant habitats and the stream (means: 9.9–14.9 dB, maxima: 25.1–30.9 dB). The spectral compositions of the ambient noise determined at different times of the year were highly correlated to each other at the river sites (mean cross-correlation coefficients: 0.85 and 0.94) and were weaker or not correlated at the other study sites (means: 0.24–0.76). The changes in ambient noise spectra were negatively correlated to changes in SPL, indicating that large changes in SPLs were accompanied by large changes in spectral composition and vice versa. Comparison of these eco-acoustical data with a preceding study (Amoser and Ladich in J Exp Biol 208:3533–3542, 2005) indicates that the auditory sensitivity in hearing specialists is affected by changes in ambient noise levels and spectra throughout a year and that this effect tends to be more pronounced in stagnant waters and the stream than at river sites. On the other hand, absolute noise levels result in a higher degree of masking in flowing waters.  相似文献   

Comparison of an Electrochemical and Luminescence‐Based Oxygen Measuring System for Use in the Biodegradability Testing According to Closed Bottle Test (OECD 301D)     

Judith Friedrich  Andreas Längin  Klaus Kümmerer 《洁净——土壤、空气、水》2013,41(3):251-257

The effects of persistent organic pollutants on the aquatic environment still can hardly be assessed. Therefore, ready biodegradability is a desired key characteristic of chemicals. Accordingly, biodegradation testing is of utmost importance and is a vital part within the risk assessment for organic chemicals. In the closed bottle test (CBT; OECD 301D) the degree of biological degradation of a chemical is assessed by monitoring the oxygen consumption so far with a Clark electrode as standard method. However, this method is time‐consuming, laborious and led to frequent fluctuations of the test results, which limited their validity. Due to these shortcomings, an optical system that enables contact‐free measuring of the oxygen concentration in a closed bottle, based on dynamic luminescence quenching, was examined. The test results were compared to those obtained with the “classical” electrochemical technique (Clark electrode). At first, a basic standard for the handling of the new instrument in the laboratory was developed. Then possible influencing factors, reliability, comparability, and reproducibility of the test results were investigated by running the CBTs in parallel with the electrode and optode method. The findings proved the new optode method to be unambiguously superior to the electrode technique. The frequency of fluctuations of the test results and time and effort necessary for the test run could considerably be reduced. The degradation kinetics of the test substances could be followed easily in the CBT by increasing the measuring points without much additional effort.  相似文献   

A Raman spectroscopic study on the structural disorder of monazite–(Ce)     

Katja Ruschel  Lutz Nasdala  Andreas Kronz  John M. Hanchar  Daniel M. T?bbens  Radek ?koda  Friedrich Finger  Andreas M?ller 《Mineralogy and Petrology》2012,105(1-2):41-55

This study addresses whether Raman spectra can be used to estimate the degree of accumulated radiation damage in monazite-(Ce) samples whose chemical composition was previously determined. Our results indicate that the degree of disorder in monazite–(Ce), as observed from increasing Raman band broadening, generally depends on both the structural state (i.e., radiation damage) and the chemical composition (i.e., incorporation of non-formula elements). The chemical effects were studied on synthetic orthophosphates grown using the Li-Mo flux method, and non radiation-damaged analogues of the naturally radiation-damaged monazite–(Ce) samples, produced by dry annealing. We found that the “chemical” Raman-band broadening of natural monazite–(Ce) can be predicted by the empirical formula, $$ {\hbox{FWHM}} {\hbox{[c}}{{\hbox{m}}^{ - {1}}}{]} = {3}{.95} + {26}{.66} \times {\hbox{(Th}} + {\hbox{U}} + {\hbox{Ca}} + {\hbox{Pb)}} {\hbox{[apfu]}} $$ where, FWHM = full width at half maximum of the main Raman band of monazite–(Ce) (i.e., the symmetric PO₄ stretching near 970?cm^?1), and (Th+U+Ca+Pb) = sum of the four elements in apfu (atoms per formula unit). Provided the chemical composition of a natural monazite–(Ce) is known, this “chemical band broadening” can be used to estimate the degree of structural radiation damage from the observed FWHM of the ν₁(PO₄) band of that particular sample using Raman spectroscopy. Our annealing studies on a wide range of monazite–(Ce) reference materials and other monazite–(Ce) samples confirmed that this mineral virtually never becomes highly radiation damaged. Potential advantages and the practical use of the proposed method in the Earth sciences are discussed.  相似文献   

Magmato-sedimentary Carboniferous to Jurassic evolution of the western Tauern window,Eastern Alps (constraints from U-Pb zircon dating and geochemistry)     

Veselá Petra  Söllner Frank  Finger Friedrich  Gerdes Axel 《International Journal of Earth Sciences》2011,100(5):993-1027

New geochronological U-Pb (LA-ICP-MS) zircon data and geochemical analyses from the Variscan orthogneisses and metavolcanic rocks in the western Tauern window are presented and used to reconstruct the pre-Alpine evolution of this area. The late- and post-Variscan stage in the Tauern window was characterised by distinct magmatic pulses accompanied by the formation of volcano-sedimentary basins. The magmatic activity started in the Visean (335.4 ± 1.5 Ma) with the intrusion of a K-rich, durbachitic biotite-granite (protolith of the Ahorn gneiss). Following a period of exhumation and erosion, Westfalian–Stefanian volcanics were deposited (Grierkar meta-rhyodacite: 309.8 ± 1.5 Ma; Venntal meta-rhyolite: 304.0 ± 3.0 Ma). A renewed magmatic pulse occurred in the Early Permian, producing large volumes of tonalites and granodiorites (Tux meta-granodiorite: 292.1 ± 1.9 Ma). The youngest magmatism is characterised by pyroclastic and tuffitic deposits (Pfitsch meta-rhyolite: 280.5 ± 2.6 Ma; Schönach valley meta-andesite: 279.0 ± 4.8 Ma). This volcanism was probably related to crustal extensional faulting within an intra-continental graben and horst setting, asthenospheric upwelling and heat flow increase due to the onset of the Permian rifting. The Permo-Triassic peneplanation and subsidence is documented by shallow marine and evaporitic deposits. Probably in the Middle Jurassic times, the area was flooded and in the Late Jurassic the whole area was covered by limestones, representing post-rift sediments on the southern European continental margin.  相似文献   

Long-term plankton studies at the lower Rhine/Germany     

Günther Friedrich  Marlies Pohlmann 《Limnologica》2009,39(1):14-39

The river Rhine has lain under considerable anthropogenic stress of its water quality for 100 years. As early as 1905 the first results of studies of the plankton in the Rhine were published. Due to the long residence time of the water a real potamoplankton can develop and at the end of the Lower Rhine it reaches its highest density. The paper consists of two parts. At first an overview is given about the history of plankton studies in the Rhine. The second part is the presentation of results from a monitoring at the Lower Rhine from 1979 to 2004.First systematic studies started at the beginning of the 20th century at the beginning of pollution. Our studies started during a phase of recreation from extreme pollution and eutrophication. Samples were taken at four stations: Bad Honnef, km 640, entrance to North Rhine-Westphalia, Düsseldorf, km 732, Duisburg, km 792 downstream large industrial effluents and big cities, Kleve-Bimmen, km 865 at the border to the Netherlands.In the 1970s nutrients were high, especially phosphate 0.65 mg PO₄-P L⁻¹ in 1979. After 1980 phosphate dropped to 0.11 mg PO₄-P L⁻¹ in 2004 (mean values of the growing season). Ammonia was reduced from about 0.52 (1979) to 0.02 (2004) mg NH₄-N L⁻¹. Nitrate remained between 3.72 (1989) and 2.26 (2004) mg NO₃-N L⁻¹ at a relatively high level. Oxygen concentrations were very low during the 1960s and 1970s, sometimes only 4 mg L⁻¹ O₂. During our studies the oxygen increased up to 9 mg L⁻¹ O₂ with a tendency to 11 mg L⁻¹ O₂ in the last years. Chlorophyll a was estimated to be between 59 (1979) and 31μg L⁻¹ (1986) with short peaks up to 170 μg L⁻¹ (1989). Since 1992 the mean values have varied between 30 (1993) and 21 μg L⁻¹ (2004).The floristic phytoplankton composition is characterised by the dominance of the centric diatom Stephanodiscus hantzschii. Other diatoms like Skeletonema subsalsum, Skeletonema potamos and Asterionella formosa were regularly present in smaller quantities. The second dominant group was coccale green algae. During the 1980s they formed up to 35% of the biomass. Since the 1990s their contribution to the phytoplankton became much smaller. This change corresponds with the increase of wastewater treatment and the diminution of nutrients. All the other groups of algae were present in minor quantities. During the time of higher trophy in the 1970s and 1980s the phytoplankton formed two peaks, in recent years only one peak has developed, depending on different flow conditions during the growing season and lower trophic state in the upstream parts of the river.Excellent correspondence exists between cell number, biovolume and chlorophyll a content and the results of delayed fluorescence (DF) measurement. The trophic status in the Lower Rhine may be estimated as (moderate) eutrophic. The ecological status of the phytoplankton is good based on the requirements of the European Water Framework Directive (WFD).The zooplankton consists mainly of rotatoria and larvs of Dreissena polymorpha. Grazing on phytoplankton seems to be mainly due to the large quantities of benthic Dreissena and the newly introduced mussel Corbicula.  相似文献