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71.
Coupling between landslides and eroding stream channels reconstructed from spruce tree rings (examples from the Carpathians and Sudetes – Central Europe) 下载免费PDF全文
Małgorzata Wistuba Ireneusz Malik Krzysztof Wójcicki Patrycja Michałowicz 《地球表面变化过程与地形》2015,40(3):293-312
The analysis of the positive feedback between landslides and erosion requires determination of the precise temporal and spatial relations between events of colluvium delivery and fluvial erosion. In our study we use decennial datasets on the occurrence of landsliding and erosion achieved through dendrochronological methods. Four sites covering areas of landslide slopes and adjacent valley floors with stream channels were studied. Landsliding on slopes was dated from the tree‐ring eccentricity developed in stems tilted due to bedrock instability. Erosion in channels was dated using the wood anatomy of roots exposed by erosion of the soil cover. Analysis of the temporal relations between dated landsliding, erosion and precipitation record has revealed that two types of repeating sequences can be observed: (1) rainfall → landsliding → erosion; (2) rainfall → erosion → landsliding. These sequences are an indication of the occurrence of slope‐channel positive feedback in the sites studied. In the first type, landsliding triggered by rainfall delivers colluvia into the valley floor and causes its narrowing, which in turn causes increased erosion. In the second type erosion triggered by rainfall disturbs the slope equilibrium and causes landsliding. Landsliding and erosion, once triggered by precipitation, can occur alternately in years with average precipitation and reinforce one another. Bidirectional coupling between landsliding and channel erosion was shown notably through the effects of channel shifting and forced sinuosity and by increased erosion of the slopes opposite the active landslides. Observations also suggest that the repetition of sequences described over longer periods of time can lead to a general widening of the valley floor at the expense of slopes and to a gradual change of the valley cross‐profile from narrow, V‐shaped into a wide flat‐bottomed. Thus landsliding–erosion coupling/positive feedback was recognized as an important factor shaping hillslope–valley topography of the mid‐mountain areas studied. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
72.
Wojciech Tylmann Kamila Szpakowska Christian Ohlendorf Micha? Woszczyk Bernd Zolitschka 《Journal of Paleolimnology》2012,47(1):55-70
A three-year field study was conducted in Lake Suminko, Poland, to gain an understanding of the limnological variables that
influence the formation and spatial extent of annually laminated sediments in the lake. The water body is divided into three
depth strata, the mixolimnion, chemocline and monimolimnion, each defined by distinct values of temperature, electrical conductivity
and oxygen concentration. Typical for meromictic lakes, the monimolimnion remains perennially anoxic and is rich in dissolved
solids and nutrients. The annual pattern of particle flux in Lake Suminko is closely related to biochemical processes in the
euphotic zone. During winter months we observed very low accumulation rates of non-carbonaceous matter, while during the rest
of the year, three periods of calcite deposition were recorded (April, July–August, and October–November). The periods of
high calcite deposition corresponded with algal blooms and oxygen concentration maxima. Two principal factors contribute to
meromixis in Lake Suminko: (1) the basin is sheltered, preventing wind mixing, and (2) biochemical processes in the lake associated
with high lacustrine productivity. Meromictic conditions must control the formation and preservation of laminated sediments
in the lake because preserved laminations occur only in areas where the lake floor lies below monimolimnetic waters. 相似文献
73.
V. I. Sotnikov H. Ruhl R. Presura T. Cowan J. N. Leboeuf P. Hellinger P. Travnicek 《Astrophysics and Space Science》2005,298(1-2):369-374
The problem of producing collisionless shocks in the laboratory is of great interest for space and astrophysical plasmas.
One approach is based on the idea of combining strong magnetic field (up to 100 Tesla) created during a Z-pinch discharge
with a plasma flow produced in the process of the interaction of a laser pulse with a solid target. In support of laboratory
experiments we present hybrid simulations of the interaction of the plasma flow with frozen in it magnetic field, with the
spherical obstacle. Parameters of the flow correspond to a laser plasma ablation produced in the laboratory during irradiation
of the target by a 3 J laser. Magnetic fields in the plasma flow and around the obstacle are created by the currents produced
by the pulse power ZEBRA voltage generator. With the appropriate set of initial conditions imposed on the flow collisionless
shocks can be created in such a system. Using independent generators for plasma flow and magnetic field allows for the exploration
of a wide range of shock parameters. We present simulations of the formation of supercritical collisionless shock relevant
to the experiment, performed with the 2D version of the hybrid code based on the CAM-CL algorithm [Planet. Space Sci. 51,
649, 2003]. 相似文献
74.
Hybrid event beds dominated by transitional‐flow facies: character,distribution and significance in the Maastrichtian Springar Formation,north‐west Vøring Basin,Norwegian Sea 下载免费PDF全文
Sarah J. Southern Ian A. Kane Michał J. Warchoł Kristin W. Porten William D. McCaffrey 《Sedimentology》2017,64(3):747-776
Hybrid event beds comprising clay‐poor and clay‐rich sandstone are abundant in Maastrichtian‐aged sandstones of the Springar Formation in the north‐west Vøring Basin, Norwegian Sea. This study focuses on an interval, informally referred to as the Lower Sandstone, which has been penetrated in five wells that are distributed along a 140 km downstream transect. Systematic variations in bed style within this stratigraphic interval are used to infer variation in flow behaviour in relatively proximal and distal settings, although individual beds were not correlated. The Lower Sandstone shows an overall reduction in total thickness, bed amalgamation, sand to mud ratio and grain size in distal wells. Turbidites dominated by clay‐poor sandstone are at their most common in relatively proximal wells, whereas hybrid event beds are at their most common in distal wells. Hybrid event beds typically comprise a basal clay‐poor sandstone (non‐stratified or stratified) overlain by banded sandstone, with clay‐rich non‐stratified sandstone at the bed top. The dominant type of clay‐poor sandstone at the base of these beds varies spatially; non‐stratified sandstone is thickest and most common proximally, whereas stratified sandstone becomes dominant in distal wells. Stratified and banded sandstone record progressive deposition of the hybrid event bed. Thus, the facies succession within hybrid event beds records the longitudinal heterogeneity of flow behaviour within the depositional boundary layer; this layer changed from non‐cohesive at the front, through a region of transitional behaviour (fluctuating non‐cohesive and cohesive flow), to cohesive behaviour at the rear. Spatial variation in the dominant type of clay‐poor sandstone at the bed base suggests that the front of the flow remained non‐cohesive, and evolved from high‐concentration and turbulence‐suppressed to increasingly turbulent flow; this is thought to occur in response to deposition and declining sediment fallout. This research may be applicable to other hybrid event bed prone systems, and emphasizes the dynamic nature of hybrid flows. 相似文献
75.
Nancy B. Rybicki Dwilette G. McFarland Henry A. Ruhl Justin T. Reel John W. Barko 《Estuaries and Coasts》2001,24(3):407-424
The establishment of submersed aquatic vegetation (SAV) at unvegetated sites in the freshwater tidal Potomac River was limited primarily by factors other than propagule availability. For two years, traps were used to quantify the amount of plant material reaching three unvegetated sites over the growing season. The calculated flux values provided a gross estimate of the flux of propagules that could potentially survive if other site factors were suitable. The mean flux ofHydrilla verticillata and all other species (≥0.01 gdw m?2 d?1) appeared sufficient to favor the establishment of vegetation, particularly considering the high viability (70–100%) of whole plants and fragments under controlled conditions. However, median water clarity values (i.e., for light attenuation, Secchi depth, total suspended solids, and chlorophylla) were below SAV restoration goals at all unvegetated sites. Additionally, sediments from unvegetated sites showed a potential for nitrogen limitation of the growth ofH. verticillata. Our findings support the hypothesis that in the tidal Potomac River, water clarity and nutrient (especially nitrogen) levels in sediment are key to plant community establishment. 相似文献
76.
A temporal analysis of the number and duration of exceedences of high- and low-flow thresholds was conducted to determine the number of years required to detect a level shift using data from Virginia, North Carolina, and South Carolina. Two methods were used—ordinary least squares assuming a known error variance and generalized least squares without a known error variance. Using ordinary least squares, the mean number of years required to detect a one standard deviation level shift in measures of low-flow variability was 57.2 (28.6 on either side of the break), compared to 40.0 years for measures of high-flow variability. These means become 57.6 and 41.6 when generalized least squares is used. No significant relations between years and elevation or drainage area were detected (P>0.05). Cluster analysis did not suggest geographic patterns in years related to physiography or major hydrologic regions. Referring to the number of observations required to detect a one standard deviation shift as ‘characterizing’ the variability, it appears that at least 20 years of record on either side of a shift may be necessary to adequately characterize high-flow variability. A longer streamflow record (about 30 years on either side) may be required to characterize low-flow variability. 相似文献
77.
Christian Ruhlé 《Aquatic Sciences - Research Across Boundaries》1989,51(4):296-305
Within one decade the yield in arctic char fishery of Lake Walenstadt (Switzerland) has dropped from 1.7 kg · ha–1 to 0.7 kg · ha–1. In the same period growth decreased by about 15%. The fish considered as degenerated dwarfed chars by fishermen are supposed to mature in very young ages.The change of the growth pattern can be related to reoligotrophication of the lake and impoverishments of the food source. Compared to other arctic char populations in Switzerland maturation takes place at least one year later. 相似文献
78.
Micha J.A. Rijkenberg Loes J.A. Gerringa Ilona Velzeboer 《Geochimica et cosmochimica acta》2006,70(11):2790-2805
In laboratory experiments, we investigated the effect of five individual Fe-binding ligands: phaeophytin, ferrichrome, desferrioxamine B (DFOB), inositol hexaphosphate (phytic acid), and protoporphyrin IX (PPIX) on the Fe(II) photoproduction using seawater of the open Southern Ocean. Addition of 10-100 nM Fe(III) to open Southern Ocean seawater without the model ligands and containing; 1.1 nM dissolved Fe(III), 1.75 ± 0.28 equivalents of nM Fe of natural ligands with a conditional stability constant (log K′) of 21.75 ± 0.34 and a concentration DOC of 86.8 ± 1.13 μM C leads to the formation of amorphous Fe(III) hydroxides. These amorphous Fe(III) hydroxides are the major source for the photoproduction of Fe(II). The addition of the model ligands changed the Fe(II) photoproduction considerably and in various ways. Phaeophytin showed higher Fe(II) photoproduction than ferrichrome and the control, i.e., amorphous Fe(III) hydroxides. Additions of phytic acid between 65 and 105 nM increased the concentration of photoproduced Fe(II) with 0.16 nM Fe(II) per nM phytic acid, presumably due to the co-aggregation of Fe(III) and phytic acid leading via an increasing colloidal surface to an increasing photoreducible Fe(III) fraction. DFOB and PPIX strongly decreased the photoproduced Fe(II) concentration. The low Fe(II) photoproduction with DFOB confirmed reported observations that Fe(III) complexed to DFOB is photo-stable. The PPIX hardly binds Fe(III) in the open Southern Ocean seawater but decreased the photoproduced Fe(II) concentration by complexing the Fe(II) with a binding rate constant of kFe(II)PPIX = 1.04 × 10−4 ± 1.53 × 10−5 s−1 nM−1 PPIX. Subsequently, PPIX is suggested to act as a photosensitizing producer of superoxide, thus increasing the dark reduction of Fe(III) to Fe(II). Our research shows that the photochemistry of Fe(III) and the resulting photoproduced Fe(II) concentration is strongly depending on the identity of the Fe-binding organic ligands and that a translation to natural conditions is not possible without further characterization of the natural occurring ligands. 相似文献
79.
Daniela Müller Rik Tjallingii Mateusz Płóciennik Tomi P. Luoto Bartosz Kotrys Birgit Plessen Arne Ramisch Markus J. Schwab Mirosław Błaszkiewicz Michał Słowiński Achim Brauer 《Boreas: An International Journal of Quaternary Research》2021,50(2):535-555
The sediment profile from Lake Gościąż in central Poland comprises a continuous, seasonally resolved and exceptionally well-preserved archive of the Younger Dryas (YD) climate variation. This provides a unique opportunity for detailed investigation of lake system responses during periods of rapid climate cooling (YD onset) and warming (YD termination). The new varve record of Lake Gościąż presented here spans 1662 years from the late Allerød (AL) to the early Preboreal (PB). Microscopic varve counting provides an independent chronology with a YD duration of 1149+14/–22 years, which confirms previous results of 1140±40 years. We link stable oxygen isotopes and chironomid-based air temperature reconstructions with the response of various geochemical and varve microfacies proxies especially focusing on the onset and termination of the YD. Cooling at the YD onset lasted ~180 years, which is about a century longer than the terminal warming that was completed in ~70 years. During the AL/YD transition, environmental proxy data lagged the onset of cooling by ~90 years and revealed an increase of lake productivity and internal lake re-suspension as well as slightly higher detrital sediment input. In contrast, rapid warming and environmental changes during the YD/PB transition occurred simultaneously. However, initial changes such as declining diatom deposition and detrital input occurred already a few centuries before the rapid warming at the YD/PB transition. These environmental changes likely reflect a gradual increase in summer air temperatures already during the YD. Our data indicate complex and differing environmental responses to the major climate changes related to the YD, which involve different proxy sensitivities and threshold processes. 相似文献
80.
Sabiela Musabelliu Michał Zatoń 《Proceedings of the Geologists' Association. Geologists' Association》2018,129(2):227-234
Upper Devonian brachiopods from the Central Devonian Field, Russia have been investigated with respect to encrustation patterns of cornulitids. These Palaeozoic encrusters were very characteristic component of shelly substrates during the Devonian and may serve as valuable objects for palaeoecological studies, especially those concerning the encrusters-host relationships. On the basis of rich material, it has been shown that cornulitid abundance significantly increased in the lower Famennian. Their disparity also increased from one morphotype (robust) present in the upper Frasnian to two distinct morphotypes (robust and slender, presumably representing two different species) in the lower Famennian. The analysis of the spatial occurrence of cornulitids on two dominant lower Famennian brachiopod species (Cyrtospirifer zadonicus and Ripidiorhynchus huotinus) showed, that these encrusters preferred the marginal parts of the shells. This, together with a dominant directional growth of cornulitids toward the commissure, indicates that cornulitids benefited from food brought by feeding currents produced by the brachiopod’s lophophore. The unequivocal evidence for syn vivo association of the cornulitids and their brachiopod hosts is the presence of distinct shell malformations caused by cornulitid growth affecting the host’s shell-secreting epithelium, preserved on some lower Famennian specimens. Interestingly, these malformations were caused by a slender cornulitid morphotype only. Thus, it is evident that during food gathering from the inhalant currents, this cornulitid species exerted a distinct, negative effect on the brachiopod – a deviation of its shell growth. In this case, we may consider this particular cornulitid species as ectoparasite rather than commensal with respect to its host. 相似文献