Regional coda Q in Costa Rica, Central America     

Victor Gonzalez  Leif Persson 《Journal of Seismology》1997,1(3):269-287

Regional seismic apparent attenuation was estimated for Costa Rica, Central America, by using a time domain single scattering model of the shear wave coda decay of local earthquakes. The sensitivity of coda Q (Q_c) measurements with respect to geological differences in the crust is demonstrated in eight sub-regions with a large variety of tectonic and geologic properties. The Q_c estimations were performed for 96 selected local earthquakes recorded at 13 sites during a period of three months. In order to model the scattering as a weak process and to avoid short distance nonlinear effects, we made use of the S-wave coda data only from events within a hypocentral distance of 12 to 106 km with a lapse time between 9 and 53 s. Seismograms were also divided into groups with three different focal depths d, namely d<21 km, 21 kmc values are frequency dependent in the range 1–9 Hz, and are approximated by a least-squares fit to the power law Q_c(f) = Q₀(f/f₀)ⁿ. The estimated parameters of the power-law dependence of Q_c for the whole region, including all depths and possible wave paths, are Q₀ = 91 (± 8.4) and n = 0.72 (±0.071). Differences in the parameter of Q_c for different depths intervals are small, ranging from Q₀ = 90 (±0.7) and n = 0.70 (±0.006) for the uppermost group, with focal depths less than 21 km, to Q₀ = 97 (±0.7) and n = 0.79 (±0.005) for the deepest group with focal depths larger than 43 km. The regional differences in Q_c for the eight sub-regions are significantly larger when compared with the differences between the three focal depth groups. An attempt is made to interpret the variation of Q_c in terms of spatial variations in the geologic and tectonic properties of the crust. Other authors have found that the frequency exponent n might be larger in active tectonic areas and smaller in more stable regions. In the northern region of the Pacific coast we obtain a value of n = 0.52 (±0.011), which might indicate a lower level of tectonic activity when compared with n = 0.85 (±0.015) and 0.83 (±0.031), respectively, for the central and southern sub-regions along the Pacific coast. The latter two sub-regions are located closer to the active area near the Cocos ridge. We obtain the frequency exponent n = 0.72 (±0.052) along a major shear zone in central Costa Rica characterized by high volcanic activity and large geologic complexity. Values of n along the Panamean border are 0.62 (±0.029) in the north and 0.86 (±0.009) and 0.83 (±0.031) in two regions adjacent to the subduction zone and the Cocos Ridge, respectively.  相似文献   

A 25-year Survey of Geoecological Change in the Scandes Mountains of Sweden     

Leif Kullman 《Geografiska Annaler: Series A, Physical Geography》1997,79(3):139-165

The paper reviews various kinds of geoecological change in the tree-limit ecotone of the Scandes Mountains during the period 1970–95. The focus of the study is a part of a regional network of sites intended for long-term tree-limit monitoring, with special stress on effects of climatic variability. The elevational tree-limits of Betula pubescens sp. Sortuosa, Picea abies and Pinus sylvestris, which rose in response to the climatic amelioration earlier this century, now show clear symptoms of increasing climatic stress and disturbance. This manifests as defoliation, growth recession and reproductive failure, locally leading to some initial elevational tree-limit retraction (unbalanced mortality). Defoliation was preceded by decades of weak summer cooling and an increasingly maritime climate, but recently it correlates significantly with low winter soil temperatures, causing death of needles, shoots and buds. In some habitats, Betula pubescens has suffered from mechanical stress and disturbance by increased snow accumulation. Tree-limit decline is paralleled by analogous responses of high-elevation boreal forests as well as the ground cover, encompassing elevational range-limit retraction of certain plant species, deterioration of alpine/subalpine dwarf-shrub heaths and terricolous lichen mats. These processes coincide with indications of enhanced periglacial activity, chiefly wind deflation of frost-heaved top-soils at exposed sites. Presumably, reindeer trampling and grazing play a certain role in the latter context, although this disturbance interacts with climate cooling and increased storminess. Short-term extreme events, particularly concerning winter climate (e.g. ground frost), represent previously underrated disturbance mechanisms in cold-stressed, high-altitude boreal forest. The results suggest mechanisms of tree-layer regression, which lag behind the most severe stresses and disturbances by decades and make cold-marginal trees increasingly sensitive to climatic extremes and, in addition, unable to respond progressively to later positive weather anomalies, due to major defoliation and hypothetical xylem cavitation. The recorded changes are logical in consequence of the irregular climatic cooling and a more maritime climate since the late 1930s. In a wider perspective, the results fit a current pattern of natural geoecological destabilization and rapid vegetation change in the North Atlantic region. In addition, the results are discussed in the perspective of global climate change and biogeographical records over the past few decades.  相似文献   

Survival of crucian carp,Carassius carassius,exposed to a high low-molecular weight inorganic aluminium challenge     

Antonio B. S. Poléo  Sigurd A. Øxnevad  Kjartan Østbye  Ronny A. Andersen  Deborah H. Oughton  Leif A. Vøllestad 《Aquatic Sciences - Research Across Boundaries》1995,57(4):350-359

Crucian carp were exposed to three combinations of pH and inorganic aluminium concentrations for 25 days. Mortality, plasma chloride and haematocrit, and gill aluminium content were investigated. No mortality was observed in any of the three exposures. Crucian carp survived acidic Al-rich water (pH 5.17) with a high concentration of low-molecular weight inorganic Al species (242 µgl^–1) for 25 days. These fish had lowered plasma chloride levels and high amounts of aluminium on their gills compared to fish exposed to acidic Al-poor water (pH 5.16 and 12 µgl^–1 Al_i) and control water (pH 6.29 and 4 µgl^–1 Al_i). Haematocrit was the same in all three exposures. Because aluminium was not acutely toxic to the anoxia tolerant crucian carp, the present results give support to the hypothesis that Al polymerization with subsequent hypoxia is of high importance for the mechanism of acute Al toxicity to fish.  相似文献   

Late Quaternary sedimentation and palaeohydrology in the Acre foreland basin, SW Amazonia     

Kronberg  Fralick  & Benchimol 《Basin Research》1998,10(3):311-323

Our study explores the geohydraulic history of the Acre retroarc foreland basin by gathering both spatial and temporal information from the upper 400 m of sediments. We also inquire into controls on sediment accommodation space as well as on stream vs. lacustrine domination. The Acre basin is located in south-west Amazonia, proximal to the Serra do Divisor which demarcates the eastern edge of the Andean fold–thrust belt. Radiocarbon ages from a range of materials indicate that the upper 50–250 m of the Solimôes Formation accumulated during the past 50 000 years. Both surficial and drill-core sediment records show lacustrine–fluvial transitions throughout the Late Quaternary. These shifts in depositional environments are in response to episodic changes in hydrological conditions as well as to geodynamic activity, such as subsidence. Juxtaposition of lacustrine and fluvial systems in the vertical Acre basin record mimics the regional-scale trends in the modern, upper and middle Solimôes–Amazon floodplains. In the Acre basin record lacustrine successions are characterized by increasing calcium contents up-section. This is also manifested, in the upper portions of lacustrine sequences outcropping at the surface, as alternating clastic and calcareous layers. The up-section increase in carbonate content is related to increasing salinities brought about by drier hydrodynamic conditions. Desiccation cracks are typically infilled with gypsum as are cavities of fossils in bone-beds. The latter represent isolated ponds in which the original fauna died as aridity intensified and waters became increasingly saline. Modern trunk river systems in the Acre basin flow from south-west to north-east with tributaries entering from the south-west, suggesting the influence of a domino-style, basement, fault regime. Fault or, at least, fracture control on stream channels is also suggested throughout the greater Amazon basin in the orthogonal dispositions and asymmetric terrace systems of trunk rivers as well as of major tributaries.  相似文献   

A sound-probe for real-time checking of physical bottom characteristics     

Leif Lundgren  Sven Blomqvist 《Aquatic Sciences - Research Across Boundaries》1998,60(1):28-32

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Redox Processes in Groundwater Impacted by Landfill Leachate   总被引：6，自引：0，他引：6  

Leif Basberg  David Banks  Ola M. Sæther 《Aquatic Geochemistry》1998,4(2):253-272

Groundwater downgradient from Trandum municipallandfill was investigated in summer 1996, with a viewto assessing the distribution of redox-sensitivespecies. The water table at Trandum is 10–26 metersbelow the surface. Monitoring well filters have beeninstalled at depths of up to 15 m below water table,covering an area of 400 m by 200 m. Groundwaterinfluenced by the landfill seems to be confined tothis area. Pristine groundwater has a chemicalsignature indicative of calcite and pyrite weathering.Groundwater influenced by leachate from the landfillexhibits an excess of alkalinity relative to calcium,which is likely to be derived from degradation oforganic matter in the landfill. Groundwaterimmediately below and downstream of the landfillcontains elevated concentrations of Fe and Mn, largelymobilized under reducing conditions from the aquifermatrix and reflected in depleted HNO₃-extractable Feand Mn in sediment samples from the same area.Groundwater samples allow the tentative identificationof redox zones based on oxidized and reduced forms ofFe, Mn, N and S. A methanogenic zone is notobserved.  相似文献   

Pluto: New photometry and a determination of the axis of rotation     

Leif E. Andersson  John D. Fix 《Icarus》1973,20(3):279-283

New photoelectric observations of Pluto, made in 1971, 1972, and 1973, show that the amplitude of the 6-day rotational light variation is larger (about 0.22 mag) and the mean magnitude is fainter $\text{(}\text{V}{}_0\text{= 15.1)}$ than during earlier periods of observation. The change is interpreted as due to a large obliquity of the rotational axis (probably larger than 50°). The phase coefficient of Pluto is about 0.05 mag/deg at 1° solar phase angle.  相似文献   

Photochemical production of ammonium and transformation of dissolved organic matter in the Baltic Sea   总被引：13，自引：1，他引：13  

Colin A. Stedmon  Stiig Markager  Lars Tranvik  Leif Kronberg  Tove Sltis  Winnie Martinsen 《Marine Chemistry》2007,104(3-4):227-240

The release of ammonium from the photochemical degradation of dissolved organic matter (DOM) has been proposed by earlier studies as a potentially important remineralisation pathway for refractory organic nitrogen. In this study the photochemical production of ammonium from Baltic Sea DOM was assessed in the laboratory. Filtered samples from the Bothnian Bay, the Gulf of Finland and the Arkona Sea were exposed to UVA light at environmentally relevant levels, and the developments in ammonium concentrations, light absorption, fluorescence and molecular size distribution were followed. The exposures resulted in a decrease in DOM absorption and loss of the larger sized fraction of DOM. Analysis of the fluorescence properties of DOM using parallel factor analysis (PARAFAC) identified 6 independent components. Five components decreased in intensity as a result of the UVA exposures. One component was produced as a result of the exposures and represents labile photoproducts derived from terrestrial DOM. The characteristics of DOM in samples from the Bothnian Bay and Gulf of Finland were similar and dominated by terrestrially derived material. The DOM from the Arkona Sea was more autochthonous in character. Photoammonification differed depending on the composition of DOM. Calculated photoammonification rates in surface waters varied between 121 and 382 μmol NH₄⁺ L^− 1 d^− 1. Estimated areal daily production rates ranged between 37 and 237 μmol NH₄⁺ m^− 2 d^− 1, which are comparable to atmospheric deposition rates and suggest that photochemical remineralisation of organic nitrogen may be a significant source of bioavailable nitrogen to surface waters during summer months with high irradiance and low inorganic nitrogen concentrations.  相似文献   

Optimal management of European eel in the Imsa River, Norway     

Vollestad  Leif Asbjorn 《ICES Journal of Marine Science》1990,46(2):115-120

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Herring at the Arctic front: influence of temperature and prey on their spatio-temporal distribution and migration     

Leif Nøttestad  Ole Arve Misund  Webjørn Melle  Bente K. Hoddevik Ulvestad  & Kjell Arild Orvik 《Marine Ecology》2007,28(S1):123-133

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