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  • 1
    ISSN: 1432-0495
    Keywords: 210Pb dating ; Lead ; Sediments
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract Two sediment cores (BO90/13b and BO90/17b) from Lake Constance were investigated byγ-spectrometry for210Pb,134Cs,137Cs,241Am,234Th, and other members of the238U decay chain. The sediments were dated using the constant-flux model for210Pb, and accumulation rates were determined. These range from 0.04 to 0.65g/cm 2/yr (BO90/13b) and 0.04 to 0.8g/cm 2/yr (BO90/17b), respectively. The mean accumulation rate amounts to 0.16g/cm 2/yr for both cores. The cores had already been dated by lamination counting and reconstruction of high-water events at the Limnological Institute at Constance, so that a very precise time scale was available. Both ages derived are in agreement within statistical error up to 1900, which means dating with the constant-flux model for210Pb was confirmed up to that age. The position of the maxima of bomb cesium and americium confirm the stratigraphic and210Pb datings. With241Am a further radioactive isotope is available, which can, due to the half-life of241Pu (t 1/2=14.4yr) be detected now byγ-spectrometry and can serve as an additional time indicator, the maximum being dated at 1963. By applying the various time scales, the depth profiles of stable lead and zinc of core BO90/13b were dated. Both heavy metals show a very significant maximum located beneath the layer of the maxima of bomb cesium and americium, showing that these maxima are older than those of the bomb isotopes. It is remarkable in this context that the maximum of zinc concentration occurs a little later than that of stable lead. Similar concentration profiles are observable in core BO90/17b and other, older sediment cores (CS6-CS10) on a transect across the lake. In contrast to a former assumption, the depth profile of stable lead in Lake Constance sediments does not reflect the anthropogenic gasoline lead emissions into the atmosphere for Germany, their maximum being dated at 1971.
    Type of Medium: Electronic Resource
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  • 2
    Publication Date: 2012-07-06
    Description: Ancient Lake Ohrid is a steep-sided, oligotrophic, karst lake that was tectonically formed most likely within the Pliocene and often referred to as a hotspot of endemic biodiversity. This study aims on tracing significant lake level fluctuations at Lake Ohrid using high-resolution acoustic data in combination with lithological, geochemical, and chronological information from two sediment cores recovered from sub-aquatic terrace levels at ca. 32 and 60m water depth. According to our data, significant lake level fluctuations with prominent lowstands of ca. 60 and 35m below the present water level occurred during Marine Isotope Stage (MIS) 6 and MIS 5, respectively. The effect of these lowstands on biodiversity in most coastal parts of the lake is negligible, due to only small changes in lake surface area, coastline, and habitat. In contrast, biodiversity in shallower areas was more severely affected due to disconnection of today sublacustrine springs from the main water body. Multichannel seismic data from deeper parts of the lake clearly image several clinoform structures stacked on top of each other. These stacked clinoforms indicate significantly lower lake levels prior to MIS 6 and a stepwise rise of water level with intermittent stillstands since its existence as water-filled body, which might have caused enhanced expansion of endemic species within Lake Ohrid.
    Type: Article , PeerReviewed
    Format: text
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  • 3
    Publication Date: 2019-05-02
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 4
    Publication Date: 2019-07-17
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 5
    Publication Date: 2019-07-17
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 6
    Publication Date: 2019-07-17
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 7
    Publication Date: 2019-07-17
    Description: The role of lakes in the global methane budget seems to be higher than previously thought. Numerous pockmarks have been described for the marine environment, but at freshwater fluid seeps the geological, chemical and biological processes operating are largely unknown. Based on different observations of intense gas flow through the water column at Lake Constance two joint research cruises were conducted in winter 2005/06 to systematically search and study these structures. Several large fields of pockmark like structures were found in the eastern part of the lake (side scan sonar, echo sounder). Water and gas was selectively sampled and based on these preliminary data a joint DFG-funded research project was arranged. The objectives are to (1) locate, describe and map pockmark areas at Lake Constance, (2) identify the pockmark formation mechanism, (3) quantify the methane flux and the temporal variability and (4) identify the source of methane.So far more than 450 pockmarks larger 〉 2 m have been identified by side-scan sonar. The pockmarks vary largely in size (dm-range to maximum diameters of 15m) and spatial distribution. At some places, they are irregular spaced, but now and then smaller decimetre sized pockmarks are evenly spaced along small lineaments. Often, large pockmarks are located at morphological highs, such as channel rims or little hills at the lake floor. There is no morphological evidence for a catastrophic gas release (solid discharge at the rim, or irregular sediment patterns near the pockmarks). The observed structures point to a constant gas release from a deeper reservoir.Two representative locations (water depth 12m and 85m) close to the former Rhine estuary have been selected for further geochemical examinations. After deploying a digital horizontal sonar system at the lake floor to allow for exact positioning, water and sediment samples were taken within the pockmark with niskin bottles from a rosette and multicorer. In addition, sediment samples and gas samples were taken by divers across the shallow pockmark. Gas concentration and isotope ratios of dissolved gas in water were measured using standard GC-FID and GC-irMS techniques. Methane concentration in sediment samples was measured directly after coring by means of by a diffusion-based methane sensor.Preliminary results of the isotope analysis of methane (δ13C, δD) and CO2 (δ13C) as well as the absence of higher molecular weight alkanes strongly indicate a bacterial formation of the gases rather than a thermogenic origin of the methane. The results of the analysis of free gas of the sediments indicate the methyl formation as dominant pathway. However, up to now available data suggest a certain difference in the gases located at the deep pockmark, the corresponding reference site and gas sampled at the shallow pockmark. In the sediments of the deep pockmark high methane concentrations were recorded, however with no significant differences between the pockmark and control cores. At the shallow pockmark sediment cores could be positioned more precisely. Here methane concentrations vary within meters in order of magnitude. Several autonomous devices for gas flow measurements using different approaches were deployed in March 2008 at selected gas emanating pockmarks. Data are not yet available but will hopefully presented
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 8
    Publication Date: 2019-07-17
    Description: The role of lakes in the global methane budget seems to be higher than previously thought. Numerous pockmarks have been described for the marine environment, but at freshwater fluid seeps the geological, chemical and biological processes operating are largely unknown. In Lake Constance, so far more than 450 pockmarks larger 〉 2m have been identified by side-scan sonar. The pockmarks vary largely in size (dm-range to maximum diameters of 15m) and spatial distribution. Two representative locations (water depth 12m and 85m) close to the former Rhine estuary have been selected for further geochemical examinations. Under-water videos demonstrate continuous release of gas bubbles. Preliminary results of the gas analysis indicate 99% pure methane formed by bacterial acitivtiy rather than a thermogenic origin of the methane. In the sediment the escaping methane gas results in small scale heterogenity of most geochemical parameters. In the water column increased methane concentrations are observed in large areas. The further impact of this massive methane flux from deep reservoir(s) on sediments and water remains to be investigated.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 9
    Publication Date: 2019-07-13
    Description: Active flow control technology is increasingly used in aerospace applications to control flow separation and to improve aerodynamic performance. In this paper, PowerFLOW is used to simulate the flow through a sweeping jet actuator at two different pressure ratios. The lower pressure ratio leads to a high subsonic flow, whereas the high pressure ratio produces a choked flow condition. Comparison of numerical results with experimental data is shown, which includes qualitatively good agreement of pressure histories and spectra. PIV measurements are also available but the simulation overestimates mean and fluctuation quantities outside the actuator. If supply pressure is matched at one point inside the mixing chamber a good qualitative agreement is achieved at all other monitor points.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: NF1676L-21725 , 2016 AIAA SciTech Forum and Exposition; 4-8 Jan. 2016; San Diego, CA; United States
    Format: application/pdf
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  • 10
    Publication Date: 2019-07-16
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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