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981.
B. G. Lottermoser U. Schütz J. Boenecke R. Oberhänsli B. Zolitschka J. F. W. Negendank 《Environmental Geology》1997,31(3-4):236-247
The accumulation of heavy metals and trace elements has been investigated in a well laminated sequence of Holocene and late
Pleistocene lake sediments composed of diatomaceous gyttja, tuff and silt and clay sediments. Varve chronology of the annually
deposited gyttja yielded a continuous high-resolution time sequence and allowed the absolute age dating of the sediment.
Fluxes of elements remained largely uniform from the late Pleistocene into the Holocene (12 867–2 364 VT years ago; VT: varve
time, years before 1950). Higher trace element and heavy metal fluxes occur from 2 322 to 862 VT years ago and reached their
maxima in the uppermost sediments (<845 VT years ago). These increasing element fluxes correlate with increasing inputs of
clastic material. The changing accumulation rates are the result of elevated soil erosion in the lake catchment caused by
human settlement, deforestation and agricultural activities. Thus disturbances of the natural geochemical cycles of the Holzmaar
region have occurred since the beginning of the Iron Age and especially since the beginning of the Middle Ages.
Received: 29 May 1996 · Accepted: 19 August 1996 相似文献
982.
ColomboC.G.Tassinari JoseM.U.Munha WilsonTeixeira TeresaPalacios AllenP.Nutman CesarSosaS. AdjairP.Santos BrunoO.Calado 《《幕》》2004,27(1):3-12
SHRIMP U/Pb-zircon data and Nd mean crustal residence ages indicate that the lmataca Complex developed from an Archean (≥3.2Ga) continental protolith which has undergone considerable isotopic disturbance plus and juvenile accretion during late-Archean (-2.8Ga) times. Transamazonian granulites experienced peak metamorphic conditions of 750-800℃, 6-8kbar with associated transpressive thrusting and tectonic imbrication. Geochronology on zircon, pyroxene and garnet constrains the timing of peak metamorphism at 1.98-2.05Ga. Diffusion modeling of Fe-Mg exchange between biotite inclusions and host garnet yields (near metamorphic peak) cooling rates of 50-100℃/Ma, with petrological cooling rates being generally consistent with cooling rates determined from geochronology. Combining the retrograde P-T path with cooling rates suggests that after the metamorphic peak, large portions of the lmataca Complex were exhumed from 30 to 17km at a rate of 7-2km/Ma.After this, exhumation rates progressively decreased as the rocks approached the surface. Rapid overall upliftlerosion had ceased when the rocks passed below 600-550℃ at 2.01-1.96 Ga ago. Observed variations in mineral cooling ages are interpreted as to reflect episodic differential tectonic exhumation within major fault systems. Inferred (maximum) ages of fault re-activation generally coincide with major continental accretion events in the Amazonian Craton and reflect long-term thermal evolution of the lmataca terrane, as conditioned by variable response to continued continental convergence during the Proterozoic. 相似文献
983.
MUHAMMAD SHARAF GARY ARROY RON PERKINS Spectra-Physics Inc. North First Street San Jose CA U.S.A. 《地理学报(英文版)》1991,(3)
Ratio measurements are commonly used to address a variety of analytical problems in environmental,forensic and pharmaceutical laboratories.In absorbance ratioing techniques,analytical chemists rely onthe spectral features of the analyte(s)of interest.The absorbances at two wavelengths are monitored andthe ratio of these two absorbances is computed.This ratio is then used to confirm the identity of theanalyte(s)of interest,the purity of a product or the overlap of chromatographic peaks.These decisionsoften have far-reaching consequences(e.g.the identification of the source,biogenic or petrogenic,ofhydrocarbons in biological tissues or water).Given the cost and the liabilities associated with suchdecisions,it is unfortunate that these ratios are seldom reported with any statistical confidence.Thepurpose of this study is to delineate the parameters that affect absorbance ratio measurements.Themodels that can be used to estimate the statistical confidence in these measurements are derived andevaluated experimentally.The results show that these models can estimate the relative standard deviationsin absorbance ratios accurately.They can also estimate the effect of signal-to-noise ratio and the choiceof wavelengths on the precision of absorbance ratios. 相似文献
984.
U. B. Mallikarjuna 《Journal of the Geological Society of India》2011,78(3):281-294
985.
U. Grubenmann 《International Journal of Earth Sciences》1912,3(8):576-576
Ohne Zusammenfassung 相似文献
986.
Salt lakes and salt efflorescences are a common phenomenon of many arid zones. Rocks weather rapidly in the presence of saline materials, and the rate of such weathering may be controlled by the mineralogy of the salts concerned. The distribution of the main salt types is presented for the polar deserts of Antarctica and the Arctic, and for the warmer deserts of Australia, North America, South America, Africa and Asia. There is great variety in the types of salts encountered and marked differences between different regions, with, for example, Australia being dominated by sodium chloride (halite) and southern Canada by the sulphates of magnesium and sodium. The possible causes of such variability and its pattern are analysed in terms of the nature of inputs into drainage basins and the various changes that take place within basins. Finally, it is apparent that in those situations when weathering has been observed as an active process there are a great many different salts involved. 相似文献
987.
A study on the mixing proportion in groundwater samples by using Piper diagram and Phreeqc model 总被引:1,自引:0,他引:1
Karmegam U. Chidambaram S. Prasanna M.V. Sasidhar P. Manikandan S. Johnsonbabu G. Dheivanayaki V. Paramaguru P. Manivannan R. Srinivasamoorthy K. Anandhan P 《中国地球化学学报》2011,30(4):490-495
Piper (1944) diagram has been the basis for several important interpretations of the hydrogeochemical data. As seen in this diagram, most natural waters contain relatively few dissolved constituents, with cations (metals or bases) and anions (acid radicles) in chemical equilibrium with one another. Apart from the facies representation, the composition of the mixed sample can be identified in terms of the composition of the parental solution. To bring out this advantage of the Piper diagram, a study was conducted in the Kalpakkam region of Tamilnadu, South India. By taking the geology and water table into consideration, two sample locations were selected as parent solution and third one as the mixture sample. All three samples were analyzed for calcium (Ca), magnesium (Mg), sodium (Na), potassium (K), chloride (Cl), sulphate (SO4) and phosphate (PO4) by Ion Chromatograph (Metrohm IC 861). HCO3 was determined by volumetric titration. The Piper diagram shows that parent solutions clustered towards Na-Mg-Ca-HCO3-Cl and Na-HCO3 facies, and the mixing sample belongs to Na-Mg-HCO3 facies. Phreeqc interac-tive (Ver 2.8) along with the original composition of the mixture sample was used to correlate the mixing proportion identified by the Piper diagram. 相似文献
988.
989.
Periodic changes in Universal Time and polar motion have been derived from a numerical ocean model for a number of small partial tides. Combined with the results reported earlier our model now includes the 34 most important constituents of degree 2 (measured by the amplitude of the tidal potential). The results show that the terms neglected in all existing models amount to 80 μs in UT1 and to 0.3 mas in polar motion. 相似文献
990.
An analysis has been carried of the correlation of the occurrence of type III bursts and flares in spotless regions over the past ten years. As a comparison, the same study has been performed also for flares covering major sunspot umbrae (i.e. in magnetic conditions presumably opposite to the above).The results show a very low correlation of the former flares with type III bursts (8%) and a higher than average value for the latter flares (36% against the normally accepted 25%). Thus an important role of the ambient magnetic field on the generation of type III events seems well established.The effect of the presence of surges and of rapid brightness rises (flash-phases) on the correlation with these bursts has been examined: both features appear to improve the correlation.Some considerations regarding the results of other researches as compared to this one are outlined. 相似文献