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541.
Mu Ramkumar Doris Stüben Zsolt Berner Jens Schneider 《Journal of Earth System Science》2010,119(1):1-17
The Ariyalur-Pondicherry sub-basin of the Cauvery basin comprises a near complete stratigraphic record of Upper Cretaceous-Lower
Tertiary periods. Earlier studies have documented variations of clay mineral assemblages, change in microtexture of siliciclasts
and many geochemical and stable isotopic anomalies far below the Cretaceous-Tertiary boundary (KTB) in these strata. This
paper documents the occurrences of two positive 87Sr/86Sr anomalies preceding K-T boundary in this basin and discusses plausible causes. Analysis of trace elemental and stable isotopic
profiles, sedimentation history, petrography and mineralogy of the rocks reveal that while both the anomalies may be due to
increased detrital influx caused by sea level and climatic changes, the second anomaly might have been influenced by Deccan
volcanism which in turn predated KTB. Record of such anomalies preceding K-T boundary supports the view of multi-causal step-wise
extinction of biota across KTB. 相似文献
542.
Jens M. Skei 《Marine pollution bulletin》1978,9(7):191-193
Thirteen surface sediment samples from a shallow, semi-enclosed bay, Gunnekleivfjorden, South-West Norway, were analysed for total mercury. The concentrations ranged between 90 and 350 ppm dried sediment. The median value (250 ppm mercury) is more than twice the median value of sixteen sediment samples collected in Minamata Bay in 1963. The surface area of Gunnekleivfjorden is approximately 1 km2, implying that minimum 10 tons of mercury is accumulated in the upper 5 cm of the sediments of the bay. The extensive contamination is caused by discharge of inorganic mercury from a chlor-alkali plant during the last 25 years. 相似文献
543.
Avery L. C. Shinneman Danuta M. Bennett Sherilyn C. Fritz Jens Schmieder Daniel R. Engstrom Aris Efting John Holz 《Journal of Paleolimnology》2010,44(2):443-464
The Nebraska Sand Hills are a distinctive eco-region in the semi-arid Great Plains of the western United States. The water
table underlying the Sand Hills is part of the High Plains/Ogallala aquifer, an important water resource for the central Great
Plains. Lake levels are affected directly by fluctuations in the water table, which is recharged primarily by local precipitation
and responds quickly to climatically induced changes in regional water balance. Instrumental records are available for only
50–100 years, and paleolimnological data provide important insights into the extremes and variability in moisture balance
over longer time scales. A set of 69 lakes from across Nebraska was used to establish a statistical relationship between diatom
community composition and water depth. This relationship was then used to develop a diatom-based inference model for water
depth using weighted averaging regression and calibration techniques. Development of the inference model was complicated by
strong intra-seasonal variability in water depth and the linkages between depth and other limnologic characteristics, including
alkalinity, water clarity and nutrient concentrations. Analysis of historical diatom communities from eight lakes allowed
for the reconstruction of lake-level fluctuations over the past several thousand years. Comparisons of the more recent portion
of these reconstructions with the instrumental Palmer Drought Severity Index (PDSI) showed that sediment records may not faithfully
reflect short-term fluctuations in water level, except where sedimentation rates are very high. However, large and persistent
changes in moisture availability were discernible even in longer, low-resolution records. Thus, diatoms are a useful addition
to the tools available for understanding past drought in the central Great Plains, especially when trajectories of change
are constrained by data from multiple sites or other proxies. 相似文献
544.
We report isotope analyses of helium, neon, argon, and xenon using different extraction techniques such as stepwise dynamic and static crushing, and high-resolution stepwise heating of three mantle xenoliths from Réunion Island. He and Ne isotopic compositions were similar to previously reported Réunion data, yielding a more radiogenic composition when compared to the Hawaiian or Icelandic mantle plume sources. We furthermore observed correlated 129Xe/130Xe and 136Xe/130Xe ratios following the mantle trend with maximum values of 6.93 ± 0.14 and 2.36 ± 0.06, respectively. High-resolution argon analyses resulted in maximum 40Ar/36Ar ratios of 9000–11,000, in agreement with maximum values obtained in previous studies. We observed a well-defined hyperbolic mixing curve between an atmospheric and a mantle component in a diagram of 40Ar/36Ar vs. 20Ne/22Ne. Using a mantle 20Ne/22Ne of 12.5 (Ne–B) a consistent 40Ar/36Ar value of 11,053 ± 220 in sample ILR 84-4 was obtained, whereas extrapolations to a higher mantle 20Ne/22Ne ratio of 13.8 (solar wind) would lead to a much higher 40Ar/36Ar ratio of 75,000, far above observed maximum values. This favours a mantle 20Ne/22Ne of about 12.5 considered to be equivalent to Ne–B. Extrapolated and estimated 40Ar/36Ar ratios of the Réunion, Iceland, Loihi, and MORB mantle sources, respectively, tend to be linearly correlated with air corrected 21Ne/22Ne and show the same systematic sequence of increasing relative contributions in radiogenic isotopes (Iceland–Loihi–Réunion–MORB) as observed for 4He/3He. In general, He–Ne–Ar isotope systematics of the oceanic mantle can be explained by following processes: (i) different degree of mixing between pure radiogenic and pure primordial isotopes generating the MORB and primitive plume (Loihi-type) endmembers; (ii) relatively recent fractionation of He relative to Ne and Ar, in one or both endmembers; (iii) after the primary fractionation event, different degrees of mixing between melts or fluids of MORB and primitive plume affinity generate the variety of observed OIB data, also on a local scale; (iv) very late-stage secondary fractionation during magma ascent and magma degassing leads to further strong variation in He/Ne and He/Ar ratios. 相似文献
545.
Ekaterina V. Korochantseva Mario Trieloff Alexei I. Buikin Jens Hopp Hans‐Peter Meyer 《Meteoritics & planetary science》2005,40(9-10):1433-1454
Abstract— We performed high‐resolution 40Ar‐39Ar dating of mineral separates and whole‐rock samples from the desert meteorites Dhofar 300, Dhofar 007, and Northwest Africa (NWA) 011. The chronological information of all samples is dominated by plagioclase of varying grain size. The last total reset age of the eucrites Dhofar 300 and Dhofar 007 is 3.9 ± 0.1 Ga, coeval with the intense cratering period on the Moon. Some large plagioclase grains of Dhofar 007 possibly inherited Ar from a 4.5 Ga event characteristic for other cumulate eucrites. Due to disturbances of the age spectrum of NWA 011, only an estimate of 3.2–3.9 Ga can be given for its last total reset age. Secondary events causing partial 40Ar loss ≤3.4 Ga ago are indicated by all age spectra. Furthermore, Ar extractions from distinct low temperature phases define apparent isochrons for all samples. These isochron ages are chronologically irrelevant and most probably caused by desert alterations, in which radiogenic 40Ar and K from the meteorite and occasionally K induced by weathering are mixed, accompanied by incorporation of atmospheric Ar. Additional uptake of atmospheric Ar by the alteration phase(s) was observed during mineral separation (i.e., crushing and cleaning in ultrasonic baths). Consistent cosmic‐ray exposure ages were obtained from plagioclase and pyroxene exposure age spectra of Dhofar 300 (25 ± 1 Ma) and Dhofar 007 (13 ± 1 Ma) using the mineral's specific target element chemistry and corresponding 38Ar production rates. 相似文献
546.
The idea of degradation of arid and semi-arid lands, often termed desertification in its irreversible form, due to human impact and/or climatic change has been much debated since the mid-1970s. From the time of the United Nation's Conference On Desertification in Nairobi, 1976, certain areas of northern Burkina Faso have been pointed out as examples of severe desertification. Several studies demonstrated that revitalization of a series of E–W oriented fossille dunes in the Oudalan province was ongoing. The present study includes an analysis of the trends of vegetation development in the region, covering the period 1955 to 1994, with emphasis on the fossile dunes. It is demonstrated that desertification and revitalization of dunes were phenomena associated with the period between the early 1970s and the mid-1980s, and that the decline in vegetation cover on the dunes seems to have been reversed in recent years. The analysis is based upon time series of aerial photos and satellite images, field studies of vegetation, interviews with local people and review of relevant literature. The findings are discussed with reference to the debate concerning desertification and land degradation, as well as to the current revisions of the ‘range management paradigm’. The observations indicate that the environmental history of the region is complex and cannot be boiled down to ‘human-induced irreversible degradation’. Rather they support the idea of semi-arid cultural landscapes undergoing constant change in response to both human impact and climatic trends and fluctuations. 相似文献
547.
René?LehmannEmail author Jean?Bachmann Bilgin?Karaoglan Jens?Lacker Christian?Polleichtner Hans?Toni?Ratte Monika?Ratte 《Stochastic Environmental Research and Risk Assessment (SERRA)》2018,32(1):213-222
Binary data such as survival, hatching and mortality are assumed to be best described by a binomial distribution. This article provides a simple and straight forward approach for derivation of a no/lowest observed effect level (NOEL/LOEL) in a one-to-many control versus treatments setup. Practically, NOEL and LOEL values can be derived by means of different procedures, e.g. using Fisher’s exact test in coherence with adjusted p values. However, using adjusted p values heavily decreases statistical power. Alternatively, multiple t tests (e.g. Dunnett test procedure) together with arcsin-square-root transformations can be applied in order to account for variance heterogeneity of binomial data. Arcsin-square-root transformation, however, violates normal distribution because transformed data are constrained, while normal distribution provides data in the range \((-\infty ,\infty )\). Furthermore, results of statistical tests relying on an approximate normal distribution are approximate too. When testing for trends in probabilities of success (probs), the step down Cochran–Armitage trend test (CA) can be applied. The test statistic used is approximately normal. However, if probs approach 0 or 1, normal approximation of the null-distribution is suboptimal. Thus, critical values and p values lack statistical accuracy. We propose applying the closure principle (CP) and Fisher–Freeman–Halton test (FISH). The resulting CPFISH can solve the problems mentioned above. CP is used to overcome \(\alpha\)-inflation while FISH is applied to test for differences in probs between the control and any subset of treatment groups. Its applicability is presented by means of real data sets. Additionally, we performed a simulation study of 81 different setups (differing numbers of control replicates, numbers of treatments etc.), and compared the results of CPFISH to CA allowing us to point out the advantages and disadvantages of the CPFISH. 相似文献