绿色撒哈拉

占非洲面积将近三分之一的撒哈拉沙漠一度是热带稀树大草原,野生动植物种类丰富,充满鱼类的湖泊星罗棋布,而且有许多古人类生活在那里。“在约5 550~10 500年以前,撒哈拉是理想的生存地带,”斯特凡·克洛普林这样说过。克洛普林是德国科隆大学的地质考古学家,在过去的30年里,他     

Precise Determination of Cadmium, Indium and Tellurium Using Multiple Collector ICP-MS

New sample preparation and ion-exchange separation methods as well as instrumental measurement protocols were established for the determination of trace-level Cd, In, and Te concentrations in geological materials by isotope-dilution mass spectrometry. High precision isotope ratio measurements were performed with a multiple collector inductively coupled plasma-mass spectrometer (MC-ICP-MS). The mass biases incurred for In and Te were corrected by adding and monitoring Pd and Sb standard solutions, respectively. Mass fractionation of Cd was corrected by using the mass fractionation factor calculated from the measurement of a standard solution. The measurement precision was better than 1 % for Cd, In and Te. Detection limits were < 1 ng g^-1 for Cd, < 0.02 ng g^-1 for In and Te. Using these new analytical techniques, the concentrations of Cd, In and Te were determined in six international geological reference materials. Concentrations could be reproduced within 3% for Cd, 4% for In and 10% for Te. Sample heterogeneity and volatility problems might have been the reason for the relatively large differences between Te replicates. Our results displayed excellent reproducibility compared with those of other techniques and agree well with data from previously published recommended values.     

Statistically derived morphological signatures of large river channels extracted from topo-bathymetric LiDAR data

With the development of LiDAR technology and the availability of topo-bathymetric data of high quality, new methods are emerging to describe and understand more accurately fluvial geomorphology. We explore the capacities of probability density functions (PDFs) of detrended dimensionless elevations extracted from digital elevation models (DEMs) to document channels morphology and configuration. These DEMs were obtained from a topo-bathymetric LiDAR survey of 450 km performed on the middle and lower reaches of the largest river of France: the Loire River. The objective is to propose a thorough and complete method for PDF analysis in order to assess the vertical structuration of river reaches presenting various fluvial patterns. The analysis was conducted on the PDFs of active channel belt of four sites selected for the diversity of their morphological configurations (anabranching, braided, channelized and meandering). Results showed that PDFs appear to be specific of a reach and that they are useful for describing its vertical structure. The simplification with Gaussian mixture model (GMM) is effective in slicing PDF and the resulting Gaussians components are related to morphological units (MUs). The distance between Gaussian curves obtained using GMMs can be considered as an indicator of topo-bathymetric connectivity between MUs (e.g., main and side channels) on sites presenting multichannel or channelized channel configurations. The study showed the high potential of PDF and GMM analysis in the field of fluvial geomorphology and suggests interesting developments for river management and restoration.