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61.
62.
Jost Eikenberg Guido Vezzu Irene Zumsteg Sixto Bajo Max Ruethi Georg Wyssling 《Quaternary Science Reviews》2001,20(18):129
In order to determine the geochemical evolution of a freshwater limestone cave system located in central Switzerland (Hell Grottoes at Baar/Zug,) young postglacial tufaceous limestone and travertine precipitates were investigated using the 230Th/234U ingrowth system. Additional analyses of further radionuclides within the 238U decay chain, i.e. 226Ra and 210Pb, showed that the Th/U chronometer started with insignificant inherited 230Th over the entire formation period of the travertine setting (i.e. 230Th(0)=0). A contribution from detrital impurities with 230Th/234U in secular equilibrium could be precisely subtracted by applying isochron dating of cogenetic phases and recently formed travertine. The resulting precise 230Th/234U formation ages were found to be consistent with the geological stratigraphy and were furthermore used to demonstrate the applicability of the next geologically important chronometer in the 238U-decay series, based on decay of excess 226Ra normalized to the initial, i.e.226Raex/226Ra(0). This system is suitable for dating phases younger than 7000 yr when the correction of a detritus component increasingly limits the precision of the 230Th/234U chronometer. Analytical solutions of the coupled 234U/230Th/226Ra radionuclide system predicted that the 226Raex/226Ra(0) chronometer is independent of the actual 230Th activity build up from decay of 234U, if the systems starts with zero inherited 230Th(0). The data set confirmed this hypothesis and showed furthermore that the initially incorporated 226Ra excess must have remained almost uniform in all limestone over a period of at least 7000 yr, i.e. 4–5 half-lives of 226Ra. This is concluded because (i) the 226Raex/226Ra(0) ages agreed well with those derived from 230Th/234U, (ii) all data plot within uncertainty on the 226Raex/226Ra(0) decay curve and (iii) the atomic Ba/Ca ratio was found to be constant in the travertine material independent of the sample ages. Provided that such boundary conditions hold, 226Raex/226Ra(0) should be applicable to materials which are suitable for 230Th/234U dating in sedimentology and oceanography, i.e. travertine, corals, phosphorites, etc., and should strongly support 230Th/234U for samples that have been formed a few thousand years ago. 相似文献
63.
A two‐dimensional numerical model of the saltation process was developed on a parallel computer in order to investigate the temporal behaviour of transport rate as well as its downwind distribution. Results show that the effects of unsteady flow on the transportation of particulates (sediment) have to be considered in two spatial dimensions (x, y). Transport rate Q(x, t) appears in the transport equation for mass M(x, t): where A = ΔxW denotes unit area composed of unit streamwise length Δx and width W. S(x, t) (units kg m−2 s−1) stands for the balance over the splash process. A transport equation for transport rate itself is suggested with U c (x, t) a mean particle velocity at location x as the characteristic velocity of the grain cloud. For a steadily blowing wind over a 50 m long sediment bed it was found that downwind changes in Q cease after roughly 10–40 m, depending on the strength of the wind. The onset of stationarity (∂/∂t=0) was found to be a function of the friction velocity and location. The local equilibrium between transport rate and wind was obtained at different times for different downstream locations. Two time scales were found. One fast response (in the order of 1) to incipient wind and a longer time for equilibrium to be reached throughout the simulation length. Transport rate also has different equilibrium values at different locations. A series of numerical experiments was conducted to determine a propagation speed of the grain cloud. It was found that this velocity relates linearly to friction velocity. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献