We have determined the production yields for radionuclides in Al2O3, SiO2, S, Ar, K2SO4, CaCO3, Fe, Ni and Cu targets, which were irradiated with slow negative muons at the Paul Scherrer Institute in Villigen (Switzerland). The fluences of the stopped negative muons were determined by measuring the muonic X-rays. The concentrations of the long-lived and short-lived radionuclides were measured with accelerator mass spectrometry (AMS) and γ-spectroscopy, respectively. Special emphasis was put on the radionuclides 10Be, 14C and 26Al produced in quartz targets, 26Al in Al2O3 and S targets, 36Cl in K2SO4 and CaCO3 targets, and 53Mn in Fe2O3 targets. These targets were selected because they are also the naturally occurring target minerals for cosmic ray interactions in typical rocks. We also present results of calculations for depth-dependent production rates of radionuclides produced after cosmic ray μ− capture, as well as cosmic ray-induced production rates of geologically relevant radionuclides produced by the nucleonic component, by μ− capture, by fast muons and by neutron capture. 相似文献
We have measured the concentration of in situ produced cosmogenic 10Be and 26Al from bare bedrock surfaces on summit flats in four western U.S. mountain ranges. The maximum mean bare-bedrock erosion rate from these alpine environments is 7.6 ± 3.9 m My−1. Individual measurements vary between 2 and 19 m My−1. These erosion rates are similar to previous cosmogenic radionuclide (CRN) erosion rates measured in other environments, except for those from extremely arid regions. This indicates that bare bedrock is not weathered into transportable material more rapidly in alpine environments than in other environments, even though frost weathering should be intense in these areas. Our CRN-deduced point measurements of bedrock erosion are slower than typical basin-averaged denudation rates ( 50 m My−1). If our measured CRN erosion rates are accurate indicators of the rate at which summit flats are lowered by erosion, then relief in the mountain ranges examined here is probably increasing.
We develop a model of outcrop erosion to investigate the magnitude of errors associated with applying the steady-state erosion model to episodically eroding outcrops. Our simulations show that interpreting measurements with the steady-state erosion model can yield erosion rates which are either greater or less than the actual long-term mean erosion rate. While errors resulting from episodic erosion are potentially greater than both measurement and production rate errors for single samples, the mean value of many steady-state erosion rate measurements provides a much better estimate of the long-term erosion rate. 相似文献
For about the last 30 years it has been recognized that the high frequency component of the tree rings 14C/12C record is dominated by the modulation of the cosmic ray flux by the solar wind. In particular, it has been demonstrated that the three most recent periods of low sunspot occurrence were characterized by high values of atmospheric 14C/12C. During the last millennium other periods of high 14C/12C values were observed but their solar origin is still debatable. In the present work we compare these fluctuations with an independent record of cosmogenic 10Be measured in ice from the South Pole to check the solar origin of the observed 14C/12C variations. In order to compare quantitatively the results obtained on 10Be and 14C, it is necessary to take into account the different behaviour of these two cosmogenic isotopes, and especially the damping effect of the carbon cycle in the case of 14C. As an input to a 12-box numerical model we used the relative fluctuations of the 10Be concentrations record measured in South Pole ice and converted it into a synthetic 14C record. We took into account the fact that 10Be modulation is enhanced in polar regions due to the orientation of the geomagnetic field. As expected, the fluctuations of the modelled 14C record are much smaller (a factor of 20) than those observed for the raw 10Be record. In addition, the variations are smoother and shifted in time by a few decades. The 10Be-based 14C variations closely resemble the 14C measurements obtained on tree rings (R = 0.81). In particular, it is easy to identify periods of maximal 14C/12C which correspond to solar activity minima centred at about 1060, 1320 (Wolf), 1500 (Spörer), 1690 (Maunder) and 1820 (Dalton) yr A.D. Cross-correlation calculations suggest that there is no significant lag between the 10Be-based 14C and the tree-ring 14C records. Our study strongly suggests the dominance of the solar modulation on the cosmonuclide production variations during the last millennium. 相似文献
A granitic rock avalanche, one of the largest Quaternary landslides in Arizona outside the Grand Canyon with a volume of approximately 5.25 M m3 and a width a little under 0.5 km, ran 1 km from the eastern McDowell Mountains. With lateral levees and pressure ridges, the rock avalanche deposit displays many features found on classic sturzstroms. Failure occurred along a major joint plane paralleling the slope with a dip of 44°, when a major base level lowering event in the Salt River system would have undermined the base of the failed slope, and probably during a period of more moisture than normally available in the present-day arid climate. Failure at the subsurface weathering front highlights the importance of the dramatic permeability change between grussified regolith and relatively fresh bedrock. Rock varnish microlaminations (VMLs) dating, in concert with other geomorphic evidence, suggests that the rock avalanche deposit is slightly older than 500 ka. The rock vanish results also have important implications for sampling strategies designed to use cosmogenic nuclide to date Quaternary landslide deposits. Discovery of a large landslide in close proximity to the extending urban fringe of metropolitan Phoenix argues for a more careful analysis of landslide hazards in the region, especially where rapid development excavates bedrock at the base of steep mountain slopes and where the subsurface weathering front is near the surface. 相似文献
Earlier work in northeast Greenland has suggested a limited advance of the Greenland Ice Sheet during the Last Glacial Maximum (LGM). However, this concept has recently been challenged by marine geological studies, indicating grounded ice on the continental shelf at this time. New 10Be‐ages from the Store Koldewey island, northeast Greenland, suggest that unscoured mountain plateaus at the outer coast were covered at least partly by cold‐based ice during the LGM. It is, however, still inconclusive whether this ice was dynamically connected to the Greenland Ice Sheet or not. Regardless of the LGM ice sheet extent, the 10Be results from Store Koldewey add to a growing body of evidence suggesting considerable antiquity of crystalline unscoured terrain near present and Pleistocene ice sheet margins. 相似文献