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1.
Cosmogenic nuclide exposure dating of glacial clasts is becoming a common and robust method for reconstructing the history of glaciers and ice sheets. In Antarctica, however, many samples exhibit cosmogenic nuclide ‘inheritance’ as a result of sediment recycling and exposure to cosmic radiation during previous ice free periods. In-situ cosmogenic 14C, in combination with longer lived nuclides such as 10Be, can be used to detect inheritance because the relatively short half-life of 14C means that in-situ 14C acquired in exposure during previous interglacials decays away while the sample locality is covered by ice during the subsequent glacial. Measurements of in-situ 14C in clasts from the last deglaciation of the Framnes Mountains in East Antarctica provide deglaciation ages that are concordant with existing 26Al and 10Be ages, suggesting that in this area, the younger population of erratics contain limited inheritance.  相似文献   

2.
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3.
It is generally considered that four-times ice age happened during the Quaternary epoch on the Tibetan Plateau. However, the research on the chronology of the four-times ice age is far from enough. The Shaluli Mountain on the Southeastern Tibetan Plateau is an ideal place for plaeo-glacier study, because there are abundant Quaternary glacial remains there. This paper discusses the ages of the Quaternary glaciations, based on the exposure dating of roche moutonnée, moraines and gla- cial erosion surfaces using in situ cosmogenic isotopes 10Be. It is found that the exposure age of the roche moutonnée at Tuershan is 15 ka, corresponding to Stage 2 of the deep-sea oxygen isotope, suggesting that the roche moutonnée at Tuershan is formed in the last glacial maximum. The expo- sure age of glacial erosion surface at Laolinkou is 130―160 ka, corresponding to Stage 6 of the deep-sea oxygen isotope. The oldest end moraine at Kuzhaori may form at 421―766 kaBP, corre- sponding to Stages 12―18 of the deep-sea oxygen isotope. In accordance with the climate charac- teristic of stages 12,14,16 and 18 reflected by the deep-sea oxygen isotope, polar ice cores and loess sequence, the oldest end moraine at Kuzhaori may form at stage 12 or stage 16, the latter is more possible.  相似文献   

4.
In situ Terrestrial Cosmogenic Nuclides (hereafter TCNs) are increasingly important for absolutely dating terrestrial events and processes. This study aimed at improving our knowledge of the production rate of Terrestrial Cosmogenic 3He formed in situ in rock surfaces at low latitude and sea level as well as re-evaluation of the Canary Islands as a calibration site for TCNs. For this purpose, we sampled basaltic lava flows from some of the youngest and yet undated volcanic sites and used the 40Ar/39Ar incremental heating method on groundmass samples and in situ cosmogenic 3He on olivine and clinopyroxene phenocrysts. 40Ar/39Ar analysis was done on a Hiden HAL Series 1000 triple filter quadrupole mass spectrometer with extraction furnace. Incremental heating data shows ages in the Late Pleistocene from 52.7 ± 21.6 ka to 398.6 ± 27.6 ka.We measured cosmogenic 3He concentrations in olivine and clinopyroxene phenocrysts from flow top samples on a MAP 215-50 sector mass spectrometer with a crushing device and a diode laser extraction system. Exposure age calculations yielded ages in the range 38.9 ± 4.0 ka to 62.3 ± 6.7 ka for the youngest lava flow and the data series is in broad agreement with the argon data up to 250 ka and reveals a more continuous time line of volcanism during the late Pleistocene on the island. However, the dataset was not sufficient for calculation of production rates for in situ Terrestrial Cosmogenic 3He as many samples showed signs of erosion. Calculated erosion rates range from none to as high as 7.3 mm/kyr assuming a rock density of 2.9 g/cm2. This finding puts a constraint on the use of Fuerteventura as a calibration site for exposure histories older than 50–100 ka. A comparison with cosmogenic 36Cl data supports these findings and indicates substantial weathering.  相似文献   

5.
Data on the content of the 14C cosmogenic isotope in tree rings, which were obtained as a result of laboratory measurements, are often used when solar activity (SA) is reconstructed for previous epochs, in which direct observations are absent. However, these data contain information not only about SA variations but also about changes in the Earth climatic parameters, such as the global temperature and the CO2 content in the Earth’s atmosphere. The effect of these variations on the 14C isotope content in different natural reservoirs after the last glacial termination to the middle of the Holocene is considered. The global temperature and the CO2 content increased on this time interval. In this case the 14C absolute content in the atmosphere increased on this time interval, even though the 14С to 12С isotope concentration ratio (as described by the Δ14С parameter) decreased. These variations in the radiocarbon absolute content can be caused by its redistribution between natural reservoirs. It has been indicated that such a redistribution is possible only when the rate of carbon exchange between the ocean and atmosphere depends on temperature. The values of the corresponding temperature coefficient for the 17–10 ka BC time interval, which make it possible to describe the carbon redistribution between the ocean and atmosphere, have been obtained.  相似文献   

6.
The CRONUS-Earth and CRONUS-EU Projects have recently examined the reproducibility of cosmogenic nuclide measurements and the ability of cosmogenic-production models to fit high-quality global calibration data sets. In both cases, although results are adequate for present geochronological needs, they must be improved to meet future demands. We recommend that the cosmogenic-nuclide community embark on a continuing effort to understand the sources of the remaining model-versus-data discrepancy, and that in the meanwhile all cosmogenic ages be benchmarked against reference ages calculated in a consistent fashion. We also recommend that interlaboratory reference materials be routinely analyzed along with samples and blanks, that the results be tracked, and that methods of achieving consistency and precision in sample preparation and analysis be shared within the community. We also recommend additional organization of the community to facilitate the use of intercomparison materials, foster analytical improvements, oversee the evolution of the community age calculator, and synthesize ongoing calibration efforts.  相似文献   

7.
Existing methods of cosmogenic nuclide burial dating perform well provided that sediment sources undergo steady rates of erosion and the samples experience continuous exposure to cosmic rays. These premises exert important limitations on the applicability of the methods. And yet, high mountain sediment sources are rife with transient processes, such as non-steady erosion by glacial quarrying and/or landsliding, or temporary cosmic-ray shielding beneath glaciers and/or sediment. As well as breaching the premises of existing burial dating methods, such processes yield samples with low nuclide abundances and variable 26Al/10Be ratios that may foil both isochron and simple burial-age solutions. P–PINI (Particle-Pathway Inversion of Nuclide Inventories) is a new dating tool designed for dating the burial of sediments sourced from landscapes characterized by abrupt, non-steady erosion, discontinuous exposure, and catchments with elevation-dependent 26Al/10Be production ratios. P–PINI merges a Monte Carlo simulator with established cosmogenic nuclide production equations to simulate millions of samples (10Be–26Al inventories). The simulated samples are compared statistically with 10Be–26Al measured in field samples to define the most probable burial age. Here, we target three published 10Be–26Al datasets to demonstrate the versatility of the P–PINI model for dating fluvial and glacial sediments. (1) The first case serves as a robust validation of P–PINI. For the Pulu fluvial gravels (China), we obtain a burial age of 1.27 ± 0.10 Ma (1σ), which accords with the isochron burial age and two independent chronometers reported in Zhao et al. (2016) Quaternary Geochronology 34, 75–80. The second and third cases, however, reveal marked divergence between P–PINI and isochron-derived ages. (2) For the fluvial Nenana Gravel (USA), we obtain a minimum-limiting burial age of 4.5 ± 0.7 Ma (1σ), which is compatible with unroofing of the Alaska Range starting ∼ 6 Ma, while calling into question the Early Pleistocene isochron burial age presented in Sortor et al. (2021) Geology 49, 1473–1477. (3) For the Bünten Till (Switzerland), we obtain a limiting burial age of <204 ka (95th percentile range), which conforms with the classical notion of the most extensive glaciation in the northern Alpine Foreland assigned to the Riss glaciation (sensu marine isotope stage 6) contrary to the isochron burial age presented in Dieleman et al. (2022) Geosciences, 12, 39. Discrepancies between P–PINI and the isochron ages are rooted in the challenges posed by the diverse pre-burial 26Al/10Be ratios produced under conditions characteristic of high mountain landscapes; i.e., non-steady erosion, discontinuous cosmic-ray exposure, and elevation-dependent 26Al/10Be production ratios in the source region, which are incompatible with the isochron method, but easily accommodated by the stochastic design of P–PINI.  相似文献   

8.
Terrestrial Cosmogenic Nuclides (TCNs) have been widely used to date the exposure of alluvial surfaces and to estimate catchment-scale erosion rates. However, TCN concentration differences in samples of different grain sizes remain to be fully understood. In order to explore the possibility that river processes generate such differences, we develop a numerical model to calculate along-stream clast-scale TCN concentrations. Using the hillslope model, there is a progressive detachment of successive clasts of specific sizes followed by their instantaneous fall into the river. In the river, transport velocity and TCN concentration evolution in a clast depend 1) on the probability of being trapped in the sediment mixing layer of the river or within an adjacent terrace; 2) on its size which decreases downstream by attrition. The size-dependent transport law corresponds to the partial transport state in a river. We model the distribution of TCN concentrations in different clast size fractions in the 0–5 cm radius range for catchments in steady-state erosion, and for catchments experiencing sedimentation.We propose that clast attrition tends to increase the variance of TCN concentrations of the small clast size fractions because these fractions incorporate initially big clasts that travelled a long distance in addition to small clasts contributed near the outlet. We obtained numerous clast size–TCN concentration correlations, positive or negative, the significance of which depends on the initial clast size distribution, hillslope erosion rate, river length and lithology. For an equilibrium catchment, even large, we found that the addition of TCN concentration acquired during river transport is negligible compared to TCN concentration acquired on a hillslope, although a clast size–TCN concentration relationship can result from or be modified by clast attrition. On the contrary, aggrading catchments may show a significant clast size-dependent TCN concentration increase during river transport. This may introduce a small bias in the TCN-derived catchment erosion rate, but it could be used positively to quantify the mean transport velocity of clasts of different sizes over thousands of years. In addition, the lack of correlation between TCN concentration and clast size does not imply that the mean transport velocity is the same for all clast size fractions. Overall, our study provides an alternative explanation for observed clast size-dependent TCN concentrations and brings to the fore the need for measuring TCN concentration in larger clast size fractions than is usually done. To see if the byproducts of abrasion dilute or increase the TCN concentration of sand, all products should be included in a future study.  相似文献   

9.
Cosmogenic nuclide depth-profiles are used to calculate the age of landforms, the rates at which erosion has affected them since their formation and, in case of deposits, the paleo-erosion rate in the source area. However, two difficulties are typically encountered: 1) old deposits or strongly affected by cosmogenic nuclide inheritance often appear to be saturated, and 2) a full propagation of uncertainties often yields poorly constrained ages. Here we show how to combine surface-exposure-dating and burial-dating techniques in the same profile to get more accurate age results and to constrain the extent of pre-depositional burial periods. A 10Be–26Al depth-profile measured in an alluvial fan of SE Iberia is presented as a natural example.  相似文献   

10.
K–Ar ages of young basalts (<500 ka) are often higher than the actual eruption age, due to low potassium contents and the frequent presence of excess Ar in olivine and pyroxene phenocrysts. Geological studies in the San Francisco and Uinkaret volcanic fields in Arizona have documented the presence of excess 40Ar and have concluded that K–Ar ages of young basalts in these fields tend to be inaccurate. This new study in the San Francisco volcanic field presents 3Hec and 21Nec ages yielded by olivine and pyroxene collected from three Pleistocene basalt flows – the South Sheba (∼190 ka), SP (∼70 ka), and Doney Mountain (∼67 ka) lava flows, – and from one Holocene basalt, the Bonito Lava Flow (∼1.4 ka) at Sunset Crater. These data indicate that, in two of three cases, 40Ar/39Ar and K–Ar ages of the young basalts agree well with cosmic-ray surface exposure ages of the same lava flow, thus suggesting that excess 40Ar is not always a problem in young basalt flows in the San Francisco volcanic field. The exposure age of the Bonito lava flow agrees within uncertainty with dendrochronological and archeological age determinations. K–Ar and cosmogenic 3He and 21Ne ages from the SP flow are in agreement and much older than the OSL age (5.5–6 ka) reported for this lava flow. Furthermore, if the non-cosmogenic ages are assumed to be accurate, the subsequent calculated production rates at South Sheba and SP flow sample sites agree well with values in the literature.  相似文献   

11.
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12.
Geomagnetism and Aeronomy - Analysis of images of the full solar disk obtained by ground-based (BST-2, CrAO RAS) and space-based (SDO/AIA, SDO/HMI) instruments and the potential approximation...  相似文献   

13.
INTRODUCTIONThe method of probabilistic seismic risk analysis was proposed by Cornell in1968(Cornell,1968).After more than30years development,it has become the main method for seismic riskassessment of engineering sites and seismic zonation,and has been u…  相似文献   

14.
Ａｎａｌｙｓｉｓｏｆｆｏｒｅｓｈｏｃｋｓｅｑｕｅｎｃｅｏｆｔｈｅ１９７５ＨａｉｃｈｅｎｇｅａｒｔｈｑｕａｋｅｏｆＭｓ７．３Ｚｈａｏ－ＲｏｎｇＺＵＯ（左兆荣）；Ｊｉａｎ－ＰｉｎｇＷＵ（吴建平）ａｎｄＺｈｉ－ＬｉｎｇＷＵ（巫志玲）（Ｉｎｓｔｉｔｕｔｅｏｆ...  相似文献   

15.
In the past decade, the most authoritative catalogues of Chinese earthquakes and the most popular with seismologists in China are the following:(1) Gu Gongxu, 1983, Catalogue of Chinese Earthquakes;(2) Min Ziqun, 1988, Concise Catalogue of Chinese Earthquakes;(3) Xie Yusuou, 1989, Catalogue of Chinese Earthquakes (M≥ 4.7) from 1900-1980 with Uniform Magnitudes; and(4) Min Ziqun, 1995, Catalogue of Chinese Historical Strong Earthquakes. Earthquakes that occurred before 1900 are mainly documented in historical records.Since 1950s, more recent earthquakes were documented in two major compilations of historical records finished in 1956 and 1983-1987. Separately this effort resulted in two chronicles: two volumes for the first one and five volumes for the second one. The magnitudes are converted from the maximum intensity. These magnitudes, by convention, are connected with surface wave magnitudes. However, it is clear that they do not have any strict seismological definition.The period of 1900-1962 documen  相似文献   

16.
Ｍａｔｈｅｍａｔｉｃａｌｍｏｄｅｌｌｉｎｇｏｆｎｏｎｌｉｎｅａｒｂｅｈａｖｉｏｕｒｏｆｓｅｉｓｍｉｃｉｔｙ杨立明，石特临，郭大庆Ｌｉ－ＭｉｎｇＹＡＮＧ；Ｔｅ－ＬｉｎＳＨＩａｎｄＤａ－ＱｉｎｇＧＵＯ（ＥａｒｔｈｑｕａｋｅＲｅｓｅａｒｃｈＩｎｓｔｉｔｕｔ...  相似文献   

17.
A method of estimation of occurrence probability of earthquake intensity at a given site from the results of a ten-year scale of earthquake prediction described with a probability of occurrence in a given "prediction cell" is proposed in this paper. 2316 cities and towns in China were analyzed by using this method. The probability of intensity Ⅵ-Ⅸ were given for every city. These results can be used for the earthquake insurance, loss estimation, and planning of disaster protection.  相似文献   

18.
Current practice uses predictive models to extrapolate long-period response spectra based on far-field recordings in moderate and weak earthquakes. However, the spectra are not long enough and the data are often not reliable, which means that the seismic design code cannot accurately define seismic design requirements for long-period structures. The near-field recordings in the main-shock of the Chi-Chi earthquake have a large signal-to-noise ratio (SNR), which makes them suitable for studying the long-period acceleration response spectrum up to 20 sec. The acceleration response spectra from 246 stations within 120 km of the causative fault are statistically analyzed in this paper. The influence of distance and site conditions on long-period response spectrum is discussed, and the shapes of the amplification spectra are compared with the standard spectra specified in the seismic design code of China. Finally, suggestions for future revisions to the code are proposed.  相似文献   

19.
Estimation of seismic hazard for the fast developing coastal area of Pakistan is carried out using deterministic and probabilistic approaches. On the basis of seismotectonics and geology, eleven faults are recognized in five seismic provinces as potential hazard sources. Maximum magnitude potential for each of these sources is calculated. Peak ground acceleration (PGA) values at the seven coastal cities due to the maximum credible earthquake on the relevant source are also obtained. Cities of Gwadar and Ormara with acceleration values of 0.21g and 0.25g respec-tively fall in the high seismic risk area. Cities of Turbat and Karachi lie in low seismic risk area with acceleration values of less than 0.1g. The Probabilistic PGA maps with contour interval of 0.05g for 50 and 100 years return period with 90% probability of non-exceedance are also compiled.  相似文献   

20.
Although it has been shown that the implementation of the HHT-α method can result in improved error propagation properties in pseudodynamic testing if the equation of motion is used instead of the difference equation to evaluate the next step acceleration, this paper proves that this method might lead to instability when used to solve a nonlinear system. Its unconditional stability is verified only for linear elastic systems, while for nonlinear systems, instability occurs as the step degree of convergence is less than 1. It is worth noting that the step degree of convergence can frequently be less than 1 in pseudodynamic testing, since a convergent solution is achieved only when the step degree of convergence is close to 1 regardless of whether its value is greater or less than 1. Therefore, the application of this scheme to pseudodynamic testing should be prohibited, since the possibility of instability might incorrectly destroy a specimen. Consequently, the implementation of the HHT-α method by using the difference equation to determine the next step acceleration is recommended for use in pseudodynamic testing.  相似文献   

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