Exploring the potential of luminescence methods for dating Alpine rock glaciers     

《Quaternary Geochronology》2013

Rock glaciers contain valuable information about the spatial and temporal distribution of permafrost. The wide distribution of these landforms in high mountains promotes them as useful archives for the deciphering of the environmental conditions during their formation and evolution. However, age constraints are needed to unravel the palaeoclimatic context of rock glaciers, but numerical dating is difficult. Here, we present a case study assessing the potential of luminescence techniques (OSL, IRSL) to date the inner sand-rich layer of active rock glaciers. We focus on the signal properties and the resetting of the signal prior to deposition by investigating single grains. While most quartz shows low signal intensities and problematic luminescence characteristics, K-feldspar exhibits much brighter and well-performing signals. Most signals from plagioclases do not show suitable properties. Luminescence signals far below saturation indicate distinct but differential bleaching. The finite mixture model was used to determine the prominent populations in the equivalent dose distributions. The luminescence ages represent travel times of grains since incorporation into the rock glacier and hence, minimum ages of rock glacier formation. Luminescence ages between 3 ka and 8 ka for three rock glaciers from the Upper Engadine and Albula region (Swiss Alps) agree well with independent age estimates from relative and semi-quantitative approaches. Therefore, luminescence seems to have the potential of revealing age constraints about processes related to the formation of rock glaciers, but further investigations are required for solving some of the problems remaining and reducing the dating uncertainties.  相似文献   

Mechanisms for avulsion on alluvial fans: Insights from high-frequency topographic data     

Anya Leenman  Brett Eaton 《地球表面变化过程与地形》2021,46(6):1111-1127

Avulsion is a key process in building alluvial fans, but it is also a formidable natural hazard. Based on laboratory experiments monitored with novel high-frequency photogrammetry, we present a new model for avulsion on widely graded gravel fans. Previous experimental studies of alluvial fans have suggested that avulsion occurs in a periodic autogenic cycle, that is thought to be mediated by the gradient of the fan and fan-channel. However, those studies measured gradients at low spatial or temporal resolutions, which capture temporally or spatially averaged topographic evolution. Here, we present high-resolution (1 mm), high-frequency (1-minute) topographic data and orthophotos from an alluvial fan experiment. Avulsions in the experiment were rapid and, in contrast to some previous experimental studies, avulsion occurrence was aperiodic. Moreover, we found little evidence of the back-filling observed at coarser temporal and spatial resolutions. Our observations suggest that avulsion is disproportionately affected by sediment accumulation in the channel, particularly around larger, less mobile grains. Such in-channel deposition can cause channel shifting that interrupts the autogenic avulsion cycle, so that avulsions are aperiodic and their timing is more difficult to predict.  相似文献   

The southernmost record of tropical pollen grains in the mid-Cretaceous of Patagonia, Argentina   总被引：1，自引：0，他引：1  

Viviana Barreda  Sergio Archangelsky   《Cretaceous Research》2006,27(6):778-787

Pollen grains characteristic of tropical Northern Gondwana (Schrankipollis, Brenneripollis and Pennipollis peroreticulatus) have been recorded from the Cretaceous of Patagonia. They were recovered from the Late Albian–Cenomanian Kachaike Formation in Santa Cruz Province, southern Argentina. The palynological assemblages are dominated by gymnosperm pollen and bryophyte-pteridophyte spores, whereas angiosperms are poorly represented. The angiospermoid type Schrankipollis has not been reported previously from Argentina, and a new species S. kachaikensis is described. Two species of Brenneripollis (of uncertain affinity) and Pennipollis peroreticulatus (related to the Alismatales) are also reported. The first appearances of Pennipollis peroreticulatus are clearly diachronous from north to south, ranging from the Barremian in tropical regions to the Coniacian–Santonian in Antarctica. S. kachaikensis and P. peroreticulatus show restricted stratigraphic distributions through the Kachaike Formation and may be stratigraphically useful markers. A tetrad of Walkeripollis (related to modern Winteraceae) is also recognized. This is the oldest record of Winteraceae in the southern temperate region where the family lives today. Evidence for migration of Winteraceae from tropical Gondwana to Antarctica and Australia throughout South America, is provided by this new finding. The presence of tropical elements in the austral margin of South America gives support to previous studies on the expansion of warm temperatures towards high latitudes during the mid Cretaceous.  相似文献   

Agribusiness and farmers in Mexico: the importance of contractual relations   总被引：1，自引：0，他引：1  

FLAVIA ECHÁNOVE  CRISTINA STEFFEN† 《The Geographical journal》2005,171(2):166-176

In recent decades contract farming as a mechanism that integrates and subordinates agricultural production to export oriented agribusiness has expanded in Third World countries. In Mexico contract farming dominates the production of horticultural crops for processing and export. It is also used for barley and to grow some varieties of wheat for Cargill . The contract farming labour agreement represents an institution that links local and global processes where agribusiness is the key actor that integrates local farmers to national and world markets. The purpose of this paper is to analyse the implications of contract farming for productive relations by examining its impact upon farmers and firms. The study found that despite the disadvantages of contract farming for growers, and the disproportional risks born by producers, they enter into contract farming labour agreements because they lack alternatives for financing, technical assistance and access to markets. The experience of contract farming in Mexico and several Third World countries shows that its growth is related to the implementation of neo-liberal policies that are accompanied by the withdrawal of state support and regulation of agricultural production. In this context, contract farming represents a livelihood strategy for many Mexican growers.  相似文献   

海南岛乐东盆地白垩系研究新进展     

符国祥  符策锐  汪迎平 《地层学杂志》2001,25(1):63-68

对海南岛乐东盆地白垩系进行了深入研究 ,将原报万组上部一套冲积相划分出来 ,新建长茅组 ;厘订后的报万组限定在长茅组之下 ,为一套河湖相沉积 ,二者为整合接触 ,含孢粉化石 ,时代为早白垩世最晚期到晚白垩世。  相似文献   

Unprecedented last-glacial mass accumulation rates determined by luminescence dating of loess from western Nebraska     

Helen M. Roberts  Daniel R. Muhs  Ann G. Wintle  Geoff A. T. Duller  E. Arthur Bettis  III 《Quaternary Research》2003,59(3):411-419

A high-resolution chronology for Peoria (last glacial period) Loess from three sites in Nebraska, midcontinental North America, is determined by applying optically stimulated luminescence (OSL) dating to 35–50 μm quartz. At Bignell Hill, Nebraska, an OSL age of 25,000 yr near the contact of Peoria Loess with the underlying Gilman Canyon Formation shows that dust accumulation occurred early during the last glacial maximum (LGM), whereas at Devil’s Den and Eustis, Nebraska, basal OSL ages are significantly younger (18,000 and 21,000 yr, respectively). At all three localities, dust accumulation ended at some time after 14,000 yr ago. Mass accumulation rates (MARs) for western Nebraska, calculated using the OSL ages, are extremely high from 18,000 to 14,000 yr—much higher than those calculated for any other pre-Holocene location worldwide. These unprecedented MARs coincide with the timing of a mismatch between paleoenvironmental evidence from central North America, and the paleoclimate simulations from atmospheric global circulation models (AGCMs). We infer that the high atmospheric dust loading implied by these MARs may have played an important role, through radiative forcing, in maintaining a colder-than-present climate over central North America for several thousand years after summer insolation exceeded present-day values.  相似文献   

单轴受拉时冻土结构变化及其机理初析     

沈忠言  王家澄 《冰川冻土》1996,18(3):262-268

土体地单轴拉应力作用下，轴向经历变形，结构发生本质性的变化，径向同时出现收缩。这种应变特性改变了矿物颗粒间的微应力场，它是土体在径向上发生颗粒错位，定向排列，微裂隙等发育等结构变化的源动力。  相似文献   

糟醉鲳鱼的工艺研究   总被引：1，自引：0，他引：1  

张芝芬  吴汉民  黄晓春  王海洪 《海洋学研究》2001,19(2):69-72

采用正交试验设计 ,研究鱼体水分含量、食盐和酒糟添加量等因素对糟鱼品质的影响。结果表明 ,酒糟用量为鱼体用量的 1 .5倍 ,食盐和白酒的用量分别为酒糟用量的 7%和 4%是制作糟醉鲳鱼的最佳组合。  相似文献   

淀粉系列海上溢油凝油剂的制备与凝油性能   总被引：4，自引：1，他引：4  

孙云明  刘会峦  陈国华  宋金明 《海洋科学》2001,25(8):37-41

利用玉米淀粉的羧甲基化与酯化等方法改性合成了一系列海上溢油凝油剂，实验发现，当羧甲基取代度足够时，凝油剂的凝油性能随酯化度的增加而增大，当酯化度足够时，羧甲基淀粉酯凝油剂的凝油性能随羧甲羧甲基取代度的增加而增大，凝油性能还与凝油剂中多价金属离子的种类，油品的种类和脂肪酸的碳原子数目有关，凝油性能随水盐度变化很小，即凝油剂对淡水水面溢面仍然有效，凝油性能随油品不同差异较大，对具有一定极性的油的胶凝作用比对非极性油的胶凝作用更强，对水面的原因及植物油比对燃料油具有更好的胶凝效果。  相似文献   

化探找金中几个问题的讨论     

刘庆成 《华东地质学院学报》1987,(3)

本文讨论了化探找金中指示元素选择、取样层位、富集粒度等问题,并提出了解决这些问题的办法,这些问题都直接影响找矿效果。  相似文献