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91.
Several cryptotephra layers that originate from Icelandic volcanic eruptions with a volcanic explosivity index (VEI) of ≤ 4 and tephra volumes of < 1 km3 have previously been identified in Northern Europe, albeit within a restricted geographical area. One of these is the Hekla 1947 tephra that formed a visible fall-out in southern Finland. We searched for the Hekla 1947 tephra from peat archives within the previously inferred fall-out zone but found no evidence of its presence. Instead, we report the first identification of Hekla 1845 and Hekla 1510 cryptotephra layers outside of Iceland, the Faroe Islands, Ireland and the UK. Additionally, Hekla 1158 tephra was found in Finland for the first time. Our results confirm that Icelandic eruptions of moderate size can form cryptotephra deposits that are extensive enough to be used in inter-regional correlations of environmental archives and carry a great potential for refining regional tephrochronological frameworks. Our results also reveal that Icelandic tephra has been dispersed into Finnish airspace at least seven times during the past millennium and in addition to a direct eastward route the ash clouds can travel either via a northerly or a southerly transport pathway. 相似文献
92.
Victorian farmers have experienced significant impact from climate change associated with drought and more recently flooding. These factors form a convergence with a complex of other factors to change production systems physically; and farmers’ decision making is variously described as adaptive or maladaptive to these drivers of change. Recently updated State Government policies on farming, climate and water have immediate and long term implications for food production systems but are not readily interpreted at a local scale. Further, peak oil and energy security are only partially integrated into either climate or water policy discourse. In effect, despite some far-sighted words about the meaning of climate change, uncertainty is largely met with a ‘business as usual’ mantra. Farmer narratives are used to demonstrate their systemic and increasing vulnerability and likelihood of perverse outcomes. The Future Farming strategy and Our Water Our Future are briefly analyzed, as are potential implications of the rhetoric of newly elected conservative government. Using ideas from Bourdieu and Bhabha we suggest that the reliance on farmers being able to innovate and take up opportunities associated with the uncertainty of large scale changes in climate and energy availability are misguided. It is more likely that current policy directions entrench the values of the global market and its elite, leaving farmers locked-in to historical structural responses that will not be successful in the long-term and will diminish their ability to imagine radical and diverse ways of avoiding the maladaptive structures currently surrounding their production systems. 相似文献
93.
<正> 穿山是桂林峰林平原上一座联座石峰(图1),在其不同高度上发育着许多洞穴。本文所涉及的古地磁样品则主要采集于穿山最西部月岩附近的19、20、21和28号洞内(图2)。与漓江高河漫滩高程相当的洞穴,其中沉积物均是细颗粒的,质地较软粘土。根据未经干扰的残留沉积物来分析,洞内沉积物充填的高度高出附近高河漫滩约13—14米。其它洞穴中残留的水平顶板亦表明曾经受过一个高水位的溶蚀作用。 相似文献
94.
Ruth Elaine Blake 《Geochimica et cosmochimica acta》2003,67(13):2315-2316
95.
Quaternary glaciation of Mount Everest 总被引:1,自引:0,他引:1
Lewis A. Owen Ruth Robinson Douglas I. Benn Robert C. Finkel Nicole K. Davis Chaolu Yi Jaakko Putkonen Dewen Li Andrew S. Murray 《Quaternary Science Reviews》2009,28(15-16):1412-1433
The Quaternary glacial history of the Rongbuk valley on the northern slopes of Mount Everest is examined using field mapping, geomorphic and sedimentological methods, and optically stimulated luminescence (OSL) and 10Be terrestrial cosmogenic nuclide (TCN) dating. Six major sets of moraines are present representing significant glacier advances or still-stands. These date to >330 ka (Tingri moraine), >41 ka (Dzakar moraine), 24–27 ka (Jilong moraine), 14–17 ka (Rongbuk moraine), 8–2 ka (Samdupo moraines) and ~1.6 ka (Xarlungnama moraine), and each is assigned to a distinct glacial stage named after the moraine. The Samdupo glacial stage is subdivided into Samdupo I (6.8–7.7 ka) and Samdupo II (~2.4 ka). Comparison with OSL and TCN defined ages on moraines on the southern slopes of Mount Everest in the Khumbu Himal show that glaciations across the Everest massif were broadly synchronous. However, unlike the Khumbu Himal, no early Holocene glacier advance is recognized in the Rongbuk valley. This suggests that the Khumbu Himal may have received increased monsoon precipitation in the early Holocene to help increase positive glacier mass balances, while the Rongbuk valley was too sheltered to receive monsoon moisture during this time and glaciers could not advance. Comparison of equilibrium-line altitude depressions for glacial stages across Mount Everest reveals asymmetric patterns of glacier retreat that likely reflects greater glacier sensitivity to climate change on the northern slopes, possibly due to precipitation starvation. 相似文献
96.
Bernhard Chapligin Melanie J. Leng Anne Alexandre Akira Ijiri Aldo Shemesh Ulrike Herzschuh Hanno Meyer Yusuke Okazaki Zachary D. Sharp Corinne Sonzogni Florence Sylvestre Ruth Yam 《Geochimica et cosmochimica acta》2011,75(22):7242-7256
Several techniques have been introduced in the last decades for the dehydration and release of O2 from biogenic silica (opal-A) for oxygen-isotope analysis. However, only one silica standard is universally available: a quartz standard (NBS28) distributed by the IAEA, Vienna. Hence, there is a need for biogenic silica working standards. This paper compares the existing methods of oxygen-isotope analyses of opal-A and aims to characterize additional possible working standards to calibrate the δ18O values of biogenic silica. For this purpose, an inter-laboratory comparison was organized. Six potential working standard materials were analysed repeatedly against NBS28 by eight participating laboratories using their specific analytical methods. The materials cover a wide range of δ18O values (+23 to +43‰) and include diatoms (marine, lacustrine), phytoliths and synthetically-produced hydrous silica. To characterize the proposed standards, chemical analyses and imaging by scanning electron microscopy (SEM) were also performed. Despite procedural differences at each laboratory, all methods are in reasonable agreement with a standard deviation (SD) for δ18O values between 0.3‰ and 0.9‰ (1σ). Based on the results, we propose four additional biogenic silica working standards (PS1772-8: 42.8‰; BFC: 29.0‰; MSG60: 37.0‰; G95-25-CL leaves: 36.6‰) for δ18O analyses, available on request through the relevant laboratories. 相似文献
97.
Nine tephra layers in marine sediment cores (MD99‐2271 and MD99‐2275) from the North Icelandic shelf, spanning the Late Glacial and the Holocene, have been investigated to evaluate the effectiveness of methods to detect tephra layers in marine environments, to pinpoint the stratigraphic level of the time signal the tephra layers provide, and to discriminate between primary and reworked tephra layers in a marine environment. These nine tephra layers are the Borrobol‐like tephra, Vedde Ash, Askja S tephra, Saksunarvatn ash, and Hekla 5, Hekla 4, Hekla 3, Hekla 1104 and V1477 tephras. The methods used were visual inspection, magnetic susceptibility, X‐ray photography, mineralogical counts, grain size and morphological measurements, and microprobe analysis. The results demonstrate that grain size measurements and mineralogical counts are the most effective methods to detect tephra layers in this environment, revealing all nine tephra layers in question. Definition of the tephra layers revealed a 2–3 cm diffuse upper boundary in eight of the nine tephra layers and 2–3 cm diffuse lower boundary in two tephra layers. Using a multi‐parameter approach the stratigraphic position of a tephra layer was determined where the rate of change of the parameters tested was the greatest compared with background values below the tephra. The first attempt to use grain morphology to distinguish between primary and reworked tephra in a marine environment suggests that this method can be effective in verifying whether a tephra layer is primary or reworked. Morphological measurements and microprobe analyses in combination with other methods can be used to identify primary tephra layers securely. The study shows that there is a need to apply a combination of methods to detect, define (the time signal) and discriminate between primary and reworked tephra in marine environments. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
98.
Tracing sources and cycling of phosphorus in Peru Margin sediments using oxygen isotopes in authigenic and detrital phosphates 总被引:4,自引:0,他引:4
Many (bio)geochemical processes that bring about changes in sediment chemistry normally begin at the sediment-water interface, continue at depth within the sediment column and may persist throughout the lifetime of sediments. Because of the differential reactivity of sedimentary phosphate phases in response to diagenesis, dissolution/precipitation and biological cycling, the oxygen isotope ratios of phosphate (δ18OP) can carry a distinct signature of these processes, as well as inform on the origin of specific P phases. Here, we present results of sequential sediment extraction (SEDEX) analyses combined with δ18OP measurements, aimed at characterizing authigenic and detrital phosphate phases in continental margin sediments from three sites (Sites 1227, 1228 and 1229) along the Peru Margin collected during ODP Leg 201. Our results show that the amount of P in different reservoirs varies significantly in the upper 50 m of the sediment column, but with a consistent pattern, for example, detrital P is highest in siliciclastic-rich layers. The δ18OP values of authigenic phosphate vary between 20.2‰ and 24.8‰ and can be classified into at least two major groups: authigenic phosphate precipitated at/near the sediment-water interface in equilibrium with paleo-water oxygen isotope ratios (δ18Ow) and temperature, and phosphate derived from hydrolysis of organic matter (Porg) with subsequent incomplete to complete re-equlibration and precipitated deeper in the sediments column. The δ18OP values of detrital phosphate, which vary from 7.7-15.4‰, suggest two possible terrigenous sources and their mixtures in different proportions: phosphate from igneous/metamorphic rocks and phosphate precipitated in source regions in equilibrium with δ18Ow of meteoric water. More importantly, original isotopic compositions of at least one phase of authigenic phosphates and all detrital phosphates are not altered by diagenesis and other biogeochemical changes within the sediment column. These findings help to understand the origin and provenance of P phases and paleoenvironmental conditions at/near the sediment-water interface, and to infer post-depositional activities within the sediment column. 相似文献
99.
Carbonation and decarbonation of eclogites: the role of garnet 总被引:3,自引:0,他引:3
Ruth Knoche Russel J. Sweeney Robert W. Luth 《Contributions to Mineralogy and Petrology》1999,135(4):332-339
Carbonates are potentially significant hosts for primordial and subducted carbon in the Earth's mantle. In addition, the
coexistence of carbonate with silicates and reduced carbon (diamond or graphite), allows constraints to be placed on the oxidation
state of the mantle. Carbonate-silicate-vapor reactions control how carbonate + silicate assemblages may form from carbon-bearing
vapor + silicate assemblages with increasing pressure. In olivine-bearing rocks such as peridotite, considered the dominant
rock type in the upper mantle, the lowest-pressure carbonate-forming reactions involve olivine (±clinopyroxene) reacting with
CO2 (e.g., Wyllie et al. 1983). In eclogitic rocks, the essential mineral assemblage is omphacitic clinopyroxene + garnet, without
olivine. Therefore, alternative carbonate-forming reactions must be sought. The carbonation of clinopyroxene via the reaction
dolomite + 2 coesite = diopside + 2 CO2 was studied experimentally by Luth (1995). The alternative possibility that garnet reacts with CO2 is explored here by determining the location of the reaction 3 magnesite + kyanite + 2 coesite = pyrope + 3 CO2 between 5 and 11 GPa in multi-anvil apparatus. At the temperatures ≥1200 °C, carbonation of eclogitic rocks with increasing
pressure will proceed initially by reaction with clinopyroxene, because the pyrope-carbonation reaction lies at higher pressures
for a given temperature than does the diopside-carbonation reaction. Diluting the pyrope component of garnet and the diopside
component of clinopyroxene to levels appropriate for mantle eclogites does not change this conclusion. At lower temperatures,
appropriate for “cold” slabs, it is possible that the converse situation will hold, with initial carbonation proceeding via
reaction with garnet, but this possibility awaits experimental confirmation. Decarbonation of an eclogite under “normal mantle”
geothermal conditions by a decrease in pressure, as in an ascending limb of a mantle convection cell, would be governed by
the formation of clinopyroxene + CO2. At higher pressure than this reaction, any CO2 produced by the breakdown of magnesite reacting with kyanite and coesite would react with clinopyroxene to produce dolomite + coesite.
Release of CO2 from eclogite into mantle peridotite would form carbonate at sub-solidus conditions and produce a dolomitic carbonate melt
if temperatures are above the peridotite-CO2 solidus.
Received: 4 May 1998 / Accepted: 23 December 1998 相似文献
100.
Following the well-established literature on women’s fear in urban contexts, a small but important literature has also begun to document accounts of boldness, fearlessness and empowerment. We extend this work by considering ways in which women live with, and beyond, experiences of fear. We argue that fear and fearlessness are not discrete and separate states, but rather they are often simultaneous conditions that women negotiate in complex ways. Moving away from a sense of victims and passivity, we suggest that women have spatial and social strategies that can be adopted when they face fear or take up forms of action that might be termed ‘bold’ or ‘courageous’. Consequently, this work draws on Koskela’s [Gender, Place and Culture 4 (1997) 301] previous discussion of ‘bold women’ in Finland to develop a notion of agency and highlight strategies that some rural women adopt in New Zealand. 相似文献