共查询到20条相似文献，搜索用时 15 毫秒
1.
B. Beddoe-Stephens J. A. Aspden T. J. Shepherd 《Contributions to Mineralogy and Petrology》1983,83(3-4):278-287
Glass (melt) inclusions in quartz, plagioclase and K-feldspar phenocrysts in Toba Tuff ignimbrites all exhibit highly evolved, rhyolitic compositions, identical to glass forming the matrix of the rocks. About 4% H2O is present, dissolved in the glass, suggesting a water saturation pressure ( \(P_{{\text{H}}_{\text{2}} {\text{O}}}\) ) of about 1 kbar. Melt compositions are consistent with phase relations for the condition \(P_{{\text{H}}_{\text{2}} {\text{O}}}\) =P total = 1 kbar. The residual rhyolitic melt formed as the result of fractional crystallisation from a more basic, possibly rhyodacitic melt, leading to the development of zoned feldspars. Water saturation in the melt probably arose as a result of this process. Melt temperatures prior to eruption and quenching were probably less than 800° C. However, hot-stage homogenisation experiments yield entrapment temperatures significantly higher (>900° C). This discrepancy is not clearly understood but indicates care must be taken in the interpretation of such experiments. Ignimbritic magmas at Toba, from pressure estimates, appear to have been erupted from about 3–4 kms depth and represent the silicic cap to a batholithic body consolidating beneath the Toba caldera. 相似文献
2.
3.
A general feedback between volcanism and climate at times of transition in the Quaternary climate record is suggested, exemplified by events accompanying the Toba eruption (74,000 yr ago), the largest known late Quaternary explosive volcanic eruption. The Toba paroxysm occurred during the δ18O stage 5a-4 transition, a period of rapid ice growth and falling global sea level, which may have been a factor in creating stresses that triggered the volcanic event. Toba is estimated to have produced between 1015 and 1016 g of fine ash and sulfur gases lofted in co-ignimbrite ash clouds to heights of at least 32 ± 5 km, which may have led to dense stratospheric dust and sulfuric acid aerosol clouds. These conditions could have created a brief, dramatic cooling or "volcanic winter," followed by estimated annual Northern Hemisphere surface-temperature decreases of 3° to 5°C caused by the longer-lived aerosols. Summer temperature decreases of 10°C at high northern latitudes, adjacent to regions already covered by snow and ice, might have increased snow cover and sea-ice extent, accelerating the global cooling already in progress. Evidence for such climate-volcanic feedback, following Milankovitch periodicities, is found at several climatic transitions. 相似文献
4.
Determination of the area density of spontaneous fission tracks (ρs) in glass shards of Toba tephra is a reliable way to distinguish between the Youngest Toba Tuff (YTT) and the Oldest Toba Tuff (OTT). The ρs values for YTT, uncorrected for partial track fading, range from 70 to 181 tracks/cm2 with a weighted mean of 108 ± 5 tracks/cm2, based on 15 samples. Corrected ρs values for YTT are in the range of 77–140 tracks/cm2 with a weighted mean of 113 ± 8 tracks/cm2, within the range of uncorrected ρs values. No significant difference in ρs exists between YTT samples collected from marine and continental depositional settings. The uncorrected ρs for OTT is 1567 ± 114 tracks/cm2 so that confusion with YTT is unlikely. 相似文献
5.
72 ka BP左右的强降温事件是末次冰期最大的气候突变事件,且同时期发生了第四纪以来最强的火山喷发——Toba火山喷发,后者在此次强降温事件中扮演着什么角色一直是古气候学界研究的热点。对此进行深入的研究,有助于全面评价火山喷发的环境效应,进一步完善末次冰期突变事件的成因机制。系统回顾深海岩芯、极地冰芯、洞穴石笋、黄土、湖泊等古气候载体的研究成果,这些记录对72 ka BP左右的强降温事件和/或Toba火山喷发进行了不同程度的描述,并从不同的角度分析二者的关系。从72 ka BP左右的强降温事件和Toba火山喷发的时间上,以及火山喷发对气候变化影响的机理上看,Toba火山喷发确实对这次强降温事件产生了实质性影响。这个影响在格陵兰冰芯记录中表现得尤为明显,并得到了气候模拟的进一步支持。然而,部分低纬海洋记录研究表明,Toba火山喷发前后气候没有发生显著变化,至少低纬地区的气候不如高纬地区变化明显。古生物研究同时显示出Toba火山喷发并未对其生存环境产生灾难性的影响,由此可见,72 ka BP左右的强降温事件驱动机制及其对Toba火山喷发的响应程度尚存争议。今后的研究应重视提高气候记录分辨率和探测Toba火山喷发证据的研究,从解剖事件的内部结构特征入手,进而建立气候模式与火山效应之间的联系,以此来明确“72 ka事件”驱动机制及其对Toba火山喷发的响应关系。 相似文献
6.
《地球科学进展》2012,27(1)
72 ka BP左右的强降温事件是末次冰期最大的气候突变事件,且同时期发生了第四纪以来最强的火山喷发———Toba火山喷发,后者在此次强降温事件中扮演着什么角色一直是古气候学界研究的热点。对此进行深入的研究,有助于全面评价火山喷发的环境效应,进一步完善末次冰期突变事件的成因机制。系统回顾深海岩芯、极地冰芯、洞穴石笋、黄土、湖泊等古气候载体的研究成果,这些记录对72 ka BP左右的强降温事件和/或Toba火山喷发进行了不同程度的描述,并从不同的角度分析二者的关系。从72 ka BP左右的强降温事件和Toba火山喷发的时间上,以及火山喷发对气候变化影响的机理上看,Toba火山喷发确实对这次强降温事件产生了实质性影响。这个影响在格陵兰冰芯记录中表现得尤为明显,并得到了气候模拟的进一步支持。然而,部分低纬海洋记录研究表明,Toba火山喷发前后气候没有发生显著变化,至少低纬地区的气候不如高纬地区变化明显。古生物研究同时显示出Toba火山喷发并未对其生存环境产生灾难性的影响,由此可见,72 kaBP左右的强降温事件驱动机制及其对Toba火山喷发的响应程度尚存争议。今后的研究应重视提高气候记录分辨率和探测Toba火山喷发证据的研究,从解剖事件的内部结构特征入手,进而建立气候模式与火山效应之间的联系,以此来明确＂72 ka事件＂驱动机制及其对Toba火山喷发的响应关系。 相似文献
7.
ROB WESTAWAY SHEILA MISHRA SUSHAMA DEO DAVID R BRIDGLAND 《Journal of Earth System Science》2011,120(3):503-530
Tephra, emplaced as a result of Pleistocene eruption of the Indonesian ‘supervolcano’ Toba, occurs at many localities in India.
However, the ages of these deposits have hitherto been contentious; some workers have argued that these deposits mark the
most recent eruption (eruption A, ca 75 ka), although at some sites they are stratigraphically associated with Acheulian (Lower
Palaeolithic) artefacts. Careful examination of the geochemical composition of the tephras, which are composed predominantly
of shards of rhyolitic glass, indicates that discrimination between the products of eruption A and eruption D (ca 790 ka)
of Toba is difficult. Nonetheless, this comparison favours eruption D as the source of the tephra deposits at some sites in
India, supporting the long-held view that the Lower Palaeolithic of India spans the late Early Pleistocene. In principle,
these tephra deposits should be dateable using the K–Ar system; however, previous experience indicates contamination by a
small proportion of ancient material, resulting in apparent ages that exceed the true ages of the tephras. We have established
the optimum size-fraction in which the material from Toba is concentrated, 53–61 μm, and have considered possible origins for the observed contamination. We also demonstrate that Ar–Ar analysis of four out
of five of our samples has yielded material with an apparent age similar to that expected for eruption D. These numerical
ages, of 809 ± 51, 714 ± 62, 797 ± 45 and 827 ± 39 ka for the tephras at Morgaon, Bori, Gandhigram and Simbhora, provide a
weighted mean age for this eruption of 799 ± 24 ka (plus-or-minus two standard deviations). However, these numerical ages
are each derived from no more than 10–20% of the argon release in each sample, which is not ideal. Nonetheless, our results
demonstrate that it is feasible, in principle, to date this difficult material using the Ar–Ar technique; future follow-up
studies will therefore be able to refine our preparation and analysis procedures to better optimize the dating. 相似文献
8.
《Quaternary Research》2013,79(2):228-241
Two fundamental issues for tephrostratigraphic work are the differentiation of primary from reworked tephra and the characterization of reworking mechanisms. We study the depositional processes of four deposits of Youngest Toba Tuff in the Lenggong valley, Malaysia. We focus on site stratigraphy, particle-size distributions, magnetic susceptibility and mineralogical associations. Reworked tephra display variable sedimentological characteristics including polymodal and unimodal, very fine to coarse-grained distributions, and variable concentrations of ash. Particle-size distributions from this study are similar to published analyses for primary deposits, demonstrating that particle size alone cannot distinguish primary from secondary tephra. The tephra sequences are associated with fluvial and colluvial deposition. Three facies are identified: flood flow, mudflow and slumping. The ash accumulated rapidly, over a period of a few days to months. In this valley the ideal site for paleoenvironmental reconstructions is Kampung Luat 3, where ash accumulated at least in two distinct phases. Despite the rapid accumulation, the Lenggong sites are not well-suited for paleoenvironmental studies of the YTT impact. The time lag between the primary deposition and the floods is unknown and the records could have been modified by site-specific characteristics. Such variables should be considered when proposing paleo-environmental reconstructions based on reworked tephra. 相似文献
9.
Christine Lane Michael Haslam Michael Petraglia Peter Ditchfield Victoria Smith Ravi Korisettar 《Quaternary Science Reviews》2011,30(15-16):1819-1824
The ～74 ka BP Youngest Toba Tuff (YTT), from the largest known Quaternary volcanic eruption, has been found for the first time as a non-visible (crypto-) tephra layer within the Billa Surgam caves, southern India. The occurrence of the YTT layer in Charnel House Cave provides the first calendrical age estimate for this much debated Pleistocene faunal sequence and demonstrates the first successful application of cryptotephrochronology within a cave sequence. The YTT layer lies ～50 cm below a major sedimentological change, which is related to global cooling around the MIS 5 to MIS 4 transition. Using this isochronous event layer the Billa Surgam Cave record can be directly correlated with other archaeological sites in peninsular India and palaeoenvironmental archives across southern Asia. 相似文献
10.
11.
J. N. Pattan M. Shyam Prasad E. V. S. S. K. Babu 《Journal of Earth System Science》2010,119(4):531-539
We have identified an ash layer in association with Australasian microtektites of ∼0.77 Ma old in two sediment cores which
are ∼450 km apart in the central Indian Ocean Basin (CIOB). Morphology and chemical composition of glass shards and associated
microtektites have been used to trace their provenance. In ODP site 758 from Ninetyeast Ridge, ash layer-D (13 cm thick, 0.73–0.75
Ma) and layer-E (5 cm thick, 0.77–0.78 Ma) were previously correlated to the oldest Toba Tuff (OTT) eruptions of the Toba
caldera, Sumatra. In this investigation, we found tephra ∼3100 km to the southwest of Toba caldera that is chemically identical
to layer D of ODP site 758 and ash in the South China Sea correlated to the OTT. Layer E is not present in the CIOB or other
ocean basins. The occurrence of tephra correlating to layer D suggests a widespread distribution of OTT tephra (∼3.6 × 107 km2), an ash volume of at least ∼1800 km3, a total OTT volume of 2300 km3, and classification of the OTT eruption as a super-eruption. 相似文献
12.
Limited effect of the Quaternary's largest super-eruption (Toba) on land mammals from Southeast Asia
The effect of the Toba super-eruption at 74 kyr on the mammals of Southeast Asia is examined. Although few Late Pleistocene sites from Southeast Asia have been described, an analysis of those which pre- and post-date Toba reveals relatively few species became extinct following the eruption. It is suggested that species survived in refugia immediately following the eruption, and that they repopulated vast areas following a probable short period (i.e. decades to century) of environmental devastation. This study suggests that mammals are more robust at coping with catastrophic events than previously acknowledged, and questions the perceived human monopoly in overcoming ecological adversity. 相似文献
13.
Nicholas J.G. Pearce John A. Westgate Guilherme A.R. Gualda Emma Gatti Ros F. Muhammad 《第四纪科学杂志》2020,35(1-2):256-271
The Youngest Toba Tuff contains five distinct glass populations, identified from Ba, Sr and Y compositions, termed PI (lowest Ba) – PV (highest Ba), representing five compositionally distinct pre-eruptive magma batches that fed the eruption. The PI–PV compositions display systematic changes, with higher FeO, CaO, MgO, TiO2 and lower incompatible element concentrations in the low-SiO2 PIV/PV, than the high-SiO2 PI–PIII compositions. Glass shard abundances indicate PIV and PV were the least voluminous magma batches, and PI and PIII the most voluminous. Pressure estimates using rhyolite-MELTS indicate PV magma equilibrated at ~6 km, and PI magma at ~3.8 km. Glass population proportions in distal tephra and proximal (caldera-wall) material describe an eruption which commenced by emptying the deepest PIV and PV reservoirs, this being preferentially deposited in a narrow band across southern India (possibly due to jet-stream and/or plinian eruption transport), and as abundant pumice clasts in the lowermost proximal ignimbrites. Later, shallower magma reservoirs erupted, with PI being the most abundant as the eruption ended, sourcing the majority of distal ash from co-ignimbrite clouds (PI- and PIII-dominant), where associated ignimbrites isolated earlier (PIV- and PV-rich) deposits. This study shows how analysis of tephra glass compositional data can yield pre-eruption magma volume estimates, and enable aspects of magma storage conditions and eruption dynamics to be described. 相似文献
14.
15.
Sacha Claire Jones 《Quaternary Research》2010,73(2):336-104
Distal deposits of rhyolitic volcanic ash from the ∼74 ka “supervolcanic” eruption of Toba, in northern Sumatra, are preserved in numerous river valleys across peninsular India. The Toba eruption is hypothesized to have resulted in climate change and the devastation of ecosystems and hominin populations. This study reports the results of the analysis of sediments and stratigraphical sequences from sites in the Jurreru and Middle Son valleys in southern and north-central India. The aim of the study is to determine the extent of palaeoenvironmental change in both valleys as a result of the ash-fall. Inferences based on evidence from the Jurreru valley are more detailed, where pre- and post-Toba palaeoenvironmental changes are divided into seven phases. The results indicate that ash-fall deposits in both valleys underwent several phases of reworking that possibly lasted for several years, indicating that ash was mobile in the landscape for a considerable period of time prior to burial. This could have enhanced and lengthened the detrimental effects of the ash on vegetation and water sources, as well as animal and hominin populations. 相似文献
16.
17.
18.
19.
Police, politics, and the right to the city 总被引：1，自引：0，他引：1
Mustafa Dikeç 《GeoJournal》2002,58(2-3):91-98
Space and spatiality have recently been at the core of debates concerning the political possibilities of the city. The arguments advanced in this paper might be seen as an attempt to contribute to these debates through a reflection on the spatiality of (in) justice, politics, and the right to the city. The case of French urban policy, with its focus on distressed urban areas, and the `suburban problem' in France are used as examples to make arguments more concrete. 相似文献
20.
E. M. Prasolov S. A. Sergeev B. V. Belyatsky E. S. Bogomolov K. A. Gruzdov I. N. Kapitonov R. Sh. Krymsky V. O. Khalenev 《Geochemistry International》2018,56(1):46-64
This paper reports the first results of a study of 11 isotope systems (3He/4He, 40Ar/36Ar, 34S/32S, 65Cu/63Cu, 62Ni/60Ni, 87Sr/86Sr, 143Nd/144Nd, 206–208Pb/204Pb, Hf–Nd, U–Pb, and Re–Os) in the rocks and ores of the Cu–Ni–PGE deposits of the Norilsk ore district. Almost all the results were obtained at the Center of Isotopic Research of the Karpinskii All-Russia Research Institute of Geology. The use of a number of independent genetic isotopic signatures and comprehensive isotopic knowledge provided a methodic basis for the interpretation of approximately 5000 isotopic analyses of various elements. The presence of materials from two sources, crust and mantle, was detected in the composition of the rocks and ores. The contribution of the crustal source is especially significant in the paleofluids (gas–liquid microinclusions) of the ore-forming medium. Crustal solutions were probably a transport medium during ore formation. Air argon is dominant in the ores, which indicates a connection between the paleofluids and the atmosphere. This suggests intense groundwater circulation during the crystallization of ore minerals. The age of the rocks and ores of the Norilsk deposits was determined. The stage of orebody formation is restricted to a narrow age interval of 250 ± 10 Ma. An isotopic criterion was proposed for the ore-bearing potential of mafic intrusions in the Norilsk–Taimyr region. It includes several interrelated isotopic ratios of various elements: He, Ar, S, and others. 相似文献