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Pyroclastic deposits exposed in the caldera walls of Santorini Volcano (Greece), contain several prominent horizons of coarse-grained andesitic spatter and cauliform volcanic bombs. These deposits can be traced around most of the caldera wall. They thicken in depressions and are intimately associated with ignimbrite and co-ignimbrite lithic lag breccias. They are interpreted as a proximal facies of pyroclastic flow deposits. Evidence for a flow origin includes the presence of a fine-grained pumiceous matrix, flow deformation of ductile spatter clasts, exceedingly coarse grain sizes several kilometres from any plausible vent, imbrication of flattened spatter clasts, intimate interbedding with normal pyroclastic flow deposits and the presence of inversely graded basal layers. The deposits contain hydrothermally altered, rounded lithic ejecta including gabbro nodules. The andesitic ejecta and the fine matrix are typically moderately to poorly vesicular indicating that magmatic gas had a subordinate role in the eruptive process. The andesitic clasts contain abundant angular lithic inclusions and some clasts are themselves formed of pre-existing agglutinate. We propose that these eruptions occurred when external water gained access to the vents, causing large-scale explosions which formed pyroclastic flows rich in coarse, semifluid but poorly vesicular ejecta. We postulate that large volumes of coarse pyroclastic ejecta and degassed lava accumulated in a deep crater prior to being disrupted by these large explosions to form pyroclastic flows. 相似文献
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CL de Moor SJ Johnston A Brandão RA Rademeyer JP Glazer LB Furman 《African Journal of Marine Science》2015,37(3):285-311
The waters off South Africa's coastline boast a rich mix of commercially fished species. Quantitative assessments of these marine resources have developed from simple methods first applied in the 1970s, to models that encompass a wide range of methodologies. The more valuable resources have undergone regular assessments in recent decades, with frequencies closely related to the management approach employed for each fishery. Many of these assessments form the operating models used to simulation-test candidate management procedures. This paper provides a comprehensive review of the assessments of 11 of the most important fisheries resources in South Africa. Some assessments use simple biomass dynamics models, whereas others are a hybrid of age- and length-based models, each designed to model the specific characteristics of the resource and fishery concerned. Many of the assessments have been disaggregated by species/stock and/or area as related multispecies/stock/ distribution hypotheses have arisen. This paper explores the similarities and differences in the data available and the methods applied. The review indicates that, whereas the status of three of these resources cannot be estimated reliably at present, the status of six resources is considered to be reasonable to good, whereas that of abalone Haliotis midae and West Coast rock lobster Jasus lalandii remains poor. 相似文献
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RAÚL DE LA HORRA Ma ISABEL BENITO JOSE LÓPEZ‐GÓMEZ ALFREDO ARCHE JOSÉ F. BARRENECHEA JAVIER LUQUE 《Sedimentology》2008,55(6):1849-1873
The Late Permian (Wuchiapingian) Alcotas Formation in the SE Iberian Ranges consists of one red alluvial succession where abundant soil profiles developed. Detailed petrographical and sedimentological studies in seven sections of the Alcotas Formation allow six different types of palaeosols, with distinctive characteristics and different palaeogeographical distribution, to be distinguished throughout the South‐eastern Iberian Basin. These characteristics are, in turn, related to topographic, climatic and tectonic controls. The vertical distribution of the palaeosols is used to differentiate the formation in three parts from bottom to top showing both drastic and gradual vertical upwards palaeoenvironmental changes in the sections. Reconstruction of palaeoenvironmental conditions based on palaeosols provides evidence for understanding the events that occurred during the Late Permian, some few millions of years before the well‐known Permian‐Triassic global crisis. 相似文献
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In Malaysia, the main land change process is the establishment of oil palm plantations on logged‐over forests and areas used for shifting cultivation, which is the traditional farming system. While standing carbon stocks of old‐growth forest have been the focus of many studies, this is less the case for Malaysian fallow systems and oil palm plantations. Here, we collate and analyse Malaysian datasets on total carbon stocks for both above‐ and below‐ground biomass. We review the current knowledge on standing carbon stocks of 1) different forest ecosystems, 2) areas subject to shifting cultivation (fallow forests) and 3) oil palm plantations. The forest ecosystems are classified by successional stage and edaphic conditions and represent samples along a forest succession continuum spanning pioneer species in shifting cultivation fallows to climax vegetation in old‐growth forests. Total carbon stocks in tropical forests range from 4 to 384 Mg C/ha, significantly wider than the range of total carbon stocks of oil palm plantations, 2 to 60 Mg C/ha. Conversion of old‐growth forest areas to oil palm plantations leads to substantial reduction in carbon storage, while conversion of forest fallows to oil palm plantations may sustain or even increase the standing carbon stock. 相似文献
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A re-evaluation of the threat status of New Zealand's marine invertebrates was undertaken in 2009, following earlier review of New Zealand's Threat Classification System and subsequent refinement of the national criteria for classifying threat of extinction to New Zealand's flora and fauna. Sufficient information was available to enable 295 marine invertebrate taxa to be fully evaluated and assigned to a national threat category. The 10 taxa at most risk of extinction (‘nationally critical’) were the giant seep clam Calyptogena sp., the primitive acorn barnacle Chionelasmus crosnieri, O'Shea's vent barnacle Volcanolepas osheai, the stalked barnacle Ibla idiotica, the four-blotched umbrella octopus Cirroctopus hochbergi, the roughy umbrella octopus Opisthoteuthis chathamensis, the giant squid Idioteuthis cordiformis, the large-egged polychaete Boccardiella magniovata and two gravel maggots, Smeagol climoi and Smeagol manneringi. The key threatening processes identified for marine invertebrates were fishing and land-use associated impacts such as sedimentation. We identified no taxa that had improved in threat status as a result of past or ongoing conservation management action, nor any taxa that had worsened in threat status because of known changes in their distribution, abundance or rate of population decline. We evaluated a small fraction of New Zealand's marine invertebrate fauna for their threat status. Many taxa remain ‘data deficient’ or unlisted. In addition to the most threatened taxa, we recommend these taxa and their habitats as priorities for further survey and monitoring. 相似文献
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Stig Roar Svenningsen Gregor Levin Martin Rudbeck Jepsen 《Geografisk tidskrift / udgivet af Bestyrelsen for Det Kongelige danske geografiske selskab》2015,115(2):157-166
During the past century, the western hemisphere has seen a general trend of agricultural expansion on the behalf of semi-natural habitat types, such as heathlands and meadows. This has been documented in numerous studies of land use change. This trend is reflected in today’s European rural landscapes, which are dominated by intensive agriculture. However, many of these studies are based on cartographical sources, such as topographical and thematic maps, and thus prone to uncertainties regarding classification systems over time, variations in definitions of land use categories and lacking documentations of land use definitions. For this study, we conducted two change detection studies, covering the same four study areas in Denmark. The first study was based on topographic maps, and indicates a strong decline in the amount of semi-natural grassland (SNG). This was contrasted by the second study, which was based on an interpretation of aerial photos, and which indicated a much less pronounced reduction of SNGs. Year to year comparisons of these two analyses also revealed large discrepancies in the coverage of SNGs. Our results indicate that change detection studies must account for uncertainties in classifications and results should be interpreted with caution. 相似文献
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B.?G.?J.?UptonEmail author O.?T.?R?m? L.?M.?Heaman J.?Blichert-Toft F.?Kalsbeek T.?L.?Barry H.?F.?Jepsen 《Contributions to Mineralogy and Petrology》2005,149(1):40-56
The lavas of the Zig-Zag Dal Formation of eastern North Greenland constitute a Mesoproterozoic tholeiitic flood basalt succession up to 1,350 m thick, extending >10,000 km2, and underlain by a sill complex. U–Pb dating on baddeleyite from one of the sills thought to be contemporaneous with the lava extrusion, gives an age of 1,382±2 Ma. The lavas, subdivided from oldest to youngest into Basal, Aphyric and Porphyritic units, are dominantly basaltic (>6 wt.% MgO), with more evolved lavas occurring within the Aphyric unit. The most magnesian lavas occur in the Basal unit and the basaltic lavas exhibit a generalised upward decrease in Mg number (MgO/(MgO + Fe2O3T)) through the succession. All of the lavas are regarded as products of variable degrees of olivine, augite and plagioclase fractionation and to be residual after generation of cumulates in the deep crust. The basaltic lavas display an up-section fall in the ratio of light to heavy rare-earth elements (LREE/HREE) but an up-section rise in Zr/Nb, Sc, Y and HREE. The older lavas (Basal and Aphyric units) are characterised by low Nd and Hf in contrast to higher values in the younger (Porphyritic unit) lavas. The Porphyritic Unit basalts are characterised by a notable enrichment in Fe and Ti. The Zig-Zag Dal succession is inferred to reflect an increase in melt fraction in the sub-lithospheric mantle, with melting commencing in garnet–lherzolite facies peridotites and subsequently involving spinel-facies mantle at increasingly shallow depths. Melting is deduced to have occurred beneath an attenuating continental lithosphere in conjunction with ascent of a mantle plume. Lithospheric contamination of primitive melts is inferred to have diminished with time with the Porphyritic unit basalts being products of essentially uncontaminated plume-source magmas. The high iron signature may reflect a relatively iron-rich plume source. 相似文献
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