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1.
In Poland, loess mainly occurs in the southern part of the country, in the Polish Uplands, and on the northern foreland of the Sudetes and the Carpathians. Most of these sediments come from the time of the last glaciation–the Vistulian (Weichselian) glaciation. Thin covers of loess from the penultimate glaciation have been preserved in Upper Silesia. Only a few sites from the Middle Palaeolithic remain in the loess. In the eastern part of Poland loess creates profiles several metres high. Discoveries in this zone represent the cultural sequence from the transition from the Middle to Upper Palaeolithic. We know more traces of various cultures from the Upper Palaeolithic exist. The study attempts to synchronise profiles from selected archaeological open-air sites, outlines of the history of research on loess sites, and attempts to explain terminological and conceptual issues. The issue of loess in the cave site profiles will not be discussed in the article.  相似文献   

2.
A thin mantle of cover-loam over much of North-East Essex has been recognised as consisting largely of loess. The cover-loam represents the intermixing, to a varying degree, of a layer of loessial silt with a thin layer of underlying sand which is also of likely aeolian origin. The heavy mineral content of the coarse silt fraction of North-East Essex loess is generally similar to that of last glaciation age loesses elsewhere in Eastern England, Belgium and the Netherlands. This suggests the North-East Essex loess is part of a single loess sheet deposited over Eastern England and parts of Western Europe. Nevertheless, detailed examination of the heavy minerals content from all of these areas reveals slight areal variations especially in the proportion of hornblende. These differences show the coarse silt from North-East Essex loess to have closest affinities with that from Norfolk. A distant source for the loess within the present North Sea Basin is proposed on textural evidence. Loess accumulation in North-East Essex probably commenced in the few thousand years leading up to the maximum extent of Devensian ice (about 18,000 years B.P.) and may have continued to about 14,000 years B.P.  相似文献   

3.
Geochemical and isotopic studies of aeolian sediments in China   总被引:5,自引:0,他引:5  
The Sr and Nd isotopic, rare earth element (REE) and major element compositions, together with mineral and grain‐size proportions, are reported for aeolian loess deposits and desert sands from several Chinese localities. The study was carried out in order to examine regional variations in the isotopic and geochemical features of these aeolian sediments, and to constrain the provenance of Chinese loess. Samples include loesses from the Tarim and Junggar basins and desert sands from the Taklimakan desert in north‐west China, loess from the Ordos area and desert sands from the Tengger and Mu‐us deserts in north‐central China, as well as loess and desert sands from the Naiman area, north‐east China. REE distributions show minimal variation among the Chinese loess deposits, whereas those for the desert sands show regional variations. New isotopic data document a latitudinal variation in Sr and Nd isotopic features for the loesses and desert sands. The Naiman and Junggar loesses have distinctly lower 87Sr/86Sr ratios and higher εNd(0) values than the loesses from the Tarim Basin, the Ordos area and the Loess Plateau. Among the desert sands, the Naiman samples have higher εNd(0) values than the Taklimakan, Tengger and Mu‐us samples. Isotopic data suggest that loesses of the Loess Plateau were supplied from the Tarim Basin loesses and Taklimakan Desert sand, and that the Naiman loesses were supplied from the Junggar Basin loesses. The latitudinal variation in the loesses and desert sands may be partly explained by isotopic variations reported previously for moraines from the Tianshan and west Kunlun Mountains, which are possible sources for the loesses and desert sands. These inferences on the provenance of the loesses and desert sands are consistent with the dust transport pattern over East Asia.  相似文献   

4.
《Earth》2009,94(3-4):47-76
At least six glaciations are purported to have affected North Africa and the Middle East region over the last one billion years, including two in the Cryogenian (Neoproterozoic), Hirnantian (Late Ordovician), Silurian, Carboniferous and Early Permian events. The sedimentary record associated with these glaciations, together with the intensity to which each has been investigated, is highly variable. As hydrocarbon exploration proceeds aggressively across the North Africa and Middle East regions, we review the relationship between glaciation and hydrocarbon accumulations.With the exception of Oman, and locally Egypt, which were tectonically active both during the Neoproterozoic and Early Palaeozoic all glaciations took place along an essentially stable passive continental margin. During the Neoproterozoic, two glaciations are recognised, referred to as older and younger Cryogenian glaciations respectively. Both of these Cryogenian events are preserved in Oman; only the younger Cryogenian has been reported in North Africa in Mauritania and Mali at the flanks of the Taoudenni Basin. The process of initial deglaciation in younger Cryogenian glaciations resulted in incision, at least locally producing large-bedrock palaeovalleys in Oman, and the deposition of glacial diamictites, gravels, sandstones and mudstones. As deglaciation progressed “cap carbonates” were deposited, passing vertically into shale with evidence for deposition in an anoxic environment. Hence, younger Cryogenian deglaciation may be associated with hydrocarbon source rock deposits.Hirnantian (Late Ordovician) glaciation was short lived (< 0.5 Myr) and affected intracratonic basins of Mauritania, Morocco, Algeria, Libya, Egypt and Saudi Arabia. The organisation of the glacial sedimentary record is considered to be controlled at the basin-scale by the location of fast-flowing ice streams active during glacial maxima, and by the processes of meltwater release during glacial recession. In these latter phases, subglacial tunnel valley networks were cut at or near the ice margin. These tunnel valleys were filled in two main phases. The initial phase was characterised by debris flow release, whereas during later phases of ice retreat a range of glaciofluvial, shallow glaciomarine to shelf deposits were laid down, depending on the water depth at the ice front. Production of linear accumulations of sediment, parallel to the ice front, also occurred between tunnel valleys at the grounding line. In Arabia, the geometry of these features may have been influenced by local tectonic uplift. As glaciogenic reservoirs, Hirnantian deposits are already of great economic significance across central North Africa. Therefore, an appreciation of the processes of ice sheet growth and decay provides significant insights into the controls on large-scale heterogeneities within these sediments, and in analogue deposits produced by glaciations of different ages.Deglacial, Early Silurian black shale represents the most important Palaeozoic source rock across the region. Existing models do not adequately explain the temporal and spatial development of anoxia, and hence of black shale/deglacial source rocks. The origins of a palaeotopography previously invoked as the primary driver for this anoxia is allied to a complex configuration of palaeo-ice stream pathways, “underfilled” tunnel valley incisions, glaciotectonic deformation structures and re-activation of older crustal structures during rebound. A putative link with the development of Silurian glaciation in northern Chad is suggested. Silurian glaciation appears to have been restricted to the southern Al Kufrah Basin in the eastern part of North Africa, and was associated with the deposition of boulder beds. Equivalent deposits are lacking in shallow marine deposits in neighbouring outcrop belts.Evidence for Carboniferous–Permian glaciation is tentative in the eastern Sahara (SW Egypt) but well established on the Arabian Peninsula in Oman and more recently in Saudi Arabia. Pennsylvanian–Sakmarian times saw repeated glaciation–deglaciation cycles affecting the region, over a timeframe of about 20 Myr. Repeated phases of deglaciation produced a complex stratigraphy consisting, in part, of structureless sandstone intervals up to 50 m thick. Some of these sandstone intervals are major hydrocarbon intervals in the Omani salt basins. Whilst studies of the Hirnantian glaciation can provide lessons on the causes of large-scale variability within Carboniferous–Permian glaciogenic reservoirs, additional factors also influenced their geometry. These include the effects of topography produced during Hercynian orogenesis and the mobilisation and dissolution of the Precambrian Ara Salt. Deglacial or interglacial lacustrine shale, with abundant palynomorphs, is also important. Whilst both Cryogenian intervals and the Hirnantian–Rhuddanian deglaciation resulted in the deposition of glaciomarine deposits, Carboniferous–Permian deglaciation likely occurred within a lacustrine setting. Hence, compared to shales of other glacial epochs, the source rock potential of Carboniferous–Permian deglacial deposits is minimal.  相似文献   

5.
Dust transport and the question of desert loess formation   总被引:24,自引:0,他引:24  
H. TSOAR  K. PYE 《Sedimentology》1987,34(1):139-153
Although contemporary dust storms are frequent in arid and semi-arid areas, desert loess deposits are poorly developed. Much of the World's loess occurs in mid-latitude areas which experienced glaciation during the Pleistocene. Ocean core evidence indicates that dust transport from sub-tropical deserts increased during cold stages of the Pleistocene, but loess formed only on certain desert margins, for reasons which have not been fully explained. This paper re-examines the mechanisms of dust transport and deposition, and the circumstances leading to the accumulation of thick loess. Typical loess is composed mainly of medium silt grains which are transported in short-term suspension a few metres above the ground. Significant thicknesses of loess form only when dust is trapped within a limited area, often relatively close to the source. Dust particles finer than 20 μm are transported mainly in long-term suspension over a greater height range and may be widely dispersed. The availability of silt and the frequency, magnitude and direction of dust-transporting winds are important factors governing the potential for loess formation, but the existence of a suitable dust trap is particularly important. Traps may be formed by topographic obstacles, areas of moist ground, or vegetated surfaces. Vegetation adjacent to glacial outwash plains and braided meltwater streams trapped dust in mid-latitudes during the Pleistocene. Dust blown during glacial periods from certain deserts, notably in Sinai, Soviet Central Asia and China, accumulated as loess in neighbouring semi-arid regions. On the margins of other deserts loess formation was inhibited partly by the absence of vegetation traps. During most of the Holocene net dust deposition rates in all desert-marginal areas have been too low for significant loess accumulation. This is mainly due to a reduction in silt availability and a tendency towards landscape stability. Reported dust storm frequencies during the past 50 years over-estimate the longer-term Holocene dust flux due to the effects of human activities. Much modern dust owes its origin to erosion of cultivated soils in semi-arid areas and is finer than typical loess.  相似文献   

6.
Along the northern coasts of Ellesmere Island, at least two glaciations are recognized on the basis of morphostratigraphy. The early Holocene ice limit lay only 5 to 60 km beyond present glaciers due to constraints imposed by aridity and calving. This limited ice advance likely extended beyond any Wisconsinan glacial limit. Marine limits established during, retreat from the last glacial maximum reach 148 m a.s.l. In contrast, earlier, more extensive glaciations inundated the coastlines and are associated with former relative sea levels now reaching 286 m a.s.l. Correlation of these pre-Wisconsinan glaciations is based upon amino acid ratios. However, this approach is severely limited by slow rates of racemization, a lack of in situ samples, and complex thermal histories owing to multiple transgressions. Models favoring extensive regional glaciation of northern Ellesmere Island and Greenland must include a glacioclimatic scenario recognizing the constraint that aridity places on glaciation. We suggest that the large ice volume associated with the oldest recognized glaciation relates to a period of reduced sea-ice cover, possibly >400,000 BP, and may correlate with an interglacial stage of the marine oxygen isotope record.  相似文献   

7.
It has been suggested that during the last glaciation the Innuitian Ice Sheet existed over the eastern Queen Elizabeth Islands. This is based on the pattern of postglacial emergence over this area and the timing of driftwood penetration into the interisland channels. Alternative interpretations of both sets of data raise questions about the presence of the Innuitian Ice Sheet at this time. Field observations on northeastern Ellesmere Island, plus additional data pertaining to the presence of multiple tills and “old” radiometric dates on lacustrine deposits, shelly tills, and raised marine features suggest that the maximum glaciation over this region, equivalent to the Innuitian Ice Sheet, predates the last glaciation, Palaeoclimatic conditions are also discussed in relation to these data. It is suggested that during the last glaciation of the Queen Elizabeth Islands there was a convergent but not coalescent advance of the existing upland ice-fields. This noncontiguous ice cover over the Queen Elizabeth Islands is termed the Franklin Ice Complex. It is suggested that the term Innuitian Ice Sheet be reserved for contiguous older glaciations over this same area.  相似文献   

8.
The late Cenozoic deposits of central Yukon contain numerous distal tephra beds, derived from vents in the Wrangell Mountains and Aleutian arc–Alaska Peninsula region. We use a few of these tephra beds to gain a better understanding on the timing of extensive Pleistocene glaciations that affected this area. Exposures at Fort Selkirk show that the Cordilleran Ice Sheet advanced close to the outer limit of glaciation about 1.5 myr ago. At the Midnight Dome Terrace, near Dawson City, exposed outwash gravel, aeolian sand, and loess, related to valley glaciers in the adjacent Ogilvie Mountains, are of the same age. Reid glacial deposits at Ash Bend on the Stewart River are older than oxygen isotope stage (OIS) 6 and likely of OIS 8 age, that is, about 250,000 yr B.P. Supporting evidence for this chronology comes from major peaks in the rates of terrigeneous sediment input into the Gulf of Alaska at 1.5 and 0.25 myr B.P.  相似文献   

9.
Don Hallett 《Geology Today》2002,18(5):182-185
Just a few kilometres south of Cracow, in southern Poland, the scenery changes abruptly from the fluvial plain of the Vistula river, mantled by tills and loess deposits of the Pleistocene glaciation, to a boldly undulating topography representing the outermost thrust sheet of the Carpathian mountains. These northern foothills of the Carpathians are known in Poland as the Beskid Mountains. The Carpathian thrust front can be traced from Upper Silesia through Cracow and Przemysl in southern Poland as far as the Ukraine and Romania. At several places along the thrust front, salt deposits have been found, which have been mined over many centuries. One of the best known mines is located at Wieliczka, 15 km south-east of Cracow. The mine is a popular tourist location, but it also illustrates some spectacular geological features.  相似文献   

10.
The relief of the expedition area in NW-Sichuan and SE-Qinghai can simply be separated into two main areas: the catchment area of the Yangtze with deeply incised rivers in narrow V-shaped valleys up to about 3,000 m and the plateau area with old peneplains in the Huang He catchment with meandering rivers up to more than 4,000 m. Precipitation and temperature decrease towards the NW. All zonal boundaries, eg the vegetation boundaries or the snowline, are ascending in elevation towards the interior of the plateau. The natural vegetation, especially the forests, are modified by anthropo-zoogenic influences. Within the zone of alpine meadows and the uppermost region of natural forest as well as in the Red Basin evidence of pasture and agriculture goes back by at least 2,000 years. During the last 20 years within the transition zone of the deeply incised valleys towards the plateau one third of the forests have been cut down.The main investigation area of Nianbaoyeze is built up by a granite-dome being about 500 to 800 m higher than the surrounding peneplains. The extent of Pleistocene glaciations can be mapped by the distribution of terminal and lateral moraines and erratic granite boulders on the slopes of the surrounding schists. The altitude of terminal moraines indicates a snowline of the last glaciation at about 4,300 m, which is about 800 m below the present snowline. Three main Pleistocene glaciations can be separated due to palaeosols, the overlying stratum being either aeolian sandy silt or solifluction debris. The ice free areas shows several Pleistocene terraces and in addition loess deposits with palaeosols in the deeper basins. In summary there is a rising of the snowline of the last glaciation at the E margin of the Tibetan Plateau from about 4,000 m in the E (Minshan) to 4,300 m at Nianbaoyeze and to about 4,700 m in the W (Bayan Har Shan). The hypothesis of an extensive plateau glaciation could not be verified for this region.  相似文献   

11.
最后4个冰期旋回中国黄土记录的东亚冬季风变化   总被引:6,自引:0,他引:6  
开展高分辨率的黄土记录研究, 是揭示过去气候变迁和变率的主要途径.选择黄土高原中部的灵台和赵家川剖面, 对L5以上的黄土-古土壤序列进行了石英颗粒的提取和粒度分析.结果表明, 石英粗颗粒(> 43μm) 体积分数变化揭示了最后4个冰期旋回东亚冬季风存有的快速变化特征, 并同深海氧同位素记录具有良好的对应关系; 然而, 在每一个冰期时段, 石英粗颗粒体积分数的变化幅度和频率存有较大差异, 可能意味着对应的冰期下垫面状况和气候系统内部的非线性响应机制并不相同.值得注意的是由石英粗颗粒体积分数反映的东亚冬季风变化, 在某些时段不仅同磁化率记录的夏季风变化不协调, 而且同深海氧同位素记录存有明显差异.深入研究这种由不同代用指标反映的古气候变化间的差异, 对理解黄土记录的全球性意义显得尤为重要.   相似文献   

12.
祁连山冷龙岭南坡晚第四纪冰川演化序列   总被引:11,自引:4,他引:7  
康建成  朱俊杰 《冰川冻土》1992,14(4):352-359
  相似文献   

13.
Many researchers consider loess as solely the product of glacial activity. Investigations throughout South America, however, have demonstrated that loess and loess-like sediments can be formed during geological periods when glaciation was absent. Loess deposits may be derived from volcanic action, from weathering processes, and as a result of pedogenic activity. Production of silt, regardless of the geologic process, can result in loess formation under suitable climatic conditions.  相似文献   

14.
The late Middle Pleistocene fluvial terrace sequence of the lower Trent system, Lincolnshire (eastern England), provides an excellent record of environmental change, including evidence for the last two interglacial episodes. It also provides important stratigraphical evidence for the timing and extent of three separate glaciations. Two of these, the Anglian and Devensian, are well-established correlatives of Marine Isotope Stages (MIS) 12 and 2 respectively; the third is a hitherto un-named post-Anglian-pre-Devensian glaciation in eastern England that has been the subject of much previous speculation, but can now be attributed with some confidence to MIS 8. Crucially, the recognition of MIS 7 interglacial deposits within the Balderton-Southrey terrace of the proto-Trent indicates that the underlying Wragby Till, which is ascribed to this additional glaciation, was emplaced no later than MIS 8. The oldest terrace preserved within the lower Trent staircase, the Eagle Moor-Martin Terrace, is considered to be a complex glacial outwash terrace related to the Wragby Till glaciation. It is suggested that deposits representing MIS 11-9, which are conspicuously absent throughout the Trent system, were removed by this glaciation. This is a departure from previous interpretations, which have suggested MIS 10 or MIS 6 as the most likely stages in which an extensive post-Anglian-pre-Devensian lowland glaciation might have occurred in Britain. However, the widespread preservation of undisrupted post-MIS 8 fluvial sequences throughout the Trent valley and in neighbouring systems, within which MIS 7 interglacial deposits have now been recognized at a number of localities, indicates that ice sheets are unlikely to have advanced further into this catchment during MIS 6 than during the Devensian (MIS 2). Recognition of a British glaciation during MIS 8 corresponds with widespread evidence in Europe, which suggests that glacial deposits classified as ‘Saalian’ represent both MIS 8 and MIS 6; in many areas, distinguishing these remains controversial, as confident correlation with either stage is often only possible where glacial sediments interdigitate with well-constrained fluvial records.  相似文献   

15.
The Suru, Doda and Zanskar river valleys in the semi-arid region of Southern Zanskar Ranges (SZR) preserve a rich repository of the glacial and fluvial landforms, alluvial fans, and lacustrine deposits. Based on detailed field observations, geomorphic mapping and limited optical ages, we suggest four glaciations of decreasing magnitude in the SZR. The oldest Southern Zanskar Glaciation Stage (SZS-4) is inferred from glacially polished bedrock and tillite pinnacles. The SZS-4 is ascribed to the Marine Isotopic Stage (MIS)-4/3. The subsequent SZS-3 is represented by obliterated and dissected moraines, and is assigned to MIS-2/Last Glacial Maximum. The multiple recessional moraines of SZS-2 glaciation are assigned the early to mid Holocene age whereas, the youngest SZS-1 moraines were deposited during the Little Ice Age. We suggest that during the SZS-2 glaciation, the Drang-Drung glacier shifted its course from Suru Valley (west) to the Doda Valley (east). The study area has preserved three generations of outwash gravel terraces, which broadly correlate with the phases of deglaciation associated with SZS-3, 2, and 1. The alluvial fan aggradation, lacustrine sedimentation, and loess deposition occurred during the mid-to-late Holocene. We suggest that glaciation was driven by a combination of the mid-latitude westerlies and the Indian Summer Monsoon during periods of cooler temperature, while phases of deglaciation occurred during enhanced temperature.  相似文献   

16.
The loamy ?brickearth’? deposits overlying coarse cold climate Kempton Park, Lynch Hill and Taplow Gravels of the R. Thames in west London are dated by thermoluminescence and mineralogical comparisons with loesses of three known ages. Although much of the ?brickearth’? resembles loess, it often contains more sand or clay than typical loess, and the mineralogical studies suggest that even the coarse silt (16–63 μm) fraction is rarely similar in composition to any known English loess. The thermoluminescence measurements suggest that most of the ?brickearth’? on the Taplow and Lynch Hill Gravels was deposited in the late Devensian, whereas that on the Park Gravel is mainly Flandrian. It is suggested that the latter is mainly floodloam, whereas the earlier ?brickearth’? is a mixed aeolian sediment derived from various local sources in and close to the Thames Valley.  相似文献   

17.
刘亮  翟羿杰  张威  初智豪 《冰川冻土》2022,44(4):1283-1297
对北美洲晚第四纪冰川10Be暴露年代进行汇编与聚集程度置信等级划分,构建了北美洲各区域的冰川演化序列,并进一步对比和分析了冰川演化序列对高分辨率气候事件的响应情况,使用较好和中等聚集程度的暴露年代漂砾组对北美洲各区域进行冰期划分。结果表明:北美洲冰川序列横跨6个深海氧同位素阶段(marine isotope stage,MIS),保守估计至少在MIS 6/5、MIS 4/3、MIS 2时期出现了规模性冰进,其记录可信区间为约150 ka至今。末次冰盛期(Last Glacial Maximum,LGM)之前的冰川演化存在区域性,这可能与劳伦泰德冰盖的大气效应存在一定的相关性。LGM的冰川达到最大范围的时间不同步,揭示了不同地区冰川演化的影响因素不尽相同。LGM以来的冰川作用对于高分辨率气候事件,如海因里希事件1(Heinrich Stadial 1,HS1)、B-A事件(B?lling-Aller?d,B-A)、新仙女木事件(Younger Dryas,YD)的响应存在同步性。对于全新世更高分辨率的邦德事件(Bond),北美洲的冰川10Be暴露年代同样具有很好的对应关系。  相似文献   

18.
J. Šajgalik 《GeoJournal》1991,24(2):165-174
The Slovak Carpathians in Pleistocene presented a very articulate territory, so that loesses, as well as loess-like deposits could occur here with various lithofacial types.Based on a complex evaluation of a great number of loess samples of the Danubian lowland and their analysis by methods of mathematical statistics, it was possible to create a realistic model of the base soil. For the statistical processing we picked 4 selective sets of the Danubian lowland hilly country, where each set can be considered as a homogeneous engineering geological type.By comparing the loess properties of the Danubian lowland with the loesses of other European countries (mainly USSR), it can be stated that the studied loesses are not so dangerous from the sagging standpoint. The increase of the oedometric modulus with the depth was shown. Similar relationships were derived also for the parametres of shear strength.The regional characteristics of the loess mechanical properties will serve well in the initial phases of investigation, planning and designing, but also in more exact geoo technical calculations.  相似文献   

19.
青藏高原东缘晚新生代地质特征与古环境变化   总被引:2,自引:0,他引:2  
青藏高原东缘地区发育了完整的晚新生代湖相、黄土、红土和冰川沉积系列,不同成因的沉积物记录了晚新生代以来古环境时空变迁的信息。上新世昔格达湖相沉积发育于4.2~2.6MaBP,具有9个冷—暖气候环境变化阶段。川西风尘堆积始于1.15MaBP,连续记录了14个古季风变化旋回,成都平原红土记录了1.13Ma来的5个古环境演化阶段。青藏高原东缘发育了约4.3MaBP的老冰期,第四纪时期出现了5次极端古气候事件,对应为5期冰期。  相似文献   

20.
The application of both thermoluminescence and infrared stimulated luminescence dating to the extensively studied “classical” Hungarian loess/paleosol sequences from Basaharc, Mende, and Paks provides a reliable chronological framework and climatostratigraphic reconstruction for the last interglacial/glacial cycle. Based on this combined luminescence dating study a new chronology is proposed for the “Young Loess” in Hungary. Luminescence dating suggests that the loess below the MF2 horizon formed during the penultimate glaciation. The MF1 horizon probably formed during an interstade within oxygen isotope stage 3. For the youngest loess, overlying MF1, a very high accumulation rate was determined. Large time gaps occur above MF2 and MF1, indicating that most of the record of the last glaciation is missing in the standard sections at Basaharc, Mende, and Paks. Either large discontinuities or a very low accumulation rate occurred in all three type sections during the soil-forming periods. High-resolution studies of climatic proxies using this combined luminescence dating approach provide a reliable chronological framework for loess and loess derivatives of the last glacial cycle in Hungary, although a precise and complete chronostratigraphic reconstruction cannot be achieved from the incomplete records found at these sites.  相似文献   

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