Palynological evidence for the astronomical origin of lignite-detritus sequence in the Middle Pleistocene Marathousa Member, Megalopolis, SW Greece     

M Okuda  N van VugtT Nakagawa  M IkeyaA Hayashida  Y YasudaT Setoguchi 《Earth and Planetary Science Letters》2002,201(1):143-157

The Marathousa Member, Middle Pleistocene strata in the fluvio-lacustrine Megalopolis basin, southwest Greece, displays distinct but complicated lithological cycles comprising first-order alternation of lignites and detrital muds and second-order alternation expressed by frequent intercalation of organic layers. Palynological evidence indicates that the lithological cycles are driven by the Earth’s orbital forcing. All the lignite seams yield temperate oak forest whereas the detrital beds provide semi-arid steppe mainly of Artemisia. This means that the first-order lithological cycle represents the glacial/interglacial cycle (i.e., the 100-kyr eccentricity cycle), providing a timescale of at least 350 kyr to the Marathousa Member. Pollen also detects smaller-scale climate fluctuations in many of the subordinate organic layers, with the total number of fluctuations being five in a complete lignite-detritus couplet. This means that the second-order lithological cycle reflects the 21-kyr insolation cycle. A tentative phase relation between the lithological cycles and the astronomical cycles is shown based on palynostratigraphy and electron spin resonance dating. Lacustrine environments with increased water tables are implied for the glacial periods sedimentologically, in contrast to local swamp vegetation for the interglacial periods. The subordinate organic layers were formed under intermediate environments (climate, water depth, etc.) between full glacials and interglacials.  相似文献   

试论百色手斧文化由西方扩散的可能性     

吴文祥 《地质科技情报》2002,21(2):23-28

更新世中期是全球气候和环境变化的一个重要时期，当时气候周期转型，全球冰量增加，海平面下降，哺乳动物迁徙或灭绝，这个时代也是直立人地理大扩张和手斧文化扩散的一个重要时期，这些事件在发生时间上的统一性指示它们之间的某种联系，推测更新世中期全球性的气候转变是这次手斧文化扩散的主要驱动力，中国百色盆地发现的手斧可能是西方阿舍科技术向东渗透或扩散的结果。  相似文献   

辽西凌源全身长有羽毛奔龙化石的再研究   总被引：8，自引：0，他引：8  

季强  姬书安  袁崇喜  季鑫鑫 《地学前缘》2002,9(3):57-63

对辽西凌源发现的全身长有羽毛的奔龙化石 (NGMC91) (Ji等 ,2 0 0 1)进行了再研究。根据头颅和头后骨骼的特征 ,其应归于中国鸟龙属 (SinornithosaurusXuetal.,1999)。由于研究的材料是一块处于非成年期的标本 ,虽然其有些特征明显与千禧中国鸟龙有别 ,但仍难以确定这些差异是否受个体发育的影响。因此 ,文中仍将其作为未定种 ,置于中国鸟龙属中。此外 ,文章还简要讨论了羽毛的发生和早期演化 ,认为羽毛出现的初始功能就是为了保持体温 ,与后期的飞行功能无关  相似文献   

Climatic and oceanographic changes in the Northeast Atlantic reflected by magnetic properties of sediments deposited on the Portuguese Margin during the last 340 ka   总被引：1，自引：0，他引：1  

Eva Moreno  Nicolas Thouveny  Doriane DelangheI.Nick McCave  Nick J Shackleton 《Earth and Planetary Science Letters》2002,202(2):465-480

Rock magnetic parameters measured along two giant piston cores MD95-2040 (40°34′N, 9°51′W) and MD95-2042 (37°47′N, 10°09′W) collected off the Portuguese Margin, related to other proxy-climatic data, have been used to reconstruct magnetic mineralogical changes of, in relation to environmental and climatic conditions over the North Atlantic, Western Europe and Northwest Africa during the last three climatic cycles (since isotope stage 10). Thin discrete layers containing coarse grains of titano-magnetite are associated with events of iceberg discharge during Heinrich events 1-6 [Heinrich, Quat. Res. 29 (1988) 142] that have equivalent events in isotope stages 5-8. Concentrations of fine-grained (Ti-) magnetite and hematite/goethite, varying in phase opposition, are directly linked with alternations of cold and warm climatic periods. Spectral analyses of the rock magnetic signals reveal Milankovitch periods at 100 and 41 ka, confirming the relationship with long-term climatic changes at high latitudes. The nature (Ti-magnetite) and size range of the finest ferrimagnetic fraction as well as its variation, suggest a control by deep currents carrying a colloidal/clayey fraction from remote sources (Iceland, Faeroes, mid-Atlantic Ridge). Variation of hematite/goethite contents is linked with transport by rivers and winds from the neighbouring continent. A tight correlation with the D-O cycles in Greenland ice records confirms that North Atlantic oceanic regimes and continental wind regimes were strongly influenced by millennial scale climatic changes throughout the last 350 ka.  相似文献   

Origin and palaeo‐environments of calcareous sediments in the Moshaweng dry valley,southeast Botswana     

S. Ringrose  A. B. Kampunzu  B. W. Vink  W. Matheson  W. S. Downey 《地球表面变化过程与地形》2002,27(6):591-611

Quaternary sedimentation in the Moshaweng dry valley of southeastern Botswana is evaluated on the basis of geomorphological evolution and sedimentological analyses. Stratigraphic evidence reveals an upper surface (1095 m) containing abundant sil‐calcrete, an intermediate surface (1085 m) in which sil‐calcrete underlies nodular calcrete and lower (1075 m) surface in which sil‐calcrete and nodular calcrete are interbedded. This subdivision is reflected in the geochemical composition of the sediments which show an overall trend of decreasing SiO₂ content (and increasing CaCO₃ content) with depth from the highest to the lowest surface levels. The calcretes and sil‐calcretes represent modifications of pre‐existing detrital Kalahari Group sand and basal Kalahari pebbles which thinned over a Karoo bedrock high. Modification took place during wet periods when abundant Ca⁺⁺‐rich groundwater flowed along the structurally aligned valley system. With the onset of drier conditions, water table fluctuations led to the precipitation of nodular calcretes in the phreatic layer to a depth of about 20 m. A major geochemical change resulted in the preferential silicification of the nodular calcrete deposits. Conditions for silica mobilization may be related to drying‐induced salinity and in situ geochemical differentiation brought about by pebble dissociation towards the top of the sediment pile. As calcretization and valley formation progressed to lower levels, silica release took place on a diminishing scale. Thermoluminescence dating infers a mid‐Pleistocene age for sil‐calcrete formation suggesting that valley evolution and original calcrete precipitation are much older. Late stage dissolution of CaCO₃ from pre‐existing surface calcretes or sil‐calcretes led to the formation of pedogenic case‐hardened deposits during a time of reduced flow through the Moshaweng system possibly during the upper or late Pleistocene. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

许家窑组及许家窑文化层年代问题的磁性地层学证据   总被引：5，自引：2，他引：5  

樊行昭  苏朴  Reidar LOVLIE 《地层学杂志》2002,26(4):248-252

许家窑文化遗址的发现将传统上被认为是早更新世前后的泥河湾层的年代范围推至晚更新世 ,由此建立的许家窑组被确认为我国北方晚更新世湖相沉积的标准地层。近几年对包含许家窑文化层在内的泥河湾层用高分辨率磁性地层学方法研究结果表明 ,在文化层以下 3m处发现了厚度达 6 m以上且分布连续的反向极性带 (未见底 ) ,综合分析认为 ,该反向极性带为松山反向极性带。这一发现否定了在文化层以下 4— 5 m处存在布莱克反向极性偏移的报道。根据松山反向极性时的结束时间为距今 78万年及泥河湾层的沉积速率推算 ,许家窑泥河湾层的年代范围应为早更新世晚期至中更新世 ,而许家窑文化层应为中更新世早期。由此可见 ,泥河湾层的年代范围及分组方案尚需用多种手段进一步研究才能定论  相似文献   

华北克拉通早前寒武纪基性火山作用与地壳增生   总被引：17，自引：3，他引：17       下载免费PDF全文

耿元生  万渝生  沈其韩 《地质学报》2002,76(2):199-208

大量的年代学资料表明,华北克拉通在早前寒武纪阶段有两个主要的基性火山活动时期,一期发生在2.7Ga左右,另一期发生在2.5Ga左右,代表了两期强烈的地壳增生事件。太古宙末期基性火山岩的分布、地球化学特征、基性火山岩与其他岩石的关系和组合特征表明,华北克拉通在新太古代时期,在陆块之间基性火山岩的喷溢使地壳面积增大并把原本分离的小陆块拼合到一起,造成地壳的增生。在陆块内部,地壳的增生主要通过地幔柱的方式进行,在较均匀的地壳部分主要通过基性岩浆的垫托方式使地壳增厚,部分岩浆侵位到地壳较浅部位,甚至溢出地表。这两种地壳增生方式是相辅相成的,它们的联合作用形成了太古宙末的华北古大陆。  相似文献   

Chironomid responses to long-term metal contamination: a paleolimnological study in two bays of Lake Imandra, Kola Peninsula, northern Russia     

Boris Ilyashuk  Elena Ilyashuk  Vladimir Dauvalter 《Journal of Paleolimnology》2003,30(2):217-230

Short sediment cores from two gulfs, Monche Bay and Kunchast Bay, of Lake Imandra (Kola Peninsula, northern Russia) were analysed for sediment chemistry and chironomid head capsule remains. Monche Bay has been receiving metals from the Severonikel copper-nickel smelter since the late 1930's. Kunchast Bay was selected in the remotest lake basin as an internal reference site. There were no pronounced changes in the chironomid assemblages with the beginning of slight metal contamination of Kunchast Bay. Based on the reconstructed environmental variables and chironomid assemblages, three developmental stages were distinguished from the chironomid fauna history of Monche Bay: (1) A natural development stage; (2) the early warning stage; and (3) the developing crises stage. During the first period, the changes in the chironomid fauna reflect an anthropogenically undisturbed assemblage, with Micropsectra insignilobus dominating (17–23%). The changes during the second period reflect the initial phase of anthropogenic succession associated with the beginning of metal pollution. The main species showed opposite distributional patterns in this period: the abundance of M. insignilobus decreased, whereas the abundance of Chironomus, Procladius and Sergentia coracina increased. At the same time, maximal numbers were attained for species richness (45) and Shannon-Weaver diversity (4.85) of chironomid assemblages, and the highest head capsule concentration (75 head capsules · g^–1 of dry sediment). The third period was characterized by a major shift in the faunal assemblages, from M. insignilobus to other dominant species, including Chironomus (22–44%), Procladius (10–30 %) and S. coracina (15–18%). Besides fauna changes, assemblages of the third period are distinguished by the occurrence of mouthpart deformities in Chironomus head capsules.  相似文献   

准噶尔盆地北部古近系挤压推覆构造与沉积耦合分析   总被引：5，自引：0，他引：5  

师志龙 《新疆地质》2002,20(2):115-117

通过对准噶尔盆地北部古近系沉积充填特征及古近纪-新近纪挤压推覆构造的分析，初步确定了准噶尔盆地北缘同沉积挤压推覆作用下构造与沉积的联系，讨论了的挤压背景下后生轴成矿的问题，并试图确定古近系沉积和准噶尔盆地北缘逆冲挤压断裂带的时空对应关系。  相似文献   

耕地存量临界警戒和耕地非农占用成本的警度修正方法初探   总被引：13，自引：4，他引：13  

葛向东  张侠  彭补拙  濮励杰 《地理科学》2002,22(2):166-170

城市扩张实际上是城乡交错带的耕地因子被城市土地系统不断捕获的过程，耕地地价作为城市土地系统对耕地系统感知反应的灵敏度而影响这种捕获力。耕地占用成本偏低、耕地损失造成的社会影响没有纳入市场成本，导致耕地比较效益低下，是耕地丧失的深层次原因。耕地的社会效益体现为其对粮食安全保障能力的状况，从耕地总量动态平衡的角度来看，耕地预警系统判定的耕地警度反映了这种保障能力。在耕地非农化过程中，据此可对耕地存量进行临界警戒，并同时设立警度修正系数，对耕地非农占用成本进行实时修正。耕地存量临界警戒和耕地非农占用成本的警度修正均应建立在耕地分等定级工作的基础之上。给出了耕地警度判定系数和警度修正系数的计算方法。  相似文献