首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
The distribution of quartz in the surface sediments of the Atlantic Ocean reflects derivation from continents by means of rivers, wind, ice, and coastal erosion. Enrichment of quartz thus supplied has occurred in some deep basins of especially the southern high latitudes from winnowing of finegrained clays by bottom currents. Although similar modes of quartz transport may have operated both during the Holocene and the last glacial maximum (18,000 yr B.P.), significant differences in the intensity of transport and in the locii of deposition, which are attributable to climatic variations during these times, exist in some areas of the Atlantic. In Holocene sediments of the eastern equatorial Atlantic, a band of high percent quartz exists directly off the present Saharan Desert and Sahel region and reflects the trade-wind transport of dusts from these arid and semiarid regions. During the last glacial maximum (18,000 yr B.P.), this high quartz band expanded southward by about 8° of latitude. This expansion was caused not only by the southward expansion of aridity and desert dunes but also by the southward migration of the northerly belt of trade winds during the last glaciation. Relatively high abundances and accumulation rates of quartz during the last glaciation suggest higher intensities of trade winds during that time compared to the Holocene. In the North Atlantic, the abundances of quartz in Holocene sediments are high adjacent to Greenland-Iceland and in the areas off Newfoundland-Labrador, and gradually decrease toward the central areas. The polar front and limit of sea-ice melting are at present confined to the northern part of the North Atlantic. The ice-rafting of quartz grains is, therefore, effective in the areas adjacent to Greenland and to some extent off Labrador causing high abundances in these areas. In contrast to this, during the last glaciation, the quartz abundances and accumulation rates are high in the central areas of North Atlantic around 45°N and decrease toward Greenland-Newfoundland. The migration of the polar front to as far south as 45°N and the consequent southward migration of sea-ice melting and ice-rafting during the last glaciation apparently caused this change in distribution. In addition to ice-rafting at present, wave or current reworking of relict glacial-marine detritus may have caused the high abundance of quartz in the surface sediments off Newfoundland-Labrador. In 18,000 yr B.P. sediments of the Norwegian Sea, the area of high percent quartz (>10%) is more extensive than that in Holocene sediments. This reflects the greater influence of ice-rafting or glacier activity in the sediment dispersal in the Norwegian Sea during the last glacial times.  相似文献   

2.
High‐resolution chirp sonar profiling in the northeastern Skagerrak shows acoustically stratified sediments draping a rough‐surfaced substratum. A 32 metre long sediment core retrieved from the survey area encompasses the entire Holocene and latest Pleistocene. The uppermost seismo‐acoustic units in the chirp profiles represent Holocene marine sediments. The lowermost unit is interpreted as ice‐proximal glacial‐marine sediments rapidly deposited during the last deglaciation. The end of ice‐proximal sedimentation is marked by a strong reflector, interpreted to have been formed during latest Pleistocene time as a consequence of rapid ice retreat and drastically lowered sedimentation rate. The subsequent distal glacial‐marine sediments were deposited with initially high sedimentation rates caused by an isostatic rebound‐associated sea‐level fall. Based on correlation between the core and the chirp sonar profiles using measured sediment physical properties and AMS 14C dating, we propose a revised position for the Pleistocene/Holocene boundary in the seismo‐acoustic stratigraphy of the investigated area. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

3.
A sedimentological study of Quaternary sediments from the northwestern part of the Barents Sea shows that their composition is controlled by the underlying Mesozoic bedrock and that very little sediment has been supplied from outside sources. The Quaternary sediments consist of Pleistocene glacial clays (moraines) and Holocene gravel, sand and mud, derived by erosion of the clay-rich moraines, which again have been derived from underlying Mesozoic rocks. On the shallow Spitsbergen Bank (30-100 m depth) we find a high energy facies of bioclastic carbonate sand and gravel and lag deposits of Mesozoic rock fragments from the underlying moraine. 14C-datings of the bioclastic carbonates (Molluscs and Barnacles) suggest that soft bottom conditions with Mya truncata prevailed in early Holocene time, succeeded by a hard bottom high energy environment with Barnacles in the last 2000-3000 years. This may be due to a southward movement of the oceanic polar front into the Spitsbergen Bank due to colder climate in Late Holocene (subatlantic) time, which at present day produces strong bottom currents down to 100 m depth. On the Spitsbergen Bank carbonate sedimentation has succeeded glacial sedimentation as a result of withdrawal of clastic sediment supply in Holocene time and high organic productivity because of upwelling. A similar mechanism may have been operating during earlier glaciations, i.e. in Late Precambrian time to produce an association of glacial and carbonate sediments although the biological precipitation was different at that time. In Late Precambrian time precipitation or carbonate by algaes may have occurred in colder water on the shelves due to higher saturation of carbonate in the sea water.  相似文献   

4.
Deposits within the floor of the Norwegian Basin were sampled to characterize the deposition from the Storegga Slide, the largest known Holocene‐aged continental margin slope failure complex. A 29 to 67 cm thick veneer of variable‐coloured, finely layered Holocene sediment caps a homogeneous, extremely well‐sorted, poorly consolidated, very fine‐grained, grey‐coloured sediment section that is >20 m thick on the basin floor. This homogeneous unit is interpreted to represent the uppermost deposits generated by a gravity flow associated with the last major Storegga Slide event. Sediments analogous to the inferred source material of the slide deposits were collected from upslope on the Norwegian Margin. Sediments sampled within the basin are distinguishable from the purported source sediments, suggesting that size sorting has significantly altered this material along its flow path. Moreover, the very fine grain size (3·1 ± 0·3 μm) suggests that the >20 m thick homogeneous unit which was sampled settled from suspension after the turbulent flow was over. Although the turbulent phase of the gravity flow that moved material out into the basin may have been brief (days), significantly more time (years) is required for turbid sediments to settle and dewater and for the new sea floor to be colonized with a normal benthonic fauna. Pore water sulphate concentrations within the uppermost 20 m of the event deposit are higher than those normally found in sea water. Apparently the impact of microbial sulphate reduction over the last ca 8·1 cal ka bp since the re‐deposition of these sediments has not been adequate to regenerate a typical sulphate gradient of decreasing concentration with sub‐bottom depth. This observation suggests low rates of microbial reactions, which may be attributed to the refractory carbon composition in these re‐deposited sediments.  相似文献   

5.
黄河源区第四纪地质研究的新进展   总被引：8，自引：1，他引：8  
通过对黄河源区的钻孔、自然露头的研究, 建立了黄河源区的第四纪地层层序。第四纪地层可划分为下更新统、中更新统、上更新统和全新统。下更新统为河湖相沉积; 中更新统主要有湖积物、冰碛物和冰水沉积物; 上更新统主要有湖积物、冰碛物、冰水沉积物、洪积物和河流沉积物; 全新统主要由河流沉积物、洪积物和湖积物构成。黄河源区的冰期可划分为3期, 即末次冰期、倒数第二次冰期、倒数第三次冰期, 末次冰期又可分为2个冰阶。黄河源区的湖泊演化可划分为早更新世、中更新世和晚更新世—全新世3个阶段: 早更新世的湖泊范围小; 中更新世的湖泊范围明显扩大, 在位置上也较早更新世的湖泊南移; 晚更新世的湖泊经历了两次的扩张—收缩变化, 到了全新世, 除现今还发育的几个湖泊外, 大多数地区的湖水已退出, 基本上转变为河流环境。在晚更新世末期到全新世初期, 封闭黄河源区的多石峡被切开, 湖水外泄, 现今的黄河形成了, 同时发生了袭夺长江水系的水流。  相似文献   

6.
对南海北部陆坡MD05-2905站9个沉积物样品稀土元素进行分析,结果显示MD05-2905站沉积物中稀土含量总量(ΣREE)变化范围较大,其分布范围为60.66×10-6～350.37×10-6,平均值为174.59×10-6,其中全新世样品的稀土含量总量明显低于末次冰期样品,其球粒陨石分布模式与上地壳基本一致。对比发现,其ΣREE平均值相对接近中国黄土和珠江口,而与深海粘土中稀土元素的丰度相差较大,说明其主体可能来自陆源。稀土元素分馏参数δEu和δCe以及富集因子(EF)和判别函数(DF)揭示南海北部MD05-2905站沉积物与黄土、珠江口和台湾浅滩都有联系,显示了多物源多传输方式的特征。另外发现富集因子(EF)和判别函数(DF)数值在末次冰期时期与全新世时期差别相对较大,末次冰期时期黄土、珠江口和台湾浅滩的DF值都远远小于全新世时期,这可能反映末次冰期时黄土、珠江口和台湾浅滩对南海北部的物源贡献更大,也反映末次冰期与全新世期间各个物源供给/传输方式可能发生变化。  相似文献   

7.
Late Weichselian and Holocene sediment flux and sedimentation rates in a continental‐shelf trough, Andfjord, and its inshore continuation, Vågsfjord, North Norway, have been analysed. The study is based on sediment cores and high‐resolution acoustic data. Andfjord was deglaciated between 14.6 and 13 14C kyr BP (17.5 and 15.6 calibrated (cal.) kyr BP), the Vågsfjord basin before 12.5 14C kyr BP (14.7 cal. kyr BP), and the heads of the inner tributary fjords about 9.7 14C kyr BP (11.2 cal. kyr BP). In Andfjord, five seismostratigraphical units are correlated to a radiocarbon dated lithostratigraphy. Three seismostratigraphical units are recognised in Vågsfjord. A total volume of 23 km3 post‐glacial glacimarine and marine sediments was mapped in the study area, of which 80% are of Late Weichselian origin. Sedimentation rates in outer Andfjord indicate reduced sediment accumulation with increasing distance from the ice margin. The Late Weichselian sediment flux and sedimentation rates are significantly higher in Vågsfjord than Andfjord. Basin morphology, the position of the ice front and the timing of deglaciation are assumed to be the reasons for this. Late Weichselian sedimentation rates in Andfjord and Vågsfjord are comparable to modern subpolar glacimarine environments of Greenland, Baffin Island and Spitsbergen. Downwasting of the Fennoscandian Ice Sheet, and winnowing of the banks owing to the full introduction of the Norwegian Current, caused very high sedimentation rates in parts of the Andfjord trough at the Late Weichselian–Holocene boundary. Holocene sediment flux and sedimentation rates in Andfjord are about half the amount found in Vågsfjord, and about one‐tenth the amount of Late Weichselian values. A strong bottom current system, established at the Late Weichselian–Holocene boundary, caused erosion of the Late Weichselian sediments and an asymmetric Holocene sediment distribution. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

8.
In this paper, nine sediment sequences in the inner desert, desert-loess transitional zone (agro-pastoral transitional zone) and the northern margin of the Loess Plateau were investigated to study the sediments of Mu Us (Maowusu) and Otindaq (Hunshandake) sandy land in northern China since the last glaciation. All of these sequences consisted of the upper Holocene dark paleosol and the lower aeolian sand formed during the last glacial stage. The Mu Us and Otindaq sandy land became the active desert since the last glaciation and covered a larger sandy area than at present. Mu Us sandy land began to show a landform of sandy desert and expanded southward towards the northern boundaries of the Loess Plateau. The Holocene was characterized by the advent of warmer and wetter climatic conditions in northern China, resulting in the rapid growth of surface vegetation and Mu Us and Otindaq sandy land became a steppe. The thickness of surface soil reached up to as much as 50–80 cm. Thick paleosol covered the drift sands, and served as a protective layer for the steppe and farmland. Although the last glacial cycle is far away, paleo-aeolian sands formed during that period are still the material sources of regional desertification. Unreasonable cultivation destroying the Holocene paleosol, combined with dry climate and strong wind would result in activation of paleo- aeolian sand leading to the desertification of the study areas.  相似文献   

9.
Comparison of catchment geomorphology and lithostratigraphical analysis of sediments in two small neighbouring alpine lakes show that the minerogenic influx into the lakes has varied significantly during the Holocene, despite similarities in environmental setting. One lake contains a homogeneous organic‐rich sediment sequence whereas the sediment of the other lake is laminated and has a higher minerogenic content. X‐ray radiographs are used to visualise lithostratigraphical structures and provide high‐resolution density data. We find that moderate differences in geomorphology and process activity in the lakeshore region around alpine lakes can significantly affect the lake sediment composition. Minerogenic sediment accumulation rates vary strongly over time, owing to different depositional processes, which complicate temporal reconstructions. We also find that non‐glacial processes deposit minerogenic sediment layers with similar characteristics (high density, low organic content) as layers interpreted as having a glaciofluvial origin. This has implications for palaeoclimate studies based on proglacial lacustrine sediment. Our results indicate that erosion of surface sediments in the catchments characterised the early Holocene. A low and constant minerogenic inflow indicates that stable environmental conditions (with little fluvial erosion) were established in the catchments during the middle Holocene. The variability in sediment composition increased again in the late Holocene, possibly as a result of short‐term climate fluctuations superimposed on a general climate deterioration trend. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

10.
The Hitura open pit exposes a sedimentary sequence up to 50 m thick representing Late Saalian to Holocene glacial and non-glacial sediments. The sequence was investigated using sedimentological methods, OSL-dating and pollen and diatom analyses to reconstruct the Middle Weichselian (MWG) glacial event in the central part of the Scandinavian Ice Sheet (SIS). The results indicate that the sediment succession represents two entire glacial advance and retreat cycles. The lowermost deposits are Late Saalian esker and delta sediments overlain by sediments that correlate with the early Eemian lacustrine phase. Remnants of the Eemian soil post-dating the lacustrine phase were also observed. The area was ice-free during the entire Early Weichselian (EWG). The first glacial advance recorded in the sediments is related to the MWG. It started 79 kyr ago, deformed underlying sediments and deposited an immature till, including large detached sediment pods containing remains of organic material, soils and fluvial sediments representing allochthonous material from EWG ice-free stadials and interstadials. The glacial deposits are conformably overlain by glaciolacustrine and littoral accumulations, indicating MWG deglaciation between 62 and 55 kyr ago. Based on the fabric measurements from the till unit overlying the MWG sediments, ice advance during the Late Weichselian (LWG) was initially from the west and later from a north-northwesterly direction. The Hitura strata provide the first dating of the MWG deglaciation (55 to 62 kyr ago) from central parts of the SIS. It can be considered as a key site for studying the growth and decay of SIS during the poorly known early parts of the glaciation.  相似文献   

11.
Fluctuations in benthic foraminiferal faunas over the last 130,000 yr in four piston cores from the Norwegian Sea are correlated with the standard worldwide oxygen-isotope stratigraphy. One species, Cibicides wuellerstorfi, dominates in the Holocene section of each core, but alternates downcore with Oridorsalis tener, a species dominant today only in the deepest part of the basin. O. tener is the most abundant species throughout the entire basin during periods of particularly cold climate when the Norwegian Sea presumably was ice covered year round and surface productivity lowered. Portions of isotope Stages 6, 3, and 2 are barren of benthic foraminifera; this is probably due to lowered benthic productivity, perhaps combined with dilution by ice-rafted sediment; there is no evidence that the Norwegian Sea became azoic. The Holocene and Substage 5e (the last interglacial) are similar faunally. This similarity, combined with other evidence, supports the presumption that the Norwegian Sea was a source of dense overflows into the North Atlantic during Substage 5e as it is today. Oxygen-isotope analyses of benthic foraminifera indicate that Norwegian Sea bottom waters warmer than they are today from Substage 5d to Stage 2, with the possible exception of Substage 5a. These data show that the glacial Norwegian Sea was not a sink for dense surface water, as it is now, and thus it was not a source of deep-water overflows. The benthic foraminiferal populations of the deep Norwegian Sea seem at least as responsive to near-surface conditions, such as sea-ice cover, as they are to fluctuations in the hydrography of the deep water. Benthic foraminiferal evidence from the Norwegian Sea is insufficient in itself to establish whether or not the basin was a source of overflows into the North Atlantic at any time between the Substage 5e/5d boundary at 115,000 yr B.P. and the Holocene.  相似文献   

12.
Southwestern Barents Sea sediments contain important information on Lateglacial and Holocene environmental development of the area, i.e. sediment provenance characteristics related to ice‐flow patterns and ice drifting from different regional sectors. In this study, we present investigations of clay, heavy minerals, and ice‐rafted debris from three sediment cores obtained from the SW Barents Sea. The sediments studied are subglacial/glaciomarine to marine in origin. The core sequences were divided into three lithostratigraphical units. The lowest, Unit 3, consists of laminated glaciomarine sediments related to regional deglaciation. The overlying Unit 2 is a diamicton, dominated by mud and oversized clasts. Unit 2 reflects a more ice‐proximal glaciomarine sedimentary environment or even a subglacial depositional environment; its deposition may indicate a glacial re‐advance or stillstand during an overall retreat. The uppermost Unit 1 consists of Holocene marine sediments and current‐reworked sedimentary material with a relatively high carbonate content. A significant proportion of the sedimentary material could be derived from Svalbard and transported by sea ice or icebergs to the Barents Sea during the late deglacial phase. The Fennoscandian sources and local Mesozoic strata from the bottom of the Barents Sea are the likely provenances of sediments deposited during the deglacial and ice re‐advance phases. Bottom currents and sea‐ice transport were the main mechanisms influencing sedimentation during the Holocene. Our results indicate that the provenance areas can be reliably related to certain ice‐flow sectors and transport mechanisms in the deglaciated Barents Sea.  相似文献   

13.
The sedimentary records of Nulhegan Pond and Beecher Pond in the Nulhegan Basin of north‐eastern Vermont were analyzed to yield a history of environmental change since the latest Pleistocene. Shoreline landforms indicate that part of the Nulhegan Basin was inundated by Glacial Lake Nulhegan (GLN), which was impounded behind a dam of glacial sediment. Outwash derived from stagnant ice forms the bottom 176 cm of the Nulhegan Pond core. Fine‐grained inorganic sediment deposited between 13.4 and 12.2k cal a BP is interpreted as a deep‐water facies representing GLN, while coarser sediment from 12.2 to 11.8k cal a BP records draining of the glacial lake. Rapid, simultaneous increases in organic matter and biogenic silica signal the onset of productivity following the Younger Dryas. Beecher Pond formed c. 11.3k cal a BP through surface collapse over a buried ice block; buried stagnant ice may have persisted in the vicinity of the pond into the early Holocene. From 8.9 to 5.5k cal a BP, sediment in both lakes became coarser and richer in aquatic organic matter, suggesting a low‐water phase in which previously deposited lacustrine sediments were reworked and the littoral zone shifted basinward. Low water levels at this time are consistent with other records from Maine and southern Quebec, but contrary to records from ～325 km to the south. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

14.
The development of a glacial lake impounded along the retreating, northeastern ice margin of the Fennoscandian Ice Sheet during the last deglaciation and environmental conditions directly following the early Holocene deglaciation have been studied in NE Finland. This so‐called Sokli Ice Lake has been reconstructed previously using topographic and geomorphologic evidence. In this paper a multiproxy approach is employed to study a 3‐m‐thick sediment succession consisting of laminated silts grading into gyttja cored in Lake Loitsana, a remnant of the Sokli Ice Lake. Variations in the sediment and siliceous microfossil records indicate distinct changes in water depth and lake size in the Loitsana basin as the Sokli Ice Lake was drained through various spillways opening up along the retreating ice front. Geochemical data (XRF core‐scanning) show changes in the influence of regional catchment geochemistry (Precambrian crystalline rocks) in the glacial lake drainage area versus local catchment geochemistry (Sokli Carbonatite Massif) within the Lake Loitsana drainage area during the lake evolution. Principal component analysis on the geochemical data further suggests that grain‐size is an additional factor responsible for the variability of the sediment geochemistry record. The trophic state of the lake changed drastically as a result of morphometric eutrophication once the glacial lake developed into Lake Loitsana. The AMS radiocarbon dating on tree birch seeds found in the glaciolacustrine sediment indicates that Lake Loitsana was deglaciated sometime prior to 10 700 cal. a BP showing that tree Betula was present on the deglaciated land surrounding the glacial lake. Although glacial lakes covered large areas of northern Finland during the last deglaciation, only few glaciolacustrine sediment successions have been studied in any detail. Our study shows the potential of these sediments for multiproxy analysis and contributes to the reconstruction of environmental conditions in NE Finland directly following deglaciation in the early Holocene.  相似文献   

15.
The retreat of the Barents Sea Ice Sheet on the western Svalbard margin   总被引：1，自引：0，他引：1  
The deglaciation of the continental shelf to the west of Spitsbergen and the main fjord, Isfjorden. is discussed based on sub-bottom seismic records and scdirncnt cores. The sea lloor on the shelf to the west of Isfjorden is underlain by less than 2 m of glaciomarine sediments over a firm diamicton interpreted as till. In central Isfjordcn up to 10 m of deglaciation sediments were recorded, whereas in cores from the innermost tributary, Billefjorden, less than a meter of ice proximal sediments was recognized between the till and the 'normal' Holocene marine sediments. We conclude that the Barents Sea Ice Sheet terminated along the shelf break during the Late Weichselian glacial maximum. Radiocarbon dates from thc glaciomarine sediments above the till indicate a stepwise deglaciation. Apparently the ice front rctrcatcd from the outermost shelf around 14. 8 ka A dramatic increase in the flux of line-grained glaciomarine sediments around 13 ka is assumed to reflect increased melting and/or current activity due to a climatic warming. This second stage of deglaciation was intcrruptcd by a glacial readvance culminating on the mid-shelf area shortly after 12.4 ka. The glacial readvance, which is correlated with a simultaneous readvance of the Fennoscundian ice sheet along the western coast of Norway, is attributed to the so-called 'Older Dryas' cooling event in the North Atlantic region. Following this glacial readvance the outer part of Isljorden became rapidly deglaciated around 12.3 ka. During the Younger Dryas the inner fjord branches were occupied by large outlet glaciers and possibly the ice liont terminated far out in the main fjord. The remnants of the Harcnts Sea Ice Shcet melted quickly away as a response to the Holocene warming around 10 ka.  相似文献   

16.
The legacy of repeated Pleistocene glaciations has endowed many Welsh river valleys with locally thick successions of glacial and alluvial sediments. Investigations of a well-preserved flight of terraced sediments with good exposures at Capel Bangor, on the Afon Rheidol, mid-Wales, has allowed its Quaternary valley fill stratigraphy to be examined in detail. Study has revealed five terraced fills consisting of seven distinct sedimentary units. These range from Late Devensian ice-contact and ice-marginal deposits, to Holocene high-sinuosity stream sediments with episodes of man-induced accelerated deposition of fine-grained alluvium, and to aggradation and subsequent incision associated with historic metal mining. Examination of general sedimentary properties (e.g., granulometry, sedimentary structures, terrace surface morphology) show both differences in the pattern and controls of deposition and also progressive changes over Late Devensian and Holocene times. The sediments of the Rheidol Valley record the response and subsequent recovery of a drainage basin to glaciation, and the increasing influence of man on sediment yields, channel processes, and sediment quality.  相似文献   

17.
The lightness (L*) and concentrations of Rb, Sr and organic carbon (Corg) have been measured in the age-constrained lake sediment cores recovered from Co Ngoin in the central Tibetan Plateau. Dissolved Sr flux is a dominant control on the variation of Rb/Sr ratios in the sediments. Variations in color and geochemical proxies of Co Ngoin sediments display a continuous history of late glacial to mid-Holocene chemical versus physical weathering intensity in response to past climatic changes between approximately 13,500 and 4500 cal yr B.P. A lower chemical weathering under a late glacial climate was followed by a higher weathering during the Holocene Optimum. Weathering intensity in the central Tibetan Plateau catchment also responds to well-known climatic events, such as the Younger Dryas (YD), and possibly the Holocene Event 5 (HE-5). Although there are differences in time or duration of the climatic events, many of the well-known late glacial to mid-Holocene events occurred in high-elevation Co Ngoin where atmospheric circulation might play a hemispherical role in climatic forcing. The sediment hiatus since c. 4200 14C yr B.P. in the Co Ngoin indicates a period of desiccation that was probably associated with a sharp decrease in summer monsoon strength. Our lascustrine results not only imply catchment weathering variations in response to late glacial to mid-Holocene climatic conditions in the central plateau, but also provide further evidence for global connections between regional climates.  相似文献   

18.
南海陆坡中新世以来沉积物特性与气体水合物分布初探   总被引：27，自引：12，他引：15  
通过对南海陆坡地区中新世以来沉积物的一些属性和特征的研究,获得了下面的初步认识：（1）南海陆坡有几个沉积速率较高的地区, 如东沙一带、西沙海槽、中建南地区,以及南沙西南部和西北部（曾母、万安盆地）;（2）大洋钻探钻井资料的研究表明,中新世与早上新世为高碳酸盐比值的低速堆积期,而近200万a以来为低碳酸盐比值的高速堆积期,上新世晚期至全新世沉积物以粘土质粉砂和粉砂质粘土为最主要成分;（3）南海不同时期的沉积速率不同,全新世为沉积速率最高的时期,其次为更新世;上新世和中新世为沉积速率较低的时期。南海利于气体水合物存藏的沉积可能为全新世和更新世的沉积;（4）对南海晚第四纪以来沉积物的综合研究表明南海陆坡的A区（东沙附近）、B区（西沙海槽）、D区（南海西部、越南以东）等地区可能利于气体水合物的形成和分布;而E区（南沙西南部）、F区（南沙中上部）和G区(南沙海槽)则是气体水合物调查值得关注的地区。  相似文献   

19.
Tephra abundance data and geochemistry in Late‐glacial and Holocene sediments on the East Greenland shelf are presented. Two well‐known tephras were identified from electron microprobe analysis of tephra shards picked from ash peaks in the cores. These are the Vedde Ash and Saksunarvatn Ash, which probably were deposited on the shelf after transport on drifting ice. The radiocarbon dates (marine reservoir corrected by −550 yr) that constrain the timing of deposition of the tephra layers compare well with the terrestrial and ice‐core ages of the tephras without requiring additional reservoir correction to align them with the known tephra ages. Several prominent tephra layers with a composition of Ash Zone 2 tephra punctuate the deglacial sediments. These tephra peaks coincide with significant light stable isotope events (signifying glacial meltwater) and fine‐grained sediments poor in ice‐rafted detritus. We interpret the Ash Zone 2 tephra peaks as sediment released from the Greenland Ice Sheet during strong melting pulses of the deglaciation. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

20.
Based on high‐resolution TOPAS acoustic data, bathymetric data sets and sediment cores from the Norwegian Channel, the last retreat of the Norwegian Channel Ice Stream has been investigated. Mapping of ice‐marginal features such as grounding‐zone wedges and terminal moraines off western Norway suggest that the retreat of the grounding line in this part of the channel was interrupted by frequent stillstands, whereas the channel south of the threshold at Jæren does not have crossing ice‐marginal landforms. Three main seismic units have been identified, and, based on their seismic characteristics, in addition to study of sediment cores, these units are interpreted as till (U1), glacial marine sediment (U2) and Holocene hemipelagic sediment (U3). Based on new and published radiocarbon dates of the lower part of U2, combined with dates from the adjacent areas, it is concluded that the grounding line started to retreat from the shelf edge at about 19 ka and that the inner part of Skagerrak was ice free at 17.6 ka. This gives an average retreat rate of 450 m a−1, which is generally higher than mean retreat rates estimated for other palaeo‐ice streams (15–310 m a−1).  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号