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1.
Sediments from the last interglacial, marine isotope stage 5e (MIS 5e), have been studied for their dinoflagellate cyst content in a core retrieved from the Vøring Plateau, Norwegian Sea. Qualitative and quantitative analyses of the data, and comparison with the surface sample and published Holocene data from the core, reveal distinct differences in hydrological surface conditions between the late Holocene and MIS 5e. A higher number of co-dominant, subordinate species in the last interglacial samples suggests there was a more pronounced seasonality of the surface water at this time. This is supported by the significant presence of Bitectatodinium tepikiense , a species that was virtually absent from the area for most of the Holocene. The seasonality signal is further substantiated by transfer-function reconstructions, which also indicates a stronger stratification of the upper water column during MIS 5e. Moreover, the assemblage data clearly show that optimal, fully marine interglacial conditions prevailed only late in MIS 5e (between ca. 117.5 and 116.5 Kya), which is in contrast with the climatic optimum early in the Holocene. Stable oxygen isotope values from planktic foraminifera for this MIS 5e optimum are comparable with the average Holocene values, but are generally ca. 0.3‰ higher than those of the earlier part of the last interglacial ( sensu stricto ). These higher δ18O values are likely to be the result of the enhanced and prolonged influence of Saalian deglacial meltwater, thus corroborating the existence of a quite differently structured sea surface, as suggested by the dinocyst data.  相似文献   

2.
The Holocene glacial and climatic development in Antarctica differed considerably from that in the Northern Hemisphere. Initial deglaciation of inner shelf and adjacent land areas in Antarctica dates back to between 10-8 Kya, when most Northern Hemisphere ice sheets had already disappeared or diminished considerably. The continued deglaciation of currently ice-free land in Antarctica occurred gradually between ca. 8-5 Kya. A large southern portion of the marine-based Ross Ice Sheet disintegrated during this late deglaciation phase. Some currently ice-free areas were deglaciated as late as 3 Kya. Between 8-5 Kya, global glacio-eustatically driven sea level rose by 10-17m, with 4-8 m of this increase occurring after 7 Kya. Since the Northern Hemisphere ice sheets had practically disappeared by 8-7 Kya, we suggest that Antarctic deglaciation caused a considerable part of the global sea level rise between 8-7 Kya, and most of it between 7-5 Kya. The global mid-Holocene sea level high stand, broadly dated to between 8-4 Kya, and the Littorina-Tapes transgressions in Scandinavia and simultaneous transgressions recorded from sites e.g. in Svalbard and Greenland, dated to 7-5 Kya, probably reflect input of meltwater from the Antarctic deglaciation.  相似文献   

3.
The discussion on climatic instability observed in Greenland ice cores during the Eemian period (substage 5e) resulted in discovery of a pronounced mid-Eemian cooling event. We report that the mid-Eemian cooling is found for the first time in the biogenic silica climatic record and microfossil abundance record of Lake Baikal. Timing of this event in Lake Baikal correlates well with timing of the European pollen records and marine sedimentary records. The presence of the mid-Eemian cooling signal in the Lake Baikal record suggests a much closer link between Asian climate influenced by strong pressure fields over the vast land masses and the climate-controlling processes in the North Atlantic during interglacial periods, than what was generally believed. Furthermore, the Lake Baikal record suggests that after the mid-Eemian cooling, the climatic conditions returned close to the warmth of the 5e optimum and thus argues that the warm conditions of the last interglacial persisted in Siberia throughout 5e, and did not end with the mid-Eemian cooling as suggested by several published marine records.  相似文献   

4.
A 36 Ky record of iceberg rafting and sedimentation from north-west Iceland   总被引：2，自引：0，他引：2  
Evidence from north-west Iceland's shelf and fjords is used to develop a scenario for environmental change during the last 36 cal Ky. The retreat history of the Iceland Ice Cap during the last deglaciation is delineated through lithofacies studies, carbon analyses and magnetic susceptibility, and studies of ice-rafted debris (IRD) in sediment cores. Sedimentological data from lake Efstadalsvatn, Vestfirdir peninsula, trace the glacier retreat on land. In two of the high resolution shelf cores we detect near continuous IRD accumulation from 36 to 11 cal Kya. However, IRD is absent in the cores from ca. 22 to 19 cal Kya, possibly indicating more extensive landfast sea ice conditions. All cores show intensified IRD during the Younger Dryas chronozone; the fjord cores show a continuous IRD record until 10 cal Kya. Magnetic susceptibility and carbon analyses from Efstadalsvatn reveal the disappearance of local ice in the basin just before 10.5 cal Kya. No IRD was detected in the sediment cores during 10 to Ø4 cal Kya. Some indication of cooling occurs between 4 and 3 cal Kya, with a fresh input of IRD in fjord cores after 1 cal Kya.  相似文献   

5.
ABSTRACT. This paper examines new geomorphological, chronological and modelling data on glacier fluctuations in southernmost South America in latitudes 46–55°S during the last glacial–interglacial transition. Establishing leads and lags between the northern and southern hemispheres and between southern mid‐latitudes and Antarctica is key to an appreciation of the mechanisms and resilience of global climate. This is particularly important in the southern hemisphere where there is a paucity of empirical data. The overall structure of the last glacial cycle in Patagonia has a northern hemisphere signal. Glaciers reached or approached their Last Glacial Maxima on two or more occasions at 25–23 ka (calendar) and there was a third less extensive advance at 17.5 ka. Deglaciation occurred in two steps at 17.5 ka and at 11.4 ka. This structure is the same as that recognized in the northern hemisphere and taking place in spite of glacier advances occurring at a time of high southern hemisphere summer insolation and deglaciation at a time of decreasing summer insolation. The implication is that at orbital time scales the‘northern’ signal dominates any southern hemisphere signal. During deglaciation, at a millennial scale, the glacier fluctuations mirror an antiphase 'southern’ climatic signal as revealed in Antarctic ice cores. There is a glacier advance coincident with the Antarctic Cold Reversal at 15.3–12.2 ka. Furthermore, deglaciation begins in the middle of the Younger Dryas. The implication is that, during the last glacial–interglacial transition, southernmost South America was under the influence of sea surface temperatures, sea ice and southern westerlies responding to conditions in the 'southern’ Antarctic domain. Such asynchrony may reflect a situation whereby, during deglaciation, the world is more sensitized to fluctuations in the oceanic thermohaline circulation, perhaps related to the bipolar seesaw, than at orbital timescales.  相似文献   

6.
Lake Ohrid is probably of Pliocene age, and the oldest extant lake in Europe. In this study climatic and environmental changes during the last glacial-interglacial cycle are reconstructed using lithological, sedimentological, geochemical and physical proxy analysis of a 15-m-long sediment succession from Lake Ohrid. A chronological framework is derived from tephrochronology and radiocarbon dating, which yields a basal age of ca. 136 ka. The succession is not continuous, however, with a hiatus between ca. 97.6 and 81.7 ka. Sediment accumulation in course of the last climatic cycle is controlled by the complex interaction of a variety of climate-controlled parameters and their impact on catchment dynamics, limnology, and hydrology of the lake. Warm interglacial and cold glacial climate conditions can be clearly distinguished from organic matter, calcite, clastic detritus and lithostratigraphic data. During interglacial periods, short-term fluctuations are recorded by abrupt variations in organic matter and calcite content, indicating climatically-induced changes in lake productivity and hydrology. During glacial periods, high variability in the contents of coarse silt to fine sand sized clastic matter is probably a function of climatically-induced changes in catchment dynamics and wind activity. In some instances tephra layers provide potential stratigraphic markers for short-lived climate perturbations. Given their widespread distribution in sites across the region, tephra analysis has the potential to provide insight into variation in the impact of climate and environmental change across the Mediterranean.  相似文献   

7.
Novaya Zemlya was covered by the eastern part of the Barents–Kara ice sheet during the glacial maximum of marine isotope stage 2 (MIS 2). We obtained 14C ages on 37 samples of mollusc shells from various sites on the islands. Most samples yielded ages in the range of 48–26 14C Ky. Such old samples are sensitive to contamination by young 14C, and therefore their reliability was assessed using replicate analyses and amino acid geochronology. The extent of aspartic acid racemization (Asp D/L) indicates that many of the 14C ages are correct, whereas some are minimum ages only. The results indicate that a substantial part of Novaya Zemlya was ice-free about 35–27 14C Kya, and probably even earlier. Corresponding shorelines up to >140 m a.s.l. indicate a large Barents–Kara ice sheet during early MIS 3. These results are consistent with findings from Svalbard and northern Russia: in both places a large MIS 4/3 Barents–Kara ice sheet is postulated to have retreated about 50 Kya, followed by an ice-free interstadial that lasted until up to ca. 25 Kya. The duration of the MIS 2 glaciation in Novaya Zemlya was calculated by applying the D/L values to a kinetic equation for Asp racemization. This indicates that the islands were ice covered for less than 3000 years if the basal temperature was 0oC, and for less than 10 000 years if it was −5oC.  相似文献   

8.
Non-glaciated Arctic lowlands in north-east Siberia were subjected to extensive landscape and environmental changes during the Late Quaternary. Coastal cliffs along the Arctic shelf seas expose terrestrial archives containing numerous palaeoenvironmental indicators (e.g., pollen, plant macro-fossils and mammal fossils) preserved in the permafrost. The presented sedimentological (grain size, magnetic susceptibility and biogeochemical parameters), cryolithological, geochronological (radiocarbon, accelerator mass spectrometry and infrared-stimulated luminescence), heavy mineral and palaeoecological records from Cape Mamontov Klyk record the environmental dynamics of an Arctic shelf lowland east of the Taymyr Peninsula, and thus, near the eastern edge of the Eurasian ice sheet, over the last 60 Ky. This region is also considered to be the westernmost part of Beringia, the non-glaciated landmass that lay between the Eurasian and the Laurentian ice caps during the Late Pleistocene. Several units and subunits of sand deposits, peat–sand alternations, ice-rich palaeocryosol sequences (Ice Complex) and peaty fillings of thermokarst depressions and valleys were presented. The recorded proxy data sets reflect cold stadial climate conditions between 60 and 50 Kya, moderate inderstadial conditions between 50 and 25 Kya and cold stadial conditions from 25 to 15 Kya. The Late Pleistocene to Holocene transition, including the Allerød warm period, the early to middle Holocene thermal optimum and the late Holocene cooling, are also recorded. Three phases of landscape dynamic (fluvial/alluvial, irregular slope run-off and thermokarst) were presented in a schematic model, and were subsequently correlated with the supraregional environmental history between the Early Weichselian and the Holocene.  相似文献   

9.
The extent of ice, thickness and dynamics of the Last Glacial Maximum (LGM) ice sheets in the Antarctic Peninsula region, as well as the pattern of subsequent deglaciation and climate development, are not well constrained in time and space. During the LGM, ice thickened considerably and expanded towards the middle–outer submarine shelves around the Antarctic Peninsula. Deglaciation was slow, occurring mainly between >14 Ky BP (14C kilo years before present) and ca. 6 Ky BP, when interglacial climate was established in the region. After a climate optimum, peaking ca. 4 - 3 Ky BP, a cooling trend started, with expanding glaciers and ice shelves. Rapid warming during the past 50 years may be causing instability to some Antarctic Peninsula ice shelves.  相似文献   

10.
The Andrews site represents one of countless prairie potholes found in areas of hummocky moraine on the northern Great Plains. Sediments from a depth of 5.8 to 3.1 m at this 'kettle-fill' site in the Missouri Coteau upland of southern Saskatchewan, Canada, provides a record of vegetation, climate, and hydrologic changes within a small, ca 30 m diameter, closed-drainage basin from ca 10.2 to 5.8 ka. Plant macrofossil analyses of 67 samples, 6 14C ages, and stratigraphy were used to identify 5 zones, representing the paleohydrological changes that followed deglaciation in southern Saskatchewan.Results of this study indicate that with the melting of residual stagnant ice a pond (>2 m deep) with abundant aquatic, emergent, and shoreline plants developed in the basin at ca 10.2 ka and persisted until at least ca 8.8 ka. During this time there was a shift in upland vegetation from a white spruce forest (Zone II) to a deciduous parkland at ca 10 ka (Zone III). As climate warmed, brackish and alkaline conditions developed coincident with shallowing of the pond at the end of Zone III. The perennial water phase ended at ca 8.8 ka and was followed by a low-water stand lasting ca 1100 years. Prairie fires and slopewash from unstable slopes were dominant sedimentological processes until ca 7.7 ka (Zone IV). Water levels began to rise and between ca 7.7 and 5.8 ka a semi-permanent pond was established in a grassland setting (Zone V). After ca 5.8 ka this prairie pothole wetland became ephemeral, to the point that plant macrofossils could not be preserved, a situation continuing today. Interactions between climate change, variability in local groundwater supply, and sedimentological processes likely account for the paleohydrologic events reconstructed at the Andrews site.  相似文献   

11.
High-resolution oxygen-isotope records of benthic ostracods and molluscs from Ammersee, southern Germany, show high-frequency climatic changes during the last deglaciation and parallel in great detail published faunal and floral variations reconstructed from Norwegian Sea sediments and isotope variations in Greenland ice cores. The marine and the terrestrial records give evidence of a synchronous late glacial climatic development in Greenland, NW- and Mid-Europe. However,14C-ages of the supraregional climatic events and of two tephra layers in the marine sediments of the northeastern Atlantic Ocean are significantly older than the14C-ages of the corresponding horizons on land. These differences strongly suggest that major short-term events have affected the exchangeable carbon on earth during the dramatic environmental changes related to the deglaciation and in particular have affected the CO2-distribution within the ocean and between ocean and atmosphere. Dating methods independent of climatic variations and of the global carbon budget should be given priority to refine the timescales of the marine and atmospheric processes during the last deglaciation.This is the fourth paper in a series of papers published in this issue on high-resolution paleolimnology. These papers were presented at the Sixth International Palaeolimnology Symposium held 19–21 April, 1993 at the Australian National University, Canberra, Australia. Dr. A. F. Lotter and Dr. M. Sturm served as guest editors for these papers.  相似文献   

12.
陕西北部榆林第四纪地层剖面的粒度分析与讨论   总被引：24，自引：1，他引：24  
本文根据榆林剖面及其粒度分析结果,讨论了剖面中地层的沉积相、古气候和环境演化.  相似文献   

13.
The early Russian researchers working in central Siberia seem to have preferred scenarios in which glaciations, in accordance with the classical glaciological concept, originated in the mountains. However, during the last 30 years or so the interest in the glacial history of the region has concentrated on ice sheets spreading from the Kara Sea shelf. There, they could have originated from ice caps formed on areas that, for eustatic reasons, became dry land during global glacial maximum periods, or from grounded ice shelves. Such ice sheets have been shown to repeatedly inundate much of the Taymyr Peninsula from the north-west. However, work on westernmost Taymyr has now also documented glaciations coming from inland. On at least two occasions, with the latest one dated to the Saale glaciation (marine isotope stage 6 [MIS 6]), warm-based, bedrock-sculpturing glaciers originating in the Byrranga Mountains, and in the hills west of the range, expanded westwards, and at least once did such glaciers, after moving 50–60 km or more over the present land areas, cross today's Kara Sea coastline. The last major glaciation affecting south-western Taymyr did, however, come from the Kara Sea shelf. According to optically stimulated luminescence dates, this was during the Early or Middle Weichselian (MIS 5 or 4), and was most probably not later than 70 Kya. South-western Taymyr was not extensively glaciated during the last global glacial maximum ca. 20 Kya, although local cold-based ice caps may have existed.  相似文献   

14.
This is the first integrated multiproxy study to investigate climate, catchment evolution and lake ecology in South Greenland. A 4-m-long sedimentary sequence from Lake Igaliku (61o 00?? N, 45o 26?? W, 15?m asl) documents major environmental and climatic changes in south Greenland during the last 10?ka. The chronology is based on a 210Pb and 137Cs profile and 28 radiocarbon dates. The paleoenvironmental history is interpreted on the basis of magnetic susceptibility, grain size, total organic carbon, total nitrogen and sulphur, sedimentation rates, pollen, and diatom assemblages. The basal radiocarbon date at ca. 10?cal?ka BP provides a minimum age for the deglaciation of the basin, which is followed by ~500?years of high sedimentation rates in a glacio-marine environment. After the glacio-isostatic emergence of the basin ca. 9.5?cal?ka BP, limnological and terrestrial proxies suggests early warmth, which may have been interrupted by a cold, dry and windy period between 8.6 and 8.1?cal?ka BP. A dry and windy event ~5.3?C4.8?cal?ka BP preceded the Neoglacial transition at Lake Igaliku, which is characterized by a shift toward moister and perhaps cooler conditions ~4.8?cal?ka BP, causing major changes in terrestrial and aquatic ecological conditions. Significant cooling is documented after ~3?cal?ka BP. Since ~1?cal?ka BP the climatic-driven changes were overprinted by the human influence of Norse and recent agriculture.  相似文献   

15.
通过对祁连山东段沙沟河黄土剖面CaCO3含量、色度代用指标的综合分析,结合14C和热释光测年结果,揭示出了沙漠边缘区夏季风在末次间冰期(S1)表现出5次增强时期。从指标曲线上能够反映出千年尺度的气侯变化,与GRIP孔的气候记录有很好的耦合性。这种气候不稳定性可能受控于高纬冰盖的变化,体现出全球气候性特点。在末次冰期,夏季风的反映就弱得多,气候长期表现为干冷,可能由于青藏高原的抬升,以及靠近沙漠边缘区,使夏季风在冰期难以到达,表现出区域性气候特点。  相似文献   

16.
西藏“一江两河”中游地区风成地层沉积时代、沉积相和磁化率等研究结果表明，早在８００ＫａＢ．Ｐ．前西南季风就已存在，受全球气候波动和青藏高原隆起的影响，其盛行衰变与东南季风具有较好的一致性，主要表现为本区地层所记录的气候变化信息不如东南季风区详细；而且西南季风因高原屏障作用给本区带来的降水愈来愈少，气候明显地向干冷化发展。  相似文献   

17.
黄土高原黄土稳定同位素与古季风研究进展   总被引：1，自引：0，他引：1  
庞奖励 《中国沙漠》1998,18(3):273-282
黄土高原黄土-古土壤完好地记录了过去环境信息,进入90年代以来,在利用碳、氧同位素作为气候替代指标研究古植被、降雨量、古环境温度及古季风演化方面取得了长足的进展,文章就这方面的主要研究成果作了综合分析与评述。  相似文献   

18.
The first marine sediment cores from the unexplored Independence Fjord system and the Wandel Sea, North Greenland, have been investigated to reveal the glacial marine history of the region. Two key sites in the Independence Fjord system, and an earlier analysed site from the Wandel Sea continental slope, off the mouth of Independence Fjord, are presented. The Independence Fjord sites reveal an early Holocene record (10.0–8.9 Kya) of fine-grained reddish muds with calcareous microfossils, dominated by the benthic foraminifera Cassidulina neoteretis . We suggest that a semi-permanent fast ice cover characterized the region in the early Holocene, and that the deeper troughs in the mouth region of the Independence Fjord system were intruded by subsurface Atlantic water. A stiff diamicton, at least 1.3 m thick, with coal and sandstone clasts of mainly local origin, and a 0.5-m-thick Holocene cover, are found in one of the sites. The diamicton is assumed to represent a subglacial till predating the early Holocene sediments (>10 Kya). Shallow seismic records off the mouth of Independence Fjord reveal kilometre-sized troughs with signs of glacial erosion, till deposition and a Holocene glaciomarine deposition. These features could indicate that glacial ice debouching from the Independence Fjord system at some time during the last glacial period extended to the mid-outer Wandel Sea shelf. Data from a high-resolution sediment core previously retrieved from the adjacent Wandel Sea slope indicate that the maximum ice sheet advance in this area culminated about 25–20 Kya.  相似文献   

19.
Age determinations of bivalve shells indicate that Bockfjorden, a fjord in north-western Spitsbergen, Svalbard, was deglaciated shortly before 10 Kya, and that the upper marine limit in this area, with an altitude of about 50 m a.s.l., has the same age. During most of the Holocene, the glaciers in Bockfjorden were less extensive than they are today. Their maximum Holocene extension occurred during the Little Ice Age. The initial shoreline emergence after the deglaciation was rapid, and former shorelines younger than 8.5 Ky are below the present sea level. A mid-Holocene transgression of the sea is traced as well as a transgression during the last thousand years.  相似文献   

20.
榆林剖面的热释光测年及其对这一地区干旱事件的讨论   总被引：2，自引：2，他引：2  
用热释光测年方法对陕西榆林地区蔡家沟和石峁剖面的风成砂－黄土－古土壤序列进行了系统的测年，结果表明末次冰期的风成沉积中夹有三层古风成砂，由此反映了末次冰期中有三次沙漠扩大的气候干旱时期。根据年代测定结果最末一次干旱事件出现在１２～＜２７ｋａＢ．Ｐ．之间，基本上与末次冰期极盛期相当，可以与深海氧同位素阶段２对应。第二次干旱期出现在４８ｋａＢ．Ｐ．左右，大致与氧同位素阶段３的中期相当。最早的干旱期出现在５５～＜７５ｋａＢ．Ｐ．之间，与阶段４对应。令人感兴趣的是虽然与阶段２、阶段４对应的干旱期在世界上其它地区同样有记录，但是大致相当于阶段３中期的这期干旱事件却以榆林剖面反映最好，表现为末次冰期的间冰段期间，毛乌素沙漠也曾经大规模扩大，这从一个侧面说明了中国的沙漠－黄土边界带对气候变化的敏感性。此外，热释光测年结果还表明了末次冰期的沉积由风成砂与黄土迭覆而成，而末次间冰期则由古土壤与黄土迭覆而成，因此，无论是冰期还是间冰期，气候条件都不是以持续的干冷或温暖为特点，而是叠加了更次一级的气候变化。毛乌素沙漠在第四纪时期并非持续存在，而是在气候的调节下经历了沙漠出现与固定的多次转变  相似文献   

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