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1.
8.4 ka 以来纳木错湖芯介形类组合的环境变化意义   总被引:1,自引:1,他引:0  
对青藏高原中南部纳木错长332cm的NMLC1孔湖芯研究发现,沉积物的介形类动物群计有6属15种。介形类属种生态特征和组合变化分析表明这些介形类对环境条件具有敏感性,其组合能够很好地反映过去环境变化的特征。结果表明,8.4ka以来具有三个不同的环境变化时期:早期在8400~6800aBP,湖泊由浅向深发展,环境具有相对冷湿的特征:中期在6800~2500aBP,湖泊深度逐渐加大,环境经历了暖湿-冷湿-冷干的变化.其中由冷湿向冷干的转化奠定了纳木错现今环境条件的基础:晚期在2500--9aBP,湖泊深度继续增加,这个时期的较早阶段,继续保持了前一时期的冷干特点,但湖水盐度可能开始增加.较晚阶段的冷干化加剧,陆面流水的活跃性大大降低。研究发现,纳木错NMLC1钻孔介形类黑色壳体的高峰值与介形类的最大生产量相一致,并且与沉积水动力条件增强相适应.指示了这些黑壳的产生与介形类的大量繁殖处于同一阶段.并且主要为异地搬运为主。钻孔中出现大量Candona幼虫壳体.其原因可能与沉积环境的水动力条件迅速改变有关。  相似文献   
2.
Ostracode analysis was carried out on samples from ice-rich permafrost deposits obtained on the Bykovsky Peninsula (Laptev Sea). A composite profile was investigated that covers most of a 38-m thick permafrost sequence and corresponds to the last ca. 60 kyr of the Late Quaternary. The ostracode assemblages are similar to those known from European Quaternary lake deposits during cold stages. The ostracode habitats were small, shallow, cold, oligotrophic pools located in low centred ice wedge polygons or in small thermokarst depressions. In total, 15 taxa, representing 7 genera, were identified from 65 samples. The studied section is subdivided into six ostracode zones that correspond to Late Quaternary climatic and environmental stadial-interstadial variations established by other paleoenvironmental proxies: (1) cold and dry Zyrianian stadial (58–53 kyr BP); (2) warm and dry Karginian interstadial (48–34 kyr BP); (3) transition from the Karginian interstadial to the cold and dry Sartanian stadial (34–21 kyr BP); (4) transition from the Sartanian stadial to the warm and dry Late Pleistocene period, the Allerød (21–14 kyr BP); (5) transition from the Allerød to the warm and wet Middle Holocene (14–7 kyr BP); and (6) cool and wet Late Holocene (ca. 3 kyr BP). The abundance and diversity of the ostracodes will be used as an additional bioindicator for paleoenvironmental reconstructions of the Siberian Arctic.  相似文献   
3.
The Dvuyakornaya Formation section in the eastern Crimea is described and subdivided into biostratigraphic units based on ammonites, foraminifers, and ostracodes. The lower part of the formation contains first discovered ammonites of the upper Kimmeridgian (Lingulaticears cf. procurvum (Ziegler), Pseudowaagenia gemmellariana Olóriz, Euvirgalithacoceras cf. tantalus (Herbich), Subplanites sp.) and Tithonian (?(Lingulaticeras efimovi (Rogov), Phylloceras consaguineum Gemmellaro, Oloriziceras cf. schneidi Tavera, and Paraulacosphinctes cf. transitorius (Oppel)). Based on the assemblage of characteristic ammonite species, the upper part of the formation is attributed to the Berriasian Jacobi Zone. Five biostratigraphic units (zones and beds with fauna) distinguished based on foraminifers are the Epistomina ventriosa-Melathrokerion eospirialis Beds and Anchispirocyclina lusitanica-Melathrokerion spirialis Zone in the upper Kimmeridgian-Tithonian, the Protopeneroplis ultragranulatus-Siphoninella antiqua, Frondicularia cuspidiata-Saracenaria inflanta zones, and Textularia crimica Beds in the Berriasian. The Cyrherelloidea tortuosa-Palaeocytheridea grossi Beds of the Upper Jurassic and Raymoorea peculiaris-Eucytherura ardescae-Protocythere revili Beds of the Berriasian are defined based on ostracodes. A new biostratigraphic scale is proposed for the upper Kimmeridgian-Berriasian of the eastern Crimea. The Dvyyakornaya Formation sediments are considered as deepwater facies accumulated on the continental slope.  相似文献   
4.
Sediment lithology and mineralogy, as well as ostracode, plant macrofossil and stable isotope stratigraphies of lake sediment cores, are used to reconstruct late Holocene hydrologic changes at Kenosee Lake, a relatively large, hyposaline lake in southeastern Saskatchewan. Chronological control is provided by AMS radiocarbon ages of upland and shoreline plant macrofossils. All indicators outline an early, low-water, saline phase of lake history (4100–3000 BP), when the basin was occupied by a series of small, interconnected, sulfate-rich brine pools, as opposed to the single, topographically-closed lake that exists today. A rapid rise in lake-level (3000–2300 BP) led to the establishment of carbonate-rich, hyposaline lake conditions like those today. Lithostratigraphic data and ostracode assemblages indicate peak salinities were attained early in this period of lake infilling, suggesting that the lake-level rise was initially driven by an influx of saline groundwater. Lake-level and water chemistry have remained relatively stable over the last 2000 years, compared to earlier events. Because of a lack of datable organic material in sediments deposited during the last 2000 years, the chronology of recent events is not well resolved. Plant macrofossil, lithostratigraphic and ostracode evidence suggests that lake draw-down, accompanied by slightly higher than present salinites, occurred sometime prior to 600 BP, followed by peak lake-level and freshwater conditions. This most recent high lake stand, indicative of a high water table on the surrounding upland, may also have led to the establishment of an extensive cover of Betula in the watershed, possibly in response to paludification. Ostracode assemblages indicate that peak freshwater conditions occurred within the last 100 years. Since historically documented lake-level fluctuations correlate with decadal scale climatic fluctuations in the meteorological record, and late-Holocene hydrologic dynamics correspond to well documented climatic excursions of the Neoglacial and Little Ice Age, Kenosee Lake dynamics offer insight into the susceptibility of the region's water resources to climate change.  相似文献   
5.
宋高  王海雷  郑绵平  李军 《湖泊科学》2015,27(5):962-974
于2008和2012年先后2次从西藏阿里和那曲地区61个不同水体,包括不同盐度的湖泊、湿地、浅水坑和河流等,共采集了78个水体表层沉积物样品,用以研究该地区现生介形类的生态分布及其环境指示意义.经鉴定,阿里和那曲地区现生介形类共计11属38种.除Ilyocypris neoaspera等5个种外,本次调查的介形类各属种均在湖泊中出现,尤以Limnocythere dubiosa的总壳瓣数最为丰富.Ilyocypris属中除Ilyocypris subdunshanensis和Ilyocypris xizangensis外,其余各种均喜浅水坑的小型浅水环境.Leucocythere dilitata、Limnocythere dubiosa、Limnocytherellina kunlunensis和Eucypris rischtanica为该地区的广盐种.其中Limnocythere dubiosa和Leucocythere dilitata适应盐度范围最广,在不同盐度的各类水体中均有出现.本次研究中的介形类均喜碱性水体,在p H值为8~9的水体中介形类各属种的丰度值达到最大.  相似文献   
6.
正1 Introduction Ostracodes are good materials for studying palaeoenvironmental evolution because of their sensitive response to environment changes.At present,the environmental implications of ostracodes are mainly based  相似文献   
7.
Sedimentological parameters and stable O- and C-isotopic composition of marl and ostracode calcite selected from a 17.7-m-long core from the 8-m-deep center of Pickerel Lake, northeastern South Dakota, provide one of the longest (ca. 12ky) paleoenvironmental records from the northern Great Plains. The late Glacial to early Holocene climate in the northern Great Plains was characterized by changes from cold and wet to cold and dry, and back to cold and wet conditions. These climatic changes were controlled by fluctuations in the positions of the Laurentide ice sheet and the extent of glacial Lake Agassiz. We speculate that the cold and dry phase may correspond to the Younger Dryas event. A salinity maximum was reached between 10.3 and 9.5 ka, after which Pickerel Lake shifted from a system controlled by atmospheric changes to a system controlled by groundwater seepage that might have been initiated by the final withdrawal of Glacial Lake Agassiz. A prairie lake was established at approximately 8.7 ka, and lasted until about 2.2 ka. During this mid-Holocene prairie period, drier conditions than today prevailed, interrupted by periods of increased moisture at about 8, 4, and 2.2 ka. Prairie conditions were more likely dry and cool rather than dry and warm. The last 2.2 ka are characterized by higher climatic variability with 400-yr aridity cycles including the Medieval Warm Period and the Little Ice Age.Although the signal of changing atmospheric circulation is overprinted by fluctuations in the positions of the ice sheet and glacial Lake Agassiz during the late Glacial-Holocene transition, a combination of strong zonal circulation and strong monsoons induced by the presence of the ice sheet and high insolation may have provided mechanisms for increased precipitation. Zonal flow introducing dry Pacific air became more important during the prairie period but seems to have been interrupted by short periods of stronger meridional circulation with intrusions of moist air from the Gulf of Mexico. More frequent switching between periods of zonal and meridional circulation seem to be responsible for increased climatic variability during the last 2.2 ka.  相似文献   
8.
Lake Neuchåtel is a medium sized, hard-water lake, lacking varved sediments, situated in the western Swiss Lowlands at the foot of the Jura Mountains. Stable isotope data (18O and 13C) from both bulk carbonate and ostracode calcite in an 81 cm long, radiocarbon-dated sediment core represent the last 1500 years of Lake Neuchåtel's environmental history. Comparison between this isotopic and other palaeolimnologic data (mineralogical, geochemical, palynological, etc.) helps to differentiate between anthropogenic and natural factors most recently affecting the lake. An increase in lacustrine productivity (450–650AD ca), inferred from the positive trend in 13C values of bulk carbonate, is related to medieval forest clearances and the associated nutrient budget changes. A negative trend in both the bulk carbonate and ostracode calcite 18O values between approximately 1300 and 1500AD, is tentatively interpreted as due to a cooling in mean air temperature at the transition from the Medieval Warm Period to the Little Ice Age. Negative trends in bulk carbonate 18O and 13C values through the uppermost sediments, which have no equivalent in ostracode calcite isotopic values, are concomitant with the recent onset of eutrophication in the lake. Isotopic disequilibrium during calcite precipitation, probably due to kinetic factors in periods of high productivity is postulated as the mechanism to explain the associated negative isotopic trends, although the effect of a shift of the calcite precipitation towards the warmer months cannot be excluded.  相似文献   
9.
We report here on the first detailed ostracode stratigraphic record to be obtained from late Holocene sediments of Lake Tanganyika. We analyzed four cores, three from the northern lake region and a fourth from a more southern lake locality, that collectively record ostracode assemblages under a variety of disturbance regimes. These cores provide a stratigraphic record of ostracode abundance and diversity, as well as depositional changes over time periods of decades to millennia. We have investigated the fossil ostracodes in these cores by looking at temporal changes of species diversity and population structure for the species present. All four cores provided distinct patterns of ostracode diversity and abundance. BUR-1, a northern lake core obtained close to the Ruisizi River delta, yielded a sparse ostracode record. Karonge #3, another northern core from a site that is closely adjacent to a river delta with high sediment loading, yielded almost no ostracodes. The third core 86-DG-14, taken from a somewhat less disturbed area of the lake, suggests that there have been recent changes in ostracode populations. Through most of the lower portion of this core, ostracode abundance is low and species richness is relatively constant. Above 7 cm there is a marked increase in ostracode abundance and a corresponding decrease in species richness, probably signaling the onset of a major community disturbance, perhaps due to human activities. The southernmost core, 86-DG-32, is from a site that is well removed from influent rivers. Ostracode abundance varies erratically throughout the core, whereas species richness is relatively constant and high throughout the core. The temporal variation evident in ostracode community makeup both within and between the studied cores may be a result of naturally patchy distributions among ostracodes, coupled with local extinctions and recolonizations, or it may reflect inadequate sampling of these high diversity assemblages. In either case, these cores illustrate the potential to obtain high resolution ostracode records from the rich, endemic fauna of Lake Tanganyika that can be used to address questions about the history of community structure and human impacts in this lake.  相似文献   
10.
The modern ostracode distribution in lakes of the north-central United States shows how ostracode abundances are related to concentrations of major ions such as calcium, sulfate, and bicarbonate. These relationships are quantified for species living in lakes that range from fresh water (200 S cm-1) to saline water (17 000 S cm-1). Lakes located in the Minnesota forests have different ostracode assemblages and different water chemistry than lakes located on the prairie of North Dakota and South Dakota. These differences are attributed to differences in precipitation and rock-water interactions. Multivariate analyses of the ostracode and water chemistry data set indicate that different ostracode species are associated with different water types. For example, Limnocythere sappaensis and Heterocypris glaucus are found in bicarbonate-enriched sulfate-dominated waters, whereas Limnocythere staplini is found in bicarbonate-depleted sulfate-dominated waters. Candona ohioensis and Limnocythere itasca are found in fresh water, and Candona rawsoni is eurytopic and found in both bicarbonate-enriched and bicarbonate-depleted sulfate-dominated waters. Ostracodes can be used to identify changes in both ionic composition (solutes) and ionic concentration.This publication is the second of a series of papers presented at the Conference on Sedimentary and Paleolimnological Records of Saline Lakes. This Conference was held August 13–16, 1991 at the University of Saskatchewan, Saskatoon, Canada. Dr Evans is serving as Guest Editor for this series.  相似文献   
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