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11.
T.M. Wells A.S. Cohen L.E. Park D.L. Dettman B.A. McKee 《Journal of Paleolimnology》1999,22(3):259-276
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
12.
Stable isotopes and sediments from Pickerel Lake, South Dakota, USA: a 12ky record of environmental changes 总被引：2，自引：0，他引：2
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
13.
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
14.
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
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16.
B. L. Nikitenko 《Stratigraphy and Geological Correlation》2008,16(1):59-80
Stages in evolution of the Early Jurassic to Aalenian foraminifers and ostracodes are established based on the analyzed diversity dynamics of respective microfauna associations. Evolution of foraminifers is divided in two, the Hettangian-initial early Toarcian and the late early Toarcian-Aalenian stages, while the identical first stage in evolution of ostracodes has been followed by the late early Toarcian-Callovian stage. During the Jurassic, periodic migrations of foraminiferal and ostracod genera and species, which were interrelated with large transgressions and climatic changes, took place in the initial late and mid-late Pliensbachian, initial early Toarcian, and the late Toarcian-early Aalenian. Being isolated to the maximum extent in the second half of the late Aalenian, the Arctic basin lost connections with seas of northwestern Europe. The Early Jurassic to Aalenian biogeography of the Arctic basin is established based on the results of cluster analysis (group average link method, Jaccard coefficient, presence or absence of foraminifers and ostracodes genera) with due account for preceding and subsequent formation history of microbenthos structure in biochores. The distinguished biochores are ranked as realms and provinces of foraminifers and ostracodes. As is established, contours of the realms and provinces populated by different groups of microbenthos did not coincide and changed with time. Ecotones between the realms (e.g., the North Sea province) changed their localitization to be a part of the Arctic or Boreal Atlantic realms in different epochs. The Early-Middle Jurassic sedimentary successions of the Arctic basins reveal several levels of sharp taxonomic changes in composition of microbenthos under influence of the first-order abiotic factors. 相似文献
17.
O. D. Naidina 《Stratigraphy and Geological Correlation》2006,14(3):331-340
The Holocene paleogeography of the Laptev Sea eastern shelf is reconstructed based on mollusks, foraminifers, ostracodes, spores and pollen found for the first time. Environmental changes in the course of the postglacial (Holocene) transgression are detectable from quantitative analysis of three ostracod assemblages. Three phases of the transgression are established based on ostracode assemblages from the AMS 14C-dated core from the ancient valley of Yana River, central part of the Laptev Sea eastern shelf. After the first phase of coastal brackish-water environment (11.3–11.1 ka), shallow-water marine taxa appeared in the transitional phase (11.1–10.3 ka), and the third phase of modern sea conditions commenced 10.3 ka ago. Spore and pollen assemblages from the core indicate that the early Holocene land vegetation was dominated by tundra plant communities adapted to climatic conditions not warmer than at present. In the interval of 9.3–8.0 ka, climate was increasingly humid and warm, and forest-tundra vegetation occupied more favorable zones of the region. 相似文献
18.
DENG Mei-yin CHUN Ping LI Jia-gui GE Rui-quanGeological Logging Branch of Shengli Oilfield Company SINOPEC Dongying 《地层学杂志》2007,(Z2)
在对济阳坳陷老斜452井的古生物分析中,发现穴状女星介(相似种)Cypridea(Cypridea)cf.cavernosa、蒙阴女星介C.(C.)mengyinensis等介形类化石,说明该地区发育了白垩纪地层。济阳坳陷中生代地层的细分主要是依据区域岩性、电性及孢粉化石等地质资料,但白垩系的岩性、电性与上下地层相似,且化石稀少,缺乏充足的古生物资料,因此难以系统、准确地划分。老斜452井发现介形类化石给济阳坳陷白垩系的细分提供了重要的古生物学依据,为该区的深入研究提供了翔实的基础资料。 相似文献
19.
Preliminary results of a multidisciplinary study of cores in southwestern Lake Michigan suggest that the materials in these cores can be interpreted in terms of both isostatically and climatically induced changes in lake level. Ostracodes and mollusks are well preserved in the Holocene sediments, and they provide paleolimnologic and paleoclimatic data, as well as biogenic carbonate for stable-isotope studies and radiocarbon dating. Pollen and diatom preservation in the cores is poor, which prevents comparison with regional vegetation records. New accelerator-mass spectrometer 14C ages, from both carbon and carbonate fractions, provide basin-wide correlations and appear to resolve the longstanding problem of anomalously old ages that result from detrital organic matter in Great Lakes sediments.Several cores contain a distinct unconformity associated with the abrupt fall in lake level that occurred about 10.3 ka when the isostatically depressed North Bay outlet was uncovered by the retreating Laurentide Ice Sheet. Below the unconformity, ostracode assemblages imply deep, cold water with very low total dissolved solids (TDS), and bivalves have 18O (PDB) values as light as — 10 per mil. Samples from just above the unconformity contain littoral to sublittoral ostracode species that imply warmer, higher-TDS (though still dilute) water than that inferred below the unconformity. Above this zone, another interval with 18O values more negative than — 10 occurs. The isotopic data suggest that two influxes of cold, isotopically light meltwater from Laurentide ice entered the lake, one shortly before 10.3 ka and the other about 9 ka. These influxes were separated by a period during which the lake was warmer, shallower, but still very low in dissolved solids. One or both of the meltwater influxes may be related to discharge from Lake Agassiz into the Great Lakes.Sedimentation rates appear to have been constant from about 10 ka to 5 ka. Bivalve shells formed between about 8 and 5 ka have 18O values that range from-2.3 to-3.3 per mil and appear to decrease toward the end of the interval. The ostracode assemblages and the stable isotopes suggest changes that are climatically controlled, including fluctuating water levels and increasing dissolved solids, although the water remained relatively dilute (TDS < 300 mg/l).A dramatic decrease in sedimentation rates occurred at about 5 ka, about the time of the peak of the Nippissing high lake stage. This decrease in sedimentation rate may be associated with a large increase in effective wave base as the lake approached its present size and fetch. A dramatic reduction in ostracode and mollusk abundances during the late Holocene is probably due to this decrease in sedimentation rates, which would result in increased carbonate dissolution. Ostracode productivity may also have declined due to a reduction in bottom-water oxygen caused by increased epilimnion algal productivity.Woods Hole Oceanographic Institute Contribution No. 7492 相似文献
20.
The stable isotopic records of ostracode valves deposited during the last interglaciation in Raymond Basin, Illinois, have 13C and 18O values as high as +16.5 and +9.2 respectively, the highest values yet reported from continental ostracodal calcite. Located in south-central Illinois, Raymond, Pittsburg, Bald Knob, and Hopwood Farm basins collectively have yielded important long pollen and ostracode records that date from about 130000 years ago to the present. Although fossils from the present-day interglaciation are not well preserved, these records constitute the only described, conformable, fossiliferous successions of this age from the interior of glaciated North America.The high 13C values from Raymond Basin are attributed to the residual effects of methane loss either by ebullition or by emission through the stems of senescent emergent aquatic vegetation. A mass balance model suggests that an increase in 13C of dissolved inorganic carbon on the order of +15 is possible within a few hours given modest rates of methanogenesis of about 0.02 mol m-2 d-1. The 13C records from other studies of ostracode valves have values approaching, but not exceeding about +14 suggesting a limiting value to 13C enrichment due to simultaneous inputs and outputs of dissolved inorganic carbon.Values of 18O in ostracodal calcite are quite variable (–4 to +9) in sediment from the late Sangamon subepisode. A model of isotopic enrichment in a desiccating water body implies that a reduction in reservoir volume of 20% could produce this range of isotopic values. High humidity and evaporation probably account for most of the 18O variability. 相似文献