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Avery L. C. Shinneman Mark B. Edlund James E. Almendinger Nergui Soninkhishig 《Journal of Paleolimnology》2009,42(3):373-389
Mongolia is an arid land with limited freshwater availability. Recent changes in climate as well as changes in livestock management
in the post-Soviet era may threaten the quality and availability of these freshwater resources and emphasize the need to characterize
current conditions relative to a long-term baseline. Because diatoms are responsive to changes in water chemistry and are
commonly preserved in lake sediments, they can be used as indicators of both present and past water quality. To these ends,
we have developed a diatom-based calibration set and quantitative inference models for the region. Physical and chemical factors
and modern diatom communities were sampled at 54 sites in western Mongolia in the Altai and Khangai mountains and the Valley
of the Great Lakes. Canonical correspondence analysis (CCA) demonstrated that both salinity and total phosphorus accounted
for statistically significant fractions of the variation in 151 diatom taxa across the 54 sampled sites. Diatom-based inference
models were developed with weighted averaging (WA) techniques and showed strong predictive capabilities and low prediction
errors for salinity and total phosphorus. These models are now available to help identify and interpret historical and future
disturbances to this sensitive and globally important eco-region. 相似文献
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The retro‐arc foreland Andean Amazon Basin records sedimentary infill from the South American craton and the emerging Northern Andean chain from the middle Cretaceous until Present day. The U/Pb ages of detrital zircons indicate significant reworking of Archean‐Proterozoic (max. 2.9 Ga) and Paleozoic crust and sediments, which were eroded on both sides. Heavy mineral associations show that the material derived from Proterozoic craton was supplied by Cretaceous reworking of non‐metamorphosed (unannealed) Paleozoic and older sedimentary rocks, which cover the Amazon Craton. Following latest Cretaceous switch of the dominant sediment source to the Andean cordillera, the influx of Precambrian zircons persisted, and these zircons were derived from the metamorphosed basement and Paleozoic sediments of the Cordillera Real (Loja terrane). Re‐evaluation of existing detrital zircon fission‐track record proves that the rise of the Cordillera Real at the Cretaceous‐Tertiary transition was initiated by the collision of Caribbean Oceanic Plateau and associated arc elements from 75–65 Ma. A further important exhumation event also occurred in the Late Oligocene, which is correlated with the break‐up of the Farallon plate. 相似文献
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Thorsten Geisler Roland Vinx Nergui Martin-Gombojav Robert T. Pidgeon 《International Journal of Earth Sciences》2005,94(3):369-384
The Eckergneiss Complex (EGC) is a geologically unique medium- to high-grade metamorphic unit within the Rhenohercynian domain of the Mid-European Variscides. A previously, poorly defined conventional lower U–Pb intercept age of about 560 Ma from detrital zircons of metasedimentary rocks has led to speculations about an East Avalonian affinity of the EGC. In order to unravel the provenance and to constrain the age of the sediment protolith, we carried out sensitive high-resolution ion microprobe U–Pb analyses on detrital zircons from five different EGC quartzite occurrences. The obtained age spectrum indicates a SW Baltica provenance of the detritus. Sveconorwegian ages between 0.9–1.2 Ga are particularly well represented by analyses from metamorphic recrystallization/alteration zones penetrating into igneous zircon. Cadomian (Pan-African) ages, which might reflect a metamorphic event, could not be substantiated. Instead, zircons of igneous origin yielded concordant Lower Devonian and Silurian ages of 410±10, 419±10, and 436±6 Ma (1), implying that sedimentation of the EG protolith must have taken place after 410±10 Ma. The lower age limit of the EGC metamorphism is constrained by 295 Ma intrusion ages of the adjacent, nonmetamorphosed Harzburg Gabbronorite and Brocken Granite. Sedimentation and metamorphism must thus have taken place between about 410 Ma and 295 Ma. Given that this time span coincides with most of the sedimentation within the virtually nonmetamorphosed (lowest grade) Rhenohercynian in the Harz Mountains, including the direct vicinity of the EGC, along with the high-grade metamorphism, the EGC can hardly be seen as uplifted local basement. A possible candidate for the root region is an easterly, concealed marginal segment of the Rhenohercynian domain of the Variscides, which is tectonically overridden and suppressed by the Mid-German Crystalline Rise during continent collision. However, based on the concept of strike-slip movement of Variscan terranes with different P–T–t histories as a result of postaccretion intraplate deformation, the EGC could also represent a fault-bounded complex with an origin located far east or south east of the present location.
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Thorsten GeislerEmail: |
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Qiang Dai Dawei Han Miguel A. Rico‐Ramirez Lu Zhuo Nergui Nanding Tanvir Islam 《水文研究》2015,29(7):1704-1716
Radar‐based estimates of rainfall are affected by many sources of uncertainties, which would propagate through the hydrological model when radar rainfall estimates are used as input or initial conditions. An elegant solution to quantify these uncertainties is to model the empirical relationship between radar measurements and rain gauge observations (as the ‘ground reference’). However, most current studies only use a fixed and uniform model to represent the uncertainty of radar rainfall, without consideration of its variation under different synoptic regimes. Wind is such a typical weather factor, as it not only induces error in rain gauge measurements but also causes the raindrops observed by weather radar to drift when they reach the ground. For this reason, as a first attempt, this study introduces the wind field into the uncertainty model and designs the radar rainfall uncertainty model under different wind conditions. We separate the original dataset into three subsamples according to wind speed, which are named as WDI (0–2 m/s), WDII (2–4 m/s) and WDIII (>4 m/s). The multivariate distributed ensemble generator is introduced and established for each subsample. Thirty typical events (10 at each wind range) are selected to explore the behaviours of uncertainty under different wind ranges. In each time step, 500 ensemble members are generated, and the values of 5th to 95th percentile values are used to produce the uncertainty bands. Two basic features of uncertainty bands, namely dispersion and ensemble bias, increase significantly with the growth of wind speed, demonstrating that wind speed plays a considerable role in influencing the behaviour of the uncertainty band. On the basis of these pieces of evidence, we conclude that the radar rainfall uncertainty model established under different wind conditions should be more realistic in representing the radar rainfall uncertainty. This study is only a start in incorporating synoptic regimes into rainfall uncertainty analysis, and a great deal of more effort is still needed to build a realistic and comprehensive uncertainty model for radar rainfall data. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
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