Lake Manitoba, the largest lake in the Prairie region of North America, contains a fine-grained sequence of late Pleistocene and Holocene sediment that documents a complex postglacial history. This record indicates that differential isostatic rebound and changing climate have interacted with varying drainage basin size and hydrologic budget to create significant variations in lake level and limnological conditions. During the initial depositional period in the basin, the Lake Agassiz phase (12–9 ka), 18O of ostracodes ranged from –16 to –5 (PDB), implying the lake was variously dominated by cold, dilute glacial meltwater and warm to cold, slightly saline water.Candona subtriangulata, which prefers cold, dilute water, dominates the most negative 18O intervals, when the basin was part of proglacial Lake Agassiz. At times during this early phase, the 18O of the lake abruptly shifted to higher values; euryhaline taxa such asC. rawsoni orLimnocythere ceriotuberosa, and halobiont taxa such asL. staplini orL. sappaensis are dominant in these intervals. This positive covariance of isotope and ostracode records implies that the lake level episodically fell, isolating the Lake Manitoba basin from the main glacial lake.18O values from inorganic endogenic Mg-calcite in the post-Agassiz phase of Lake Manitoba trend from –4 at 8 ka to –11 at 4.5 ka. We interpret that this trend indicates a gradually increasing influence of isotopically low (–20 SMOW) Paleozoic groundwater inflow, although periods of increased evaporation during this time may account for zones of less negative isotopic values. The 18O of this inorganic calcite abruptly shifts to higher values (–6) after 4.5 ka due to the combined effects of increased evaporative enrichment in a closed basin lake and the increased contribution of isotopically high surface water inflow on the hydrologic budget. After 2 ka, the 18O of the Mg-calcite fluctuates between –13 and –7, implying short-term variability in the lake's hydrologic budget, with values indicating the lake varied from outflow-dominated to evaporation-dominated. The 13C values of Mg-calcite remain nearly constant from 8 to 4.5 ka and then trend to higher values upward in the section. This pattern suggests primary productivity in the lake was initially constant but gradually increased after 4.5 ka.This is the sixth in a series of papers published in this issue on the paleolimnology of arid regions. These papers were presented at the Sixth International Palaeolimnology Symposium held 19–21 April, 1993 at the Australian National University, Canberra, Australia. Dr A. R. Chivas served as guest editor for these papers. 相似文献
An original theoretical model has been devised to simulate mass flow over hill slopes due to gravitational sliding. The sliding mass is discretized into a sequence of contiguous blocks which are subjected to gravitational forces, to bottom friction and to surface resistance stresses that are generally negligible for subaerial flows, but are relevant for submarine slides. The blocks interact with each other while sliding down the hill flanks because of internal forces that dissipate mechanical energy and produce a momentum exchange between the individual blocks, yet conserving the total momentum of the mass. Internal forces are expressed in terms of interaction coefficients depending on the instantaneous distance between the block centers of mass, which is a measure of the deformation experienced by the blocks: the functional dependence includes three parameters, namely the interaction intensity ¯, the deformability parameter and the shape parameter , by means of which a wide range of interaction types can be fully accounted for. The time integration is performed numerically by solving the equations for the block velocities and positions at any time ti by means of the block accelerations at the previous time ti-1, and by subsequently updating the block accelerations, which allows to proceed iteratively to the following times. The model has been tested against laboratory results available from literature and by means of several numerical experiments involving a simplified geometry both for the sliding body and the basal surface, with the purpose of clarifying the influence of the model parameters on the slide dynamics. The model improves the performance of the existing kinematic models for slides, moreover preserving an equivalent numerical simplicity. Future applications and possible improvements of this model are suggested. 相似文献
Dissolved organic matter (DOM) is an important chemical component in natural water. Chromophoric dissolved organic matter (CDOM), a fraction of optical properties, plays art important role in the biogeochemical cycle of nutrients in aquatic environment. People realized that DOM cycle is crucial in the global carbon and nitrogen flux, and also is inherently related to nutrients and trace metal elements. Therefore, CDOM was concerned by scientists in global oceanography and limnology fields. Water samples were collected from three sections (North Channel, South Channel and Zhuyuan) of the Yangtze (Changjiang River) estuary in March 2006 Three-dimensional excitation emission matrix (3-DEEM) fluorescence spectra were analyzed for those filtrates through Whatman GF/F filters. Dissolved organic carbon (DOC) was also measured by TOC analyzer. The tidal variety was also taken into account. The 3-D EEM fluorescence scans suggested the fluorescence characteristics of humic acid (Ex=332-344 nm, Em=439-451 nm) and fulvic acid (Ex=250-254 nm, Em=472-478 nm) were obvious, and the fluorescence group of protein-like and tyrosine (Ex=230 nm, Em=283 nm) was also found. They are mainly composed of CDOM in the Yangtze estuary. Further data analysis, especially the fluorescence index (f 450/500), showed that terrestrial signal was rather strong (1.41-1.65) in the surface water, however, some terrestrial CDOM signals of bottom water showed excursions (1.28-1.39). On the other hand, anthropogenic sign was impressed in the waters of Zhuyuan, which is one of the main drain outlets of Shanghai Metropolis. DOC concentrations ranged from 2.2 mg/L to 3.4 mg/L in Zhuyuan and South Channel, and from 2.0 mg/L to 2.4 mg/L in North Channel. The tide effect played a role in the composition of the CDOM measured by 3-D fluorescence scan technology. 相似文献
We report here a multiphase mineral inclusion composed of quartz, plagioclase, K-feldspar, sapphirine, spinel, orthopyroxene, and biotite, in porphyroblastic garnet within a pelitic granulite from Rajapalaiyam in the Madurai Granulite Block, southern India. In this unique textural association, hitherto unreported in previous studies, sapphirine shows four occurrences: (1) as anhedral mineral between spinel and quartz (Spr-1), (2) subhedral to euhedral needles mantled by quartz (Spr-2), (3) subhedral to anhedral mineral in orthopyroxene, and (4) isolated inclusion with quartz (Spr-4). Spr-1, Spr-2, and Spr-4 show direct grain contact with quartz, providing evidence for ultrahigh-temperature (UHT) metamorphism at temperatures exceeding 1000 °C. Associated orthopyroxene shows high Mg/(Fe + Mg) ratio ( 0.75) and Al2O3 content (up to 9.6 wt.%), also suggesting T > 1050 °C and P > 10 kbar during peak metamorphism.
Coarse spinel (Spl-1) with irregular grain morphology and adjacent quartz grains are separated by thin films of Spr-1 and K-feldspar, suggesting that Spl-1 and quartz were in equilibrium before the stability of Spr-1 + quartz. This texture implies that the P–T conditions of the rock shifted from the stability field of spinel + quartz to sapphirine + quartz. Petrogenetic grid considerations based on available data from the FMAS system favour exhumation along a counterclockwise P–T trajectory. The irregular shape of the inclusion and chemistry of the inclusion minerals are markedly different from the matrix phases suggesting the possibility that the inclusion minerals could have equilibrated from cordierite-bearing silicate-melt pockets during the garnet growth at extreme UHT conditions. 相似文献