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971.
Lawrence J. Jackson Christopher Ellis Alan V. Morgan John H. McAndrews 《Geoarchaeology》2000,15(5):415-440
This article investigates changing lake levels in the late Pleistocene eastern Great Lakes in order to gain insights into the Early Palaeo‐Indian occupations. Significant new information bearing on lake level history is provided, notably the first well‐documented deposits of a high water level above modern in the ca. 11,000–10,300 B.P. period in the southern Lake Huron basin. The lake level information, along with paleoenvironmental and site data, reinforces site age estimates to the 11th millennium B.P.; suggests significant numbers of sites have been inundated by rising water levels; provides specific information on the setting of archaeological sites such as placing the Parkhill site adjacent to a large lake estuary; indicates reasons for the attractiveness of shorelines to Palaeo‐Indians including persistence of more open areas conducive to higher game productivity; and points to ideal areas for future archaeological site survey, particularly in the Lake Erie drainage. © 2000 John Wiley & Sons, Inc. 相似文献
972.
Integer ambiguity resolution in precise point positioning: method comparison 总被引:24,自引:10,他引:14
Integer ambiguity resolution at a single receiver can be implemented by applying improved satellite products where the fractional-cycle
biases (FCBs) have been separated from the integer ambiguities in a network solution. One method to achieve these products
is to estimate the FCBs by averaging the fractional parts of the float ambiguity estimates, and the other is to estimate the
integer-recovery clocks by fixing the undifferenced ambiguities to integers in advance. In this paper, we theoretically prove
the equivalence of the ambiguity-fixed position estimates derived from these two methods by assuming that the FCBs are hardware-dependent
and only they are assimilated into the clocks and ambiguities. To verify this equivalence, we implement both methods in the
Position and Navigation Data Analyst software to process 1 year of GPS data from a global network of about 350 stations. The
mean biases between all daily position estimates derived from these two methods are only 0.2, 0.1 and 0.0 mm, whereas the
standard deviations of all position differences are only 1.3, 0.8 and 2.0 mm for the East, North and Up components, respectively.
Moreover, the differences of the position repeatabilities are below 0.2 mm on average for all three components. The RMS of
the position estimates minus those from the International GNSS Service weekly solutions for the former method differs by below
0.1 mm on average for each component from that for the latter method. Therefore, considering the recognized millimeter-level
precision of current GPS-derived daily positions, these statistics empirically demonstrate the theoretical equivalence of
the ambiguity-fixed position estimates derived from these two methods. In practice, we note that the former method is compatible
with current official clock-generation methods, whereas the latter method is not, but can potentially lead to slightly better
positioning quality. 相似文献
973.
Piero Bellanova Mike Frenken Bruce Richmond Jan Schwarzbauer Seanpaul La Selle Frances Griswold Bruce Jaffe Alan Nelson Klaus Reicherter 《Sedimentology》2020,67(3):1230-1248
Far-field tsunami deposits observed in the Kahana Valley, O‘ahu, Hawai‘i (USA), were investigated for their organic-geochemical content. During short high-energy events, (tsunamis and storms) organic and chemical components are transported with sediment from marine to terrestrial areas. This study investigates the use of anthropogenic based organic geochemical compounds (such as polycyclic aromatic hydrocarbons, pesticides and organochlorides) as a means to identify tsunami deposits. Samples were processed by solid–liquid extraction and analyzed using gas chromatography–mass spectrometry. A total of 21 anthropogenic marker compounds were identified, of which 11 compounds were selected for detailed analysis. Although the tsunami deposits pre-date industrial activity in Hawai‘i by several hundred years, distinct changes were found in the concentrations of anthropogenic marker compounds between sandy tsunami deposits and the surrounding mud/peat layers, which may help in identifying tsunami deposits within cores. As expected, low overall concentrations of anthropogenic markers and pollutants were observed due to the lack of industrial input-sources and little anthropogenic environmental impact at the study site. This geochemical characterization of tsunami deposits shows that anthropogenic markers have significant potential as another high-resolution, multi-proxy method for identifying tsunamis in the sedimentary record. 相似文献
974.
Numerical studies of flow over a sill: sensitivity of the non-hydrostatic effects to the grid size 总被引:1,自引:1,他引:0
A non-hydrostatic terrain-following model in cross sectional form is applied to study the processes in the lee of a sill in
an idealized stratified fjord during super-critical tidal inflow. A sequence of numerical studies with horizontal grid sizes
in the range from 100 to 1.5625 m are performed. All experiments are repeated using both hydrostatic and non-hydrostatic versions
of the model, allowing a systematic study of possible non-hydrostatic pressure effects and also of the sensitivity of these
effects to the horizontal grid size. The length scales and periods of the internal waves in the lee of the sill are gradually
reduced and the amplitudes of these waves are increased as the grid size is reduced from 100 down to 12.5 m. With a further
reduction in grid size, more short time and space scale motions become superimposed on the internal waves. Associated with
the internal wave activity, there is a deeper separation point that is fairly robust to all parameters investigated. Another
separation point nearer to the top of the sill appears in the numerical results from the high-resolution studies with the
non-hydrostatic model. Associated with this shallower separation point, an overturning vortex appears in the same set of numerical
solutions. This vortex grows in strength with reduced grid size in the non-hydrostatic experiments. The effects of the non-hydrostatic
pressure on the velocity and temperature fields grow with reduced grid size. In the experiments with horizontal grid sizes
equal to 100 or 50 m, the non-hydrostatic pressure effects are small. For smaller grid sizes, the time mean velocity and temperature
fields are also clearly affected by the non-hydrostatic pressure adjustments. 相似文献
975.
976.
Direct numerical simulation (DNS) is applied to investigate properties of katabatic and anabatic flows along thermally perturbed
(in terms of surface buoyancy flux) sloping surfaces in the absence of rotation. Numerical experiments are conducted for homogeneous
surface forcings over infinite planar slopes. The simulated flows are the turbulent analogs of the Prandtl (1942) one-dimensional
laminar slope flow. The simulated flows achieve quasi-steady periodic regimes at large times, with turbulent fluctuations
being modified by persistent low-frequency oscillatory motions with frequency equal to the product of the ambient buoyancy
frequency and the sine of the slope angle. These oscillatory wave-type motions result from interactions between turbulence
and ambient stable stratification despite the temporal constancy of the surface buoyant forcing. The structure of the mean-flow
fields and turbulence statistics in simulated slope flows is analyzed. An integral dynamic similarity constraint for steady
slope/wall flows forced by surface buoyancy flux is derived and quantitatively verified against the DNS data. 相似文献
977.
In Ethiopia and Sierra Leone, recent social, political and environmental transformations have precipitated the intensification of wetland use, as local people have sought to safeguard and strengthen their livelihoods. Concurrent decentralization policies in both countries have also seen the government strengthen its position at the local level. Drawing upon recent field-based evidence from Ethiopia and Sierra Leone, this paper examines the compatibility between community-based local institutions for wetland use, and the process of decentralization. It argues that decentralization has in fact restricted the development of mature local institutional arrangements, due to its intrinsically political interventionist nature. 相似文献
978.
Sorptive barrier technologies have emerged as useful tools for addressing a wide range of remediation problems. When simulating barrier performance, numerous isotherm expressions are available for relating aqueous and sorbed concentrations. However, isotherm selection is non-trivial because alternative expressions may yield comparable fits to experimental data. Additionally, concentration data collected for parameter fitting is often outside the range of concentrations relevant to simulating barrier performance. This incompatibility necessitates extrapolation of isotherm behavior during simulation–optimization. Consequently, equally plausible isotherms may predict significantly different barrier performance.Numerical experiments involving organic contaminants were performed to examine the influence of isotherm selection and extrapolation on optimal barrier design. Ten isotherms were calibrated to existing experimental data and evaluated using information-theoretic selection criteria. When incorporated into simulation–optimization, extrapolation effects were clearly evident and optimal designs varied according to the chosen isotherm. To ensure robust barrier design in the presence of such variability, a simple methodology is proposed that utilizes a piecewise-minimum isotherm concept. By favoring plausible isotherms that predict the least amount of sorption, the methodology encourages conservative barrier design while respecting available data. 相似文献
979.
David A. Osleger Alan C. Heyvaert Joseph S. Stoner Kenneth L. Verosub 《Journal of Paleolimnology》2009,42(1):103-122
Sediment cores from Lake Tahoe permit the discrimination of turbidites initiated by seismic-induced debris flows from those
generated by severe storms and associated hyperpycnal currents over the last 7000 years using integrated textural, magnetic,
and geochemical signatures. Relative to fine-grained ‘background’ sediments, the majority of Tahoe turbidites exhibit coincident
trends of increased mean grain size, increased magnetic susceptibility, decreased TOC, higher δ13Corg and variable C/N. We interpret these characteristics to record the rapid influx of terrigenous sediments within runoff from
the watershed triggered by high-intensity storms. Correlation of multiple, individual turbidites between cores suggests a
synchronicity of occurrence, supporting the model of extreme hydrologic events as the trigger for most turbidity currents
into Lake Tahoe. In contrast, turbidites generated by seismic collapse of steep lake margins would have textural, magnetic
and geochemical signatures that would reflect a homogenized mix of autochthonous biogenic debris and multiple older turbidites.
Only one of the turbidites in the cores appears to be seismically generated. A second component of this study tested the hypothesis
that turbidite clustering reflects phases of increased storminess, paleoprecipitation and lake level. We correlated broad
patterns of turbidite frequency in the Tahoe cores with climate proxies from (1) elsewhere in the Tahoe watershed, (2) the
western Great Basin (primarily Pyramid Lake) and (3) the San Francisco bay estuary. The reasonable degree of temporal overlap
suggests that apparent trends in severe storm frequency recorded by clusters of turbidites provides a measure of long-term
regional paleoprecipitation and lake level. A key finding is an extended phase of dryness and a near absence of major storms
between ~3000 and ~900 cal yr B.P. in the Tahoe watershed. 相似文献
980.
Depositional history of the Miocene Lake Sinj (Dinaride Lake System,Croatia): a long-lived hard-water lake in a pull-apart tectonic setting 总被引:1,自引:0,他引:1
Oleg Mandic Davor Pavelić Mathias Harzhauser Jožica Zupanič Doris Reischenbacher Reinhard F. Sachsenhofer Neven Tadej Alan Vranjković 《Journal of Paleolimnology》2009,41(3):431-452
Early Miocene transpressional wrenching yielded a series of NW–SE-elongated pull-apart basins in the Dinarides of Croatia
and Bosnia and Herzegovina. They accommodated a huge lake system that gave rise to spectacular endemic mollusk radiation.
Lake Sinj, moderately sized at 342 km2, flooded the south-westernmost basin of this system. Due to the karstic environment, the hard-water, alkaline, long-lived
lake developed a sediment infill with an average thickness of 370 m, dominated by authigenic limestone. The studied section
represents the upper third of the basinal infill and provides detailed insights into the critical period of the lake and of
the basinal evolution during the final stages of its filling. It comprises two large-scale, shallowing-upward cycles, both
starting with fossil-poor limestones, gradually passing into coal-bearing carbonate rocks and coal seams. The fossil-poor
intervals are interpreted as phases of repetitive acidification events due to changing lake level, which induced periodic
drying and flooding of the uppermost littoral zone inhabited by starfruit (Damasonium) meadows. The flooding of the aerated, limy mud plain introduced H+ ions from organic-matter decay reactions into the shallow
lake. This decreased its pH level, with catastrophic consequences for its biota. The ecosystem then stabilized during the
orbitally-forced, dry climate phases. Based on the mollusk record, streams still influenced the marginal lake environment
and rich organic-matter production created swamps and mires. The onset of mollusk radiation in the section correlates with
stabilized lake alkalinity, as indicated by the disappearance of starfruits, ongoing authigenic carbonate production and by
coal seams representing textbook examples for coal formation in alkaline environments. The inferred basinal setting fits well
with the pull-apart basin model, pointing to the presence of an extended shallow ramp in front of a steep, fault-induced hillside
of the hinterland. 相似文献