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排序方式: 共有576条查询结果,搜索用时 40 毫秒
121.
Samuel D. Chamberlain Joseph Verfaillie Elke Eichelmann Kyle S. Hemes Dennis D. Baldocchi 《Boundary-Layer Meteorology》2017,165(2):197-210
Corrections accounting for air density fluctuations due to heat and water vapour fluxes must be applied to the measurement of eddy-covariance fluxes when using open-path sensors. Experimental tests and ecosystem observations have demonstrated the important role density corrections play in accurately quantifying carbon dioxide \((\hbox {CO}_{2})\) fluxes, but less attention has been paid to evaluating these corrections for methane \((\hbox {CH}_{4})\) fluxes. We measured \(\hbox {CH}_{4}\) fluxes with open-path sensors over a suite of sites with contrasting \(\hbox {CH}_{4}\) emissions and energy partitioning, including a pavement airfield, two negligible-flux ecosystems (drained alfalfa and pasture), and two high-flux ecosystems (flooded wetland and rice). We found that density corrections successfully re-zeroed fluxes in negligible-flux sites; however, slight overcorrection was observed above pavement. The primary impact of density corrections varied over negligible- and high-flux ecosystems. For negligible-flux sites, corrections led to greater than 100% adjustment in daily budgets, while these adjustments were only 3–10% in high-flux ecosystems. The primary impact to high-flux ecosystems was a change in flux diel patterns, which may affect the evaluation of relationships between biophysical drivers and fluxes if correction bias exists. Additionally, accounting for density effects to high-frequency \(\hbox {CH}_{4}\) fluctuations led to large differences in observed \(\hbox {CH}_{4}\) flux cospectra above negligible-flux sites, demonstrating that similar adjustments should be made before interpreting \(\hbox {CH}_{4}\) cospectra for comparable ecosystems. These results give us confidence in \(\hbox {CH}_{4}\) fluxes measured by open-path sensors, and demonstrate that density corrections play an important role in adjusting flux budgets and diel patterns across a range of ecosystems. 相似文献
122.
Samuel J. Oltmans Wolfgang E. Raatz Walter D. Komhyr 《Journal of Atmospheric Chemistry》1989,9(1-3):245-253
A series of nearly daily ozone vertical profiles obtained at station T-3 on Fletcher's Ice Island (85°N, 90°W) during the period January-March 1971 shows several significant ozone intrusions into the troposphere. These intrusions are not only associated with enhanced ozone amounts in the stratosphere but also require tropopause folding events to transport ozone into the troposphere. These folds in the Arctic tropopause appear to be capable of contributing significantly to the ozone budget of the Arctic troposphere during the late winter and spring seasons. The importance of tropopause folding for bringing ozone into the troposphere seen in the daily ozone profiles confirms the results found in the Arctic Gas and Aerosol Sampling Program aircraft flights. 相似文献
123.
Paul M. Myrow Jahandar Ramezani Anne E. Hanson Samuel A. Bowring Grzegorz Racki Michał Rakociński 《地学学报》2014,26(3):222-229
Precise U–Pb zircon dates from three volcanic ash beds that bracket the Hangenberg Shale in the Holy Cross Mountains, Poland, constrain the age and duration of one of the most significant palaeobiological events of the Palaeozoic Era, the Hangenberg Event. It is linked to a terminal Devonian global shift from greenhouse to icehouse climate conditions, a global transgression, and widespread black shale deposition. Our results constrain the Hangenberg Event to between 358.97 ± 0.11 Ma and 358.89 ± 0.20 Ma, with a calculated duration of 0.05 +0.14/?0.05 Ma. A third, underlying ash bed yielded a distinctly older age of 359.97 ± 0.46 Ma. The duration of ~50–100 ka. for the event is comparable to those of Quaternary glaciations, and is consistent with both a glacio‐eustatic origin for the eustatic fluctuations and changes in ocean chemistry that led to this major reorganization of the biosphere. 相似文献
124.
Johanna Lomax Markus Fuchs Pierre Antoine Denis‐Didier Rousseau France Lagroix Christine Hatt Samuel N. Taylor Jessica L. Till Maxime Debret Olivier Moine Diana Jordanova 《Boreas: An International Journal of Quaternary Research》2019,48(1):179-194
The Harletz loess‐palaeosol sequence is located in northwestern Bulgaria and represents an important link between well‐studied loess sequences in eastern Romania and further sites to the west of the Carpathians (e.g. Serbia and Hungary). The aim of this study was to establish a chronostratigraphy of the deposits, using various methods of luminescence dating, together with basic stratigraphical field observations as well as magnetic properties. Luminescence dating was carried out using the quartz fine grain fraction and a SAR protocol, and the feldspar coarse grain fraction, applying the MET‐pIRIR protocol. Due to underestimation of the quartz fine grain fraction in the lower parts of the sequence, the resulting chronology is mainly based on the feldspar ages, which are derived from the stimulation temperature at 150 °C. A comparison with nearby sequences from Serbia, Hungary and Romania, and interpretations obtained through the stratigraphical and sedimentological signature of the sequence, supports the established chronology. Our data suggest that the prominent palaeosol (soil complex) in the upper quarter of the sequence was formed during MIS 5. It would follow that large parts of the Last Glacial loess overlying this palaeosol were probably eroded, and that the thick loess accumulation underlying this soil complex can be allocated to the penultimate glacial (MIS 6). A prominent MIS 6 tephra, which has been reported from other sequences in the area, is also present at Harletz. 相似文献
125.
Samuel M. Hudson Samuel A. Hillam Joel Barker Stephen T. Nelson Kevin A. Rey 《Boreas: An International Journal of Quaternary Research》2019,48(3):810-824
Rock‐Eval pyrolysis provides a quick, relatively inexpensive means of characterizing organic‐rich strata, and has been used for decades to understand global petroleum systems. Although designed to characterize ancient kerogens, pyrolysis is increasingly being used to understand Holocene systems as well. The ability of this technique to distinguish between types of preserved organic matter is useful in characterizing climatic evolution, particularly in systems sensitive to climatic fluctuation such as isolated fens and bogs. Cores collected from the Tokewanna and Garden Basin Cattail fens in central/eastern Utah exhibit variability of organic source, with the mixture of terrestrial and algal sources varying through time, as shown through the hydrogen index (HI) and oxygen index pyrolysis parameters. A sediment core was collected at each fen, and 176 samples were taken from the cores at 6‐cm intervals. Total organic carbon (TOC) for all samples ranges from 1.3 to 44.2%, with an average of 18.2% TOC. Samples range from 84 to 687 HI, equivalent to Type I (lacustrine algal) to Type III (terrestrial) organic material (OM). Variability in HI response represents mixing of the two OM sources, and the relative amount of aqueous organic input can be estimated through time based on age‐calibrated HI curves at the two sites. The balance of organic input serves as an accurate, high‐resolution proxy for climate, and calibration with palynological data near both sites confirms patterns shown by pyrolysis, showing the utility of this method in quickly, affordably and accurately characterizing Holocene sediments for use in understanding palaeoclimate. 相似文献
126.
Samuel R. Green 《Environmental Geology》1985,7(1-2):9-13
When the Tennessee-Tombigbee Waterway of Mississippi and Alabama opened to commercial and recreational traffic on January
16, 1985—some 21 months ahead of schedule—it signaled the completion of the largest Army Corps of Engineers Civil Works project
ever undertaken, and perhaps the last of its kind After more than a century of studies and re-studies and major funding battles
in Congress, the Tennessee-Tombigbee construction faced, predictably, great opposition on environmental grounds The first
major project to be completed under the National Environmental Policy Act, it brought environmental design to the same level
of prominence as engineering design. 相似文献
127.
M. Koenig T. Vinci A. Benuzzi-Mounaix S. Lepape N. Ozaki S. Bouquet L. Boireau S. Leygnac C. Michaut C. Stehle J. -P. Chièze D. Batani T. Hall K. Tanaka M. Yoshida 《Astrophysics and Space Science》2005,298(1-2):69-74
We present the set-up and the results of a supercritical radiative shock experiment performed with the LULI nanosecond laser
facility. Using specific designed targets filled with xenon gaz at low pressure, the propagation of a strong shock with a
radiative precursor is evidenced. The main measured quantities related to the shock (electronic density, propagation velocities,
temperature, radial dimension) are presented and compared with various numerical simulations. 相似文献
128.
Summary. We investigate the issues of stability and conditioning for the one-dimensional seismic inverse problem. We show that these issues are distinct; i.e. that numerically stable implementations of solutions to the inverse problem will not give accurate results if the problem is ill-conditioned. In addition, we identify the factors which determine the condition of the inverse problem. These are the total variation of the acoustic impedance profile being sought and the accuracy of the low-frequency content of the reflection data. We illustrate these results on implementations of two numerically stable algorithms for the inverse problem, one of which has a reputation for being unstable. A comparison shows nearly identical results for the two methods on noise-contaminated and frequency band limited reflection data. In fact, we conjecture that all of the well-known 'layer-stripping' inverse scattering methods share the same mathematical stability characteristics. On the other hand, we also show that ill-conditioning can lead to failure of such algorithms, through amplification of error due either to inaccurate data or to discretization or roundoff. Finally, we observe that appropriate smoothing of the seismic data for an ill-conditioned inverse problem (high-variation impedance profile) can cause the problem to become well-conditioned (lower-variation profile). As is typical with regularizations, the price paid for the newly-acquired ability to solve the problem is a loss of accuracy in the solution. 相似文献
129.
U-Pb systematics of monazite and xenotime: case studies from the Paleoproterozoic of the Grand Canyon, Arizona 总被引:1,自引:0,他引:1
Monazite is accepted widely as an important U-Pb geochronometer in metamorphic terranes because it potentially preserves
prograde crystallization ages. However, recent studies have shown that the U-Pb isotopic system in monazite can be influenced
by a variety of processes that partially obscure the early growth history. In this paper, we attempt to interpret complex
monazite and xenotime U-Pb data from three Paleoproterozoic granite dikes exposed in the Grand Canyon. Single-crystal monazite
analyses from an unfoliated granite dike spread out along concordia from the crystallization age of the dike (defined by U-Pb
zircon data to be 1685 ± 1 Ma) to 1659 ± 2 Ma, a span of 26 million years. Back-scattered electron (BSE) imaging reveals
that magmatic domains within most crystals from this sample are truncated by secondary domains associated with prominent embayments
at the grain margin. Fragments of a single crystal yield contrasting, concordant dates and fragments from the edges and tips
of crystals yield the youngest dates. Based on these observations we suggest that the secondary domains formed at least 26
million years after the crystal formed. Monazite and xenotime dates from the second sample, a sheared dike that cross-cuts
the previous dike, spread out along concordia over 16 million years and range up to 2.4% normally discordant. Again, BSE imaging
reveals secondary domains that truncate both magmatic zoning and xenocrystic cores. Fragments sliced from specific domains
of a previously imaged monazite crystal demonstrate that the secondary domain is 13 million years younger than the core domain.
Textures revealed in BSE images suggest that the secondary domains formed by fluid-mineral interaction. Normal discordance
appears to result from both radiation damage accumulated at temperatures below 300 °C and water-mineral interaction. Monazite
data from the third sample exhibit dispersion in both the 207Pb/206Pb dates (1677–1690 Ma) and discordance (+ 1.6% to − 3.1%). Reverse discordance in these monazites cannot be explained by
incomplete dissolution or excess (thorogenic) 206Pb. Sliced fragments from several crystals reveal dramatic intragrain U-Pb disequilibrium that does not correlate with either
Th or U concentration or position within the crystal. We suggest that reverse discordance resulted from mechanisms that involve
exchange or fractionation of elemental U or elemental Pb, and that neither the U-Pb dates nor the 207Pb/206Pb dates are reliable indicators of the rock's crystallization age. Given the large number of processes proposed in the recent
literature to explain monazite U-Pb systematics from rocks of all ages, our results can be viewed as another cautionary note
for single-crystal and multi-crystal monazite geochronometry. However, we suggest that because individual crystals can preserve
a temporal record of primary and secondary monazite growth, micro-sampling of individual monazite crystals may provide precise
absolute ages on a variety of processes that operate during the prograde, peak and/or retrograde history of metamorphic terranes.
Received: 9 June 1996 / Accepted: 18 October 1996 相似文献
130.
There has been a great deal of discussion about global warming from accumulation of anthropogenic greenhouse gases in the atmosphere (Houghton et al., 1990). Relatively less attention has been paid to spatial and/or temporal climatic variations that may be associated with a warmer climate (Rind et al., 1989) or with anthropogenic activities (Schneider, 1994). In this article, we show that an increase in climatic variability may have started. Fourteen isotopic time series of tree rings are presented. These trees were randomly collected from world-wide locations and cover time periods of 120 to over 200 years. The isotopic records show increasing D values that suggest a consistent and progressive warming occurred in the 19th century in all locations where the trees were sampled. The rate of warming is greater at relatively cold locations than at warm locations with two exceptions. The records also suggest greater climatic variations both temporally and spatially in the 20th century than in the 19th century. 相似文献