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981.
John A. Williams 《Estuarine, Coastal and Shelf Science》1983,16(3):291-298
The diurnal burrow depth distribution of Talitrus saltator, a sand-beach talitrid amphipod was examined in relation to moisture content and temperature profiles of the substrate. Individuals burrow to depths maintaining a moisture content of between 2·0 and 4·0% irrespective of the temperature profile, and therefore burrow depth is suggested to be regulated primarily by the requirements of the amphipods for an optimal humidity environment. 相似文献
982.
A chemical network determining the formation and destruction of C+, C, and CO and other species is set up and applied to spherical clouds in the normal interstellar radiation field. The spherical geometry adopted gives results which are different from those for slab models. The sensitivity of chemical species to chemical and astronomical parameters is explored. The relevance of this work to the observations is discussed. 相似文献
983.
A current research project is addressing the problem of finding appropriate geographical information by developing a (geo)information realization resource based upon the concepts of the Literate Traveller. It has as its main goal to provide tools for geographical knowledge building and exploration, by providing conventional spatial information plus geographical interpretation and appreciation artifacts. These artifacts are delivered in an interactive multimedia environment. This research examines the formulation of an initial prototype component to assist in providing geographical knowledge. The prototype is based on a study site in Townsville, Australia, and is being used as a vehicle to explore enhanced spatial information provision through the use of rich media. The research examines new ways to prospect for, discover and disseminate spatial knowledge within an intelligence‐briefing context. The paper reports on the concepts behind the design of the application developed for the test site, and presents initial findings from the early evaluation of the prototype. 相似文献
984.
Influence of surface heterogeneity on scalar dissimilarity in the roughness sublayer 总被引:1,自引:8,他引:1
Christopher A. Williams Todd M. Scanlon John D. Albertson 《Boundary-Layer Meteorology》2007,122(1):149-165
While it is generally known that surface heterogeneity weakens the application of Monin–Obukhov similarity (MOS), few studies
have investigated how seasonal changes in the degree of surface heterogeneity at a particular site may influence the validity
of the similarity application. Exploiting seasonal changes in forest function associated with senescence, we conduct a unique
evaluation of the effects of surface heterogeneity on the validity of similarity theory at two sites through time. Using high
frequency (10 Hz) velocity and scalar time series collected within the roughness sublayer over mixed hardwood deciduous and
coniferous forests during both periods of peak leaf area and senescence of deciduous foliage, we examined conformity with
proposed universal flux-variance predictions and agreement amongst normalized standard deviations of different scalars (temperature,
water vapour and carbon dioxide concentrations). Normalized scalar standard deviations were elevated above MOS flux-variance
predictions, with more pronounced deviations observed during and following senescence, particularly in the case of CO2. Power-law scaling of normalized standard deviations as a function of stability was upheld and robust to seasonal changes
in surface heterogeneity. However, dissimilarity of normalized standard deviations for the scalars increased during senescence,
as heterogeneity in the source/sink field increased. Scalewise decomposition of scalar time series using wavelet analysis
indicated that correlations between scalars were conservative through much of the inertial cascade but decayed for eddies
< 10 m. Senescence lowered correlations between scalars over a wide range of eddy sizes. These results demonstrate how seasonal
changes in surface physiology can cause a temporal production of heterogeneity in the source/sink field, thus weakening similarity
applications in the roughness sublayer. 相似文献
985.
David A Williams, Wendy A Brown, Stephen D Price, Jonathan M C Rawlings and Serena Viti of University College London review how far experimental work on molecular synthesis can be applied to the interstellar medium – and how it is becoming a tool for the understanding of astrophysical processes. 相似文献
986.
R. C. Izaurralde J. R. Williams W. M. Post A. M. Thomson W. B. McGill L. B. Owens R. Lal 《Climatic change》2007,80(1-2):73-90
The soil C balance is determined by the difference between inputs (e.g., plant litter, organic amendments, depositional C)
and outputs (e.g., soil respiration, dissolved organic C leaching, and eroded C). There is a need to improve our understanding
of whether soil erosion is a sink or a source of atmospheric CO2. The objective of this paper is to discover the long-term influence of soil erosion on the C cycle of managed watersheds
near Coshocton, OH. We hypothesize that the amount of eroded C that is deposited in or out of a watershed compares in magnitude
to the soil C changes induced via microbial respiration. We applied the erosion productivity impact calculator (EPIC) model to evaluate the role of erosion–deposition
processes on the C balance of three small watersheds (∼1 ha). Experimental records from the USDA North Appalachian Experimental
Watershed facility north of Coshocton, OH were used in the study. Soils are predominantly silt loam and have developed from
loess-like deposits over residual bedrock. Management practices in the three watersheds have changed over time. Currently,
watershed 118 (W118) is under a corn (Zea mays L.)–soybean (Glycine max [L.] Merr.) no till rotation, W128 is under conventional till continuous corn, and W188 is under no till continuous corn.
Simulations of a comprehensive set of ecosystem processes including plant growth, runoff, and water erosion were used to quantify
sediment C yields. A simulated sediment C yield of 43 ± 22 kg C ha−1 year−1 compared favorably against the observed 31 ± 12 kg C ha−1 year−1 in W118. EPIC overestimated the soil C stock in the top 30-cm soil depth in W118 by 21% of the measured value (36.8 Mg C
ha−1). Simulations of soil C stocks in the other two watersheds (42.3 Mg C ha−1 in W128 and 50.4 Mg C ha−1 in W188) were off by <1 Mg C ha−1. Simulated eroded C re-deposited inside (30–212 kg C ha−1 year−1) or outside (73–179 kg C ha−1 year−1) watershed boundaries compared in magnitude to a simulated soil C sequestration rate of 225 kg C ha−1 year−1 and to literature values. An analysis of net ecosystem carbon balance revealed that the watershed currently under a plow
till system (W128) was a source of C to the atmosphere while the watersheds currently under a no till system (W118 and W188)
behaved as C sinks of atmospheric CO2. Our results demonstrate a clear need for documenting and modeling the proportion of eroded soil C that is transported outside
watershed boundaries and the proportion that evolves as CO2 to the atmosphere. 相似文献
987.
Paleoenvironmental proxy records from Lake Hovsgol, Mongolia, and a synthesis of Holocene climate change in the Lake Baikal watershed 总被引:1,自引:0,他引:1
Alexander A. Prokopenko Galina K. Khursevich Mikhail I. Kuzmin Douglas F. Williams Nataliya V. Kulagina Anna A. Abzaeva 《Quaternary Research》2007,68(1):2-17
Here we discuss paleoenvironmental evolution in the Baikal region during the Holocene using new records of aquatic (diatom) and terrestrial vegetation changes from Hovsgol, Mongolia's largest and deepest lake. We reconcile previous contradictory Baikal timescales by constraining reservoir corrections of AMS dates on bulk sedimentary organic carbon. Synthesis of the Holocene records in the Baikal watershed reveals a northward progression in landscape/vegetation changes and an anti-phase behavior of diatom and biogenic silica proxies in neighboring rift lakes. In Lake Baikal, these proxies appear to be responsive to annual temperature increases after 6 ka, whereas in Lake Hovsgol they respond to higher precipitation/runoff from 11 to 7 ka. Unlike around Lake Baikal, warmer summers between 6 and 3.5 ka resulted in the decline, not expansion, of forest vegetation around Lake Hovsgol, apparently as a result of higher soil temperatures and lower moisture availability. The regional climatic proxy data are consistent with a series of 500-yr time slice Holocene GCM simulations for continental Eurasia. Our results allow reevaluation of the concepts of ‘the Holocene optimum’ and a ‘maximum of the Asian summer monsoon’, as applied to paleoclimate records from continental Asia. 相似文献
988.
El Galeno and Michiquillay are early to middle Miocene Cu–Au–Mo porphyry-related deposits located in the auriferous Cajamarca
district of northern Peru. The El Galeno deposit (486 Mt at 0.57% Cu, 0.14 g/t Au and 150 ppm Mo) is associated with multiple
dioritic intrusions hosted within Lower Cretaceous quartzites and shales. Emplacement of the porphyry stocks (17.5–16.5 Ma)
in a hanging wall anticline was structurally controlled by oblique faults superimposed on early WNW-trending fold-thrust structures.
Early K-feldspar–biotite–magnetite (potassic) alteration was associated with pyrite and chalcopyrite mineralisation. A quartz–magnetite
assemblage that occurs at depth has completely replaced potassically altered rocks. Late- and post-mineralisation stocks are
spatially and temporally related to weak quartz–muscovite (phyllic) alteration. High Au grades are associated with early intrusive
phases located near the centre of the deposit. Highest Cu grades (~0.9% Cu) are mostly associated with a supergene enrichment
blanket, whilst high Mo grades are restricted to contacts with the metasedimentary rocks. The Michiquillay Cu–Au–Mo deposit
(631 Mt at 0.69% Cu, 0.15 g/t Au, 100–200 ppm Mo) is associated with a Miocene (20.0–19.8 Ma) dioritic complex that was emplaced
within the hanging wall of a back thrust fault. The intrusive complex is hosted in quartzites and limestones. The NE-trending
deposit is crosscut by NNW-trending prospect-scale faults that influenced both alteration and metal distribution. In the SW
and NE of the deposit, potassic alteration zones contain moderate hypogene grades (0.14 g/t Au and 0.8% Cu) and are characterised
by chalcopyrite and pyrite mineralisation. The core of the deposit is defined by a lower grade (0.08 g/t Au and 0.57% Cu)
phyllic alteration that overprinted early potassic alteration. Michiquillay contains a supergene enrichment blanket of 45–80 m
thickness with an average Cu grade of 1.15%, which is overlain by a deep leached cap (up to 150 m). Cu–Au–Mo (El Galeno-Michiquillay)
and Au-rich (Minas Conga) deposits in the Cajamarca region are of similar age (early–middle Miocene) and intrusive rock type
(dioritic) associations. Despite these geochronological and geochemical similarities, findings from this study suggest variation
in metal grade between the hybrid-type and Au-rich deposits result from a combination of physio-chemical factors. These include
variations in temperature and oxygen fugacity conditions during hypogene mineralisation resulting in varied sulphide assemblages,
host rock type, precipitation of ubiquitous hydrothermal magnetite, and late hydrothermal fluid flow resulting in a well-developed
phyllic alteration zone. 相似文献
989.
The climate of the future: clues from three million years ago 总被引:1,自引:0,他引:1
In the eighteenth century, James Hutton came up with a theory that revolutionized the science of geology, ‘The present is a key to the past’. But could the past also be a guide to our future? The world three million years ago was warmer than present. What might it tell us about global climate change in the near future? 相似文献
990.
This investigation examined plant survival and water sources for small depressions excavated to build an earthen dam and nature
trail in north-central Texas, USA. These “inadvertent wetlands” occupy part of an outdoor education facility, overlying alluvial
deposits of the Trinity River. A large lake behind the earthen dam strongly influences surface water and groundwater levels
in the area. Excavated depressions receive direct precipitation, runoff, and groundwater inputs, losing water by seepage and
evapotranspiration. Hydroperiods varied with location and water input: excavations receiving groundwater held water year round;
others periodically desiccated. Groundwater-fed depressions had higher salinity; however, the highest average value (1,304 μS/cm)
was within the freshwater range. A total of 66 to 75% of emergent and floating wetland species planted in three depressions
survived after 2 years. The developing wetlands appear viable under hydrologic conditions typical of the study period. Ultimately,
variable hydroperiods among wetlands, and water level fluctuations within individual wetlands, may foster diverse plant-species
compositions. Depressed lake levels following long-term drought or increased water demands pose the greatest threat to the
developing wetlands. 相似文献