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   

Long-term modeling of soil C erosion and sequestration at the small watershed scale   总被引：1，自引：0，他引：1  

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   

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   

Error Analysis of Weekly Station Coordinates in the DORIS Network   总被引：1，自引：2，他引：1  

Simon D. P. Williams  Pascal Willis 《Journal of Geodesy》2006,80(8-11):525-539

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Investigating New Wireless Technologies and Their Potential Impact on Children's Spatiality: A Role for GIS     

Morris Williams  Owain Jones  Lucy Wood  Constance Fleuriot 《Transactions in GIS》2006,10(1):87-102

This paper outlines exploratory research undertaken by the “A New Sense of Place?” Project in Bristol, UK, into the potential new, location sensitive, computing technologies may have for enhancing urban children's socio‐spatial practices. The paper describes a series of workshops held with children in which mapping activities and use of the technologies are supported by the use of a Geographic Information System (GIS). The paper suggests that sound‐enabled GIS could play a major role in the management of such technologies.  相似文献   

The Sensitivity of Australian Fire Danger to Climate Change   总被引：14，自引：2，他引：12  

Allyson A. J. Williams  David J. Karoly  Nigel Tapper 《Climatic change》2001,49(1-2):171-191

Global climate change, such as that due to the proposed enhanced greenhouseeffect, is likely tohave a significant effect on biosphere-atmosphere interactions, includingbushfire regimes. Thisstudy quantifies the possible impact of climate change on fire regimes byestimating changes infire weather and the McArthur Forest Fire Danger Index (FDI), an index thatis used throughoutAustralia to estimate fire danger. The CSIRO 9-level general circulation model(CSIRO9 GCM)is used to simulate daily and seasonal fire danger for the present Australianclimate and for adoubled-CO₂ climate. The impact assessment includes validation ofthe GCMs daily controlsimulation and the derivation of correction factors which improve theaccuracy of the firedanger simulation. In summary, the general impact of doubled-CO₂is to increase firedanger at all sites by increasing the number of days of very high and extremefire danger.Seasonal fire danger responds most to the large CO₂-induced changesin maximumtemperature.  相似文献   

Three routes to jet collimation by the Balbus–Hawley magnetorotational instability     

Peter T. Williams  † 《Monthly notices of the Royal Astronomical Society》2005,361(1):345-356

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The problem of small divisors in planetary motion     

Carol A. Williams 《Celestial Mechanics and Dynamical Astronomy》1984,34(1-4):395-410

Small divisors caused by certain linear combinations of frequencies appear in all analytical planetary theories. With the exception of the deep resonance between Neptune and Pluto, they can be removed at the expense of introducing secular and mixed secular terms, limiting the domain in which the solution is valid. Because of them classical solutions are known not to converge uniformly; Poincaré referred to them as asymptotic. The KAM theory shows that if one is far enough from exact commensurability and has small enough planetary masses, expansions exist which will converge to quasi-periodic orbits. Solutions showing very small divisors are excluded from this region of convergence. The question of whether they are intrinsic to the problem or are just manifestations of the method of solution is not settled. Problems with a single commensurabily that can be isolated from the rest of the Hamiltonian may have solutions with no small divisors. The problem of two or more commensurabilities remains unsolved.  相似文献   

The final Galileo SSI observations of Io: orbits G28-I33     

Elizabeth P. Turtle  Laszlo P. Keszthelyi  Jani Radebaugh  Damon P. Simonelli  David A. Williams  Windy L. Jaeger  H.Herbert Breneman  Cynthia B. Phillips  the Galileo SSI Team 《Icarus》2004,169(1):3-28

We present the observations of Io acquired by the Solid State Imaging (SSI) experiment during the Galileo Millennium Mission (GMM) and the strategy we used to plan the exploration of Io. Despite Galileo's tight restrictions on data volume and downlink capability and several spacecraft and camera anomalies due to the intense radiation close to Jupiter, there were many successful SSI observations during GMM. Four giant, high-latitude plumes, including the largest plume ever observed on Io, were documented over a period of eight months; only faint evidence of such plumes had been seen since the Voyager 2 encounter, despite monitoring by Galileo during the previous five years. Moreover, the source of one of the plumes was Tvashtar Catena, demonstrating that a single site can exhibit remarkably diverse eruption styles—from a curtain of lava fountains, to extensive surface flows, and finally a ∼400 km high plume—over a relatively short period of time (∼13 months between orbits I25 and G29). Despite this substantial activity, no evidence of any truly new volcanic center was seen during the six years of Galileo observations. The recent observations also revealed details of mass wasting processes acting on Io. Slumping and landsliding dominate and occur in close proximity to each other, demonstrating spatial variation in material properties over distances of several kilometers. However, despite the ubiquitous evidence for mass wasting, the rate of volcanic resurfacing seems to dominate; the floors of paterae in proximity to mountains are generally free of debris. Finally, the highest resolution observations obtained during Galileo's final encounters with Io provided further evidence for a wide diversity of surface processes at work on Io.  相似文献   

Effects of the passage of an IMF discontinuity on the polar cap geometry and the formation of a polar cap arc     

S.-I. Akasofu  R. Williams  M. Roederer 《Planetary and Space Science》1984,32(1):119-125

Changes of the geometry of the open field line region (namely, the polar cap) caused by the passage of a tangential IMF discontinuity are simulated using the model constructed by Akasofu and Roederer (1983). A singly-bounded open field line region tends to split into two, forming a narrow closed field line region and thus allowing the formations of a plasma sheet and of an auroral arc across the highest latitude region of the Earth. The three-dimensional geometry of some of the closed field lines in the narrow closed region is examined. In this connection, an interesting observation of the formation of an auroral arc over Thule, Greenland, is reported.  相似文献