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51.
52.
Michel E Baglin A Auvergne M Catala C Samadi R Baudin F Appourchaux T Barban C Weiss WW Berthomieu G Boumier P Dupret MA Garcia RA Fridlund M Garrido R Goupil MJ Kjeldsen H Lebreton Y Mosser B Grotsch-Noels A Janot-Pacheco E Provost J Roxburgh IW Thoul A Toutain T Tiphène D Turck-Chieze S Vauclair SD Vauclair GP Aerts C Alecian G Ballot J Charpinet S Hubert AM Lignières F Mathias P Monteiro MJ Neiner C Poretti E de Medeiros JR Ribas I Rieutord ML Cortés TR Zwintz K 《Science (New York, N.Y.)》2008,322(5901):558-560
Oscillations of the Sun have been used to understand its interior structure. The extension of similar studies to more distant stars has raised many difficulties despite the strong efforts of the international community over the past decades. The CoRoT (Convection Rotation and Planetary Transits) satellite, launched in December 2006, has now measured oscillations and the stellar granulation signature in three main sequence stars that are noticeably hotter than the sun. The oscillation amplitudes are about 1.5 times as large as those in the Sun; the stellar granulation is up to three times as high. The stellar amplitudes are about 25% below the theoretic values, providing a measurement of the nonadiabaticity of the process ruling the oscillations in the outer layers of the stars. 相似文献
53.
A full-dimensional quantum dynamics simulation of a hydrogen atom reacting with methane on an accurate ab initio potential energy surface is reported. Based on first-principles theory, thermal rate constants are predicted with an accuracy comparable to (or even exceeding) experimental precision. The theoretical prediction is within the range of the significantly varied experimental rate constants reported by different groups. This level of accuracy has previously been achieved only for smaller, three-or four-atom reactive systems. Comparison with classical transition state theory confirms the importance of quantum mechanical tunneling for the rate constant below 400 kelvin. 相似文献
54.
Spielhagen RF Werner K Sørensen SA Zamelczyk K Kandiano E Budeus G Husum K Marchitto TM Hald M 《Science (New York, N.Y.)》2011,331(6016):450-453
The Arctic is responding more rapidly to global warming than most other areas on our planet. Northward-flowing Atlantic Water is the major means of heat advection toward the Arctic and strongly affects the sea ice distribution. Records of its natural variability are critical for the understanding of feedback mechanisms and the future of the Arctic climate system, but continuous historical records reach back only ~150 years. Here, we present a multidecadal-scale record of ocean temperature variations during the past 2000 years, derived from marine sediments off Western Svalbard (79°N). We find that early-21st-century temperatures of Atlantic Water entering the Arctic Ocean are unprecedented over the past 2000 years and are presumably linked to the Arctic amplification of global warming. 相似文献
55.
Striking circular, labyrinthine, polygonal, and striped patterns of stones and soil self-organize in many polar and high alpine environments. These forms emerge because freeze-thaw cycles drive an interplay between two feedback mechanisms. First, formation of ice lenses in freezing soil sorts stones and soil by displacing soil toward soil-rich domains and stones toward stone-rich domains. Second, stones are transported along the axis of elongate stone domains, which are squeezed and confined as freezing soil domains expand. In a numerical model implementing these feedbacks, circles, labyrinths, and islands form when sorting dominates; polygonal networks form when stone domain squeezing and confinement dominate; and stripes form as hillslope gradient is increased. 相似文献
56.
Challenges of modeling ocean basin ecosystems 总被引:1,自引:0,他引:1
deYoung B Heath M Werner F Chai F Megrey B Monfray P 《Science (New York, N.Y.)》2004,304(5676):1463-1466
With increasing pressure for a more ecological approach to marine fisheries and environmental management, there is a growing need to understand and predict changes in marine ecosystems. Biogeochemical and physical oceanographic models are well developed, but extending these further up the food web to include zooplankton and fish is a major challenge. The difficulty arises because organisms at higher trophic levels are longer lived, with important variability in abundance and distribution at basin and decadal scales. Those organisms at higher trophic levels also have complex life histories compared to microbes, further complicating their coupling to lower trophic levels and the physical system. We discuss a strategy that builds on recent advances in modeling and observations and suggest a way forward that includes approaches to coupling across trophic levels and the inclusion of uncertainty. 相似文献
57.
Soil moisture strongly controls the uptake of atmospheric methane by limiting the diffusion of methane into the soil, resulting in a negative correlation between soil moisture and methane uptake rates under most non-drought conditions. However, little is known about the effect of water stress on methane uptake in temperate forests during severe droughts. We simulated extreme summer droughts by exclusion of 168 mm (2001) and 344 mm (2002) throughfall using three translucent roofs in a mixed deciduous forest at the Harvard Forest, Massachusetts, USA. The treatment significantly increased CH4 uptake during the first weeks of throughfall exclusion in 2001 and during most of the 2002 treatment period. Low summertime CH4 uptake rates were found only briefly in both control and exclusion plots during a natural late summer drought, when water contents below 0.15 g cm−3 may have caused water stress of methanotrophs in the A horizon. Because these soils are well drained, the exclusion treatment had little effect on A horizon water content between wetting events, and the effect of water stress was smaller and more brief than was the overall treatment effect on methane diffusion. Methane consumption rates were highest in the A horizon and showed a parabolic relationship between gravimetric water content and CH4 consumption, with maximum rate at 0.23 g H2O g−1 soil. On average, about 74% of atmospheric CH4 was consumed in the top 4-5 cm of the mineral soil. By contrast, little or no CH4 consumption occurred in the O horizon. Snow cover significantly reduced the uptake rate from December to March. Removal of snow enhanced CH4 uptake by about 700-1000%, resulting in uptake rates similar to those measured during the growing season. Soil temperatures had little effect on CH4 uptake as long as the mineral soil was not frozen, indicating strong substrate limitation of methanotrophs throughout the year. Our results suggest that the extension of snow periods may affect the annual rate of CH4 oxidation and that summer droughts may increase the soil CH4 sink of temperate forest soils. 相似文献
58.
W. Werner 《植物养料与土壤学杂志》1987,150(5):358-358
59.
Marion A. Sonnleitner Madeleine S. Günthardt-Goerg Inga K. Bucher-Wallin Werner Attinger Sybille Reis Rainer Schulin 《Water, air, and soil pollution》2001,126(3-4):271-290
Sixteen open-top chambers, each equipped with two non-weighablegravity-drained lysimeter compartments, were used to investigate the impacts of elevated atmospheric carbon dioxide (CO2) concentration and nitrogen (N) deposition on the water relations and growth of young model forest ecosystems on two different types of soils. The same vegetation of a mixed spruce and beech overstorey and various herbs in the understorey was planted in all treatments on both soils. The soils were repacked on top of a drainage layer. Four combinations of treatments were applied in four replicates each: ambient (370 cm3 m-3) CO2 + low (7 kg N ha-1 a-1) N deposition, ambient CO2 + high(70 kg N ha-1 a-1) N deposition, elevated (590 cm3 m-3) CO2 + low N deposition, and elevated CO2 + high N deposition. After canopy closure, treatment effects on evapotranspiration and growth during the third year of study were very different for the two soils.On the acidic sandy loam, elevated CO2 enhanced growth(leaf biomass +21%, roots +27%) at reduced evapotranspiration (–9%). High N deposition increased aboveground growth even more strongly (+50%), but also increased evapotranspiration (+16%). Together, elevated CO2 and high N had a more than additive fertilizer effect on growth, while their effects on evapotranspirationcompensated. On the calcareous loamy sand, elevated CO2not only tended to enhance growth (leaf biomass +17%, roots +20%), but also increased evapotranspiration (+5%).On this soil, aboveground growth was stimulated by N only incombination with elevated CO2, but less than on the acidic soil, while evapotranspiration (–6.5%) and root growth into the subsoil (–54%) were decreased by increased N deposition at both CO2 concentrations, in contrast to the N treatments on the acidic sandy loam. The influence of the soil on the observed ecosystem responses canbe interpreted in terms of the concept of optimal resource allocation. 相似文献
60.
Michael Ochs Ivano Brunner Werner Stumm BoŽena ćosović 《Water, air, and soil pollution》1993,68(1-2):213-229
The effect of complex natural organic ligands on the weathering kinetics of aluminum oxide was investigated in laboratory experiments. A peat-derived humic substance and root exudates obtained from ectomycorrhizal (Picea abies — Hebeloma crustuliniforme) and non-mycorrhizal Norway Spruce trees; and γ-Al2O3 were used as a model system. The experimental weathering rates are in accordance with a surface-controlled dissolution mechanism. The effect of the humic material on dissolution rates appears to depend on the degree of protonation of the humic (macro)molecules: we observed dissolution-enhancement or -inhibition at pH 3 and 4, respectively. Ectomycorrhizal exudates proved to be effective weathering agents at pH 4, as opposed to humic material and non-mycorrhizal exudates. Our results suggest that (i) the role of humic materials in mineral weathering and podzolization is different from what is commonly thought, and (ii) mineral weathering rates in the rhizosphere may be higher than in the bulk soil. 相似文献