Vegetation changes and their climatic implication for the late Pleistocene at Lake Poukawa, Hawkes Bay, New Zealand     

M. Okuda  J. Shulmeister  J. R. Flenley 《Global and Planetary Change》2002,33(3-4)

A detailed vegetation history extending back to the Last Interglacial (Marine Isotope Substage 5e) is presented for Lake Poukawa, Hawkes Bay, New Zealand. This history is based on palynological analyses of a 198-m core record, age secured by uranium/thorium (U/Th) and optically stimulated luminescence (OSL) dates as well as tephrochronology. Vegetation of the penultimate glacial (Isotope Stage 6) probably consisted of a very sparse shrubby herbland of stunted podocarps (Phyllocladus sp.), daisies, grasses and sedges. Similar floras existed in each of the succeeding cool periods. Four interglacial/interstadial floras are preserved. The peak of the Last Interglacial (Substage 5e) was dominated by extensive lowland forest of tall forest podocarps (Podocarpus/Prumnopitys), most probably matai (Prumnopitys taxifolia) with red beech (Nothofagus fusca), secondary forest trees (Coprosma spp., Myrsine spp.) and tree ferns (Cyathea spp.). In the Poukawa basin itself, a swamp forest of the podocarp (Dacrycarpus dacrydioides) with the true palm (Rhopalostylis sapida) expanded. Substage 5e was significantly warmer and/or moister than the present day. Substage 5c/5a was marked by montane forest of red and silver beeches (N. fusca and N. menziesii), Phyllocladus spp. and secondary trees. A reduced representation of Podocarpus/Prumnopitys possibly consists of montane podocarp (Podocarpus hallii). We infer the temperature of Substage 5c/5a to have been 3.5–5 °C lower than the present day. Although Stage 3 flora are superficially similar to those of Substage 5c/5a, the beeches are relatively less dominant. The regional flora during Stage 1 is very similar to those of Substage 5e, but the density is lower with no swamp forest during the present interglaciation. This may represent a change in evapotranspiration balance around Poukawa, but may also be controlled by hydrological conditions within the basin.  相似文献   

Ekman层风速随高度分布方程的数值解试验     

刘敏  郭鹏 《中国科学院上海天文台年刊》2005,(1):31-38

根据大气水平运动方程推导出Ekman层风速随高度分布方程,该分布可用二阶线性微分方程表示,用有限差分法求解该二阶线性微分方程的边值问题的数值解,并给出计算程序,输出结果。将结果与经典解析解比较,讨论了经典解的正确性和适用范围。  相似文献   

Phytochemical changes in leaves of subtropical grasses and fynbos shrubs at elevated atmospheric CO₂ concentrations     

D. Hattas  W.D. Stock  W.T. Mabusela  I.R. Green 《Global and Planetary Change》2005,47(2-4):181

The effects of elevated atmospheric CO₂ concentrations on plant polyphenolic, tannin, nitrogen, phosphorus and total nonstructural carbohydrate concentrations were investigated in leaves of subtropical grass and fynbos shrub species. The hypothesis tested was that carbon-based secondary compounds would increase when carbon gain is in excess of growth requirements. This premise was tested in two ecosystems involving plants with different photosynthetic mechanisms and growth strategies. The first ecosystem comprised grasses from a C₄-dominated, subtropical grassland, where three plots were subjected to three different free air CO₂ enrichment treatments, i.e., elevated (600 to 800 μmol mol⁻¹), intermediate (400 μmol mol⁻¹) and ambient atmospheric CO₂. One of the seven grass species, Alloteropsis semialata, had a C₃ photosynthetic pathway while the other grasses were all C₄. The second ecosystem was simulated in a microcosm experiment where three fynbos species were grown in open-top chambers at ambient and 700 μmol mol⁻¹ atmospheric CO₂ in low nutrient acid sands typical of south western coastal and mountain fynbos ecosystems. Results showed that polyphenolics and tannins did not increase in the grass species under elevated CO₂ and only in Leucadendron laureolum among the fynbos species. Similarly, foliar nitrogen content of grasses was largely unaffected by elevated CO₂, and among the fynbos species, only L. laureolum and Leucadendron xanthoconus showed changes in foliar nitrogen content under elevated CO₂, but these were of different magnitude. The overall decrease in nitrogen and phosphorus and consequent increase in C:N and C:P ratio in both ecosystems, along with the increase in polyphenolics and tannins in L. laureolum in the fynbos ecosystem, may negatively affect forage quality and decomposition rates. It is concluded that fast growing grasses do not experience sink limitation and invest extra carbon into growth rather than polyphenolics and tannins and show small species-specific chemical changes at elevated atmospheric CO₂ concentrations. Responses of fynbos species are varied and were species-specific.  相似文献   

The radial velocity of the companion star in the low-mass X-ray binary 2S 0921–630: limits on the mass of the compact object     

P. G. Jonker †  D. Steeghs  G. Nelemans  M. van der Klis 《Monthly notices of the Royal Astronomical Society》2005,356(2):621-626

In this paper we report on optical spectroscopic observations of the low-mass X-ray binary 2S 0921–630 obtained with the Very Large Telescope. We found sinusoidal radial velocity variations of the companion star with a semi-amplitude of  99.1 ± 3.1 km s⁻¹  modulated on a period of 9.006 ± 0.007 d, consistent with the orbital period found previously for this source, and a systemic velocity of  44.4 ± 2.4 km s⁻¹  . Owing to X-ray irradiation, the centre of light measured by the absorption lines from the companion star is probably shifted with respect to the centre of mass. We try to correct for this using the so-called K -correction. Conservatively applying the maximum correction possible and using the previously measured rotational velocity of the companion star, we find a lower limit to the mass of the compact object in 2S 0921–630 of   M_X sin³ i > 1.90 ± 0.25 M_⊙  (1σ errors). The inclination in this system is well constrained since partial eclipses have been observed in X-ray and optical bands. For inclinations in the range  60° < i < 90°  we find  1.90 ± 0.25 < M_X < 2.9 ± 0.4 M_⊙  . However, using this maximum K -correction we find that the ratio between the mass of the companion star and that of the compact object, q , is 1.32 ± 0.37, implying super-Eddington mass-transfer rates; however, evidence for that has not been found in 2S 0921–630. We conclude that the compact object in 2S 0921–630 is either a (massive) neutron star or a low-mass black hole.  相似文献   

X-ray absorption and re-emission from an ionized outflow in the Type 1 quasi-stellar object 2MASS 234449+1221 observed by XMM–Newton     

K. A. Pounds  B. J. Wilkes  K. L. Page 《Monthly notices of the Royal Astronomical Society》2005,362(3):784-788

  相似文献   

环境中二噁英的浓度水平及其控制技术   总被引：1，自引：0，他引：1  

段磊  杨胜科  王文科  冯秀芳 《地球科学与环境学报》2006,28(2):84-88

根据近年来二噁英的污染现状,对环境中二噁英的污染浓度水平、污染来源及途径进行了探讨,结果表明:近10年来,二噁英污染浓度水平仍高于检出标准。造纸制浆、漂白继垃圾焚烧后成为主要污染源。在人体危害方面,深入到二噁英对人体基因和染色体有诱导致变作用。根据二噁英的来源和形成机理,按前期预处理、过程控制和后期处理三个阶段,论述了二噁英的污染控制技术,同时提出了对二噁英污染控制措施。  相似文献   

Variations in dissolved CO2 and CH4 in a first‐order stream and catchment: an investigation of soil–stream linkages     

Diane Hope  Sheila M. Palmer  Michael F. Billett  Julian J. C. Dawson 《水文研究》2004,18(17):3255-3275

Spatial and seasonal variations in CO₂ and CH₄ concentrations in streamwater and adjacent soils were studied at three sites on Brocky Burn, a headwater stream draining a peatland catchment in upland Britain. Concentrations of both gases in the soil atmosphere were significantly higher in peat and riparian soils than in mineral soils. Peat and riparian soil CO₂ concentrations varied seasonally, showing a positive correlation with air and soil temperature. Streamwater CO₂ concentrations at the upper sampling site, which mostly drained deep peats, varied from 2·8 to 9·8 mg l^?1 (2·5 to 11·9 times atmospheric saturation) and decreased markedly downstream. Temperature‐related seasonal variations in peat and riparian soil CO₂ were reflected in the stream at the upper site, where 77% of biweekly variation was explained by an autoregressive model based on: (i) a negative log‐linear relationship with stream flow; (ii) a positive linear relationship with soil CO₂ concentrations in the shallow riparian wells; and (iii) a negative linear relationship with soil CO₂ concentrations in the shallow peat wells, with a significant 2‐week lag term. These relationships changed markedly downstream, with an apparent decrease in the soil–stream linkage and a switch to a positive relationship between stream flow and stream CO₂. Streamwater CH₄ concentrations also declined sharply downstream, but were much lower (<0·01 to 0·12 mg l^?1) than those of CO₂ and showed no seasonal variation, nor any relationship with soil atmospheric CH₄ concentrations. However, stream CH₄ was significantly correlated with stream flow at the upper site, which explained 57% of biweekly variations in dissolved concentrations. We conclude that stream CO₂ can be a useful integrative measure of whole catchment respiration, but only at sites where the soil–stream linkage is strong. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

基于EJB组件的分布式电子商务应用系统框架     

白清源  李成忠 《成都信息工程学院学报》2005,20(6):657-661

在分析J2EE体系结构及EJB组件技术的基础上，提出一种基于EJB组件的分布式电子商务应用系统框架；并对其中各部分所采用技术进行了分析及合理选择；最后讨论了应用服务器选取、负载均衡等关键技术问题。该系统框架具有较好的可扩展性、可重用性、开放性和兼容性。  相似文献   

Probing the wind–wind collision in γ² Velorum with high-resolution Chandra X-ray spectroscopy: evidence for sudden radiative braking and non-equilibrium ionization     

David B. Henley  Ian R. Stevens  Julian M. Pittard 《Monthly notices of the Royal Astronomical Society》2005,356(4):1308-1326

We present a new analysis of an archived Chandra HETGS X-ray spectrum of the WR+O colliding wind binary γ² Velorum. The spectrum is dominated by emission lines from astrophysically abundant elements: Ne, Mg, Si, S and Fe. From a combination of broad-band spectral analysis and an analysis of line flux ratios we infer a wide range of temperatures in the X-ray-emitting plasma (∼4–40 MK). As in the previously published analysis, we find the X-ray emission lines are essentially unshifted, with a mean FWHM of  1240 ± 30 km s⁻¹  . Calculations of line profiles based on hydrodynamical simulations of the wind–wind collision predict lines that are blueshifted by a few hundred  km s⁻¹  . The lack of any observed shift in the lines may be evidence of a large shock-cone opening half-angle (>85°), and we suggest this may be evidence of sudden radiative braking. From the R and G ratios measured from He-like forbidden-intercombination-resonance triplets we find evidence that the Mg  xi emission originates from hotter gas closer to the O star than the Si  xiii emission, which suggests that non-equilibrium ionization may be present.  相似文献   

The Orbital Evolution of Two Sounding Satellites and Analysis of the Accuracy of Orbit Determination     

Gong-you Wu  Chang-yin Zhao  Rong-zhi Zhang  Jia-song Wang  Hong-bo Wang 《Chinese Astronomy and Astrophysics》2007,31(4):420-429

The satellites TC-1 and TC-2 are the two Chinese satellites with great elliptical orbits which are still in orbit around the earth at present. Since the launch the orbits of the two satellites have continuously evolved, which has a certain effect on the orbit determination and prediction precision. The regularities of the orbital evolution of the two sounding satellites are qualitatively and quantitatively analyzed. Under the current tracking mode the corresponding prediction precision of orbit determination is analyzed based on the different stages of the orbital evolution, thereby providing the basis for the adjustment of planning mode by the satellite application departments and the guarantee of normal satellite payload. Finally, the orbital lifetimes of the two satellites are predicted through the trend of the orbital evolution.  相似文献