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991.
The Younger Dryas stadial (11 000-10 000 yr BP) was an abrupt return to a glacial climate during the termination of the last glaciation. We have reconstructed atmospheric CO2 concentrations from a high-resolution sequence of fossil Salix herbacea leaves through this climatic oscillation from Kråkenes, western Norway, using the relationship between leaf stomatal density and atmospheric CO2 concentration. High Allerød CO2 values (median 273 ppmv) decreased rapidly during 130–200 14C-years of the late Allerød to ca. 210 ppmv at the start of the Younger Dryas. They then increased steadily through the Younger Dryas, reaching typical interglacial values once more (ca. 275 ppmv) in the Holocene. The rapid late Allerød decrease in CO2 concentration preceded the Younger Dryas temperature drop, possibly by several decades. This striking pattern of changes has not so far been recorded unambiguously in temporally coarse measurements of atmospheric CO2 from ice cores. Our observed late-glacial CO2 changes have implications for global modelling of the ocean-atmosphere-biosphere over the last glacial-interglacial transition. 相似文献
992.
黄淮海平原河流的纵剖面凹度特征 总被引:1,自引:0,他引:1
本文以黄淮海平原为例,对于冲积河流纵剖面凹度的形成机理及其与不同河型的关系,进行了初步探讨。 相似文献
993.
M. Stephenson J. Klaverkamp M. Motycka C. Baron W. Schwartz 《Journal of Paleolimnology》1996,15(1):99-106
Sediments of Lake 382, Experimental Lakes Area, Canada, were sampled at six sites using a 5-cm Wildco KB core sampler (KB), a similar device incorporating a ball check valve (BC), and a 0.2 m by 1.2 m flat-faced aluminum freeze core sampler (FC). Cores were sectioned at 1-cm intervals to a depth of 15 cm. Contaminant (210Pb and 137Cs) concentrations (Bq g-1) were measured by gamma spectroscopy, and inventories (Bq cm-2) were calculated following standard methods. Sediments collected using FC, BC and KB had similar contaminant concentrations, however, cores collected by FC and BC had lower estimated inventories than KB cores. Differences between estimates appear to be caused by differences in the water content (WC) of core material. Laboratory studies confirm that FC sediments have higher WC than tube-cored sediment. We hypothesize that ice crystal formation increases the WC of freeze cores, resulting in lower contaminant inventories. Loss of surficial sediment caused by a bow wave may have a similar effect on BC samples. We conclude that KB core gear is appropriate for sampling sediments to measure contaminant concentrations and inventories in recently deposited sediments. 相似文献
994.
GUO HUA FAN TONG QI YAO KE LI HAN KE JIA ZHU ZUO WEN GU Institute of Geophysics China Seismological Bureau Beijing China 《地震学报(英文版)》1998,11(6):719-727
ntroductionThevariationmagneticfieldoftheEarthcontainsabundantinformation,suchastheinformationrelativetosolaractivity,solarte... 相似文献
995.
L. Delobbe 《Boundary-Layer Meteorology》1998,89(1):75-107
Marine stratocumulus observations show a large variability in cloud droplet number concentration (CDNC) related to variability in aerosol concentration. Changes in CDNC modify the cloud reflectivity, but also affect cloud water content, cloud lifetime, and cloudiness, through changes in precipitation. In mesoscale models and general circulation models (GCMs), precipitation mechanisms are parameterized. Here we examine how the precipitation parameterization can affect the simulated cloud. Simulations are carried out with the one-dimensional version of the hydrostatic primitive equation model MAR (Modéle Atmosphérique Régional) developed at the Université catholique de Louvain. It includes a E- turbulence closure, a wide-band formulation of the radiative transfer, and a parameterized microphysics including prognostic equations for water vapour, cloud droplets and rain drops concentrations. In a first step, the model is used to simulate a horizontally homogeneous stratocumulus deck observed during the Atlantic Stratocumulus Transition Experiment (ASTEX) on the night of 12–13 June 1992. The observations show that the model is able to realistically reproduce the vertical structure of the cloud-topped boundary layer. In a second step, several precipitation parameterizations commonly used in mesoscale models and GCMs are tested. It is found that most parameterizations tend to overestimate the precipitation, which results in an underestimation of the vertically integrated liquid water content. Afterwards, using those parameterizations that are sensitive to CDNC, several simulations are performed to estimate the effect of CDNC variations on the simulated cloud. Based upon the simulation results, we argue that currently used parameterizations do not enable assessment of such a sensitivity. 相似文献
996.
Lianhong Gu 《Boundary-Layer Meteorology》1998,87(3):515-524
This paper considers a modification to the localized near-field (LNF) theory for predicting scalar concentrations from source distributions in plant canopies. It is argued that the non-diffusive, near-field part of the transition probability Pn, and thus the near-field contribution of the scalar concentration Cn, in the original theory are simply errors. The (approximate) equality of the sum of Cn and the diffusive, far-field contribution Cf with the total scalar concentration C is not guaranteed in general by this theory. A revision is suggested. 相似文献
997.
998.
本文对求解土壤或岩石中氡浓度及其表面析出率的边界条件进行了探讨。以空气中无穷远处氡浓度为零(空气中无氡源)的自然边界条件代替空气与土壤或岩石交界面氡浓度为零的边界条件,给出了土壤和岩石中氡浓度及其表面析出率的具体表达式。与前人研究结果比较,该式中增加了一项,表面氡析出率缩小了一个倍数。通过对土壤、岩石的具体计算.表明两种边界条件的计算结果有相当的偏差。同时,文中还讨论了在两类边界条件下对流对土壤或岩石中氡浓度及其表面析出率计算的影响。 相似文献
999.
贵州西部煤层中若干有害微量元素的分布及迁移富集规律 总被引:3,自引:0,他引:3
焦建伟 《华北地质矿产杂志》1998,13(3):236-242
煤中有害微量元素不仅影响煤的加工利用,而且对生态环境及人类健康有极大危害,本文利用现代分析测试手段,对贵州西部晚二叠世主采煤层中的有害微量元素As,F,Hg,Pt,Cd等的含量,分布赋存状态及其迁移富集规律进行了研究。 相似文献
1000.
碲在官田黄铁矿矿床中的聚集及地球化学 总被引:1,自引:0,他引:1
碲是官田黄铁矿矿床中的重要伴生组分,高度聚集在寄主矿物黄铁矿中,含碲矿物是楚碲铋矿、赫碲铋矿和碲铋矿。成碲和聚碲的物理化学条件是:压力低于10MPa,温度140~220℃,含碲流体的盐度低于12ω(NaCl),离子类型为SO2-4-K+-Ca2+型,pH值为4.4~5.8,Eh值为-0.5V,fO2<10-25×105Pa,fCO2<101.8×105Pa,fS2为(10-21.34~10-15.72)×105Pa,fTe2为(10-11.93~10-8.72)×105Pa。含碲热液具多来源,成碲和聚碲时期以天水为主导。碲的可能迁移形式是[TeS3]2-和[AuTeS3]-。文章最后与大水沟独立碲矿床进行了综合对比。 相似文献