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291.
Background
The Himalayan zones, with dense forest vegetation, cover a fifth part of India and store a third part of the country reserves of soil organic carbon (SOC). However, the details of altitudinal distribution of these carbon stocks, which are vulnerable to forest management and climate change impacts, are not well known. 相似文献292.
293.
The upper millimeters of sediments are considered major locations of microbial colonization and activity. The goal of our study was to investigate variations of microbial communities in the uppermost sediment layers of the coastal Baltic Sea with emphasis on the complex interplay between microbiology and physico‐chemical sediment properties. We selected a high‐resolution methodological approach that combined the cryolanding sampling technique (spatial resolution of 250 μm) with microsensor‐, spectrophotometric and microscopic analyses. While the oxygen penetration depths in dark conditions ranged from 1.4 mm to 2.6 mm during the study period, this zone expanded by about 1 mm in light and could be divided into three micro‐horizons: (i) an upper zone with a high net O2 production, (ii) an intermediate zone with increased O2 consumption on account of light‐stimulated respiration and (iii) a lower zone with lower O2 consumption. Time‐series experiments revealed a rapid response of the benthic microbial community to altered light intensities. In May and July, the net O2 budget in the porewater of sediments was positive within 35 and 22 min after illumination, respectively, whereas in June O2 production exceeded O2 consumption after 112 min. The thickness of the O2 production and O2 consumption micro‐zones decreased from May to July coinciding with an increase in temperature. In May, sites of enhanced O2 consumption were closely associated with subsurface maxima of microbial numbers and enzymatic activities indicating a tight coupling between photoautotrophic and heterotrophic processes between 1‐ and 2‐mm depth. In June and July, the microbial abundance and enzymatic activity hardly varied with depth. Spatial and temporal microheterogeneity of microbial distribution and activity in O2 gradients was seen as a reflection of the complex interplay between microbiology and physico‐chemical sediment properties. 相似文献
294.
Changes in groundwater elevation may cause a change in the net normal stress and matric potential within the soil mass, which results in volume changes of unsaturated soil. This research investigated the relationship between the drawdown of groundwater and the characteristics of volumetric compressibility of unsaturated soil. Sand column experiments were designed and conducted to measure the volume changes of coarse and fine sands under different types of drainage conditions at fast and slow drainage rates. The finite element program FEMWATER was calibrated and used to simulate the distributions of stress, tension and moisture content within the sands. Finally, based on the changes of net normal stress and matric potential and the observed volume change of the sands, a least‐square method was applied to determine the volumetric consolidation parameters of the unsaturated soils. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
295.
Rainer Wehrse Philipp Rosenau Andrei Survernev James Liebert Christoph Leinert 《Astrophysics and Space Science》1997,255(1-2):127-133
The M dwarfs GL 832 (spectral type M2), GL 887 (M1), and GL 1 (M2) have been observed with the ISOPHOT spectrophotometers
in the range 2.5-12μm at high signal-to-noise ratios. Since optical and ground based near-infrared data are also available,
more than 95 % of the total flux is now covered for these objects. It is seen that between 2.5 μm and about 9 μm the fluxes
fall off faster than a Rayleigh-Jeans tail of a black body distribution and that for longer wavelengths the decline is smaller.
This indicates that the ISO data are probing the region around the temperature minimum and that our stars have some kind of
chromosphere although Hα emission is not observed. Comparison with a model indicates that the features are considerably weaker
than predicted.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
296.
Nadia Vogel Heinrich Baur Addi Bischoff Ingo Leya Rainer Wieler 《Meteoritics & planetary science》2004,39(5):767-778
Abstract— Calcium‐aluminum‐rich inclusions (CAIs) were among the first solids in the solar system and were, similar to chondrules, created at very high temperatures. While in chondrules, trapped noble gases have recently been detected, the presence of trapped gases in CAIs is unclear but could have important implications for CAI formation and for early solar system evolution in general. To reassess this question, He, Ne, and Ar isotopes were measured in small, carefully separated and, thus, uncontaminated samples of CAIs from the CV3 chondrites Allende, Axtell, and Efremovka. The 20Ne/22Ne ratios of all CAIs studied here are <0.9, indicating the absence of trapped Ne as, e.g., Ne‐HL, Ne‐Q, or solar wind Ne. The 21Ne/22Ne ratios range from 0.86 to 0.72, with fine‐grained, more altered CAIs usually showing lower values than coarse‐grained, less altered CAIs. This is attributed to variable amounts of cosmogenic Ne produced from Na‐rich alteration phases rather than to the presence of Ne‐G or Ne‐R (essentially pure 22Ne) in the samples. Our interpretation is supported by model calculations of the isotopic composition of cosmogenic Ne in minerals common in CAIs. The 36Ar/38Ar ratios are between 0.7 and 4.8, with fine‐grained CAIs within one meteorite showing higher ratios than the coarse‐grained ones. This agrees with higher concentrations of cosmogenic 36Ar produced by neutron capture on 35Cl with subsequent β?‐decay in finer‐grained, more altered, and thus, more Cl‐rich CAIs than in coarser‐grained, less altered ones. Although our data do not strictly contradict the presence of small amounts of Ne‐G, Ne‐R, or trapped Ar in the CAIs, our noble gas signatures are most simply explained by cosmogenic production, mainly from Na‐, Ca‐, and Cl‐rich minerals. 相似文献
297.
Rainer Beck 《Astrophysics and Space Science》2004,289(3-4):293-302
Interstellar magnetic fields are strong: up to 25μG in spiralarms and 40μG in nuclear regions.In the spiral galaxy NGC 6946 the average magnetic energy densityexceeds that of the thermal gas. Magnetic fields control the evolution of denseclouds and possibly the global star formation efficiency in galaxies.Gas flows and shocks in spiral arms and bars are modified by magneticfields. Magnetic forces instar-forming circumnuclear regions are able to drive mass inflow towardsthe active nucleus. Magnetic fields are essential for the propagationof cosmic rays and the formation of galactic winds and halos. 相似文献
298.
Thickness of the lithosphere east of the Dead Sea Transform 总被引:1,自引:0,他引:1
299.
300.
Patrick Louchouarn Rainer M.W. Amon Shuiwang Duan Christina Pondell Shaya M. Seward Noah White 《Marine Chemistry》2010,118(1-2):85-97
A series of reference materials are proposed for intercomparison and quality control purposes during the quantification of lignin oxidation products (LOP) from diverse environmental matrices. These materials are all easily accessible and certified for diverse organic constituents (NIST and IHSS). They represent a suite of natural environmental matrices (from solids to aqueous) and are characterized by a wide range of organic carbon and lignin concentrations with abundant proportions of all major LOP. The variability of LOP concentrations and signatures for all these materials averages 3–5% and does not exceed 10%. Using these standards, a new quantification method was developed and validated for the determination of low-level CuO oxidation products using capillary gas chromatography–tandem mass spectrometry (GC/MS–MS). Tandem mass spectrometry provides both the high sensitivity and selectivity required for the identification and quantification of trace levels of dissolved lignin. The method is particularly useful for removing interference from co-eluting isotopes generated from the DOM matrix and during glucose amendment procedures of low-carbon samples. Such glucose amendment is not necessary, however, when the CuO to organic carbon weight-to-weight ratio can be kept at a value < 200–300. 相似文献