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21.
A. F. Holland 《Estuaries and Coasts》1985,8(2):93-113
Macrobenthic species abundances and physical/chemical factors known of affect them were measured in a mesohaline region of the Chesapeake Bay from 1971 to 1982. Variation in species abundance due to station differences, seasonal patterns and year-to-year fluctuations in physical/chemical factors were quantified. The major source of variation in abundance of most species was that associated with seasonal recruitment cycles. Spatial variation in abundances of dominant species was small within habitats defined by sediment characteristics. However, abundance variation among sediment types was relatively large. All species showed significant year-to-year fluctuation in abundance, but no species had systematic long-term increases or decreases in abundance during the study period. Macrobenthos populations were persistent over the 11 years within abundance boundaries defined chiefly by fluctuations in salinity and dissolved oxygen concentration. Most species responded to salinity changes, especially those of an extreme nature, with predictable increases or decreases in abundance. Abundances of all species declined rapidly under conditions of low dissolved oxygen concentration (<2 ppm). Macrobenthos with planktonic developmental stages rapidly repopulated the region following periods of population decline. Repopulation by brooding species was slower. 相似文献
22.
Tropical soils could dominate the short-term carbon cycle feedbacks to increased global temperatures 总被引:4,自引:0,他引:4
Results of a simple model of the effects of temperature on net ecosystem production call into question the argument that the large stocks of soil carbon and greater projected warming in the boreal and tu ndra regions of the world willlead to rapid efflux of carbon from these biomes to the atmosphere. We show that low rates of carbon turnover in these regions and a relatively greater response of net primary production to changes in temperature may lead to carbon storage over some limited range of warming. In contrast, the high rates of soil respiration found in tropical ecosystems are highly sensitive to small changes in temperature, so that despite the less pronounced warming expected in equatorial regions, tropical soils are likely to release relatively large amounts of carbon to the atmosphere. Results for high-latitude biomes are highly sensitive to parameter values used, while the net efflux of carbon from the tropics appears robust. 相似文献
23.
Using a combination ofin-vacuo crushing and stepped heating, the40Ar39Ar technique has been applied to two K-feldspar-bearing sedimentary rocks from the Proterozoic of North China, with a view to studying ancient fluids trapped in these rocks and their chronology. Correlations between natural Ar isotopes and those produced by neutron irradiation permitted a clear distinction to be made between different components of natural argon. Crushing released Cl-correlated excess40Ar (and palaeoatmospheric36Ar) trapped in moderately saline fluid inclusions in quartz and possibly the K-feldspar. The very high40Ar/Cl ratios of these fluids (2.7 × 10−4 and1.7 × 10−4) is interpreted as indicating a metamorphic source for the40Ar. More surprisingly, crushing also released, from the K-feldspar, a K-correlated component with a very well defined40Ar/K ratio, which for both samples corresponded formally to an apparent age comparable to or slightly less than the stratigraphic age. In contrast, stepped heating yielded40Ar/K ratios corresponding to significantly younger ages which are interpreted as (low-grade) metamorphic ages. The explanation of the K-correlated component released on crushing and the significance (if any) of the distinct40Ar/K ratio is obscure, although several effects involving microporosity and a combination of39Ar recoil and diffusion are suggested. There is a contribution of this component to the stepped heating release pattern which may have implications for attempts to use K-feldspar as a40Ar39Ar thermochronometer. 相似文献
24.
The purpose of this study was to develop a diffusion model for a continuous point source which takes into account the increase
of wind speed with height, and to compare this model with short-range diffusion experiments. The main problem was to find
a good expression for the vertical diffusion coefficient. It turned out that good agreement between theory and experiment
could only be obtained by introducing a settling speed W for the tracer combined with a conventional expression for the vertical diffusion (K(Z) = K
0Z1−p). An empirical relation was found between K
0 and τ
vU and between W and bar
σ
vU2. 相似文献
25.
M. Karl H.-P. Dorn F. Holland R. Koppmann D. Poppe L. Rupp A. Schaub A. Wahner 《Journal of Atmospheric Chemistry》2006,55(2):167-187
Atmospheric oxidation of isoprene and its oxidation products methacrolein (MACR) and methyl vinyl ketone (MVK) have an important impact on the photochemical activity in the boundary layer, in particular in forested areas. The oxidation of isoprene by OH radicals was investigated in chamber experiments conducted under tropospheric conditions in the atmosphere simulation chamber SAPHIR at the Research Center Jülich. The aim was to determine the product yield of MVK and MACR in the OH-induced isoprene oxidation and the rate constant of their reaction with OH under real atmospheric conditions. The recently published updated degradation scheme for isoprene from Geiger et al. (2003) was used to determine rate constants and product yields. The fractional yields in the isoprene peroxy radical reaction with NO were found to be 0.41±0.03 for MVK and 0.27±0.03 for MACR. The rate coefficient for MACR with OH was found to be in very good agreement with the recommended value of IUPAC Atkinson (Atkinson et al., 2005). while the rate coefficient for MVK with OH was 27% lower. 相似文献
26.
27.
28.
Amphibole zonation in metabasites as a guide to the evolution of metamorphic conditions 总被引:2,自引:0,他引:2
Timothy J. B. Holland Stephen W. Richardson 《Contributions to Mineralogy and Petrology》1979,70(2):143-148
In low grade metabasites the amphibole components tremolite, glaucophane, edenite and tschermakite have their activities controlled
by interactions with the excess components albite, clinozoisite, chlorite, quartz and H2O vapor. Three types of reaction are involved, (i) Those in which only components of condensed phases take part: isopleths
of equilibrium constant are straight lines in the P-T plane. (ii) Dehydration reactions in which entropy change due to change in Al coordination is of the same sign as that due
to dehydration: isopleths of constant K are positive at low pressure and negative at high pressure. (iii) Dehydration reactions
in which entropy change due to Al coordination change is opposite in sign to that of dehydration: isopleths of constant K
loop in the P-T plane with positive slopes at low and at high pressure.
Zonation in naturally occurring amphiboles records the evolution of metamorphic conditions in particular rocks. In an example
from the eastern Alps (Austria) early conditions calculated as 15 kb, 200 ° C evolve upgrade to 6 kb, 525 ° C implying concurrent
heating and erosion. The record of evolving conditions may span some 30 Ma of geological history. 相似文献
29.
The eclogite fragments of the Tauern Window formed at pressures around 20 kbar and temperatures in the region 600–650°C; these pressures are higher by 10–12 kbar than those experienced by the units now surrounding the eclogite-bearing zone. The eclogites probably formed in a subduction zone prior to the main Austroalpine collision: downward shearing forces dominated over buoyancy forces in the subduction zone mélange, permitting the subduction of relatively light material to great depth. At the cessation of subduction this buoyant material returned to the surface, carrying eclogite blocks that were too small to sink rapidly through it. A similar mechanism could account for the emplacement of certain Alpine-type garnet peridotites. 相似文献
30.
A. Hofzumahaus U. Aschmutat U. Brandenburger T. Brauers H.-P. Dorn M. Hausmann M. Heßling F. Holland C. Plass-Dülmer D. H. Ehhalt 《Journal of Atmospheric Chemistry》1998,31(1-2):227-246
In-situ OH measurements by laser-induced fluorescence (LIF) spectroscopy and folded long-path differential optical absorption spectroscopy (DOAS) were carried out in a rural environment in North-East Germany as part of the field experiment POPCORN in August 1994. The large set of OH data obtained allowed an intercomparison of both techniques based on relative diurnal profiles and simultaneously measured absolute concentrations. Most of the time the two OH instruments encountered the same air and agreed well in the measured relative diurnal variations. Only on a few occasions the measurements significantly disagreed due to a perturbation of the DOAS measurements by a local OH source in the north-western wind sector. Excluding data from this wind direction, the statistical analysis of 137 data pairs yields a correlation coefficient of r = 0.90 and a weighted linear fit with a slope of 1.09 ± 0.12. The correlations are carefully analyzed. The comparison of both instruments is discussed in the light of newly published effective absorption cross-sections for H2O and O2 that affect the calibration of LIF. 相似文献