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871.
Anisotropy of magnetic susceptibility and structural geology of the ca. 1.45 Ga Karlshamn pluton (southern Sweden) are used
to study its emplacement and structural evolution. The Karlshamn pluton is one of the largest metaluminous A-type granitoid
intrusions in southern Sweden. It is a multiphase body made up of two suites that differ in composition but which have similar
crystallization ages. The magmatic foliation, ductile shear zones and granite–pegmatite filled fractures were mapped as well
as the metamorphic foliation and extension lineation in the metamorphic host rocks. The anisotropy of magnetic susceptibility
was used to map the magnetite petrofabric of the pluton, providing a larger data set for both the magmatic foliations and
lineations, which could not be mapped in the field. The fabrics within the pluton are continuous with the metamorphic fabrics
in the country rocks. Both the pluton and the country rock fabrics were folded during ENE–WSW compression, while the pluton
was still a magma mush. The stress field orientation during cooling of the pluton is determined on the basis of magmatic,
ductile and brittle structures in the Karlshamn pluton that formed successively as the pluton cooled. The compressional event
is referred to as the Danopolonian orogeny and therefore the Karlshamn granitoids, and other plutons of similar composition
and age in central and southern Sweden, on the Danish Island of Bornholm, and in Lithuania, may be considered as syntectonic
intrusions and not as anorogenic, as was previously thought. 相似文献
872.
Kunchithapadam Gopalan J. Douglas Macdougall Christopher Macisaac 《Geostandards and Geoanalytical Research》2007,31(3):227-236
For studies of mass-dependent fractionation of calcium isotopes in natural materials, the 48 Ca/42 Ca ratio is a superior choice to the conventionally measured 44 Ca/40 Ca ratio for two important reasons. These are (1) mass-dependent fractionation can be determined free from the effects of inherited or ingrown radiogenic 40 Ca and (2) this ratio increases the spread of measured isotopic masses by 50%, resulting in statistically better resolution of fractionation, assuming similar precision. A third, though strictly technical, advantage is the inherent ability of a mass spectrometer to measure ratios close to unity (48 Ca/42 Ca) more precisely than very small or large ratios (44 Ca/40 Ca). However, because of the very low natural abundance of both 48 Ca and 42 Ca, their ratio has been very difficult to measure, the only attempt so far being on a high mass resolution MC-ICP-MS with a precision of 0.33%. We report here determination of the 48 Ca/42 Ca ratio by the more commonly available and user-friendly multi-collector TIMS using a 43 Ca-46 Ca double-spike, with a significantly better precision of 0.18% (2s). The 48 Ca/40 Ca or 44 Ca/40 Ca ratio can also be measured in the same mass spectrometer run to provide complementary information on any radiogenic component. 相似文献
873.
Sediment microphytobenthos, such as diatoms and photosynthetic bacteria, are functionally important components of food webs
and are key mediators of nutrient dynamics in marine wetlands. The medium to long-term recovery of benthic microproducers
in restored habitats designed to improve degraded coastal wetland sites is largely unknown. Using taxon-specific photopigments,
we describe the composition of microphytobenthic communities in a large restoration site in southern California and differences
in the temporal recovery of biomass (chlorophylla), composition, and taxonomic diversity between vegetatedSpartina foliosa salt marsh and unvegetated mudflat. Visually distinct, spatially discreet, microphytobenthic patches appeared after no more
than 7 mo within the restoration site and were distinguished by significant differences in biomass, taxonomic diversity, and
the relative abundance of cyanobacteria versus diatoms. Sediment chlorophylla concentrations within the restored site were similar to concentrations in nearby natural habitat 0.2–2.2 yr following marsh
creation, suggesting rapid colonization by microproducers. RestoredSpartina marsh very rapidly (between 0.2 and 1.2 yr) acquired microphytobenthic communities of similar composition and diversity to
those in naturalSpartina habitat, but restored mudflats took at least 1.6 to 2.2 yr to resemble natural mudflats. These results suggest relatively
rapid recovery of microphytobenthic communities at the level of major taxonomic groups. Sediment features, such as pore water
salinity andSpartina density, explained little variation in microphytobenthic taxonomic composition. The data imply that provision of structural
heterogeneity in wetland construction (such as pools and vascular plants) might speed development of microproducer communities,
but no direct seeding of sediment microfloras may be necessary. 相似文献
874.
Christopher A Williams Niall P Hanan Jason C Neff Robert J Scholes Joseph A Berry Scott A Denning David F Baker 《Carbon balance and management》2007,2(1):3-13
The African continent has a large and growing role in the global carbon cycle, with potentially important climate change implications.
However, the sparse observation network in and around the African continent means that Africa is one of the weakest links
in our understanding of the global carbon cycle. Here, we combine data from regional and global inventories as well as forward
and inverse model analyses to appraise what is known about Africa's continental-scale carbon dynamics. With low fossil emissions
and productivity that largely compensates respiration, land conversion is Africa's primary net carbon release, much of it
through burning of forests. Savanna fire emissions, though large, represent a short-term source that is offset by ensuing
regrowth. While current data suggest a near zero decadal-scale carbon balance, interannual climate fluctuations (especially
drought) induce sizeable variability in net ecosystem productivity and savanna fire emissions such that Africa is a major
source of interannual variability in global atmospheric CO2. Considering the continent's sizeable carbon stocks, their seemingly high vulnerability to anticipated climate and land use
change, as well as growing populations and industrialization, Africa's carbon emissions and their interannual variability
are likely to undergo substantial increases through the 21st century. 相似文献
875.
Dust emission from wet and dry playas in the Mojave Desert,USA 总被引:1,自引:0,他引:1
Richard L. Reynolds James C. Yount Marith Reheis Harland Goldstein Pat Chavez Jr. Robert Fulton John Whitney Christopher Fuller Richard M. Forester 《地球表面变化过程与地形》2007,32(12):1811-1827
The interactions between playa hydrology and playa‐surface sediments are important factors that control the type and amount of dust emitted from playas as a result of wind erosion. The production of evaporite minerals during evaporative loss of near‐surface ground water results in both the creation and maintenance of several centimeters or more of loose sediment on and near the surfaces of wet playas. Observations that characterize the texture, mineralogic composition and hardness of playa – surfaces at Franklin Lake, Soda Lake and West Cronese Lake playas in the Mojave Desert (California), along with imaging of dust emission using automated digital photography, indicate that these kinds of surface sediment are highly susceptible to dust emission. The surfaces of wet playas are dynamic surface texture and sediment availability to wind erosion change rapidly, primarily in response to fluctuations in water‐table depth, rainfall and rates of evaporation. In contrast, dry playas are characterized by ground water at depth. Consequently, dry playas commonly have hard surfaces that produce little or no dust if undisturbed except for transient silt and clay deposited on surfaces by wind and water. Although not the dominant type of global dust, salt‐rich dusts from wet playas may be important with respect to radiative properties of dust plumes, atmospheric chemistry, windborne nutrients and human health. Published in 2007 by John Wiley & Sons, Ltd. 相似文献
876.
877.
Andrey A. Grachev Edgar L Andreas Christopher W. Fairall Peter S. Guest P. Ola G. Persson 《Boundary-Layer Meteorology》2007,124(3):315-333
Measurements of atmospheric turbulence made during the Surface Heat Budget of the Arctic Ocean Experiment (SHEBA) are used
to examine the profile stability functions of momentum, φ
m
, and sensible heat, φ
h
, in the stably stratified boundary layer over the Arctic pack ice. Turbulent fluxes and mean meteorological data that cover
different surface conditions and a wide range of stability conditions were continuously measured and reported hourly at five
levels on a 20-m main tower for 11 months. The comprehensive dataset collected during SHEBA allows studying φ
m
and φ
h
in detail and includes ample data for the very stable case. New parameterizations for φ
m
(ζ) and φ
h
(ζ) in stable conditions are proposed to describe the SHEBA data; these cover the entire range of the stability parameter
ζ = z/L from neutral to very stable conditions, where L is the Obukhov length and z is the measurement height. In the limit of very strong stability, φ
m
follows a ζ 1/3 dependence, whereas φ
h
initially increases with increasing ζ, reaches a maximum at ζ ≈ 10, and then tends to level off with increasing ζ. The effects
of self-correlation, which occur in plots of φ
m
and φ
h
versus ζ, are reduced by using an independent bin-averaging method instead of conventional averaging. 相似文献
878.
879.
Jarrod R. Hurley † Christopher A. Tout Sverre J. Aarseth Onno R. Pols 《Monthly notices of the Royal Astronomical Society》2001,323(3):630-650
We present a state-of-the-art N -body code which includes a detailed treatment of stellar and binary evolution as well as the cluster dynamics. This code is ideal for investigating all aspects relating to the evolution of star clusters and their stellar populations. It is applicable to open and globular clusters of any age. We use the N -body code to model the blue straggler population of the old open cluster M67. Preliminary calculations with our binary population synthesis code show that binary evolution alone cannot explain the observed numbers or properties of the blue stragglers. On the other hand, our N -body model of M67 generates the required number of blue stragglers and provides formation paths for all the various types found in M67. This demonstrates the effectiveness of the cluster environment in modifying the nature of the stars it contains, and highlights the importance of combining dynamics with stellar evolution. We also perform a series of N =10 000 simulations in order to quantify the rate of escape of stars from a cluster subject to the Galactic tidal field. 相似文献
880.
Anthony J. Reid Andrew P. Fowler David Phillips Christopher J.L. Wilson 《Journal of Asian Earth Sciences》2005,25(6):9
Four K-feldspar samples from the Yidun Arc, eastern Tibetan Plateau, were analysed by the 40Ar/39Ar method with the aim of recovering information on their thermal history using multiple diffusion domain (MDD) theory. Arrhenius plots for each of the samples reveal low retentivity early in the heating experiments, a property that is attributed to their recrystallised nature. This low argon retentivity appears to violate the MDD assumption that volume diffusion is the only mechanism for argon transport within the crystals, thus the thermal histories derived from these analyses are considered suspect. Nevertheless, the age spectra themselves suggest that the majority of samples had cooled below 200 °C prior to the Eocene collision of India with Asia. Thermal history modelling from apatite fission track analyses from the same and nearby samples shows slow cooling through the apatite fission track partial annealing zone during the Cenozoic in samples from the high elevation, low relief areas of the Yidun Arc, while samples from the major Jinsha River valley show rapid cooling through the partial annealing zone beginning in the Miocene. These results suggest that significant Cenozoic denudation has been localised and that most parts of the Yidun Arc have experienced very little denudation during the Cenozoic. 相似文献