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排序方式: 共有159条查询结果,搜索用时 0 毫秒
1.
Erika Sternberg Catherine Jeandel Juan-Carlos Miquel Beat Gasser Marc Souhaut Roseanna Arraes-Mescoff Roger Francois 《Marine Chemistry》2007,105(3-4):281-295
Biogenic barium, mostly in the barite (BaSO4) form, has been proposed as a tracer for export production in the ocean. Here we report on biogenic barium (Baxs) and particulate organic carbon (POC) fluxes from sediment traps deployed at the DYFAMED site in the Northwestern Mediterranean Sea. Baxs fluxes display average values of 37 ± 45 and 50 ± 58 μg/m2/d at 200 and 1000 m respectively, and are linearly correlated to POC fluxes (mean values of 7.9 ± 9.3 and 6.8 ± 6.8 mg C/m2/d at 200 and 1000 m). Export production estimates, calculated using published Baxs- or POC-based algorithms, all fall below or close to the lower limit of potential export values proposed in the literature. This work clearly demonstrates the usefulness of Baxs as a tracer of oceanic export production in the Northwestern Mediterranean Sea. However, development of a quantitative export production proxy requires a clear understanding of the underlying cause(s) for the observed spatial variations in the relationship between Baxs and POC fluxes. The present study confirms that the processes leading to barite formation differ between margin and open-ocean sites and probably account for much of the regional variability in the POC/Baxs ratio. 相似文献
2.
Norbert I. K 《冰川冻土》2004,26(Z1):310-318
The exploration of ice sheets by melting vertical holes into the ground has some tradition in terrestrial glaciology. Such probes have been used since the 1960's to investigate the vertical structure of the ice in Greenland and Antarctica and in alpine glaciers. In this paper we look into the possibility to develop similar devices for use on extraterrestrial icy bodies, like e.g. the polar areas on Mars or the icy satellites of the outer solar system. We report on some basic experiments performed in the cryo-vacuum laboratory of the Space Research Institute of the Austrian Academy of Sciences, Graz. In these experiments the penetration of a simple melting probe into compact and porous water ice (with a snow-like texture) was monitored, both under vacuum conditions and under air pressure. The observed penetration speeds for a given power supply are compared with a simple mathematical model. We conclude that a miniature melting probe with small overall dimensions and a reasonable power demand could well be part of the payload of a future planetary mission, for example to the poles of Mars. Such missions are currently under discussion in several space agencies. Moreover such probes could also e? ectively be used in terrestrial environments. A possible design is presented at the end of the paper. 相似文献
3.
Filippo Giorgi Erika Coppola Francesca Raffaele Gulilat Tefera Diro Ramon Fuentes-Franco Graziano Giuliani Ashu Mamgain Marta Pereira Llopart Laura Mariotti Csaba Torma 《Climatic change》2014,125(1):39-51
We analyze changes of four extreme hydroclimatic indices in the RCP8.5 projections of the Phase I CREMA experiment, which includes 21st century projections over 5 CORDEX domains (Africa, Central America, South America, South Asia, Mediterranean) with the ICTP regional model RegCM4 driven by three CMIP5 global models. The indices are: Heat Wave Day Index (HWD), Maximum Consecutive Dry Day index (CDD), fraction of precipitation above the 95th intensity percentile (R95) and Hydroclimatic Intensity index (HY-INT). Comparison with coarse (GPCP) and high (TRMM) resolution daily precipitation data for the present day conditions shows that the precipitation intensity distributions from the GCMs are close to the GPCP data, while the RegCM4 ones are closer to TRMM, illustrating the added value of the increased resolution of the regional model. All global and regional model simulations project predominant increases in HWD, CDD, R95 and HY-INT, implying a regime shift towards more intense, less frequent rain events and increasing risk of heat wave, drought and flood with global warming. However, the magnitudes of the changes are generally larger in the global than the regional models, likely because of the relatively low “climate sensitivity” of the RegCM4, especially when using the CLM land surface scheme. In addition, pronounced regional differences in the change signals are found. The data from these simulations are available for use in impact assessment studies. 相似文献
4.
In an elementary approach every geometrical height difference between the staff points of a levelling line should have a corresponding
average g value for the determination of potential difference in the Earth’s gravity field. In practice this condition requires as
many gravity data as the number of staff points if linear variation of g is assumed between them. Because of the expensive fieldwork, the necessary data should be supplied from different sources.
This study proposes an alternative solution, which is proved at a test bed located in the Mecsek Mountains, Southwest Hungary,
where a detailed gravity survey, as dense as the staff point density (~1 point/34 m), is available along a 4.3-km-long levelling
line. In the first part of the paper the effect of point density of gravity data on the accuracy of potential difference is
investigated. The average g value is simply derived from two neighbouring g measurements along the levelling line, which are incrementally decimated in the consecutive turns of processing. The results
show that the error of the potential difference between the endpoints of the line exceeds 0.1 mm in terms of length unit if
the sampling distance is greater than 2 km. Thereafter, a suitable method for the densification of the decimated g measurements is provided. It is based on forward gravity modelling utilising a high-resolution digital terrain model, the
normal gravity and the complete Bouguer anomalies. The test shows that the error is only in the order of 10−3mm even if the sampling distance of g measurements is 4 km. As a component of the error sources of levelling, the ambiguity of the levelled height difference which
is the Euclidean distance between the inclined equipotential surfaces is also investigated. Although its effect accumulated
along the test line is almost zero, it reaches 0.15 mm in a 1-km-long intermediate section of the line. 相似文献
5.
A time-dependent microphysical model is used to study the evolution of ethane ice clouds in Titan’s atmosphere. The model simulates nucleation, condensational growth, evaporation, coagulation, and transport of particles. For a critical saturation of 1.15 (a lower limit, determined by laboratory experiments), we find that ethane clouds can be sustained between altitudes of 8 and 50 km. Growth due to coalescence is inefficient, limiting the peak in the size distribution (by number) to 10 μm. These clouds vary with a period of about 20 days. This periodicity disappears for higher critical saturation values where clouds remain subvisible. Rainout of ethane due to methane cloud formation raises the altitude of the ethane cloud bottom to near the tropopause and may eliminate ethane clouds entirely if methane cloud formation occurs up to 30 km. However, clouds formed above the troposphere from other gases in Titan’s atmosphere could be sustained even with rainout up to 30 km. Although the optical depth of ethane clouds above 20 km is typically low, short-lived clouds with optical depths of order 0.1-1 can be created sporadically by dynamically driven atmospheric cooling. Ethane cloud particles larger than 25 μm can fall to the surface before total evaporation. However, ethane clouds remain only a small sink for tholin particles. At the peak of their cycle, the optical depth of ethane clouds could be comparable to that of tholin in the near-infrared, resulting in a 5% increase in Titan’s albedo for wavelengths between 1 and 2 μm. A number of factors limit our ablility to predict the ethane cloud properties. These factors include the mixing time in the troposphere, the critical saturation ratio for ethane ice, the existence of a surface reservoir of ethane, the magnitude and timing of dynamically driven temperature perturbations, and the abundance and life cycle of methane clouds. 相似文献
6.
Norbert I. Kömle Hui Bing Wen Jie Feng Roman Wawrzaszek Erika S. Hütter Ping He Wojciech Marczewski Borys Dabrowski Kathrin Schröer Tilman Spohn 《Acta Geotechnica》2007,2(2):127-138
A series of thermal conductivity measurements for various materials was performed in a large climate chamber. The size of
the chamber allowed the preparation of relatively large samples in a controlled thermal environment. Three types of thermal
sensors were used: (1) two needle probes; (2) a grid of temperature sensors, evenly distributed inside the sample; (3) two
additional thermal probes, which were simplified versions of an instrument originally developed for measuring thermal properties
of the ice/dust mixture expected to exist at the surface of a comet nucleus. They consist of a series of individual temperature
sensors integrated into a glass fibre rod. Each of these sensors can be operated in an active (heated) or passive (only temperature
sensing) mode. The following sample materials were used: fine-grained reddish sand, coarse-grained moist sand, gravels with
various grain size distributions from < 1 cm up to about 6 cm, and for comparison and calibration pure water (with convection
suppressed by adding agar-agar), compact ice, and compact granite. Of particular interest are the measurements with composite
samples, like stones embedded in an agar-agar matrix. We describe the evaluation methods and present the results of the thermal
conductivity measurements. 相似文献
7.
Noriko T. Kita Qing‐Zhu Yin Glenn J. MacPherson Takayuki Ushikubo Benjamin Jacobsen Kazuhide Nagashima Erika Kurahashi Alexander N. Krot Stein B. Jacobsen 《Meteoritics & planetary science》2013,48(8):1383-1400
High‐precision bulk aluminum‐magnesium isotope measurements of calcium‐aluminum‐rich inclusions (CAIs) from CV carbonaceous chondrites in several laboratories define a bulk 26Al‐26Mg isochron with an inferred initial 26Al/27Al ratio of approximately 5.25 × 10?5, named the canonical ratio. Nonigneous CV CAIs yield well‐defined internal 26Al‐26Mg isochrons consistent with the canonical value. These observations indicate that the canonical 26Al/27Al ratio records initial Al/Mg fractionation by evaporation and condensation in the CV CAI‐forming region. The internal isochrons of igneous CV CAIs show a range of inferred initial 26Al/27Al ratios, (4.2–5.2) × 10?5, indicating that CAI melting continued for at least 0.2 Ma after formation of their precursors. A similar range of initial 26Al/27Al ratios is also obtained from the internal isochrons of many CAIs (igneous and nonigneous) in other groups of carbonaceous chondrites. Some CAIs and refractory grains (corundum and hibonite) from unmetamorphosed or weakly metamorphosed chondrites, including CVs, are significantly depleted in 26Al. At least some of these refractory objects may have formed prior to injection of 26Al into the protosolar molecular cloud and its subsequent homogenization in the protoplanetary disk. Bulk aluminum and magnesium‐isotope measurements of various types of chondrites plot along the bulk CV CAI isochron, suggesting homogeneous distribution of 26Al and magnesium isotopes in the protoplanetary disk after an epoch of CAI formation. The inferred initial 26Al/27Al ratios of chondrules indicate that most chondrules formed 1–3 Ma after CAIs with the canonical 26Al/27Al ratio. 相似文献
8.
Dissolved and particulate samples were collected to study the distribution of thorium isotopes (234Th, 232Th and 230Th) in the water column of the Indian sector of the Southern Ocean (from 42°S to 47°S and from 60°E to 66°E, north of the Polar Front) during Austral summer 1999. Vertical profiles of excess 230Th (230Thxs) increases linearly with depth in surface water (0–100 m) and a model was applied to estimate a residence time relative to the thorium scavenging (τscav). Low τscav in the Polar Front Zone (PFZ) are found, compared to those estimated in the Subtropical Front Zone (STZ). Changes in particle composition between the PFZ and STZ could influence the 230Thxs scavenging efficiency and explain this difference. An innovative coupling between 234Th and 230Thxs was then used to simultaneously constrain the settling velocities of small (0.6–60 μm) and large (above 60 μm) particles. Although the different hydrological and biogeochemical regimes visited during the ANTARES IV cruise did not explain the spatial variation of sinking velocity estimates, our results indicate that less particles may reach the seafloor north (60 ± 2 m d− 1, station 8) than south of the Agulhas Return Current (119 ± 23 and 130 ± 5 m d− 1 at stations 3 and 7, respectively). This information is essential for understanding particle transport and by extension, carbon export. In the deep water column, the 230Thxs concentrations did not increase linearly with depth, probably due to lateral transport of North Atlantic Deep Water (NADW) from the Atlantic to the Indian sector, which renews the deep waters and decreases the 230Thxs concentrations. A specific 230Thxs transport model is applied in the deep water column and allows us to assess a “travel time” of NADW ranging from 2 to 15 years. 相似文献
9.
Elva Escobar Briones Erika Laura Estrada Santilln Pierre Legendre 《Deep Sea Research Part II: Topical Studies in Oceanography》2008,55(24-26):2679
The composition, density and community structure of the benthic macrofauna were investigated in sediments of the Campeche Canyon in the SW Gulf of Mexico. Total macrofaunal density ranged from 9466±2736 ind m−2 at the continental shelf station to 1550±195 ind m−2 in the canyon. Density values significantly diminished with distance from the coast and depth; only a few stations in the center of the canyon displayed larger density values (E-37 with 4666±1530 ind m−2, E-36 with 5791±642 ind m−2 and E-26 with 6925±2258 ind m−2). Densities were positively correlated to organic nitrogen in the sediment (r=0.82) and coarse silt (r=0.43), and negatively with depth (r=−0.74) and distance from the coast (r=−0.68). At all stations, the polychaete worms contributed most to the multi-species community structure. The nematodes and Foraminifera displayed their highest densities in the center of the canyon. The biomass values declined significantly with depth. We conclude that the macrofauna density and biomass changed in response to organic matter contents in the sediment, both with distance from the coast and with depth. 相似文献
10.
Public debate and prior scholarship tend to emphasize the link between gentrification and high-end chain retail, underscoring the importance of taste cultures tied to social class in making sense of gentrification’s impact on neighborhood identity. In this study, we present evidence from 10 years of change in the businesses of Brooklyn, New York City, a period of extensive gentrification across a wide variety of neighborhood and census-tract level contexts. Adopting census tracts clustered in neighborhoods as the units of analysis, we model the effects of institutional and demographic change on two separate outcomes: chain retail density and homogeneity in the types of goods and services available. We argue that changes in consumer culture embedded in broader processes of gentrification are neither “chaotic” nor “unitary” but are “segmented” according to local spatial and demographic context, taking two discrete forms: institutionally facilitated corporatization; lifestyle-driven homogenization. 相似文献