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801.
Fukashi Maeno Masashi Nagai Setsuya Nakada Rose E. Burden Samantha Engwell Yuki Suzuki Takayuki Kaneko 《Bulletin of Volcanology》2014,76(6):1-16
Constraining physical parameters of tephra dispersion and deposition from explosive volcanic eruptions is a significant challenge, because of both the complexity of the relationship between tephra distribution and distance from the vent and the difficulties associated with direct and comprehensive real-time observations. Three andesitic subplinian explosions in January 2011 at Shinmoedake volcano, Japan, are used as a case study to validate selected empirical and theoretical models using observations and field data. Tephra volumes are estimated using relationships between dispersal area and tephra thickness or mass/area. A new cubic B-spline interpolation method is also examined. Magma discharge rate is estimated using theoretical plume models incorporating the effect of wind. Results are consistent with observed plume heights (6.4–7.3 km above the vent) and eruption durations. Estimated tephra volumes were 15–34?×?106 m3 for explosions on the afternoon of 26 January and morning of 27 January, and 5.0–7.6?×?106 m3 for the afternoon of 27 January; magma discharge rates were in the range 1–2?×?106 kg/s for all three explosions. Clast dispersal models estimated plume height at 7.1?±?1 km above the vent for each explosion. The three subplinian explosions occurred with approximately 12-h reposes and had similar mass discharge rates and plume heights but decreasing erupted magma volumes and durations. 相似文献
802.
Assimilating along-track SLA data using the EnOI in an eddy resolving model of the Agulhas system 总被引:1,自引:1,他引:0
Björn C. Backeberg François Counillon Johnny A. Johannessen Marie–Isabelle Pujol 《Ocean Dynamics》2014,64(8):1121-1136
The greater Agulhas Current is one of the most energetic current systems in the global ocean. It plays a fundamental role in determining the mean state and variability of the regional marine environment, affecting its resources and ecosystem, the regional weather and the global climate on a broad range of temporal and spatial scales. In the absence of a coherent in-situ and satellite-based observing system in the region, modelling and data assimilation techniques play a crucial role in both furthering the quantitative understanding and providing better forecasts of this complicated western boundary current system. In this study, we use a regional implementation of the Hybrid Coordinate Ocean Model and assimilate along-track satellite sea level anomaly (SLA) data using the Ensemble Optimal Interpolation (EnOI) data assimilation scheme. This study lays the foundation towards the development of a regional prediction system for the greater Agulhas Current system. Comparisons to independent in-situ drifter observations show that data assimilation reduces the error compared to a free model run over a 2-year period. Mesoscale features are placed in more consistent agreement with the drifter trajectories and surface velocity errors are reduced. While the model-based forecasts of surface velocities are not as accurate as persistence forecasts derived from satellite altimeter observations, the error calculated from the drifter measurements for eddy kinetic energy is significantly lower in the assimilation system compared to the persistence forecast. While the assimilation of along-track SLA data introduces a small bias in sea surface temperatures, the representation of water mass properties and deep current velocities in the Agulhas system is improved. 相似文献
803.
The spatial and seasonal distributions of organic matter and fine grains were tested as possible determinants of fauna distribution in bed sediment of a Hercynian gravel stream. Invertebrate densities and the amounts of fine grains and organic carbon were assessed in freeze-core samples taken along 70 cm depth profiles at three different positions in the stream channel. Sampling was conducted on five occasions of low discharge over two years. The variability in invertebrate community composition was analysed using Detrended Correspondence Analysis with posterior projection of explanatory variables; Variation Partitioning was used to estimate the independent and shared effects of the explanatory variables. We found that the best predictors of the invertebrate community were spatial variables (depth, position in the channel) and then variables influenced by seasonal patterns (surface water temperature and discharge). The influence of organic matter and fine grain content was significant only after eliminating spatial autocorrelation. High amounts of organic matter, randomly accumulated in the sediment, improved the model by explaining high fauna densities. The fine grain content was not a limiting factor to fauna at our study site. It is possible that the large amount of mica flakes in the sediment has caused the arrangement of grains with a pore space sufficient for fauna even when fine grain content was high. 相似文献
804.
Channel and floodplain sediment dynamics in a reach of the tropical meandering Rio Beni (Bolivian Amazonia) 总被引:1,自引:0,他引:1
Emmanuèle Gautier Daniel Brunstein Philippe Vauchel Jean‐Marie Jouanneau Marc Roulet Coral Garcia Jean‐Loup Guyot Marcello Castro 《地球表面变化过程与地形》2010,35(15):1838-1853
The study investigates interactions, water and sediment exchanges, between a rapidly migrating meander and its associated floodplain at fine temporal and spatial scales. The Beni River, an Amazonian free meandering river, makes the transition between Andean ranges and Amazonian lowlands. For the period 2002–2006, an assemblage of tools and methods (water and sediment discharges, topometric and bathymetric surveys, sedimentation rate estimations from unsupported 210Pb and sediment trapping system) was used to jointly analyse the influence on the sediment budget of external factors (mainly water and sediment discharge) and the inherent behaviour of the system. The main issue addressed is the investigation of the complex relationship between ‘morphological conditioning’ of fluvial landform and process. The first part of the study was undertaken with the aim of linking erosion–deposition in an active meander with water and sediment fluxes. The three inter‐annual evolutions are characterized by very unequal sediment budgets; the first two intervals underwent predominant erosion, and the latter slight accumulation. Digital elevation models, evaluated for the active meander, demonstrate that sedimentation on the point bar depends more on external factors than erosion of the concave bank, which fluctuates slightly. The second part of the study, focusing on water and sediment exchanges between active bend and floodplain, examines the respective parts played by overbank flow and by an abandoned channel on the diffusion and sequestration of sediment. The association of short‐ and long‐term estimation of sedimentation rates suggests that floodplain construction is associated with two different processes and rhythms of sediment transportation. Finally, a sediment budget is proposed for the Beni River in the upper part of the Amazonian lowlands. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
805.
Matthew P. Miller Bailey E. Simone Diane M. McKnight Rose M. Cory Mark W. Williams Elizabeth W. Boyer 《Aquatic Sciences - Research Across Boundaries》2010,72(3):269-275
In their article, “New light on a dark subject: On the use of fluorescence data to deduce redox states of natural organic
matter,” Macalady and Walton-Day (2009) subjected natural organic matter (NOM) samples to oxidation, reduction, and photochemical transformation. Fluorescence spectra
were obtained on samples, which were diluted “to bring maximum uvvisible absorbance values below 1.0.” The spectra were fit
to the Cory and McKnight (2005) parallel factor analysis (PARAFAC) model, and consistent variation in the redox state of quinone-like moieties was not detected.
Based on these results they concluded that fitting fluorescence spectra to the Cory and McKnight (2005) PARAFAC model to obtain information about the redox state of quinone-like moieties in NOM is problematic. Recognizing that
collection and correction of fluorescence spectra requires consideration of many factors, we investigated the potential for
inner-filter effects to obscure the ability of fluorescence spectroscopy to quantify the redox state of quinone-like moieties.
We collected fluorescence spectra on Pony Lake and Suwannee River fulvic acid standards that were diluted to cover a range
of absorbance wavelengths, and fit these spectra to the Cory and McKnight (2005) PARAFAC model. Our results suggest that, in order for the commonly used inner-filter correction to effectively remove inner-filter
effects, samples should be diluted such that the absorbance at 254 nm is less than 0.3 prior to the collection of fluorescence
spectra. This finding indicates that inner-filter effects may have obscured changes in the redox signature of fluorescence
spectra of the highly absorbing samples studied by Macalady and Walton-Day (2009). 相似文献
806.
807.
The pattern and influence of low dissolved oxygen in the Patuxent River,a seasonally hypoxic estuary
Denise?L.?BreitburgEmail author Aaron?Adamack Kenneth?A.?Rose Sarah?E.?Kolesar Beth?Decker Jennifer?E.?Purcell Julie?E.?Keister James?H.?Cowan 《Estuaries and Coasts》2003,26(2):280-297
Increased nutrient loadings have resulted in low dissolved oxygen (DO) concentrations in bottom waters of the Patuxent River,
a tributary of Chesapeake Bay. We synthesize existing and newly collected data to examine spatial and temporal variation in
bottom DO, the prevalence of hypoxia-induced mortality of fishes, the tolerance of Patuxent River biota to low DO, and the
influence of bottom DO on the vertical distributions and spatial overlap of larval fish and fish eggs with their gelatinous
predators and zooplankton prey. We use this information, as well as output from watershed-quality and water-quality models,
to configure a spatially-explicit individual-based model to predict how changing land use within the Patuxent watershed may
affect survival of early life stages of summer breeding fishes through its effect on DO. Bottom waters in much of the mesohaline
Patuxent River are below 50% DO saturation during summer. The system is characterized by high spatial and temporal variation
in DO concentrations, and the current severity and extent of hypoxia are sufficient to alter distributions of organisms and
trophic interactions in the river. Gelatinous zooplankton are among the most tolerant species of hypoxia, while several of
the ecologically and economically important finfish are among the most sensitive. This variation in DO tolerances may make
the Patuxent River, and similar estuaries, particularly susceptible to hypoxia-induced alterations in food web dynamics. Model
simulations consistently predict high mortality of planktonic bay anchovy eggs (Anchoa mitchilli) under current DO, and increasing survival of fish eggs with increasing DO. Changes in land use that reduce nutrient loadings
may either increase or decrease predation mortality of larval fish depending on the baseline DO conditions at any point in
space and time. A precautionary approach towards fisheries and ecosystem management would recommend reducing nutrients to
levels at which low oxygen effects on estuarine habitat are reduced and, where possible, eliminated. 相似文献
808.
Measurements of CO2 fluxes from open-vent volcanos are rare, yet may offer special capabilities for monitoring volcanos and forecasting activity.
The measured fluxes of CO2 and SO2 from Mount St. Helens decreased from July through November 1980, but the record includes variations of CO2/SO2 in the emitted gas and episodes of greatly increased fluxes of CO2. We propose that the CO2 flux variations reflect two gas components: (a) a component whose flux decreased in proportion to 1/ √t with a CO2/SO2 mass ratio of 1.7, and (b) a residual flux of CO2 consisting of short-lived, large peaks with a CO2/SO2 mass ratio of 15. We propose two hypotheses: (a) the 1/ √t dependence was generated by crystallization in a deep magma body at rates governed by diffusion-limited heat transfer, and
(b) the gas component with the higher CO2/SO2 was released from ascending magma, which replenished the same magma body. The separation of the total CO2 flux into contributions from known processes permits quantitative inferences about the replenishment and crystallization
rates of open-system magma bodies beneath volcanos. The flux separations obtained by using two gas sources with distinct CO2/SO2 ratios and a peak minus background approach to obtain the CO2 contributions from an intermittent source and a continuously emitting source are similar. The flux separation results support
the hypothesis that the second component was generated by episodic magma ascent and replenishment of the magma body. The diffusion-limited
crystallization hypothesis is supported by the decay of minimum CO2 and SO2 fluxes with 1/ √t after 1 July 1980. We infer that the magma body at Mount St. Helens was replenished at an average rate (2.8×106 m3 d–1) which varied by less than 5% during July, August, and September 1980. The magma body volume (2.4–3.0 km3) in early 1982 was estimated by integrating a crystallization rate function inferred from CO2 fluxes to maximum times (20±4 years) estimated from the increase of sample crystallinity with time. These new volcanic gas
flux separation methods and the existence of relations among the CO2 flux, crystallization rates, and magma body replenishment rates yield new information about the dynamics of an open-vent,
replenished magma body.
Received: 15 February 1995 / Accepted: 30 March 1996 相似文献
809.
Preliminary shock experiments at approximately 50 and 250 kb have been carried out with lunar soil and with a dispersion of iron in quartz. The lunar soils acquire remanent magnetization in the Earth's field of order of magnitude 10?3 G cm3 g?1. The remanence exhibited considerable stability against AF demagnetization. Remanence appears to be acquired both during the passage of the shock wave through the material and during post shock cool-down. The higher shock range gave rise to an increase in magnetic viscosity and in the saturation magnetization of the soil, which is most readily explained as due to the generation of fine grained iron. 相似文献
810.
The hydrology of a small wet forested region is studied. Catchment discharge, solar radiation, air temperature, humidity, precipitation, soil moisture, and the water elevation in a swamp were monitored for several months in 1973 and 1975. These data are used to investigate various aspects of the catchment's hydrology and in particular to study the catchment's evapotranspirational requirements. The actual evapotranspiration loss is calculated from a water budget approach. The Priestley-Taylor model is used to predict the medium to long-term evapotranspiration from the basin. The predictions based on the model are in very good agreement with the actual evapotranspirational demand. Evapotranspiration is found to play a significant role in the catchment's water balance. 相似文献