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111.
During the deglaciation stages of the last glacial period a rock avalanche took place on the glacier that occupied the upper sector of the Cuerpo de Hombre Valley (Sierra de Béjar). The material displaced during the avalanche fell onto the ice, was transported by the glacier and later deposited as supraglacial ablation till. The cause of the avalanche was the decompression of the valley slopes after they were freed from the glacier ice (stress relaxation). Reconstruction of the ice masses has been carried out to quantify the stress relaxation that produced the collapse. The rock avalanche took place on a lithologically homogeneous slope with a dense fracture network. The avalanche left a 0.4 ha scar on the slope with a volume of displaced material of 623 ± 15 × 103 m3. The deposit is an accumulation of large, angular, heterometric boulders (1–100 m3 in volume) with a coarse pebble‐size matrix. The avalanche can be explained as a relaxation process. This implies rock decompression once the glacier retreat left the wall ice free (debuttressing). Calculations show that the avalanche took place where the decompression stresses were highest (130–170 kPa). In the Spanish Central System paleoglaciers the largest accumulation of morainic deposits occurred after the glacial maximum and the earliest stages of the ice retreat. The process described here is used as an example to formulate a hypothesis that the largest accumulations of tills were formed in relation to enhanced slope dynamics once some glacier retreat had occurred.  相似文献   
112.
Ancient lakes, which are important centres of biodiversity and endemism, are threatened by a wide variety of human impacts. To assess environmental impact on ancient Lake Ohrid we have taken short sediment cores from two contrasting site locations, comprising a site of urban pollution and an apparently pristine area. Recent impacts on water quality and ecology were assessed using sediment, geochemical, ostracode, and diatom data derived from analysis of two 210Pb-dated sediment cores spanning the period from 1918 to 2009. According to the index of geoaccumulation, sediments were often moderately contaminated with As. Fe and Ni concentrations often exceeded reported maximum limits above which harmful effects on sediment-dwelling organisms are expected. Productivity in the (pristine) south-eastern part of Lake Ohrid (Sveti Naum) is generally lower than in the north, probably due to the strong influence of spring discharge. Low ostracode and diatom concentrations, low abundance of the epilimnetic diatom Cyclotella ocellata, and low values of TOC and TIC indicate a lower productivity from the early 1920s to the late 1980s. Since the mid 1970s, increased relative abundance of C. ocellata and increasing diatom concentration indicate increasing productivity in the south-eastern part. Rising numbers of ostracode valves and higher TIC and TOC contents in both sediment cores indicate an increase in productivity during the late 1980s. A slight increase in productivity near Sveti Naum continued from the early 1990s until 2009, witnessed by rising TC, TIC, and TOC content and a generally high number of ostracode valves and ostracode diversity. The area near the City of Struga (site of urban pollution) is also characterized by rising TOC and TIC contents and, furthermore, by increasing Cu, Fe, Pb, and Zn concentrations since the early 1990s. The recent reduction in the number of ostracode valves and ostracode diversity is probably caused by a higher heavy metal load into the lake. This suggests that living conditions for the endemic species in Lake Ohrid have become less favourable in the northern part of the lake, which might threaten the unique flora and fauna of Lake Ohrid.  相似文献   
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Populations of the American horseshoe crab (Limulus polyphemus) differ in broad areas of their biology. We observed a non-harvested, marked Florida Gulf coast population during their spring spawning (March–May) in 11 years across a 17-year period (1992–2009). Long-term changes occurred in the number of spawning pairs: the population was stable from 1992 to 2000 but increased markedly after 2000. Short-term variation in numbers of spawning pairs, unpaired females, unpaired males, and operational sex ratios was explained by changes over the season and during each week of spring tides and by differences in actual (not predicted) maximum high tide height. Wind direction strongly affected tidal inundation and the number of spawning horseshoe crabs. Tagging individuals revealed that females returned to the nesting beach less often than males and most females were re-sighted only within 1 week of spring tides. No animals were seen across more than 6 years. Implications for management are discussed.  相似文献   
116.
Abstract— Four exposures of Chicxulub impact ejecta along the Mexico‐Belize border have been sampled and analyzed for major and trace element abundances. The ejecta deposits consist of a lower spheroid bed, containing clay and dolomite spheroids, and an upper diamictite bed with boulders and clasts of limestone and dolomite. The matrix of both beds is composed of clay and micritic dolomite. The rare earth element (REE) compositions in the matrix of both units show strong similarities in concentrations and pattern. Furthermore, the Zr/TiO2 scatter plot shows a linear correlation indicating one source. These results indicate that the basal spheroid bed has the same source and was generated during the same event as the overlying diamictite bed, which lends support to a single‐impact scenario for the Albion Formation ejecta deposits. The elevated concentrations of non‐meteoritic elements such as Sb, As, U, and Zn in the matrix of the lower spheroid bed are regarded to have been derived from the sedimentary target rocks at the Chicxulub impact site. The positive Eu and Ce anomalies in clay concretion and in the matrix of the lower part of the spheroid bed in Albion Island quarry is probably related to processes involved in the impact, such as high temperature and oxidizing conditions. Analogous trace element anomalies have been reported from the distal Cretaceous‐Paleogene (K/T) boundary clay layer at different sites. Thus, the trace element signals, reported herein, are regarded to support a genetic link between the Chicxulub impact, the ejecta deposits along the Mexico‐Belize border, and the global K/T boundary layer.  相似文献   
117.
Benthic metabolism and nutrient cycling in Boston Harbor, Massachusetts   总被引:1,自引:0,他引:1  
To gain insight into the importance of the benthos in carbon and nutrient budgets of Boston Harbor and surrounding bays, we measured sediment-water exchanges of oxygen, total carbon dioxide (DIC), nitrogen (ammonium, nitrate+nitrite, urea, N2O), silicate, and phosphorus at several stations in different sedimentary environments just prior to and subsequent to cessation of sewage sludge disposal in the harbor. The ratio of the average annual DIC release to O2 uptake at three primary stations ranged from 0.84 to 1.99. Annual average DIC:DIN flux ratios were consistently greater than predicted from the Redfield ratio, suggesting substantial losses of mineralized N. The pattern was less clear for P: some stations showed evidence that the sediments were a sink for P while others appeared to be a net source to the water column over the study period. In general, temporal and spatial patterns of respiration, nutrient fluxes, and flux ratios were not consistently related to measures of sediment oxidation-reduction status such as Eh or dissolved sulfide. Sediments from Boston Harbor metabolize a relatively high percentage (46%) of the organic matter inputs from phytoplankton production and allochthonous inputs when compared to most estuarine systems. Nutrient regeneration from the benthos is equivalent to 40% of the N, 29% of the P, and more than 60% of the Si demand of the phytoplankton. However, the role of the benthos in supporting primary production at the present time may be minor as nutrient inputs from sewage and other sources exceed benthic fluxes of N and P by 10-fold and Si by 4-fold. Our estimates of denitrification from DIC:DIN fluxes suggests that about 45% of the N mineralized in the sediments is denitrified, which accounts for about 17% of the N inputs from land.  相似文献   
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One of the important recent advances in the field of hurricane/storm modelling has been the development of high-fidelity numerical simulation models for reliable and accurate prediction of wave and surge responses. The computational cost associated with these models has simultaneously created an incentive for researchers to investigate surrogate modelling (i.e. metamodeling) and interpolation/regression methodologies to efficiently approximate hurricane/storm responses exploiting existing databases of high-fidelity simulations. Moving least squares (MLS) response surfaces were recently proposed as such an approximation methodology, providing the ability to efficiently describe different responses of interest (such as surge and wave heights) in a large coastal region that may involve thousands of points for which the hurricane impact needs to be estimated. This paper discusses further implementation details and focuses on optimization characteristics of this surrogate modelling approach. The approximation of different response characteristics is considered, and special attention is given to predicting the storm surge for inland locations, for which the possibility of the location remaining dry needs to be additionally addressed. The optimal selection of the basis functions for the response surface and of the parameters of the MLS character of the approximation is discussed in detail, and the impact of the number of high-fidelity simulations informing the surrogate model is also investigated. Different normalizations of the response as well as choices for the objective function for the optimization problem are considered, and their impact on the accuracy of the resultant (under these choices) surrogate model is examined. Details for implementation of the methodology for efficient coastal risk assessment are reviewed, and the influence in the analysis of the model prediction error introduced through the surrogate modelling is discussed. A case study is provided, utilizing a recently developed database of high-fidelity simulations for the Hawaiian Islands.  相似文献   
120.
Effective control of nonpoint source pollution from contaminants transported by runoff requires information about the source areas of surface runoff. Variable source hydrology is widely recognized by hydrologists, yet few methods exist for identifying the saturated areas that generate most runoff in humid regions. The Soil Moisture Routing model is a daily water balance model that simulates the hydrology for watersheds with shallow sloping soils. The model combines elevation, soil, and land use data within the geographic information system GRASS, and predicts the spatial distribution of soil moisture, evapotranspiration, saturation‐excess overland flow (i.e., surface runoff), and interflow throughout a watershed. The model was applied to a 170 hectare watershed in the Catskills region of New York State and observed stream flow hydrographs and soil moisture measurements were compared to model predictions. Stream flow prediction during non‐winter periods generally agreed with measured flow resulting in an average r2 of 0·73, a standard error of 0·01 m3/s, and an average Nash‐Sutcliffe efficiency R2 of 0·62. Soil moisture predictions showed trends similar to observations with errors on the order of the standard error of measurements. The model results were most accurate for non‐winter conditions. The model is currently used for making management decisions for reducing non‐point source pollution from manure spread fields in the Catskill watersheds which supply New York City's drinking water. Copyright © 1999 John Wiley & Sons, Ltd.  相似文献   
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