Geochemical Responses to a Major Transgression in Giant African Lakes     

Michael R. Talbot  Niels Bo Jensen  Tine Lærdal  Maria Letizia Filippi 《Journal of Paleolimnology》2006,35(3):467-489

Bulk organic matter (OM) in sediments that accumulated in Lakes Victoria, Albert and Tanganyika during the major transgression that transformed these basins from a desiccated or lowstand condition at the time of the Last Glacial Maximum to highstand with overflow in the terminal Pleistocene and early Holocene, show some striking compositional trends. Sediment OM content (TOC) and Rock-Eval Hydrogen Index (HI) both rise markedly, reflecting increasing primary productivity during the transgression and enhanced conditions for the preservation of labile OM, In Lake Victoria rising sedimentary sulphur content tracks the change in HI. Over the same time interval in all three lakes, δ¹³C_OM shows a falling trend of 5–8‰ . Nitrogen isotopic and C/N trends are, in contrast, more variable. Relatively high δ¹⁵N values at an early stage in the transgression may reflect isotopic effects associated with ammonia loss from an alkaline waterbody, while high δ¹⁵N later in the transgression was probably due to periods when stratification-related denitrification had a particularly strong impact upon the lakes' N cycle. The observed changes are attributed to the influence of the landscape drowned during the transgression upon the nutrient budget of the lake, a rise in river-supplied nutrients as the climate became wetter, and an increasing tendency to stable chemical stratification as the water column deepened. In the case of δ¹³C, additional contributory factors were changes in the amount and isotopic composition of river-supplied dissolved inorganic carbon (DIC) as runoff and soil–moisture turnover rates increased, and the replacement of a C4-grassland dominated flora by mixed woodland-grassland and forest with a significant presence of C3 plants. Distinct geochemical changes are also apparent in Lakes Victoria and Albert sediments at the end of the transgression which are consistent with a reduction in nutrient loading. In the case of N this resulted in biological N fixation becoming a major part of the lakes' N cycle, Some of the observed geochemical variations may be generally applicable as proxy indicators of major transgressions in large lakes. IDEAL Contribution No. 152.  相似文献   

Assessment of the Enhanced Geothermal System Resource Base of the United States     

David D. Blackwell  Petru T. Negraru  Maria C. Richards 《Natural Resources Research》2006,15(4):283-308

This paper describes an assessment of the enhanced geothermal system (EGS) resource base of the conterminous United States, using constructed temperature at depth maps. The temperature at depth maps were computed from 3 to 10 km, for every km. The methodology is described. Factors included are sediment thickness, thermal conductivity variations, distribution of the radioactive heat generation and surface temperature based on several geologic models of the upper 10 km of the crust. EGS systems are extended in this paper to include coproduced geothermal energy, and geopressured resources. A table is provided that summarizes the resource base estimates for all components of the EGS geothermal resource. By far, the conduction-dominated components of EGS represent the largest component of the U.S. resource. Nonetheless, the coproduced resources and geopressured resources are large and significant targets for short and intermediate term development. There is a huge resource base between the depths of 3 and 8 km, where the temperature reaches 150–250°C. Even if only 2% of the conventional EGS resource is developed, the energy recovered would be equivalent to roughly 2,500 times the annual consumption of primary energy in the U.S. in 2006. Temperatures above 150°C at those depths are more common in the active tectonic regions of the western conterminous U.S., but are not confined to those areas. In the central and eastern U.S. there are identified areas of moderate size that are of reasonable grade and probably small areas of much higher grade than predicted by this analyses. However because of the regional (the grid size is 5′ × 5′) scale of this study such potentially promising sites remain to be identified. Several possible scenarios for EGS development are discussed. The most promising and least costly may to be developments in abandoned or shut-in oil and gas fields, where the temperatures are high enough. Because thousands of wells are already drilled in those locations, the cost of producing energy from such fields could be significantly lowered. In addition many hydrocarbon fields are producing large amounts of co-produced water, which is necessary for geothermal development. Although sustainability is not addressed in this study, the resource is so large that in at least some scenarios of development the geothermal resource is sustainable for long periods of time.  相似文献   

Earthquake risk assessment of lifelines   总被引：4，自引：0，他引：4  

Kyriazis Pitilakis  Maria Alexoudi  Sotiris Argyroudis  Olivier Monge  Christophe Martin 《Bulletin of Earthquake Engineering》2006,4(4):365-390

This paper presents the RISK-UE methodology for the seismic risk assessment of utility systems (potable water, waste-water, gas system, telecommunication, electric power) and transportation infrastructures (port, airport, road and railway system). The proposed methodology provides a uniform basis for the reduction of the consequences of lifeline damages in urban areas and an efficient mitigation strategy and prioritization policies for pre-earthquake and post earthquake actions. A detailed inventory for every element at risk together with a reliable seismic hazard assessment, appropriate selection of fragility models, estimation of the “global value” and economical impact of lifeline damages and losses are the main steps of the proposed methodology. The consideration of European distinctive features of lifelines and utility systems in the construction of seismic scenarios and the proposition of fragility curves are among the basic aims of the proposed methodology. Different modules of the methodology were applied in seven pilot cities (Thessaloniki, Catania, Nice, Bucharest, Sofia, Barcelona, Bitola). We present herein few representative examples (case studies) in order to illustrate the methodology and to prove its efficiency.  相似文献   

Magnetic survey of topsoils in Windsor–Essex County, Canada     

Ruiping Shi  Maria T. Cioppa   《Journal of Applied Geophysics》2006,60(3-4):201-212

Windsor–Essex County is a major cross-border truck and transportation route, with significant localized industrialization as well as rural and farming areas. Magnetic property measurements (in-field and laboratory susceptibility, frequency-dependent susceptibility, hysteresis properties, thermomagnetic and thermosusceptibility curves, anhysteretic and isothermal magnetizations) were made in order to determine the potential for using such variables to distinguish between natural and anthropogenic pollutants. In-field magnetic susceptibility measured on 324 soil sampling sites on a 0.5–2 km grid spacing through Windsor–Essex County ranged from 3.7 × 10^− 6 to 305.2 × 10^− 6 SI (average 36.2 ± 35.8 × 10^− 6 SI), and showed that high magnetic susceptibility values were obtained on soil sampling sites in and around the cities/towns of Windsor, Harrow, Olinda and Oakland and near the beaches of Point Pelee National Park (PPNP) and Deerbrook, whereas lower susceptibility values were observed in near the towns of Lakeshore and Essex. On this grid spacing, Highway 401 (the major truck route) did not show anomalous susceptibility values; however, closer (1–3 m) sampling on other roads did show anomalously high values, suggesting that the coarser grid spacing may have missed anomalies. Laboratory measurements indicated that the dominant magnetic mineral in the Windsor–Essex County soils is magnetite; however, the grain size is variable. Pseudo-single domain (PSD)–multidomain (MD) magnetite is generally found on beaches and in PPNP, whereas single domain (SD)–PSD magnetite has been found near the City of Windsor and other towns. While certain correlations exist between some anthropogenic activities and the measured magnetic susceptibility and magnetic property values, no overall correlation can be made. A variety of geologic and anthropogenic factors must be considered when interpreting the origin of the magnetic signal in a particular area.  相似文献   

Supercontinent evolution and the Proterozoic metallogeny of South America   总被引：2，自引：1，他引：2  

Joo Batista Guimares  Aroldo  Maria 《Gondwana Research》2007,11(3):346-361

The cratonic blocks of South America have been accreted from 2.2 to 1.9 Ga, and all of these blocks have been previously involved in the assembly and breakup of the Paleoproterozoic Atlantica, the Mesoproterozoic to Neoproterozoic Rodinia, and the Neoproterozoic to Phanerozoic West Gondwana continents. Several mineralization phases have sequentially taken place during Atlantica evolution, involving Au, U, Cr, W, and Sn. During Rodinia assembly and breakup and Gondwana formation, the crust-dominated metallogenic processes have been overriding, responsible for several mineral deposits, including Au, Pd, Sn, Ni, Cu, Zn, Mn, Fe, Pb, U, P₂O₅, Ta, W, Li, Be and precious stones. During Rodinia breakup, epicontinental carbonate-siliciclastic basins were deposited, which host important non-ferrous base metal deposits of Cu–Co and Pb–Zn–Ag in Africa and South America. Isotope Pb–Pb analyses of sulfides from the non-ferrous deposits unambiguously indicate an upper crustal source for the metals. A genetic model for these deposits involves extensional faults driving the circulation of hydrothermal mineralizing fluids from the Archean/Paleoproterozoic basement to the Neoproterozoic sedimentary cover. These relations demonstrate the individuality of metal associations of every sediment-hosted Neoproterozoic base-metal deposit of West Gondwana has been highly influenced by the mineralogical and chemical composition of the underlying igneous and metaigneous rocks.  相似文献   

Molluscs in Kongsfjorden (Spitsbergen, Svalbard): a species list and patterns of distribution and diversity     

Maria W&#;odarska-Kowalczuk 《Polar research》2007,26(1):48-63

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Coseismic reactivation of the Samambaia fault, Brazil     

Francisco H.R. Bezerra  Mrio K. Takeya  Maria O.L. Sousa  Aderson F. do Nascimento 《Tectonophysics》2007,430(1-4):27-39

Northeastern Brazil is, within the present knowledge of historical and instrumental seismicity, one the most seismic active areas in intraplate South America. Seismic activity in the region has occurred mainly around the Potiguar basin. This seismicity includes earthquake swarms characterized by instrumentally-recorded events ≤ 5.2 m_b and paleoseismic events ≥ 7.0. Our study concentrates in the João Câmara (JC) epicentral area, where an earthquake swarm composed of more than 40,000 aftershocks occurred mainly from 1986 to 1990 along the Samambaia fault; 14 of which had m_b > 4.0 and two of which had 5.1 and 5.0 m_b. We describe and compare this aftershock sequence with the present-day stress field and the tectonic fabric in an attempt to understand fault geometry and local control of seismogenic faulting. Earthquake data indicate that seismicity decreased steadily from 1986 to 1998. We selected 2,746 epicenters, which provided a high-quality and precise dataset. It indicates that the fault trends 37° azimuth, dips 76°–80° to NW, and forms an alignment  27 km long that cuts across the NNE–SSW-trending ductile Precambrian fabric. The depth of these events ranged from  1 km to  9 km. The fault forms an echelon array of three main left-bend segments: one in the northern and two in the southern part of the fault. A low-seismicity zone, which marks a contractional bend, occurs between the northern and southern segments. Focal mechanisms indicate that the area is under an E–W-oriented compression, which led to strike–slip shear along the Samambaia fault with a small normal component. The fault is at 53° to the maximum compression and is severely misoriented for reactivation under the present-day stress field. The seismicity, however, spatially coincides with a brittle fabric composed of quartz veins and silicified-fault zones. We conclude that the Samambaia fault is a discontinuous and reactivated structure marked at the surface by a well-defined brittle fabric, which is associated with silica-rich fluids.  相似文献   

Geographical location in the information age: from destiny to opportunity?     

Aharon Kellerman  Maria Paradiso 《GeoJournal》2007,70(2-3):195-211

This article argues for a possibly new significance of geographical location in the information age, a significance amounting to more than mere spatial anchoring, on the one hand, and to less and different than absolute destiny, on the other. Hence, we show a possible change in the significance of geographical location from destiny in the industrial age to potential opportunity in the information age, a transition which applies to geographical units (i.e. cities, regions) and to individuals. First we discuss geographical location as destiny in the industrial age for both geographical entities and individuals. Second, we highlight a transitional period spanning through the 1970s and 1980s during which geographical location lost much of its absolute nature, becoming more of an anchor. Third, we present a conceptual framework for geographical location possibly turning into opportunity in the information age, again for both geographical units and individuals. We demonstrate that this potential opportunity has only partially materialized by individuals in the international use of online shopping, showing that their adoption is wider in North America, and differentiated in Europe. We further demonstrate a partial dissemination of opportunities for cities and regions by focusing on FDI. Finally, in the fourth and concluding section, we briefly discuss pertaining policy issues for the enhancement of locational opportunities.  相似文献   

Rare earth elements as tracers of sediment contamination by phosphogypsum in the Santos estuary,southern Brazil     

Sonia Maria Barros de Oliveira  Paulo Sergio Cardoso da Silva  Barbara Paci Mazzilli  Deborah Ines Teixeira Favaro  Catia Heloisa Saueia 《Applied Geochemistry》2007

In the Cubatão region, southern Brazil, sediments are transported by several rivers from the Serra do Mar Ridge into the Santos estuary. Fertilizer plants have been operating along the margins of one of these rivers (Mogi River) producing a large volume of phosphogypsum, which is stockpiled in nearby areas. Surface sediments of the Mogi River were sampled upstream and downstream in relation to the point where the effluents of the phosphogypsum piles flow into the drainage system. In the vicinity of this point one sediment core was collected. Results show that REE, Ba, Zr and Th concentrations in the non-contaminated sediments are of the same order as those present in the upper continental crust. The contaminated samples present a composition affected by that of the phosphogypsum, marked by a higher concentration of these elements and a stronger degree of REE fractionation. These phosphogypsum characteristics are inherited from the Catalão igneous phosphate ore and were moderately modified by the industrial process of phosphoric acid production. The phosphogypsum signal decreases rapidly downstream, pointing to a limited area of influence of the stacks. The deepest sediments of the core are also free of contamination, representing a time interval prior to the deposition of phosphogypsum wastes on the banks of the estuary.  相似文献   

Hydrologic response of upland catchments to wildfires     

Maria Cristina Rulli  Renzo Rosso 《Advances in water resources》2007

To which extent do wildfires affect runoff production, soil erosion and sediment transport in upland catchments? This transient effect is investigated here by combining data of long term precipitation, sediment yield and wildfire records with a fine resolution spatially distributed modeling approach to flow generation and surface erosion. The model accounts for changes in the structure and properties of soil and vegetation cover by combining the tube-flux approach to topographic watershed partition with a parsimonious parametrization of hydrologic processes. This model is used to predict hydrologic and sediment fluxes for nine small catchments in Saint Gabriel mountains of southern California under control (pre-fire) and altered (post-fire) conditions. Simulation runs using a 45 years record of hourly precipitation show the passage of fire to significantly modify catchment response to storms with a major effect on erosion and flood flows. The probability of occurrence of major floods in the post-fire season is shown to increase up to an order of magnitude under same precipitation conditions. Also, the expected anomaly of sediment yield can increase dramatically the desertification hazard in upland wildfire prone areas. One should further consider the role of firefloods produced by the combined occurrence of wildfires and storms as a fundamental source of non-stationarity in the assessment of hydrologic hazard.  相似文献