Estimates of the γ-ray flux from Sagittarius A     

Laura Maraschi  Massimo Tarenghi  Aldo Treves 《Astrophysics and Space Science》1970,8(1):12-19

Analysis of the anisotropy of the -ray background observed by Clarket al. (1968) has lead to the suggestion that part of the anisotropy be due to a source at the Galactic Centre. The complex structure of radio and infrared emission of the region of Sgr A is considered. The -ray flux arising from Compton scattering and ₀ decay is calculated. For a value of the magnetic field H10^–4 Gauss, corresponding to equipartition, the -ray flux above 100 MeV at Earth from the Sgr A source is expected to lie between 10^–6 and 10^–5 gamma/cm² sec. Such a source should be detectable in the near future.  相似文献   

Book reviews     

Fakhry A. Assaad  Laura Stafford  Philip E. LaMoreaux 《Environmental Geology》2005,48(8):1149-1150

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Entrepreneurial Regionalist Planning in a Rescaled Spain: The Cases of Bilbao and València     

David L. Prytherch  Laura Huntoon 《GeoJournal》2005,62(1):41-50

European politics and planning have recently been characterized by a shift to economic entrepreneurialism at sub-national scales, and the planned redevelopment of the city-region in pursuit of global competitiveness, which scholars have interpreted in light of political-economic “rescaling” or regionalization and the emergence of a “new regionalism.” Analyzing rescaling largely in terms of shifting economic and institutional structures, however, many accounts underestimate the complexity and enduring power of so-called ‘old’ regionalist politics of culture and identity as backdrop to urban redevelopment planning. In this paper we address how the urban planning process mediates between the seemingly dichotomous tendencies of regionalized entrepreneurialism and cultural regionalism. Using case studies of two Spanish autonomous regions and their major urban centers – the Basque Country or Euskadi (Bilbao) and the Comunitat Valenciana (València) – we review the historical geography of planning in the European region in order to explore how cultural regionalism collides with economic rescaling and entrepreneurialism, in and through the planned landscape. We propose that such emerging and hybrid politics and planning be understood as a form of entrepreneurial regionalism, a culturally inflected form of economic competitiveness characteristic of but not unique to the Spanish region. This specific notion of entrepreneurial regionalism may illuminate how planners mediate global and local imperatives within political discourse and landscapes that materialize them, and allow us to better reconceptualize the relationship between economic globalization, state restructuring, and cultural politics in a new Europe of the Regions.  相似文献   

Pleistocene geomorphology and geochronology of eastern Grand Canyon: linkages of landscape components during climate changes     

Matt D. Anders  Joel L. Pederson  Tammy M. Rittenour  Warren D. Sharp  John C. Gosse  Karl E. Karlstrom  Laura J. Crossey  Ronald J. Goble  Lisa Stockli  Guang Yang 《Quaternary Science Reviews》2005,24(23-24):2428-2448

We report new mapping, soils, survey, and geochronologic (luminescence, U-series, and cosmogenic-nuclide) data from Pleistocene deposits in the arid setting of eastern Grand Canyon. The result is a stratigraphic framework of inset fill gravels and associated terraces that provide a record of the responses of hillslopes, tributary streams, and the Colorado River to the last 400 kyr of glacial–interglacial climate change. The best-preserved last 80 kyr of this record indicates a stratigraphic–chronologic disconnect between both deposition and incision along the Colorado River versus along the trunks of local tributaries. For example, the Colorado River finished aggrading and had already begun incising before the main pulse of aggradation in the trunks of local catchments during Marine Isotope Stage 3, and then tributary incision followed during the millennial-scale fluctuations of the last glacial epoch, potentially concurrent with mainstem aggradation. The mainstem record appears to broadly correlate with regional paleoclimate and upstream geomorphic records and thus may be responding to climatic–hydrologic changes in its mountain headwaters, with aggradation beginning during full-glacial times and continuing into subsequent interglacials. The contrasting lag time in responses of the dryland catchments within Grand Canyon may be largely a function of the weathering-limited nature of hillslope sediment supply.  相似文献   

A unified global height reference system as a basis for IGGOS   总被引：1，自引：0，他引：1  

Johannes Ihde  Laura Snchez 《Journal of Geodynamics》2005,40(4-5):400

The definition of a global height reference system is based on a mean sea surface, gravity field parameters, and a three-dimensional terrestrial reference frame (TRF). Tide gauge records, satellite altimetry, gravity measurements on Earth and from space, TRF coordinates, and spirit levelling have to be combined for the realization of the vertical reference frame. Observations and parameters have to be consistent with respect to the used standards, conventions and models. They have to provide globally unified reference surfaces (geoid or quasigeoid, respectively, and mean sea surface). The continental reference systems of Europe (EUREF, ECGN) and South America (SIRGAS) are considering these requirements in their strategies. They are presented here, and slightly different definitions and realizations for a globally unified height reference system are discussed.  相似文献   

Community Oxygen and Nutrient Fluxes in Seagrass Beds of Florida Bay, USA   总被引：1，自引：0，他引：1  

Laura A. Yarbro  Paul R. Carlson Jr. 《Estuaries and Coasts》2008,31(5):877-897

We used clear, acrylic chambers to measure in situ community oxygen and nutrient fluxes under day and night conditions in seagrass beds at five sites across Florida Bay five times between September 1997 and March 1999. Underlying sediments are biogenic carbonate with porosities of 0.7–0.9 and with low organic content (<1.6%). The seagrass communities always removed oxygen from the water column during the night and produced oxygen during daylight, and sampling date and site significantly affected both night and daytime oxygen fluxes. Net daily average fluxes of oxygen (?4.9 to 49 mmol m^?2 day^?1) ranged from net autotrophy to heterotrophy across the bay and during the 18-month sampling period. However, the Rabbit Key Basin site, located in the west-central bay and covered with a dense Thalassia testudinum bed, was always autotrophic with net average oxygen production ranging from 4.8 to 49 mmol m^?2 day^?1. In November 1998, three of the five sites were strongly heterotrophic and oxygen production was least at Rabbit, suggesting the possibility of hypoxic conditions in fall. Average ammonium (NH₄) concentrations in the water column varied widely across the bay, ranging from a mean of 6.9 μmol l^?1 at Calusa in the eastern bay to a mean of 0.6 μmol l^?1 at Rabbit Key for the period of study. However, average NH₄ fluxes by site and date (?240 to 110 μmol m^?2 h^?1) were not correlated with water column concentrations and did not vary in a consistent diel, seasonal, or spatial pattern. Concentrations of dissolved organic nitrogen (DON) in the water column, averaged by site (15–25 μmol l^?1), were greater than mean NH₄ concentrations, and the range of day and night DON fluxes (?920 to 1,300 μmol m^?2 h^?1), averaged by site and date, was greater than the range of mean NH₄ fluxes. Average DON fluxes did not vary consistently from day to night, seasonally or spatially. Mean silicate fluxes ranged from ?590 to 860 μmol m^?2 h^?1 across all sites and dates, but mean net daily fluxes were less variable and most of the time contributed small amounts of silicate to the water column. Mean concentrations of filterable reactive phosphorus (FRP) in the water column across the bay were very low (0.021–0.075 μmol l^?1); but site average concentrations of dissolved organic phosphorus (DOP) were higher (0.04–0.15 μmol l^?1) and showed a gradient of increasing concentration from east to west in the bay. A pronounced gradient in average surficial sediment total phosphorus (1.1–12 μmol g DW^?1) along an east-to-west gradient was not reflected in fluxes of phosphorus. FRP fluxes, averaged by site and date, were low (?5.2 to 52 μmol m^?2 h^?1), highly variable, and did not vary consistently from day to night or across season or location. Mean DOP fluxes varied over a smaller range (?8.7 to 7.4 μmol m^?2 h^?1), but also showed no consistent spatial or temporal patterns. These small DOP fluxes were in sharp contrast to the predominately organic phosphorus pool in surficial sediments (site means?=?0.66–7.4 μmol g DW^?1). Significant correlations of nutrient fluxes with parameters related to seagrass abundance suggest that the seagrass community may play a major role in nutrient recycling. Integrated means of net daily fluxes over the area of Florida Bay, though highly variable, suggest that seagrass communities might be a source of DOP and NH₄ to Florida Bay and might remove small amounts of FRP and potentially large amounts of DON from the waters of the bay.  相似文献   

Buchbesprechungen     

Peter Perret  Laura Sigg 《Aquatic Sciences - Research Across Boundaries》1981,43(2):378-379

Ohne Zusammenfassung  相似文献   

Minimum continental velocities with respect to the pole since the Archean     

Laura Ullrich  Rob Van Der Voo 《Tectonophysics》1981,74(1-2)

Paleomagnetic apparent polar wander paths provide a method for estimating minimum plate velocities which can be extended much further into the geologic past than can be estimated based on sea-floor magnetic anomalies. Minimum velocities can be determined from the rate of change of latitude as derived relative to a fixed pole. Such latitudinal velocities have been determined for the center of mass of the North American, Baltic, Siberian and African shields since the Archean. The results indicate that plate velocities in the past have at times easily exceeded those for present-day continents, and that they often were equivalent to present-day oceanic velocities, although there are peaks and troughs through time. North American velocities are significantly greater prior to one billion years ago than those of Siberia or Baltica.  相似文献   

Oil Extraction and Benefit Sharing in an Illiberal Context: The Nenets and Komi-Izhemtsi Indigenous Peoples in the Russian Arctic     

Maria Tysiachniouk  Laura A. Henry  Machiel Lamers  Jan P. M. van Tatenhove 《社会与自然资源》2018,31(5):556-579

ABSTRACT

How can indigenous communities in illiberal regimes benefit from oil production? This paper compares the experience of two indigenous peoples in the Russian Arctic, the Nenets and the Komi-Izhemtsi, in their quest for environmental protection and the development of benefit-sharing arrangements with Lukoil, a Russian oil company. The Nenets people, recognized by the Russian state as indigenous, are marginalized political actors who identified a route to receiving compensation for loss of land and damage to the environment as well as economic benefits under the auspices of Russian law and Lukoil’s corporate policies. In contrast, the Komi-Izhemtsi, despite indigenous status in global institutions including the United Nations and the Arctic Council, are unrecognized as indigenous domestically and initially received no compensation. Their path to benefit sharing was more challenging as they partnered with local nongovernmental organizations and global environmentalists to pressure Lukoil to sign a benefit-sharing agreement. Ultimately, the comparison illustrates how transnational partnerships can empower indigenous people to gain benefits from natural resource exploitation even in illiberal political systems.  相似文献   

Temporal variability in terrestrially-derived sources of particulate organic carbon in the lower Mississippi River and its upper tributaries     

Thomas S. Bianchi  Laura A. Wysocki  Timothy R. Filley 《Geochimica et cosmochimica acta》2007,71(18):4425-4437

In this study, we examined the temporal changes of terrestrially-derived particulate organic carbon (POC) in the lower Mississippi River (MR) and in a very limited account, the upper tributaries (Upper MR, Ohio River, and Missouri River). We used for the first time a combination of lignin-phenols, bulk stable carbon isotopes, and compound-specific isotope analyses (CSIA) to examine POC in the lower MR and upper tributaries.A lack of correlation between POC and lignin phenol abundances (Λ₈) was likely due to dilution effects from autochthonous production in the river, which has been shown to be considerably higher than previously expected. The range of δ¹³C values for p-hydroxycinnamic and ferulic acids in POC in the lower river do support that POM in the lower river does have a significant component of C₄ in addition to C₃ source materials. A strong correlation between δ¹³C values of p-hydroxycinnamic, ferulic, and vanillyl phenols suggests a consistent input of C₃ and C₄ carbon to POC lignin while a lack of correlation between these same phenols and POC bulk δ¹³C further indicates the considerable role of autochthonous carbon in the lower MR POC budget. Our estimates indicate an annual flux of POC of 9.3 × 10⁸ kg y⁻¹ to the Gulf of Mexico. Total lignin fluxes, based on Λ₈ values of POC, were estimated to be 1.2 × 10⁵ kg y⁻¹. If we include the total dissolved organic carbon (DOC) flux (3.1 × 10⁹ kg y⁻¹) reported by [Bianchi T. S., Filley T., Dria K. and Hatcher, P. (2004) Temporal variability in sources of dissolved organic carbon in the lower Mississippi River. Geochim. Cosmochim. Acta68, 959-967.], we get a total organic carbon flux of 4.0 × 10⁹ kg y⁻¹. This represents 0.82% of the annual total organic carbon supplied to the oceans by rivers (4.9 × 10¹¹ kg).  相似文献