Reflections on a White Discipline     

Laura Pulido 《The Professional geographer》2002,54(1):42-49

Utilizing a partially autobiographical format, this article considers the practice and study of race within geography. I argue that the overwhelmingly white composition of the discipline has very real implications for both individual experiences and our intellectual production and disciplinary culture. I explore these issues by drawing on my own experiences as a Chicana within geography, and by examining the extent to which one area of research, environmental justice, has engaged questions of race and the consequences of that engagement. I conclude with some general remarks on what it might take to significantly diversify geography.  相似文献   

Atmospheric nitrogen inputs into the North Sea: effect on productivity     

Gerrit de Leeuw  Lucinda Spokes  Tim Jickells  Carsten Ambelas Skjth  Ole Hertel  Elisabetta Vignati  Susanne Tamm  Michael Schulz  Lise-Lotte Srensen  Britta Pedersen  Laura Klein  K. Heinke Schlünzen 《Continental Shelf Research》2003,23(17-19):1743

The ANICE (Atmospheric Nitrogen Inputs into the Coastal Ecosystem) project addressed the atmospheric deposition of nitrogen to the North Sea, with emphasis on coastal effects. ANICE focused on quantifying the deposition of inorganic nitrogen compounds to the North Sea and the governing processes. An overview of the results from modelling and experimental efforts is presented. They serve to identify the role of the atmosphere as a source of biologically essential chemical species to the marine biota. Data from the Weybourne Atmospheric Observatory (UK) are used to evaluate the effect of short episodes with very high atmospheric nitrogen concentrations. One such episode resulted in an average deposition of 0.8 mmol N m⁻² day⁻¹, which has the potential to promote primary productivity of 5.3 mmol C m⁻² day⁻¹. This value is compared to long-term effects determined from model results. The total calculated atmospheric deposition to the North Sea in 1999 is 948 kg N km⁻¹, i.e. 0.19 mmol N m⁻² day⁻¹ which has the potential to promote primary productivity of 1.2 mmol C m⁻² day⁻¹. Detailed results for August 1999 show strong gradients across the North Sea due to adjacent areas where emissions of NO_x and NH₃ are among the highest in Europe. The average atmospheric deposition to the southern part of the North Sea in August 1999 could potentially promote primary production of 2.0 mmol C m⁻² day⁻¹, i.e. 5.5% of the total production at this time of the year in this area of the North Sea. For the entire study area the atmospheric contribution to the primary production per m² is about two-third of this value. Most of the deposition occurs during short periods with high atmospheric concentrations. This atmospheric nitrogen is almost entirely anthropogenic in origin and thus represents a human-induced perturbation of the ecosystem.  相似文献   

Groundwater residence times and flow paths in fractured rock determined using environmental tracers in the Mission Tunnel; Santa Barbara County,California, USA     

Laura K. Rademacher  Jordan F. Clark  James R. Boles 《Environmental Geology》2003,43(5):557-567

Multiple geochemical tracers [ion chemistry, stable isotopes of water, chlorofluorocarbons (CFC), tritium] and a 25-year-long record of discharge were used to understand residence times and flow paths of groundwater seeps in the fractured rock aquifer surrounding the Mission Tunnel, Santa Barbara, California. Tritium data from individual seeps indicate that seep waters are a mixture of >45-year-old (recharged prior to the nuclear bomb tests) and young groundwater. CFC data support this interpretation, however, a two-end member mixing model cannot completely explain the age tracer data. Microbial degradation and partial re-equilibration complicate the CFC signal. Spectral analysis of precipitation and groundwater seepage records shows that seepage lags precipitation by 3 months. This delay is related to the advancement of the wetting front and increasing the number of active flow paths. Additionally, the amount of seepage produced by precipitation is less during extended periods of drought than during normal or wet periods, suggesting antecedent conditions strongly affect flow through this fractured rock aquifer.  相似文献   

Adsorption of Copper, Nickel, and Cadmium on Goethite in the Presence of Organic Ligands   总被引：2，自引：0，他引：2  

Diane Buerge-Weirich  Philippe Behra  Laura Sigg 《Aquatic Geochemistry》2003,9(2):65-85

Adsorption of copper, cadmium and nickel at low concentrations on goethite was studied in the presence of the simple organic ligands oxalate, salicylate, and pyromellitate. The experimental metal adsorption behavior was compared to calculations with a surface complexation model to evaluate the most important interactions. Oxalate mostly decreased Cu and Ni adsorption at high pH-values by competition between solution and surface complexation but had no effect on Cd adsorption. Cu adsorption in the presence of oxalate below pH 6 could best be described by defining a ternary complex of type A (surface-metal-ligand). Salicylate had only minor effects on metal adsorption. The adsorption of Cu in the presence of salicylate above pH 5 could be explained by a ternary complex of type A. Pyromellitate increased the adsorption of Cu and Cd in the acidic pH-range, likely by formation of ternary surface complexes of type B (surface-ligand-metal).  相似文献   

Deep to shallow lacustrine evaporites in the Libros Gypsum (southern Teruel Basin, Miocene, NE Spain): an occurrence of pelletal gypsum rhythmites     

Federico Ortí  Laura Rosell  Pere AnadÓN† 《Sedimentology》2003,50(2):361-386

ABSTRACT A number of non‐marine evaporite units composed of primary gypsum were deposited in saline lakes that developed in the southern Teruel Basin (NE Spain) during the Miocene. In the basin depocentre, a continuum of lacustrine evaporite lithofacies influenced by the activity of organisms is displayed. The Libros Gypsum was deposited in a deep lake, in which water stratification became unstable with progressive shoaling. Rhythmites, composed of laminae of pelletal gypsum and laminae of very fine lenticular gypsum crystals mixed with siliceous microorganisms, formed in addition to gypsum turbidites, intraformational gypsum breccias and slump structures. The pelletal laminae originated from the faecal activity of animals (crustaceans?) ingesting gypsum crystallites in the lake water during episodes of maximum evaporation, whereas the laminae of very fine lenticular gypsum mixed with microorganisms accumulated during episodes of relative dilution. In the wide marginal zones of the basin, the Libros Gypsum unit consists of massive to thin‐bedded bioturbated gypsum and thin‐bedded clotted gypsum, which formed in intermediate to very shallow (palustrine) water depths. The bioturbated gypsum lithofacies were produced by the action of diverse organisms, presumably worms and coleopterans, and chironomid larvae to a lesser extent; the massive lithofacies precipitated in very shallow water; and the thin‐bedded lithofacies formed in shallow to deeper settings. The thin‐bedded clotted gypsum is a relatively deep facies that may have diverse origins (e.g. bioturbation, compaction, disruption of soft sediments and early diagenesis). There is a well‐developed metre‐scale cyclicity in the marginal lake sequences, which is not observed in the inner lake deposits. This suggests a depth control in the various lacustrine subenvironments to record cyclic evaporitic processes. The isotopic composition of the gypsum indicates early sulphate‐reducing bacterial activity in the bottom of the lake and suggests that the sulphate was derived from the chemical recycling of Triassic evaporites of the country rocks.  相似文献   

Study of an unexpected convective storm     

Maria E. Saluzzi  Laura Levi  Luisa Lubart  Gustavo Carri 《Atmospheric Research》1997,44(3-4)

The occurrence of a severe storm in the Argentine littoral has been analyzed. Although general atmospheric conditions were not favorable to the development of deep convection, this storm produced strong winds and hail of considerable intensity. Its occurrence was associated with the entrance of a pulse of humid air at low levels and could be related to the development of two vorticity centers in the region. Atmospheric conditions existing above a low-level inversion are used to simulate two possible clouds, with base altitudes at 2200 and 3000 m and updraft speeds reaching maximum values of 27 and 29 m s⁻¹ respectively. It is noted that the base temperature of about 13°C, found for the first cloud, can be correlated with the prevalence of frozen drop embryos, observed in an analyzed hailstone sample. Hailstone vertical trajectories are simulated in both modeled clouds, assuming spheroidal and spherical particles and assigning different values to the drag coefficient C_d.  相似文献   

Comparison of Cu and Zn cycling in eutrophic lakes with oxic and anoxic hypolimnion     

HanBin Xue  René Gächter  Laura Sigg 《Aquatic Sciences - Research Across Boundaries》1997,59(2):176-189

Data on the cycling of Cu and Zn in two eutrophic lakes are presented: Lake Greifen that becomes seasonally anoxic in its hypolimnion and Lake Sempach that is aerated during winter and oxygenated during summer. They suggest that hypolimnetic oxygenation 1) enhances the release of copper from the sediment but 2) also accelerates the entrapment and deposition of Cu and Zn by freshly formed Mn- and Fe-oxides.  相似文献   

Relationship of Avalonian and Cadomian terranes to Grenville and Pan-African events     

Laura D. Mallard  John J. W. Rogers 《Journal of Geodynamics》1997,23(3-4)

Suturing of the supercontinent Rodinia in the Grenville event (˜ 1000 Ma) was followed by rifting in the late Proterozoic (˜ 800-700 Ma), reorganization to Gondwana in the Pan-African (˜ 700-500 Ma) and further accretion to develop Pangea at the end of the Paleozoic. One of the Rodinian rifts followed part of the Grenville suture, it produced the margin of eastern North America and southern Baltica and the contrasting margin of west Gondwana in present South America. The Paleozoic accretionary wedge against the Grenville-age margin of North America and Baltica contains Avalonian/Cadomian terranes that exhibit Pan-African erogenic events ± sediments apparently developed while the terranes were in or near Gondwana. These terranes carry lower-Paleozoic fauna (Acado-Baltic) that are not indigenous to North America and Baltica.U---Pb zircon ages range from 1500-1000 Ma in Grenville terranes and from 800–500 Ma with minor inheritance in Avalonian terranes; they are generally much older in Cadomian terranes, implying very little resetting during Pan-African events. T_DM ages are generally 2000–1200 Ma in Grenville terranes, 1300–600 Ma in Avalonian terranes and 2000–1200 in Cadomian terranes. These summary data show that: (1) the Grenville orogenic event produced almost no juvenile crust; (2) the Avalonian terranes of North America contain crust that evolved primarily in the late Proterozoic, possibly as a mixture of juvenile Pan-African material and Grenville or slightly older material; (3) the Cadomian terranes of Europe consist of old (middle-Proterozoic to Archean) crust with minor juvenile Pan-African material. The Avalonian terranes apparently evolved near, and partly on, the Grenville-age crust now in South America during the intense orogeny associated with rotation of Gondwana away from North America. The Cadomian terranes of Europe, however, appear to be fragments of other parts of Gondwana, probably West Africa.  相似文献   

Iron isotope fractionation by Fe(II)-oxidizing photoautotrophic bacteria     

Laura R. Croal  Brian L. Beard 《Geochimica et cosmochimica acta》2004,68(6):1227-1242

Photoautotrophic bacteria that oxidize ferrous iron (Fe[II]) under anaerobic conditions are thought to be ancient in origin, and the ferric (hydr)oxide mineral products of their metabolism are likely to be preserved in ancient rocks. Here, two enrichment cultures of Fe(II)-oxidizing photoautotrophs and a culture of the genus Thiodictyon were studied with respect to their ability to fractionate Fe isotopes. Fe isotope fractionations produced by both the enrichment cultures and the Thiodictyon culture were relatively constant at early stages of the reaction progress, where the ⁵⁶Fe/⁵⁴Fe ratios of poorly crystalline hydrous ferric oxide (HFO) metabolic products were enriched in the heavier isotope relative to aqueous ferrous iron (Fe[II]_aq) by ∼1.5 ± 0.2‰. This fractionation appears to be independent of the rate of photoautotrophic Fe(II)-oxidation, and is comparable to that observed for Fe isotope fractionation by dissimilatory Fe(III)-reducing bacteria. Although there remain a number of uncertainties regarding how the overall measured isotopic fractionation is produced, the most likely mechanisms include (1) an equilibrium effect produced by biological ligands, or (2) a kinetic effect produced by precipitation of HFO overlaid upon equilibrium exchange between Fe(II) and Fe(III) species. The fractionation we observe is similar in direction to that measured for abiotic oxidation of Fe(II)_aq by molecular oxygen. This suggests that the use of Fe isotopes to identify phototrophic Fe(II)-oxidation in the rock record may only be possible during time periods in Earth’s history when independent evidence exists for low ambient oxygen contents.  相似文献   

Phytoremediation: Transformation and Control of Contaminants     

Laura L. Sanders  Book Editor 《Ground water》2005,43(1):6-7

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