Microbial mass-dependent fractionation of chromium isotopes   总被引：1，自引：0，他引：1  

Eric R. Sikora  Thomas M. Johnson  Thomas D. Bullen 《Geochimica et cosmochimica acta》2008,72(15):3631-3641

Mass-dependent fractionation of Cr isotopes occurs during dissimilatory Cr(VI) reduction by Shewanella oneidensis strain MR-1. Cells suspended in a simple buffer solution, with various concentrations of lactate or formate added as electron donor, reduced 5 or 10 μM Cr(VI) to Cr(III) over days to weeks. In all nine batch experiments, ⁵³Cr/⁵²Cr ratios of the unreacted Cr(VI) increased as reduction proceeded. In eight experiments covering a range of added donor concentrations up to 100 μM, isotopic fractionation factors were nearly invariant, ranging from 1.0040 to 1.0045, with a mean value somewhat larger than that previously reported for abiotic Cr(VI) reduction (1.0034). One experiment containing much greater donor concentration (10 mM lactate) reduced Cr(VI) much faster and exhibited a lesser fractionation factor (1.0018). These results indicate that ⁵³Cr/⁵²Cr measurements should be effective as indicators of Cr(VI) reduction, either bacterial or abiotic. However, variability in the fractionation factor is poorly constrained and should be studied for a variety of microbial and abiotic reduction pathways.  相似文献   

Thermodynamic model for arsenic speciation in sulfidic waters: A novel use of ab initio computations     

G.R. Helz  J.A. Tossell 《Geochimica et cosmochimica acta》2008,72(18):4457-4468

Recent discoveries demonstrate that the chemistry of arsenic in sulfidic waters is much more complex that previously believed. One implication is that all earlier thermodynamic data on stabilities of As thioanions require revision. Previously used experimental approaches for determining As thioanion stabilities may be inadequate to deal with the full range of complexity. Here we use computational as well as empirical information to construct a provisional model for equilibrium As thioanion distributions in sulfidic waters. Whereas previous authors have argued for either As(III) or As(V) thioanions, the new model predicts that both are important and can occur simultaneously under commonly encountered pH and ΣS^−II conditions. At the order of magnitude level, the model reasonably predicts the solubility of As₂S₃ in sulfidic solutions, provides tentative peak assignments for published Raman spectroscopic data and plausibly accounts for how sulfide modifies the bacterial toxicity of As. The model yields a thermodynamic justification for how sulfide, which is usually regarded as a reducing agent, can counter-intuitively drive oxidation of As(III) to As(V), as has been observed both in the laboratory and in the field. Despite its uncertain accuracy, the model serves as a useful source of new, testable hypotheses about As geochemistry and highlights crucial experimental data needs.  相似文献   

Models in geography revisited     

N.J. Clifford 《Geoforum》2008,39(2):675-686

Agent based models (ABMs) have many applications, and illustrate a rapidly developing field of enquiry, spanning both the physical-mathematical and human-social sciences. ABMs are seen as most appropriate in situations where decisions or actions are distributed around particular locations, and in which structure is viewed as emergent from the interaction between individuals. ABMs may be used either as representational devices, to reproduce the patterns observed or desired in the system of interest, or as foundational tools contributing to the development of social or economic theory. The role and status of models and modelling is itself an instantiation of a wider debate concerning representation and explanation. Today, a case can be made that the nature of explanation and the use of scientific interpretation reflect much less definite and exclusive positions and permit more diverse approaches than hitherto. The underlying proposition of this commentary is, therefore, that the time is right for a positive application of ABMs within the discipline of geography, and for a rediscovery and reappraisal of the richness and depth of insight in the model-building enterprise more generally. First, the context for ABM development and application is set with reference to the agency-structure debate. Second, some aspects of the heritage of models in geography is presented, based upon reviews of two benchmark publications bearing that title. Next, some of the most significant characteristics, uses, potentials and limitations of ABMs, are reviewed. Finally, some constructive ways forward are suggested, as informed by theory and method from the interpretative social sciences.  相似文献   

Integrating diverse methods to understand climate-land interactions in East Africa   总被引：3，自引：0，他引：3  

Jennifer M. Olson  Gopal Alagarswamy 《Geoforum》2008,39(2):898-911

The questions of how land use change affects climate, and how climate change affects land use, require examination of societal and environmental systems across space at multiple scales, from the global climate to regional vegetative dynamics to local decision making by farmers and herders. It also requires an analysis of causal linkages and feedback loops between systems. These questions and the conceptual approach of the research design of the Climate-Land Interaction Project (CLIP) are rooted in the classical human-environment research tradition in Geography.This paper discusses a methodological framework to quantify the two-way interactions between land use and regional climate systems, using ongoing work by a team of multi-disciplinary scientists examining climate-land dynamics at multiple scales in East Africa. East Africa is a region that is undergoing rapid land use change, where changes in climate would have serious consequences for people’s livelihoods, and requiring new coping and land use strategies. The research involves exploration of linkages between two important foci of global change research, namely, land use/land cover (LULC) and climate change. These linkages are examined through modeling agricultural systems, land use driving forces and patterns, the physical properties of land cover, and the regional climate. Both qualitative and quantitative methods are being used to illustrate a diverse pluralism in scientific discovery.  相似文献   

Models of natural and human dynamics in forest landscapes: Cross-site and cross-cultural synthesis   总被引：1，自引：0，他引：1  

Miguel F. Acevedo  J. Baird Callicott  Donald Lyons  Judith Rosales  Magdiel Ablan  Giorgio Tonella  Emilio Vilanova 《Geoforum》2008,39(2):846-866

We synthesize the study of coupled natural and human systems across sites and cultures through a process of simplification and abstraction based on multiple dimensions of human-nature connectedness: satisfaction of basic needs, psycho-cultural connectedness and regulation of use of natural resources. We thus provide both a place-based and general understanding of value-driven anthropogenic environmental change and response. Two questions guide this research: what are the crucial stakeholder values that drive land use decisions and thus land cover change? And how can knowledge of these values be used to make decisions and policies that sustain both the human and natural systems in a place? To explore these questions we build simulation models of four study sites, two in the State of Texas, United States, and two in Venezuela. All include protected areas, though they differ in the specifics of vegetation and land use. In the Texas sites, relatively affluent individuals are legally converting forests to residential, commercial, and industrial uses, while in Venezuela landless settlers are extra-legally converting forests for purposes of subsistence agriculture. Contemporary modeling techniques now facilitate simulations of stakeholder and ecosystem dynamics revealing emergent patterns. Such coupled human and natural systems are currently recognized as a form of biocomplexity. Our modeling framework is flexible enough to allow adaptation to each of the study sites, capturing the essential features of the respective natural and anthropogenic land use changes and stakeholder reactions. The interactions between human stakeholders are simulated using multi-agent models that act on forest landscape models, and receive feedback of the effects of these actions on ecological habitats and hydrological response. The multi-agent models employ a formal logic-based method for the Venezuelan sites and a decision analysis approach using multi-attribute utility functions for the Texas sites, differing more in style and emphasis than in substance. Our natural-systems models are generic and can be tailored according to site-specific conditions. Similar models of tree growth and patch transitions are used for all the study sites and the differing responses to environmental variables are specified for each local species and terrain conditions.  相似文献   

Church-based social capital, networks and geographical scale: Katrina evacuation, relocation, and recovery in a New Orleans Vietnamese American community     

Christopher A. Airriess  Wei Li  Karen J. Leong  Angela Chia-Chen Chen  Verna M. Keith 《Geoforum》2008,39(3):1333-1346

This research examines the role of social capital and networks to explain the evacuation, relocation, and recovery experiences of a Vietnamese American community in New Orleans, Louisiana in the aftermath of Hurricane Katrina. As the single largest community institution, the parish church’s complex bonding and bridging social capital and networks proved particularly critical in part because of its historically based ontological security. The process of evacuation, but especially relocation and recovery, was dependent on deploying co-ethnic social capital and networks at a variety of geographical scales. Beyond the local or community scale, extra-local, regional, and national scales of social capital and networks reproduced a spatially redefined Vietnamese American community. Part of the recovery process included constructing discursive place-based collective-action frames to successfully contest a nearby landfill that in turn engendered social capital and networks crossing ethnic boundaries to include the extra-local African American community. Engaging social capital and networks beyond the local geographical scale cultivated a Vietnamese American community with an emergent post-Katrina cultural and political identity.  相似文献   

The posthumous tectonics and mechanism of exhumation of granitic plutons: The case of the Transbaikal region and the Tien Shan     

M. G. Leonov  Yu. A. Morozov  A. V. Nikitin 《Geotectonics》2008,42(2):81-104

The study of granitic plutons of the Baikal Highland and the Tien Shan has made it possible to establish new features of their posthumous (after incorporation into the consolidated Earth’s crust) structural reworking and to understand the implications of the cataclastic flow for the exhumation of the crystalline basement in the studied regions. It is shown that granitic plutons undergo appreciable structural transformation at the stages of tectonic reactivation that is significantly separated in time from the moment of formation of plutons as geological bodies. The 3D cataclastic deformation is the main mode of structural reworking of granitic plutons, while the cataclastic flow is the main form of their mobility. Newly recognized slice structures characterize the volumetric deformation of granites.  相似文献   

Lateral structural variability in zone of eocene island-arc-continent collision,Kamchatka     

M. N. Shapiro  A. V. Solov’ev  J. K. Hourigan 《Geotectonics》2008,42(6):469-487

The lateral variability of structural elements in the collision zone of the Cretaceous-Paleocene Achaivayam-Valagin island arc with the northeastern Asian margin is considered. The similarity and difference of Eocene collision structural elements in the north and the south of Kamchatka are shown. In northern Kamchatka, the continent-arc boundary is traced along the Lesnaya-Vatyn Thrust Fault, which completed its evolution about 45 Ma ago. The thin, near-horizontal allochthon of this thrust, composed of island-arc rocks, overlies the deformed but unmetamorphosed terrigeneous sequences of the Asian margin. The general structure of this suture in the Kamchatka Isthmus and southern Koryakia is comparable with the uppermost subduction zone, where a thin lithospheric wedge overlaps intensely deformed sediments detached from the plunging plate. In southern Kamchatka (Malka Uplift of the Sredinny Range), the arc-continent collision started 55–53 Ma ago with thrusting of island-arc complexes over terrigenous rocks of continental margin. However, the thickness of the allochthon was much greater than in the north. Immediately after this event, both the autochthon and lower part of allochthon were deformed and subsided to a significant depth. This subsidence gave rise to metamorphism of both the autochthon (Kolpakov and Kamchatka groups, Kheivan Formation) and lower allochthon (Andrianovka and Khimka formations). The anomalously fast heating of the crust was most likely related to the ascent of asthenospheric masses due to slab breakoff, when the Eurasian Plate was plunging beneath the Achaivayam-Valagin arc.  相似文献   

Lithospheric Origin of Oligocene-Miocene Magmatism in Central Chile: U-Pb Ages and Sr-Pb-Hf Isotope Composition of Minerals     

Montecinos  P.; Scharer  U.; Vergara  M.; Aguirre  L. 《Journal of Petrology》2008,49(3):555-580

Establishing the petrogenesis of volcanic and plutonic rocksis a key issue in unraveling the evolution of distinct subduction-relatedtectonic phases occurring along the South American margin. Thisis particularly true for Cenozoic times when large volumes ofmagma were produced in the Andean belt. In this study we havefocused on Oligo-Miocene magmatism in central Chile at 33°S.Our data include field and petrographic observations, whole-rockmajor and trace element analyses, U–Pb zircon dating,and Pb, Sr, and Hf isotope analyses of plagioclase, clinopyroxene,and zircon mineral separates. Combined with earlier dating resultsthe new zircon ages define a 28·8–5·2 Maperiod of plutonic and volcanic activity that ceased as a consequenceof flattening subduction of the Nazca–Farallon plate.Rare earth elements patterns are variable, with up to 92 timeschondrite concentrations for light rare earth elements yielding(La/Yb)_N between 3·6 and 7·0, and an absence ofEu anomalies. Initial Pb isotope signatures are in the rangeof 18·358–19·023 for ²⁰⁶Pb/ ²⁰⁴Pb, 15·567–15·700for ²⁰⁷Pb/ ²⁰⁴Pb and 38·249–39·084 for ²⁰⁸Pb/²⁰⁴Pb. Initial ⁸⁷Sr/ ⁸⁶Sr are mostly in the range of 0·70369–0·70505,with two more radiogenic values at 0·7066. Initial Hfisotopic compositions of zircons yield exclusively positiveHf_i ranging between + 6·9 and + 9·6. The newlydetermined initial isotope characteristics of the Oligo-Miocenemagmas suggest that the mantle source lithologies are differentfrom both those of Pacific mid-ocean ridge basalt and oceanisland basalt, plotting in the field of reference values forsubcontinental lithospheric mantle, characterized by moderatelarge ion lithophile element–high field strengh elementdepletion and high ²³⁸U/ ²⁰⁴Pb. A Hf model age of 2 Ga is estimatedfor the formation of the subcontinental mantle–continentalcrust assemblage in the region, suggesting that the initialSr and Pb isotope ratios inferred for the source of the Oligo-Mioceneparental magmas are the result of later Rb and U enrichmentcaused by mantle metasomatism. A time-integrated model Rb/Srof 0·039 and µ 16 are estimated for the sourceof the parental magmas, consistent with ratios measured in peridotitexenoliths from continental areas. Evolution from predominant(>90%) basaltic–gabbroic to andesitic–dioriticmagmas seems to involve a combination of (1) original traceelement differences in the metasomatized subcontinental mantle,(2) different degrees of partial melting and (3) fractionalcrystallization in the garnet- and spinel-peridotite stabilityfields. The genesis of more differentiated magmas reaching rhyolitic–graniticcompositions most probably also includes additional crystalfractionation at both shallow mantle depths and within the crust,possibly leading to some very minor assimilation of crustalmaterial. KEY WORDS: calc-alkaline magmatism; Oligo-Miocene; U–Pb dating; Sr–Pb–Hf isotopes; central Chile  相似文献   

Global Correlations of Ocean Ridge Basalt Chemistry with Axial Depth: a New Perspective   总被引：4，自引：0，他引：4  

Niu  Yaoling; O'Hara  Michael J. 《Journal of Petrology》2008,49(4):633-664

The petrological parameters Na₈ and Fe₈, which are Na₂O andFeO contents in mid-ocean ridge basalt (MORB) melts correctedfor fractionation effects to MgO = 8 wt%, have been widely usedas indicators of the extent and pressure of mantle melting beneathocean ridges. We find that these parameters are unreliable.Fe₈ is used to compute the mantle solidus depth (P_o) and temperature(T_o), and it is the values and range of Fe₈ that have led tothe notion that mantle potential temperature variation of T_P= 250 K is required to explain the global ocean ridge systematics.This interpreted T_P = 250 K range applies to ocean ridges awayfrom ‘hotspots’. We find no convincing evidencethat calculated values for P_o, T_o, and T_P using Fe₈ have anysignificance. We correct for fractionation effect to Mg^# = 0·72,which reveals mostly signals of mantle processes because meltswith Mg^# = 0·72 are in equilibrium with mantle olivineof Fo_89·6 (vs evolved olivine of Fo_{88·1–79·6}in equilibrium with melts of Fe₈). To reveal first-order MORBchemical systematics as a function of ridge axial depth, weaverage out possible effects of spreading rate variation, local-scalemantle source heterogeneity, melting region geometry variation,and dynamic topography on regional and segment scales by usingactual sample depths, regardless of geographical location, withineach of 22 ridge depth intervals of 250 m on a global scale.These depth-interval averages give Fe₇₂ = 7·5–8·5,which would give T_P = 41 K (vs 250 K based on Fe₈) beneathglobal ocean ridges. The lack of Fe₇₂–Si₇₂ and Si₇₂–ridgedepth correlations provides no evidence that MORB melts preservepressure signatures as a function of ridge axial depth. We thusfind no convincing evidence for T_P > 50 K beneath globalocean ridges. The averages have also revealed significantcorrelations of MORB chemistry (e.g. Ti₇₂, Al₇₂, Fe₇₂,Mg₇₂, Ca₇₂, Na₇₂ and Ca₇₂/Al₇₂) with ridge axial depth. Thechemistry–depth correlation points to an intrinsic linkbetween the two. That is, the 5 km global ridge axial reliefand MORB chemistry both result from a common cause: subsolidusmantle compositional variation (vs T_P), which determines themineralogy, lithology and density variations that (1) isostaticallycompensate the 5 km ocean ridge relief and (2) determine thefirst-order MORB compositional variation on a global scale.A progressively more enriched (or less depleted) fertileperidotite source (i.e. high Al₂O₃ and Na₂O, and low CaO/Al₂O₃)beneath deep ridges ensures a greater amount of modal garnet(high Al₂O₃) and higher jadeite/diopside ratios in clinopyroxene(high Na₂O and Al₂O₃, and lower CaO), making a denser mantle,and thus deeper ridges. The dense fertile mantle beneath deepridges retards the rate and restricts the amplitude of the upwelling,reduces the rate and extent of decompression melting, givesway to conductive cooling to a deep level, forces melting tostop at such a deep level, leads to a short melting column,and thus produces less melt and probably a thin magmatic crustrelative to the less dense (more refractory) fertile mantlebeneath shallow ridges. Compositions of primitive MORB meltsresult from the combination of two different, but geneticallyrelated processes: (1) mantle source inheritance and (2) meltingprocess enhancement. The subsolidus mantle compositional variationneeded to explain MORB chemistry and ridge axial depth variationrequires a deep isostatic compensation depth, probably in thetransition zone. Therefore, although ocean ridges are of shalloworigin, their working is largely controlled by deep processesas well as the effect of plate spreading rate variation at shallowlevels. KEY WORDS: mid-ocean ridges; mantle melting; magma differentiation; petrogenesis; MORB chemistry variation; ridge depth variation; global correlations; mantle compositional variation; mantle source density variation; mantle potential temperature variation; isostatic compensation  相似文献