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51.
This article explores the role of risk perception in adaptation to stress through comparative case studies of coffee farmers’ responses to climatic and non-climatic stressors. We hypothesized that farmers associating these changes with high risk would be more likely to make adaptations than those who saw the events as part of normal variation. Nevertheless, we found that farmers who associated events with high risk were not more likely to engage in specific adaptations. Adaptive responses were more clearly associated with access to land than perception of risk, suggesting that adaptation is more a function of exogenous constraints on decision making than perception. 相似文献
52.
Dr. Maurice E. Tucker 《International Journal of Earth Sciences》1973,62(1):137-153
Ferromanganese nodules (equivalent to Recent manganese nodules) are described from the Upper Devonian griotte (red pelagic limestone) of the Montagne Noire (S. France) and the Cephalopodenkalk of the Rheinisches Schiefergebirge, West Germany. They occur as encrustations, commonly exhibiting colloform structures, around skeletal material and limestone clasts. The nodules are associated with encrusting foraminifera and a development in the sublittoral environment is envisaged. Chemically, the ferromanganese nodules are depleted in manganese relative to iron, compared with Recent nodules, a loss which is attributed to diagenetic migration of manganese. Electron probe studies show that manganese covaries positively with calcium, but negatively with iron and silicon. Diagenetic enrichment of hematite occurs in the griotte at hardground horizons where two periods of mineralization can be established. The Devonian ferromanganese nodules show that solution of nodules has not occurred on burial.
Zusammenfassung Es werden Eisenmanganknollen (als Äquivalent zu den rezenten Manganknollen) aus dem oberdevonischen Griotte (rote pelagische Kalke) der Montagne Noir und den Cephalopoden-Kalken des Rheinischen Schiefergebirges beschrieben. Sie treten als Umkrustungen von Skeletten und Kalksteinbruchstückchen auf und zeigen gewöhnlich Kolloform-Strukturen. Die Knollen sind vergesellschaftet mit umkrusteten Foraminiferen, und es wird deshalb sublitorales Bildungsmilieu angenommen. Verglichen mit rezenten sind die oberdevonischen Knollen durch Auslaugung von Mangan relativ Eisen-angereichert, was auf diagenetische Mobilisation zurückzuführen ist. Untersuchungen mit der Mikrosonde haben gezeigt, daß Mangan und Calcium gleichläufig, Mangan und Eisen und Kieselsäure gegenläufig sind. Diagenetische Hämatit-Anreicherungen gibt es im Griotte an hardground-Horizonten; es wurden zwei Mineralisationsphasen festgestellt. Die oberdevonischen Eisenmangan-Knollen zeigen, im Gegensatz zur allgemein verbreiteten Meinung, daß sich die Knollen bei Überlagerung nicht unbedingt auflösen.
Résumé Les nodules de ferromanganèse (équivalents des nodules de manganèse de la période holocène) décrits ici proviennent de la griotte du Dévonien Supérieur (calcaire pélagique rouge) de la Montagne Noire (Sud de la France) et du calcaire à Céphalopodes du Massif Schisteux Rhénan, (Allemagne de l'Ouest). Ils se présentent comme des incrustations exhibant communément des structures colloformes autour de débris de squelettes et de matériaux clastiques calcaire. Les nodules sont associés à des foraminifères qui s'y incrustent; leur développement est conçu dans un milieu sous-littoral. Chimiquement les nodules de ferromanganèse, comparés aux nodules du manganèse de l'époque récente, sont de faible teneur en manganèse par rapport au fer, perte que l'on attribute à la migration diagénétique du manganèse. Des expériences faites à la sonde électronique montrent que le manganèse réagit positivement avec le calcium, mais négativement avec le fer et le silicium. L'enrichissement diagénétique de l'hématite se produit dans la griotte dans des zones de «hardgrounds», où deux périodes de minéralisation peuvent être constatées. Les nodules de ferromanganèse dévoniens montrent, contrairement à une opinion généralement répandue, que les nodules ne se dissolvent pas nécessairement par l'accumulation des sédiments.
— — Griotte ( ) Montagne Noir . . ; , . , . , , , . Griotte ; . , , .相似文献
53.
P. Tucker 《Earth and Planetary Science Letters》1981,54(1):167-172
The low-temperature magnetic hysteresis characteristics of multidomain titanomagnetite were fitted by a mixed coercivity model in which magnetocrystalline-controlled and magnetostriction-controlled domain-wall pinning play an important part. The effective contribution of the former was found to decrease systematically with increasing titanium content for compositions in the range Fe2.48Ti0.52O4Fe2.35Ti0.65O4. In agreement with the model, the observed magnetic remanence transitions and the characteristic peaks on the susceptibility vs. temperature curve became less pronounced as the titanium content increased. The problems in using these features as a means of identifying the carriers of natural remanent magnetization are discussed. 相似文献
54.
Stromatolites and cryptalgal laminites are described from the Lower Fars Formation of Middle Miocene age from northern Iraq. The Lower Fars is comprised of many cycles of the sequence marl, limestone, gypsum. The microstructures of the stromatolites are described and compared with those of Recent algal mats. An intertidal depositional environment is indicated by the features of the cryptalgal limestones and by the presence of calcite pseudomorphs after gypsum. The significance of this interpretation in terms of Lower Fars sedimentation is discussed. It is concluded that the sulphate horizons formed in a supratidal setting and that the cycles resulted from repeated sabkha progradation.
Zusammenfassung Charakteristisch für die untere Fars-Formation (Mittel-Miozän) im Nord-Irak sind Stromatolithe und kryptoalgische Plättchen. Das untere Fars wird aus zahlreichen Zyklen von Mergel, Kalk und Gips aufgebaut. Die Mikrostrukturen der Stromatolithe werden mit heutigen Algenrasen verglichen. Ein Ablagerungsraum in der intertidalZone ist durch die Merkmale des kryptoalgischen Kalkes und die Kalzit-Pseudomorphosen nach Gips angedeutet. Der Schluß folgt, daß die Sulphat-Horizonte im Supratidal entstanden und daß die Zyklen durch wiederholtes Sabkhawachstum entstanden sind.
Résumé Sont décrites ici les stromatolites et les laminites cryptalgaires de la formation du Fars Inférieur d'âge Miocène Moyen du nord de l'Irak. Les Fars Inférieur comprend beaucoup de cycles formés de marne, calcaire, gypse. Les microstructures des stromatolites sont décrites et comparées avec celles des mattes algaires récentes. Un milieu de depôt intertidal est indiqué par les caractères des calcaires cryptalgaires et par la présence de pseudomorphoses de gypse en calcite. On discute l'importance de cette interprétation en termes de sédimentation du Fars Inférieur. En conclusion: les horizons sulfatés sont formés dans un emplacement supratidal et les cycles sont le résultat d'une progradation répétée de sabkha.
, , . , . , c . , , . , , sabkha.相似文献
55.
Gregory E. Tucker 《地球表面变化过程与地形》2004,29(2):185-205
Long‐term average rates of channel erosion and sediment transport depend on the frequency–magnitude characteristics of ?ood ?ows that exceed an erosion threshold. Using a Poisson model for rainfall and runoff, analytical solutions are developed for average rates of stream incision and sediment transport in the presence of such a threshold. Solutions are derived and numerically tested for three erosion/transport formulas: the Howard–Kerby shear‐stress incision model, the Bridge–Dominic sediment transport model, and a generic shear‐stress sediment transport model. Results imply that non‐linearity resulting from threshold effects can have a ?rst‐order impact on topography and patterns of dynamic response to tectonic and climate forcing. This non‐linearity becomes signi?cant when fewer than about half of ?ood events are capable of detaching rock or sediment. Predicted morphology and uplift‐gradient scaling is more closely consistent with observations and laboratory experiments than conventional slope‐linear or shear‐linear erosion laws. These results imply that particle detachment thresholds are not details that can be conveniently ignored in long‐term landscape evolution models. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
56.
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
57.
M. Sultan R. D. Tucker Z. El Alfy R. Attia Abdel Ghany Ragab 《International Journal of Earth Sciences》1994,83(3):514-522
The first U-Pb zircon ages are reported for the gneissic bedrock inliers previously interpreted as part of the Nile Craton. The inliers crop out in the Egyptian Western Desert, east of the Uweinat area and west of the Eastern Desert. Multi- and single-grain zircon analyses of granitoid gneiss and migmatite from Gebel Um Shagir, Aswan, and another locality approximately 160 km south-west of Aswan, yield simple discordia with near modern day Pb loss trajectories, and the following Neoproterozoic crystallization ages: 626+4/–3, 634 ± 4 and 741 ± 3 Ma. In contrast, multi- and single-grain U-Pb analyses (zircon and sphene) from an anorthositic gabbro at Gebel Kamil (22°46N 26°21E) and an anorthosite at Gebel El Asr (22°46N 31°10E) yield Archean and Paleoproterozoic emplacement ages. The former yield a crystallization age of > 2.67 Ga and a metamorphic age of 2.0 Ga; the latter a metamorphic age of 0.69 Ga and an inheritance age of 1.9–2.1 Ga. Because high grade gneiss and migmatite of Neoproterozoic, Paleoproterozoic and Archean age crop out west of the Nile, pre-Neoproterozoic crust should no longer be identified by its metamorphic grade. By contrast, mapping the anorthosite and related rocks might provide first-order estimates for the extension of pre-Neoproterozoic crust in north-east Africa. It is suggested that Archean and Paleoproterozoic crust of the Uweinat and Congo Craton are contiguous because these U-Pb (zircon) data show no evidence for a Neoproterozoic thermal overprint in the Gebel Kamil area and there is no pronounced Neoproterozoic magmatic activity south of the Uweinat inlier and north of the Congo Craton. 相似文献
58.
Maria Carmen Lemos Timothy J. Finan Roger W. Fox Donald R. Nelson Joanna Tucker 《Climatic change》2002,55(4):479-507
This article examines the use of seasonal climate forecasting in public and private efforts to mitigate the impacts of drought in Ceará, Northeast Brazil. Here, forecasts have been directed towards small scale, rainfed agriculturalists as well as state and local level policymakers in the areas of agriculture, water management, and emergency drought relief. In assessing possibilities and constraints of forecast application in Ceará, the present analysis takes into account three types of variables: (a)characteristics of the forecasts; (b) policymaking systems; and (c)institutional environments. We conclude that, on the one hand, several factors in the Ceará case have limited the effectiveness of seasonal climate forecast use. First, the current level of skill of the forecasts is inadequate for the needs of policy development and farmer decisionmaking. Second, forecast information application has been subject to distortion, misinterpretation and political manipulation. Third, focus on the forecast as a product until recently neglected to take into account end users' needs and decisionmaking behavior. On the other hand, climate forecasting has the potential to offer a dramatic opportunity for state and local level bureaucracies to embark on a path of proactive drought planning. 相似文献
59.
The Las Campanas Redshift Survey, an optically selected survey which contains 26 418 galaxy redshifts, has been correlated with “The First ROSAT Source Catalogue of Pointed Observations with the PSPC,” which contains 50 408 sources from 2876 ROSAT pointed observations. Ten matches were found. The optical spectra of most of the ten matches show weak narrow emission lines. Due to their high x-ray luminosities, their high x-ray-to-optical flux ratios, and the evidence of rapid x-ray variability in the two brightest matches, we interpret the majority of these objects to be narrow-line Seyfert galaxies or “hidden” active galactic nuclei. Of the ten matches, only one galaxy shows the characteristics of a bona fide starburst. 相似文献
60.
Which way do you lean? Using slope aspect variations to understand Critical Zone processes and feedbacks 总被引:1,自引:0,他引:1
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Jon D. Pelletier Greg A. Barron‐Gafford Hugo Gutiérrez‐Jurado Eve‐Lyn S. Hinckley Erkan Istanbulluoglu Luke A. McGuire Guo‐Yue Niu Michael J. Poulos Craig Rasmussen Paul Richardson Tyson L. Swetnam Greg E. Tucker 《地球表面变化过程与地形》2018,43(5):1133-1154
Soil‐mantled pole‐facing hillslopes on Earth tend to be steeper, wetter, and have more vegetation cover compared with adjacent equator‐facing hillslopes. These and other slope aspect controls are often the consequence of feedbacks among hydrologic, ecologic, pedogenic, and geomorphic processes triggered by spatial variations in mean annual insolation. In this paper we review the state of knowledge on slope aspect controls of Critical Zone (CZ) processes using the latitudinal and elevational dependence of topographic asymmetry as a motivating observation. At relatively low latitudes and elevations, pole‐facing hillslopes tend to be steeper. At higher latitudes and elevations this pattern reverses. We reproduce this pattern using an empirical model based on parsimonious functions of latitude, an aridity index, mean‐annual temperature, and slope gradient. Using this empirical model and the literature as guides, we present a conceptual model for the slope‐aspect‐driven CZ feedbacks that generate asymmetry in water‐limited and temperature‐limited end‐member cases. In this conceptual model the dominant factor driving slope aspect differences at relatively low latitudes and elevations is the difference in mean‐annual soil moisture. The dominant factor at higher latitudes and elevations is temperature limitation on vegetation growth. In water‐limited cases, we propose that higher mean‐annual soil moisture on pole‐facing hillslopes drives higher soil production rates, higher water storage potential, more vegetation cover, faster dust deposition, and lower erosional efficiency in a positive feedback. At higher latitudes and elevations, pole‐facing hillslopes tend to have less vegetation cover, greater erosional efficiency, and gentler slopes, thus reversing the pattern of asymmetry found at lower latitudes and elevations. Our conceptual model emphasizes the linkages among short‐ and long‐timescale processes and across CZ sub‐disciplines; it also points to opportunities to further understand how CZ processes interact. We also demonstrate the importance of paleoclimatic conditions and non‐climatic factors in influencing slope aspect variations. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献