Wetlands are highly dynamic and productive systems that have been under increased pressure from changes in land use and water management strategies. In Eastern Africa, wetlands provide resources at multiple spatial and temporal levels through farming, fishing, livestock ownership and a host of other ecosystem services that sustain the local economy and individual livelihoods. As part of a broader effort to describe future development scenarios for East African coastal wetlands, this qualitative study focuses on understanding the processes by which river water depletion has affected local food production systems in Kenya's Tana River Delta over the past 50 years, and how this situation has impacted residents’ livelihoods and well-being. Interviews performed in six villages among various ethnic groups, geographical locations and resource profiles indicated that the agro-ecological production systems formerly in place were adapted to the river's dynamic flooding patterns. As these flooding patterns changed, the local population diversified and abandoned or adopted various farming, fishing and livestock-rearing techniques. Despite these efforts, the decrease in water availability affected each subcomponent of the production systems under study, which led to their collapse in the 1990s. Water depletion negatively impacted local human well-being through the loss of food security. The current study provides a detailed account of the dynamics of agro-ecological production systems facing the effects of river water depletion in a wetland-associated environment in Sub-Saharan Africa. 相似文献
Mongolia is subject to regular peaks of livestock winter mortality called dzuds. Several kinds of dzud exist and the ‘white dzud’, characterized by heavy stochastic snowfalls preventing livestock to access forage, is considered the most common. Droughts and high livestock densities are thought to be part of the dzud process by affecting body condition, which increases vulnerability to snowfalls. Guided by the equilibrium/nonequilibrium framework, we studied how rainfall, animal numbers and pasture health (defined as the integrity of ecological processes sustaining grass growth) impact livestock body condition in a case study of West Mongolia. We studied this parameter through livestock productivity (LP) as a proxy, defined as the annual number of newborns per breeding-age female. We found no significant impact of rainfall or livestock numbers, alone or combined. We found through the study of pasture use, defined as the ratio forage consumed/forage available, an impact of the combined effect of rainfall, animal numbers and pasture health. We observed in addition sharp LP decreases prior to dzuds, which suggests that the above-mentioned drivers interact to weaken livestock which increases its vulnerability to winter hazards. This tends to show that in our case study, dzuds are not the simple consequence of stochastic hazards striking randomly, but instead, the final stage of a chain of events that involves dry years, high livestock densities and pasture degradation. This also indicates that dzud early warning indicators could be designed based on LP monitoring. 相似文献
In 1990–1991 the LITHOPROBE project completed 450 km of seismic reflection profiles across the late Archaean crust of the southwestern Superior province. The results define a broad three-fold division of crust: upper crust in the Abitibi greenstone belt is non-reflective and is a 6–8 km veneer of volcanic and plutonic supracrustal rocks, whereas, in the sediment-gneiss dominated Pontiac subprovince, upper crust comprises shallow northwest-dipping turbidite sequences; mid-crust, in both the Abitibi and the Pontiac subprovinces, is interpreted as imbricate sequences of metasedimentary and metaplutonic rocks; lower crust in both subprovinces has a horizontal layer parallel strycture which may represent interleaved mafic-intermediate gneisses. The seismic signature of the northern Abitibi greenstone belt may be represented in an exposed 25 km crustal section in the Kapuskasing stuctural zone.
Preliminary tectonic models based on the seismic data are consistent with a plate-tectonic scenario involving oblique subduction and imbrication of sedimentary, plutonic and volcanic sequences. The northern Abitibi supracrustal sequences either represent an allochthon, or overlie an allochthonous underthrust metasedimentary and plutonic sequence which may be equivalent to a metasedimentary subprovince such as the Pontiac or Quetico.
Seismic velocities have yet to be defined. However, crustal thicknesses are relatively constant at 35–40 km. The thinnest crust is adjacent to the Grenville Front where Moho is very well defined. 相似文献
Résumé Les galènes et sulfoantimoniures de plomb des filons quartzeux, barytiques et fluorés du Haut-Allier (district de Brioude-Massiac) ont fait l'objet d'analyses isotopiques du plomb. Les résultats montrent l'existence de deux populations de plomb appartenant à chacune des deux phases minéralisatrices reconnues dans la région.La première population (18,108<206Pb/204Pb<18,181) est rapportée à la phase mésothermale permienne. Elle comprend les sulfosels précoces des filons antimonifères ainsi que la galène des filons plombo-stannifères. L'homogénéité isotopique confirme la liaison génétique existant entre ces différents types filoniens et leur appartenance à un même processus métallogénique. Le socle métamorphique régional apparaît comme la source possible du plomb.La relative dispersion des valeurs isotopiques de la seconde population (18,215<206Pb/204Pb<18,472), d'âge liasique, est corrélable avec la situation géographique et géologique des filons. Elle est due à deux phénomènes complémentaires: le lessivage d'un segment crustal comprenant le socle métamorphique avec une participation graduée du granite uranifère de la Margeride et la réassimilation partielle du plomb des filons permiens. Ce dernier phénomène d'héritage métallique se traduit au niveau des paragenèses par la formation de filons à sulfosels dominants.
Lead isotope analyses are reported for galena and lead sulfosalts from quartz, barite and fluorite vein deposits from the Haut-Allier (Brioude-Massiac district). The data define two lead populations reflecting the two mineralization episodes in this region.The first population (18,108<206Pb/204Pb<18,181) is related to the Permian mesothermal episode. It includes early sulfosalts from antimony veins and galena from lead-stanniferous veins. The Pb isotopic homogeneity confirms the relationship between the different vein types and their formation by a single metallogenic process. The metamorphic basement is the potential source of lead.The second population includes quartz-barite and fluorite veins. It shows much more isotopic variation (18,215<206Pb/204Pb<18,472) correlating with the geographical and geological location of the veins. Because the veins have a contemporaneous age, the isotopic dispersion therefore depends on two complementary phenomena: the leaching of a large volume of crust, including metamorphic basement, with a progressive contribution of the uraniferous granite of Margeride; and the partial remobilization of lead from Permian veins. The latter inheritance phenomena result in the genesis of major lead-sulfosalt vein deposits.
Alteration of basaltic glass to palagonite is characterized by a nearly isomolar exchange of SiO2, Al2O3, MnO, MgO, CaO, Na2O, P2O5, Zn, Cu, Ni, Cr, Hf, Sc, Co and REE for H2O and K2O, whilst TiO2 and FeO are passively accumulated during removal of the remaining cations. The network forming cations Al and Si are removed from the glass in proportion to the gain in Ti and Fe, whilst the other cations do not show a significant relationship to the amount of Ti and Fe accumulation. Sr isotopic data show that during palagonite formation approximately 85% of the basaltic Sr is lost to the hydrous solutions and 40% of seawater Sr is added to the glass, yielding an average loss of the same order of magnitude as of the network forming cations. Losses and gains of oxides yield an average increase of +105% TiO2.K, Rb, and Cs show high increases, but and ratios indicate two different alteration processes: (1) formation of palagonite involves a drastic decrease in these ratios, indicating structural similarities between palagonite and smectite; (2) surface alteration of glass is characterized by an increase in and ratios, probably best interpreted as sorption of alkalies in ratios approximating those of seawater.The total fluxes involved in alteration of glass in the upper portion of the oceanic crust are estimated from the modal abundance of palagonite in the oceanic crust and the abundance of the vein materials smectite and carbonate. Smectite and carbonates act as a sink for a significant portion of the elements liberated up during alteration of basaltic glass except for Na and Al, which are probably taken up by zeolites and/or albite, possibly hidden in the macroscopic estimate of carbonate. Formation of the observed quantity of secondary phases requires additional sources for Si, Fe. Ca and K. K is provided in excess from the inflowing seawater at reasonable water/rock ratios. The remaining excess Ca, Si and Fe required may be derived by alteration of interstitial glass and breakdown of anorthite rich plagioclase and titano-magnetite, and/or by supply of deeper seated metamorphic reactions. 相似文献