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1.
River discharge and nutrient measurements are subject to aleatory and epistemic uncertainties. In this study, we present a novel method for estimating these uncertainties in colocated discharge and phosphorus (P) measurements. The “voting point”‐based method constrains the derived stage‐discharge rating curve both on the fit to available gaugings and to the catchment water balance. This helps reduce the uncertainty beyond the range of available gaugings and during out of bank situations. In the example presented here, for the top 5% of flows, uncertainties are shown to be 139% using a traditional power law fit, compared with 40% when using our updated “voting point” method. Furthermore, the method is extended to in situ and lab analysed nutrient concentration data pairings, with lower uncertainties (81%) shown for high concentrations (top 5%) than when a traditional regression is applied (102%). Overall, for both discharge and nutrient data, the method presented goes some way to accounting for epistemic uncertainties associated with nonstationary physical characteristics of the monitoring site.  相似文献   
2.
Climate change impacts on global agriculture   总被引：1，自引：0，他引：1  
Based on predicted changes in the magnitude and distribution of global precipitation, temperature and river flow under the IPCC SRES A1B and A2 scenarios, this study assesses the potential impacts of climate change and CO2 fertilization on global agriculture. The analysis uses the new version of the GTAP-W model, which distinguishes between rainfed and irrigated agriculture and implements water as an explicit factor of production for irrigated agriculture. Future climate change is likely to modify regional water endowments and soil moisture. As a consequence, the distribution of harvested land will change, modifying production and international trade patterns. The results suggest that a partial analysis of the main factors through which climate change will affect agricultural productivity provide a false appreciation of the nature of changes likely to occur. Our results show that global food production, welfare and GDP fall in the two time periods and SRES scenarios. Higher food prices are expected. No matter which SRES scenario is preferred, we find that the expected losses in welfare are significant. These losses are slightly larger under the SRES A2 scenario for the 2020s and under the SRES A1B scenario for the 2050s. The results show that national welfare is influenced both by regional climate change and climate-induced changes in competitiveness.  相似文献   
3.
The Hili Manu peridotite occupies a key position at the outer limit of continental crust on the north coast of East Timor. Most models for the tectonic evolution of the Outer Banda Arc interpret peridotite bodies on Timor, such as Hili Manu, as fragments of young oceanic lithosphere from the Banda Arc (upper plate). However, recent workers have used major-element geochemistry to argue that the peridotite bodies on Timor were derived from the Australian subcontinental lithosphere. Our major, trace and isotopic geochemical study of the Hili Manu peridotite body supports a supra-subduction origin from either a forearc or backarc position for the Hili Manu peridotite. In particular, the wide range in Nd and Sr isotopic compositions, overlapping that of arc volcanics from the Sunda – Banda Island arc, and highly fractionated Nb/Ta values indicate a supra-subduction setting. As there is no evidence for subduction beneath the rifted Australian continental margin, it is unlikely that the Hili Manu peridotite is Australian subcontinental lithosphere. This result, along with the clear supra-subduction setting of the Ocuzzi peridotite and associated volcanics in West Timor, gives support to the interpretation that the Miocene collision between the Banda Arc and the Australian continental margin has produced widespread ‘Cordilleran’-style ophiolites on Timor.  相似文献   
4.
Karoo picrites are divided into a volumetrically dominant high Ti-Zr (HTZ) group and a less abundant low Ti-Zr (LTZ) group. Distinguished in this way, it is then possible to use the comparison of major element compositions with those of experimentally determined partial melts of mantle peridotites to constrain the depth of segregation for each magma type. This approach also identifies the nature of source and residual mantle materials from which such magma segregation may have occurred. The LTZ group show uniformly shallow pressures (ca. 13–15 kb) of magma segregation and the HTZ group show a range of pressures (10 kb to > 30 kb) of segregation. In the HTZ group a range of K2O, TiO2 and Zr contents is observed, with greater concentrations in picrites which have segregated at higher pressures (high-NaK# HTZ picrites) relative to a low-NaK# HTZ picrite group (NaK#=[Na/2O+K2O][Na2O+K2O+CaO]). If we measure the refractory character of a mantle source peridotite by its CaO and Al2O3 content then it is shown that the LTZ picrites have segregated from sources which are more refractory than MORB or OIB sources and which lack the strong K, Ti and Zr enrichments of HTZ sources. Mixing of at least two mantle components is required to fully explain the major element and isotopic data in the HTZ group and is combined with variations in composition produced by different depths of magma segregation. Consideration of the major element phase relations coupled with trace element abundances and isotope data leads us to suggest that one component is the Kaapvaal lithospheric mantle (incompatible element enriched but relatively refractory with respect to basaltic components) and the other is asthenospheric mantle. This lithospheric mantle component is most evident in the high-NaK# HTZ picrite end-member which has segregated at a greater depth (ca. 18–32 kb). The asthenospheric component is most closely represented by the low-NaK# HTZ picrites which have segregated at pressures of 10–22 kb. The LTZ source, more refractory mantle than MORB, is inferred to be the shallow sub-continental lithospheric mantle which may also be the source of the low-MgO tholeiites related to these picrites, and therefore the source for the main phase of Karoo basaltic eruptions.  相似文献   
5.
Future levels of water stress depend on changes in several key factors including population, climate-change driven water availability, and a carbon dioxide physiological-forcing effect on evaporation and run-off. In this study we use an ensemble of the HadCM3 climate model forced with a range of future emissions scenarios combined with a simple water scarcity index to assess the contribution of each of these factors to the projected population living in water stress over the 21st century.Population change only scenarios increase the number of people living in water stress such that at peak global population 65% of people experience some level of water stress. Globally, the climate model ensemble projects an increase in water availability which partially offsets some of the impacts of population growth. The result is 1 billion fewer people living in water stress by the 2080s under the high end emissions scenarios than if population increased in the absence of climate change.This study highlights the important role plant-physiological forcing has on future water resources. The effect of rising CO2 is to increase available water and to reduce the number of people living in high water stress by around 200 million compared to climate only projections. This effect is of a similar order of magnitude to climate change.  相似文献   
6.
Noble gas elemental and isotopic abundances have been analysed in eight samples of youthful basaltic glass dredged from three different locations within the Lau Backarc Basin: (1) the King's Triple Junction, (2) the Central Lau Spreading Centre at 18°S and (3) the Eastern Lau Spreading Centre at 19°S. Samples from the Lau central and eastern spreading centres have MORB-like helium isotopic ratios of approximately 1.2 × 10−5 (8.5 R/RA). In contrast, the samples from the King's Triple Junction yield helium isotopic ratios averaging 9.4 (±0.8) × 10−6 (6.7 ± 0.6 R/RA), systematically lower than the MORB-like value, which may be reflecting the addition of radiogenic 4He released from the descending slab. Neon isotopic ratios are enriched in 20Ne and 21Ne with respect to atmospheric ratios by as much as 23% and 62% respectively. These observations further confirm that non-atmospheric neon is a common characteristic of samples derived from the mantle. The helium and neon isotopic signatures in the samples can be explained by mixing of a primordial solar component, radiogenic and nucleogenic components produced by radioactive processes inside the Earth, and an atmospheric component. This reconnaissance survey of noble gases in a backarc basin indicates that current volcanism is dominated by magmas from the mantle wedge, a source similar to that from which MORBs are derived. The heavier noble gases (argon, krypton and xenon), however, show more atmosphere-like compositions, either indicating strong interaction of the magmas with the atmosphere or the presence of a recycled component derived from the underlying subducting slab.  相似文献   
7.
The origin of island arc high-alumina basalts   总被引：5，自引：1，他引：5  
A detailed examination of the hypothesis that high-alumina basalts (HAB) in island arcs are primary magmas derived by 50–60% partial melting of subducted ocean crust eclogite shows that this model is unlikely to be viable. Evidence suggests that the overwhelming majority of arc HAB are porphyritic lavas, enriched in Al2O3 either by protracted prior crystallization of olivine and clinopyroxene, or by plagioclase phenocryst accumulation in magmas of basaltic andesite to dacite composition. Experimentally-determined phase relationships of such plagioclase-enriched (non-liquid) compositions have little bearing on the petrogenesis of arc magmas, and do not rule out the possibility that arc HAB can be derived by fractionation of more primitive arc lavas. Although models invoking eclogite-melting can match typical arc HAB REE patterns, calculations indicate that the Ni and Cr contents of proposed Aleutian primary HAB are many times lower than such models predict. In contrast, Ni vs Sc and Cr vs Sc trends for arc HAB are readily explained by olivine (+Cr-sp) and clinopyroxene-dominated fractionation from more primitive arc magmas. GENMIX major element modelling of several HAB compositions as partial melts of MORB eclogite, using appropriate experimentally (26–34 kb)-determined garnet and omphacite compositions yields exceptionally poor matches, especially for CaO, Na2O, MgO and Al2O3. These mismatches are easily explained if the HAB are plagioclase-accumulative. Groundmasses of arc HAB are shown to vary from basaltic andesite to dacite in composition. Crystal fractionation driving liquid compositions toward dacite involves important plagioclase separation, resulting in development of significant negative Eu anomalies in more evolved lavas. Plagioclase accumulation in such evolved liquids tends to diminish or eliminate negative Eu anomalies. Therefore, the absence of positive Eu anomaly in a plagioclase-phyric HAB does not indicate that plagioclase has not accumulated in that lava. In addition, we show that plagioclase phenocrysts in arc HAB are not in equilibrium with the liquids in which they were carried (groundmass). Exceptional volumes of picrite and olivine basalt occur in the Solomons and Vanuatu arcs; the presence in lavas from these and other arcs (Aleutian, Tonga) of olivine phenocrysts to Fo94, finds no ready explanation in the primary eclogite-derived HAB model. We suggest that most lavas in intra-oceanic arcs are derived from parental magmas with >10% MgO; fractionation of olivine (+Cr-sp) and clinopyroxene drives liquids to basalt compositions with <7% MgO, but plagioclase nucleation is delayed by their low but significant (<1%?) H2O contents. Thus evolved liquid compositions in the basaltic andesite—andesite range may achieve relatively high Al2O3 contents (<17.5%). The majority of arc basalts, however, have Al2O3 contents in excess of 18%, reflecting plagioclase accumulation. We give new experimental data to show that HAB liquids may be generated by anhydrous, low-degree (<10%) partial melting of peridotite at P<18 kb. Relative to arc HAB, these experimental melts have notably higher Mg#(69–72) and are in equilibrium with olivine Fo87–89. Only further detailed trace element modelling will show if they might be parental magmas for some arc HAB.  相似文献   
8.
Summary Fresh back-arc basin lavas were recovered during five dives of the submersible Mir during the 1990 cruise of the research vessel Akademik Mstislav Keldysh to the Lau Basin. Three dives were conducted on the central spreading center of the King's Triple Junction (KTJ) in the northeastern part of the Lau Basin east of Niuafo'ou Island at approximately 15°S. The lavas from the KTJ can be divided into types I and II based on their similarities to N-MORB and the BABB magma type ofSinton andFryer (1987) respectively. One dive each was made on the Central Lau Spreading Center (CLSC) at 18°S and the Eastern Lau Spreading Center (ELSC) at 19°S. Lavas sampled on the CLSC were associated with active hydrothermal sulphide chimneys occurring at the base of a collapsed caldera structure on the central volcanic axial high. Sampled lavas from both the CLSC and ELSC are all of type I geochemistry.The results of the Keldysh 90 cruise are integrated with previous work to evaluate the geochemical characteristics of the Lau Basin crust as a whole and geochemical zonation models for back-arc basin development. An important part of this review of Lau Basin basalt geochemistry is the recognition of boninites and rocks of boninitic affinity which occur at off-ridge locations throughout the Lau Basin. These are suggested to erupt during the initial stages of development of new spreading ridges associated with episodes of ridge jump or the propagation of back-arc spreading ridges into arc crust.Hf-Th-Ta systematics of the Lau Basin lavas are used to identify unmodified mantle source compositions and possible subduction-related enriched components. Unmodified mantle source compositions range from D-, N-, and E-MORB to OIB. All Lau Basin lavas show some evidence of enrichment by a H2O ± LILE ± LREE-enriched component, a slab derived hydrous fluid. The slab-derived fluid is not homogeneous in composition perhaps reflecting the presence or absence of a subducted sediment component.The sampled lavas from the KTJ confirm the uniqueness of the BABB magma type ofSinton andFryer (1987).
Zusammenfassung Dieser Artikel berichtet über erste Ergebnisse einer Forschungsexpedition in das Lau Basin, ausgeführt mit dem Forschungsschiff Akademiker Mstislav Keldysh und dem Mini-U-Boot Mir. Drei Tauchfahrten führten zur zentralen Achse der King's triple junction (KTJ) im nordöstlichen Teil des Lau Basins östlich der Insel Niuafo'ou (15°S). Die dort beprobten Laven werden auf der Basis ihrer chemischen Zusammensetzung eingeteilt in die Typen I und II, entsprechend der Klassifikation vonSinton andFryer (1987) in N-MORB und BABB. Eine vierte Tauchfahrt zum zentralen Lau Spreading Center (CLSC, 18°N) erbrachte Fragmente von Sulphidschloten eines aktiven Black Smoker innerhalb einer kollabierten Calderastructur. Eine fünfte Tauchfahrt zum östlichen Lau Spreading Center (ELSC, 19°N) erbrachte Laven vom Typus I (entsprechend N-MORB).Unter Berücksichtigung früherer Arbeiten gestatten die Ergebnisse der Keldysh 90 Expedition eine geochemische Charakterisierung des Lau Basins, sowie eine Bewertung bestehender Zonierungsmodelle fur die Entstehung und Evolution von Backarc Basins. Entlang der gesamten Achse im Lau Basin, aber abseits des eigentlichen aktiven Spreading Centers, werden Boninite und Laven boninitischer Affinität beobachtet. Diese Boninite können als integraler Teil des Frühstadiums eines Backarc Spreading Centers gedeutet werden, und entstehen möglicherweise dort, wo Spreading Centers in die Kruste eines Inselbogens vordringen oder dann, wenn es zu einer abrupten Positionsänderung der Achse kommt.Die Hf-Th-Ta Konzentrationen und Verhältnisse in Lau Basin Laven gestatten die Identifikation mehrerer Mantel-Endglieder (entsprechend D-MORB, N-MORB, E-MORB, und OIB), sowie einer an H2O und LILE-LREE angereicherten Komponente, die mit Subduktionstätigkeit und anschließender Mantelmetasomatose in Zusammenhang gebracht wird. Die angereicherte Komponente war vermutlich inhomogen aufgrund variabler Anteile subduzierter Sedimente. Die Laven des Lau Basins bestätigen die vonSinton andFryer (1987) vorgeschlagene geochemische Sonderstellung von BABB Magmentypen.

With 8 Figures  相似文献   
9.
The Xikeer mafic granulite xenoliths are hosted by a Cenozoic basanite sill (∼20 Ma) in the northwest Tarim Basin, northwest China. Sulfides, identified in these xenoliths consist mainly of pyrite. Two groups of pyrite (types A and B) can be distinguished based on petrography and trace element geochemistry, as determined by laser-ablation ICP-MS analysis. Type A pyrite is subhedral and in general has lower trace element abundance than type B pyrite. Type B pyrite is fractured with ragged anhedral morphology and has extremely high Cu, Tl, Ni, and Co abundance. The low Co and Ni contents combined with a positive correlation between Cu, Cr, and Pb all indicate that the type A pyrite may have formed from a high-temperature magmatic hydrothermal fluid. In contrast, the Cu-, Tl-, Ni-, and Co-rich type B pyrite was probably formed in a relatively low-temperature basinal environment. The Cu mineralization at Xikeer can be explained when Cu, Tl, Ni, and Co were leached from the intruded basanites and subsequently enriched in circulating basin brines. Such enrichment of Cu in basin brines could provide an important Cu source for the Cu mineralization in the Xikeer district and other Cenozoic sub-basins in the Tarim Basin.  相似文献   
10.
A suite of young volcanic basaltic lavas erupted on the intra-plate island of Niuafo’ou and at active rifts and spreading centres (the King’s Triple Junction and the Northeastern Lau Spreading Centre) in the northern Lau Basin is used to examine the pattern of mantle flow and the dynamics of melting beneath this complex back-arc system. All lavas contain variable amounts of a subduction related component inherited from the Tonga subduction zone to the east. All lavas have higher 87Sr/86Sr, lower 143Nd/144Nd and more radiogenic Pb isotope compositions than basalts erupted at the Central Lau Spreading Centre in the central Lau Basin, and are interpreted as variable mixtures of subduction-modified, depleted upper mantle, and mantle residues derived from melting beneath the Samoan Islands which has leaked through a tear in the subducting Pacific Plate beneath the Vitiaz Lineament at the northern edge of the Lau Basin. Our data can be used to map out the present-day distribution of Samoan mantle in this region, and show that it influences the compositions of lavas erupted as far as 400 km from the Samoan Islands. The distribution of Samoan-influenced lavas implies south- and southwest-wards mantle flow rates of >4 cm/year. U-series disequilibria in historic Niuafo’ou lavas have average (230Th/238U) = 1.13, (231Pa/235U) = 2.17, (226Ra/230Th) = 2.11, and together with major and trace element data require ∼5% partial melting of mantle at between 2 and 3 GPa, with a residual porosity of 0.002 and an upwelling rate of 1 cm year−1. We suggest that intraplate magmatism in the northern Lau Basin results from decompression melting during southward flow of mantle from beneath old (110–120 Ma), relatively thick Pacific oceanic lithosphere to beneath young (<5 Ma), thinner oceanic lithosphere beneath the northern Lau Basin.  相似文献   
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