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81.
The Ebro Fan System consists of en echelon channel-levee complexes, 50×20 km in area and 200-m thick. A few strong reflectors in a generally transparent seismic facies identify the sand-rich channel floors and levee crests. Numerous continuous acoustic reflectors characterize overbank turbidites and hemipelagites that blanket abandoned channel-levee complexes. The interlobe areas between channel complexes fill with homogeneous mud and sand from mass flow and overbank deposition; these exhibit a transparent seismic character. The steep continental rise and sediment “drainage” of Valencia Trough at the end of the channel-levee complexes prevent the development of distributary channels and midfan lobe deposits. Margin setting represents fan and/or source area  相似文献   
82.
A Lagrangian analysis was applied to the outputs of a coupled physical-biogeochemical model to describe the redistribution of nitrate-rich and nitrate-poor surface water masses in the tropical Pacific throughout the major 1997 El Niño. The same tool was used to analyze the causes of nitrate changes along trajectories and to investigate the consequences of the slow nitrate uptake in the high nutrient low chlorophyll (HNLC) region during the growth phase of the event. Three patterns were identified during the drift of water masses. The first mechanism is well known along the equator: oligotrophic waters from the western Pacific are advected eastward and retain their oligotrophic properties along their drift. The second concerns the persistent upwelling in the eastern basin. Water parcels have complex trajectories within this retention zone and remain mesotrophic. This study draws attention to the third process which is very specific to the HNLC region and to the El Niño period. During the 1997 El Niño, horizontal and vertical inputs of nitrate decreased so dramatically that nitrate uptake by phytoplankton became the only mechanism driving nitrate changes along pathways. The study shows that because of the slow nitrate uptake characteristic of the tropical Pacific HNLC system, nitrate in the pre-El Niño photic layer can support biological production for a period of several months. As a consequence, the slow nitrate uptake delays the gradual onset of oligotrophic conditions over nearly all the area usually occupied by upwelled waters. Owing to this process, mesotrophic conditions persist in the tropical Pacific during El Niño events.  相似文献   
83.
84.
The H2CO production rates measured in Comet C/1995 O1 (Hale-Bopp) from radio wavelength observations [Biver, N., and 22 colleagues, 2002a. Earth Moon Planets 90, 5-14] showed a steep increase with decreasing heliocentric distance. We studied the heliocentric evolution of the degradation of polyoxymethylene (formaldehyde polymers: (CH2O)n, also called POM) into gaseous H2CO. POM decomposition can indeed explain the H2CO density profile measured in situ by Giotto spacecraft in the coma of Comet 1P/Halley, which is not compatible with direct release from the nucleus [Cottin, H., Bénilan, Y., Gazeau, M.-C., Raulin, F., 2004. Icarus 167, 397-416]. We show that the H2CO production curve measured in Comet C/1995 O1 (Hale-Bopp) can be accurately reproduced by this mechanism with a few percents by mass of solid POM in grains. The steep heliocentric evolution is explained by the thermal degradation of POM at distances less than 3.5 AU. This study demonstrates that refractory organics present in cometary dust can significantly contribute to the composition of the gaseous coma. POM, or POM-like polymers, might be present in cometary grains. Other molecules, like CO and HNC, might also be produced by a similar process.  相似文献   
85.
This paper synthesizes the results from the model intercomparison exercise among regionalized global energy-economy models conducted in the context of the RECIPE project. The economic adjustment effects of long-term climate policy are investigated based on the cross-comparison of the intertemporal optimization models ReMIND-R and WITCH as well as the recursive dynamic computable general equilibrium model IMACLIM-R. A number of robust findings emerge. If the international community takes immediate action to mitigate climate change, the costs of stabilizing atmospheric CO2 concentrations at 450?ppm (roughly 530?C550?ppm-e) discounted at 3% are estimated to be 1.4% or lower of global consumption over the twenty-first century. Second best settings with either a delay in climate policy or restrictions to the deployment of low-carbon technologies can result in substantial increases of mitigation costs. A delay of global climate policy until 2030 would render the 450?ppm target unachievable. Renewables and CCS are found to be the most critical mitigation technologies, and all models project a rapid switch of investments away from freely emitting energy conversion technologies towards renewables, CCS and nuclear. Concerning end use sectors, the models consistently show an almost full scale decarbonization of the electricity sector by the middle of the twenty-first century, while the decarbonization of non-electric energy demand, in particular in the transport sector remains incomplete in all mitigation scenarios. The results suggest that assumptions about low-carbon alternatives for non-electric energy demand are of key importance for the costs and achievability of very low stabilization scenarios.  相似文献   
86.
The spatio-temporal variability of Northern Hemisphere Sea Level Pressure (SLP) and precipitation over the mid-to-low reaches of the Yangtze River (PMLY) is analyzed jointly using the multi-taper/singular value decomposition method (MTM-SVD). Statistically significant narrow frequency bands are obtained from the local fractional variance (LFV) spectrum. Significant interdecadal (i.e., 16-to-18-year periods) and interannual (i.e., 3-to-6-year periods) signals are identified. Moreover, a significant quasi-biennial signal is identified but only for PMLY data. The spatial joint evolution of patterns obtained for peaks in the LFV spectrum sheds light on relationships between SLP and PMLY: the Arctic Oscillation (AO) modulates the variability of the PMLY while the interannual variability of PMLY is in phase with the Northern Atlantic Oscillation (NAO) and the Northern Pacific Oscillation (NPO).  相似文献   
87.
In the category of motions preserving the angular momentum direction, Gorringe and Leach exhibited two classes of differential equations having elliptical orbits. After enlarging slightly these classes, we show that they are related by a duality correspondence of the Arnold–Vassiliev type. The specific associated conserved quantities (Laplace–Runge–Lenz vector and Fradkin–Jauch–Hill tensor) are then dual reflections of each other.  相似文献   
88.
The structure and vibrational spectrum of boehmite have been investigated at the quantum-mechanical level with the CRYSTAL code, using a Gaussian-type basis set and the B3LYP Hamiltonian. Three space groups are considered in this study: Cmcm, Cmc21, P21/c. Cmcm turns out to correspond to a transition state, whereas Cmc21 and P21/c are minimum energy structures. The difference among them is the position of H atoms only, the Al-O frame being essentially the same. Harmonic frequencies at the Γ point have been computed. The comparison between calculated and experimental frequencies shows a good agreement for the Al-O part of the spectrum (under 790 cm−1). For the Al-OH bending modes (800–1,300 cm−1) an absolute differences of 50–100 cm−1 is observed; for the OH stretching modes (3,200–3,500 cm−1) it increases to 120–200 cm−1: anharmonicity is large because OH groups are involved in strong hydrogen bonds.  相似文献   
89.
Madagascar has one of the highest poverty rates in the world and consequently the long-term monitoring of groundwater resources is not a priority for the authorities. However, groundwater is often the only sustainable resource that has a satisfactory quality to supply the population. This is especially true in the south-west of the country, which is a semi-arid region and a global change hot spot (intense land use and climate changes). In response to the lack of data, the Groundwater Resource Observatory for Southwestern Madagascar (GROSoM) was established to monitor piezometry and meteorology over the longer term as part of a humanitarian response. The first site was setup in 2014 in a catchment located over a carbonate plateau; in 2018, a second site was installed in an alluvial setting within a crystalline basement catchment and a third site will be installed in 2020 to monitor groundwater dynamics in a coastal setting. The three sites, located between Toliara and Taolagnaro cities, are complementary and representative of various hydrogeological systems in Southwestern Madagascar. Each site includes a weather station and between 3 and 6 piezometric probes. The monitoring data indicate a strong inter-annual variability in precipitation, which induces a strong variability in aquifers recharge. One of the driest years in 2016 seems to be consistent with strong El Niño – Southern Oscillation (ENSO) effects observed at the global scale, while years with higher recharge appear to be related to cyclones such as Fundi in 2015 and Eketsang in 2019. Preliminary results of cross-disciplinary studies demonstrated a link between groundwater and health issues (i.e., admissions to basic health centres). This observatory aims to produce long-term data and has two objectives: (i) strengthening the early warning system for humanitarian crises in Madagascar; (ii) contributing to a better understanding of the effects of climate change on groundwater resources in this semi-arid region.  相似文献   
90.
Sulfates have been discovered by the OMEGA spectrometer in different locations of the planet Mars. They are strongly correlated to light toned layered deposits in the equatorial regions. West Candor Chasma is the canyon with the thickest stack of layers and one with the largest area covered by sulfates. A detailed study coupling mineralogy derived from OMEGA spectral data and geology derived from HRSC imager and other datasets leads to some straightforward issues. The monohydrated sulfate kieserite is found mainly over heavily eroded scarps of light toned material. It likely corresponds to a mineral present in the initial rock formed either during formation and diagenesis of sediments, or during hydrothermal alteration at depth, because it is typically found on outcrops that are eroded and steep. Polyhydrated sulfates, that match any Ca-, Na-, Fe-, or Mg-sulfates with more than one water molecule, are preferentially present on less eroded and darker outcrops than outcrops of kieserite. These variations can be the result of a diversity in the composition and/or of the rehydration of kieserite on surfaces with longer exposure. The latter possibility of rehydration in the current, or recent, atmosphere suggests the low surface temperatures preserve sulfates from desiccation, and, also can rehydrate part of them. Strong signatures of iron oxides are present on sulfate-rich scarps and at the base of layered deposits scarps. They are correlated with TES gray hematite signature and might correspond to iron oxides present in the rock as sand-size grains, or possibly larger concretions, that are eroded and transported down by gravity at the base of the scarp. Pyroxenes are present mainly on sand dunes in the low lying terrains. Pyroxene is strongly depleted or absent in the layered deposits. When mixed with kieserite, local observations favor a spatial mixing with dunes over layered deposits. Sulfates such as those detected in the studied area require the presence of liquid water to form by precipitation, either in an intermittent lacustrine environment or by hydrothermal fluid circulation. Both possibilities require the presence of sulfur-rich groundwater to explain fluid circulation. The elevation of the uppermost sulfate signatures suggests the presence of aquifers up to 2.5 km above datum, only 1 km below the plateau surface.  相似文献   
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