Sources of dissolved silica to the fjords of northern Patagonia (44–48°S): the importance of volcanic ash soil distribution and weathering          下载免费PDF全文

Elke Vandekerkhove  Sébastien Bertrand  Brian Reid  Astrid Bartels  Bernard Charlier 《地球表面变化过程与地形》2016,41(4):499-512

Dissolved silica (DSi) plays an important biogeochemical role in the fjords of northern Chilean Patagonia (44–48°S), where it drives high biogenic productivity and promotes carbon burial. It is generally believed that the DSi riverine input to lakes and coastal environments is controlled by a combination of factors including lithology, climate, topography, vegetation, and meltwater input. In northern Chilean Patagonia several authors have proposed that the postglacial volcanic ash soils (andosols) may play a significant role in the high supply of DSi to the regional fjords. To assess the influence of andosols on DSi concentrations in north Patagonian rivers, we mapped andosol thickness and compared our results with river chemistry. The mineralogical and geochemical composition of three representative andosol profiles was also examined to evaluate the efficiency of weathering processes. The andosol thickness map clearly demonstrates that volcanic ash was predominantly deposited on the eastern side of the regional volcanoes, reflecting the influence of the prevailing westerly winds on the distribution of pyroclastic material. Mineralogical and geochemical results show that the andosol parent material has the typical andesitic basaltic signature of the regional volcanoes, i.e. high amounts of amorphous material, plagioclase, K‐feldspar, and pyroxene. Down‐profile variations in soil mineralogy and geochemistry indicate increased leaching of silica with depth, resulting from weathering of the volcanic parent material. For the five studied watersheds, a highly positive correlation (R²=0.98) was found between average andosol thickness and DSi concentrations, suggesting that andosol thickness is the main parameter affecting DSi concentrations in north Patagonian river systems. On seasonal timescales, increased precipitation (winter) and glacial meltwater (summer) input can significantly reduce DSi concentrations. We argue that the weathering of andosols constitutes the most important source of DSi to the lakes and fjords of northern Chilean Patagonia, explaining the particularly high regional rates of biogenic silica production. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

In situ Measurement of H2O and CH4 with Telecommunication Laser Diodes in the Lower Stratosphere: Dehydration and Indication of a Tropical Air Intrusion at Mid-Latitudes     

Georges Durry  Alain Hauchecorne  Joelle Ovarlez  Henri Ovarlez  Ivan Pouchet  Virginie Zeninari  Bertrand Parvitte 《Journal of Atmospheric Chemistry》2002,43(3):175-194

Telecommunication laser diodes emitting near 1.39 m and 1.65 m in combination with direct-differential absorption spectroscopy are efficient tools to monitor in situ stratospheric H₂O andCH₄ with a good precision error (a few percents), a high temporal resolution (ranging from 10 ms to 1 s), a large dynamic range in the concentration measurements (four orders of magnitude) and a high selectivity in the analyte species. To illustrate the capability of laser probing technique, we report balloonborne H₂Oand CH₄ simultaneous measurements obtained on October 2001 atmidlatitudes (43° N). The H₂O vertical profile achieved with the lasersensor in the lower stratosphere is compared with the H₂O data yielded by a balloonborne frost-point hygrometer. The total hydrogen mixing ratio in the lower stratosphere, 2[CH₄] + [H₂O], appears to beconstant at 7.5 ± 0.1 ppmv. Nevertheless, an unexpected largedehydration of 0.5 ppmv was detected by both the laser sensor and thehygrometer between 16 km and 23 km. We suspect the occurrence of a tropicalair intrusion into mid-latitudes. We support this interpretation using a high-resolution advection model for potential vorticity.  相似文献   

Modelling a geo-composite cell using discrete analysis     

David Bertrand  Franois Nicot  Philippe Gotteland  Stphane Lambert 《Computers and Geotechnics》2005,32(8):564-577

Reinforced soil structures can be used to protect infrastructures against rockfalls. An innovative way to implement such protection is to construct the front face using a cellular assembly. This paper focuses on the cellular scale, with the cell a complex heterogeneous material, composed of a wire netting box filled with rocky particles. The behaviour of a single cell was modelled using the discrete element method, thus accounting for the interaction between the rocky particles and the interaction between the box and the rocky particles. A constitutive model was developed and calibrated along confined compression loading paths, then preliminary elements of validation were obtained from the simulation of unconfined compression tests.  相似文献   

Temporal evolution of sediment supply in Lago Puyehue (Southern Chile) during the last 600 yr and its climatic significance     

Sébastien Bertrand  Xavier Boës  François Charlet  Cristian Espinoza  Bernard Charlier 《Quaternary Research》2005,64(2):163-175

Short-term climate changes in Southern Chile are investigated by a multi-proxy analysis of a 53-cm-long sedimentary sequence selected among eight short cores retrieved in Lago Puyehue (Chile, 40°S). This core contains a 600-yr-long undisturbed record of paleo-precipitation changes. Two measurement methods for sediment density, organic matter and biogenic silica contents are compared and the most appropriate techniques are selected. Together with aluminium and titanium concentrations, grain size and geochemical properties of the organic matter, these proxies are used to demonstrate paleo-precipitation changes around 40°S. Increase of terrigenous particle supply between A.D. 1490 and A.D. 1700 suggests a humid period. Contemporaneously, δ¹³C data show increasing lake productivity, in response to the high nutrient supply. The A.D. 1700-1900 interval is characterized by a decreasing terrigenous supply and increasing δ¹³C values, interpreted as a drying period. The magnetic susceptibility signal, reflecting the terrigenous/biogenic ratio, demonstrates that similar variations occur in all the undisturbed sedimentary environments of Lago Puyehue. The A.D. 1490-1700 wet period is associated with the onset of the European Little Ice Age (LIA) and interpreted as its local signature. This work supports the fact that the LIA was a global event, not only restricted to the Northern Hemisphere.  相似文献   

4D signal enhancement using singular-value decomposition: application to mapping oil–water contact movement across the Nelson Field     

Fiona J.L. Reid  Alexandre Bertrand †  Alan T. McInally ‡  Colin MacBeth 《Geophysical Prospecting》2005,53(2):253-263

A new method for time‐lapse signal separation and enhancement using singular‐value decomposition is presented. Singular‐value decomposition is used to separate a 4D signal into its constituent parts: common geology, time‐lapse response and noise. Synthetic tests which demonstrate the advantages of the singular‐value decomposition technique over traditional differencing methods are also presented. This signal separation and enhancement technique is used to map out both the original and moved oil–water contacts across the Nelson Field. The singular‐value decomposition technique allows the oil–water contact to be mapped across regions which would have been missed using traditional differencing methods. In particular, areas toward the edges of the field are highlighted by the technique. The oil–water contact is observed to move upwards across the field, with the largest movements being associated, as anticipated, with natural production. The results obtained are broadly consistent with those predicted by the reservoir simulator model. Singular‐value decomposition is demonstrated to be a useful tool for enhancing the time‐lapse signal and for gaining confidence in areas where traditional differencing fails.  相似文献   

Quaternary faulting in the Upper Rhine Graben revealed by high-resolution multi-channel reflection seismic     

Guillaume Bertrand  Philippe Elsass  Gunther Wirsing  Alex Luz 《Comptes Rendus Geoscience》2006,338(8):574-580

A high-resolution multichannel seismic reflection river profiling campaign was completed in July 2002 in the southern Upper Rhine graben (URG), along the River Rhine. Preliminary results show apparent Quaternary vertical slip rates, on intra-graben faults that are relatively slow, of the order of a few thousandths to a few hundredths of mm/yr. Moreover, kinematical data from the Ludwigshafen area show decreasing vertical slip rates since the Middle Pleistocene and/or a migration of tectonic activity. While still preliminary, these data show inhomogeneous and relatively slow tectonic activity in the URG that could probably not alone have shaped the Quaternary graben morphology. To cite this article: G. Bertrand et al., C. R. Geoscience 338 (2006).  相似文献   

A computer simulation study of natural silicate melts. Part II: High pressure properties     

Bertrand Guillot  Nicolas Sator 《Geochimica et cosmochimica acta》2007,71(18):4538-4556

The thermodynamic, structural and transport properties of natural silicate melts under pressure are investigated by molecular dynamics simulation with the help of a force field recently introduced by us [Guillot B. and Sator N. (2007) A computer simulation study of natural silicate melts. Part I: low pressure properties. Geochim. Cosmochim. Acta71, 1249-1265]. It is shown that the simulation reproduces accurately the bulk moduli of a large variety of silicate liquids as evaluated from ultrasonic studies. The equations of state (EOS) of the simulated melts are in good agreement with the density data on mid-ocean ridge basalt, komatiite, peridotite and fayalite as obtained either by sink/float experiments or by shock-wave compression. From the structural point of view it is shown that the population of ^[5]Al and ^[6]Al species increases rapidly upon initial compression (0-50 kbar) whereas for Si these highly coordinated species are found in a significant abundance (>5%) only above ∼50 kbar for ^[5]Si and ∼100-150 kbar for ^[6]Si. This increase of the coordination of network formers is not the only response of the melt structure to the densification: there is also a large redistribution of the T-O-T (T = Si, Al) bond angles with the pressure and noticeably upon initial compression in rhyolitic and basaltic liquids. Furthermore, a detailed analysis of the population of bridging oxygens (BO) and nonbridging oxygens (NBO) points out that the polymerization of the melt generally increases when the pressure increases, the magnitude of this polymerization enhancement being all the more important that the melt is depolymerized at low pressure. The role of triclusters (threefold coordinated oxygens to network former cations) is particularly emphasized in acidic and basaltic liquids. The pressure-induced redistribution of the oxygen atoms through the melt structure is also stressed. Finally, the simulation predicts a nonmonotonic behavior of the diffusivity of network former ions when the pressure increases, a feature with depends on the melt composition. This could have a counterpart in the electrical conductivity at sufficiently high temperature when the viscosity of the liquid is low.  相似文献   

Major and Trace Element and Sr, Nd, Hf, and Pb Isotope Compositions of the Karoo Large Igneous Province, Botswana-Zimbabwe: Lithosphere vs Mantle Plume Contribution   总被引：4，自引：0，他引：4  

Jourdan  F.; Bertrand  H.; Scharer  U.; Blichert-Toft  J.; Feraud  G.; Kampunzu  A. B. 《Journal of Petrology》2007,48(6):1043-1077

We report major and trace element abundances for 147 samplesand Sr, Nd, Hf, and Pb isotope compositions for a 36 samplesubset of basaltic lava flows, sills, and dykes from the Karoocontinental flood basalt (CFB) province in Botswana, Zimbabwe,and northern South Africa. Both low- and high-Ti (TiO₂ <2 wt % and > 2 wt %) rocks are included. MELTS modeling showsthat these magmas evolved at low pressure (1 kbar) through fractionalcrystallization of gabbroic assemblages. Whereas both groupsdisplay enrichment in light rare earth elements (LREE) relativeto heavy REE (HREE) and high field strength elements, and systematicnegative Nb anomalies, they differ in terms of contrasting middleREE (MREE) to HREE fractionation, which is greater for the high-Tibasalts. This reflects different depths of melting of slightlyenriched mantle sources: calculations suggest that the low-Tibasalts were generated by melting of a shallow spinel-bearing(2 % spinel) lherzolite, whereas the high-Ti magmas originatedfrom a deeper-seated garnet-bearing (2–7% garnet) lherzolite.In most isotope plots, the high-Ti lavas together with the picritesdefine a common trend from Bulk Silicate Earth (BSE) to compositionswith strongly negative Nd_i and Hf_i akin to those of some nephelinitesand lamproites. The low-Ti rocks are shifted from BSE-like tomore radiogenic Sr isotope ratios, indicative of upper crustalcontamination. Trace element and isotope characteristics ofthe Karoo magmas require a combination of enrichment processes(subduction induced?) and long-term isolation of the mantlesources. We propose two distinct scenarios to explain the originof the Karoo province. The first calls for polybaric meltingof spatially heterogeneous, partially veined, sub-continentallithospheric mantle (SCLM). Calculations show that mixing betweenSCLM (BSE) and a strongly Nd–Hf unradiogenic nephelinite-likecomponent (sediment input?) could account for the compositionalvariations of most of the high-Ti group lavas, whereas the mantlecomposition responsible for the low-Ti magmas is more likelyto be similar to a vein-free, metasomatically enriched SCLMcomponent. The second scenario involves mixing between two end-membersrepresented by the SCLM and its deep-seated alkalic veins anda sub-lithospheric (asthenospheric- or ocean island basalt-like?)mantle plume. In this case, the data are compatible with anincreasing mantle plume contribution as the plume rises andexpands through the lithosphere. Regardless of which of thetwo scenarios is invoked, the spatial distribution of the low-and high-Ti magmas matches the relative positioning of the cratonsand the Limpopo belt in such a way that strong control of thelithosphere on magma composition and distribution is a mandatoryrequirement of any petrogenetic model applied to the Karoo CFB. KEY WORDS: Karoo; large igneous province; flood basalts; dyke swarms; major and trace elements; Sr; Nd; Hf; and Pb isotopes  相似文献   

Oceanic influence on the sub-seasonal to interannual timing and frequency of extreme dry spells over the West African Sahel     

Seyni Salack  Alessandra Giannini  Moussa Diakhaté  Amadou. T. Gaye  Bertrand Muller 《Climate Dynamics》2014,42(1-2):189-201

Intra-seasonal drought episodes (extreme dry spells) are strongly linked to crop yield loss in the West African Sahel, especially when they occur at crop critical stages such as juvenile or flowering stage. This paper seeks to expose potentially predictable features in the sub-seasonal to inter-annual occurrence of “extreme dry spells” (extDS) through their links to sea surface temperature anomalies (SSTAs). We consider two kinds of extreme dry spells: more than 2 weeks of consecutive dry days following a rain event (often found at the beginning of the rainy season, after the first rain events) and more than a week (observed towards the end of the rainy season, before the last rain events). We extract dry spells from daily rainfall data at 43 stations (31 stations in Senegal over 1950–2010 and 12 stations in Niger over 1960–2000) to identify the intra-seasonal distribution of extDS and their significant correlation with local rainfall deficits. Seasonality of distribution and high spatial coherence are found in the timing and the frequency of occurrence of extDS in different rainfall regions over Niger and Senegal. The correlation between the regional occurrence index (ROI), necessary to capture the spatial extent of extDS, and observed global sea surface temperature anomalies (SSTAs) sheds light on the influence of the external factors on the decadal, interannual and sub-seasonal variability of extDS over the West African Sahel. When the global tropics and the Atlantic are warmer than normal, more coherent and delayed June–July extDS are observed after onset of rainy season, as well as early cessation type in August–September. When the Indo-Pacific is cooler and the equatorial south Atlantic is warmer than normal little to no extDS are found in the onset sub-period of the monsoon season. Mostly late types of extDS occur in October as a result of late cessation. These results show potential predictability of extreme dry spells after onset and before cessation of monsoonal rain based on global patterns of sea surface temperature anomalies.  相似文献