Isotopic constraints on the Si-biogeochemical cycle of the Antarctic Zone in the Kerguelen area (KEOPS)     

François Fripiat  Anne-Julie Cavagna  Nicolas Savoye  Frank Dehairs  Luc André  Damien Cardinal 《Marine Chemistry》2011,123(1-4):11-22

Estimation of the silicon (Si) mass balance in the ocean from direct measurements (Si uptake-dissolution rates …) is plagued by the strong temporal and spatial variability of the surface ocean as well as methodological artifacts. Tracers with different sensitivities toward physical and biological processes would be of great complementary use. Silicon isotopic composition is a promising proxy to improve constraints on the Si-biogeochemical cycle, since it integrates over longer timescales in comparison with direct measurements and since the isotopic balance allows to resolve the processes involved, i.e. uptake, dissolution, mixing. Si-isotopic signatures of seawater Si(OH)₄ and biogenic silica (bSiO₂) were investigated in late summer 2005 during the KEOPS experiment, focusing on two contrasting biogeochemical areas in the Antarctic Zone: a natural iron-fertilized area above the Kerguelen Plateau (< 500 m water depth) and the High Nutrient Low Chlorophyll area (HNLC) east of the plateau (> 1000 m water depth). For the HNLC area the Si-isotopic constraint identified Upper Circumpolar Deep Water as being the ultimate Si-source. The latter supplies summer mixed layer with 4.0 ± 0.7 mol Si m^? 2 yr^? 1. This supply must be equivalent to the net annual bSiO₂ production and exceeds the seasonal depletion as estimated from a simple mixed layer mass balance (2.5 ± 0.2 mol Si m^? 2 yr^? 1). This discrepancy reveals that some 1.5 ± 0.7 mol Si m^? 2 yr^? 1 must be supplied to the mixed layer during the stratification period. For the fertilized plateau bloom area, a low apparent mixed layer isotopic fractionation value (?³⁰Si) probably reflects (1) a significant impact of bSiO₂ dissolution, enriching the bSiO₂ pool in heavy isotope; and/or (2) a high Si uptake over supply ratio in mixed layer at the beginning of the bloom, following an initial closed system operating mode, which, however, becomes supplied toward the end of the bloom (low Si uptake over supply ratio) with isotopically light Si(OH)₄ from below when the surface Si(OH)₄ pool is significantly depleted. We estimated a net integrated bSiO₂ production of 10.5 ± 1.4 mol Si m^? 2 yr^? 1 in the AASW above the plateau, which includes a significant contribution of bSiO₂ production below the euphotic layer. However, advection which could be significant for this area has not been taken into account in the latter estimation based on a 1D approach of the plateau system. Finally, combining the KEOPS Si-isotopic data with those from previous studies, we refined the average Si-isotopic fractionation factor to ? 1.2 ± 0.2‰ for the Antarctic Circumpolar Current.  相似文献   

The Temperature and Ionization of T-Tauri Micro-Jets     

Darren M. O'Brien  Paulo J.V. Garcia  Jonathan Ferreira  Sylvie Cabrit  Luc Binette 《Astrophysics and Space Science》2004,292(1-4):609-617

The effects of phenomenological heating functions on the flow thermodynamics of cold T-Tauri disk winds are examined. Turbulent dissipation (mechanical) heating and a warm disk corona are invoked to heat the wind. The temperature and ionization evolution are solved for along the flow. The results allow the construction of synthetic observations; emission maps, forbidden line ratios, line fluxes and line profiles; and successfully reproduce a number of observed trends. Mechanical heating produces line ratios and fluxes that fit very well with observations. Invoking a warm disk corona successfully reproduces forbidden line profile low velocity components.  相似文献   

Hydrological flowpaths and nitrate removal rates within a riparian floodplain along a fourth‐order stream in Brittany (France)     

Jean‐Christophe Clment  Luc Aquilina  Olivier Bour  Kristelle Plaine  Tim P. Burt  Gilles Pinay 《水文研究》2003,17(6):1177-1195

Three main reservoirs were identified that contribute to the shallow subsurface flow regime of a valley drained by a fourth‐order stream in Brittany (western France). (i) An upland flow that supplied a wetland area, mainly during the high‐water period. It has high N‐NO₃^? and average Cl^? concentrations. (ii) A deep confined aquifer characterized by low nitrate and low chloride concentrations that supplied the floodplain via flow upwelling. (iii) An unconfined aquifer under the riparian zone with high Cl^? and low N‐NO₃^? concentrations where biological processes removed groundwater nitrate. This aquifer collected the upland flow and supplied a relict channel that controlled drainage from the whole riparian zone. Patterns of N‐NO₃^? and Cl^? concentrations along riparian transects, together with calculated high nitrate removal, indicate that removal occurred mainly at the hillslope–riparian zone interface (i.e. first few metres of wetland), whereas dilution occurred in lower parts of the transects, especially during low‐water periods and at the beginning of recharge periods. Stream flow was modelled as a mixture of water from the three reservoirs. An estimation of these contributions revealed that the deep aquifer contribution to stream flow averaged 37% throughout the study period, while the contribution of the unconfined reservoir below the riparian zone and hillslope flow was more variable (from ca 6 to 85%) relative to rainfall events and the level of the riparian water table. At the entire riparian zone scale, NO₃^? removal (probably from denitrification) appeared most effective in winter, despite higher estimated upland NO₃^? fluxes entering the riparian zone during this period. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

The build-up of the colour–magnitude relation in galaxy clusters since z∼ 0.8     

Gabriella De Lucia  Bianca M. Poggianti  Alfonso Aragón-Salamanca  Simon D. M. White  Dennis Zaritsky  Douglas Clowe  Claire Halliday  Pascale Jablonka  Anja von der Linden  Bo Milvang-Jensen  Roser Pelló  Gregory Rudnick  Roberto P. Saglia  Luc Simard 《Monthly notices of the Royal Astronomical Society》2007,374(3):809-822

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Pluri-decadal evolution of rock glaciers surface velocity and its impact on sediment export rates towards high alpine torrents     

Mario Kummert  Xavier Bodin  Luc Braillard  Reynald Delaloye 《地球表面变化过程与地形》2021,46(15):3213-3227

The acceleration of surface velocities observed over the last two decades on monitored rock glaciers worldwide is a widespread signal of the probable control of warming air temperatures on long-term permafrost creep. Yet, the actual consequences of this acceleration on sediment availability in high mountain catchments have never been properly estimated at the pluri-decadal scale. The present study evaluates the sediment transfer activity between five rock glaciers located in the western European Alps and the headwaters of the torrential channels they are respectively connected to. It reposes on the orthorectification of aerial images available generally from the 1960s to the mid-2010, to reconstruct time-series of (i) horizontal surface velocities and (ii) frontal erosion rates. Values of horizontal velocity are retrieved by tracking the displacement of boulders on the surface of rock glaciers between consecutive images while erosion rates affecting the fronts are calculated by combining these values of displacement with the geometry of the front (mean width and rock glacier thickness) derived from recent high-resolution digital elevation models. Results confirm the general acceleration of rock glaciers surface velocities since the 1970s and indicate that this accelerating trend is causing an increase in the erosion rates calculated at the front of most studied rock glaciers. In some cases and over specific periods however, the acceleration resulted in the advance of the whole landforms over their own sediments, leading to a comparatively low sediment export towards the torrents.  相似文献   

A Downscaling Approach Toward High-Resolution Surface Mass Balance Over Antarctica     

Hubert Gall��e  C��cile Agosta  Luc Gential  Vincent Favier  Gerhard Krinner 《Surveys in Geophysics》2011,32(4-5):507-518

The Antarctic ice sheet surface mass balance shows high spatial variability over the coastal area. As state-of-the-art climate models usually require coarse resolutions to keep computational costs to a moderate level, they miss some local features that can be captured by field measurements. The downscaling approach adopted here consists of using a cascade of atmospheric models from large scale to meso-?? scale. A regional climate model (Modèle Atmosphérique Régional) forced by meteorological reanalyses provides a diagnostic physically-based rain- and snowfall downscaling model with meteorological fields at the regional scale. Although the parameterizations invoked by the downscaling model are fairly simple, the knowledge of small-scale topography significantly improves the representation of spatial variability of precipitation and therefore that of the surface mass balance. Model evaluation is carried out with the help of shallow firn cores and snow height measurements provided by automatic weather stations. Although downscaling of blowing snow still needs to be implemented in the model, the net accumulation gradient across Law Dome summit is shown to be induced mostly by orographic effects on precipitation.  相似文献   

Relationship of present saline fluid with paleomigration of basinal brines at the basement/sediment interface (Southeast basin – France)     

Luc Aquilina  Jean-Raynald De Dreuzy 《Applied Geochemistry》2011,26(12):1933-1945

This paper investigates the isotopic composition (O, D, Sr, O_SO4, S_SO4, Cl, He) of a present saline fluid sample collected at the sediment/basement interface in the Permian continental formation at 634 m depth in the SE margin of the Massif Central shield (Ardèche margin of the Southeast basin of France). The fluid sample shows clear water–rock interaction processes, such as feldspar dissolution and kaolinite precipitation, which have led to high Na concentrations and water stable isotopes above the local meteoric water line. The geological formations of the SE margin of the Massif Central shield show that intensive fluid circulation phases occurred across the margin from the late Triassic to the middle Jurassic. The fluids most probably originated from fluid expulsion during burial of the thick Permo-Carboniferous sedimentary succession. These circulation phases were responsible for cementation of the margin and for the solutes in the matrix microporosity which were extracted by leaching core samples.The chemical and isotopic composition of the saline fluid sample at 634 m in the Permian rock is very similar to that of fluids in the microporosity of the rock matrix. Their S_SO4, O_SO4 and Sr isotopic compositions are close to those of cements investigated in fracture fillings in the same geological formations. Simple diffusion computations and comparison of the chemical composition of the present free fluid sample with matrix porosity fluids indicate that the solutes in the present free fluid sample are related to solutes originating from fluid circulation events which occurred 160–200 Ma ago through their diffusion from the matrix microporosity.A two-stage fluid flow regime is proposed to interpret the chemistry of present and paleo-fluids. (1) During the extensional context (Permian to Cretaceous), basinal brines migrated along the basement/sediment interface after expulsion from the subsiding basin. This fluid migration would be responsible for the solutes in the rock matrix microporosity and the solutes in the present free fluid sample. (2) Following the Alpine and Pyrenean compressive phases, gravity-driven meteoritic fluids slowly migrated from the surface down to the basement along major faults. This fluid regime would be responsible for the meteoric water collected in the present free fluid sample. Several investigations in Europe have shown that the existence of other saline fluids sampled elsewhere could be explained by these phases of fluid circulation related to specific geodynamic events.  相似文献   

Non-stationary frequency analysis of heavy rainfall events in southern France     

Yves Tramblay  Luc Neppel  Julie Carreau  Kenza Najib 《水文科学杂志》2013,58(2):280-294

Abstract

Heavy rainfall events often occur in southern French Mediterranean regions during the autumn, leading to catastrophic flood events. A non-stationary peaks-over-threshold (POT) model with climatic covariates for these heavy rainfall events is developed herein. A regional sample of events exceeding the threshold of 100 mm/d is built using daily precipitation data recorded at 44 stations over the period 1958–2008. The POT model combines a Poisson distribution for the occurrence and a generalized Pareto distribution for the magnitude of the heavy rainfall events. The selected covariates are the seasonal occurrence of southern circulation patterns for the Poisson distribution parameter, and monthly air temperature for the generalized Pareto distribution scale parameter. According to the deviance test, the non-stationary model provides a better fit to the data than a classical stationary model. Such a model incorporating climatic covariates instead of time allows one to re-evaluate the risk of extreme precipitation on a monthly and seasonal basis, and can also be used with climate model outputs to produce future scenarios. Existing scenarios of the future changes projected for the covariates included in the model are tested to evaluate the possible future changes on extreme precipitation quantiles in the study area.

Editor Z.W. Kundzewicz; Associate editor K. Hamed

Citation Tramblay, Y., Neppel, L., Carreau, J., and Najib, K., 2013. Non-stationary frequency analysis of heavy rainfall events in southern France. Hydrological Sciences Journal, 58 (2), 280–294.  相似文献   

GRASP-UTS: an algorithm for unsupervised trajectory segmentation     

Amílcar Soares Júnior  Bruno Neiva Moreno  Valéria Cesário Times  Stan Matwin  Lucídio dos Anjos Formiga Cabral 《International journal of geographical information science》2013,27(1):46-68

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Two‐stage Variscan metamorphism in the Canigou massif: Evidence for crustal thickening in the Pyrenees     

Luc de Hoÿm de Marien  Benjamin Le Bayon  Pavel Pitra  Jean Van Den Driessche  Marc Poujol  Florence Cagnard 《Journal of Metamorphic Geology》2019,37(6):863-888

The Variscan metamorphism in the Pyrenees is dominantly of the low‐pressure–high‐temperature (LP‐HT) type. The relics of an earlier, Barrovian‐type metamorphism that could be related to orogenic crustal thickening are unclear and insufficiently constrained. A microstructural and petrological study of micaschists underlying an Ordovician augen orthogneiss in the core of the Canigou massif (Eastern Pyrenees, France) reveals the presence of two syntectonic metamorphic stages characterized by the crystallization of staurolite (M1) and andalusite (M2), respectively. Garnet is stable during the two metamorphic stages with a period of resorption between M1 and M2. The metamorphic assemblages M1 and M2 record similar peak temperatures of 580°C at different pressure conditions of 5.5 and 3 kbar, respectively. Using chemical zoning of garnet and calculated P–T pseudosections, a prograde P–T path is constrained with a peak pressure at ~6.5 kbar and 550°C. This P–T path, syntectonic with respect to the first foliation S1, corresponds to a cold gradient (of ~9°C/km), suggestive of crustal thickening. Resorption of garnet between M1 and M2 can be interpreted either in terms of a simple clockwise P–T path or a polymetamorphic two‐stage evolution. We argue in favour of the latter, where the medium‐pressure (Barrovian) metamorphism is followed by a period of significant erosion and crustal thinning leading to decompression and cooling. Subsequent advection of heat, probably from the mantle, leads to a new increase in temperature, coeval with the development of the main regional fabric S2. LA‐ICP‐MS U–Th–Pb dating of monazite yields a well‐defined date at c. 300 Ma. Petrological evidence indicates that monazite crystallization took place close to the M1 peak pressure conditions. However, the similarity between this age and that of the extensive magmatic event well documented in the eastern Pyrenees suggests that it probably corresponds to the age of monazite recrystallization during the M2 LP‐HT event.  相似文献