Éditeur(s) : HAL CCSD Springer-Verlag
Résumé : International audience
High-resolution borehole strainmeters are usually installed in tectonically active regions in order to detect slow-slip events, and to estimate slow transients related to earthquake swarms. However, they are also sensitive to other numerous influences, internal or external. Furthermore, the quality of their coupling to the rock through cementation, and the mechanical properties of the rock mass around them, have a critical influence on their records. Many of the existing strainmeters present such problems, and the correction for these effects often remains a challenge. In this paper, we present the analysis of the records of a high-resolution borehole dilatometer (Sacks–Evertson), located in the seismically active rift of Corinth (Greece) (station TRZ in the Trizonia island). We show that the instrument suffers from an only partial-solid coupling, and that the nearby sea tides have a direct (through elastic response) and indirect (through pore-pressure diffusion) effect on the dilatation signal, which adds up to the solid tidal strain source. We propose a methodology that allows, in a first step, to better separate the internal (solid tide) from the external (air pressure, sea level) influences, by calculating a frequency-dependent transfer function outside the range of the tidal periods. We then extrapolate this function, in particular at the tidal periods. In a second step, the resulting variation with frequency of the coupling coefficients with sea level led us to estimate the proportion of instrument not solidly cemented to rock (thus in contact with water pore pressure), which is about 90 % of the total height. Despite the small proportion of solid coupling, the sensor resolution remains very good up to a few tens of hours of a time period, thanks to the confining effects of the rocks on the local pore pressure. These results allow us to correct for the external effects, and reduce the associated variance by 80–90 % (in the period range of minutes to days). The empirical correction of the sea level effect could be explained using a simple Boussinesq’s approximation and 1D pore-pressure diffusion model, which contributed to better constraint of some of the poro-elastic parameters in the vicinity of the instrument. After correction, the solid tidal signal at the 24-h period is almost anti-correlated with those of the theoretical solid tide. This surprising result is consistent with a similar anti-correlation observed for the longest period surface waves (200 s) comparing the TRZ dilatometer signals to the strain measured by a nearby borehole strainmeter (MOK, 15 km). This could be related to the presence of a shallow fault close to the instrument, which would creep in response to seismic wave-related stress.
ISSN: 0033-4533

hal-01122853
https://hal.archives-ouvertes.fr/hal-01122853
DOI : 10.1007/s00024-013-0742-2

Éditeur(s) : Ieee-inst Electrical Electronics Engineers Inc
Résumé : We validate Soil Moisture and Ocean Salinity (SMOS) sea surface salinity (SSS) retrieved during August 2010 from the European Space Agency SMOS processing. Biases appear close to land and ice and between ascending and descending orbits; they are linked to image reconstruction issues and instrument calibration and remain under study. We validate the SMOS SSS in conditions where these biases appear to be small. We compare SMOS and ARGO SSS over four regions far from land and ice using only ascending orbits. Four modelings of the impact of the wind on the sea surface emissivity have been tested. Results suggest that the L-band brightness temperature is not linearly related to the wind speed at high winds as expected in the presence of emissive foam, but that the foam effect is less than previously modeled. Given the large noise on individual SMOS measurements, a precision suitable for oceanographic studies can only be achieved after averaging SMOS SSS. Over selected regions and after mean bias removal, the precision on SSS retrieved from ascending orbits and averaged over 100 km $times$ 100 km and 10 days is between 0.3 and 0.5 pss far from land and sea ice borders. These results have been obtained with forward models not fitted to satellite L-band measurements, and image reconstruction and instrument calibration are expected to improve. Hence, we anticipate that deducing, from SMOS measurements, SSS maps at 200 km $times$ 200 km, 10 days resolution with an accuracy of 0.2 pss at a global scale is not out of reach.
Ieee Transactions On Geoscience And Remote Sensing (0196-2892) (Ieee-inst Electrical Electronics Engineers Inc), 2012-05 , Vol. 50 , N. 5 , P. 1662-1675

Droits : 2012 IEEE
http://archimer.ifremer.fr/doc/00074/18557/16108.pdf
DOI:10.1109/TGRS.2012.2184546
http://archimer.ifremer.fr/doc/00074/18557/

Éditeur(s) : HAL CCSD
Résumé : International audience
The challenge is to implement research that can estimate the consequences ofclimate changes in terms of impact on terrestrial environments and resources.Emphasis should be placed on regions dependent on natural resources and forwhich demographic pressure is strong. Simulations obtained from climate modelprojections (using different Representative Concentration Pathways (RCPs))predict that the Mediterranean basin and its southern periphery are particularlyvulnerable to water resources and environmental impact (IPCC, AR5, 2013).An annual rainfall decrease by 30% is found for the projection period 2070-2099(IPCC, AR5, 2013) associated with a decrease in water resources by 30 to 50%(Milano, 2012). In addition, several studies using regional atmospheric modelsindicate an increase in the precipitation inter-annual variability with extremeevents and a spatial heterogeneous signature, superimposed on a decrease in thetotal precipitation amount (Giorgi and Lionello, 2008; Raible et al. 2010).Currently, regional climate projections are highly sensitive to the climate modelused. In particular, spatial resolution as well as local climate conditions seemto impact significantly on the simulations (Jacob et al. 2014).The Mediterranean region, at the interface between arid and temperate climateswith several mountainous areas, is a complex climate system affected by theinteractions between mid-latitude and sub-tropical processes. In this context,Morocco, located at the transition between a temperate climate to the North anda tropical climate to the south constitutes a key area for an impact and sensitivitystudy to global climate changes. The climate is influenced by the Atlantic Ocean,the Mediterranean Sea and the Sahara, together with a very steep orography inthe Atlas region. The precipitation distribution is therefore characterised by greatspatial variability, and exhibits a marked seasonality, a strong inter-annualvariability (Ouda et al. 2005) and in general a pronounced gradient from northto south and west to east. At a broader scale, Morocco is located on the subtropicalsubsidence path and between the Acores High and the Saharan Low (Agoussine,2003). Several studies have also identified strong links with inter-annualprecipitation variability and NAO index (Knippertz, 2003) as well as remoteclimate modes (Esper et al. 2007).Continental climate variability at a local/regional scale, if it is to be integratedin climate predictions, needs to be supported by long-term observation.Meteorological stations in Morocco provide climatic data mainly for the last40 years with only a few stations located in the mountainous region (Tramblayet al. 2012; 2013; Driouech et al. 2010). This climate database is also supportedby the IAEA network providing stations for which isotope tracers have beenapplied to daily/monthly rain and water vapour samples over 2 to 3 years between2000 and 2004. Besides the poor coverage of instrumented areas, lacustrinesystems can provide a climatic data set that offers access to short and long-termtime series of climate parameters when knowledge of modern lake water balanceis combined with lacustrine sedimentary-climate records. Lake sediment recordsideally provide high resolution climate/environmental information of the last10,000 years (Magny et al. 2013). This time interval (corresponding to theHolocene) is a key period to investigate short and long-term climate variabilityand to improve prediction in a warming climate.In this study we present an integrated approach focusing on a mountainous lake(Aguelmam Azigza). The modern lake system study is based on site monitoring(2012-2016) and available regional hydro-climatic data. These data show thatlake level changes during the instrumented period were mainly driven byprecipitation following the high inter-annual variability. These data are thencompared with accurately dated short sediment cores retrieved in the same lake.Micro-scale geochemical and sedimentological analyses of these sequencesenable us to identify various sedimentary facies that can be linked with periodsof high (low) lake levels over the past decades.
The Mediterranean Region under Climate ChangeLa méditerranée face au changement climatique

insu-01394690
https://hal-insu.archives-ouvertes.fr/insu-01394690
https://hal-insu.archives-ouvertes.fr/insu-01394690/document
https://hal-insu.archives-ouvertes.fr/insu-01394690/file/Vidaletal.2016-COP22-Chap1.pdf

Éditeur(s) : HAL CCSD Elsevier
Résumé : Handheld energy dispersive portable X-ray spectrometers (pXRF) are generally designed and used for qualitative survey applications. We developed shipboard quantitative analysis protocols for pXRF and employed the instrument to make over 2000 individual abundance measurements for a selection of major and trace elements on over 1200 m of recovered core during the eight weeks of the International Ocean Discovery Program (IODP) Expedition 352 to the Izu-Bonin forearc. pXRF analytical performance, accuracy and precision were found to be the same on powdered rock samples and on freshly cut rock surfaces, and sample results were similar within error to measurements made via shipboard ICP-OES analysis save at low abundance levels for a few elements. Instrument performance was optimal for elements between Z = 19 and Z = 40, and the system yielded reproducible data for K, Ca, Ti, V, Cr, Mn, Fe, Cu, Zn, Rb, Sr, and Zr on both powdered samples and rock surfaces. Working curves developed via pXRF measurement of a suite of geologic standard reference materials and well-characterized lavas permitted accurate quantitative measurements for many of the examined elements on both sample powders and rock surfaces. Although pXRF has been sporadically employed on previous cruises, Expedition 352 is the first time a detailed, high-density chemostratigraphy of recovered core samples was collected using pXRF measurements of rock core surfaces. These high-resolution data allowed the recognition of chemically distinct eruptive units in near real-time. The rapid identification of geochemical trends vastly improved our selection of samples for shipboard and shore-based analysis, permitted a more comprehensive interpretation of our Expedition results, and provided key decision-making information for drilling operations.
ISSN: 0009-2541

hal-01553188
https://hal.archives-ouvertes.fr/hal-01553188
DOI : 10.1016/j.chemgeo.2017.01.007

Éditeur(s) : HAL CCSD
Résumé : The effects of climate change on extreme events are difficult to assess because extremes present large variability and consequently, it is difficult to identify significant trends in relation to the lack of instrumental long time series. Here we present a record of these extreme storm events in the French Mediterrannean coast over the past 7000 years based on a long sediment core from lagoonal environment in Gulf of Lions. Using a high resolution multiproxies approach on core associating grain size, faunal to reconstruct Mid to Late Holocene history of backbarrier deposits in relation to landfalling activity. Even if change in lagoon geomorphological setting over the Holocene does not allow to compare storm events in terms of intensity trough time, we have recorded six periods of increase in storm activity at 6200, 5500, 4400, 3200, 1500 yr cal B.P. and over the LIA (450 and 100 yr cal B.P.). These evidences of changes in coastal hydrodynamic, inversely correlated to periods of aridification in Western Mediterranean region, are in phase with those observed over the North Atlantic and correspond to Holocene cooling events. This increase in storm activity during Holocene cold events over Mediterranean region was probably due to thermal gradient increase leading to enhanced lower tropospheric baroclinicity over a large Central Atlantic-European domain. This study demonstrates that temperatures in North Atlantic region influence the severe storm activity and therefore the Mediterranean climate at Holocene timescale.
Les effets du changement climatique sur les événements extrêmes sont difficiles à estimer car ces phénomènes présentent une large variabilité, et par conséquent il est délicat d'identifier des tendances significatives compte tenu de l'absence de longues séries de données instrumentales. Dans cette étude, nous présentons un enregistrement de ces tempêtes extrêmes sur la côte Méditerranéenne française au cours des 7000 dernières années à partir de carottes sédimentaires prélevées dans un système lagunaire du Golfe du Lion. L'utilisation d'une approche multi-traceur à haute résolution avec des analyses granulométriques, faunistiques, géochimiques (XRF), en minéraux argileux associées à des données chronologiques de radiocarbone et de 210Pb, 137Cs ; nous a permis de reconstruire l'histoire, au cours de l'Holocène, des dépôts lagunaires en relation avec l'activité des tempêtes. Bien que les paramètres morphologiques de cet environnement lagunaire aient changé durant l'Holocène, ne permettant pas de comparer ces événements au cours du temps en terme d'intensité, nous enregistrons six périodes d'augmentation de l'activité des tempêtes datées à 6200, 5500, 4400, 3200, 1500 an cal B.P. et durant le Petit Age Glaciaire (de 450 à 100 an cal B.P.). Ces changements de l'activité hydrodynamique côtière, en antiphase avec les périodes d'aridifications en Méditerranée de l'Ouest, sont corrélées avec celles observées dans le bassin Nord Atlantique et correspondent aux événements de refroidissement Holocène. L'augmentation de l'activité des tempêtes dans la cette région de la Méditerranée durant les périodes froides, associée à une diminution des températures au pôle Nord, est probablement liée à l'augmentation du gradient thermique qui, par interaction barocline, favorise les cycles d'évolution des tempêtes dans toute l'Europe et la Méditerranée de l'Ouest. Cette étude démontre ainsi que la température dans la région Nord Atlantique influence l'activité des tempêtes extrêmes et donc du climat Méditerranéen à l'échelle de l'Holocène.
https://tel.archives-ouvertes.fr/tel-00437757

tel-00437757
https://tel.archives-ouvertes.fr/tel-00437757
https://tel.archives-ouvertes.fr/tel-00437757/document
https://tel.archives-ouvertes.fr/tel-00437757/file/Sabatier_these.pdf

Éditeur(s) : HAL CCSD Wiley-VCH Verlag
Résumé : International audience
Deciphering the assembly history of the Milky Way is a formidable task, which becomes possible only if one can produce high-resolution chrono-chemo-kinematical maps of the Galaxy. Data from large-scale astrometric and spectroscopic surveys will soon provide us with a well-defined view of the current chemo-kinematical structure of the Milky Way, but will only enable a blurred view on the temporal sequence that led to the present-day Galaxy. As demonstrated by the (ongoing) exploitation of data from the pioneering photometric missions CoRoT, Kepler, and K2, asteroseismology provides the way forward: solar-like oscillating giants are excellent evolutionary clocks thanks to the availability of seismic constraints on their mass and to the tight age-initial-mass relation they adhere to. In this paper we identify five key outstanding questions relating to the formation and evolution of the Milky Way that will need precise and accurate ages for large samples of stars to be addressed, and we identify the requirements in terms of number of targets and the precision on the stellar properties that are needed to tackle such questions. By quantifying the asteroseismic yields expected from PLATO for red-giant stars, we demonstrate that these requirements are within the capabilities of the current instrument design, provided that observations are sufficiently long to identify the evolutionary state and allow robust and precise determination of acoustic-mode frequencies. This will allow us to harvest data of sufficient quality to reach a 10% precision in age. This is a fundamental pre-requisite to then reach the more ambitious goal of a similar level of accuracy, which will only be possible if we have to hand a careful appraisal of systematic uncertainties on age deriving from our limited understanding of stellar physics, a goal which conveniently falls within the main aims of PLATO's core science.
ISSN: 0004-6337

hal-01541393
https://hal.archives-ouvertes.fr/hal-01541393
ARXIV : 1706.3778
BIBCODE : 2017AN....338..644M
DOI : 10.1002/asna.201713385