The potential of ground gravity measurements to validate GRACE data

New satellite missions are returning high precision, time-varying, satellite measurements of the Earth’s gravity field. The GRACE mission is now in its calibration/- validation phase and first results of the gravity field solutions are imminent. We consider here the possibility of external validation using data from the superconducting gravimeters in the European sub-array of the Global Geodynamics Project (GGP) as ‘ground truth’ for comparison with GRACE. This is a pilot study in which we use 14 months of 1-hour data from the beginning of GGP (1 July 1997) to 30 August 1998, when the Potsdam instrument was relocated to South Africa. There are 7 stations clustered in west central Europe, and one station, Metsahovi in Finland. We remove local tides, polar motion, local and global air pressure, and instrument drift and then decimate to 6-hour samples. We see large variations in the time series of 5–10<i>µ</i>gal between even some neighboring stations, but there are also common features that correlate well over the 427-day period. The 8 stations are used to interpolate a minimum curvature (gridded) surface that extends over the geographical region. This surface shows time and spatial coherency at the level of 2– 4<i>µ</i>gal over the first half of the data and 1–2<i>µ</i>gal over the latter half. The mean value of the surface clearly shows a rise in European gravity of about 3µgal over the first 150 days and a fairly constant value for the rest of the data. The accuracy of this mean is estimated at 1<i>µ</i>gal, which compares favorably with GRACE predictions for wavelengths of 500 km or less. Preliminary studies of hydrology loading over Western Europe shows the difficulty of correlating the local hydrology, which can be highly variable, with large-scale gravity variations.<br><br><b>Key words. </b>GRACE, satellite gravity, superconducting gravimeter, GGP, ground trut

Le gisement de méga-fossiles ligneux de Bourret (moyenne vallée de la Garonne) : processus de formation et signification climatique

International audienceLe gisement de mégafossiles ligneux de Bourret est le plus important identifié à ce jour en vallée de Garonne. La datation14C de 8 des 17 troncs subfossiles de Quercus extraits s’échelonne entre 8 300 et 4 200 ans cal. BP. Elles attestent du caractèrepartiellement remanié du gisement. Certains de ces bois ont subi une ou des phases de stockage temporaire d’une durée compriseentre 4 100 et 1 000 ans avant d’être incorporés définitivement dans la basse plaine. Les bois les plus anciens ont donc connu unehistoire complexe comportant une phase d’injection dans le chenal, de stockage dans des unités intermédiaires, puis une érosion etune nouvelle phase de dépôt avec la charge de fond. Les bois les plus jeunes résultent quant à eux d’une injection directe depuis laripisylve dans le système fluvial. L’agrégation des dates obtenues sur ce site et sur les sites découverts antérieurement s’organisent en3 agrégats (env. 8 200, 5 200 et 4 200 ans cal. BP). Ceci suggère que l’injection des bois dans le système fluvial est liée à des phasesde forte activité hydrosédimentaire. Le synchronisme avec les Rapid Climate Changes (RCC) suggère un contrôle climatique sur ceprocessus, ce qui nous amène à proposer un schéma conceptuel de formation des gisements de bois fossiles

L'ARCHITECTURE MORPHO-SÉDIMENTAIRE DE LA BASSE PLAINE DE LA GARONNE MOYENNE EN AVAL DE TOULOUSE : PREMIERS RÉSULTATS

International audienceLa moyenne Garonne, entre Toulouse et la confluence du Tarn, correspond à la zone d'élargissement de la basse plaine alluviale inondable, jusqu'à plus de 4 km. Peu de travaux ont été menés sur l'architecture sédimentaire et la chronologie de la mise en place de cette unité géomorphologique. Dans ce travail, nous montrons que l'édification de la plaine en aval de Toulouse s'est effectuée en deux temps. Tout d'abord, une phase d'apports sédimentaires réduits, correspondant aux " limons jaunes ", dont la mise en place s'effectue durant la phase alluviale pléniglaciaire entre 16 000 ans cal. BP et au plus tard 8 900 ans cal. BP. Cet épisode est encore très mal documenté. Une phase d'incision majeure remanie et érode les limons jaunes entre 8 900 et 5 000-4 500 ans cal. BP. Cet épisode pourrait être contrôlé par le climat et la biostabilisation de la plaine en réponse à la reconquête par la chênaie mixte. Les accumulations de charbons alluviaux vers 4 400 ans cal. BP pourraient signer les premiers impacts anthropiques majeurs sur la plaine, en relation avec les sites de Fontréal et Cassagna. Ces impacts anthropiques pourraient avoir favorisé la reprise de l'alluvionnement notamment sur l'effluent Hers-Girou à la confluence avec la Garonne. Après 4 000 ans cal. BP, trois phases de mise en place des limons bruns de couverture sont identifiées vers 3 700-3 400 ans cal. BP, 2 500-2 000 BP et entre la fin du Moyen Âge et de la période Moderne

Reconstructing past terrace fields in the Pyrenees: Insights into land management and settlement from the Bronze Age to the Early Modern era at Vilalta (1650 masl, Cerdagne, France)

© Trustees of Boston University 2015. The building of a solar power station at Thémis, at 1650 masl on the south-facing slope of the Carlit massif in the eastern Pyrenees, led to an archaeological evaluation from April-June 2009. This evaluation covered a surface of 10 ha that included a medieval village as well as the surrounding agricultural land in terraces. Non-destructive archaeological methods were used for the village. A detailed study of the 6 ha of terraces began with a fieldwalking survey, mapping every visible feature, followed by systematic trial trenches. Fifty-five trenches, 11 in the village and 44 in the fields, were opened. The stratigraphies were then compared with a series of 22 radiocarbon dates and eight relative dates provided by ceramic typologies. This combination of surface and buried evidence supported our preliminary hypothesis about the dynamics of the slope. The results suggest the existence of agrarian features beginning in the Bronze Age and reveal that the field patterns were frequently transformed, both in the Medieval and Early Modern periods. The transformations in the terrace fields after the village was abandoned are as interesting as those during occupation because, contrary to the idea of a fixed, unchanging landscape after the end of the Middle Ages, they challenge the idea that mountain zones are marginal spaces by nature, or were marginalized later.Peer Reviewe

The subfossil tree deposits from the Garonne Valley and their implications on Holocene alluvial plain dynamics

International audienceSubfossil tree trunks deposits are common in large rivers, but their status as a source for dating alluvial sequences and palaeoenvironmental studies is still discussed. Particularly their origin and the process(es) of deposition as well as a possible remobilization were pointed as a limit to their use to document river alluvial changes. In this work we report the discovery of the largest subfossil trunks deposits in the Garonne valley. These new data are compared to the previous ones. A set of 17 tree trunks and more than 300 smaller wood fragments were collected. The xylologic study shows the prevalence of Quercus and a single occurrence of Ulmus. These two hardwood species are commonly associated with riparian forest. The 14C dating carried out on seven trunks and a single branch of Quercus on the outermost identified growth rings, indicates age ranging from 8400-8000 cal. BP for the oldest fragment (bough) to 4300-4000 cal. BP for the most recent tree trunk. Radiocarbon ages of the trunks are aggregated into two main periods: 5300-5600 cal. BP (four trunks) and 4300-4000 cal. BP (three trunks). The radiocarbon (charcoal) dating of the top of the alluvial sequence overlaying the trunks gives an age between 1965-1820 and 1570-1810 cal. BP, i.e. between the 2nd and the 5th c. AD. In addition, the discovery of two unpublished subfossil tree trunks deposits in Finhan are reported (six trunks). At the light of these results, we discuss previously proposed models for the Garonne floodplain building

Non-Invasive Time-Lapse Imaging of Rainfall Infiltration Levels in the Sedimentary Soils of Central Panama

The economic and commercial worldwide impact of the Panama Canal makes this route one of the most important international maritime routes. The quality of water in communities located near the vicinity of the Canal makes conservation and understanding of water resources a priority. In order to identify the stratigraphy and to understand the infiltration processes caused by the rains in the area of Gamboa between the transition (dry to rainy season) and rainy seasons, a time-lapse electrical resistivity tomography analysis was performed. The results showed two horizons constituted mainly by clay material. The first one composed of clay and accumulation of rocks in some areas and the second, the same clay material with certain level of moisture. About the time-lapse results, the negative percent change of resistivity (between 0 and -46.5%) obtained are associated with the rainfall that reached 19 mm during the last days of the second survey; this fact demonstrates the remarkable contribution of the rains to the groundwater that feed the Panama Canal. Moreover, the positive percent change of resistivity (between 0 and 50%) are linked to artefacts which can be common linked to inversion scheme, electrode array or electrode removal and relocation process