La mer d’Aral reviendra-t-elle ?

Au mois de septembre 2001, un véhicule kaki parcourait les routes et les pistes entre Tachkent, Noukous, Mojnaq et le fond asséché de la mer d’Aral. Il s’arrêtait souvent en des lieux insolites : ses cinq passagers et le chauffeur descendaient munis d’accessoires tels que marteaux, spatules, couteaux, petites bouteilles en plastique, conductimètre pour mesurer des propriétés de l’eau. Ils étudiaient et prélevaient les roches, l’eau des rivières ou des captages de la nappe phréatique. De temps..

Apports de la géochimie isotopique de bivalves dulcicoles à la reconstitution paléoenvironnementale en milieu urbain : le cas de la ville de Lyon

International audienceThe human settlement and development of the city of Lyon since Antiquity have been partly controlled by the topography of the site, particularly by the presence of the Saone and the Rhône rivers, which enclosed the peninsula. Bivalves and few gastropods shells have been collected from archaeological levels, to conduct an extensive δ18O and δ13C study. In fact, malacofauna shells potentially record, during the growth period, the impact of the major hydrological and climatic changes on the aquatic ecosystem. The study is focused on two extreme climatic periods: the Medieval Climate Optimum and the Little Ice Age. A third, less documented period, corresponds to the High Roman Empire (Antiquity). The shells were collected from several sites on the peninsula: both near the left bank of the Saone river, close to the old confluence area and near the right bank of the Rhône river; on the Fourvière hill near the right bank of the Saone.The isotopic data are close to those of the modern Saone water and modern mollusk shells from the river. This seems to indicate a privileged use of the alluvial deposits of the Saône river for human activity. Temperatures of the water were estimated between 17.2°C and 18.5°C. An average paleotemperature difference of nearly 1.3°C is observed between the MCO and the LIA. The shells of the High Empire record intermediate temperatures (18.1°C). The δ13C of Middle Age shells suggest more favorable climatic period (warming and decreasing flows) and episodes of swirling and paludification of the banks. This study shows the importance of the geochemical record from the mollusk shells as a climate archive with high temporal resolution (year), on the continent and particularly in urban areas. This latter can be indeed characterized by rapid and frequent variabilities and the disappearance of many markers due to human activity.L’installation et le développement de la ville de Lyon depuis l’Antiquité ont été en partie influencés par le fonctionnement des deux cours d’eau qui encadrent la presqu’île : la Saône et le Rhône. Des coquilles de lamellibranches et de gastéropodes dulçaquicoles ont été récoltées dans des niveaux archéologiques afin d’étudier l’enregistrement isotopique (δ13C et δ18O) de ces biominéralisations. En effet, la malacofaune enregistre plus ou moins directement les changements hydrologiques et climatiques. L’étude s’est focalisée sur deux périodes climatiques extrêmes : l’Optimum Climatique Médiéval et le Petit Âge Glaciaire. Une troisième période moins documentée correspond au Haut-Empire Romain (Antiquité). Les sites étudiés sont localisés sur la presqu’île : près de la rive gauche de la Saône, à proximité de l’ancienne zone de confluence et près de la rive droite du Rhône, en contrebas de la colline de Fourvière à proximité de la rive droite de la Saône.Toutes les coquilles ont une signature isotopique caractéristique de la Saône. Cela semble indiquer une utilisation privilégiée des alluvions de cette rivière par l’Homme. Des paléotempératures de l’eau de la Saône ont été estimées entre 17,2 et 18,5 °C. Elles montrent un écart moyen de près de 1,3 °C entre l’OCM et le PAG. Les coquilles du Haut-Empire enregistrent des températures intermédiaires (18,1 °C). Au Moyen Âge, autour de l’an mil, le δ13C très négatif des coquilles correspondrait à des épisodes de tourbification et de paludification des berges dû à une accalmie du régime hydrologique de la rivière et à un réchauffement climatique. Cette étude montre l’intérêt de l’enregistrement géochimique des coquilles de mollusques comme archive climatique à haute résolution temporelle (année), en domaine continental et notamment en milieu urbain. Ce milieu peut en effet être marqué par une variabilité rapide et fréquente et par la disparition de nombreux autres marqueurs du fait de l’activité humaine

Apports de la géochimie isotopique de bivalves dulcicoles à la reconstitution paléoenvironnementale en milieu urbain : le cas de la ville de Lyon

L’installation et le développement de la ville de Lyon depuis l’Antiquité ont été en partie influencés par le fonctionnement des deux cours d’eau qui encadrent la presqu’île : la Saône et le Rhône. Des coquilles de lamellibranches et de gastéropodes dulçaquicoles ont été récoltées dans des niveaux archéologiques afin d’étudier l’enregistrement isotopique (δ13C et δ18O) de ces biominéralisations. En effet, la malacofaune enregistre plus ou moins directement les changements hydrologiques et climatiques. L’étude s’est focalisée sur deux périodes climatiques extrêmes : l’Optimum Climatique Médiéval et le Petit Âge Glaciaire. Une troisième période moins documentée correspond au Haut-Empire Romain (Antiquité). Les sites étudiés sont localisés sur la presqu’île : près de la rive gauche de la Saône, à proximité de l’ancienne zone de confluence et près de la rive droite du Rhône, en contrebas de la colline de Fourvière à proximité de la rive droite de la Saône. Toutes les coquilles ont une signature isotopique caractéristique de la Saône. Cela semble indiquer une utilisation privilégiée des alluvions de cette rivière par l’Homme. Des paléotempératures de l’eau de la Saône ont été estimées entre 17,2 et 18,5 °C. Elles montrent un écart moyen de près de 1,3 °C entre l’OCM et le PAG. Les coquilles du Haut-Empire enregistrent des températures intermédiaires (18,1 °C). Au Moyen Âge, autour de l’an mil, le δ13C très négatif des coquilles correspondrait à des épisodes de tourbification et de paludification des berges dû à une accalmie du régime hydrologique de la rivière et à un réchauffement climatique. Cette étude montre l’intérêt de l’enregistrement géochimique des coquilles de mollusques comme archive climatique à haute résolution temporelle (année), en domaine continental et notamment en milieu urbain. Ce milieu peut en effet être marqué par une variabilité rapide et fréquente et par la disparition de nombreux autres marqueurs du fait de l’activité humaine.The human settlement and development of the city of Lyon since Antiquity have been partly controlled by the topography of the site, particularly by the presence of the Saone and the Rhône rivers, which enclosed the peninsula. Bivalves and few gastropods shells have been collected from archaeological levels, to conduct an extensive δ18O and δ13C study. In fact, malacofauna shells potentially record, during the growth period, the impact of the major hydrological and climatic changes on the aquatic ecosystem. The study is focused on two extreme climatic periods: the Medieval Climate Optimum and the Little Ice Age. A third, less documented period, corresponds to the High Roman Empire (Antiquity). The shells were collected from several sites on the peninsula: both near the left bank of the Saone river, close to the old confluence area and near the right bank of the Rhône river; on the Fourvière hill near the right bank of the Saone. The isotopic data are close to those of the modern Saone water and modern mollusk shells from the river. This seems to indicate a privileged use of the alluvial deposits of the Saône river for human activity. Temperatures of the water were estimated between 17.2°C and 18.5°C. An average paleotemperature difference of nearly 1.3°C is observed between the MCO and the LIA. The shells of the High Empire record intermediate temperatures (18.1°C). The δ13C of Middle Age shells suggest more favorable climatic period (warming and decreasing flows) and episodes of swirling and paludification of the banks. This study shows the importance of the geochemical record from the mollusk shells as a climate archive with high temporal resolution (year), on the continent and particularly in urban areas. This latter can be indeed characterized by rapid and frequent variabilities and the disappearance of many markers due to human activity

Late Cenomanian-Turonian isotopic stratigraphy in the chalk of the Paris Basin (France): a reference section between the Tethyan and Boreal realms

A chemostratigraphic study (δ13C and δ18O) of the Late Cenomanian and Turonian chalk succession from the “Craie 701” Poigny borehole (near Provins in the Paris Basin, France) provides new high-resolution stable carbon and oxygen isotope data. Correlation of the bentonite horizons and the isotopic trends from Poigny with its English Chalk equivalent allows the development of a precise stratigraphic framework. δ13C and δ18O variations are synchronous and widespread throughout the European epicontinental seas and Tethyan Ocean. In the Poigny borehole, the Oceanic Anoxic Event 2 (OAE 2) is marked by a large and brief positive excursion of carbon isotopes (a carbon isotope excursions: CIE of 3‰ amplitude) without any apparent anoxia in the Late Cenomanian Chalk. Comparisons between different key sections on a North-South transect from the Anglo-Paris Basin to the Umbria-Marche Basin (Gubbio Section, Italy) and the Vocontian Basin (South-East France), suggests that the OAE 2 is linked to an increase in marine organic matter production, modulated by a regional effect on the organic carbon burial rate. Thus, the large positive carbon isotope increase spanning the Middle Cenomanian through to the Middle Turonian, including the salient CIE associated with the OAE 2, reflects a global scale increase in marine productivity that would be concomitant with a major long-term sea level rise. The stratigraphic position of the Turonian-Coniacian boundary can also be better defined by this isotopic study. A comparison of δ18O data between the Anglo-Paris Basin and Tethyan Basin shows high-amplitude, long-term synchronous variations reflecting primary paleo-environmental changes which are independent of local facies, sediment thickness and diagenesis. In particular, a negative shift (−1‰ of amplitude) reflects a warmer climate regime, marking the onset of OAE 2. Two colder phases (+1‰ amplitude each) occurred in the Early Turonian and the beginning of the Late Turonian

Siliceous phytoplankton in middle Eocene sediments of ODP Hole 207-1260A

The Middle Eocene diatom and silicoflagellate record of ODP Site 1260A (Demerara Rise) is studied quantitatively in order to throw light on the changes that siliceous phytoplankton communities experienced during a Middle Eocene warming event that occurred between 44.0 and 42.0 Ma. Both Pianka's overlap index, calculated per couple of successive samples, and cluster analysis, point to a number of significant turnover events highlighted by changes in the structure of floristic communities. The pre-warming flora, dominated by cosmopolitan species of the diatom genus Triceratium, is replaced during the warming interval by a new and more diverse assemblage, dominated by Paralia sulcata (an indicator of high productivity) and two endemic tropical species of the genus Hemiaulus. The critical warming interval is characterized by a steady increase in biogenic silica and a comparable increase in excess Ba, both reflecting an increase in productivity. In general, it appears that high productivity not only increased the flux of biogenic silica, but also sustained a higher diversity in the siliceous phytoplankton communities. The microflora preserved above the critical interval is once again of low diversity and dominated by various species of the diatom genus Hemiaulus. All assemblages in the studied material are characterized by the total absence of continental and benthic diatoms and the relative abundance of neritic forms, suggesting a transitional depositional environment between the neritic and the oceanic realms

Trace organic contaminants within solid matrices along an anthropized watercourse: Organo-mineral controls on their spatial distribution

International audienceAlthough numerous studies have determined significant contamination in terms of trace organic contaminant (TrOC) diversity and concentration, the occurrence of TrOCs within solid matrices as suspended solids and sediments flies under the radar. In this study, the occurrence of 35 TrOCs of various classes (i.e. pharmaceutical products and pesticides) was investigated in three compartments, namely dissolved phase, suspended particulate matter (SPM) and sediments, within an anthropized river in France. The sampling was performed to assess the spatial contamination dynamics and the impact of a major wastewater treatment plant (WWTP), under two contrasted hydrological conditions, i.e. base level and flood conditions. Solid samples were finely characterized (XRD, grainsize, TOC) in order to assess the impact of organic and mineral composition on the sorption extent of TrOCs. The study reveals that the clear spatial pattern of contamination in water samples, mostly generated by the effluent discharge of WWTPs, is less clear in solid matrices as the variability of the organo-mineral composition of such samples strongly impacts their favourability for sorption. Moreover, the flood event strongly impacted the sedimentary compartment, remobilizing fine and TrOC contaminated particles that were further found in suspended particulate matter. Lastly, the representativeness of contaminant 2 diversity and concentration within the solid matrices displayed more favourable insights for SPM