Caractérisation et classification de sols dérivés de cendres volcaniques récentes (volcan Pichincha, Equateur)

Dans le cadre d'une étude sur le fonctionnement hydrologique et les risques de coulées boueuses des versants du volcan Pichincha qui domine la ville de Quito (Equateur), une séquence de sols volcaniques dérivés de cendres récentes est étudiée. Les profils correspondent à des séries de sols formés sur une succession de dépôts quaternaires. L'ensemble du versant est constitué de sols jeunes en début de pédogenèse. Selon les conditions climatiques apparaissent des Thaptics et Humics Vitricryands en haut de versant et des Vitriandic Troporthents en bas de versant (Soil Survey Staff, 1994). Les syhthèses de minéraux allophaniques sont très limitées pour le type Andisol. Les sols de bas de versant sont dépourvus de tous minéraux secondaires aussi bien d'allophanes que d'argiles. Le facteur limitant à une pleine évolution des sols semble être ici le temps : les sols actuels dérivent de matériaux trop jeunes et la succession trop rapide des éruptions a empêché l'évolution des horizons enfouis jusqu'au stade andique. Les propriétés andiques, et en particulier les caractéristiques physiques particulières des Andosols, sont donc trop peu exprimées pour pouvoir être considérées comme responsables d'une instabilité des versants en condition de fortes pluies. (Résumé d'auteur)

Les sols des paramos d'Equateur sur couverture pyroclastique : diversité, genèse et propriétés physiques

Dans les Andes centrales et septentrionales (Colombie, Equateur), l'étage des paramos, situé entre les neiges permanentes (+/- 4800 m d'altitude) et la forêt andine (+/- 3000 m d'altitude) est soumis à une pression anthropique croissante. Cette étude vise à préciser la nature, la diversité et la genèse des sols des paramos d'Equateur sur couverture pyroclastique puis à étudier leurs principales propriétés physiques (pouvant contrôler les comportements hydrodynamiques et érosifs à l'état naturel et après mises un usage). Cinq profils sont choisis pour rencontrer les principales catégories des sols volcaniques (Andosols) de ce milieu. Nous montrons que les profils retenus s'incrivent dans une séquence au sein de laquelle l'âge et la texture des pyroclastes parentaux contrôlent le degré d'évolution des sols. Dans ce milieu montagnard et humide, tous les sols sont très humifères et cela quel que soit l'âge de la mise en place de leur dernier niveau pyroclastique. Les Andosols les plus âgés (+/- 3000 ans) de cette séquence sont non-allophaniques et présentent une combinaison inhabituelle de propriétés avec des teneurs en carbone proches de celles de sols organiques (150-230g/kg) est des capacités de rétention en eau à 1500 kPa qui sont celles d'Andosols perhydratés (greater than 1000 g/kg). Nos résultats montrent que la porosité totale de ces Andosols est essentiellement constituée par des pores de rayon less than 0,1 micromètres et que le développement de cette nanoporosité est dû, pour l'essentiel, à l'accroissement de leurs teneurs en carbone humifié et en complexes organo-aluminiques. Nous mettons également en évidence que plus la nanoporosité de ces matériaux est élevée, plus la dessiccation entraîne des modifications irréversibles de leur architecture porale... (D'après résumé d'auteur

Evolution of the alpine Critical Zone since the Last Glacial Period using Li isotopes from lake sediments

Comprehending and predicting the way humans affect the Earth's Critical Zone remains a challenge. An understanding of the past changes resulting from human and non-human influences in the dynamics of the Critical Zone is crucial. Here, we use a retrospective approach to address this question based on a new lithium (Li) isotope record from the Late Glacial Period to the present from a pre-Alpine lake sediment sequence (Lake La Thuile, France). Coupled with the lake sediment archive, the investigation of present-day soils in the lake catchment suggests that lake sediments are not necessarily recording the erosoin of topsoil in the catchment. Our findings indicate that soil particles can be sorted during transportation to the lake, with finer particles being preferentially mobilized, highlighting the influence of fine particle transport on the Li isotope signature of soils and lake sediments. Characterized by low Li isotope signatures, changes in weathering signatures in lake sediments can be amplified by the combined effect of soil development and selective transport. In the La Thuile catchment, soil development was limited during the Late Glacial Period, whereas it became a dominant process during the Holocene climatic optimum together with enhanced selective transport of fine particles. Human activities since 3,000–4,000 yr cal BP induced a strong perturbation hindering both soil formation and selective transport by reinforcing erosion rates. After a period of topsoil destruction caused by intense deforestation and agriculture, lake Li isotopes record the evolution of soil profiles associated with changes in agricultural practices

Approche archéologique et environnementale des premiers peuplements alpins autour du col du Petit-Saint-Bernard (Savoie, vallée d’Aoste) : un bilan d’étape

De 2003 à 2007, une étude archéologique et sédimentaire a été réalisée sur les versants du col du Petit-Saint-Bernard (2188 m, Alpes occidentales), par de grandes séries de sondages manuels effectués dans des contextes sélectionnés. Les avantages et les inconvénients de cette méthode sont exposés et discutés. Ce travail a permis d’identifier, interstratifiés dans les remplissages holocènes, des sols bruns fersiallitiques sur les versants et des sols hydromorphes en altitude. Les répartitions chronologique et altitudinale des découvertes archéologiques sont analysées, puis comparées à une compilation des connaissances préexistantes dans les vallées alpines du Beaufortin, de la Maurienne et de la Tarentaise. Les analyses paléo-environnementales et archéologiques en cours sont présentées.An archaeological and sedimentary study was realized on the hillsides of the Petit-Saint-Bernard pass (2188 m, western Alps), from 2003 to 2007. It consisted of great series of manual boreholes in selected contexts. The efficiency of this method is discussed. This work allowed identifying brown fersiallitics soils in hillsides and hyrdromorphic soils in the heights, interstratified in the Holocene fillings. The chronological and altitudinal distributions of the findings are analysed and compared to previous data from Beaufortin, Maurienne and Tarentaise. An additional project of palaeo-environmental and archaeological analyses is presented

The precursors of chocolate aroma.

The subject of this research was an experimental study of the development of chocolate aroma during the commercial processing of cocoa beans. The components of unroasted aroma cocoa beans have been fractionated, and the various groups of substances obtained tested for the development of aroma when roasted. The results obtained provide further evidence of the involvement of simple amino acids, reducing sugars and flavonoids in the formation of the primary aroma of chocolate products, and a mechanism for the formation and mixing of these substances during cocoa fermentation is described. Model mixtures of these compounds have been examined in order to assess the extent of the contribution of individual compounds to the production of the aroma. The reaction of these model mixtures have also been examined under the conditions obtaining during commercial bean roasting, and combined gas chromstography-mase spectrometry has been used to compare their volatile reaction products with these present in roasted cocoa beans. The results of these studies indicate the importance of the natural environment of the precursors in the control of the extent of subsequent reactions, and the consequences of the 'dry state' reaction conditions in this respect. Various types of compound formed in these reactions are described and their possible importance in the recognition of chocolate aroma is discussed. The experimental results are discussed in terms of literature surveys of the current state of knowledge of both chocolate aroma development, and the aroma potential of amino acid-reducing sugar reactions

New insights on lake sediment DNA from the catchment: importance of taphonomic and analytical issues on the record quality

Over the last decade, an increasing number of studies have used lake sediment DNA to trace past landscape changes, agricultural activities or human presence. However, the processes responsible for lake sediment formation might affect DNA records via taphonomic and analytical processes. It is crucial to understand these processes to ensure reliable interpretations for “palaeo” studies. Here, we combined plant and mammal DNA metabarcoding analyses with sedimentological and geochemical analyses from three lake-catchment systems that are characterised by different erosion dynamics. The new insights derived from this approach elucidate and assess issues relating to DNA sources and transfer processes. The sources of eroded materials strongly affect the “catchment-DNA” concentration in the sediments. For instance, erosion of upper organic and organo-mineral soil horizons provides a higher amount of plant DNA in lake sediments than deep horizons, bare soils or glacial flours. Moreover, high erosion rates, along with a well-developed hydrographic network, are proposed as factors positively affecting the representation of the catchment flora. The development of open and agricultural landscapes, which favour the erosion, could thus bias the reconstructed landscape trajectory but help the record of these human activities. Regarding domestic animals, pastoral practices and animal behaviour might affect their DNA record because they control the type of source of DNA (“point” vs. “diffuse”)

Global maps of soil temperature

Research in global change ecology relies heavily on global climatic grids derived from estimates of air temperature in open areas at around 2 m above the ground. These climatic grids do not reflect conditions below vegetation canopies and near the ground surface, where critical ecosystem functions occur and most terrestrial species reside. Here, we provide global maps of soil temperature and bioclimatic variables at a 1-km2 resolution for 0–5 and 5–15 cm soil depth. These maps were created by calculating the difference (i.e. offset) between in situ soil temperature measurements, based on time series from over 1200 1-km2 pixels (summarized from 8519 unique temperature sensors) across all the world\u27s major terrestrial biomes, and coarse-grained air temperature estimates from ERA5-Land (an atmospheric reanalysis by the European Centre for Medium-Range Weather Forecasts). We show that mean annual soil temperature differs markedly from the corresponding gridded air temperature, by up to 10°C (mean = 3.0 ± 2.1°C), with substantial variation across biomes and seasons. Over the year, soils in cold and/or dry biomes are substantially warmer (+3.6 ± 2.3°C) than gridded air temperature, whereas soils in warm and humid environments are on average slightly cooler (−0.7 ± 2.3°C). The observed substantial and biome-specific offsets emphasize that the projected impacts of climate and climate change on near-surface biodiversity and ecosystem functioning are inaccurately assessed when air rather than soil temperature is used, especially in cold environments. The global soil-related bioclimatic variables provided here are an important step forward for any application in ecology and related disciplines. Nevertheless, we highlight the need to fill remaining geographic gaps by collecting more in situ measurements of microclimate conditions to further enhance the spatiotemporal resolution of global soil temperature products for ecological applications