Vincent Bichet

Les decouvertes archeologiques du Mormont (Vaud, Suisse) temoignent d’une occupation humaine au 1er siecle avant notre ere. Afin de definir le contexte environnemental, une etude alliant approches sedimentologiques et palynologiques a ete conduite sur une sequence de 6 metres sur la tourbiere d’En Deleze a Dizy. La dynamique lacustre est attestee du Tardiglaciaire au Preboreal avec un remplissage de silts et de craie. Un episode palustre avec accumulation de tourbe se developpe ensuite jusqu’a nos jours. Au Tardiglaciaire, la region etait couverte de pelouses parsemees de tapis de bouleaux, saules et genevriers nains. Le rechauffement du Bolling permet au genevrier et a d’autres arbustes d’envahir ces pelouses ; de rares forets claires a bouleau se developpent. A l’Allerod, le bouleau cede sa place au pin. La sedimentation du Dryas recent est carbonatee, ce qui distingue Dizy des sites jurassiens d’altitude, probablement en raison de conditions estivales suffisamment temperees favorisant la precipitation de carbonates. Au debut de l’Holocene, les pinedes regressent au profit d’une foret mesophile qui perdurera avant de subir les defrichements des groupes humains successifs. Les signes d’agro-pastoralisme au Neolithique sont tenus. Au-dela de l’Âge du Bronze, la tourbe n’est pas exploitable par les analyses mises en œuvre.

Aujourd’hui, une majorite de chercheurs s’accordent sur le fait que le rechauffement climatique est la consequence d’une responsabilite combinee entre l’evolution naturelle du climat et la part croissante prise par l’homme dans l’ampleur de ce rechauffement. Comment rester indifferent au fait que 2014 fut l’annee la plus chaude enregistree sur le globe, la temperature terrestre ayant ete superieure de 0,69 °C a la moyenne au XXe siecle ? Comment ne pas s’inquieter que les dix annees les plus chaudes des deux derniers siecles sont toutes posterieures a l’an 2000, a l’exception de 1998 ? En 2015, selon l’Agence americaine oceanique et atmospherique (NOAA), les mois de fevrier, mars, mai, juin, juillet et aout ont battu les records de temperatures mensuelles jamais enregistrees a la surface du globe depuis 1880 ; en aout, la temperature moyenne sur les terres et les oceans etait de 1,14 °C superieure a la moyenne du XXe siecle. En decembre 2015, la Conference internationale sur le clim...

ABSTRACT Palaeoenvironmental studies from continental and marine sedimentary archives have been conducted over the last four decades in the archaeologically rich Norse Eastern Settlement in Greenland. Those investigations, briefly reviewed in this paper, have improved our knowledge of the history of the Norse colonization and its associated environmental changes. Although deep lakes are numerous, their deposits have been little used in the Norse context. Lakes that meet specific lake-catchment criteria, as outlined in this paper, can sequester optimal palaeoenvironmental records, which can be highly sensitive to both climate and/or human forcing. Here we present a first synthesis of results from a well-dated 2000-year lake-sediment record from Lake Igaliku, located in the center of the Eastern Settlement and close to the Norse site Garðar. A continuous, high-resolution sedimentary record from the deepest part of the lake provides an assessment of farming-related anthropogenic change in the landscape, as well as a quantitative comparison of the environmental impact of medieval colonization (AD 985—ca. AD 1450) with that of recent sheep farming (AD 1920—present). Pollen and non-pollen palynomorphs (NPPs) indicate similar magnitudes of land clearance marked mainly by a loss of tree-birch pollen, a rise in weed taxa, as well as an increase in coprophilous fungi linked to the introduction of grazing livestock. During the two phases of agriculture, soil erosion estimated by geochemical proxies and sediment-accumulation rate exceeds the natural or background erosion rate. Between AD 1010 to AD 1180, grazing activities accelerated soil erosion up to ≈8 mm century-1, twice the natural background rate. A decrease in the rate of erosion is recorded from ca. AD 1230, indicating a progressive decline of agro-pastoral activities well before the end of the Norse occupation of the Eastern Settlement. This decline could be related to possible climate instabilities and may also be indirect evidence for the shift towards a more marine-based diet shown by archaeological studies. Mechanization of agriculture in the 1980s caused unprecedented soil erosion up to ≈21 mm century-1, five times the pre-anthropogenic levels. Over the same period, diatom assemblages show that the lake has become steadily more mesotrophic, contrary to the near-stable trophic conditions of the preceding millennia. These results reinforce the potential of lake-sediment studies paired with archaeological investigations to understand the relationship between climate, environment, and human societies.

In Niamey region, ponds and lakes formed in the past six decades are being filled with a sedimentation rate of 4 cm per year. The cultivated sandy soils covering their watershed are prone to wind and water erosions. This work aimed to estimate the proportion of aeolian deposits in the filling of one of these lakes, Bangou Bi, by the means of aeolian deposition flux measurements on the lake and wind erosion flux measurements on cultivated and bare soils. It appeared that more than 70% of aeolian deposits on Bangou Bi were provided by locale wind erosion that is controlled by the annual cycle of cultivated and bare soils erosion. Anyway, these deposits only reached a maximum thickness of 400µm per year which appears negligible in the filling of the lake.

In Niamey region, the annual dust cycle is well documented by ground base, airborne and remotesensing measurements. They allow the description of sources, transport pattern and dust characteristics. Even at the beginning of the rainy season, Saharan dusts, transported in elevated atmospheric layer are prevalent in the Sahel. Nevertheless, in this area, local emissions are currently clearly linked to agricultural activities. Do these activities confuse the climatic information generally attributed to desert dust?

Plateaus on the western bank of the River Niger are covered with heaps and lines of dry stone although these tabular areas are now without any agriculture. A sandy clay soil horizon is present only in the areas where these structures are located, suggesting that they result from removing stones to improve the soil. The materials extracted were placed in lines or in heaps when there were too many stones. These plots could be contemporary with metallurgical activities located at the foot of plateaus and dated from the second half of the first millennium.

Chaillexon lake has been created by a rock collapse that has dammed the valley of the Doubs river 12,000 years ago. The available space was and is still filling up by the river and that trap represents a continuous record for the Post-glacial. The study is based on the characterization of geochemical and optical organic markers in the actual watershed (soils and bedrock) and their occuring in the sedimentary organic matter. It appears that the Preboreal/Boreal transition (9000 BP) characterizes a major event in the story of this lacustrin system. A forest and soils development permits a significant protection of the bedrock from erosional processes.

A multiproxy approach to a sediment sequence at Lake Saint-Point in the French Jura Mountains gives evidence of a strong coupling between changes in terrestrial and lacustrine ecosystems throughout the Holocene. The early Holocene (11,700–10,200 cal. BP) is characterised by the recovery of terrestrial and lake ecosystems favoured by climatic warming. During the middle Holocene (10,600–6200 cal. BP), the climatic optimum coincided with an extension of deciduous forests into the catchment area, while lake sedimentation is dominated by authigenic carbonates and low detrital inputs. After 6200 cal. BP, the Neoglacial favoured expansion of Abies-Fagus forests and increasing detrital inputs to the lake where ostracod fauna declined and changed in composition. After 1200 cal. BP, human impact was responsible for extensive forest clearings in the catchment area, while the lake basin shows contrasting pictures with increasing detrital input, resuming sedimentation of authigenic carbonates an...

We performed a high-resolution study of chironomid assemblages in a sediment core retrieved from Lake Igaliku in southern Greenland. The well-dated core is located within the former Norse Eastern Settlement and covered the last 1500 yr. The comparison of chironomid stratigraphy (PCA axis scores) with instrumental temperature data, land use history and organic matter in the sediment over the last 140 yr suggested that the primary changes in chironomid fauna in 1988 ± 2 yr were driven by the shift to modern agriculture in the catchment. This unprecedented change in chironomid fauna was most likely triggered by a shift in in-lake processes. Within the instrumental period, subtle variations in the chironomid assemblages that occurred before 1988 ± 2 yr were significantly correlated with summer temperatures even in times of traditional extensive sheep farming in the catchment. The relevance of the chironomid-derived climate signal over the last 1500 yr was supported by its good concordan...

In the Sahel, the rapid increase of the population during the last decades and the climate variation lead to an important environmental degradation. This work aims to measure the impacts of the human pressure on ecosystem during the six last decades. A diachronic cartography of a 100 km² area close to Niamey was done with aerial photographs (1950 and 1975) and GPS measurements (2009). Results showed that the tiger bush vegetation was completely cleared between 1950and 2009 while the fallow decreases from 7 % to 1 %. In the asndy valley, the increase of cultivated fields from 20,7 % (1950) to 69,4 % (1975) favoured wind and water erosions which allowed surface soil crusting. Between 1975 and 2009, the bare crusted soil dramatically developed at the expense of the cultivated area which represents only 54,4 % of the studied area. The valleys are going to be overloaded because of high sedimentation rate (> 4 cm per year).

International audienceIn the framework of climate changes, peatland ecosystems are compartments of the Critical Zone of growing importance for greenhouse gas (GHG) exchanges with the atmosphere. Peatlands contain about 30 % of the total organic soil carbon worldwide. Interactions of GHG between atmosphere and peatland are potentially controlled by organic matter production and degradation controlled by biotic functiuns, which are in turn influenced by diverse abiotic factors including (1) water saturation, (2) peat humification degree, and (3) water quality conditioning electron acceptor availability. Delineating the detailed roles of these mechanisms is therefore challenging for long-term peatland management, requiring a detailed spatio-temporal monitoring of a wide range of hydrological and biogeochemical parameters.The Frasne peatland (Jura mountains, eastern France), belonging to a regional natural reserve is an outstanding site for such studies as it is monitored since 2008 in the framework of the national observatory of peatlands (SNO Tourbières),and belongs to the Zones Ateliers (RZA) and French Critical Zone (OZCAR) networks supplying the Europe Long-Term Ecosystem Research (eLTER) database (DEIMS-SDR). The peatland took place in a periglacial context on groundmoraines covering a large-scale karstified synclinal structure. This complex geological setting is combined with eco-physical (peat thicknesses and maturity), and hydrological (water-table depth) heterogeneities at the peatland scale.Based on daily hydrometeorological data (P, T, potential evapotranspiration, groundwater level) and monthly monitoring (hydrochemistry; peat lability, GHG fluxes) in 25 piezometers, the goal of the CRITICAL PEAT project is to identify the hydrological and biogeochemical drivers controlling GHG exchanges between peatland and atmosphere. In this perspective, this contribution aims at presenting preliminary results on the hydroclimatic sensitivity of the system inferred from correlation analyses, and its relationships with water origin and chemistry

Dans la région de Niamey, le cycle annuel des poussières terrigènes est bien documenté tant par des mesures directes, au sol ou aéroportées, que par télédétection. Il est possible de décrire à la fois l’origine des poussières, leurs conditions de transport et leurs caractéristiques. Même au début de la saison des pluies, l’atmosphère sahélienne est encore dominée par les poussières sahariennes transportées en altitude. Cependant, dans cette zone, les émissions locales qui traduisent l’aptitude du sol à s’éroder sont actuellement liées aux activités humaines. Dans quelle mesure ces activités peuvent-elles brouiller le signal climatique généralement attribué aux aérosols terrigènes ?

Depuis 2006, une equipe constituee de paleoenvironnementalistes du laboratoire Chrono-environnement de Besancon travaille sur les sequences lacustres du sud du Groenland. Le secteur etudie est au cœur de la zone occupee par les viking (l’Eastern settlement) entre 986 et 1450 cal. AD. Le but premier de cette etude etait de mettre en evidence l’impact de cette colonisation sur un environnement vierge. L’etude des parametres biotiques (grains de pollen, microfossiles non polliniques et diatomees) et abiotiques (sedimentologie, geochimie et isotopes) a ete etendue a l’ensemble de certaines sequences afin de replacer la partie viking dans son contexte holocene. Cinq missions de terrain ont permis de collecter une vingtaine de sequences lacustres. La sequence sedimentaire prelevee dans le lac d’Igaliku (N61°00’22”, W45°26’28), a cote du site medieval de Garðar debute aux environs de 8000 cal. BC et l’enregistrement pollinique commence a partir de 6100 cal. AD ; elle fait partie des rares sequences completes et bien datees (28 dates radiocarbone) de ce secteur subarctique. La premiere moitie de l’Holocene, jusque vers 2500 cal. BC, est marquee par un couvert vegetal domine par le genevrier et le saule. Ensuite l’apparition des conditions plus humides du Neoglacial favorise l’essor du bouleau glanduleux, puis du bouleau pubescent. Des la fin du premier millenaire apres Jesus-Christ, la colonisation viking est marquee par une pression pastorale qui se traduit par l’augmentation brutale des taux de spores de champignons coprophiles, la diminution des pourcentages de bouleau et l’apparition de plantes importees comme l’oseille, le bouton d’or ou le pissenlit. Le developpement de ces activites agropastorales entraine une erosion moderee des sols mais les diatomees montrent que le systeme lacustre n’est en rien affecte par ces changements. Des 1300 cal. AD la pression pastorale et l’erosion diminuent, donnees corroborees par l’archeologie. A l’oree du Petit Âge Glaciaire, les viking se tournent vers une alimentation tiree de la mer aux depends de l’elevage. Aux environs de 1450 cal. AD, le peuplement disparait mais la resilience est moderee du point de vue de la vegetation car les plantes importees se maintiennent. La reintroduction de l’elevage au debut du XXe siecle est parfaitement marquee par les differents parametres d’etude : si l’impact sur la vegetation n’est guere different de ce qui a ete observe pendant la periode viking, l’erosion est largement superieure et l’emploi d’engrais bouleverse definitivement l’ecosysteme lacustre.

On the west bank of the Niger River, archaeological prospecting has mapped more than 4000 low shaft furnaces on an area of 32km². These single-use furnaces have been identified by either a shaft or a block of slag in the erosion surface. This smelting activity is dated between 4th and 14th century AD. Spatial data were processed by a Geographic Information System (GIS) and mathematical tools (Ripley’s K function, Kernel density estimation). The results of this analysis show that smiths built low shaft furnaces near ores and in sandy soils. However, the choice of location for the furnaces seems also to depend on sociocultural factors, thus leading to the formation of clusters in specific places.

Recent lakes appeared in the vicinity of Niamey in response to environmental changes in Sahel are good recorders of recent climate dynamics. This work is a preliminary analysis of sediments cored in a recent lake (<50 years) aiming to understand the effects of global change and human pressure on its watershed. In the long term, it should produce a paleoenvironmental model to interpret similar oldest lake sediments in the same area. The BK-08-02 core was setting apart in the lake Bangou Kirey (N13°30’ E2°13’) with a UWITEC corer. XRF, spectrophotometry, micro-particle size analysis and magnetic susceptibility were applied to the core. Laminated sediments from Bangou Kirey shows a good time resolution since sediment fluxes are higher than 2 cm yr-1. The lamination is supposed to correspond to seasonal and intra-seasonal dynamics of sediment flux. The increase of rainy season layer thickness on the top of the core should indicate an increase in runoff intensity linked to anthropogen...

Le remplissage sédimentaire des mares de Bangou-Bi et de Tankalawal (SW Niger) a fait l’objet d’une étude de son contenu organique qui fournit des informations sur l’origine et les conditions de dépôt du matériel sédimentaire. Deux carottes mesurant respectivement 140 cm (carotte BB2) et 60 cm (carotte TK1) ont été étudiées. Les échantillons ont été soumis à une pyrolyse Rock-Eval 6 renseignant sur la géochimie globale de la Matière Organique (MO) et le signal S2 de la pyrolyse a été exploité afin de mieux préciser la nature de ces MO. Le niveau argilo-limoneux de la carotte BB2 contient une MO aquatique et terrestre dont la composition en composés hydrocarbonés se répartit entre les biopolymères (F1+F2), et les géopolymères matures (F3) et immatures (F4). Cette MO serait déposée dans un environnement calme. En revanche le niveau inférieur sableux renferme une MO terrestre très altérée et enrichie en géopolymères matures qui serait amenée par des pulses détritiques grossiers (colluv...

During the last two millennia, global climate warming events allowed two phases of agricultural expansion in Southwestern Greenland (Massa et al., 2012a). The first phase coincided with the medieval Norse colonization between 986 AD and the mid-fifteenth century; the second, corresponds to the modern reestablishment of farmers since 1920, after the end of the Little Ice Age. This context appears as an exceptional study model to examine the transition from a pristine to an anthropogenic landscape. To this respect, lake deposits provide a sensitive recorder of environmental changes due to land use. In order to assess the history and impacts of grazing activities, a first molecular inventory was conducted on a sedimentary sequence retrieved from Lake Igaliku (61°00'N, 45°26'W, 15m asl). A well-dated core spanning the last two millennia was analyzed in order to identify biomarkers that attest to pastoralism such as fecal sterols (Bull et al., 2001) and bile acids (Tyagi et al., 2008). 40 samples were extracted, separated into neutral, acid and polar fractions. The neutral fraction was further separated into aliphatics, aromatics, ethers ketones and alcohols by flash chromatography. After addition of an internal standard, fractions were analysed by Gas Chromatography-Mass Spectrometry. 205 compounds were identified and quantified including n-alkanes, n-alkan-2-ones, mid-chain ketones, wax esters, sterols, hopanoids, fernenes, triterpenyl acetates, polycyclic aromatic hydrocarbons (PAH), tricyclic diterpenes, fatty acids, bile acids, diacids and hydroxy-acids. Principle Components Analysis of the data allowed us distinguishing three main groups of distinct behaviour: (i) fatty acids, bile acids, diacids and hydroxy-acids; (ii) tricyclic diterpenes; (iii) PAH and sterols. The evolution of biomarker concentrations were compared to other parameters such as total organic carbon, total nitrogen and sulfur contents, sediment accumulation rate and non-pollens palynomorph (Gauthier et al., 2010; Massa et al., 2012b; Perren et al., 2012). The evolution of deoxycholic acid, bile acid that can be attributed to sheep (Tyagi et al., 2008) matches the evolution of spores of coprophilous fungi (Gauthier et al., 2010) and that of the C/N, established as an erosion marker (Perren et al., 2012). Indeed, deoxycholic acid concentration is weak before the Norse settlement and reaches a peak at 1100 AD when other indicators attest to a stronger Human impact on lands. Deoxycholic acid concentrations, standardized to the ration C/N showed the same trends which indicate that variations are not only due to the erosion rates but also to the importance of pastoralism in the watershed. These preliminary results provide a new insight into the Norse history in Greenland, completing the archaeological knowledge, and a first quantified molecular inventory for biomarkers in the lacustrine sediments in Southern Greenland. REFERENCES Bull, I. D., Betancourt, P. P., Evershed R. P., 2001. An organic geochemical investigation of the practice of manuring at a Minoan site on Pseira Island, Crete. Geoarchaeology 2, 223-242. Gauthier, E., Bichet, V., Massa, C., Petit, C., Vanniere, B., Richard, H., 2010. Pollen and non-pollen palynomorph evidence of medieval farming activities in southwestern Greenland. Vegetation History and Archeobotany 19, 427-438. Massa, C., Bichet, V., Gauthier, E., Perren, B. B., Mathieu, O., Petit, C., Monna, F., Giraudeau J., Losno, R., Richard, H., 2012a. A 2500 year record of natural and anthropogenic soil erosion in South Greenland. Quaternary Science Review, 32, 119-130. Massa, C., Perren, B. B., Gauthier, E., Bichet, V., Petit, C., Richard, H., 2012. A multiproxy evaluation of Holocene environmental change from Lake Igaliku, South Greenland. Journal of paleolimnology 48, 241-258. Perren, B. B., Massa, C., Bichet V., Gauthier, E., Mathieu, O., Petit, C., Richard, H., 2012. A paleoecological perspective on 1450 years of Human impacts from a lake in Southern Greenland. The Holocene, 22, 1025-1034. Tyagi, P., Edwards, D. R., Coyne M. S., 2008. Use of sterols and bile acids biomarkers to identify domesticated animal sources of fecal pollution. Water Air Soil Pollution 187, 263-274.

ABSTRACT Artic is the most sensitive region to the effects of global warming. Indeed, it warms three times faster than the rest of the world. Nowadays, it is an important issue to characterize the responses of these ecosystems to a rise in temperature and the part that Human plays in this mechanism. For several millennia, agriculture and husbandry are known as one of the major anthropogenic impacts on environment. During the last two millennia, global climate warming events allowed two phases of agricultural expansion in South Greenland. The first phase coincided with the medieval Norse colonization between 986 AD and the mid-fifteenth century; the second, corresponds to the modern reestablishment of farmers since 1920, at the very end of the Little Ice Age. This context appears as an exceptional study model to examine the transition from a pristine to an anthropogenic landscape. To this respect, lake deposits provide a sensitive recorder of environmental changes due to land use. In order to assess the history and impacts of grazing activities on the environment, a first molecular inventory was conducted on a sedimentary sequence retrieved from Lake Igaliku (61°00’N, 45°26’W, 15m asl). The analysis of molecular biomarkers retrieved from a well-dated core spanning the last two millennia was confronted to pollen, non pollen palynomorph and elements, allowing us discussing the evolution of agropastoralism and its impacts on the local ecosystems. When found in soils or sediments, fecal sterols and bile acids can help identifying the former presence of Humans and their livestock. In our case, only deoxycholic acid (DCA) is detected with high fluxes recorded during the two phases of agricultural expansion, coincident with high percentages of coprophilous spores. DCA is produced by all herbivores but, because it is the unique bile acid found in Igaliku sediments, we propose that it mainly derives from sheep. Thus, it could be possible to distinguish predominant livestock species raised in the catchment of Igaliku during the two phases of settlements. Sheep breeding induce an increase in trimethyl tetra hydro chrysene (TTHC) and in Ti, considered as tracers of soil erosion during the last centuries. This increase erosion is accompagnied by an increase in n-C17 alkane fluxes and mesotrophic diatoms percentages that attest to an eutrophication of the lake waters in recent times, potentially due to an important supply of fertilizers. During the last centuries as well as the medieval period, sheep breeding has also involved a change in vegetation, pointed by lows percentages of trees and shrubs pollens associated with an increase in triterpenyl acetates fluxes, and an increase of the average chain length of n-alkanes, molecular biomarkers of an open area. The combined analysis of molecular biomarkers and pollen allows tracing the compared evolution of farming activities during the two major phases of anthropisation. The impacts of these activities on local ecosystems allow us providing a new insight into the Norse history in Greenland, completing the archaeological and palaeoenvironmental knowledge.