Gilles Bossuet

Land use change, soil erosion and alluvial dynamic in the lower Doubs Valley over the 1st millenium AD (Neublans, Jura, France)

Abstract Geochemical and particle size analyses, surface scanning magnetic susceptibility, microscopic charcoal counting and pollen analysis, have been carried out on two cores from the lower Doubs valley in order to reconstruct the land use history. The interpretation of anthropogenic pollen indicators and micro-charcoal deposits is discussed in relation to the lithological variation of sedimentary accumulation and evolution of the settlement. The environmental changes which occurred during the 1st and the 6th century AD reflect the impact of human activities on the landscape in the immediate surroundings of the site. Multiproxy indicators indicate that soil erosion as a consequence of agricultural activities including fire practices. A reactivation of hydrologic flow of the river recorded in the sedimentary sequences attests to a larger environmental perturbation between 1st–3rd and 7th–8th century AD.

SEDIMENT YIELD DURING LATE GLACIAL AND HOLOCENE PERIODS IN THE LAC CHAMBON WATERSHED, MASSIF CENTRAL, FRANCE

A sediment budget for the Late Glacial and Holocene periods was calculated for the Lac Chambon watershed which is located in a formerly glaciated temperate crystalline mountain area. It appears that over 15 500 years: (1) 69 per cent of eroded particles have been displaced by gravity processes and then stored within the watershed, compared to 31 per cent that have been displaced by running water and evacuated outward; (2) the mean mechanical erosion due to gravity processes on the slopes amounted to 16·1± 6 m and only developed on a quarter of the watershed surface, whereas the mean mechanical erosion due to running water amounted 1·24± 0·37 m and involved the whole watershed surface. The mean sediment yields due to gravity processes on slopes were 2300 ±1360, 1770± 960 and 380±100 m 3 km -3 a -1 , respectively, for basalts, and basic and acidic trachyandesites. Values of sediment yield due to running water were 49 ±15, 120 ± 36 and 79± 24 m 3 km -2 a -1 , respectively, during the Bolling‐Allerod, the Younger Dryas and the Pre-Boreal‐Boreal periods. They were 56±17 and 166± 50 m 3 km -2 a -1 during the Sub-Atlantic period before and after 1360 a BP, respectively. These values reflect variations in the natural environment and the impact of human-induced deforestation.

Landuse and soil degradation in the southern Maya lowlands, from Pre-Classic to Post-Classic times: The case of La Joyanca (Petén, Guatemala)

This work focuses on the impact of Maya agriculture on soil degradation. In site and out site studies in the area of the city of La Joyanca (NW Petén) show that “Maya clays” do not constitute a homogeneous unit, but represent a complex sedimentary record. A high resolution analysis leads us to document changes in rates and practices evolving in time in relation with major socio-political and economic changes. It is possible to highlight extensive agricultural practices between Early Pre-classical to Late Pre-classical times. Intensification occurs in relation with reduction of the fallow duration during Pre-classic to Classic periods. The consequences of these changes on soil erosion are discussed. However, it does not seem that the agronomic potential of the soils was significantly degraded before the end of the Classic period.

Magnetic prospecting of diachronic structures (antiquity to First World War) on the site of the sanctuary of Ribemont-sur-Ancre (Somme, France)

The site of the sanctuary of Ribemont-sur-Ancre (Somme, France) is of major scientific interest for the knowledge of the Celtic civilization in Europe. It also presents other peculiarities: two clearly identifiable periods of occupation—the first one as early as third century BC- to the second century AD and the second one from 1914–1918. In the area disrupted through military operations, the remains of several epochs are intermingled. In order to obtain a precise knowledge of the space time organisation of each occupation, a magnetic survey was carried out. The comparison of geophysical results with the aerial photographs shot during the war allows an exact determination of the origin of the anomalies. It becomes possible to separate the magnetic anomalies which are contemporary, anterior or probably posterior to the conflict of 1914–1918. The range of magnitude and the sign of polarity of anomalies appear as a means to facilitate the identification of the features of each period. Among the anomalies detected, some of them, such an the elliptic enclosure, have been identified by the excavations as unknown archaeological remains. Generally speaking, the ability of the magnetic method for surveying any structure in such a disrupted environment suggests a more frequent use in prospecting of the sites which were battlefields in Historic periods. Copyright

Détection par méthode radar de niveaux anthropiques du Néolithique final sous le niveau actuel du lac de Chalain (France)

Abstract Several GPR profiles arc carried out on Lake Chalain (Jura) in order to detect several anthropic layers dating from Middle Neolithic times. The geometry of the sedimentary deposits is imaged by several GPR reflectors. Several auger borings allow the GPR reflectors to be correlated with anthropic levels (strata interleafed within lacustrine chalk). In particular, the oldest and finest (5 cm) level (of Horgen culture from the 32nd century BC) is detected. This experiment proves that it is possible to characterize the 3-dimensional geometry of the deposits from Lake Chalain built up in Middle Neolithic times.

Original Detection Methods for Tephra Layers and Cryptotephras

The building of independent and accurate chronologies is crucial to correlate environmental and climatic changes as reflected in biotic as well as abiotic signals from continental lacustrine sediments. The use of tephras, which are instantaneous and synchronous atmospheric deposits, as time-parallel markers, allows independent, high-precision regional and inter-regional correlations to be established. The development of effective, rapid methods to detect tephra layers in sedimentary records of various compositions is a challenge. Many traditional methods for the detection of tephra layers, such as regular sampling or magnetic susceptibility measurements, have shown their limits. Regular sampling takes a long time, and finding tephra layers is an uncertain process. Moreover, magnetic susceptibility is ineffective when tephra layers are composed of volcanic glass shards of differentiated magma composition. This study proposes three original methods to detect tephra layers: XRF, CT imaging, and quantitative sulphur analysis. XRF enables tephra layer composition elements and sediment to be differentiated. CT imaging may be used to observe 3D sedimentary structures. Sulphur analysis allows the environmental impact of degassing in volcanic eruptions to be examined. These methods can be used successfully to detect tephra layers of various chemical compositions in both peaty and carbonated deposits.

Un enregistrement à haute résolution des variations paléohydrologiques au paléolac du Locle dans le Jura suisse, de la fin du Dryas récent au milieu de l'Holocène

Abstract A high-resolution record of palaeohydrological changes from Late Younger Dryas to Mid-Holocene is reconstructed from palynological and sedimentological investigations of lacustrine infillings at the palaeolake Le Locle in the Swiss Jura. The Late Younger Dryas is characterized by a general trend towards a fall in lake-level and a large climatic instability. The Early to Mid-Holocene period shows two large lowering phases developing during the Preboreal and the first half of the Older Atlantic, Each of these phases was interrupted by a short-term rise in lake-level, and followed by a major and durable high lake-level event occurring during the Boreal and the second half of the Older Atlantic. This regional pattern of climatic changes presents similarities with climatic changes recorded both in western and central Europe and in the Atlantic ocean.