Pierre Giresse

Le linteau en marbre blanc de l’eglise abbatiale de Saint-Genis-des-Fontaines, dans le sud du Rou... more Le linteau en marbre blanc de l’eglise abbatiale de Saint-Genis-des-Fontaines, dans le sud du Roussillon, est reconnu comme la plus ancienne sculpture romane datee de l’Occident (1020-1021). Il presente de fortes analogies stylistiques avec le linteau et la table d’autel de l’eglise voisine de Saint-Andre-de-Sorede. Ce sont ces trois pieces sculptees qui ont ici fait l’objet d’etudes petrographiques et isotopiques afin d’identifier la provenance des marbres dont elles sont composes. Contrairement aux premieres idees, il ne s’agit pas de marbres pyreneens, Saint-Beat ou plus localement Ceret, mais de marbres de Carrare. Cette conclusion s’appuie a la fois sur les caracteres texturaux des cristaux calcitiques et sur les rapports isotopiques δ18O et δ13C. Ces derniers coincident avec le champ pourtant assez restreint des marbres de Carrare tel qu’il a ete defini dans les graphiques de caracterisation des marbres antiques. De plus, le caractere tres groupe des points obtenus laisse presumer d’une meme origine pour tous ces marbres roussillonnais. Ces resultats s’inserent dans un schema d’ateliers narbonnais qui œuvraient sur des materiaux importes de Rome ou encore sur des reemplois de monuments antiques de la cite.

Understanding the 2500 yr BP rainforest crisis in West and Central Africa in the framework of the Late Holocene: Pluridisciplinary analysis and multi-archive reconstruction

Global and Planetary Change, 2020

Abstract Numerous palaeoclimatic and prehistoric reconstructions have been reported at both the l... more Abstract Numerous palaeoclimatic and prehistoric reconstructions have been reported at both the local scale and across the very large area of tropical Central Africa, clearly highlighting the Late Holocene Rainforest Crisis (LHRC) which mainly developed from ca. 2500 to 2000 yr BP. The broad synchroneity of this interval is striking and has been revealed in many different deep lakes from humid or montane forest, lakes or swamps marginal to tropical forest, and swamp forests where the oscillations of the surface water table produce temporary emersions. In parallel, a chronological review is presented of the Bantu arrival in these areas, including indicators for burning, metallurgy and land clearance. Nevertheless, evidence of human occupation, such as artefacts or deposits containing charcoal is exceptional and generally absent in most areas. Although the archaeological data exhibit a gradual southward densification of human occupation throughout Central Africa, the increase of settlements clearly began after 2350 yr BP, not before 2500 BP. However, some authors have interpreted the geochemical signal of increased erosion in the Congo Basin or the opening up of the forest around Lake Barombi Mbo in Cameroon as being attributed to agricultural clearance, or even the supply of charcoal required for metallurgy. In short, these early Bantu settlers (in such modest densities) may have been responsible for some local landscape degradation (clearance, fire, metallurgy), but these same settlers could not, under any circumstances and across all Central Africa, be held responsible directly or indirectly for the synchronous changes of lake levels, draining vast swamps and opening up of the tropical forest canopy, which was due to an increased dry season, while the recovery with the recolonization of light demanding species, including oil palms (Elaeis guineensis) assisted by dispersers such as chimpanzees, was likewise due to natural processes. Therefore, it can be concluded that no data available validate the hypothesis that the major erosion or vegetation destruction ca. 2500 yr BP, was the result of large population movements. Indeed, the evolution of the environments of Central Africa are linked to the natural responses induced by general palaeoclimatic processes, observed synchronously not only in northern and eastern Africa, but globally. The natural recovery and resilience of these forests until the last centuries contrasts with the situation currently being faced.

Le fer et les glauconies au large de l'embouchure du fleuve Congo / Iron and glauconltes In front of the Congo River mouth

Sciences Géologiques. Bulletin, 1985

Les apports a l'ocean en solution et en suspension du fleuve Congo determinent des milieux de... more Les apports a l'ocean en solution et en suspension du fleuve Congo determinent des milieux de sedimentation marine et fluvio-marine ou plusieurs mineralogeneses ferrugineuses se developpent (goethite, berthierine, nontronite potassique et glauconie). L'etude sedimentologique et stratigraphique de la couverture meuble du plateau permet de separer plusieurs phases chronologiques de depot de la fin du Quaternaire. Chacun des paleoenvironnements controle des etapes distinctes de la mineralogenese : (1) vers 18 000 ans B.P. concentrations biologiques et mecaniques des pelotes fecales sur la bordure externe du plateau lors du dernier bas-niveau marin ; ce sont les glauconies les plus anciennes du plateau et les plus proches du pole mica glauconitique ; (2) vers 12 000 ans B.P. accumulation de debris calcaires biogenes ou les grains verts peu abondants sont composes surtout de nontronite potassique ; (3) pendant l'Holocene, les grains verts sont d'autant plus abondants que l'on s'eloigne des zones de sedimentation active ; dans les depots organiques du delta sous-marin, des berthierines sont neoformees alors qu'a distance de l'embouchure des nontronites potassiques se constituent au sein de depots recents. L'analyse est faite a l'echelle globale de la fraction glauconieuse des depots, mais aussi a l'echelle du tri granulometrique, densitometri-que ou de la susceptibilite magnetique et a celle du grain. Les echelles d'observation permettent de preciser le scenario d'une evolution mineralogique d'environ 20 000 ans dont on ne connait ailleurs qu'une resultante globale et actuelle.

Chemistry of smectite and volcanic glass of ODP Site 159-959, supplement to: Giresse, Pierre; Wiewióra, Andrzej (1999): Origin and diagenesis of blue-green clays and volcanic glass in the Pleistocene of the Côte d'Ivoire-Ghana Marginal Ridge (ODP Leg 159, Site 959). Sedimentary Geology, 127(3-4),...

The Ocean Drilling Program (ODP) Site 959 was drilled in the northern border of the Côte d'Iv... more The Ocean Drilling Program (ODP) Site 959 was drilled in the northern border of the Côte d'Ivoire–Ghana Ridge at a water depth of 2100 m. Pleistocene total thickness does not exceed 20 m. Winnowing processes resulted in a low accumulation rate and notable stratigraphic hiatuses. During the Late Pleistocene, bottom circulation was very active and controlled laminae deposition (contourites) which increased the concentration of glauconitic infillings of foraminifera, and of volcanic glass and blue-green grains more rarely, with one or several subordinate ferromagnesian silicates. Volcanic glass generally was X-ray amorphous and schematically classified as basic to intermediate (44–60% SiO2). Opal-A or opal-CT suggested the beginning of the palagonitisation process, and previous smectitic deposits may have been eroded mechanically. The blue-green grains presented two main types of mineralogic composition: (1) neoformed K, Fe-smectite associated with zeolite (like phillipsite) and un...

Quaternary Glauconitization on Gulf of Guinea, Glauconite Factory: Overview of and New Data on Tropical Atlantic Continental Shelves and Deep Slopes

Minerals

For a long time, particular attention was paid to glauconitization in the surficial sediments lyi... more For a long time, particular attention was paid to glauconitization in the surficial sediments lying on the outer continental shelves of present oceans. Subsequently, the processes observed and analyzed may have served as models for studies of glauconite in Cenozoic or even Mesozoic shelf deposits. Access to the sedimentary domains of deep oceans, particularly those of contouritic accumulation fields, has made it possible to discover unexpected processes of glauconitization. Thus, the long-term prevalence of control using fairly high-temperature water has become obsolete, and the prerequisite influence of continental flows has come to be considered on a new scale. Frequently, sediments from contouritic accumulation provide a condensed and undisturbed sedimentary record without periods of sediment erosion. Glauconitic grains could possibly integrate the signatures of bottom-water masses over prolonged periods of time, which, while preventing their use in high-resolution studies, would...

Pierre Giresse

Uploads

Papers by Pierre Giresse

Log In