Louis Bonnet

Holocene climatic changes in the western Mediterranean: installation of the Mediterranean climate

Abstract In the western Mediterranean during the Holocene, summer drought occurred progressively from south to north. In south eastern Spain, between latitudes 36 and 39 °N, it prevailed from about 10 000 BP and a first arid episode is recorded around 7,000 yr BP. A Mediterranean climate reached a latitude 39–40 °N near 7 000 BP and near 41 °N before 5 000 BP. The climatic change that began near 4 500–4 200 BP, resulted in changes in the seasonal pattern of precipitation and the installation of summer drought between about 3 300 and 1 000 BP in the zone 40 °–44°N.

Sequence stratigraphy and biological signals: statistical studies of benthic foraminifera from Liassic series

Abstract A study of benthic Foraminifera from the Toarcian in Quercy (Aquitaine Basin, France) shows that micropaleontological assemblages are influenced by sea level changes. The percentages of coiled, and straight Nodosarridae (Lenticulina, Citharina) differ during periods of low sea level to high sea level. Statistical analysis of all the taxa in the successive levels of a section (Multiple factorial Analysis of Correspondences, methods of automatic clustering) show that there is a perfect distribution of the samples into groups corresponding to the type of systems tract: differences within the foraminiferal benthic biocoenosis allow the successive systems tracts to be separated. The first axis in the Multiple Analysis of Correspondences coincides with a gradient of biotic capacity and seems to be connected with the sea level changes. The second axis could be linked with depth. The renewal rate of taxa and the factorial diversity of the sampling levels show fluctuations associated with 4th, 3rd and 2nd order variations of the sea level. These connections are caused by intricate processes with ecological, biogeographical and bioevolutionary influence. However, the micropaleontological assemblages can be good indicators of systems tracts and unconformities. This is particularly important within deposits that have homogeneous lithology.

Fluctuations of environmental factors as seen by means of statistical analyses in micropaleontological assemblages from a Liassic series

A comparative statistical study of foraminiferal and ostracode assemblages in Domerian marls from the Aquitaine basin (France) shows that the taxonomic structure of the populations fluctuates through time. Multiple Correspondences Analyses (M.C.A.) processing presence-absence data (table of taxa in the successive sample levels) were conducted. The first axis (F1) of this M.C.A. is related to faunal diversity and expresses a biotic capacity gradient. The F2 axis coincides with accomodation space and can be used to identify systems tracts and sequence boundaries. The F3 axis is interpreted as representing an oxygenation gradient. Short-term fluctuations at the systems tract or half systems tract level appear in the residuals of regressions and the distance between samples.

Opportunistic development of Ogmoconcha amalthei (ostracod) in the lower Liassic of Quercy (SW France): an indicator of sedimentary disturbance

Abstract Ostracod assemblages have been studied in the sedimentary units of the lower Liassic of Quercy (SW France), at the Carixian–Domerian boundary (Pliensbachian). Ogmoconchella transversa was found to decrease in frequency towards the base of the Domerian while at the same time the O. pseudospina population increased. It is also noteworthy that the occurrence of small-sized species coincides with dominance of species of the Ogmoconcha amalthei group. This large-sized species makes up half or more of the ostracod assemblages in the periods of environmental disturbance indicated by sequence stratigraphy. The biocoenotic indices (number of taxa, faunal turnover and equitability) seem to confirm environmental disturbance at these levels. Correspondence analysis (CA) distinguishes such samples from those in which Ogmoconchella and Polycope species are more common. The appearance or development in the Domerian of species that were rare or absent throughout the Carixian indicates a change of environmental conditions. The opportunism of species of the Ogmoconcha amalthei group can be used to determine periods of disturbance in sedimentary units.

La comparaison à un niveau de référence : un nouveau traitement statistique de données paléoécologiques

The successive levels of a section are compared independently with a reference level (referential) having well identified characters. The comparison is realized by means of a distance or dissimilarity index. The choice of the index as function of the nature of data is argued, as well as the choice of the referential. This new approach is applied to the study of two examples from liassic series of southern France. The treatment of micropaleontological or faciological datas reveals cycles that are coincident with 3rd order eustatic cycles.