Paleocene-Eocene Thermal Maximum impacts on terrestrial environments. Insights from the organic matter evolution in the Vasterival section (Dieppe-Hampshire Basin, France)
Résumé
The Paleocene-Eocene Thermal Maximum (PETM, 55.8 Ma [1]) is regarded as one of the most rapid global warming of the Cenozoic era, with temperature increase of 4-8°C in about 10-20 ka [2]. Thus, it is often proposed as a potential analogue of future climatic conditions expected in the IPCC screenplays. The PETM is recorded in both marine and continental deposits by an abrupt negative Carbon Isotope Excursion (CIE) associated with other sedimentary and biological anomalies. The consequences of the PETM on terrestrial environments are less documented than in marine ones. This limits our regional- and global-scale understanding of the impact of such a climate change on continents and the associated response of ecosystems. This study focuses on the Vasterival section (Seine- Maritime, Upper Normandy) located in the southern part of the Dieppe-Hampshire Basin, in which the PETM is attested by a negative 13Corg shift in the organic matter (OM), and confirmed by the stratigraphic record in this locality belonging to the Cap dAilly composite section [3]. The 2 m thick section, which presents a notably well preserved OM, is mainly constituted by terrestrial sediments from lacustrine to coastal swamp environments, and in which OM-poor clays are followed by OM-rich clays, centimetric lignite beds and clays with roots evidences. The uppermost part of the section is constituted by 50 cm thick lagoonal clay with shell debris that records the Apectodinium acme. Global organic geochemical, palynofacies and isotopic analyses were performed on thirty samples. The total organic carbon of this section is ranging from 0.5 % for OM-poor-clays to 45 % for lignite levels. Hydrogen Index (from 6 to 210 mg HC/g TOC), Oxygen Index (from 80 to 630 mgCO2/g TOC) and Tmax values (from 410 to 430°C) show that the OM is of Type III (terrestrial higher plants), and immature. These results are reinforced by palynofacies observations that show a large amount of ligno- cellulosic phytoclasts in most of the samples. In the uppermost shale the presence of many Apectodinium species would suggest the continuity of the PETM in marine deposits. Thus, the continental deposits would represent a time interval that includes the uppermost Paleocene and the basal part of the PETM. Lipid biomarkers extracted from twenty five samples were quantified and their hydrogen and carbon isotopic composition were determined by GC-irMS. Important changes in palynofacies, biomarker assemblages and compound-specific isotopic data are coincident with the CIE onset interval. This is consistent with an important environmental modification in the Vasterival area during the Early Eocene that could be linked to the PETM climatic change. References [1] Aubry, M. P., Ouda, K., Dupuis, C ., Berggren, W. A., Van Couvering, J A., and the Members of the Working Group on the Paleocene/Eocene Boundary, 2007. The Global Standard Stratotype-section and Point (GSSP) for the base of the Eocene Series in the Dababiya section (Egypt). Episodes 30 (4): 271-286. [2] Zachos, J. C., Pagany, N., Sloan, L., Thomas, E., Billups, K., 2001. Trends, rythms, and aberrations in global climate 65 Ma to Present. Science 292: 686 693. [3] Dupuis, C., Steurbaut, E., De Coninck, J. and Riveline, J., 1998. The Western Argiles à Lignites facies. In: M. Thiry and C. Dupuis (Eds.), The Paleocene/Eocene boundary in Paris basin: the Sparnacian deposits. Field trip guide. ENSMP Mém. Sc. De la Terre, 34, 60-71.