Environmental and hydrological changes at the Paleocene-Eocene boundary in the terrestrial sediments of the Cap d'Ailly core (Upper Normandy, France)
Résumé
The Paleocene-Eocene Thermal Maximum (PETM, 55.8 Ma, Aubry et al., 2007) 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. Thus, it is often proposed as a potential analogue of future climatic conditions expected in the screenplays provided by the International Panel on Climate Change (IPCC). The PETM is recorded in both marine and terrestrial deposits by an abrupt negative Carbon Isotope Excursion (CIE) associated with other sedimentary and biological anomalies. The consequences of the PETM in terrestrial environments are less documented than in marine ones. This limits our regional- and global-scale understanding of the impact of such a drastic climate change on continents and the ecosystems response. This study focuses on the Cap d'Ailly core (Seine-Maritime, France) drilled by the BRGM in 2008 in the southern part of the Dieppe-Hampshire Basin, an area where the PETM has already been attested in a few sections by the presence of the CIE and the Apectodinium acme (Magioncalda et al, 2001; Dupuis et al, 2006; Storme et al, 2012). These sections consist of terrestrial and lagoonal organic matter-rich deposits that are typical of the Sparnacian facies (latest Paleocene - earliest Eocene; Aubry et al., 2005). The lower part of the "Sparnacian" is dominated by fluvial, lacustrine and swamp deposits such as marls and lignites. It is overlain by two units of lagoonal deposits rich in shell debris separated by terrestrial deposits. The uppermost part of the section is constituted by a 1 m-thick marine clay rich in glauconite (Dupuis & Steurbaut, 1987). Global organic geochemical, palynofacies and isotopic analyses reveal that the onset of the CIE is located in the lowermost swamp deposits. The total organic carbon ranges from 0.02 % for paleosols to 43 % for lignite beds. The organic matter is mainly of Type III (terrestrial higher plants) and immature. These interpretations are reinforced by palynofacies observations that show a large amount of ligno-cellulosic phytoclasts in many samples. In marine and lagoonal deposits, the presence of many Apectodinium specimens would suggest the continuation of the PETM up to the top of the Sparnacian deposits, although this has to be confirmed by carbon isotopes analyses. Palynofacies, distribution of specific lipid biomarkers as well as their hydrogen and carbon isotopic compositions show important changes coincident with the CIE onset. This is consistent with important environmental and hydrological changes in the Cap d'Ailly area during the earliest Eocene that could be linked to the PETM climatic change.