Paleocene-Eocene Thermal Maximum consequences on terrestrial environments. Insights from the evolution of organic matter in the Vasterival section (Paris Basin, France)

The Paleocene-Eocene Thermal Maximum (PTEM) is often proposed as an analogue of future climatic conditions expected in the screenplays provided by the International Panel on Climate Change (IPCC). The PETM is characterized by a 3°C estimated rise of global temperature and is recorded in marine and terrestrial deposits by a carbon isotopic excursion (CIE). Most of published works related to this paleoclimatic crisis focus on marine sediments. This presently limits our regional- and global-scale understanding of the impact of such a climate change in terrestrial environments and the deciphering of the ecosystems response. This study focuses on the Vasterival section (Seine-Maritime, Upper Normandy, France) located in the NW of the Paris Basin, and in which the PETM is recorded in the organic matter (OM) by the d13C negative shift of the CIE. The 2m thick section is mainly constituted by terrestrial sediments deposited in fluvial and lacustrine to coastal swamp environments and in which OM-poor shales are followed by OM-rich shales, centimetric lignite beds, paleosols, and two beds containing carbonate nodules. The uppermost part of the section is constituted by a 50cm thick shale with shell debris that attests to a lagoonal influence. This section is constantly refreshed by a little stream, thus facilitating the sampling of fresh material, notably a well preserved OM. Global geochemical, palynofacies and isotopic analyses focused on thirty samples, and seventeen lipid extracts were analyzed by gas chromatography-mass spectrometry. The association of these methods allows us to characterize the OM of the different deposits encountered. Sulfur contents are particularly high (up to 15% of S), thus indicating an euxinic depositional environment. Total Organic Carbon (TOC) varies from 1% for OM-poor shales and paleosols up to 45% for lignite levels. From 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) we can show that the OM is of Type III (terrestrial higher plants), and immature. These results are reinforced by palynofacies observations, as a large amount of ligno-cellulosic phytoclastes is present in most of the samples. In the uppermost shale the continuity of the PETM in marine deposits can be inferred by the presence of many Apectodinium species. Thus, the terrestrial deposits would represent a time interval that includes the uppermost Paleocene and the basal part of the PETM. Molecular analyses underline the contribution of terrestrial higher plants by the presence of long chain n-alcanes with odd-over-even predominance, and Onocerane I, a compound rarely detected in sediments, attests to a contribution of lycopods or plants affiliated to the genus Pseudofagus. A notable bacterial contribution is also inferred by the presence of hopanoids in the extracts. This is confirmed by organic petrography results that reveal alginites laminae, often interpreted as a witness of well-preserved bacterial and/or algal mats. From our early results we show that significant environmental changes are recorded during the CIE. High resolution analyses and complementary analyses (X-ray tomography, isotopic, thin section petrography...) are currently in progress. This approach is complemented by a sedimentological study and they both will be extended to other sites along a transect crossing the terrestrial to lagoonal paleoenvironments of the Paleocene-Eocene transition in the Paris Basin. They aim at defining possible spatial heterogeneities in the PETM's impact on terrestrial environments and ecosystems, at a regional scale. To the west of the transect, the Cap d'Ailly sections and core will be first studied, because of their position very close to the Vasterival site, the relative richness in outcrops and the continuation of the PETM recorded in around 10m of lagoonal deposits, comprising various facies often OM rich: tidal fine sand, silt and clay, shell beds, and few cemented carbonate beds