Unravelling the PETM record in the Sparnacian facies of NW Europe: new data from the north-eastern Paris Basin
Résumé
The geological archives record "hyperthermic" crises, along with their consequences on the biota and physical environment. Among these, the PETM (Paleocene-Eocene Thermal Maximum) is considered as the closest analogue to the current climate crisis due to its global character and the speeds at which the CO2 rate and average temperatures increased. Some 55.8 Ma ago (Aubry et al., 2007), it affected the Earth for a period of almost 200,000 years; the terrestrial and marine paleoenvironments were marked by a negative δ13C anomaly (or Carbon Isotopic Excursion, CIE) coinciding with a negative δ18O anomaly indicative of a drastic temperature rise (+ 3 to + 8 °C). The causes envisaged to explain the major global disturbances of the atmosphere and oceans are linked with the massive emission of 12C-enriched greenhouse gases whose origins are still debated and probably multiple. Moreover the PETM left a deep and long-lasting imprint on the living animal and plant world, both terrestrial and marine. The preserved paleoenvironments of the sedimentary record also show a recrudescence of clastic influx or are distinguished by specific sediments such as coal and lignite, phosphates, black shales, diatomites, etc. The shallow to deep marine environments from all the paleolatitudes have been studied intensively, the data collected allowing to refine climate models and ocean/atmosphere responses. However, the terrestrial record was seldom examined and very few studies aimed at checking if the drastic rises of greenhouse gases and temperature had a real impact on the fluvial, lacustrine, palustrine sediments and on the paleosols and weathering profiles development during the PETM. Apart notable paleontological studies, its impact on the terrestrial realm at a regional scale and on different interconnected contemporary paleoenvironments has probably not been studied and integrated enough (Zachos et al., 2008). We propose such a regional study in the Sparnacian facies of the Paris Basin. To ensure correlation of the events and processes identified with a confidence level as precise as possible, high resolution temporal framework is essential. Historically, the Paris and adjacent basins are the cradle of stratigraphy, where the notion of "Sparnacian" took shape (Dollfus 1880), pointing terrestrial to brackish deposits with particular facies and faunas, interstratified between two easily distinguishable Late Paleocene (Thanetian) and Early Eocene (Ypresian) mainly sandy and marine formations. Since that time stratigraphy has evolved, especially regarding the Paleocene and Eocene epochs, and we refer to the lithostratigraphies of Aubry et al. (2005) and Steurbaut (1998) for the Paris and Belgian Basins. In Avesnois, close to Belgium, the Paleocene and Eocene lithostratigraphy has been revised thanks to a geological mapping project supported by drillings (Quesnel, 2006). Detailed sampling and various analyses have been performed: granulometry and XRD mineralogy, heavy mineral assemblages, carbonate and organic carbon contents, biostratigraphy, palynofacies, pyrolyse rock-eval, and chemostratigraphy (d13C of the dispersed organic matter). In the AVE 007 drilling (Mormal Forest), the CIE has been identified in a lignitic clay. The clay content is dominated by the illite-smectite mixed layers, with 10 to 20 % of illite and kaolinite. The palynological study of the pollen and spores assemblages confirms the Early Eocene age. The Paleocene markers are absent and using the palynozonation of Roche et al. (2009), the lignitic clay can be correlated to the lower part of the Tienen Fm and to the SP3 to SP4 units of the Cap d'Ailly section. The depositional environment is palustrine-lacustrine, with sporo-pollen fluvial inputs from the hinterland in a humid subtropical climate. The lignitic clay overlies fluvial flint gravels and sands, above the Coniacian chalk. Those clastic deposits contain mainly illite-smectite mixed layers, a few kaolinite and illite and are richer in pyrite downwards. Paleocene pollens and dinocysts are present. The onset of the CIE is recorded at the top of this sandy unit, below the lignitic clay. Those fluvial flint gravels and sands are widespread in Avesnois, and named “Cailloutis à silex de Mormal” (new name) and “Sables et Grès du Quesnoy”. The lignitic clay corresponds to a palustrine deposit, filling the fluvial channel after the river bed migration. Similar units are well known in northern France and Belgium, they belong to the Upper Landenian fluvial sands, often display cross stratification, paleoweathering (oxidation and/or silcrete), lignite or marl lenses, vertebrate fauna and sometimes a rich flora in silicified slabs or beds. They can be attributed to the Tienen Fm. Similar sandy, clayey and lignitic units are also recognized southwards in the Paris Basin, in the first clastic units of the Mortemer Fm. In Northern France and Belgium those terrestrial units often cap or incise Upper Thanetian marine sands. In Avesnois the latter are named “Sables verts de l'Avesnois” and correlated to the “Sables de Grandglise” (NP8) and “Sables de Bois Gilles” (NP9). In the western part of the Avesnois, the AVE 31 drilling has yielded a shallow marine shelly sand (“Falun de Viesly”), with Upper Thanetian benthic foraminifera (P4c to P5), and a shelly clay (“Argile de Louvil”), containing Thanetian benthic foraminifera (P4). They are overlain by a sandy laminated clay (“Tuffeau de Valenciennes”), then a laminated silt at the top. In the AVE007 drilling, the lignitic clay is also overlain by a silty laminated unit, containing the same clay minerals as the clay plus chlorite and vermiculite at the top. Lower Ypresian dinocysts (Wetzeliella sp.), phytoliths, rootlets moulds and agglutinated foraminifera similar to those contained in the Orchies Clay (NP10 – NP 11?) are present. That silty unit is widespread in Avesnois and Valenciennois. We name it “Silt et Sablon de l'Avesnois” and we correlate it to a part of the Kortrijk Clay Fm in the Ieper Gp. The new units described around the Paleocene-Eocene boundary in Avesnois allow correlation between the Belgian and Paris Basins and help to precise the landscape evolution during this critical interval. The PETM record has also been recently identified around 80km southwestwards, at Sinceny (Aisne), a key locality for the Sparnacian facies. The data obtained in this drilling will be presented and compared to those obtained in Avesnois and integrated in the Paris Basin stratigraphic framework.