Marine mollusk shells record the seasonal variations of temperature during the Mid Eocene Climatic Optimum in the Paris Basin

During the Middle-Late Eocene, the Earth transitioned from a greenhouse to icehouse period. Within this period, a warming phase of 500 kyr called MECO (Middle Eocene Climatic Optimum) took place at the beginning of the Bartonian (from 40.5 Ma to 40 Ma - C18n). This event is characterized by a negative shift in the δ¹⁸O profile of benthic foraminifera associated with an increase of 4 to 6 °C in surface and deep ocean waters. The peak of the MECO is also characterized by a short δ¹³C negative excursion at 40.0 Myr during an overall increasing trend of δ¹³C. This positive trend of the δ¹³C curve appears to be related to an atmospheric increase in the pCO², but the causes remains unclear.

Unlike the oceanic domain, few datas exist for the characterization of the MECO in coastal areas. Additionally, important component of the climatic context, such as the seasonal gradient of temperature, remain unknown. To unravel these uncertainties, this work focuses on the nearshore Eocene sedimentary records of the Paris Basin, which presents an important and remarkably well preserved paleobiodiversity of marine mollusk shells. Previous studies have confirmed that the MECO event is well recorded in Bartonian sediments, but due to several uncertainties, its stratigraphic position remains to be specified. Here we present a composite section that spans a stratigraphic interval covering the middle Lutetian (falunière de Grignon outcrop) and the Bartonian (Horizon de Mont-Saint Martin Formation, le Guépelle section and the Sables de Cresnes Formation). Thus, we combine different proxies provided by ¹⁸O, ¹³C and ∆₄₇ analyses of marine mollusk shells sampled in these sections in order to clarify the stratigraphic position of the MECO in the sedimentary succession of the Paris Basin and to constrain the climatic expression of this hyperthermal event in shallow marine environment. 

Isotopic analyses were performed on the shells of 3 Bartonian mollusks species: 2 bivalves represented by Bicorbula gallica and Crassostrea cucullaris and 1 gastropod represented by Torquesia sulcifera. Stable isotope (δ¹⁸O and δ¹³C) results both show a characteristic negative excursion at the end of the Sables du Guépelle formation, in the lower part of the Bartonian. Clumped isotope analyses were performed on some specimens of B. gallica and T. sulcifera in order to better constrain the composition of δ¹⁸O_w throughout the stratigraphic interval studied. These results indicate significant decreases in local δ¹⁸O_w over the lifetime of most individuals, interpreted as large infra-annual variations in salinity. Paleotemperatures calculated from the previously constrained δ¹⁸O_carbonate increase by 4 to 10 °C during the MECO event , while the seasonal temperature variation decreases from 11-13 °C to 8 °C during the negative isotopic excursion of the end of the Sables du Guépelle formation.

Based on these new results, we propose that the MECO is recorded in the top of the Sables du Guépelle formation marked by a warming period and a lower seasonal temperature gradient. These results lead to a better chimio-chronostratigraphic calibration of the Bartonian deposits of the Paris Basin.