Unravelling the impact of the Eocene Thermal Maximum 2 (ETM2) : A high-resolution shallow marine record from Belgium

The early Eocene long-term warming was punctuated by relatively short (50 to 200 kyr) and abrupt warming events, which are used as analogues to understand current anthropogenic global warming. Among these hyperthermal events, the Eocene Thermal Maximum 2 (ETM2) corresponds to an orbitally paced release of 2,600 to 3,800 Gt of carbon at 54.1 Ma. It has primarily been identified in deep oceanic settings, while terrestrial and coastal records of this event remain scarce. Indeed, the ETM2 has only been identified in shallow marine settings in a few locations (Arctic, USA Atlantic coast, Egypt, India, and New Zealand), hindering a full understanding of its environmental impact.

Here, we present a high-resolution multi-proxy record from a newly drilled 25-m-long core in southwest Belgium (Mons Basin), at a paleolatitude of ~40°N, combining Gamma-ray spectrometry, mineralogy (XRD bulk-rock and clays, TEM, grain-size) and organic geochemistry (δ¹³C_org, Rock-Eval^®, and palynofacies). Sedimentological interpretation indicates a siliciclastic tidal shallow marine environment (a few tens of meters water depth). Using an age model based on nannofossils, dinocysts and cyclostratigraphy, the ETM2 is recorded over approximately 2.5 m by a ~1‰ negative carbon isotope excursion (CIE) located within the NP11 nannofossil biozone. The peak of this CIE corresponds, with a slight delay, to an increase in carbonate content and nannofossil abundance, suggesting enhanced primary productivity related to an intensified hydrological cycle and higher nutrient inputs during the ETM2. An increase in detrital input is also suggested by the transition to coarser grain size. After a progressive decline in kaolinite, illite, and chlorite contents in the clay fraction, smectite becomes the dominant clay mineral during the CIE, possibly pointing to a transgressive event in relation with the ETM2, as also suggested by palynofacies and dinocyst assemblages.

This study presents the first high-resolution record of the ETM2 in the coastal environments of the southern North Sea Basin, preserved in the Mons Basin. In these settings, the ETM2 is associated with a deepening trend, possibly related to sea-level rise, as well as with increased primary productivity and detrital inputs, which point to an enhanced hydrological cycle.