An Early Cretaceous lacustrine record: Organic matter and organic carbon isotopes at Bernissart (Mons Basin, Belgium)

The early Cretaceous Bernissart lake deposits (Mons Basin, Belgium) constitute an important historical site in the field of palaeontology. In this study, we focus on the organic matter (OM) content of the 50 m-long borehole core, "Ber 3", drilled at Bernissart. Organic data (Rock-Eval, palynofacies, C/N ratios) and carbon-isotope measurements on bulk OM and selected, isolated wood fragments were collected. The OM distribution exhibits two total organic carbon (TOC) cycles (12 m and 38 m-thick, respectively) that are stacked in a lower frequency cyclic pattern. A general increase of in situ aquatic production and preservation of OM through time is evidenced within the freshwater masses. A spectral analysis of a high-resolution gamma-ray record demonstrates that the sedimentation was controlled by orbitally-induced climatic changes. Precession, obliquity and short- and long-term eccentricity cycles are recognized. The 12 m-thick and the 38 m-thick TOC cycles fit well with a 100 kyr short-term and a 400 kyr long-term eccentricity cycle, respectively. Long-term organic fluctuations are interpreted as resulting from orbitally-induced fluctuating lake levels through time. Carbon-isotope evolution of bulk OM is thought to reflect firstly two local patterns of sedimentation: (1) the changing contribution of in situ amorphous organic matter versus terrestrial OM, and (2), the possible influence of enhanced productivity. The negative δ¹³C_WOOD trend recorded at Bernissart seems equivalent to published coeval δ¹³C_WOOD and δ¹³C_CARB in the Upper Barremian to Lower Aptian. This trend noted at Bernissart is thus regarded as reflecting changes of the carbon-isotope ratios of atmospheric CO₂. Previous presented palynological data suggest a "middle" Barremian to Lowermost Aptian stratigraphic range for the succession at Bernissart. According to this new dataset, we refine the age-assignment for Bernissart: the duration of deposition is now estimated to range between 0.55 and 2.2 myr in the Upper Barremian to Lowermost Aptian.