Carbon isotope excursions during the Oxfordian: multi-proxy constraints on carbon cycle dynamics

The Oxfordian interval is characterized by a long-term (~6 Myr) increase in carbon isotope values, punctuated by several short-lived (<1 Myr) positive carbon isotope excursions (CIEs) occurring in the lower Oxfordian, and in the middle Oxfordian. These excursions have only been recognized in a limited number of sections and their spatial extent, stratigraphic reproducibility, and paleoenvironmental significance remain poorly constrained, and their potential relationship to oceanic anoxic events (OAEs) remains uncertain.

Here, we present a high-resolution, multi-proxy chemostratigraphic dataset from shallow-marine Oxfordian successions of northwestern Europe, integrating inorganic and organic carbon isotopes (δ¹³C_inorg and δ¹³C_org), palynofacies analysis, and Rock-Eval pyrolysis. The dataset combines subsurface data from the Konrad #101 borehole (southeastern Lower Saxony Basin, northern Germany) with new outcrop data from the northern Paris Basin (Normandy, France). Both successions are constrained by robust biostratigraphic frameworks, enabling detailed intra- and interbasinal correlations.

Our results reveal pronounced and reproducible carbon isotope trends, including a ~3.0‰ positive CIE recorded in both δ¹³C_inorg and δ¹³C_org within the lower to middle Oxfordian interval. Comparison with available records from Europe, western Asia, and the Gulf of Mexico suggests that these excursions may reflect regionally synchronous perturbations of the exogenic carbon cycle, although the degree of global synchronicity remains equivocal. The integration of geochemical and palynofacies data provides new insights into the paleoenvironmental context of these events by demonstrating that the observed carbon isotope fluctuations are not driven by changes in organic matter preservation or mixing of organic matter sources (e.g., marine versus terrestrial inputs). This multi-proxy approach allows a critical assessment of whether Oxfordian CIEs constitute robust chemostratigraphic markers and whether they can be plausibly linked to episodes of widespread marine oxygen depletion.