New insights into global carbon cycle disturbances: a shallow marine record of the Middle Oxfordian Excursion (MOxE) in the Lower Saxony Basin

The Oxfordian period is characterized by a long-term (ca. 6 Myrs) trend of increasing stable carbon isotope values, punctuated by three short-lived (ca. <1 Myrs) carbon isotope excursions (CIEs) at the Callovian/Oxfordian boundary, in the lower Oxfordian (Quenstedtoceras mariae ammonite zone), and in the middle Oxfordian (Gregoryceras transversarium ammonite zone), also known as the MOxE. This pattern is evident worldwide in both organic and inorganic carbon (δ¹³C_org and δ¹³C_carb) in terrestrial and shallow marine environments, indicating recurrent global carbon cycle disturbances affecting the entire ocean-atmosphere system. However, previous sedimentological and chemostratigraphic studies on Oxfordian strata in the Lower Saxony Basin (LSB) of northern Germany, have failed to identify the three CIEs cited above, hindering their correlation with the global carbon isotope record. In this study, we provide, for the first time, a high-resolution δ¹³C_carb record revealing the MOxE expressed in a positive CIE of 3.8‰. We collected data from drilling core samples of the Korallenoolith Formation in the Konrad 101 borehole, located in the southwestern part of the LSB. This exploration drilling is biostratigraphically well constrained, unlike many other sections in the LSB that are characteristic of shallow tropical marine depositional settings. This particularity enables us to compare the Konrad 101 δ¹³C_carb pattern with other localities distributed worldwide incl. Europe, western Asia, and the Gulf of Mexico. Our high-resolution δ¹³C_carb reflects a major synchronous change in the exogenic carbon cycle, with no satisfying explanation so far for the triggering mechanisms, in the context of already published datasets.