Unraveling short- and long-term carbon cycle variations during the Oceanic Anoxic Event 2 from the Paris Basin Chalk

摘要

The Oceanic Anoxic Event 2 (OAE2, ca. 94.6 Ma) is one of the major perturbations in the global carbon cycle during the Phanerozoic. Stable carbon isotopes (delta C-13) from marine and continental sedimentary environments document this carbon cycle perturbation with a pronounced ( > 2 parts per thousand) positive carbon isotope excursion (CIE). Although the OAE2 stratigraphic interval has been intensively studied in terms of paleoceanography and paleoclimatology, several climatic and carbon cycle aspects are not yet well-understood. In particular, cyclic shortterm Milankovitch-scale delta C-13 variations within the OAE2 and their potential implications for the global carbon cycle have been rarely addressed. Here, we present high-resolution (5 cm, similar to 2 kyr) delta C-13 data spanning the OAE2 from the Paris Basin Chalk (Poigny Craie-701 drill-core) to show high amplitude short-term delta C-13 oscillations, superimposed on the major CIE. Time-series analysis indicates that short-term oscillations are astronomically paced, with eccentricity cycles being the most prominent. Orbital forcing of delta C-13 variations is further supported by time-series analysis of the English Chalk (Eastbourne section). We suggest that orbitally paced carbon cycle oscillations were amplified by considerable emission of greenhouse gases from volcanism that caused the overall CIE. Astronomical calibration of the whole OAE2 (the perturbation and recovery phases) from the Poigny record provides a duration equivalent to eight to eight and a half short eccentricity cycles.Cyclostratigraphic correlations among several OAE2 key records indicate the same duration of the whole CIE. However, duration of the interval from the onset of CIE till the Cenomanian-Turonlan boundary (CTB) is significantly different from one basin to another. In particular, a difference of almost two short eccentricity cycles is highlighted between the Anglo-Paris and Western Interior basins. According to cyclostratigraphic approach and correlations, the entry of W. devonense was at least 200 kyr later in the Western Interior Basin (WIB, USA) than in Europe. Key calcareous nannofossil biohorizons (e.g., Quadnmt gartnen) are also stratigraphically upshifted in the WE with respect to the European sections, hence concurring with the hypothesis of a younger CTB in the WIB. We ascribe such significant temporal offset to diachroneity of the CTB, which is likely the result of different, regional biotic responses to the global OAE2 paleoenvironmental perturbation.