Carbon cycle during the late Aptian–early Albian OAE 1b: A focus on the Kilian–Paquier levels interval

Abstract

The Oceanic Anoxic Event (OAE) 1b took place over a protracted time interval during the Aptian/Albian boundary interval, at the dawn of the mid-Cretaceous climatic optimum. OAE 1b is characterized by the occurrence of several sub-events recorded by organic matter-rich levels, which can be traced on regional-to-global scale. Previous studies have highlighted that the climax of the event occurred around the Kilian – Paquier interval, with this latter sub-event being the most extended and pronounced stratigraphic interval. Numerous studies on OAE 1b have however only focused on high-precision investigations of the Kilian and/or Paquier levels, leaving vast uncertainties about the environmental changes and their drivers during the entire OAE 1b, and hence also on the mechanisms leading to the formation of the sub-events themselves. In this study, we have performed a high-resolution multi-proxy analysis of the Briers section, a well-exposed section in the Blue Marls Formation of the SE France Vocontian Basin, continuously covering the Kilian – Paquier interval. Pyrolysis analyses show that most of the organic matter in this section is immature and of continental origin, averaging 1.5% TOC. The Kilian and Paquier levels are characterized by higher TOC values and a substantial increase in the amount of marine organic matter. Comparing TOC values with changes in Al or Ti concentration (proxies for continental weathering) and Hydrogen index (HI values, tracer for the type of organic matter) reveals that the background long-term change in TOC is linked to change in the continental organic matter delivery to the Vocontian Basin tied to changes in continental weathering rates. Sporadic increases in TOC values associated with the Kilian, HN 8, and Paquier levels, are most likely the result of short-lived events of increased marine primary productivity and organic matter preservation superimposed on the background influx of continental organic matter. A high-resolution bulk organic matter carbon isotope record shows that, apart from the Paquier level, all the fluctuations observed in the carbonate carbon isotope ratios are also mirrored in the organic matter record, although with higher amplitudes. This discrepancy in amplitude can be resolved by correcting the bulk organic matter carbon isotope record for fluctuations in the type of organic matter, demonstrating that both oceanic and atmospheric reservoirs were affected by similar carbon isotope fluctuations, which were hence of global extent. The abnormal bulk organic matter carbon isotope record of the Paquier level further confirms the large geographical expansion of unusual organic matter production and/or accumulation during this peculiar event. Overall, this study suggests that Milankovitch-paced (long eccentricity) changes in monsoonal activity and their effect on the accumulation of organic matter in continental wetlands best explains the rhythmic change in the global carbon isotope record across the OAE 1b interval.