Latitude-dependant climate changes across the Aptian Oceanic Anoxic Event 1a

Abstract

The understanding of the climatic evolution during the Early Cretaceous in general, and across Oceanic Anoxic Event 1a (OAE-1a) in particular, has generally been derived from Tethyan localities, implying large uncertainties about their significance at a global scale. In this study, high-resolution clay-mineral assemblage analyses have been performed on the Hauterivian to lower Aptian cored section of the North Jens-1 well, located in the Danish Central Graben, North Sea, in the Boreal Realm. Large amounts of detrital kaolinite are observed throughout the core, indicating the presence of a local, kaolinite-rich source. A long-term decline in kaolinite content is recorded from the upper Hauterivian to the lowermost pre-OAE-1a Aptian, followed by a sharp rise within the OAE-1a interval. This trend is similar to that observed in the Tethyan Realm, where a supra-regional climatic evolution is observed, including: (i) relatively humid conditions in the late Hauterivian; (ii) a shift towards overall drier conditions in the latest Barremian – earliest Aptian; and (iii) renewed humid conditions during the unfolding of OAE-1a. However, the precise timing of climate change across OAE-1a differs between the Tethyan and Boreal Realms. The shift towards humid conditions coincides with the onset of OAE-1a (segment C3) in the Tethyan Realm, followed by a return to drier conditions in the second half of the event. In the Boreal Realm, however, the onset of OAE-1a was characterised by a relatively dry climate, followed by an increase in humidity within its middle part (segments C4–C5) that persisted through the remainder of the early Aptian. Consequently, there was a non-linear change in precipitation patterns across latitudinal belts during the unfolding of OAE-1a. Similar conclusions have been drawn for other OAEs, suggesting a more complex weathering feedback mechanism during hyperthermal events than generally assumed.