Evolution of Early Cretaceous (Hauterivian–Aptian) seawater oxygenation in the Boreal Realm

Oceanic anoxic events (OAEs) are defined by widespread deposition of organic-rich sediments resulting from transient episodes of oceanic deoxygenation in response to global environmental changes. This study contributes to the understanding of the evolution of seawater oxygenation during the late Hauterivian – early Aptian in the Boreal Realm by introducing a high-resolution dataset of rare-earth element (REE) plus yttrium (Y; REY) patterns and redox-sensitive trace element (RSTE) concentrations from the Danish Central Graben (DCG), North Sea. Changes in oxygenation in the seawater column can be deduced from the Ce anomaly (Ce/Ce*) whereas uranium (U), vanadium (V) and manganese (Mn) concentrations reflect oxygenation conditions at the sediment-water interface. Decreasing Ce/Ce* values and low RSTE concentrations indicate a long-term trend from an oxygen-depleted to a more oxygenated water column from the late Hauterivian to earliest Aptian. This long-term trend was interrupted by two shorter-lived deoxygenation events controlled by two contrasting styles of relative sea-level change. First, an early Barremian sea-level fall led to semi-isolated depocentres in the DCG, causing reduced ocean ventilation that triggered stratification of the water column and ultimately anoxic conditions at the sediment-water interface. This resulted in the deposition of the organic-rich, laminated Munk Marl Bed. Second, in the early Aptian, the unfolding of the OAE 1a which was coeval to a global eustatic sea-level highstand resulted in the deposition of the regionally persistent, organic-rich, laminated Fischschiefer Member. Contrary to the oxygen-depleted/ferruginous seawaters in the Tethyan Realm during OAE 1a, the Ce anomaly of the Fischschiefer Member indicates an oxygen-depleted/manganous water column in the Boreal Realm. Regional environmental conditions, such as ocean ventilation, are therefore an important factor with regards to ocean oxygenation, explaining the heterogenous spatial distribution of organic-matter-rich levels associated with OAEs.