Taking your breath away with Early Cretaceous Boreal seawater oxygenation

Abstract

Mesozoic Oceanic Anoxic Events (OAEs) were linked to the accumulation of organic-rich sediments in response to global climatic and environmental changes, resulting in transient episodes of oceanic deoxygenation. To better understand the evolution of seawater oxygenation in the Boreal Realm during the Early Cretaceous (late Hauterivian – early Aptian), this study presents high-resolution datasets of Rare Earth Element plus Yttrium (REE + Y; REY) patterns and redox-sensitive trace elemental (RSTE) concentrations in the Danish Central Graben (DCG). Oxygenation in the seawater column can be derived from the Cerium (Ce) anomaly (Ce/Ce*), which is based on the premise that Ce acts differently in well-oxygenated environments compared to the other REYs. At the seafloor, anoxia is indicated by the enrichment of RSTEs Vanadium (V) and Uranium (U). A decline in Ce anomaly values and low RSTE concentrations from the late Hauterivian to late Barremian indicates a shift from an oxygen-depleted to a more oxygenated seawater column and seafloor conditions in the Boreal Realm. A similar trend is observed in the Tethyan Realm, suggesting the existence of a global long-term driver of seawater oxygenation level. In the DCG, this long-term trend is interrupted by a brief drop in relative sea level, leading to partial isolation of the basin, reduced ventilation, stratification of the water column and consequently short-term anoxic conditions at the sediment–water interface. This resulted in the deposition and preservation of an organic-rich layer (Munk Marl Bed) during the early Barremian. During the early Aptian, an organic-rich layer (Fischschiefer Member) of regional extent, linked to the global OAE-1a, was deposited. Significantly elevated Ce anomaly values along with increased concentrations of U and V signify anoxic conditions within this Boreal basin, whereas the seawater column in the Tethyan Realm exhibited dysoxic conditions. Variations in anoxia and the fact that some OAEs are only observed locally, leads to the conclusion that short-term regional or local factors can overprint global changes.