Secular change in northwestern Tethyan water-mass oxygenation during the late Hauterivian–early Aptian

The Mesozoic is punctuated by several extreme episodes of oxygen drawdown in the ocean, known as Oceanic Anoxic Events (OAE). Beside these discrete events, little is however known about longer-term oxygen level fluctuations and their controls. A high-resolution dataset of Rare Earth Elements (REE) content in carbonate deposited in the Vocontian Basin (SE France) has allowed to survey the evolution of the Cerium anomaly (Ce/Ce*) during part of the Early Cretaceous (latest Hauterivian–early Aptian). This ratio is used as a proxy for the degree of oceanic oxygenation. The Cerium anomaly is compared to the coeval evolution of relative sea level, organic-rich occurrences, and nutrient level proxies, in order to infer a cause-and-consequence relationship between these parameters and the evolution in sea-water oxygenation. The long-term evolution of the Cerium anomaly shows that northwestern Tethyan water masses have evolved from less oxygenated during the latest Hauterivian–early Barremian interval to more oxygenated during the late Barremian–earliest Aptian time, before being strongly oxygen-depleted during the early Aptian OAE 1a time interval. This trend is correlated with both the long-term trend in nutrient levels, as well as with the frequency of organic-rich layers within the rock record. On a medium-term scale (sub-stage level), there is a good correlation between the Cerium anomaly and tectono-eustatism (as deduced by the Mn content of the basinal carbonate and sequence stratigraphic observation on surrounding shallow-water areas). This suggests that sea-level changes, by altering ocean stratification and currents, as well as modulating the amount of nutrient delivered from the continent, exert control on oceanic oxygenation via changes in primary productivity.