Variations in oxygenation, primary productivity, and sea level: Investigating the Faraoni Oceanic Anoxic Event in the Lower Cretaceous

The Faraoni Event is considered one of the oldest oceanic anoxic events of the Cretaceous, occurring in the Upper Hauterivian. This event, of short duration (200 to 300 ka) and not globally recognized, exhibits atypical characteristics when compared to larger anoxic events like the OAE 1a. Due to these peculiarities, an in-depth study of this event is of great importance, as it suggests the possibility of the existence of other relevant dysoxic to anoxic events, yet to be recognized and adequately investigated. This study focuses on the Faraoni Event in the Western Tethys, from the Puez section located in the Dolomites Mountains, Northern Italy. It is noteworthy that the studied samples were not black shales, as generally expected in anoxic events, but rather . The research included analyses of elements, elemental ratios, rare earth element analyses, grain size, mineralogy, and magnetic mineralogy of discrete samples.

From the conducted analyses, it was possible to comprehend the Faraoni Event in more detail, with a focus on variations in redox conditions and primary productivity. These analyses highlighted distinctive features of predominantly dysoxic conditions during the event, as well as anoxic and euxinic conditions, occurring punctually. A significant perturbation was observed at the beginning of the event, possibly associated with a rapid sea-level rise, triggering a substantial nutrient input, causing a disturbance of considerable magnitude in the environment, and culminating in euxinic conditions. This nutrient input does not appear to be related to increased chemical weathering or volcanic activity, as proposed by some authors in the literature.

Euxinic conditions occurred only for a short period. Through self-regulation of the system, primarily, and the decrease and/or stabilization of sea-level rise, there is no further evidence of euxinic conditions, but rather . There is evidence that the self-regulation of the ecosystem is crucial for returning to oxic conditions in the face of external variations. High productivity conditions would increase oxygen consumption through the decomposition of organic matter, leading to reducing conditions. This, combined with lower quantities of limiting nutrients such as phosphorus, would, in turn, decrease productivity. This would result in improved oxygenated conditions until the system reached equilibrium, maintaining oxygenation levels as observed before the initial disturbance. This system variation has been termed the "string vibration" and is clearly recognized in this context.

Peculiar deposition conditions of a dysoxic event with euxinic and anoxic moments in limestones, the possible initial trigger of the Faraoni Event, and the importance of the self-regulation of the system are highlighted conclusions in this study.

Key words: Faraoni Event, Lower Cretaceous, OAE