GEOCHEMICAL EVIDENCE FOR LOCAL VARIABILITY IN REDOX AND DEPOSITIONAL CONDITIONS IN A DEEP-WATER BONARELLI EQUIVALENT SECTION FROM SOUTHERN TETHYS(FONTANA VALLONETO SECTION, SOUTHERN ITALY)

摘要

Identifying the depositional redox conditions is useful to evaluate the interplay between climate changes, biological feedbacks and de-oxygenation processes in the oceans during the Oceanic Anoxic Events (OAEs). Here, we focus on the about 56 m-thick Albian-Turonian Fontana Valloneto stratigraphic section cropping out in Southern Italy (Potenza, Basilicata), belonging to the "Flysch Rosso" Formation, and containing an equivalent of the Bonarelli Horizon (globally called OAE2 which occurred at～ 94 Ma). Inorganic geochemical compositions and Total Organic Carbon contents obtained from this section are here used to assess depositional environment and redox conditions. The paucity of carbonates within the entire sequence and a gradual decrease in Y, Zr and Al contents along the section suggest a deep depositional environment (below the Calcite Compensation Depth) and an overall decrease in the terrigenous supply. Samples within the Bonarelli Horizon (BH-e) show highly variable TOC contents (～0 to ～ 30 wt%) that, mirrored by variations in redox sensitive and nutrient-related elements (e.g. V, Mo and U) and Mn, suggest variation of the seawater primary productivity associated to changes of the local redox conditions between suboxic to strongly euxinic. We infer that during OAE2 the accumulation of the black shales was associated to high Organic Matter (OM) productivity, high biogenic silica production and fine-grained sedimentation (mainly aeolian dust and illite) in a period of "sluggish" oceanic circulation and stagnant conditions. These periods were alternated by moments of more active oceanic circulation and enhanced runoff, leading to the local deposition of ra-diolarites with very low TOC contents. Finally, a comparison with other section from the proto-Atlantic Ocean and the Mesozoic Tethys sustains the idea that the drawdown of redox-sensitive elements (V, Mo and U) was a global process during the deposition of OAE2, providing a link between the environmental changes detected in our section with the global perturbations developed during this oceanic anoxic event.