Anoxia in the Permian Irati-Whitehill Ocean of southern Gondwana: A possible link with uprising of the Cape and San Rafael mountain belts

Abstract

The Permian section of the Paraná-Etendeka basin is represented by the Palermo and Irati formations, comprising a shallow sea that occupied ca. 5 million km2 of southern Gondwana before completely drying out around 277 million years ago (Irati-Whitehill ocean). This is broadly coincident with the uprising of the Cape Fold Belt of southern Africa and the San Rafael orogeny of the paleo-Pacific margin of South America, leading to the interpretation that basin restriction and the major ecosystem changes that followed were ultimately caused by uprising of mountainous domains surrounding the shallow sea. We combine new iron speciation, organic carbon isotope and trace element data with previous biomarker, organic carbon and nitrogen isotope data to unravel the biogeochemical and redox changes during this transition from an open marine realm to a restricted setting, and to test the hypothesis of external controls on the biogeochemical cycles of southern Gondwana. Mudstones and shales of the Palermo Formation yielded FeHR/FeT around or below 0.2, suggesting oxic bottom water conditions, reinforced by muted redox-sensitive element (RSE) concentrations and overall low Total Organic Carbon (TOC) contents, with δ13Corg around -25‰. Black shales of the overlying Irati Formation, on the other hand, record an abrupt shift to anoxic conditions, with FeHR/FeT between 0.3 and 0.9, representing mostly ferruginous conditions with sporadic euxinic incursions (FePy/FeHR > 0.8), higher concentrations of RSE such as Mo, higher TOC contents and d13Corg rapidly oscillating from ca. -29 up to ca. -19‰. The euxinic intervals are associated with the Assistência Member, containing tephra layers dated at 277 Ma and thus coeval to the Cape and San Rafael orogenies. Our results reinforce the hypothesis of mountain belt formation as the main external driver of biogeochemical changes, leading to toxic conditions for complex life forms in the Permian internal basins and to the accumulation of important organic-rich source rocks in the shallow seas of southern Gondwana.