Early development of anoxia prior to the main euxinic phase of the Jenkyns Event on the margin of the Western Tethys (Sachrang Basin, Eastern Alps)

Abstract

ABSTRACT Although the global expansion of oxygen‐depleted marine environments during the Early Toarcian Jenkyns Event (or Toarcian Oceanic Anoxic Event) is primarily inferred from successions in the NW European epicontinental basins, the timing of the expansion of anoxia in Tethyan basins, where hydrography potentially played a minor role, is poorly known owing to hiatuses and the sparsity of index fossils. The Sachrang section in the Northern Calcareous Alps deposited in an intrashelf basin on the Neo‐Tethyan margin shows that anoxia developed in the Tethys Ocean earlier than on the NW European shelf. Stratigraphic variability in geochemical data, including δ 13 C org , TOC, Mn, V, U, Mo, DOP T and TOC/P, supported by microfacies analyses as well as ammonite and calcareous nannofossil biostratigraphy, subdivides the section into three distinct lithological units. The basal Unit 1 (~18 m) was deposited under suboxic but non‐sulfidic conditions that were associated with the precipitation of authigenic Mn‐rich carbonates and with the inhibition of benthic assemblages in the NJT5c Zone (≈ early Tenuicostatum Zone), thus prior to the Jenkyns Event. The black shale facies of Unit 2 (~6 m) was deposited under euxinic conditions in the NJT6 Zone, representing the main phase of the event. The Unit 3 (~11 m), corresponding to part of the NJT6 and Serpentinum zones, was formed in anoxic but non‐sulfidic conditions, generating submillimetric lamination and a near‐absence of benthic fauna. Although the peak of euxinia occurred during the Serpentinum Zone, similar to the NW European Shelf, anoxic conditions were initiated earlier in the Sachrang Basin, during the early Tenuicostatum Zone.