Euxinia and hydrographic restriction in the Tethys Ocean: Reassessing global oceanic anoxia during the early Toarcian

Despite carbon-cycle perturbations at a global scale during the early Toarcian, the extent of anoxia during the-182-Ma Toarcian Oceanic Anoxic Event (T-OAE) remains in debate. A common factor in the development of oceanic anoxia is watermass restriction, which is thought to have been important in the NW European Seaway, but whose influence elsewhere is relatively unstudied. Here, we analyze Mo/TOC (a proxy for watermass re-striction) and redox proxies (e.g., Corg/P) in two sections of the Asturian Basin (northern Iberian Paleomargin), and we integrate these results with data from a suite of global Toarcian sections in order to reassess the rela-tionship of euxinia and local hydrographic restriction during the T-OAE. The Asturian Basin study sections accumulated in oxic to dysoxic waters, punctuated by two episodes of intermittently euxinic bottomwaters or porewaters around the Tenuicostatum/Serpentinum zonal boundary. This area was an unrestricted carbonate ramp during the T-OAE that was not sufficiently reducing for its Mo/TOC ratios (2.3-4.5) to accurately record the degree of watermass restriction. Regionally, Lower Toarcian black shales exhibit elevated Mo and TOC con-centrations (21-42 ppm Mo, 12-18% TOC) along with exceptionally low mMo/TOC (e.g.,-0.3 in the Paris Basin,-0.4 in the Cleveland Basin, and -2.0 in the Qiangtang Basin), providing evidence of highly restricted, euxinic bottomwaters in Northwestern (epicontinental intrashelf troughs) and Eastern (barrier-lagoon) Tethyan settings. In contrast, in other basins across the SW Tethys and Panthalassic oceans, low contents of Mo and TOC (mostly <5 ppm Mo, <3% TOC) indicate unrestricted watermass conditions and at most intermittent bottomwater or porewater euxinia in pelagic to hemipelagic environments, although intermittently euxinic bottomwaters developed also in deep open-marine settings. The results of our analysis support both (1) a major role for hy-drographic restriction in modulating the local expression of the T-OAE in various regions globally, and (2) a substantial drawdown of aqueous Mo in the global ocean during the T-OAE, indicating that existing hypotheses regarding the nature of the T-OAE event are not mutually exclusive.