Reconstructing marine redox conditions during the Toarcian Oceanic Anoxic Event constrained by combined U-Mo isotopes in black shales

Oceanic anoxic events represent major perturbations of marine redox conditions with varying spatial extents of ocean deoxygenation through Earth’s history. The isotopic composition of redox-sensitive elements, preserved in sedimentary archives, particularly molybdenum (Mo) and uranium (U) isotopes, are powerful proxies for reconstructing past ocean oxygenation. However, Mo and U isotope compositions can be influenced by both global ocean anoxia and local depositional conditions. Both isotope systems show opposite isotope fractionation behavior under variable local redox conditions but are expected to be shifted in the same direction (towards lower values) at a global expansion of seafloor anoxia, allowing combined U-Mo isotope analyses to discriminate between local and global redox signals. The Toarcian Oceanic Anoxic Event (T-OAE; ~183 Ma) represents an Early Jurassic interval of marine deoxygenation and environmental perturbation, but it remains incompletely understood whether ocean anoxia was globally extensive or locally restricted.

Here, we present combined U-Mo isotope data from black shales deposited during and after the T-OAE at two locations within the European Epicontinental Sea (Schandelah, North German Basin, and Metzingen, South German Basin). During the T-OAE, all sections are characterized by light Mo and U isotope compositions, reaching values as low as 0.61-0.73 ‰ for δ⁹⁸Mo and -0.19 to -0.13 ‰ for δ²³⁸U. Following the T-OAE, both isotope systems show an increase towards heavier δ⁹⁸Mo values between 1.66 and 1.73 ‰ and δ²³⁸U values between 0.12 and 0.19 ‰ across both sites. This observed positive correlation between Mo and U isotope compositions is consistent with a global expansion of seafloor anoxia. To further exclude potential local effects, we used redox- and salinity-sensitive proxies, such as Fe/Al, Sr/Ba, B/Ga, and TS/TOC ratios. These proxies show no significant variations across the T-OAE interval and beyond, indicating stable depositional conditions at both localities. Therefore, the U-Mo isotope shifts in the black shales likely reflect a global expansion of seafloor anoxia during the T-OAE.