Uranium isotope evidence for extensive seafloor anoxia after the end-Triassic mass extinction

Abstract

The end-Triassic extinction (ETE) ranks as one of the ‘Big Five’ biotic crises of the Phanerozoic and is thought to be triggered by volcanism of the Central Atlantic Magmatic Province (CAMP). However, the proximal causes of the extinction and the factors responsible for the delayed biotic recovery remain debated. Here we use uranium isotopes and geochemical models to constrain the evolution of global seafloor anoxia during the latest Triassic and earliest Jurassic. We document a major negative uranium isotope anomaly from carbonates in a western Tethyan continuous marine Triassic-Jurassic boundary section. The onset of the δ238U anomaly is coincident with the initial negative carbon isotope anomaly that is correlative with the extinction horizon and a major pulse of intrusive CAMP volcanism. The U isotope values remain low throughout most of the Hettangian indicating persistent, widespread anoxia. Our coupled C-P-U Earth system and U-cycle box model results show that the maximum extent of anoxia (∼13%) was reached 200–250 kyr after the extinction, probably as a consequence of extrusive CAMP pulses. The anoxic extent remained high (1–6%) throughout the Hettangian. We suggest that the continuing volcanic activity, recorded by the successive negative carbon isotope anomalies and Hg peaks in the section, inhibited any rapid recovery from anoxia. Our results indicate that the spread of marine anoxia and the ETE have a common cause rather than a cause-and-effect relationship but anoxia played a key role in hindering the biotic recovery of benthic ecosystems following the extinction.