Two pulses of extinction of larger benthic foraminifera during the Pliensbachian-Toarcian and early Toarcian environmental crises

Abstract

The early Toarcian (Early Jurassic, ~183 Ma) was characterized by a pronounced oceanic anoxic event (OAE), global warming, major changes in hydrological cycling and a second order mass extinction. Most studies of early Toarcian geobiology have focused on the Northern Hemisphere, and only a few studies have been carried out on strata from the southwestern Tethys Ocean and Southern Hemisphere. This limits our understanding of the geographic extent of the early Toarcian mass extinction (ETME). To address this issue, in this study we have analyzed foraminiferal assemblages from the East Selong and Nianduo sections of South Tibet, China. Our results show that 14 species disappeared during the ETME, accounting for 21.9% (14/64) of overall taxonomic richness. Of these, nine of the victims were larger benthic foraminifera (LBF), accounting for 75% (9/12) of LBF species. We also find that close to the Pliensbachian-Toarcian boundary, six species of foraminifera disappear, and five of these victims were LBF (5/17). Our work thus demonstrates two extinction pulses of LBF in South Tibet, i.e. at the Pliensbachian-Toarcian boundary, and coeval the early Toarcian OAE. The results suggest LBF were more vulnerable to environmental stress than smaller foraminifera, which is consistent with other mass extinctions such as the Permian-Triassic extinction. Our findings confirm that the Southern Hemisphere, like the Northern Hemisphere, was severely affected by environmental changes across the late Pliensbachian to early Toarcian interval, and that extinctions were synchronous with the Northern Hemisphere.