Benthic foraminiferal assemblage changes during OAE 2 in the southern high latitudes - IODP Site U1516, Indian Ocean

At Site U1516 (Mentelle Basin, southeast Indian Ocean, offshore western Australia), the International Ocean Discovery Program (IODP) Expedition 369 recovered an almost complete record of the Upper Cretaceous, including the Oceanic Anoxic Event 2 (OAE 2). Benthic foraminiferal assemblages indicate a distal, outer neritic to bathyal depositional environment. For a better understanding of the paleoenvironmental changes across the OAE 2, we analysed 32 samples for benthic foraminiferal abundance data. The dataset represents one of the few deep water benthic foraminiferal records of the OAE 2 in the southern high latitudes.

The record of the OAE 2 can be subdivided in pre OAE 2, pre max-CIE, low CaCO₃, and post low CaCO₃ intervals. The interval of low CaCO₃ content covers a prominent positive Carbon Isotope Excursion (CIE). The Cenomanian-Turonian boundary records an extreme decline in benthic foraminifera during OAE 2. The post low CaCO₃ interval documents a strong repopulation event in benthic foraminifera.

During the pre OAE2 and pre-max-CIE intervals, taxa like the calcareous deep-water gavelinellids, lingulogavelinellids and gyroidinids are dominant. In the overlying low-carbonate interval, the microfossil record documents a substantial increase in Radiolaria while foraminifera become a rare faunal element. Remarkable changes in benthic foraminiferal assemblage composition are recorded in the initial low CaCO₃ interval directly underlying the maximum CIE. Rather than acting as the trigger, it appears that the max CIE merely interrupted environmental change in the bottom waters.  Furthermore, the comparison of pre- and post-CIE benthic foraminiferal assemblages highlights a distinct repopulation event during the post max- CIE interval mainly represented by a profound increase in the abundance of agglutinated foraminifera and the calcareous epibenthic taxon Osangularia. The dataset collected at Site U1516 documents one of the most complete benthic foraminiferal records across the OAE 2 that registers the Late Cretaceous environmental changes in the Southern Hemisphere.