Mid-Tertiary Braarudosphaera-rich sediments on the Exmouth Plateau

Abstract

Nannofossil assemblages enriched in Braarudosphaera occur in lower Oligocene to lower Miocene sediments at Sites 762 and 763. Braarudosphaerids appear here rather abruptly in the lower Oligocene (in Zone NP21). They reach their greatest numbers in the lower Oligocene (in Zones NP22 and NP23), where they comprise up to 10% of some samples. Braarudosphaera bigelowii is the overwhelmingly dominant species, occurring together with rare specimens of B. discula and Micrantholithus pinguis. The holococcoliths Peritrachelina joidesa and Lanternithus minutus are also associated with the Braarudosphaera enrichment. There are two populations of B. bigelowii: one of normal size (10-14 μm) and one of large specimens (20-22 μm). The larger braarudosphaerids are more common than the smaller forms. Braarudosphaera-rich sediments are absent at Wombat Plateau sites during the same time interval. This is attributed to latitudinal control, because the Wombat sites are about 4°N of the central Exmouth Plateau sites. The occurrence of braarudosphaerids is probably related to an Oligocene oceanographic event on the Exmouth Plateau. Mid-ocean upwelling of cool, low-salinity, nutrient-rich water along a divergent zone may have created the Braarudosphaera-rich sediments in the South Atlantic and Indian oceans. -from Authors