On the Recurrence of Enigmatic Nannoplankton Blooms in the Subtropical South Atlantic during the Early Oligocene

Abstract:

Climatic cooling and expansion of Antarctic ice sheets was accompanied by a global reorganization in ocean circulation during the early Oligocene. Such a change in the ocean-climate system is expected to alter the pelagic ecosystem through elevated rates of extinction and increased biogeographic provincialism. A well documented, but poorly understood, example of this provincialism is the recurrence of unusual chalks composed of the nannofossil genus Braarudosphaera across the subtropical South Atlantic Ocean. Here we present preliminary findings from a study of the paleoceanographic conditions that fostered these Braarudosphaera “blooms” at Deep Sea Drilling Site 516 (Rio Grande Rise, southwestern Atlantic). Within the early Oligocene stratigraphy at this site, there are four chalky (recrystallized) layers in which braarudosphaerids compose ~70% of the nannofossil assemblages. Astronomical tuning was performed on conventional benthic foraminiferal δ¹⁸O and δ¹³C records encompassing the four layers to determine the timing of their recurrence. A strong astronomical rhythm is preserved with the blooms occurring during nodes in the theoretical obliquity solution. In addition, planktic foraminiferal stable isotope (δ¹⁸O, δ¹³C) records were generated for the study section using both secondary ion mass spectrometry (SIMS) and conventional gas-source isotope ratio mass spectrometry (IRMS). The SIMS-based δ¹³C record for the thermocline-dwelling genus Catapsydrax registers substantial (~1.5‰) decreases during the blooms, signaling pulsed increases in the upwelling of ¹³C-depleted waters. By contrast, the IRMS-based δ¹³C record for this same genus show no appreciable change in hydrographic conditions during the blooms. We attribute the invariant nature of the IRMS-based δ¹³C record to the smoothing effects of diagenesis. These results demonstrate how marine plankton respond to changing oceanographic conditions driven by astronomical forcing of ice-sheet dynamics.