Late Paleogene ocean and climate dynamics of the Southwest Indian Ocean: onset and variability of the Agulhas Current as a harbinger or response to climate change?

• Applicants: Professor Dr. Steve Bohaty; Professorin Denise Kulhanek, Ph.D.
• Subject Area: Geology ; Oceanography
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 546924479

The Eocene–Oligocene Transition (EOT, ~34 Ma) marked a profound shift in global climate characterized by abrupt ocean cooling and establishment of a large Antarctic Ice Sheet (AIS). It also resulted in a major reorganization of the world’s ocean system, in concert with global changes in carbon and nutrient cycling, as well as major turnover in marine plankton assemblages including extinctions of calcareous nannoplankton, planktonic foraminifera, and radiolarians. However, many of these extinctions precede geochemical evidence for cooling and glaciation. Following the establishment of the AIS, Oligocene climate was exceptionally dynamic and may have included intervals with a larger-than-present AIS despite pCO2 >450 ppm. Calcareous plankton diversity underwent a long-term decline during this time, while diatom diversity increased. Both cooling and increased nutrient supply/invigoration of Oligocene upwelling systems have been proposed as drivers for these changes in plankton communities. Yet, open questions remain concerning the timing of ocean circulation changes and resultant disruption to plankton communities. With our inability to curb greenhouse gas emissions (current pCO2 422 ppm), the Earth is rapidly approaching conditions similar to those during the Oligocene, making this an ideal time period to study the impact of changing climate on ocean- and ecosystems. Here we propose to investigate this time interval using geochemical proxy records and nannoplankton data collected during IODP Expedition 392, which recovered the first Paleogene records from the Agulhas Plateau (AP) in the Southwest Indian Ocean. The AP is positioned in a critical gateway between Africa and the Southern Ocean and today is influenced by the Agulhas Current (AC), which is an important component of meridional overturning circulation. Site U1579 recovered a nearly continuous nannofossil ooze/chalk sequence spanning the EOT to lower Miocene on the central AP. Low resolution benthic foraminiferal stable isotopes and X-ray fluorescence records from this site show orbital scale cyclicity; development of a high resolution stable isotope record will allow us to develop the astronomically turned age model necessary for examining past climate and oceanographic events. We also propose to develop geochemical proxy records (foraminiferal 18O and 13C, carbonate content, Nd) and biotic records (nannoplankton assemblage data) to examine changes in ocean structure and circulation, frontal system dynamics, nutrients, and productivity from the latest Eocene to late Oligocene (~35–24 Ma). This will allow us to test hypotheses focused on: (1) the timing of AC initiation and impact on plankton assemblages, (2) orbital control on frontal system, circulation, and ocean structure dynamics, and impact on plankton, and (3) changes in ocean temperature, nutrient supply, and primary productivity during times of a dynamic AIS.

• DFG Programme: Infrastructure Priority Programmes
• Subproject of: SPP 527: Bereich Infrastruktur - "International Ocean Discovery Program" (IODP)
• International Connection: Spain
• Co-Investigators: Eleni Anagnostou, Ph.D.; Edoardo Dallanave, Ph.D.; Dr. Thomas Westerhold
• Cooperation Partner: Sietske Josephien Batenburg, Ph.D.