How gateways shape the world – An ocean perspective on the Eocene-Oligocene-Transition

The Eocene-Oligocene Transition (EOT) (~34.4–33.7 Ma) is not only known for its drastic shift from greenhouse to icehouse climate, but also for a dynamic ocean gateway configuration. The Southern Ocean gateways are expected to open during this period, while the Arctic Ocean likely remains largely isolated, resulting in distinct ocean circulation patterns.

Using the AWI-Earth System Model (AWI-ESM) coupled to the Parallel Ice Sheet Model (PISM), we explore the ocean dynamics under an ocean straight configuration markedly different from today’s, providing a detailed depiction of global climate during the EOT. Low-latitudinal seaways, which are absent in the present continental configuration, and opening Southern gateways change the global ocean circulation fundamentally. This also has profound impacts on the continental climate, such as the formation of deserts.

With a targeted study of the Southern Ocean, we show that deep Southern gateways alone are insufficient to allow an Antarctic Circumpolar Current (ACC). Whether the Antarctic glacial inception came before or after the onset of the ACC is broadly debated. Here we observe, the onset of the ACC is not a necessary condition for East Antarctic glaciation. Instead of the ACC, a large Weddel-Australian gyre dominates the Southern Ocean. This gyre creates mixing and deep water formation and thus influences an Atlantic Ocean that faces very different boundary conditions than today.

This study enhances our understanding of Southern Ocean dynamics prior to the establishment of a strong ACC and underscores the critical role of oceanic gateway configurations in assessing their impact on regional and global climate.