Tectonic shift in the Southern Ocean caused sudden cooling event

The results shed new light on a 50-year-old question about how and why the Antarctic ice sheets formed.

During the Early Cenozoic, the Earth underwent one of the most fundamental global climate changes known in geological history. Almost 34 million years ago, the Earth underwent dramatic ancient cooling events- from hot Greenhouse conditions to cold Icehouse conditions.

This cooling event leads to the formation of the Antarctic ice sheets. However, what caused this cooling event remains uncertain.

A new study by an international team of researchers from the University of Leicester, the Netherlands, Australia, Germany, and Norway, shed light on the event, suggesting that the tectonic shift in the Southern Ocean caused this dramatic event 34 million years ago.

For this work, scientists used increased available computational capabilities to run high-resolution ocean model simulations. The simulations show that the tectonic opening of Southern Ocean seaways caused a fundamental reorganization of ocean currents. This caused the strong Antarctic surface water cooling of up to 5°C.

Along with declining atmospheric carbon dioxide (CO2) concentrations, this event played a crucial role in the first glaciation of Antarctica and the Earth’s transition into an Icehouse world.

Dr. Katharina Hochmuth, International Ocean Discovery Program (IODP) Research Associate at the University of Leicester, and co-author of the study, said: “In the last week and the lead up to COP26, we have heard a lot about modeling projections on our planet’s future. In this paper, we show that it is crucial to include atmospheric CO2 conditions as well as appropriate geographies from the past to successfully model changing climates.”

“A 600m change in the depth of an ocean gateway can cause a dramatic drop in coastal temperatures and, therefore, the fate of the Antarctic ice sheet.”

The simulation also shows that this tectonic event did not only leave the polar continent isolated by other land masses; it also led to a significant reorganization of ocean currents in the Southern Ocean.

The flowing of circumpolar current prevented subpolar gyres from passing warm surface waters to the Antarctic coast. At the same time, ice sheets started to build on Antarctica. The Earth underwent one of its most fundamental climate change events, transitioning from warm Greenhouse to cold Icehouse conditions.

Dr. Sauermilch added“When we started this project, I was surprised to see how much high-resolution matters in an ocean model. These simulations are sensitive to minimal changes in the depth of these seaways of a few hundred meters and react very differently than their low-resolution counterparts.”

“On top, they resolve ‘eddies,’ turbulent ocean currents that are smaller than 100 km and which are crucial for the accurate temperature distribution in the Southern Ocean.”

The study has answered a five-decades-old question- how and why the Antarctic ice sheets formed? It also demonstrates the importance of tectonically-driven processes in the changing oceanographic and climatic conditions of the Southern Ocean.

Journal Reference:

  1. Sauermilch, I., Whittaker, J.M., Klocker, A. et al. Gateway-driven weakening of ocean gyres leads to Southern Ocean cooling. Nat Commun 12, 6465 (2021). DOI: 10.1038/s41467-021-26658-1

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