Antarctica, with an annual average temperature anywhere between minus ten to minus sixty, is one of the most inhospitable places on the Earth. However, this was not the case forty million years ago; a mild climate had prevailed there. Towards the end of the Late Eocene, approximately 33.7 million years ago, temperatures plunged rapidly, allowing permanent continental-scale glaciation in Antarctica. How did such drastic climatic change occur? Dr. Sudipta Sarkar at IISER Pune probed this question.
ACC, the defining factor for Antarctica
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| Modern configuration of the Antarctic Circumpolar Current (Image Courtesy: Sudipta Sarkar) |
Today the mightiest ocean current, the Antarctic Circumpolar Current (ACC) separates Antarctica from Australia and South America, facilitating the thermal isolation, cooling, and stabilization of the Antarctic glaciers. Approximately 40 million years ago, Antarctica was connected to the continental landmasses, Australia and South America. The clearance of continental obstacles was crucial for the onset of the ACC. Evidence from previous research suggests that the onset of the ACC could have played a role in the Late Eocene cooling in Antarctica. But the timing of its onset and consequences for the expansion of the Antarctic Ice Sheet during the Late Eocene-Early Oligocene are uncertain.
Exploring the marine sediments
A popular method of examining what might have occurred millions of years ago is to analyze the marine sedimentary record recovered by ocean drilling. Marine reflection seismic data also help to map out the sub-seabed sedimentary deposits and changes in sediment properties through time. Seismic imaging uses the sound waves that bounce off from the subsurface. The same technique aids in locating oil and gas reservoirs. Combined analysis of seismic data and geological information from drill holes provides a powerful way to decode the changes in Earth’s past.
The early development of the ACC
In the current study published in the journal Scientific Reports, Sarkar and co-authors analyzed the seismic lines and information from drill holes to unravel the early development of the ACC. The team examined marine sedimentary record that was deposited approximately 36 million years ago in the deep-water basins east of New Zealand. They found evidence for extensive siliceous microfossils, such as diatoms and radiolarians in the sediments. They studied the neodymium isotopes from the teeth of fossil fish preserved in the marine sediments of the Pacific region. The record confirmed a major Late Eocene oceanographic transformation in the Pacific. The deep Equatorial Pacific waters could arrive at the high latitudes of the South Pacific during Late Eocene.
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| Changes in paleo-ocean circulation (Image Courtesy: Sudipta Sarkar) |
Based on these observations and analysis of the seismic lines, the team proposed the sequence of events that led to the establishment of the ACC during the Late Eocene and Early Oligocene. As the Tasman Gateway, the seaway between Australia and Antarctica started to widen and deepen during Late Eocene, 35 Million years ago, the westerlies-driven nascent proto-ACC could flow across the gateway. The onset of the proto-ACC could aid in pulling up the Equatorial Pacific deep waters to the shallow depths of southern high-latitudes. The arrival of nutrient-rich waters from the equator boosted primary productivity, such as diatom blooms and carbon burial in the southern high-latitudes. These processes facilitated atmospheric carbon dioxide reduction contributing to the expansion of the Antarctic Ice Sheet at the Eocene-Oligocene Transition 33.7 Million Years ago. The transition to the continental-scale Antarctic glaciation in the Late Eocene-Early Oligocene occurred in concert with a critical reorganization in the ocean, underpinning the role of the ACC in these events.
This study titled “Late Eocene onset of the Proto-Antarctic Circumpolar Current” was authored by Sudipta Sarkar, Chandranath Basak, Martin Frank, Christian Berndt, Mads Huuse, Shray Badhani, and Joerg Bialas. The article was published in Scientific Reports (2019) 9: 10125, DOI:10.1038/s41598-019-46253-1
- with inputs from Dr. Sudipta Sarkar; Edited by Shanti Kalipatnapu
