Why is Antarctica frozen? Mississippi mud helps solve enigma

To piece together a more comprehensive understanding of what caused this cooling event, scientists in this new study meticulously analyzed materials within Mississippian mud core layers.

They studied marine clay samples that spanned approximately 137 meters (449 feet) in depth and compared these samples with vital records from the Pacific Ocean. This analysis enabled them to precisely pinpoint the sea level drop that marked the inception of ice sheet formation.

Additionally, they uncovered a link between carbon transfer and the emergence of Antarctic ice caps. According to the authors, as the ice caps began to take shape, sea levels dramatically dropped by approximately 40 meters, leading to a notable carbon transfer from plant remnants in coastal environments into the atmosphere. 

This intriguing interplay between nature's elements triggered a 300,000-year pause in the ongoing climate cooling process.

How did Antartica become frozen?

The consequence of this sea level drop went beyond the expected. Coastal regions, laid bare by the receding waters, faced the relentless forces of rain and rivers. 

Organic carbon, once nestled within these sediments and environments—reminiscent of today's vibrant tropical mangrove ecosystems—was suddenly exposed to atmospheric oxygen. 

Bacteria feasted on this released carbon, converting it back into carbon dioxide, which then reentered the atmosphere.

"We've unearthed information from the Mississippi mud to answer a key question about how Antarctic ice massively expanded to continental scale," said senior author Dr. Tom Dunkley Jones from the University of Birmingham, in a press release.

"The Eocene-Oligocene transition is probably the planet's biggest climate cooling event and has had a major impact on the Earth's history," he added.

"As sea levels fell over this transition, we can observe how a temporary brake on atmospheric cooling took place with the release of large amounts of carbon dioxide sequestered in coastal regions around the basin of the Mississippi River."

The researchers assert that this finding resolves a puzzle concerning the transition's timeline. It indicates that the initial stages of this event and the formation of Antarctic ice sheets actually commenced around 300,000 years earlier. 

After the organic carbon brake was exhausted, the transition proceeded unhindered, leading to the colder state observed over the last 34 million years.

“Our paper gives us a valuable new clue about how Earth's climate can undergo dramatic shifts and how this is often strongly linked to the biosphere and carbon cycle," concluded Dr. Kirsty Edgar, the University of Birmingham.

“Understanding these past events gives us a clearer picture of the beauty and complexity of the Earth’s climate and ecology.”

The complete study was published in Nature on August 8 and can be found here.

Study abstract:

Continental-scale expansion of the East Antarctic Ice Sheet during the Eocene-Oligocene Transition (EOT) is one of the largest non-linear events in Earth’s climate history. Declining atmospheric carbon dioxide concentrations and orbital variability triggered glacial expansion and strong feedbacks in the climate system. Prominent among these feedbacks was the repartitioning of biogeochemical cycles between the continental shelves and the deep ocean with falling sea level. Here we present multiple proxies from a shallow shelf location that identify a marked regression and an elevated flux of continental-derived organic matter at the earliest stage of the EOT, a time of deep ocean carbonate dissolution and the extinction of oligotrophic phytoplankton groups. We link these observations using an Earth System model, whereby this first regression delivers a pulse of organic carbon to the oceans that could drive the observed patterns of deep ocean dissolution and acts as a transient negative feedback to climate cooling.