Scientists have discovered evidence of a large river system that crossed Antarctica before it became an isolated frozen continent.

Until about 100 million years ago, Antarctica constituted the central part of the supercontinent Gondwana. After Gondwana’s breakup, Antarctica established itself as an independent continent.

Despite its south polar location, temperate climate conditions prevailed in Antarctica until the end of the Eocene around 34 million years ago, and the continent was crossed by extensive river systems.

An international research team, involving a Lancaster University environmental scientist, and led by the University of Bremen in Germany, has discovered the largest of these river systems and described it in a paper published the journal Science Advances.

The team examined sediment samples which they obtained from the Amundsen Sea off the West Antarctic coast during an expedition of the research icebreaker Polarstern. Their analyses show that most of the minerals and rock fragments in these samples do not derive from West Antarctica, but from the Transantarctic Mountains on the edge of East Antarctica, thousands of kilometers away.

This mountain range has been uplifting since the late Eocene as the steep shoulder of a continental rift, the West Antarctic Rift System, which today divides Antarctica into the two land masses of East and West Antarctica.

Uplift and erosion of the Transantarctic Mountains have since produced large quantities of erosion debris, which the newly discovered river transported over a distance of more than 1,500 km through the West Antarctic Rift System into what is now the Amundsen Sea, and deposited it there in a swampy river delta. Modern examples of large river systems in a similar geological setting are the Rio Grande in the Rio Grande Rift or the Rhine in the Upper Rhine Graben.

The existence of such a transcontinental river system shows that – unlike today – large parts of West Antarctica must have been located above sea level as extensive, flat coastal plains. Due to the low topography, West Antarctica was still ice-free at the end of the Eocene, while the mountainous regions of East Antarctica were already beginning to glaciate.

Co-author of the paper, Yani Najman, Professor of Tectonics at Lancaster University, was on the Polarstern Expedition which collected the sedimentary core used in the study.

She said: “It’s so hard to imagine the Antarctic being anything other than freezing cold and covered in ice. What I think is particularly cool about this study is that it reconstructs what the region was like, as a swampy environment in temperate conditions, millions of years ago.”