Before the ice, the rivers

West Antarctica

Today, much of the rock surface in West Antarctica lies below sea level. But that wasn’t always the case. © Leamus/ iStock

Today, West Antarctica is covered by a thick sheet of ice and large glaciers. But a good 34 million years ago, this area was a marshy, flat coastal plain through which a large, branching river flowed. This transcontinental river system stretched more than 1,500 kilometers and transported alluvial material from the newly forming Transantarctic Mountains to the coast, as a research team has determined using core analysis and seismic data. This river flowed into the sea where the Amundsen Sea is today. The existence of the river system thus provides new insights into the past development of Antarctica and its landscapes.

Today, Antarctica is an isolated continent covered by kilometers-thick ice. But it wasn’t always like this: until around 100 million years ago, it formed the central part of the supercontinent Gondwana. When this disintegrated, Antarctica initially remained connected to Australia and parts of South America until it separated from them around 45 million years ago. Unlike today, Antarctica’s climate was mild at that time, and despite its south polar location, there were no extensive ice sheets. It was only 34 million years ago, at the end of the Eocene, that the increasing glaciation of the South Pole continent began and an almost continuous ice sheet buried the Antarctic landscapes. Radar mapping of the Antarctic subsurface shows, however, that enormous gorges, extensive volcanic areas and mountains lie beneath the ice sheet. Geologists have also discovered the first evidence of prehistoric river systems.

Drill core as a window into the past

But what the Antarctic landscape looked like shortly before the continent glaciated has only been partially clarified so far. Maximilian Zundel from the University of Bremen and his colleagues have now gained new insights. For their study, they examined sediment samples that were obtained during an expedition of the research icebreaker “Polarstern” in the Amundsen Sea off the West Antarctic coast. The core samples taken there come from one of the southernmost and oldest sediment sequences in the Amundsen Sea, as the team explains. The layers of the core date back to the Cretaceous period, but also contain sandstone layers from the Eocene and thus from the time immediately before the West Antarctic glaciated. To determine the origin and age of the rock, the researchers carried out chemical and mineralogical analyses and dated the layers using isotope analyses and uranium-lead dating of zircon crystals in the sediment.

The analyses showed that most of the minerals and rock fragments in these samples were deposited in this part of West Antarctica between 44 and 34 million years ago. From the composition of the deposited minerals, Zundel and his team conclude that these grains and pebbles did not come from West Antarctica, but from the Transantarctic Mountains on the edge of East Antarctica, a good 1,500 kilometers away. This mountain range began to rise due to volcanic and tectonic activity around 44 million years ago. “An origin in the Transantarctic Mountains is also evident in the occurrence of arkose pebbles in the sandstone, which show remarkable similarities with the rock of the Kukri Mountains of the Transantarctic Mountains,” explain the researchers. Geologists refer to arkose as a dark-colored sandstone that contains a particularly high amount of feldspar.

A river from the mountains to the sea

Further analysis also revealed that this sediment could not have been washed into the area of ​​today’s Amundsen Sea by ocean currents. Instead, a freshwater river must have transported the grains, as Zundel and his team report. Evidence of this includes residues of organic molecules made of sugars and hydrocarbons that were once produced by freshwater blue-green algae. “The data suggest that a large transcontinental river system transported the sediment from the young and rising Transantarctic Mountains all the way through West Antarctica to the South Pacific,” the scientists write. This river system, which is more than 1,500 kilometers long, flowed into an extensive, swampy river delta in the area of ​​today’s Amundsen Sea. In addition to the seismic reflection patterns in the subsoil, evidence of the latter was also provided by grains of the mineral kaolinite. This silicate mineral often forms in the acidic environment of moors.

Taken together, these results suggest that before the glaciation of West Antarctica, there was an extensive river system that once flowed across this part of the continent – probably parallel to the Transantarctic Mountains, as the team explains. The presence of such a river provides important insights into what the landscape looked like before the glaciation – and corrects a previous assumption. Today, large parts of West Antarctica lie below sea level. “The existence of a transcontinental river system, however, proves that the interior of West Antarctica must have been above sea level at that time,” say Zundel and his colleagues. In the Eocene, the region was characterized by extensive, flat coastal plains that only glaciated relatively late because of their mild climate.

Source: Maximilian Zundel (University of Bremen) et al., Science Advances, doi: 10.1126/sciadv.adn6056

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