German and British scientists have found unexpected fossil traces of a temperate rainforest near the South Pole from 90 million years ago.
Writing in the journal Nature, they say analysis of pristinely preserved soil from the Cretaceous period suggests prehistoric rainforests in Antarctica were similar to those in New Zealand today.
The samples come from a core of sediment taken from the seabed near West Antarctica’s Pine Island Glacier in 2017.
“During the initial shipboard assessments the unusual colouration of the sediment layer quickly caught our attention; it clearly differed from the layers above it,” says first author Johann Klages, a geologist from Germany’s Alfred Wegener Institute (AWI).
CT scans revealed a dense network of roots throughout the soil layer, with countless traces of pollen and spores, and remnants of flowering plants. The researchers say they could even make out individual cell structures.
The mid-Cretaceous period – considered the age of the dinosaurs – was the warmest in the past 140 million years. Sea levels were 170 metres higher than today and sea surface temperatures in the tropics are believed to have been as high as 35 degrees Celsius.
Until now, however, little has been known about the environmental conditions south of the Polar Circle.
Palaeoecologist Ulrich Salzmann, from Northumbria University, UK, used the preserved pollen and spores to reconstruct the past vegetation and climate, revealing that “the coast of West Antarctica was, back then, a dense temperate, swampy forest…”
The team’s analysis suggests that, at the time, the annual mean air temperature 800 kilometres from the South Pole was about 12 degrees Celsius – roughly the same as the Australian city of Hobart today.
Summer temperatures averaged 19 degrees and water temperatures in rivers and swamps reached up to 20. This was despite a four-month polar night, when there was no sunlight.
The amount and intensity of rainfall in West Antarctica was similar to that in Wales today.
Such climate conditions, the researchers say, could only be achieved with a dense vegetation cover on the Antarctic continent and the absence of any major ice-sheets in the South Pole region.
Carbon dioxide concentration in the atmosphere was also far higher than previously assumed.
“Before our study, the general assumption was that the global carbon dioxide concentration in the Cretaceous was roughly 1000 ppm [parts per million],” says AWI climate modeller Gerrit Lohmann.
“But in our model-based experiments, it took concentration levels of 1120 to 1680 ppm to reach the average temperatures back then in the Antarctic.”
Accordingly, the researchers say, the study shows both the enormous potency of carbon dioxide and how essential the cooling effects of today’s ice sheets are.
“We now know that there could easily be four straight months without sunlight in the Cretaceous, but because the carbon dioxide concentration was so high, the climate around the South Pole was nevertheless temperate, without ice masses,” says geoscientist Torsten Bickert from the University of Bremen.
The big question now is: if it became so warm in the Antarctic back then, what caused the climate to subsequently cool so dramatically to form ice sheets again? “Our climate simulations haven’t yet provided a satisfactory answer,” says Lohmann.
CT scan of the sediment core, showing sand at the top and tree roots and pollen with roots in-situ approximately 30 metres below seabed. Credit: Awi/Bremen