The mid-Cretaceous period (115mn to 80mn years ago) was the heyday of the dinosaurs but was also the warmest period in the past 140mn years, with temperatures in the tropics as high as 35C and sea level 558ft higher than today. However, little was known about the environment south of the Antarctic Circle at this time.
“The preservation of this 90mn-year-old forest is exceptional, but even more surprising is the world it reveals,” said Professor Tina van de Flierdt, a researcher in the Department of Earth Science & Engineering at Imperial College London.
“Even during months of darkness, swampy temperate rainforests were able to grow close to the South Pole, revealing an even warmer climate than we expected.” This scorching climate allowed a rainforest to take root in Antarctica, the researchers said. The rainforest’s remains were discovered under the ice in a sediment core that a team of international researchers collected from a seabed near Pine Island Glacier in West Antarctica in 2017. The layer that had formed about 90mn years ago was a different colour, study lead researcher Johann Klages, a geologist at the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research in Bremerhaven, Germany, said. A CT (computed tomography) image of the core showed an intact 3m-long network of fossil roots, so well preserved that they could make out individual cell structures.
The 90mn-year-old sample also contained countless traces of pollen and spores from plants, including the first remnants of flowering plants ever found at these high Antarctic latitudes. By analysing the pollen and spores, study co-researcher Ulrich Salzmann, a palaeoecologist at Northumbria University in England, was able to reconstruct West Antarctica’s 90mn-year-old vegetation and climate. “The numerous plant remains indicate that the coast of West Antarctica was, back then, a dense temperate, swampy forest, similar to the forests found in New Zealand today,” Salzmann said in the statement.
The sediment core revealed that during the mid-Cretaceous, West Antarctica had a mild climate, with an annual mean air temperature of about 12C. Summer temperatures were warmer, with an average of 19C. In rivers and swamps, the water would have reached up to 20C. These temperatures are impressively warm, given that Antarctica had a four-month polar night, or no sunlight for a third of every year. However, the world was warmer back then, in part, because the carbon dioxide concentration in the atmosphere was high — even higher than previously thought, according to the analysis of the sediment core, the researchers said.
“Before our study, the general assumption was that the global carbon dioxide concentration in the Cretaceous was roughly 1,000 ppm [parts per million],” study co-researcher Gerrit Lohmann, a climate modeller at Alfred Wegener Institute, said in the statement. “But in our model-based experiments, it took concentration levels of 1,120 to 1,680 ppm to reach the average temperatures back then in the Antarctic.”
These findings show how potent greenhouse gases like carbon dioxide can cause temperatures to skyrocket, so much so that today’s freezing West Antarctica once hosted a rainforest. Moreover, it shows how important the cooling effects of today’s ice sheets are, the researchers added.
