(CN) — Researchers combing through soil samples in the South Pole found evidence that a vast rainforest stretched across West Antarctica nearly 90 million years ago, offering a first glimpse into prehistoric climate in Earth’s southernmost region.
On their Antarctic expedition, geoscientists from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research in Germany studied sediment samples from the continent’s core in order to learn about its climate history.
Inside the sediment cores were ancient plant pollen, spores and dense root systems; pristinely preserved elements of forest soil from the Cretaceous period.
Researchers believe the plant remains confirm that 90 million years ago the Earth’s southernmost continent was home to a swampy and warm rainforest, according to the study published Wednesday in the journal Nature.
"The preservation of this 90-million-year-old forest is exceptional, but even more surprising is the world it reveals,” study co-author Tina van de Flierdt of Imperial College London said in a statement. “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."
Study first author Johann Klages added the core samples were collected in West Antarctica’s Amundsen Sea in February 2017 using the University of Bremen's seafloor drill rig MARUM-MeBo70.
"During the initial shipboard assessments, the unusual coloration of the sediment layer quickly caught our attention; it clearly differed from the layers above it,” said Klages, who is with Alfred Wegener Institute. “Moreover, the first analyses indicated that, at a depth of 27 to 30 meters (88 to 98 feet) below the ocean floor, we had found a layer originally formed on land, not in the ocean.”
X-ray-computed tomography scans of the cores revealed dense networks of roots and first remnants of flowering plants ever found at these high Antarctic latitudes, according to study author Ulrich Salzmann, a paleoecologist at Northumbria University.
“The numerous plant remains indicate that 93 to 83 million years ago the coast of West Antarctica was a swampy landscape in which temperate rainforests grew — similar to the forests that can still be found, say, on New Zealand's South Island,” said Salzmann.
The rainforest’s climate — considerably warm for the South Pole — likely resulted from the absence of an Antarctic ice sheet and a higher concentration of atmospheric carbon dioxide than previous climate models predicted, the study said.
"Before our study, the general assumption was that the global carbon dioxide concentration in the Cretaceous was roughly 1,000 ppm,” said study co-author Gerrit Lohmann. “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.”
The mid-Cretaceous period, between 115 million and 80 million years ago, was a time when dinosaurs roamed the planet and was also the warmest period in the past 140 million years, with sea surface temperatures rising as high as 95 degrees Fahrenheit.
But the region also experienced an annual, four-month polar night in which no life-sustaining sunlight touched the surface, leaving researchers to ponder how plants remains in the core samples could have thrived.
"To get a better idea of what the climate was like in this warmest phase of the Cretaceous, we first assessed the climatic conditions under which the plants' modern descendants live,” Klages said.
Using geochemical temperature and precipitation indicators in the soil samples, researchers reconstructed the prehistoric rainforest’s climate.
The new climate models determined that in the Cretaceous, the average temperature at the South Pole was about two degrees warmer than the mean temperature today in Germany and rainfall was on par with contemporary averages in Wales, the study said.
The rainforest’s existence relied on the key climate factors uncovered in the study: no massive ice sheets in the South Pole, a continent covered in dense vegetation and high levels of the greenhouse gas carbon dioxide, according to study coauthor Torsten Bickert of the University of Bremen.
"We now know that there could easily be four straight months without sunlight in the Cretaceous,” Bickert said. “But because the carbon dioxide concentration was so high, the climate around the South Pole was nevertheless temperate, without ice masses."
The remaining question for researchers is how the climate cooled so dramatically over time to allow ice sheets to form again in the South Pole.
“Our climate simulations haven't yet provided a satisfactory answer,” Lohmann said.
Researchers did not immediately respond to a request for further comment on the study.
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