By Israh Ghobbar
Although Antarctica is currently cold and frozen, this was not always the case. Buried sediment extracted from the seafloor in West Antarctica, 900km from the South Pole, contains fossilised pollen, roots, spores, and other chemical evidence proving the existence of a diverse forest millions of years ago.
Simplified overview map of the South Polar region at time of deposition 90 million years ago.
(credit: J.P. Klages/Alfred Wegener Institute)
In 2017, a team led by geoscientists from the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research in Germany and researchers from Imperial College London went on an expedition aboard the RV Polarstern in the Amundsen Sea. There, using the MeBo seabed drilling system with remote technology, the researchers drilled deep into the ground underneath the seabed of West Antarctica, near the melting Pine Island and Thwaites glaciers. This robotic drilling system is used for obtaining soft sediment and hard rock cores in the sea. The team retrieved a 30m-long core, which upon initial shipboard assessments had an unusual colouration of the sediment layer, which differed from the layers resting closer to the surface. These analyses indicated that at a depth of 27m to 30m below the ocean floor, a layer had originally formed on land, not in the ocean. This layer corresponds to a time in the mid-Cretaceous Period.
Researchers led by Johann Klages (right) analysed seafloor sediment from off the coast of West Antarctica.
(credit: Thomas Ronge/Alfred Wegener Institute)
During further investigation on land, the team put the core into a CT (computed tomography) scanner, resulting in a digital image showing a dense network of roots throughout the mudstone layer. The dirt contained traces of ancient pollen, spores and remnants of flowering plants from the Cretaceous Period. The soil was so well-preserved that individual cell structures could be made out. The pollen in the core showed that the ancient forest was home to conifers, ferns, flowering shrubs and bacteria. The sediment analysis showed no traces of salt, suggesting it was a freshwater swamp. This proved that during the mid-Cretaceous Period, Antarctica was largely ice-free and had dense, temperate and swampy forests similar to forests found in New Zealand today.
Scientists, including co-researcher, Ulrich Salzmann, a paleoecologist at Northumbria University, UK reconstructed the environment of this preserved rainforest by analyzing descendant plants, temperature and precipitation indicators within the sample. An animated film of the discovery was created, presenting the diverse forest flora (vegetation) and warmer conditions.
An illustration of the temperate rainforest that thrived in West Antarctica about 90 million years ago.
(credit: James McKay/Alfred Wegener Institute; Creative Commons licence CC-BY 4.0)
The presence of these conditions was narrowed down to the Turonian-Santonian Age (92 to 83 million years ago) in the middle of the Cretaceous Period (145 to 65 million years ago). This is known to have been one of the warmest periods on Earth in the last 140 million years, based on analyses of fossils and sediment collected from the seafloor closer to the equator. Dinosaurs roamed the Earth and sea-surface temperatures in the tropics were as warm as 35°C, resulting in sea levels 170m higher than they are today. Average annual temperatures in West Antarctica were 12°C, and summertime temperatures reached 19°C. In rivers and swamps, the temperature would have reached up to 20°C. The rainfall then was comparable to the rainfall of current-day Wales (2464mm annually).
Today’s atmospheric carbon dioxide levels average around 407 parts per million (ppm). Atmospheric carbon dioxide levels were thought to be 1000 ppm during the mid-Cretaceous Period. But for a forest to reach Antarctica, more potent greenhouse conditions must have existed than previously thought. According to lead researcher Johann Klages, a marine geologist and Gerrit Lohmann, a climate modeler at the Alfred Wegener Institute, atmospheric carbon dioxide levels were between 1120 and 1680 ppm. Due to the drifting of tectonic plates, the drill site would have been several hundred kilometres closer to the South Pole, subjecting the South Pole to a four-month polar night (no sunlight). But Antarctica was able to maintain a temperate climate, showing the extreme potency of carbon dioxide. However, high carbon dioxide levels alone would not have been enough to keep mild temperatures close to the pole. If a white ice sheet were present, it would have reflected much of the incoming sunlight back into space, keeping the land cold. But the vegetation cover absorbs more solar heat and increases the greenhouse warming.
This discovery is significant because it shows how important the cooling effects of ice sheets are in preventing greenhouse warming. This extreme climate change warns society of what a greenhouse climate could look like, as the use of fossil fuels increase concentrations of carbon dioxide. The research will inform scientists what caused the climate to cool dramatically, allowing the ice sheets seen today to form.
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Harry Cockburn (2020, April 6), Fossilised remains of 90-million-year-old rainforest discovered under Antarctic ice. Retrieved from https://www.independent.co.uk/news/science/antarctica-rainforest-ancient-fossils-cretaceous-period-climate-change-a9449791.html
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Klages, J.P., Salzmann, U., Bickert, T. et al. (2020, April 1), Temperate rainforests near the South Pole during peak Cretaceous warmth. Nature 580, 81–86 (2020, April 1). Retrieved from https://doi.org/10.1038/s41586-020-2148-5
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