90 million years ago, when the dinosaurs were at their peak, the west coast of Antarctica was a very different place from what we know today: its average annual temperature was 12 ° C and it was covered with temperate forests. This is the fascinating discovery that an international team of scientists has made after analyzing a sample of Cretaceous sediments found in the Amundsen Sea in 2017.
It is known that the middle Cretaceous was the warmest period on our planet in the last 140 million years. At that time, when dinosaurs were still roaming around, the sea level was 170 times higher than it is today and it is estimated that the temperature of the ocean at the height of the tropics would have reached 35 ° C, but much is known little about what the conditions were in the polar circles.
The team of scientists, from different institutions in Germany and the United Kingdom, was near the Pine Island and Thwaites glaciers taking sediment samples with a drilling platform 30 meters deep from the seabed. ” When we did a first evaluation of the samples on board the ship, we were struck by the coloration of the core of the sediment, it was clearly different from the upper layers ,” explains Johann Klages, geologist at the Helmholtz Center for Polar and Marine Research at the Alfred Institute. Wegener (Germany) and first author of the work, which is published in Nature .
90 million year old plants
Once in the laboratory, the scientists analyzed the sediments with X-ray computed tomography. The results revealed that the sample contained a dense tangle of fossil roots, so well preserved that they even allowed them to identify cellular structures. The sample also contained remains of pollen, spores and even flowering plants that had never before been found in these latitudes . “It is fascinating to see fossil pollen and other plant remains so well preserved in a sediment deposited 90 million years ago,” explains Ulrich Salzmann, a paleoecologist at the University of Northumbria. “All this tells us that the west Antarctic coast was, at that time, a temperate and swampy forest, very similar to what we can find in New Zealand today.”
Reconstruction of the climate of the past
To reconstruct the environment of these prehistoric forests, the scientists looked at the climatic conditions in which the plants descended from those found in the sediment sample live. They also used other biological and geochemical indicators of temperature and precipitation that they found in the sample.
In this way, they concluded that, 90 million years ago, the climate of this region 900 kilometers from the South Pole was temperate, with average annual temperatures of 12 ° C (two degrees higher than the average temperature in Germany, for example). . On average, in summer it would reach 19 ° C and the water in the rivers would reach 20 ° C. Furthermore, the amount and intensity of annual precipitation would be similar to that of Wales.
Carbon dioxide concentration was higher than expected
This is an extraordinary discovery, especially considering that at this latitude there are approximately four months a year of absolute darkness. These conditions of precipitation and temperature were possible thanks to three factors: the Antarctic continent would have been covered with dense vegetation, there were no large masses of ice on the earth’s surface and, finally, the amount of carbon dioxide in the atmosphere was much higher. which had been previously assumed for the Cretaceous period. “Until now, the global concentration in the Cretaceous was thought to be about 1000 parts per million (ppm). But according to the calculations of our paleoclimatic models, concentration levels of 1120 to 1680 ppm were needed to reach the average temperatures that were at that time in Antarctica, “explains co-author and climate modeler Gerrit Lohmann.
These results also reveal the enormous capacity of greenhouse gases to heat the atmosphere, as well as the importance of the cooling effect produced by today’s ice sheets. “In the Cretaceous there were also four months in a row without light. But, because the concentration of carbon dioxide was so high, the climate around the South Pole was temperate, with no ice masses, ” explains Torsten Bickert, a geoscientist at the center of MARUM research from the University of Bremen.
The question now is to know what was the cause that motivated the drastic cooling of this area, so powerful that it caused the formation of the current ice sheets. Finding the explanation is one of the great challenges facing the international community of climate scientists.
Referencia: Klages, J.P. et al, “Temperate rainforests near the South Pole during peak Cretaceous warmth”, Nature, 1 April 2020, DOI: 10.1038/s41586-020-2148-5
