Today Antarctica is the most hostile and coldest continent on our planet. But that was not always the case, as a drill core from West Antarctica now reveals. In it, scientists discovered relics of a temperate rainforest that are around 90 million years old. Tree roots, pollen and other plant remains prove that there was a mild, humid climate at that time even near the South Pole. The researchers conclude that Antarctica must have been largely ice-free in the Middle Cretaceous and that the carbon dioxide levels in the atmosphere were even higher than previously thought.
The Cretaceous period includes some of the warmest climatic periods in the history of the earth. Especially in the period from 94 to 84 million years ago there was an almost global greenhouse climate, as fossil finds and isotope measurements suggest. At that time the tropical seas were around 35 degrees warm and it was mild and humid even in our latitudes. Scientists suspect that the carbon dioxide content of the atmosphere at that time could have been around 1000 parts per million (ppm) – today the CO2 value is around 400 ppm. Because the ice deposits in the polar regions were strongly decimated, sea levels in this phase of the Middle Cretaceous were also up to 170 meters higher than today. However, it was still unclear to what extent there was still polar ice at that time: Were there any larger glaciers at the north and south poles? “Because geological evidence from the area south of the Antarctic Arctic Circle is almost completely absent, it is controversial whether polar ice could still exist under such environmental conditions,” explain Johann Klages from the Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research (AWI) and his colleagues.
Rich flora 90 million years ago
In order to close this knowledge gap, Klages and his team have now examined a drill core that the research vessel Polarstern extracted in 2017 from the sea floor of the Amundsensee off Western Antarctica. The sediment core drilled in front of the ice edge of the Pine Island Glacier extends to a depth of 30.70 meters and at its lower end also traverses some rock layers deposited during the Middle Cretaceous Period. Already when the researchers brought this drill core on board, they noticed the first abnormalities: “When we first examined it on board, we immediately noticed the unusual color of this sediment layer. It was very different from the deposits above it, ”reports Klages. “Initial analyzes also suggested that we encountered a layer at a depth of 27 to 30 meters below the sea floor that must have formed on land and not in the sea.”
In order to learn more about this striking layer, the researchers subjected this part of the core to intensive chemical, paleontological and mineralogical investigations. A fluoroscopy using high-resolution computed tomography (CT) also revealed even fine structures inside the drill core. These pictures revealed surprising things, because they showed a petrified root network, which ran through the entire layer of soil made of very fine-grained clay and silt and was so well preserved that the scientists could even recognize individual woody cell groups. The soil sample also contained numerous pollen and spores from various vascular plants, including traces of the first flowering plants within these high Antarctic latitudes. “The many plant remains indicate that the coastal area of West Antarctica 93 to 83 million years ago formed a swamp and bog landscape in which a temperate rainforest with many conifers and tree ferns grew – as is still the case today, for example South Island of New Zealand, ”says co-author Ulrich Salzmann from Northumbria University in Newcastle upon Tyne.
Mild climate even at the South Pole
This proves that the climate in this part of the Cretaceous period must have been mild enough to allow a moderate rainforest to grow even near the South Pole. Because, as additional studies have shown, the drilling site was located near the poles some 90 million years ago, despite the continental drift that has since expired. “We determined a paleo-latitude of 81.9 ° south,” the researchers report. At that time, the rainforest was only around 900 kilometers from the South Pole and experienced a polar night of around four months. In around a third of the year, this ecosystem had to do without the sun – how did it do it? In order to get a more precise idea of the climate at that time in this region, the researchers examined, among other things, the climatic conditions under which today’s relatives of fossil plants grow. In addition, they also used chemical analyzes to infer the temperature and precipitation conditions at that time.
The analyzes showed that Antarctica must have been largely ice-free around 90 million years ago – and extremely warm. Because the average annual air temperature at that time was around 13 degrees – this is two degrees more than the average annual temperature in today’s Germany. In the summer, the air in the Antarctic could have warmed to around 19 degrees, the surface temperature of the rivers and swamps was probably even around 20 degrees, as Klages and his colleagues report. Rain fell abundantly and often in this area at around 1120 millimeters per year. “Our data thus provide, as far as we know, the southernmost evidence of the Cretaceous environmental conditions,” say the researchers. “They reveal a ‘greenhouse climate’ that allowed mild climatic conditions far further south than previously documented.”
Using a climate model, the scientists reconstructed how such a warm climate could have been possible so close to the South Pole. It was clear from this that such conditions could only be achieved if there were no ice sheets in the South Pole region at that time and the entire Antarctic was covered by dense vegetation. In addition, the CO2 values of the atmosphere in the Middle Cretaceous period must have been even higher than previously thought. “Until our study, it was assumed that the global carbon dioxide concentration in the chalk era was around 1000 ppm. In our model tests, however, values from 1120 to 1680 ppm were necessary to reach the temperatures in the Antarctic at that time, ”explains co-author Gerrit Lohmann from AWI. Only then would the greenhouse effect have been strong enough to keep the climate mild even during a four-month polar night.
Source: Johann Klages (Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research (AWI), Bremerhaven) et al., Nature, doi: 10.1038 / s41586-020-2148-5