Evidence of ancient wildfires near the South Pole

When the international team led by marine geologist Dr Johann Klages of the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI)published the results of a Polarstern expedition in Nature in 2020, the discovery caused quite a stir. “In a sediment core from the Amundsen Sea in West Antarctica, we found an extremely well-preserved forest soil dating back around 90 million years, containing abundant pollen and spores and a dense network of roots,” explains Johann Klages. “During the Cretaceous, tectonic conditions meant that this temperate rainforest lay even farther south, just 900 kilometres from the South Pole. Today, average annual temperatures at this location are around -30°C and everything is covered by an ice sheet several kilometres thick. Our discovery showed that during the warmest period of the Cretaceous, when atmospheric CO₂ levels were four to six times higher than today, a relatively warm and humid climate prevailed – even close to the South Pole.”

The researchers have now followed up with a new study in Communications Earth & Environment. Johann Klages, co-first author of the study, notes: “We took another very close look at the core using a different range of methods and found clear evidence of recurring wildfires. The sediments recovered contain evidence of the southernmost wildfires ever recorded on Earth.”

Throughout the Late Cretaceous section of the core, the team found microscopic charcoal particles that became increasingly abundant in the younger sediments. Analysis of the particles showed that mainly soft conifer wood had burned at relatively low temperatures, as is typical of surface fires that do not spread through the entire forest canopy. Further evidence of fire came from amber – fossilised tree resin – which appears to have formed a kind of protective seal over fire-damaged areas of tree trunks. Finally, countless spores of peat moss (genus: Sphagnum) provided evidence of the earliest raised bogs, the formation of which was closely linked to the occurrence of recurring fires. 

From these clues, the researchers were able to develop a scenario that may also be highly relevant today. “90 million years ago, just 900 kilometres from the South Pole, a temperate and very swampy Antarctic rainforest with pronounced dry and monsoon seasons gradually silted up and developed into a peatland dominated by peat mosses,” summarises co-first author Prof. Dr Ulrich Salzmann of Northumbria University in England. “Wildfires, which became increasingly frequent, clearly played a decisive role in this process. They kept the vegetation open and enabled the development of a peat bog where ground-level smouldering fires then occurred repeatedly, similar to those that have become increasingly common in German peatlands in recent years.” Because volcanically active areas were at least 400 kilometres away at the time, it was probably lightning from thunderstorms, rather than lava or volcanic ash flows, that triggered the large-scale fires.

“Peatlands play a key role in the climate system, both then and now, as highly effective long-term stores of carbon,” adds co-first author Prof. Dr Thorsten Bauersachs of RWTH Aachen University. “In a warm climate, wetlands dry up more quickly and fires that prevent forest development occur more frequently. This can promote the formation of peatlands, where carbon-rich plant material accumulates over long periods.” Johann Klages puts this into context: “The Late Cretaceous was extremely warm and prone to frequent fires. We were surprised to find evidence in our core of an early Antarctic ecosystem that, in many respects, resembled today’s Arctic raised bogs and also experienced regular fires.”