42,000 years ago, Earth’s magnetic field temporarily reversed for a few centuries. With the help of a tree trunk from this period preserved in the swamp, researchers have now made a more accurate history of the so-called Champagne event. At the same time they found that the Earth’s magnetic field was very weak before the reflection. Its field strength could be as low as six percent of the normal range. This had consequences not only for the atmosphere, but also for the climate and environment on our planet, as scientists have shown using simulations.
Earth’s magnetic field is the most important protection for us against solid radiation from space. But their polarity and intensity do not always remain constant. Several times over the course of Earth’s history, the Earth’s magnetic field has completely reversed its polarity – the magnetic north pole has become the south pole and vice versa. In some of these events, this reversal of polarity lasted for millions of years, and in others, so called trips, the magnetic field jumped back to its ancient poles after a few hundred years. The last of these magnetic trips was about 42,000 years ago. At that time, there was a polar reversal that lasted about 800 years, as the magnetic field strength dropped to about 28 percent of its original value. During the previous stage of the chaotic redirection of the magnetic field, it would have weakened to as little as ten percent.
Fossil trees as contemporary witnesses
Alan Cooper of the Museum of South Australia in Adelaide and colleagues have now studied closely when this vulnerability occurred, how long it lasted and what the consequences would be. A lucky coincidence came to their aid: in a wetland in northern New Zealand, the marshes have preserved some kauri trees (Agathis australis) that grew during the time of the polar reversal. One of these fossil trees was 1,700 years old, and thus their tissues contained isotopic evidence from every time that happened to Champagne and the centuries that followed. For their study, the researchers analyzed the carbon-14 isotope content in the tree’s annual rings. This isotope is increasingly forming in Earth’s atmosphere when the magnetic field weakens and cosmic radiation splits nitrogen atoms into carbon.
Through these analyzes, scientists were able to more accurately date the time and duration of the Champagne event. “For the first time, we can accurately determine the time and effects of the recent reversal of the polarity of the Earth’s magnetic field,” says co-author Chris Turney of the University of New South Wales in Sydney. Accordingly, this reversal of polarity occurred 41,500 years ago. But even before that, about 42,350 years ago, there was a strong increase in C14 values in Earth’s atmosphere. The researchers take this as an indication of a longer phase of chaotic reorganization of the magnetic field, as its intensity decreased to very low values. They called this stage the “Adams Transitional Geomagnetic Event” – after author Douglas Adams. “During the Adams event, the intensity of the Earth’s magnetic field decreased to only zero to six percent,” says Tierney. “In principle, at that time we no longer had a magnetic field – the cosmic radiation shielding screen had completely disappeared.”
Northern lights, UV exposure and climate change around the world
Scientists have used various past climate and environmental archives and simulation models to investigate the consequences of this. Because the new data from the Kauri tree also enabled a more accurate synchronization of the geochemical witnesses for each era. “These curie trees are like Rosetta Stone,” Cooper explains. “They help us collect evidence of environmental change from caves, ice cores and peat swamps around the world.” Assessments have shown that weakening Earth’s magnetic field some 42,000 years ago has had major impacts on Earth’s climate and environmental conditions. Among other things, the increased bombardment by cosmic rays has significantly thinned the protective ozone layer. “The unfiltered radiation from space has torn apart oxygen molecules in the Earth’s atmosphere, separating electrons and causing light emissions – a process called ionization,” explains Tierney. The resulting charged particles destroyed the ozone layer and, along with the weakening of the magnetic field, caused auroras to appear over the entire globe.
At the same time, these processes in the atmosphere led to widespread changes in air currents and climate, as the researchers determined using models. In the temperate latitudes of the Southern Hemisphere, including Australia, the ultraviolet flux increased, the climate became cooler and drier, and the glaciers advanced in the Northern Hemisphere. “The Adams event appears to represent a significant climate, environmental and archaeological change that has yet to be discovered,” says Cooper and colleagues. In her opinion, these changes may also be related to some disturbances in the animal world and human development that occurred around this time: in Europe and Southeast Asia, people suddenly began to visit caves and paint their rock walls. In Australia the megafauna died and in Europe Neanderthals died. The scientists concluded that “these findings generally raise important questions about the evolutionary effects of magnetic polar reversals and trips over the course of Earth’s history.”
Cowell: Alan Cooper (Museum of South Australia, Adelaide) et al., Science, Doi: 10.1126 / science. abb8677