A new study has suggested that the unexpected reversal of Earth’s magnetic poles approximately 41,000 - 42,000 years ago may have drastically decreased the magnetic field’s strength, thus precipitating a number of environmental crises on Earth.
New, accurate carbon dating procedures obtained data from ancient tree fossils through which scientists were able to observe correlations between shifts in climate patterns, large mammal extinctions, and changes in human behavior and the Laschamps excursion. The Laschamps excursion was a brief reversal of Earth’s magnetic field that occurred during the Last Glacial Period. The field was reversed for approximately 400 years and this reduced its strength by 75%.
Earth’s protective magnetic field protects the planet from an outpour of charged particles streaming from the sun. During a reversal, the field can diminish in strength and researchers have suggested that this decrease in strength could be connected to mass extinction events. However, it has been difficult to procure conclusive evidence for these suggestions because according to evolutionary biologist Alan Cooper, “the general belief had been that geomagnetic changes had no impact on climate or anything else”. This belief has been supported by a lack of precise dates for the length of the reversal event to correlate with environmental, magnetic rock, and ice core records.
Currently, the Australian Research Council is funding research to observe New Zealand’s kauri tree, one of the oldest trees in the world. Carbon dating has revealed that the tree was alive during the Laschamps excursion (41,000-42,000 years ago). Cooper and his team acquired cross-sections from four ancient trees retrieved from a swamp at Ngāwhā Springs in New Zealand. These cross-sections were analyzed for carbon-14 which is a radioactive form of carbon. A giant log dating to approximately 14,000 years ago showed a 1,700 year-long carbon-14 record which unveiled significant changes in carbon-14 before and during the Laschamps excursion. These changes have been attributed to the fact that increasing incoming cosmic rays, which would have been seen with the weakened magnetic field, would have produced more carbon-14. This increase in carbon-14 would have then been absorbed into the tree’s tissues.
Cooper and his colleagues simulated how a weakened magnetic field would have impacted weather patterns and computer analysis suggested that the rise in charged particles entering the atmosphere would have also increased the release of atmospheric hydrogen and nitrous oxides. These molecules usually take in ozone which would have decreased the availability of stratospheric ozone that shields Earth’s inhabitants from UV radiation. The changes in the atmosphere would have also affected the amount of sunlight that was incorporated into the different layers of the atmosphere which would have lead to dramatic changes in weather patterns that would have cooled the Earth. The carbon-14 data that was taken from the kauri trees, as well as sediment, pollen, and other data from before and during the Laschamps excursion, directed Cooper’s team to sudden cooling locations from Australia to the Andes. The team discovered that the most extreme effects did not take place during the actual reversal event but instead in the years leading up to it (41,600 - 42,300 years ago).
Scientists have debated who is to blame for the extinction of giant mammals such as the woolly mammoth for years, a discussion that, coauthor Chris Turney, says was one of the motivators for the study. In a study from 2015 by Cooper, Turney, and their colleagues, they noticed that “some of the megafaunal extinctions appear to cluster” and they questioned why.
The team compared the dates of the magnetic reversal with previous records from ice cores that reflect changes in solar activity, which suggested that solar activity was at a minimum during the event. They concluded that the combination of the weakened magnetic field and a decrease in the sun’s output “created the perfect storm” of climate and environmental conditions that contributed to the extinction of megafauna populations. More evidence lies in a diminished ozone layer and an increase in red ochre handprints made by humans in cave paintings. Red ochre is said to have been used as a sunscreen. Also, the increase in the use of caves 40,000 - 42,000 years ago indicates that the sun may have been more intense, thus increasing the need for shelter.
Monika Korte says that “this is the first study to consider such a broad range of environmental consequences of extreme magnetic field changes” and that the “suggested links seem conceivable” to her. She also says that the paper contributes many ideas that must be further explored.
Turner says that while “whether other magnetic reversals may have prompted similar upheavals in the past remains unknown”, they hope that the scientific community will analyze biological and archaeological datasets with a new perspective. He also feels that the key to discovering the effects of magnetic reversals on similar environmental disruptions will be increasing the accuracy of isotope dating for these events. He adds that radiocarbon dates observed from the kauri trees may be able to assist with investigations into other recent reversals, such as the Mono Lake Excursion that took place approximately 34,000 years ago. Cooper says that the big question is “what long-term environmental havoc may have been wreaked by much longer reversal events recorded in ancient rocks, such the 20,000-year-long Brunhes-Matuyama reversal that began 781,000 years ago”. Continued research into this field will shed more light on these unknowns, furthering our understanding of magnetic field reversals and their effects on the environment.
Gramling, C. (2021, February 19). A magnetic field Reversal 42,000 years ago may have contributed to mass extinctions. Retrieved March 12, 2021, from https://www.sciencenews.org/article/earth-magnetic-field-reversal-mass-extinctions-environment-crisis