42,000-year-old trees allow accurate analysis of the last reversal of Earth’s magnetic field


Ancient kauri tree log from Ngawha, New Zealand. Credit: Nelson Parker

Radiocarbon measurements on the 42,000-year-old New Zealand kauri tree remains provide the basis for a better calibration of geological archives from this period.

The last complete reversal of the Earth’s magnetic field, the so-called Laschamps event, took place 42,000 years ago. Radiocarbon analyzes of New Zealand kauri tree remains now allow for the first time to accurately time and analyze this event and its associated effects, as well as calibrate geological records such as sediments and ice cores from this period. Simulations based on this show that the strong reduction in the magnetic field had considerable effects on the Earth’s atmosphere. This is demonstrated by an international team led by Chris Turney from the Australian University of New South Wales, with the participation of Norbert Nowaczyk from the German Research Center for Geosciences in Potsdam and Florian Adolphi from the Alfred Wegener Institute, in a study that now appears in the newspaper Sciences.

The Earth’s magnetic field undergoes permanent fluctuations and occasionally there are even polarity reversals. Its causes, course, and effects are not yet fully understood. Researchers have now investigated the so-called Laschamps event in more detail. It refers to the last complete reversal of the polarity of the Earth’s magnetic field about 42,000 years ago. The magnetic field not only changed direction, it also lost strength dramatically over a period of several hundred years.

About 42,000 years ago, the magnetic north pole moved south. Within this process, which lasted about 500 years, the magnetic field weakened between six and zero percent. Over a period of roughly 500 years, the poles remained reversed, with field strengths varying below 28 percent of today’s value, only to reverse again over the course of roughly 250 years.

Ancient Kauri Tree Log from Ngawha, New Zealand

Ancient kauri tree log from Ngawha, New Zealand. Credit: Nelson Parker

This exact chronological classification is now possible by linking different data sets. First, the researchers used the Earth’s magnetic field results from Black Sea sediment cores by Norbert Nowaczyk and his team from 2013, which were compared to Greenland ice cores through climate variation documented at the same time. .

Second, accurate analysis and dating of events was only possible thanks to radiocarbon (14C) analysis of a subfossil kauri tree that grew in the Ngawha wetlands in northern New Zealand for approximately 1700 years during the period in question and was subsequently very well preserved in the swamps.

Chris Turney had reported on this find about 40,000 years ago during a visit to the German Research Center for Geosciences in Potsdam (GFZ) a few years ago. “As a geomagnetic scientist, I immediately had a link to the Laschamps event in mind and suggested analyzes of 14C, which had not yet been done on trees at that time,” says Nowaczyk, who directs the Laboratory for Rock Paleo- and Magnetism at the GFZ.

The background: With the decrease in the magnetic field, the Earth is losing an important protective shield against cosmic radiation, at least in part. This is also reflected in increased levels of the radioactive carbon isotope 14C in trees. The reason for this is the increased formation of 14C in the Earth’s atmosphere during the bombardment of nitrogen by high-energy electrically charged cosmic particles.

“Subfossil kauri trees are a fascinating archive of atmospheric composition,” says Florian Adolphi, paleoclimatologist at the Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research (AWI). These trees can live for several thousand years and record annual variations in atmospheric radiocarbon content as they grow, which the research team accurately measured.

“These data improve the calibration curve for radiocarbon dating, allowing for more accurate dating of a wide range of fossil and climate archives. They also allow a direct comparison with ice cores: beryllium isotopes measured there show variations similar to radiocarbon in trees, since the production of both isotopes in the Earth’s atmosphere depends on the intensity of the cosmic rays that hit the Earth ” explains the study co. -Author. Use this effect to synchronize trees and ice cores with high precision and reduce the uncertainty of comparing the two files from several thousand years to about 100 years.

To further investigate the effects of the weak Earth’s magnetic field on the atmosphere and thus also on the global climate, the researchers carried out simulations of atmospheric chemistry. Among other things, they found a decrease in ozone. “Unfiltered radiation from space was breaking up air particles in Earth’s atmosphere, separating electrons and emitting light, a process called ionization,” Turney explains. “Ionized air ‘sizzled’ the ozone layer.” This set off a wave of changes in the atmosphere, including an increase in the dazzling light shows we know as the Northern Lights, which at the time may have been observed not just near the poles but around the world.

It is important to further analyze the effects of the weak magnetic field in this direction in view of current developments, Nowaczyk says. Because the Earth’s magnetic field has already been weakening for about 2000 years. Compared to the first direct measurements 170 years ago, a weakening of nine percent was observed, in the South Atlantic area even thirty percent. It is debatable whether this means that a pole reversal is in sight for the next thousand or two thousand years. However, a collapse of the natural radiation shield would pose a great challenge to our current society, which is largely based on electronics.

On the basis of these new possibilities for the chronological classification of the events of 42,000 years ago, the main authors of the study put forward even more momentous hypotheses about the effects of the reversal of the Earth’s magnetic field, for example regarding the extinction of Neanderthals or the beginning of cave paintings. Nowaczyk does not rule out the possibility that there are causal connections here, but considers it quite unlikely.

To learn more about this research, read Earth’s magnetic field reversal 42,000 years ago triggered a global environmental crisis.

Reference: “A Global Environmental Crisis 42,000 Years Ago” by Alan Cooper, Chris SM Turney, Jonathan Palmer, Alan Hogg, Matt McGlone, Janet Wilmshurst, Andrew M. Lorrey, Timothy J. Heaton, James M. Russell, Ken McCracken, Julien G. Anet, Eugene Rozanov, Marina Friedel, Ivo Suter, Thomas Peter, Raimund Muscheler, Florian Adolphi, Anthony Dosseto, J. Tyler Faith, Pavla Fenwick, Christopher J. Fogwill, Konrad Hughen, Mathew Lipson, Jiabo Liu, Norbert Nowaczyk, Eleanor Rainsley, Christopher Bronk Ramsey, Paolo Sebastianelli, Yassine Souilmi, Janelle Stevenson, Zoë Thomas, Raymond Tobler and Roland Zech, February 19, 2021, Sciences.
DOI: 10.1126 / science.abb8677



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