Ancient kauri tree from Ngawha, New Zealand. Credit: Nelson Parker
Measurement of radiocarbon on the remains of 42,000-year-old New Zealand kauri trees provides the basis for better calibration of geological archives of 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 the remains of kauri trees from New Zealand now make it possible for the first time to precisely time and analyze this event and its associated consequences, as well as to calibrate geological archives such as sediment and ice cores from this period. Simulations based on this show that the strong reduction of the magnetic field had significant effects in the earth’s atmosphere. This is shown by an international team led by Chris Turney of the Australian University of New South Wales, with the participation of Norbert Nowaczyk of the German Research Center for Earth Sciences Potsdam and Florian Adolphi of the Alfred Wegener Institute, in a study now published in the journal Science.
The earth’s magnetic field undergoes permanent fluctuations and sometimes even reversal of polarity occurs. The causes, course and consequences thereof are not yet fully understood. Researchers have now investigated the so-called Laschamps event in more detail. This refers to the last complete reversal of the polarity of the Earth’s magnetic field about 42,000 years ago. Not only did the magnetic field change direction, it also lost dramatic power 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 to between six and zero percent. For a period of about 500 years, the poles remained inverted, with a field strength varying below 28 percent of the present value, but only again over the course of about 250 years.
Ancient kauri tree from Ngawha, New Zealand. Credit: Nelson Parker
This exact chronological classification is now possible by linking different data sets. First, the researchers used results on Earth’s magnetic field of sediment cores from the Black Sea by Norbert Nowaczyk and his team from 2013, which correspond to the Greenlandic ice cores with the climate variation documented at the same time.
Second, the precise analysis and dating of the events was only made possible by the radiocarbon (14C) analysis of a subfossil kauri tree that grew during the period in question about 1700 years in the wetlands of Ngawha in northern New Zealand. . thereafter very well preserved in the swamps.
Chris Turney reported this finding about 40,000 years ago during a visit to the German Research Center for Earth Sciences in Potsdam (GFZ) a few years ago. “As a geomagnetic scientist, I immediately had in mind a link to the Laschamps event and suggested 14C analyzes, which had not yet been done on trees at that time,” says Nowaczyk, who heads the Laboratory for Paleo and Rock Magnetism. GFZ.
The background: With the waning of the magnetic field, the earth loses an important protective shield against cosmic radiation, at least in part. This is also reflected in elevated levels of the radioactive carbon isotope 14C in the 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.
“The subfossil kauri trees are an exciting archive of atmospheric composition,” said Florian Adolphi, paleoclimatologist at the Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research (AWI). These trees can live for thousands of years and absorb annual variations in atmospheric radiocarbon content as they grow, which the research team accurately measured.
“These data improve the calibration curve for dating of radiocarbon, which can more accurately date a wider range of climate archives and fossils. They also allow a direct comparison with ice cores: beryllium isotopes measured there show similar variations as the radiocarbon in the trees, since the production of both isotopes in the atmosphere of the earth depends on the intensity of cosmic rays that the hit the earth, ‘explains the study author. He uses this effect to synchronize trees and ice cores with high precision and to reduce the uncertainty of comparing the two archives of a few thousand years with about 100 years.
To investigate further effects of the weak Earth’s magnetic field on the atmosphere and therefore also on the world climate, the researchers performed simulations of atmospheric chemistry. Among other things, they found a decrease in ozone. “Unfiltered radiation from space breaks up air particles in the Earth’s atmosphere, emitting electrons and emitting light – a process called ionization,” Turney explains. “The ionized air has hissed the ozone layer.” It caused a wave of changes in the atmosphere, including increased glittering light showing that we know the aurora borealis, which was observed not only near the poles at the time, but all over the world.
It is important to further analyze the effects of the weak magnetic field in this direction in the light of current developments, says Nowaczyk. Because the Earth’s magnetic field has been weakening for about 2000 years. Compared to the first direct measurements 170 years ago, a weakening of nine percent was observed, in the area of the South Atlantic even thirty percent. Whether this means that a reversal of the pole is going on the next one to two thousand years can be discussed. However, a collapse of the natural radiation screen would pose a major challenge to our contemporary society, which is very much based on electronics.
Based on these new possibilities for the chronological classification of events 42,000 years ago, the lead authors of the study made even more profound hypotheses about the consequences of reversing the Earth’s magnetic field – for example, with regard to the extinction of the Neanderthals or the commencement of cave paintings. Nowaczyk does not rule out the possibility that causal links exist here, but considers them unlikely.
For more information on this research, read the reversal of the Earth’s magnetic field 42,000 years ago in 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, Science.
DOI: 10.1126 / science.abb8677