42,000-year-old trees allow a more accurate study of the Earth’s final magnetic field rotation

The last complete revolution of the Earth’s magnetic field, the so-called Laschamps event, occurred 42,000 years ago. Radiocarbon analyzes of the remains of New Zealand kauri trees now make it possible for the first time to study and analyze this event and its associated effects, as well as ‘capitalization of geological archives such as sediment and ice rights from this period. Symbols based on this show that the strong reduction of the magnetic field had a significant effect on the Earth ‘s atmosphere. This is illustrated by an international team led by Chris Turney from the University of New South Wales in Australia, with Norbert Nowaczyk participating from the German Study Center for Geosciences Potsdam and Florian Adolphi from the Alfred Wegener Institute, in a study that is now appearing in a journal Science.

There are permanent variations in the Earth’s magnetic field and sometimes even a reversal of polarity occurs. Their causes, course and effects are not yet fully understood. Researchers have now studied the Laschamps incident in more detail. It refers to the last complete reversal of the polarity of the Earth’s magnetic field around 42,000 years ago. Not only did the magnetic field change direction, it lost much strength over a period of several hundred years.

About 42,000 years ago, the pole moved north to south. In this process, which lasted about 500 years, the magnetic field weakened to between six and zero percent. Over a period of about 500 years, the poles stayed back, with field strength varying below 28 percent of today’s value, only to return over about 250 years.

This exact time classification is now possible by combining different datasets. The researchers first used results on the Earth’s magnetic field from Black Sea sedimentary rocks by Norbert Nowaczyk and his team from 2013, which were matched by Greenland’s ice cores through climate change recorded at the one time.

Second, the analysis and date of the events were only made possible by the radiocarbon analysis (14C) of a sub-fossil kauri tree grown in Ngawha wetlands in northern New Zealand for approx. on 1700 years during that period and was later well preserved in the bogs.

Chris Turney had reported this discovery some 40,000 years ago when he visited the German Geological Survey in Potsdam (GFZ) a few years ago. “As a geomagnetic scientist, I was immediately connected to the Laschamps event in my mind and suggested 14C analyzes, which have not yet been performed on trees since,” says Nowaczyk, who is in charge of the laboratory for Palaeo- and Rock Magnetism at the GFZ.

The background: With the erosion of the magnetic field, the Earth loses an important protective shield against cosmic radiation, at least in part. This is also reflected in the higher levels of the radioactive carbon isotope 14C in the trees. The reason for this is that more 14C is created in the Earth’s atmosphere during the explosion of nitrogen by cosmic particles full of energy, lightning.

“The subfossil kauri trees are an interesting archive of atmospheric creation,” says Florian Adolphi, palaeoclimatologist at the Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research (AWI). These trees can live for several thousand years and record annual changes in atmospheric radiocarbon content as they grow, the research team measured in detail.

“These data improve the calibration curve for radiocarbon dating, allowing a more accurate selection of a wide range of climate and fossil archives. They also allow direct comparisons to ice cores: beryllium isotopes are present. the measure there reveals radiocarbon-like changes in the trees, as the production of the two isotopes in the Earth’s atmosphere is dependent on the intensity of cosmic rays hitting the Earth, “the study’s co-author explains . He uses this effect to synchronize ice trees and corries with high precision and to reduce uncertainty comparing the two archives from several thousand years to about 100 years.

To study the other effects of the Earth’s weak magnetic field on the atmosphere and thus also on global climate, the researchers made simulations of atmospheric chemistry. Among other things, they found a reduction in ozone. “Unchanged radiation from space broke air particles in the Earth’s atmosphere, separated from electricity and released light – a process known as ionization,” Turney explains. “The ionized air showers the ozone layer.” This triggered a wave of changes in the atmosphere, including more bright light indicating that we know it as the aurora borealis, which at the time seen not only near the poles but all over the globe.

It is important to further analyze the effects of the weak magnetic field towards this path according to conventional developments, says Nowaczyk. Because the Earth’s magnetic field has already been weakening for about 2000 years. Compared to the first measurements just 170 years ago, a narrowing of nine per cent was observed, in the Atlantic region even thirty per cent. There is debate as to whether this means that pole turning is going on for the next mile or two. However, the collapse of the natural radiation shield would be a major challenge for our society today, which is largely based on electronics.

Based on these new opportunities for the chronological classification of events 42,000 years ago, the study’s lead authors put forward even broader ideas about the effects of the Earth’s magnetic field revolution – for example on the extinction of the Neanderthals. or the beginning of cave paintings. Nowaczyk does not rule out that there are causal links here, but he believes that it is unlikely.

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