A few years ago, workers paving the way for a power station in New Zealand unearth record for lost time: a chest of 60 tons of a kauri tree, the largest tree species in New Zealand The tree, who grew there 42000 years, was preserved in a bog and its rings lasted 1700 years, capturing a tumultuous time when the world was turned upside down – at least magnetically speaking

The radiocarbon levels in this piece of wood and several others indicate an increase in radiation from space, as the earth's protective magnetic field weakened and its poles turned, reports a team of scientists today in Science Modeling the effect of this radiation on the atmosphere, the team suggests that Earth's climate has briefly changed, possibly contributing to the extinction of large mammals in Australia and Neanderthals in Europe “We are only scratching the surface of what geomagnetic change has done”, says Alan Cooper, former DNA researcher at the South Australian Museum and one of the lead authors of the study.

The study not only describes in detail the timing and extent of the magnetic exchange, the most recent in Earth's history, but is also among the first to present a credible case, although speculative, that these reversals may affect the global climate , says Quentin Simon, paleomagnetist at the European Center for Research and Teaching in Environmental Geosciences in Aix-en-Provence, France But some paleoclimatic scientists are skeptical of the team's more general claims, claiming that other documents show little evidence of climate change

Earth's magnetic field is created by the flow of molten iron in the outer core, which is subject to chaotic oscillations which not only weaken the field, but also cause wandering and sometimes the complete reversal of the poles The magnetic orientations of the minerals in the rock record long-lasting inversions, but cannot capture the details of a turnaround that spans hundreds of years, like the one there is 42 000 years.

Carbon 14 radioactive, however, may mark these shorter fluctuations The isotope is produced when cosmic rays – charged particles from outer space – pass beyond the magnetic field and strike the atmosphere. It is absorbed by living things and its specific half-life makes it a standard clock The team used radiocarbon to date kauri wood by aligning it with accurate records, but rude, of radiocarbon caves from China. And measuring the finer carbon-14 changes in the rings, they tracked how his output varied over intervals of 40 years, as the magnetic field rose and grew. «It’s just amazing that you can do this ago 42 000 years», says Lawrence Edwards, geochemist at the University of Minnesota, Twin Cities, who worked on Chinese cave records.

Radiocarbon spikes indicate that the magnetic field has weakened to about 6% of its current power there is 41 500 years. At this moment, the poles have shifted and the field has regained strength, before crashing and going back 500 years later. Cooper notes that not only was Earth's cosmic ray shield being shot down; the sun was also Evidence from ice cores suggests that, around the same time, the Sun suffered several «great minima» – episodes of weak magnetic activity The resulting cosmic ray onslaught charged the atmosphere to a level that would have knocked out today's power grid and created auroras in the subtropics, said Cooper. «What happens when the atmosphere is so ionized?» he asks “God only knows»(The newspaper is the first that Cooper has managed since his dismissal in 2019 of the University of Adelaide following allegations that he intimidated staff and students; Cooper denied the allegations)

To explore the consequences, the team ran a climate model, which suggested that the bombardment of cosmic rays would have eroded the ozone layer, reducing the heat it normally picks up from ultraviolet rays. High altitude cooling would have altered wind flows, which in turn could have led to «drastic changes» surface, including a warmer North America and a colder Europe, says Marina Friedel, member of the team and doctoral student in stratospheric chemistry at ETH Zurich

This is where other scientists say the study gets too speculative Ice cores from Greenland and Antarctica that cover the 100000 recent years capture large temperature variations every few thousand years But they show no change ago 42000 years A few records from the Pacific Ocean show oscillations But even though the change has mainly occurred in the tropics, as Cooper and colleagues suggest, it should be seen in the mirror, declare Anders Svensson, glaciologist at the University of Copenhagen. «We just don't see it”

The study team goes further by claiming that climate change could explain a series of curious events ago 42 000 years More particularly, large Australian mammals disappeared around this time Neanderthals disappeared from Europe and elaborate cave paintings began to appear in Europe and Asia Yet, none of the milestones of human evolution correspond well to the flip of ago 42000 years, and neither was suddenly, this Thomas Higham, archaeologist and radiocarbon expert at the University of Oxford. Connect them to the field inversion, he says, «seems to me to push the proof too far”

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Terre, earth's magnetic field, Kauri, geomagnetic inversion, research

News – FR – Ancient kauris capture the latest collapse of Earth's magnetic field

Source: https://www.sciencemag.org/news/2021/02/ancient-kauri-trees-capture-last-collapse-earth-s-magnetic-field

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