We’re all probably aware by now that the Earth undergoes magnetic reversals, when the North and South poles swap positions. Discovery of evidence of the reversals – preserved in volcanic rocks on the ocean floor – were instrumental in the 1960s in confirming the process of plate tectonics. At the time, the sequence of repeatedly alternating magnetism either side of the mid-Atlantic ridge was likened to tree-rings, counting the Earth’s history away.
Now, in a fascinating twist of chance, the record of an almost complete magnetic reversal has been discovered in real tree rings, in a New Zealand kauri.
The tree was found during excavation work on a geothermal power plant in Ngawha, near Kaikohe in North Island. Buried in nine metres of soil, the log measured some 2.5 metres in diameter and 16 metres in length and weighed about 60 tonnes. Carbon dating showed that it lived from about 42,500 to 41,000 years ago, and its more than 1500 tree-rings contain evidence of a period of wandering magnetic poles that persisted for the whole of the tree's life.
Known as the Laschamp event, this geomagnetic excursion was first recognised in volcanic lavas at two sites in France. Since then, it has been found at thirty or more sites across the world, including in Australia and New Zealand. It seems to have lasted from about 43,000 to 38,000 years BP. During this period, the two poles wandered erratically in a broadly clockwise direction, and for about 400 years, around 41,000 years ago, were almost completely reversed. Throughout the period, the strength of the geomagnetic field was also considerably weakened, reaching only 25 percent of the current level at its lowest. This significantly lowered Earth’s protection against cosmic rays, which in turn led to an increased exposure to radiation. Evidence for this is shown by peaks of radioactive beryllium in ice cores recovered from the Greenland ice sheet.
Samples of the tree are now being analyzed by scientists, led by Chris Turney from the University of New South Wales—an expert in paleoclimatology and climate change. "Because the Earth's magnetic field has a major effect on how much radiocarbon carbon is formed in the upper atmosphere, these precious analyses will allow us to investigate the magnitude and rate of change when the magnetic field reversed during the Laschamp; something not possible before and of great interest given recent changes in the Earth's magnetic field," Turney said.
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For more information about the geomagnetic reversals and the Laschamp event, see:
Evidence for a rapid reversal of the geomagnetic field 41,000 years ago. https://www.iflscience.com/environment/evidence-rapid-reversal-geomagnetic-field-41000-years-ago/
Roberts, A. 2008 Geomagnetic excursions: Knowns and unknowns. Geophysical Research Letters 35, L17307, doi:10.1029/2008GL034719 (people.rses.anu.edu.au/roberts_a/AR_Publications/111.%20Roberts%20GRL%202008.pdf)
The NMRC Blog
An occasional article about things geological, inspired by things Club members have done or found, or by recent scientific discoveries.